Patent Publication Number: US-11655611-B2

Title: Shovel

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation of International Application No. PCT/JP2018/023151, filed on Jun. 18, 2018, which claims priority to Japanese Application No. 2017-121776, filed on Jun. 21, 2017, Japanese Application No. 2017-121777, filed on Jun. 21, 2017, Japanese Application No. 2017-121778, filed on Jun. 21, 2017, and Japanese Application No. 2017-143522, filed on Jul. 25, 2017, the entire content of each of which is incorporated herein by reference. 
    
    
     BACKGROUND 
     Technical Field 
     The disclosures herein relate to a shovel. 
     Description of Related Art 
     Conventionally, in order to prevent the movement of a shovel not intended by an operator (hereinafter simply referred to as an “unintended movement”), a technique that corrects the movement of an attachment of the shovel is known. 
     Patent Document 1 describes the technique that controls the pressure of a hydraulic cylinder, which drives the attachment of the shovel, not to exceed a predetermined maximum allowable pressure, thereby minimizing an unintended movement such as the dragging or lifting of the shovel. 
     However, it is desirable to minimize an unintended movement whatever the operating state of the attachment. Therefore, the movement of the attachment is required to be corrected regardless of the operating state of the attachment. 
     SUMMARY 
     According to an embodiment of the present invention, a shovel includes a traveling body, a turning body turnably mounted on the traveling body; an attachment attached to the turning body, a hydraulic actuator configured to drive the attachment, and a controller. The controller is configured to control the hydraulic actuator to minimize a change in orientation of the traveling body or of the turning body, in response to a change in moment caused by an aerial movement of the attachment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
         FIG.  1    is a drawing illustrating a shovel according to an embodiment of the present invention; 
         FIG.  2    is a block diagram illustrating an example configuration of a drive system of the shovel according to the embodiment of the present invention; 
         FIG.  3    is a drawing illustrating an example of a forward dragging movement of the shovel; 
         FIG.  4 A  is a drawing illustrating an example of an backward dragging movement of the shovel; 
         FIG.  4 B  is a drawing illustrating an example of the backward dragging movement of the shovel; 
         FIG.  5    is a drawing illustrating an example of a front lifting movement of the shovel; 
         FIG.  6    is a drawing illustrating an example of a rear lifting movement of the shovel; 
         FIG.  7 A  is a drawing illustrating an example of a vibration movement of the shovel; 
         FIG.  7 B  is a drawing illustrating the example of the vibration movement of the shovel; 
         FIGS.  8 A and  8 B  are graphs illustrating the example of vibration movement of the shovel; 
         FIG.  9 A  is a drawing schematically illustrating a method for preventing an unintended movement of the shovel; 
         FIG.  9 B  is a drawing schematically illustrating the method for preventing the unintended movement of the shovel; 
         FIG.  9 C  is a drawing schematically illustrating the method for preventing the unintended movement of the shovel; 
         FIG.  9 D  is a drawing schematically illustrating the method for preventing the unintended movement of the shovel; 
         FIG.  10    is a drawing illustrating an example mechanical model of forward dragging; 
         FIG.  11    is a drawing illustrating an example mechanical model of backward dragging; 
         FIG.  12    is a drawing schematically illustrating an example mechanical model of the lifting of the front of the shovel; 
         FIG.  13    is a drawing schematically illustrating an example mechanical model of the lifting of the rear of the shovel; 
         FIG.  14 A  is a drawing illustrating the relationship between a tipping fulcrum and the direction of an upper turning body; 
         FIG.  14 B  is a drawing illustrating the relationship between the tipping fulcrum and the direction of the upper turning body; 
         FIG.  14 C  is a drawing illustrating the relationship between the tipping fulcrum and the direction of the upper turning body; 
         FIG.  15    is a drawing illustrating the relationship between a tipping fulcrum and the conditions of the ground surface; 
         FIG.  16    is a flowchart illustrating an example of a process performed by a controller to set a control condition when lifting is detected, 
         FIG.  17 A  is a drawing illustrating examples of waveforms related to vibration of the shovel; 
         FIG.  17 B  is a drawing illustrating examples of waveforms related to vibration of the shovel; 
         FIG.  17 C  is a drawing illustrating examples of waveforms related to vibration of the shovel; 
         FIG.  18    is a drawing illustrating a method for acquiring a limit thrust; 
         FIG.  19 A  is a drawing illustrating a first example of a method for determining the occurrence of dragging; 
         FIG.  19 B  is a drawing illustrating the first example of the method for determining the occurrence of dragging; 
         FIG.  20    is a drawing illustrating a second example of the method for determining the occurrence of dragging; 
         FIG.  21 A  is a drawing illustrating a third example of the method for determining the occurrence of dragging; 
         FIG.  21 B  is a drawing illustrating the third example of the method for determining the occurrence of dragging; 
         FIG.  22 A  is a drawing illustrating a fourth example of the method for determining the occurrence of dragging; 
         FIG.  22 B  is a drawing illustrating the fourth example of the method for determining the occurrence of dragging; 
         FIG.  23 A  is a graph illustrating a first example of a method for determining the occurrence of lifting; 
         FIG.  23 B  is a graph illustrating the first example of the method for determining the occurrence of lifting; 
         FIG.  23 C  is a graph illustrating the first example of the method for determining the occurrence of lifting; 
         FIG.  24    is a drawing illustrating a second example of the method for determining the occurrence of lifting; 
         FIG.  25 A  is a drawing illustrating a third example of the method for determining the occurrence of lifting; 
         FIG.  25 B  is a drawing illustrating the third example of the method for determining the occurrence of lifting; 
         FIG.  26 A  is a drawing illustrating a fourth example of the method for determining the occurrence of lifting; 
         FIG.  26 B  is a drawing illustrating the fourth example of the method for determining the occurrence of lifting; 
         FIG.  27    is a drawing schematically illustrating a first example of a characteristic configuration of the shovel; 
         FIG.  28    is a drawing schematically illustrating a second example of the characteristic configuration of the shovel; 
         FIG.  29    is a drawing schematically illustrating a third example of the characteristic configuration of the shovel; 
         FIG.  30    is a drawing schematically illustrating a fourth example of the characteristic configuration of the shovel; 
         FIG.  31    is a drawing schematically illustrating a fifth example of the characteristic configuration of the shovel; 
         FIG.  32    is a drawing schematically illustrating a sixth example of the characteristic configuration of the shovel; 
         FIG.  33    is a drawing schematically illustrating a seventh example of the characteristic configuration of the shovel; 
         FIG.  34    is a drawing schematically illustrating an eighth example of the characteristic configuration of the shovel; 
         FIG.  35    is a drawing schematically illustrating a ninth example of the characteristic configuration of the shovel; 
         FIG.  36    is a flowchart schematically illustrating an example of a process (predetermined movement minimizing process) for minimizing an unintended movement of the shovel; 
         FIG.  37    is a drawing illustrating a first variation of the shovel; 
         FIG.  38    is a drawing illustrating the first variation of the shovel; 
         FIG.  39    is a drawing illustrating a second variation of the shovel; 
         FIG.  40    is a drawing illustrating a third variation of the shovel; 
         FIG.  41    is a drawing illustrating an example configuration of a drive system of a shovel according to a fourth variation; 
         FIG.  42    is a drawing illustrating the relationship between forces that act on the shovel when excavation is performed; 
         FIG.  43    is a drawing illustrating an example configuration of a hydraulic circuit installed in the shovel; 
         FIG.  44    is a flowchart illustrating a flow of a first support process; 
         FIG.  45    is a drawing illustrating changes in physical quantities over time during arm excavation work; 
         FIG.  46    is a drawing illustrating a configuration example of another hydraulic circuit installed in the shovel; 
         FIG.  47    is a flowchart illustrating a flow of a second support process; and 
         FIG.  48    is a flowchart illustrating a flow of a third support process. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     It is desirable to provide a shovel that corrects the movement of an attachment regardless of the operating state of the attachment by an operator. 
     In the following, embodiments of the present invention will be described with reference to the accompanying drawings. 
     In the drawings, the same or corresponding elements are denoted by the same reference numerals and a duplicate description thereof may be omitted. 
     [Overview of Shovel] 
     First, referring to  FIG.  1   , an overview of a shovel  100  will be described. 
       FIG.  1    is a side view of the shovel  100  according to an embodiment of the present invention. 
     The shovel  100  according to the present embodiment includes a lower traveling body  1 , an upper turning body  3  turnably mounted on the lower traveling body  1  via a turning mechanism  2 , a boom  4 , an arm  5 , a bucket  6 , and a cabin  10  in which an operator is located. The boom  4 , the arm  5 , and the bucket  6  serve as an attachment. 
     The lower traveling body  1  (an example of a traveling body) includes a pair of left and right crawlers. The crawlers are hydraulically driven by respective traveling hydraulic motors  1 L and  1 R (see  FIG.  2   , for example) to move the shovel  100 . 
     The upper turning body  3  (an example of a turning body) is driven by a turning hydraulic motor  21  (see  FIG.  2   ), which will be described below, and is rotated with respect to the lower traveling body  1 . 
     The boom  4  is pivotally attached to the front center of the upper turning body  3 , the arm  5  is pivotally attached to the end of the boom  4 , and the bucket  6  is pivotally attached to the end of the arm  5 , in such a manner that the boom  4 , the arm  5 , and the bucket  6  are raised and lowered. The boom  4 , the arm  5 , and the bucket  6  are hydraulically driven by a boom cylinder  7 , an arm cylinder  8 , and a bucket cylinder  9 , respectively. The boom cylinder  7 , the arm cylinder  8 , and the bucket cylinder  9  serve as hydraulic actuators. 
     The cabin  10  is mounted on the front left of the upper turning body  3 , and the operator is located in the cabin  10 . 
     [Basic Configuration of Shovel] 
     Next, referring to  FIG.  2   , a configuration of the shovel  100  according to the present embodiment will be described. 
       FIG.  2    is a block diagram illustrating an example configuration of a drive system of the shovel  100  according to the present embodiment. 
     In  FIG.  2   , a mechanical power system is indicated by a double line, a hydraulic oil line (high-pressure hydraulic line) is indicated by a thick continuous line, a pilot line is indicated by a dashed line, and an electric drive control system is indicated by a thin continuous line. 
     A hydraulic drive system of the shovel  100  according to the present embodiment includes an engine  11 , a main pump  14 , and a control valve  17 . As described above, the hydraulic drive system according to the present embodiment includes the traveling hydraulic motors  1 L and  1 R, the turning hydraulic motor  21 , the boom cylinder  7 , the arm cylinder  8 , the bucket cylinder  9 , which hydraulically drive the lower traveling body  1 , the upper turning body  3 , the boom  4 , the arm  5 , and the bucket  6 , respectively. 
     The engine  11  is a drive power source of the shovel  100 , and is mounted on the rear of the upper turning body  3 , for example. The engine  11  is a diesel engine using diesel fuel as fuel. The main pump  14  and a pilot pump  15  are connected to the output shaft of the engine  11 . 
     The main pump  14  is installed at the rear of the upper turning body  3 , for example, and supplies hydraulic oil to the control valve  17  via a hydraulic oil line  16 . The main pump  14  is driven by the engine  11  as described above. The main pump  14  is, for example, a variable displacement hydraulic pump, and the inclination angle of a swash plate is controlled by a regulator  14 A (see  FIG.  29   ), which will be described below, thereby adjusting the length of stroke of a piston and controlling a discharge flow rate (discharge pressure). 
     The control valve  17  is a hydraulic control unit that is installed, for example, at the center of the upper turning body  3 , and that controls the hydraulic drive system of the shovel  100  in accordance with the operation performed by the operator with an operation device  26 . Hydraulic actuators such as a left-side traveling hydraulic motor  1 L, a right-side traveling hydraulic motor  1 R, the boom cylinder  7 , the arm cylinder  8 , the bucket cylinder  9 , and the turning hydraulic motor  21  are connected to the control valve  17  via hydraulic oil lines. The control valve  17  is provided between the main pump  14  and the hydraulic actuators. The control valve  17  is a valve unit that includes a plurality of hydraulic control valves, namely direction control valves (such as a boom direction control valve  17 A as will be described below) that control the flow rate and the direction of hydraulic oil supplied to each of the hydraulic actuators. 
     Next, an operation system of the shovel  100  according to the present embodiment includes the pilot pump  15 , the operation device  26 , and a pressure sensor  29 . 
     The pilot pump  15  is installed, for example, at the rear of the upper turning body  3 , and applies a pilot pressure to a mechanical brake  23  and the operation device  26  via a pilot line  25 . For example, the pilot pump  15  is a fixed displacement hydraulic pump, and is driven by the above-described engine  11 . 
     The operation device  26  includes levers  26 A and  26 B, and a pedal  26 C. The operation device  26  is provided near an operator&#39;s seat of the cabin  10 , and allows the operator to perform operations of operational elements (such as the lower traveling body  1 , the upper turning body  3 , the boom  4 , the arm  5 , and the bucket  6 ). In other words, the operation device  2  enables operations of the hydraulic actuators (such as the traveling hydraulic motors  1 L and  1 R, the boom cylinder  7 , the arm cylinder  8 , the bucket cylinder  9 , and the turning hydraulic motor  21 ), which drive the respective operational elements. The operation device  26  (the levers  26 A and  26 B, and the pedal  26 C) is connected to the control valve  17  via a pilot line  27 . The control valve  17  receives a pilot signal (pilot pressure) corresponding to the state of an operation of each of the lower traveling body  1 , the upper turning body  3 , the boom  4 , the arm  5 , and the bucket  6  performed with the operation device  26 . Accordingly, the control valve  17  can drive each of the hydraulic actuators in accordance with the state of an operation performed with the operation device  26 . The operation device  26  is connected to the pressure sensor  29  via a pilot line  28 . 
     The levers  26 A and  26 B are respectively provided on the left side and on the right side of the operator seated on the operator&#39;s seat within the cabin  10 . The levers  26 A and  26 B are configured to be tilted forward and backward and to the left and right from the neutral position (a state in which no operation is performed by the operator). Operations of tilting the lever  26 A forward, backward, to the left, and to the right, and operations of tilting the lever  26 B forward, backward, to the left, and to the right are set as appropriate so as to operate the upper turning body (turning hydraulic motor  21 ), the boom  4  (boom cylinder  7 ), the arm  5  (arm cylinder  8 ), and the bucket  6  (bucket cylinder  9 ). 
     Further, the pedal  26 C is provided on the floor ahead of the operator seated on the operator&#39;s seat within the cabin  10 . The pedal  26 C is configured to be stepped by the operator to operate the lower traveling body  1  (traveling hydraulic motors  1 L and  1 R). 
     As described above, the pressure sensor  29  is connected to the operation device  26  via the pilot line  28 , detects the secondary-side pilot pressure of the operation device  26 , namely the pilot pressure corresponding to the state of an operation of each of the operational elements performed with the operation device  26 . The pressure sensor  29  is connected to the controller  30 . The controller  30  receives a pressure signal (a detected pressure value) corresponding to the state of an operation of each of the lower traveling body  1 , the upper turning body  3 , the boom  4 , the arm  5 , and the bucket  6  performed with the operation device  26 . Accordingly, the controller  30  can identify the state of an operation of each of the lower traveling body  1 , the upper turning body  3 , and the attachment of the shovel. 
     Next, a control system of the shovel  100  according to the present embodiment includes various types of sensors  32 . 
     The controller  30  is a main controller that controls the driving of the shovel  100 . The controller  30  may be implemented by any hardware, software, or a combination thereof. The controller  30  may be configured mainly by a microcomputer including a central processing unit (CPU), a random-access memory (RAM), a read-only memory (ROM), an auxiliary storage device, and an input-output (I/O) interface. The controller  30  controls the driving by causing the CPU to execute various types of programs stored in the ROM, the auxiliary storage device, and the like. 
     In the present embodiment, the controller  30  determines the occurrence of a predetermined movement of the shovel  100  not intended by the operator (hereinafter simply referred to as an unintended movement). Namely, the controller  30  determines the occurrence of a movement of the shovel  100  not desired by the operator. If the controller  30  determines that an unintended movement has occurred, the controller  30  corrects the movement of the attachment of the shovel  100  to minimize the movement of the attachment. Accordingly, the unintended movement of the shovel  100  is minimized. 
     Examples of the unintended movement include a forward dragging movement in which the shovel  100  is dragged forward by an excavation reaction force, a backward dragging movement in which the shovel  100  is dragged backward by a reaction force from the ground when leveling the ground. The unintended movement occurs without the lower traveling body  1  being operated by the operator. In the following, the term “forward dragging movement” and the term “backward dragging movement” may be correctively referred to as a “dragging movement” without being distinguished. The examples of the unintended movement further include a lifting movement in which the front or the rear of the shovel  100  is lifted by an excavation reaction force. In the following, the lifting movement may be distinguished between a front lifting movement in which the front of the shovel  100  is lifted and a rear lifting movement in which the rear of the shovel  100  is lifted. The examples of the unintended movement further include vibration of the body (the lower traveling body  1 , the turning mechanism  2 , or the upper turning body  3 ) of the shovel  100  caused by a change in the moment of inertia during in-air movement of the attachment of the shovel  100  (namely, during the movement of the attachment without the bucket  6  contacting the ground). Details of the unintended movement will be described below. 
     The controller  30  includes a movement determining unit  301  and a movement correcting unit  302  as functional units implemented by causing the CPU to execute one or more of the programs stored in the ROM and the auxiliary storage device. 
     The movement determining unit  301  determines the occurrence of an unintended movement, based on sensor information on various states of the shovel  100 . The sensor information is input from the pressure sensor  29  and the various types of sensors  32 . Details of determination methods will be described below. 
     When the movement determining unit  301  determines that an unintended movement has occurred, the movement correcting unit  302  corrects the movement of the attachment to minimize the unintended movement. Details of a correction method will be described below. 
     The various types of sensors  32  are known detectors for detecting various states of the shovel  100  and various states in the vicinity of the shovel  100 . The various types of sensors  32  may include an angle sensor that detects an angle at a joint between the upper turning body  3  and the boom  4  relative to a reference plane of the boom  4  (a boom angle), an angle sensor that detects an angle of the arm  5  relative to the arm  5  (an arm angle), and an angle sensor that detects an angle of the bucket  6  relative to the arm  5  (a bucket angle). Further, the various types of sensors  32  may include pressure sensors that detect the pressure of hydraulic oil in hydraulic actuators. More specifically, the pressure sensors detect the pressure in a rod-side oil chamber and the pressure in a bottom-side oil chamber of a hydraulic cylinder. Further, the various types of sensors  32  may include sensors that detect movement states of the lower traveling body  1 , the upper turning body  3 , and the attachment. For example, the various types of sensors  32  may include an acceleration sensor, an angular acceleration sensor, and an inertial measurement unit (IMU) capable of outputting three-axis acceleration and three-axis angular acceleration. Further, the various types of sensors  32  may also include a distance sensor or an image sensor that detects a relative position of the ground surface or an obstacle in the vicinity of the shovel  100 . 
     [Movement of Shovel Unintended by Operator] 
     Next, referring to  FIG.  3    through  FIG.  8 B , details of the movement of the shovel  100  unintended by the operator will be described. 
     &lt;Forward Dragging Movement&gt; 
       FIG.  3    is a drawing illustrating an example of the forward dragging movement of the shovel  100 . More specifically,  FIG.  3    is a drawing illustrating a work situation in which the shovel  100  is dragged forward. 
     As illustrated in  FIG.  3   , the shovel  100  is excavating a ground surface  30   a . Mainly because of the closing movement of the arm  5  and the bucket  6 , a force F 2  is exerted on the ground surface  30   a  by the bucket  6  in an obliquely downward direction toward the body (the lower traveling body  1 , the turning mechanism  2 , and the upper turning body  3 ) of the shovel  100 . At this time, a reaction force F 3  of the force F 2  against the bucket  6  acts on the body (the lower traveling body  1 , the turning mechanism  2 , and the upper turning body  3 ) of the shovel  100  through the attachment. Namely, the reaction force F 3  corresponding to a horizontal component F 2   a H of an excavation reaction force F 2   a  acts on the body of the shovel  100  through the attachment. If the reaction force F 3  exceeds the maximum static friction force F 0  between the shovel  100  and the ground surface  30   a , the body of the shovel  100  would be dragged forward. 
     &lt;Backward Dragging Movement&gt; 
     Next,  FIG.  4 A  and  FIG.  4 B  are drawings illustrating an example of the backward dragging movement of the shovel  100 . More specifically,  FIG.  4 A  and  FIG.  4 B  are drawings illustrating work situations in which the shovel  100  is dragged backward. 
     As illustrated in  FIG.  4 A , the shovel  100  is leveling a ground surface  40   a . A force F 2  is generated mainly by opening the arm  5  so that the bucket  6  pushes sediment  40   b  forward. At this time, a reaction force F 3  of the force F 2  against the bucket  6  acts on the body of the shovel  100  through the attachment. If the reaction force F 3  exceeds the maximum static friction force F 0  between the shovel  100  and the ground surface  40   a , the body of the shovel  100  would be dragged backward. 
     Further, as illustrated in  FIG.  4 B , the shovel  100  is performing river construction work. More specifically, in order to solidify sediment, the shovel  100  is pushing the bucket  6  against the surface  40   c  of a sloped bank by opening the arm  5 . In such a construction work, a reaction force F 3  of a force F 2  against the bucket  6  acts on the body of the shovel  100  through the attachment. As a result, the body of the shovel  100  may be dragged backward. 
     &lt;Front Lifting Movement&gt; 
     Next,  FIG.  5    is a drawing illustrating an example of the front lifting movement of the shovel  100 . More specifically,  FIG.  5    is a drawing illustrating a work situation in which the front of the shovel  100  is lifted. 
     As illustrated in  FIG.  5   , the shovel  100  is excavating a ground surface  50   a . Mainly because of the closing movement of the arm  5  and the bucket  6 , a force F 2  is exerted on the ground surface  50   a  by the bucket  6  in an obliquely downward direction toward the body of the shovel  100 . At this time, a reaction force F 3  (a moment of force, which is hereinafter simply referred to as a “moment”) of the force F 2  against the bucket  6  acts on the body of the shovel  100  through the attachment which causes the body of the shovel  100  to be tiled backward. Namely, the reaction force F 3  corresponding to a vertical component F 2   a V of an excavation reaction force F 2   a  acts on the body of the shovel  100  through the attachment. Specifically, the reaction force F 3  acts on the body of the shovel  100  as a force F 1  that lifts the boom cylinder  7 . If the moment caused by the force F 1  exceeds a force (a moment) that pushes the body of the shovel  100  to the ground by gravity, the body of the shovel  100  would be lifted. 
     &lt;Rear Lifting Movement&gt; 
     Next,  FIG.  6    is a drawing illustrating an example of the rear lifting movement of the shovel  100 . More specifically,  FIG.  6    is a drawing illustrating a work situation in which the rear of the shovel  100  is lifted. 
     As illustrated in  FIG.  6   , the shovel  100  is excavating a ground surface  60   a . A force F 2  (a moment) that causes the bucket  6  to excavate a sloped surface  60   b  is generated. In addition, a force F 3  (a moment) that causes the boom  4  to push the bucket  6  against the sloped surface  60   b  is generated. In other words, the force F 3  (the moment) that causes the body of the shovel  100  to be tilted forward is generated. At this time, a force F 1  that lifts the rod of the boom cylinder  7  is generated, and the force F 1  acts to tilt the body of the shovel  100 . If the moment, caused by the force F 1 , that tilts the body of the shovel  100  forward exceeds a force (a moment) that pushes the body of the shovel  100  to the ground by gravity, the rear of the shovel  100  would be lifted. 
     If the bucket  6  is in contact with the ground surface or an object, and is caught by or partially embedded into the ground surface or the object, the boom  4  does not move even if a force is exerted on the boom  4 . Thus, the rod of the boom cylinder  7  would not be displaced. If the pressure in a contraction-side (in the present embodiment, rod-side) oil chamber of the boom cylinder  7  increases, the force F 1  that lifts the boom cylinder  7  would increase, that is, the force that tilts the body of the shovel  100  forward would increase. 
     The above-described situation may occur when the bucket  6  is located below the body (lower traveling body  1 ) of the shovel  100  during deep excavation work, in addition to the leveling work of the front sloped surface as illustrated in  FIG.  6   . Further, the above-described situation may occur not only when the boom  4  is operated, but also when the arm  5  or the bucket  6  is operated. 
     &lt;Vibration Movement&gt; 
     Next,  FIG.  7 A  and  FIG.  7 B  and  FIG.  8 A  and  FIG.  8 B  are drawings illustrating examples of vibration of the shovel  100 . More specifically,  FIG.  7 A  and  FIG.  7 B  are diagrams illustrating an example situation in which the shovel  100  is vibrated when the attachment is being moved in the air.  FIG.  8 A  is a graph illustrating a waveform of an angle about the pitch axis (a pitch angle) over time, and  FIG.  8 B  is a graph illustrating a waveform of angular velocity (pitch angular velocity) over time during an discharge operation of the shovel  100  illustrated in  FIG.  7 A  and  FIG.  7 B . In the present embodiment, as an example of the in-air movement of the attachment, a discharge movement for discharging a load placed in the bucket  6  will be described. 
     As illustrated in  FIG.  7 A , in the shovel  100 , the bucket  6  and the arm  5  are closed, the boom  4  is raised, and load DP such as sediment is placed in the bucket  6 . 
     When the shovel  100  performs a discharge operation from the state illustrated in  FIG.  7 A , the bucket  6  and the arm  5  are largely opened, the boom  4  is lowered, and the load DP is discharged from the bucket  6  to the outside, as illustrated in  FIG.  7 B . At this time, a change in the moment of inertia of the attachment causes the body of the shovel  100  to be vibrated in the pitch direction indicated by an arrow A in  FIG.  7 B . 
     As is seen from  FIG.  8 A  and  FIG.  8 B , an overturning moment that causes the shovel  100  to turn over is generated during the aerial movement of the attachment, specifically during the discharge operation, thereby causing the body of the shovel  100  to be vibrated about the pitch axis. 
     [Method for Minimizing Unintended Movement of Shovel] 
     Next, referring to  FIG.  9 A  through  FIG.  18   , a method for minimizing the above-described unintended movements of the shovel  100  will be described. 
     &lt;Overview of Method for Minimizing Unintended Movement of Shovel&gt; 
     First,  FIG.  9 A  through  FIG.  9 D  are drawings schematically illustrating methods for minimizing unintended movements of the shovel  100 . More specifically,  FIG.  9 A  through  FIG.  9 D  are plan views of the shovel  100  viewed from above, in which combinations of the direction of the lower traveling body  1  and the turning angle of the upper turning body  3  are different from each other. 
     In plan view, the attachment, configured by the boom  4 , the arm  5 , and the bucket  6 , is always operated on a line L 1  that corresponds to the extending direction of the attachment, namely operated in the same vertical plane, regardless of the orientation and the operation of the attachment. Thus, it can be said that, when the attachment is in motion, a reaction force F 3  is exerted on the body of the shovel  100  by the attachment in the vertical plane. This does not depend on the positional relationship (turning angle) between the lower traveling body  1  and the upper turning body  3 . As illustrated in  FIG.  3    through  FIG.  7 B , the direction of the reaction force F 3  in plan view may differ depending on the operation content. That is, when the shovel  100  is subjected to an unintended movement such as dragging, lifting, or vibration, the unintended movement is caused by the movement of the attachment. Accordingly, the above-described unintended movements can be minimized by controlling the attachment. 
     &lt;Method for Minimizing Dragging Movements&gt; 
       FIG.  10    is a drawing schematically illustrating an example method for minimizing the forward dragging movement of the shovel  100 . More specifically,  FIG.  10    is a drawing illustrating an example mechanical model of the shovel  100  dragged forward. Similar to  FIG.  3   ,  FIG.  10    depicts a force acting on the shovel  100  when the shovel  100  is excavating a ground surface  100   a .  FIG.  11    is a drawing schematically illustrating an example method for minimizing the backward dragging movement of the shovel  100 . More specifically,  FIG.  11    is a drawing illustrating an example mechanical model of the shovel  100  dragged backward. Similar to  FIG.  4 A ,  FIG.  11    depicts a force acting on the shovel  100  when the shovel  100  is leveling a ground surface  110   a  by pushing sediment  110   b  forward. 
     As illustrated in  FIG.  10    and  FIG.  11   , a force F 3  that pushes the body (upper turning body  3 ) of the shovel  100  in the horizontal direction (either forward or backward) is expressed by the following equation (1).
 
 F 3 =F 1 sin η1  (1)
 
     In the above equation, η 1  represents an angle formed by the boom cylinder  7  and a vertical axis  100   c  or  110   c , F 1  represents a force exerted on the upper turning body  3  by the boom cylinder  7 , namely exerted on the body of the shovel  100  by the attachment. 
     The maximum static friction force F 0  is expressed by the following equation (2).
 
 F 0 =μMg   (2)
 
     In the above equation, μ represents a static friction coefficient between the lower traveling body  1  and each of the ground surfaces  100   a  and  110   a , M represents a body mass, and g represents gravitational acceleration. 
     A condition in which the shovel  100  is not dragged by the reaction force F 3  is expressed by the following inequality (3).
 
 F 3 &lt;F 0  (3)
 
     By substituting the equations (1) and (2) into the inequality (3), the following inequality (4) is obtained.
 
 F 1 sin η1&lt;μ Mg   (4)
 
     That is, the movement correcting unit  302  may correct the movement of the boom cylinder  7  such that the inequality (4) is established. As a result, it is possible to prevent the shovel  100  from being dragged backward. 
     For example, as indicated by the following equation (5), the force F 1  is expressed by a function f with an argument PR that represents the pressure in the rod-side oil chamber (rod pressure) and an argument P B  that represents the pressure in the bottom-side oil chamber (bottom pressure).
 
 F 1= f ( PR,P   B )  (5)
 
     The movement correcting unit  302  (force estimating unit) calculates (estimates) the force F 1  by using the equation (5) based on the rod pressure P R  and the bottom pressure P B . At this time, the movement correcting unit  302  may obtain the rod pressure P R  and the bottom pressure P B , based on output signals of pressure sensors that detect the rod pressure and the bottom pressure of the boom cylinder  7 . The pressure sensors may be included in the various types of sensors  32 . 
     By way of example, the force F 1  may be expressed by the following equation (6).
 
 F 1= AR·P   R   −AB·P   B   (6)
 
     In the above equation, AR represents a rod-side pressure receiving area, and AB represents a bottom-side pressure receiving area. 
     Accordingly, the movement correcting unit  302  (force estimating unit) may calculate (estimate) the force F 1  based on the equation (6). 
     Further, the movement correcting unit  302  (angle calculating unit) calculates the angle η 1  formed by the boom cylinder  7  and the vertical axis  100   c  or  110   c . The angle η 1  may be geometrically calculated based on the extension length of the boom cylinder  7 , the size of the shovel  100 , and the tilt of the body of the shovel  100 . For example, the movement correcting unit  302  may calculate the angle η 1  based on the output of a sensor that detects the boom angle. The sensor that detects the boom angle may be included in the various types of sensors  32 . 
     Note that the angle η 1  may be obtained from the output of a sensor that directly measures the angle η 1 . The sensor that directly measures the angle η 1  may be included in the various types of sensors  32 . 
     The movement correcting unit  302  (pressure controlling unit) controls the pressure of the boom cylinder  7 , based on the obtained (calculated) force F 1  and the angle η 1 , such that the inequality (4) is established. More specifically, the movement correcting unit  302  controls excessive one of either the pressure of the rod-side oil chamber or the pressure of the bottom-side oil chamber. That is, the movement correcting unit  302  (pressure controlling unit) controls either the rod pressure P R  or the bottom pressure P B , such that the inequality (4) is established. More specifically, by employing various configurations (see  FIG.  26 A  through  FIG.  34   ), which will be described below, it becomes possible for the movement correcting unit  302  to control the pressure of the boom cylinder  7  by outputting a control command to a control target. Accordingly, the dragging of the shovel  100  is minimized. 
     Note that the static friction coefficient μ in the inequality (4) may be a given typical value, or may be input by the operator in accordance with the conditions of the ground surface at the work site. Alternatively, the shovel  100  may further include an estimation device for estimating the static friction coefficient μ. Specifically, the estimation device may calculate the static friction coefficient μ, based on the force F 1  exerted by the attachment and causing the stationary shovel  100  to slide (to be dragged). As will be described below, the occurrence of dragging can be determined by mounting an acceleration sensor or any other sensor on the upper turning body  3 , as necessary. 
     &lt;Method for Minimizing Lifting Movement&gt; 
     Next,  FIG.  12    is a drawing schematically illustrating an example method for minimizing the lifting movement in which the front of the shovel  100  is lifted. More specifically,  FIG.  12    is a drawing illustrating a mechanical model of the lifting movement in which the front of the shovel  100  is lifted. Similar to  FIG.  5   ,  FIG.  12    depicts a force acting on the shovel  100  when the shovel  100  is excavating a ground surface  120   a.    
     As illustrated in  FIG.  12   , a tipping fulcrum P 1  of the shovel  100  may be regarded as the rearmost end of an effective grounding area  120   b  of the lower traveling body  1  in the extending direction of the attachment (the direction of the upper turning body  3 ). Accordingly, a moment τ 1  that lifts the front of the shovel  100  about the tipping fulcrum P 1  is expressed by the following equation (7), based on the force F 1  and also the distance D 3  between an extension line  12  of the boom cylinder  7  and the tipping fulcrum P 1 .
 
τ1= D 3· F 1  (7)
 
     A moment τ 2  that pushes the body of the shovel  100  to the ground about the tipping fulcrum P 1  is expressed by the following equation (8), based on the distance D 1  between the center of gravity P 3  and the rear tipping fulcrum P 1  of the lower traveling body  1 , the body mass M, and the gravitational acceleration g.
 
τ2= D 1· Mg   (8)
 
     A condition for stabilizing the body of the shovel  100  without lifting the front of the shovel  100  is expressed by the following inequality (9).
 
τ1&lt;τ2  (9)
 
     By substituting the equations (7) and (8) into the inequality (9), the following inequality (10) is obtained as a stability condition.
 
 D 3· F 1&lt; D 1· Mg   (10)
 
     That is, the movement correcting unit  302  may correct the movement of the attachment such that the inequality (10) serving as the stability condition is established. As a result, the lifting of the front of the shovel  100  is prevented. 
     Further,  FIG.  13    is a drawing illustrating a mechanical model of the movement in which the rear of the shovel  100  is lifted. Similar to  FIG.  6   ,  FIG.  13    depicts a force acting on the shovel  100  when the shovel  100  is excavating a ground surface  130   a.    
     A tipping fulcrum P 1  of the shovel  100  may be regarded as the frontmost end of an effective grounding area  130   b  of the lower traveling body  1  in the extending direction of the attachment (the direction of the upper turning body  3 ). Accordingly, a moment τ 1  that lifts the rear of the shovel  100  about the tipping fulcrum P 1  is expressed by the following equation (11), based on the force F 1  and the distance D 4  between an extension line  12  of the boom cylinder  7  and the tipping fulcrum P 1 .
 
τ1= D 4· F 1  (11)
 
     A moment τ 2  that pushes the body of the shovel  100  to the ground about the tipping fulcrum P 1  is expressed by the following equation (12), based on the distance D 2  between the center of gravity P 3  and the front tipping fulcrum P 1  of the lower traveling body  1 , the body mass M, and the gravitational acceleration g.
 
τ2= D 2· Mg   (12)
 
     Similar to the inequality (9), a condition for stabilizing the body of the shovel  100  without lifting the rear of the shovel  100  is expressed by the following inequality (13).
 
τ1&lt;τ2  (13)
 
     By substituting the equations (11) and (12) into the inequality (13), the following inequality (14) is obtained as a stability condition.
 
 D 4· F 1&lt; D 2· Mg   (14)
 
     That is, the movement correcting unit  302  may correct the movement of the attachment such that the inequality (14) serving as the stability condition is established. As a result, the lifting of the rear of the shovel  100  is prevented. 
     Further, by replacing the distances D 1  and D 3  with DA, replacing the distances D 2  and D 4  with DB, and using the front tipping fulcrum P 1  and the rear tipping fulcrum P 1 , a condition for controlling (stabilizing) the front lifting and the rear lifting are expressed by the following expression (15).
 
 DB·F 1&lt; DA·Mg   (15)
 
     For example, similar to the above-described equation (5), as indicated by the following equation (16), the force F 1  is expressed by a function f with the arguments of the rod pressure P R  and the bottom pressure P B  of the boom cylinder  7 .
 
 F 1= f ( P   R   ,P   B )  (16)
 
     The movement correcting unit  302  (force estimating unit) calculates (estimates) the force F 1  exerted on the upper turning body  3  by the boom cylinder  7 , based on the rod pressure P R  and the bottom pressure P B . At this time, the movement correcting unit  302  may obtain the rod pressure P R  and the bottom pressure P B , based on output signals of pressure sensors that detect the rod pressure and the bottom pressure of the boom cylinder  7 . The pressure sensors may be included in the various types of sensors  32 . 
     By way of example, similar to the above-described equation (6), the force F 1  may be expressed by the following equation (17).
 
 F 1= AR·P   R   −AB·P   B   (17)
 
     In the above equation, AR represents a rod-side pressure receiving area, and AB represents a bottom-side pressure receiving area. 
     Accordingly, the movement correcting unit  302  (force estimating unit) may calculate (estimate) the force F 1  based on the equation (17). 
     Further, the movement correcting unit  302  (distance obtaining unit) obtains the distances D 2  and D 4 . Alternatively, the movement correcting unit  302  (distance obtaining unit) may obtain the ratio of D 1  to D 3  or the ratio of D 2  to D 4 . 
     The position of the center of gravity P 3  of the body of the shovel  100  excluding the attachment is fixed, irrespective of the turning angle θ of the upper turning body  3 , while the position of the tipping fulcrum P 1  changes in accordance with the turning angle θ. Accordingly, the distances D 1  and D 2  may actually vary in accordance with the turning angle θ of the upper turning body  3 . However, in the simplest manner, the distances D 1  and D 2  may be treated as constants. 
     The distances D 3  and D 4  may be geometrically calculated based on the position of the tipping fulcrum P 1  and the angle of the boom cylinder  7  (for example, an angle η 1  formed by the boom cylinder  7  and a vertical axis  130   c ). 
     The angle η 1  may be geometrically calculated based on the extension length of the boom cylinder  7 , the size of the shovel  100 , and the tilt of the body of the shovel  100 . For example, the movement correcting unit  302  may calculate the angle η 1  based on the output of a sensor that detects the boom angle. The sensor that detects the boom angle may be included in the various types of sensors  32 . 
     Note that the angle η 1  may be obtained from the output of a sensor that directly measures the angle η 1 . The sensor that directly measures the angle η 1  may be included in the various types of sensors  32 . 
     The movement correcting unit  302  (pressure controlling unit) controls the pressure of the boom cylinder  7 , specifically controls excessive one of the pressure of the rod-side oil chamber or the pressure of the bottom-side oil chamber, based on the obtained force F 1  and either the distances D 1  and D 3  or the distances D 2  and D 4 , such that the inequality (15), namely the inequality (10) or (14) is established. That is, the movement correcting unit  302  (pressure controlling unit) controls either the rod pressure P R  or the bottom pressure P B  of the boom cylinder  7 , such that the inequality (15) is established. More specifically, by employing various configurations (see  FIG.  26 A  through  FIG.  34   ), which will be described below, it becomes possible for the movement correcting unit  302  to control the pressure of the boom cylinder  7  by outputting a control command to a control target, as necessary. Accordingly, the lifting of the shovel  100  is minimized. 
     &lt;Method for Minimizing Lifting Movement by Taking into Account Changes in Tipping Fulcrum&gt; 
     In the above description, changes in the tipping fulcrums P 1  are not considered. However, because the positions of the tipping fulcrums P 1  may change as described above, changes in the positions of the tipping fulcrums P 1  may be taken into account. In the following, referring to  FIG.  14 A  through  FIG.  16   , a method for minimizing the lifting movement by taking into account a change in a tipping fulcrum will be described. 
     As described above, the control condition (stability condition) in which the front and the rear of the shovel  100  are not lifted is the inequality (15), namely the inequality (10) and the inequality (14). In the inequality (10) and the inequality (14), the distances D 1 , D 2 , D 3 , and D 4  are used as parameters, and these distances depend on the position of a tipping fulcrum P 1 . 
       FIG.  14 A  through  FIG.  14 C  are drawings illustrating the relationship between a tipping fulcrum P 1  and the direction (turning angle θ) of the upper turning body  3 . In  FIG.  14 A  through  FIG.  14 C , the turning angle θ is assumed to be 0° when the extending direction of the attachment (the direction of the attachment) is the same as the direction (the traveling direction) of the lower traveling body  1 , and turning to the right is assumed to be the positive direction. More specifically,  FIG.  14 A ,  FIG.  14 B , and  FIG.  14 C  respectively depict the tipping fulcrum P 1  when the turning angle θ is 0°, 30°, and 90°. Further,  FIG.  15    is a drawing illustrating the relationship between the tipping fulcrum P 1  and conditions of a ground surface  150   a  (work site). 
     In  FIG.  14 A  through  FIG.  14 C , it is assumed that the rear of the shovel is lifted, and the tipping fulcrum P 1  is located on the front of the shovel. Further, a line  11  is orthogonal to the extending direction of the attachment (the direction of the upper turning body  3 ), and passes through the frontmost end of an effective ground contact area  140   a  in the extension direction of the attachment  12 . The tipping fulcrum P 1  is on the line  11 . Further, in  FIG.  15   , the continuous line indicates the hard ground surface  150   a , and the dash-dot line indicates the soft ground surface  150   b.    
     As illustrated in  FIG.  14 A  through  FIG.  14 C  and  FIG.  15   , the tipping fulcrum P 1  moves in accordance with the direction of the upper turning body  3  and also the conditions of the ground surface. 
     For example, as illustrated in  FIG.  14 A  through  FIG.  14 C , as the tipping fulcrum P 1  moves, the distance D 2  changes. Similarly, as the tipping fulcrum P 1  moves, the distance D 4  changes. 
     Further, as illustrated in  FIG.  15   , on the hard ground surface  150   a , the tipping fulcrum is located at a position P 1  indicated by the continuous triangle. On the soft ground surface  150   b , the tipping fulcrum is located at a position P 1   a  indicated by the dash-dot line triangle. Moreover, if there is a hard obstacle near the tipping fulcrum P 1  at the work site, or if the lower traveling body  1  rides on an obstacle, the tipping fulcrum P 1  may be moved further. 
     The change in the position of the tipping fulcrum P 1  affects the distances D 1  to D 4 , and affects the mechanical stability condition in which the body of the shovel  100  does not fall. Accordingly, the movement correcting unit  302  may set the control condition (stability condition) in accordance with the position of the tipping fulcrum P 1 , and correct the movement of the attachment based on the set control condition, so as to minimize the lifting of the body of the shovel  100 . 
     For example, as will be described below, the movement determining unit  301  monitors the state of the body or the attachment based on the inputs from the various types of sensors  32 , and identifies a moment of time when the front or the rear of the lower traveling body  1  is lifted. Then, the movement correcting unit  302  dynamically changes the control condition (stability condition) used to correct the movement of the attachment, that is, the inequality (10) and the inequality (14), based on the state of the shovel  100  at a moment of time when the body of the shovel  100  (the lower traveling body  1 ) is lifted. 
     A moment of time when the body of the shovel  100  is lifted may be approximated as the state in which the moment τ 1 , caused by the force F 1  exerted by the attachment and tilting the body, is balanced with the moment τ 2 , caused by gravity acting against the force F 1 . Therefore, by monitoring the state of the shovel  100  and identifying a moment of time when the body of the shovel  100  is lifted, it is possible to minimize the lifting of the body of the shovel  100  in a variety of applications. 
     The movement determining unit  301  identifies (detects) a moment of time when the shovel  100  (the lower traveling body  1 ) is lifted, based on the outputs of the various types of sensors  32 . For example, a sensor may detect the rotation about the pitch axis and identify a moment of time when the body of the shovel  100  is lifted, based on the outputs of an orientation sensor (an inclination angle sensor), a gyro sensor (an angular acceleration sensor), an acceleration sensor, and an IMU, which may be mounted on the upper turning body  3  and included in the various types of sensors  32 . 
     For example, the movement correcting unit  302  (condition setting unit) sets the control condition for minimizing the lifting of the rear of the body, if the movement determining unit  301  detects the angular acceleration or the angular velocity in the forward direction, based on the outputs of the various types of sensors  32 . Further, the movement correcting unit  302  (the control condition setting unit) sets the control condition for minimizing the lifting of the front of the body, if the movement determining unit  301  (condition setting unit) detects the angular acceleration or the angular velocity in the backward direction, based on the outputs of the various types of sensors  32 . 
     The movement correcting unit  302  (condition setting unit) acquires the force F 1  (F 1 _INIT) exerted by the boom cylinder  7  on the upper turning body  3  at a moment of time when lifting is detected (identified) by the movement determining unit  301 . Then, the movement correcting unit  302  (condition setting unit) acquires parameters related to the position of the tipping fulcrum P 1  based on the acquired F 1 _INIT, and also sets the control condition based on the parameters. 
     For example, as the control condition for minimizing the lifting of the front of the body, the above-described inequality (10) is used. 
     If backward rotation about the pitch axis, which corresponds to the lifting of the front of the body, is detected by the movement determining unit  301 , the moment τ 1  and the moment τ 2  are balanced at a moment when the front of the body is lifted. Therefore, the following equation (18) is established.
 
 D 3· F 1_INIT= D 1· Mg   (18)
 
     Because the force F 1 _INIT, the body mass M, and the gravitational acceleration g are known, the equation (18) is considered to be satisfied by the distances D 1  and D 3  in the current situation where the shovel  100  is used. 
     With the known equation (18), the distances D 1  and D 3  are geometrically uniquely determined. Therefore, the movement correcting unit  302  (condition setting unit) acquires the current distances D 1  and D 3  (distances D 1  DET and D 3  DET), based on the equation (18) and the orientation of the attachment. 
     Note that acquiring the distance D 1  is equivalent to acquiring position information of the tipping fulcrum P 1 . Because the position of the center of gravity P 3  does not change, the position of the tipping fulcrum P 1  can be uniquely determined once the distance D 1  is acquired. 
     The movement correcting unit  302  (condition setting unit) sets the following inequality (19) as the subsequent control condition.
 
 D 3_DET· F 1&lt; D 1_DET· Mg   (19)
 
     The movement correcting unit  302  (condition setting unit) corrects the movement of the attachment based on the control condition represented by the inequality (19). 
     As long as the direction of the upper turning body  3  does not change and also the conditions of the ground do not change, the distance D 1  does not change, and thus, the same value can be used, once acquired. Conversely, the distance D 3  varies in accordance with the raising and lowering of the boom  4 . Therefore, when the angle of the boom  4  changes, the movement correcting unit  302  (condition setting unit) changes the distance D 3  accordingly, and applies the change to the control condition. 
     The lifting of the rear of the body is controlled in a similar manner. For example, the above-described inequality (14) is used as the control condition for minimizing the lifting of the rear of the body. 
     If forward rotation about the pitch axis, which corresponds to the lifting of the rear of the body, is detected by the movement determining unit  301 , the moment τ 1  and the moment τ 2  are balanced at a moment of time when the rear of the body is lifted. Therefore, the following equation (20) is established.
 
 D 4· F 1_INIT= D 2· Mg   (20)
 
     Because the F 1 _INIT, the body mass M, and the gravitational acceleration g are known, the equation (20) is considered to be satisfied by the distances D 2  and D 4  in the current situation where the shovel  100  is used. 
     The movement correcting unit  302  (condition setting unit) acquires the current distances D 2  and D 4  (distances D 2 _DET and D 4 _DET) based on the equation (20) and the orientation of the attachment. 
     Note that acquiring the distance D 2  is equivalent to acquiring position information of the tipping fulcrum P 1 . 
     Then, the movement correcting unit  302  (condition setting unit) sets the following inequality (21) as the subsequent control condition, based on the above-described inequality (14).
 
 D 2_DET· F 1&lt; D 4_DET· Mg   (21)
 
     The movement correcting unit  302  corrects the movement of the attachment based on the control condition represented by the inequality (21). 
     As long as the direction of the upper turning body  3  does not change and also the conditions of the ground do not change, the distance D 2  does not change, and thus, the same value can be used, once acquired. Conversely, the distance D 4  varies in accordance with the raising and lowering of the boom  4 . Therefore, when the angle of the boom  4  changes, the movement correcting unit  302  (condition setting unit) changes the distance D 4  accordingly, and applies the change to the control condition. 
       FIG.  16    is a flowchart schematically illustrating a process (condition setting process) performed by the controller  30  (the movement determining unit  301  and the movement correcting unit  302 ) to set a control condition. This process may be performed periodically or at predetermined intervals after the shovel is started to be operated until stopped. 
     In step S 1600 , the movement determining unit  301  determines whether excavation work using the attachment is being performed. The movement determining unit  301  may determine that excavation work using the attachment is being performed when the shovel is not traveling and turning, and the pressure of any or all of the boom cylinder  7 , the arm cylinder  8 , and the bucket cylinder  9  are greater than or equal to a predetermined pressure. When the movement determining unit  301  determines that excavation work using the attachment is being performed, the process proceeds to step S 1602 . When it is determined that excavation work using the attachment is not being performed, the process ends. 
     Note that the excavation work includes leveling work and backfilling work. 
     In step S 1602 , the movement determining unit  301  monitors the occurrence of lifting of the shovel  100 . When the movement determining unit  301  identifies (detects) lifting, the process proceeds to step S 1604 . When the movement determining unit  301  identifies (detects) no lifting, the process ends. 
     In step S 1602  in which the control condition has not been set, the body of the shovel  100  is lifted for a moment. If an appropriate combination of a processor and a software program is used in the controller  30 , the control condition can be set in a very short period of time after the lifting of the body is identified (detected) in step S 1602 , without causing the body of the shovel  100  to be largely tilted. The movement correcting unit  302  can start to correct the movement of the attachment before the body of the shovel  100  is largely tilted. 
     In step S 1604 , the movement correcting unit  302  acquires information related to the state of the shovel  100  at a moment of time when the body of the shovel  100  is lifted. Examples of the information related to the state of the shovel  100  include the above-described F 1 _INIT. 
     In step S 1606 , the movement correcting unit  302  calculates parameters related to the tipping fulcrum P 1 , such as the distances D 1  through D 4 , and sets a control condition based on the information related to the state of the shovel  100  acquired in step S 1604 . Thereafter, the movement correcting unit  302  corrects the movement of the attachment based on the set control condition until the excavation work is completed, as long as the control condition is not updated in S 1610 . 
     In step S 1608 , the movement determining unit  301  determines whether the orientation of the boom  4  is changed. When the movement determining unit  301  determines that the orientation of the boom  4  is changed, the process proceeds to step S 1610 . When the movement determining unit  301  determines that the orientation of the boom  4  is not changed, the process proceeds to step S 1612 . 
     In step S 1610 , because the distances D 3  and D 4  are changed in accordance with the change in the orientation of the boom  4 , the movement correcting unit  302  updates the control condition. 
     In step S 1612 , the movement determining unit  301  determines whether the excavation work is completed. When the movement determining unit  301  determines that the excavation work is not completed, the process returns to step S 1608 . When the movement determining unit  301  determines that the excavation work is completed, the process ends. 
     In the present embodiment, the control condition is defined by calculating the distances D 1  through D 4 ; however, the present invention is not limited thereto. For example, by changing the inequality (10) and the inequality (14), the following inequality (22) and (23) are obtained.
 
 F 1&lt; D 1/ D 3· Mg   (22)
 
 F 1&lt; D 2/ D 4· Mg   (23)
 
     The following equations (24) and (25) are established at a moment of time when the body is lifted.
 
 F 1_INIT= D 1/ D 3· Mg   (24)
 
 F 1_INIT= D 2/ D 4· Mg   (25)
 
     Accordingly, the movement correcting unit  302  (condition setting unit) may acquire the force  1 _INIT exerted at a moment of time when the body is lifted, and may set the following inequality (26) as the subsequent control condition.
 
 F 1&lt; F 1_INIT  (26)
 
     Note that, although the distances D 1  through D 4  and the position of the tipping fulcrum  21  are not explicitly calculated, accurate position information of the tipping fulcrum  21  is, of course, applied to the control condition expressed by the inequality (26). 
     Further, in the present embodiment, the force F 1  is explicitly included in the control condition for minimizing the lifting of the body; however, the present invention is not limited thereto. For example, instead of the force F 1 , another force or moment having correlation with the force F 1  may be used to define the control condition. 
     &lt;Method for Minimizing Vibration&gt; 
       FIG.  17 A  through  FIG.  17 C  are drawings illustrating examples of waveforms related to vibration of the shovel  100 . More specifically,  FIG.  17 A through  17 C  are drawings illustrating one example, another example, and yet another example of waveforms when in-air movement of the attachment is repeatedly performed.  FIG.  17 A through  17 C  depict, from the top, pitch angular velocity (namely, vibration of the body of the shovel), boom angular acceleration, arm angular acceleration, a boom angle, and an arm angle. 
     In  FIG.  17 A through  17 C , an X symbol indicates a point corresponding to a negative peak of the pitch angular velocity. 
     As illustrated in  FIG.  17 A through  17 C , vibration is induced when the boom angle stops changing. In other words, it can be said that the boom angular acceleration has the largest effect on the generation of vibration. Namely, this means that controlling the boom angular acceleration is effective in minimizing vibration. This can be intuitively understood because the moment of inertia with respect to the bucket angle is affected only by the mass of the bucket  6 , and the moment of inertia with respect to the arm angle is affected by the mass of the bucket and the mass of the arm, whereas the moment of inertia with respect to the boom angle is affected by the total mass of the boom  4 , the arm  5 , and the bucket  6 . 
     Therefore, it is preferable for the movement correcting unit  302  to correct the movement of the boom cylinder  7 , which serves as a control target. That is, the movement correcting unit  302  operates so that the thrust of the boom cylinder  7  does not exceed the upper limit (thrust limit F MAX ) based on the state of the attachment. 
     The thrust F of the boom cylinder  7  is expressed by the equation (27), based on the pressure receiving area AR of the rod-side oil chamber, the rod pressure P R  of the rod-side oil chamber, the pressure receiving area AB of the bottom-side oil chamber, and the bottom pressure P B  of the bottom-side oil chamber.
 
 F=AB·P   B   −AR·P   R   (27)
 
     The thrust F of the boom cylinder  7  is required to be smaller than the thrust limit F MAX . Thus, the following inequality (28) is required to be established.
 
 F   MAX   &gt;AB·P   B   −AR·P   R   (28)
 
     From the inequality (28), the following inequality (29) is obtained.
 
 P   B &lt;( F   MAX   +AR·P   R )/ AB   (29)
 
     The right side of the inequality (29) corresponds to the upper limit P BMAX  of the bottom pressure P B , which corresponds to the thrust limit F MAX . Therefore, the following equation (30) is obtained.
 
 P   BMAX =( F   MAX   +AR·P   R )/ AB   (30)
 
     The movement correcting unit  302  corrects the movement of the attachment, namely the movement of the boom cylinder  7  so that the equation (30) is established. That is, the movement correcting unit  302  controls the bottom pressure P B  of the boom cylinder  7  so that the equation (30) is established. More specifically, by employing various configurations (see  FIG.  27    through  FIG.  35   ), which will be described below, it becomes possible for the movement correcting unit  302  to control the bottom pressure P B  of the boom cylinder  7  by outputting a control command to a control target, as necessary. Accordingly, the vibration of the shovel  100  is minimized. 
     The movement correcting unit  302  acquires the thrust limit F MAX , based on detection signals output from the various types of sensors  32 . In one embodiment, a thrust limit obtaining unit  586  receives the state of the attachment, namely detection signals from the various types of sensors  32 , and acquires the thrust limit F MAX  by calculation. The movement correcting unit  302  calculates the upper limit P BMAX  of the bottom pressure P B  based on the equation (30), and controls the bottom pressure P B  of the boom cylinder  7  not to exceed the calculated upper limit P BMAX . 
     If the thrust limit F MAX  is too small, the boom  4  is lowered. Therefore, the movement correcting unit  302  may acquire a thrust (holding thrust F MIN ) that can hold the orientation of the boom  4 , and may set the thrust limit F MAX  in a range greater than the holding thrust F MIN . 
       FIG.  18    is a drawing illustrating a method performed by the movement correcting unit  302  to acquire the thrust limit F MAX . More specifically,  FIG.  18    is a block diagram illustrating a functional configuration in which the movement correcting unit  302  acquires the thrust limit F MAX . 
     As illustrated in  FIG.  18   , the movement correcting unit  302  acquires the thrust limit F MAX  based on table reference. The movement correcting unit  302  includes a first lookup table  600 , a second lookup table  602 , a table selector  604 , and a selector  606 . 
     The first lookup table  600  receives a boom angle θ 1 , output from a boom angle sensor included in the various types of sensors  32 , and outputs the thrust limit F MAX . The first lookup table  600  may include a plurality of tables provided corresponding to a plurality of different predetermined states of the shovel  100 . 
     The second lookup table  602  receives the boom angle θ 1  and an arm angle θ 2 , output from the boom angle sensor and an arm angle sensor included in the various types of sensors  32 , and outputs the holding thrust F MIN . Similar to the first lookup table  600 , the second lookup table  602  may include a plurality of tables provided corresponding to a plurality of different predetermined states of the shovel  100 . 
     The table selector  604  uses any or all of a bucket angle θ 3 , a body pitch direction θ P , and a swing angle θ S  as parameters, which are output from a bucket angle sensor, a pitch direction sensor mounted on the body (upper turning body  3 ), and a swing angle sensor included in the various types of sensors  32 , to select an optimum table in the first lookup table  600 . 
     Further, the table selector  604  uses any or all of the bucket angle θ 3 , the body pitch direction θ P , and the swing angle θ S  as parameters to select an optimum table in the second lookup table  602 . 
     The selector  606  outputs the larger one of the thrust limit F MAX  and the holding thrust F MIN . Accordingly, it is possible to minimize vibration while also preventing the lowering of the boom. 
     Note that the movement correcting unit  302  may acquire the thrust limit F MAX  by calculation instead of table reference. Similarly, the movement correcting unit  302  may acquire the holding thrust F MIN  by calculation instead of table reference. 
     [Method for Determining Occurrence of Unintended Movement of Shovel] 
     Next, referring to  FIG.  19 A  through  FIG.  26 B , a method for determining the occurrence of an unintended movement will be described. 
     &lt;Method for Determining Occurrence of Dragging Movement&gt; 
       FIG.  19 A  and  FIG.  19 B  are drawings illustrating a first example of a method for determining the occurrence of dragging of the shovel  100 . To be more specific,  FIG.  19 A  and  FIG.  19 B  are drawings illustrating an example position of an acceleration sensor  32 A mounted on the upper turning body  3  of the shovel  100 . 
     In this example, the various types of sensors  32  of the shovel  100  include the acceleration sensor  32 A. 
     As illustrated in  FIG.  19 A  and  FIG.  19 B , the acceleration sensor  32 A is mounted on the upper turning body  3 . 
     The acceleration sensor  32 A has a detection axis in the direction along a straight line L 1  corresponding to the extending direction of the attachment of the shovel  100  in plan view. The point of action at which a force is exerted by the attachment on the upper turning body  3  is located at the bottom  3 A of the boom  4 . Therefore, it is preferable to provide the acceleration sensor  32 A at the bottom of the boom  4 . In this manner, the movement determining unit  301  can suitably identify the occurrence of the dragging of the shovel  100  caused by the movement of the attachment, based on an output signal of the acceleration sensor  32 A. 
     If the acceleration sensor  32 A is located away from a turning axis  3 B, the acceleration sensor  32 A may be affected by the centrifugal force when the upper turning body  3  is rotated. Therefore, it is desirable to provide the acceleration sensor  32 A in the vicinity of the bottom  3 A of the boom  4  and also in the vicinity of the turning axis  3 B. 
     Namely, the acceleration sensor  32 A is desirably provided in a region R 1  located between the bottom  3 A of the boom  4  and the turning axis  3 B of the upper turning body  3 . Accordingly, it becomes possible to reduce the influence of rotation, thereby allowing the movement determining unit  301  to suitably detect the occurrence of dragging caused by the movement of the attachment, based on an output signal of the acceleration sensor  32 A. 
     Further, if the acceleration sensor  32 A is located far away from the ground surface, acceleration components due to pitch and roll tend to be included in the output of the acceleration sensor  32 A. In light of the above, the acceleration sensor  32 A is preferably mounted as low as possible on the upper turning body  3 . 
     Further, in this example, a velocity sensor, which may be included in the various types of sensors  32 , may be mounted at a similar position on the upper turning body  3 , instead of the acceleration sensor  32 A. Accordingly, the movement determining unit  301  can identify the occurrence of dragging of the shovel  100 , based on the output corresponding to the velocity along the straight line L 1  detected by the velocity sensor. 
     Further, in this example, the various types of sensors  32  may include an angular velocity sensor mounted on the upper turning body  3 , in addition to the acceleration sensor  32 A. In this case, the output of the acceleration sensor  32 A may be corrected based on the output of the angular velocity sensor. The output of the acceleration sensor  32 A includes components of not only linear motion (dragging movement) in a particular direction, but also of rotational motion in the pitch direction, the yaw direction, and the roll direction. By using the angular velocity sensor together, the influence of rotational motion can be excluded, thereby extracting linear motion corresponding to the dragging movement only. As a result, the accuracy of determining the dragging movement by the movement determining unit  301  can be improved. 
     Further, in this example, the acceleration sensor  32 A is mounted on the upper turning body  3 , but may be mounted on the lower traveling body  1 . In this case, the movement determining unit  301  may also use the output of an angle sensor together, which detects a turning angle (turning position) of the upper turning body  3  and may be included in the various types of sensors  32 . In this manner, the movement determining unit  301  can identify linear motion along the extending direction (straight line L 1 ) of the attachment, based on the output of the acceleration sensor  32 A of the lower traveling body  1 , thereby identifying the occurrence of dragging in that direction. 
     Next,  FIG.  20    is a drawing illustrating a second example of the method for determining the occurrence of dragging. 
     In this example, the various types of sensors  32  include a distance sensor  32 B. 
     As illustrated in  FIG.  20   , the distance sensor  32 B is mounted to the front end of the upper turning body  3  of the shovel  100 , and measures the distance between the body (upper turning body  3 ), on which the distance sensor  32 B is mounted, and the ground surface, an obstacle, or any other object located in front of the upper turning body  3  of the shovel  100  within a predetermined range. The distance sensor  32 B may be light detection and ranging (LIDAR), a millimeter wave radar, a stereo camera, or the like. 
     The movement determining unit  301  determines the occurrence of dragging of the shovel  100 , based on a change in the relative positional relationship between the upper turning body  3  and a fixed reference object around the shovel  100 , which is measured by the distance sensor  32 B. More specifically, the movement determining unit  301  determines that the shovel  100  has been dragged, when the relative position of a ground surface  200   a  viewed from the upper turning body  3  is moved approximately in the horizontal direction, more specifically, approximately parallel to the surface on which the shovel  100  is located, based on the output of the distance sensor  32 B. For example, as illustrated in  FIG.  20   , the movement determining unit  301  determines that the shovel  100  has been dragged forward, when the relative position of the ground surface  200   a  viewed from the upper turning body  3  is moved towards the upper turning body  3  (towards a dotted line  200   b ) approximately in the horizontal direction, based on the output of the distance sensor  32 B. Conversely, the movement determining unit  301  determines that the shovel  100  has been dragged backward, when the relative position of the ground surface  200   a  viewed from the upper turning body  3  is moved away from the upper turning body  3  approximately in the horizontal direction. 
     Instead of the distance sensor  32 B, the movement determining unit  301  may use any other sensor such as an image sensor (a monocular camera) capable of detecting the relative position between the upper turning body  3  and a fixed reference object around the shovel  100  to determine the occurrence of dragging. 
     Further, the fixed reference object around the shovel  100  is not limited to the ground surface, and may be a building or may be an object intentionally disposed around the shovel  100  to be used as the reference object. 
     Further, the distance sensor  32 B is not required to be mounted on the upper turning body  3 , and may be mounted on the attachment. In this case, the movement determining unit  301  may be able to measure the distance between the attachment and the upper turning body  3 , in addition to the distance between the attachment and a reference object. Accordingly, the movement determining unit  301  can identify the relative position of the reference object and the relative position of the upper turning body  3  with respect to the attachment, based on the output of the distance sensor  32 B. That is, the movement determining unit  301  can determine the relative position between the reference object and the upper turning body  3  in an indirect manner. Accordingly, the movement determining unit  301  determines that the shovel  100  has been dragged, when the relative position between the reference object and the upper turning body  3  is changed, namely when the reference object is moved approximately parallel to the surface on which the upper turning body  3  is located, based on the output of the distance sensor  32 B mounted on the attachment. 
     Next,  FIG.  21 A  and  FIG.  21 B  are drawings illustrating a third example of the method for determining the occurrence of dragging. To be more specific,  FIG.  21 A  depicts the shovel  100  that is not dragged, and  FIG.  21 B  depicts the shovel  100  that is being dragged. 
     In this example, the various types of sensors  32  include an IMU  32 C. 
     As illustrated in  FIG.  21 A  and  FIG.  21 B , the IMU  32 C is mounted on the boom  4 . 
     As illustrated in  FIG.  21 A , when the shovel  100  is not dragged, the IMU  32 C of the boom  4  detects rotational motion in accordance with the raising and lowering of the boom  4 . Thus, an acceleration component in the front-back direction of the shovel  100  detected by the IMU  32 C is output as a relatively small value because of the rotational motion. 
     Conversely, as illustrated in  FIG.  21 B , at the time of dragging, the shovel  100  moves in the front-back direction. Thus, an acceleration component in the dragging direction, namely an acceleration component in the front-back direction of the shovel  100  detected by the IMU  32 C is output as a relatively large value. 
     Therefore, when an acceleration component detected by the IMU  32 C becomes greater than or equal to a predetermined threshold, the movement determining unit  301  may determine that the dragging of the shovel  100  has occurred. The predetermined threshold may be set as appropriate based on experiments, simulation analyses, and the like. Further, the movement determining unit  301  can determine whether the shovel  100  is dragged forward or backward, based on the direction of the detected acceleration component. 
     Further, in this example, any other sensor such as a velocity sensor or an acceleration sensor may be used instead of the IMU  32 C, as long as the motion in the front-back direction of the boom  4  can be detected. In this case, as with the IMU  32 C, the movement determining unit  301  may determine that the dragging of the shovel  100  has occurred when the output value of the sensor becomes relatively large. 
     Next,  FIG.  22 A  and  FIG.  22 B  are drawings illustrating a fourth example of the method for determining the occurrence of dragging. To be more specific,  FIG.  22 A  depicts the shovel  100  that is not dragged, and  FIG.  22 B  depicts the shovel  100  that is being dragged. 
     In this example, the various types of sensors  32  include two IMUs  32 C. 
     As illustrated in  FIG.  22 A  and  FIG.  22 B , one IMU  32 C is mounted on the arm  5 , and the other IMU  32 C is mounted on the bucket  6 . 
     As illustrated in  FIG.  22 A , when the shovel  100  is not dragged, an acceleration component in the front-back direction detected by the IMU  32 C of the bucket  6  is represented as a combination of an acceleration component of the arm  5  and an angular acceleration component about the drive axis of the bucket  6 . Therefore, the acceleration component detected by the IMU  32 C of the bucket  6  becomes relatively larger than the acceleration component in the front-back direction detected by the IMU  32 C of the arm  5 . 
     Conversely, as illustrated in  FIG.  22 B , when the shovel  100  is being dragged, the arm  5  is moved in the front-back direction of the shovel  100 . Because the bucket  6  makes contact with the ground surface for excavation work, the bucket  6  does not readily move. Therefore, an acceleration component in the front-back direction detected by the IMU  32 C of the bucket  6  becomes somewhat smaller than an acceleration component in the front-back direction detected by the IMU  32 C of the arm  5 . 
     Thus, when the difference between an acceleration component detected by the IMU  32 C of the arm  5  and an acceleration component detected by the IMU  32 C of the bucket  6  becomes greater than or equal to a predetermined threshold, the movement determining unit  301  may determine that the dragging of the shovel  100  has occurred. The predetermined threshold may be set as appropriate based on experiments, simulation analyses, and the like. Further, the movement determining unit  301  can determine whether the shovel  100  is dragged forward or backward, based on the direction of the acceleration component of the arm  5 . 
     Further, the IMU  32 C mounted on the arm  5  is preferably disposed closer to the position where the arm  5  is coupled to the boom  4  than to the position where the arm  5  is coupled to the bucket  6 . Accordingly, with the position where the arm  5  is coupled to the bucket  6  being used as the fulcrum, the amount of movement of the arm  5  at the position where the IMU  32 C is mounted can be increased as much as possible when the dragging of the shovel  100  has occurred. Thus, the movement determining unit  301  can readily determine the occurrence of dragging, based on the difference between the acceleration component detected by the IMU  32 C of the arm  5  and the acceleration component detected by the IMU  32 C the IMU  32 C of the bucket  6 . 
     Further, in this example, instead of the IMUs  32 C, any other sensors such as velocity sensors or acceleration sensors may be employed, as long as the sensors are capable of detecting the motion in the front-back direction of the arm  5  and the bucket  6 . Further, in this example, the IMUs  32 C are mounted on the arm  5  and the bucket  6 ; however, an additional IMU  32 C may be mounted on the boom  4 . Accordingly, the movement determining unit  301  can determine the occurrence of dragging, based on the difference between output values of the respective IMUs  32 C mounted on the boom  4  and the bucket  6 , in addition to the difference between output values of the respective IMUs  32 C mounted on the arm  5  and the bucket  6 , thereby improving determination accuracy. Further, the IMU  32 C is not required to be mounted on the arm  5 , and the IMUs  32 C may be mounted on the boom  4  and the bucket  6 . In this case, the movement determining unit  301  may determine the occurrence of dragging, based on the difference between output values of the respective IMUs  32 C mounted on the boom  4  and the bucket  6 . 
     &lt;Method for Determining Occurrence of Lifting&gt; 
       FIG.  23 A  through  FIG.  23 C  are drawings illustrating a first example of a method for determining the occurrence of lifting of the shovel  100 . To be more specific,  FIG.  23 A  is a graph illustrating changes in the inclination angle in the front-back direction of the body of the shovel  100  (in the pitch direction) over time,  FIG.  23 B  is a graph illustrating changes in the angular velocity over time, and  FIG.  23 C  is a graph illustrating changes in the angular acceleration over time when the shovel  100  is lifted. 
     In this example, the movement determining unit  301  determines the occurrence of lifting of the shovel  100  based on the outputs of sensors included in the various types of sensors  32 . The sensors are capable of outputting information related to the inclination angle in the front-back direction of the body of the shovel  100 , namely the inclination angle in the pitch direction. 
     Examples of the sensors capable of outputting information related to the inclination angle in the pitch direction of the body of the shovel  100  include an inclination angle sensor (angle sensor), an angular velocity sensor, and an IMU. 
     For example, as illustrated in  FIG.  23 A  through  FIG.  23 C , at the time of the occurrence of lifting, the inclination angle, the angular velocity, and the angular acceleration in the pitch direction become somewhat large. Therefore, when these values exceed predetermined thresholds (constant values indicated by dotted lines), the movement determining unit  301  can determine that the lifting has occurred. In addition, the movement determining unit  30  can determine whether the front of the shovel  100  has lifted or the rear of the shovel  100  has lifted, based on the direction of the inclined angle, the angular velocity, and the angular acceleration, namely based on the forward inclination or the backward inclination about the pitch axis. 
     Next,  FIG.  24    is a drawing illustrating a second example of the method for determining the occurrence of lifting. 
     In this example, similar to  FIG.  20   , the various types of sensors  32  include the distance sensor  32 B. 
     As illustrated in  FIG.  24   , similar to  FIG.  20   , the distance sensor  32 B is mounted to the front end of the upper turning body  3  of the shovel  100 , and measures the distance from the body (upper turning body  3 ), on which the distance sensor  32 B is mounted, to the ground surface, an obstacle, or any other object located in front of the upper turning body  3  of the shovel  100  within a predetermined range. 
     Similar to  FIG.  20   , the movement determining unit  301  determines the occurrence of lifting of the shovel  100 , based on a change in the relative positional relationship between the upper turning body  3  and a fixed reference object around the shovel  100 , which is measured by the distance sensor  32 B. More specifically, the movement determining unit  301  determines that the shovel  100  has been lifted, when the relative position of a ground surface  240   a  viewed from the upper turning body  3  is moved approximately in the vertical direction, more specifically, approximately perpendicular to the surface on which the shovel  100  is located, based on the output of the distance sensor  32 B. For example, as illustrated in  FIG.  24   , the movement determining unit  301  determines that the front of the shovel  100  has been lifted, when the relative position of the ground surface  240   a  viewed from the upper turning body  3  is moved approximately downward (toward a dotted line  240   b ), based on the output of the distance sensor  32 B. Conversely, the movement determining unit  301  determines that the rear of the shovel  100  has been lifted, when the relative position of the ground surface  240   a  viewed from the upper turning body  3  is moved away from the upper turning body  3  approximately upward. 
     Instead of the distance sensor  32 B, the movement determining unit  301  may use any other sensor such as an image sensor (a monocular camera) capable of detecting the relative position between the upper turning body  3  and a fixed reference object around the shovel  100  to determine the occurrence of lifting. 
     Further, the fixed reference object around the shovel  100  is not limited to the ground surface, and may be a building or may be an object intentionally disposed around the shovel  100  to be used as the reference object. 
     Further, the distance sensor  32 B is not required to be mounted on the upper turning body  3 , and may be mounted on the attachment. In this case, the movement determining unit  301  may be able to measure the distance between the attachment and the upper turning body  3 , in addition to the distance between the attachment and a reference object. Accordingly, the movement determining unit  301  can identify the relative position of the reference object and the relative position of the upper turning body  3  with respect to the attachment, based on the output of the distance sensor  32 B. That is, the movement determining unit  301  can determine the relative position between the reference object and the upper turning body  3  in an indirect manner. Accordingly, the movement determining unit  301  determines that the shovel  100  has been lifted, when the relative position between the reference object and the upper turning body  3  is changed, namely when the reference object is moved approximately perpendicular to the surface on which the upper turning body  3  is located, based on the output of the distance sensor  32 B mounted on the attachment. 
     Next,  FIG.  25 A  and  FIG.  25 B  are drawings illustrating a third example of the method for determining the occurrence of lifting. To be more specific,  FIG.  25 A  depicts the shovel  100  that is not lifted, and  FIG.  25 B  depicts the shovel  100  that is being lifted. 
     In this example, the various types of sensors  32  include the IMU  32 C, similar to  FIG.  21 A  and  FIG.  21 B . 
     As illustrated in  FIG.  25 A  and  FIG.  25 B , the IMU  32 C is mounted on the boom  4 , similar to  FIG.  21 A  and  FIG.  21 B . 
     As illustrated in  FIG.  25 A , when the shovel  100  is not lifted, the IMU  32 C of the boom  4  detects rotational motion in accordance with the relatively slow raising and lowering of the boom  4 . Thus, an angular acceleration component detected by the IMU  32 C is output as a relatively small value. 
     Conversely, as illustrated in  FIG.  25 B , at the time of the lifting of the shovel  100 , an angular acceleration component in the lifting direction is detected by the IMU  32 C and output as a relatively large value. 
     Therefore, when an angular acceleration component detected by the IMU  32 C becomes greater than or equal to a predetermined threshold, the movement determining unit  301  may determine that the lifting of the shovel  100  has occurred. The predetermined threshold may be set as appropriate based on experiments, simulation analyses, and the like. Further, the movement determining unit  301  can determine whether the front or the rear of the shovel  100  is lifted, based on the direction of the detected acceleration component. 
     Further, with only the absolute value of angular acceleration generated in the boom  4 , it may be difficult to determine the occurrence of the lifting of the shovel  100 , when the lifting direction of the shovel  100  is opposite to the moving direction of the boom  4 . Therefore, the movement determining unit  301  may determine that the shovel  100  has lifted, when the amount of change or the rate of change in angular acceleration detected by the IMU  32 C of the boom  4  becomes greater than or equal to a predetermined threshold. 
     Further, in this example, any other sensor such as a velocity sensor or an acceleration sensor may be employed instead of the IMU  32 C, as long as the motion in the rotation direction of the boom  4  can be detected. In this case, as with the IMU  32 C, the movement determining unit  301  may determine that the lifting of the shovel  100  has occurred, when the output value of the sensor or the rate of change becomes relatively large. 
     Next,  FIG.  26 A  and  FIG.  26 B  are drawings illustrating a fourth example of the method for determining the occurrence of lifting. To be more specific,  FIG.  26 A  depicts the shovel  100  that is not lifted, and  FIG.  26 B  depicts the shovel  100  that is being lifted. 
     In this example, similar to  FIG.  22 A  and  FIG.  22 B , the various types of sensors  32  include two IMUs  32 C. 
     As illustrated in  FIG.  26 A  and  FIG.  26 B , one IMU  32 C is mounted on the arm  5 , and the other IMU  32 C is mounted on the bucket  6 . 
     As illustrated in  FIG.  26 A , when the shovel  100  is not lifted, an acceleration component in the front-back direction detected by the IMU  32 C of the bucket  6  is represented as a combination of an acceleration component of the arm  5  and an angular acceleration component about the drive axis of the bucket  6 . Therefore, the acceleration component detected by the IMU  32 C of the bucket  6  becomes relatively larger than the acceleration component in the front-back direction detected by the IMU  32 C of the arm  5 . 
     Conversely, as illustrated in  FIG.  26 B , when the shovel  100  is lifted, the arm  5  is moved (rotated) centered on the point at which the bucket  6  makes contact with the ground. Because the bucket  6  makes contact with the ground surface for excavation work, the bucket  6  does not readily move. Therefore, an acceleration component in the front-back direction and an angular acceleration component about the drive axis detected by the IMU  32 C of the bucket  6  become somewhat smaller than an acceleration component in the front-back direction and an angular acceleration component detected by the IMU  32 C of the arm  5 . 
     Thus, when the difference between acceleration components or between angular acceleration components about an axis parallel to the drive axis of the attachment, detected by the respective IMUs  32 C of the arm  5  and the bucket  6 , becomes greater than or equal to a predetermined threshold, the movement determining unit  301  may determine that the lifting of the shovel  100  has occurred. The predetermined threshold may be set as appropriate based on experiments, simulation analyses, and the like. Further, the movement determining unit  301  can determine whether the front or the rear of the shovel  100  is lifted, based on the direction of the acceleration component of the arm  5 . 
     Further, the IMU  32 C mounted on the arm  5  is preferably disposed closer to the position where the arm  5  is coupled to the boom  4  than to the position where the arm  5  is coupled to the bucket  6 . Accordingly, with the position where the arm  5  is coupled to the bucket  6  being used as the fulcrum, the amount of movement of the arm  5  at the position where the IMU  32 C is mounted can be increased as much as possible when the lifting of the shovel  100  has occurred. Thus, the movement determining unit  301  can readily determine the occurrence of lifting based on the difference between acceleration components detected by the respective IMUs  32 C of the arm  5  and the bucket  6 . 
     Further, in this example, instead of the IMUs  32 C, any other sensors such as velocity sensors or acceleration sensors may be employed, as long as the sensors are capable of detecting the motion in the front-back direction of the arm  5  and the bucket  6  as well as in the rotational direction about the axis parallel to the drive axis. Further, in this example, the IMUs  32 C are mounted on the arm  5  and the bucket  6 ; however, an additional IMU  32 C may be mounted on the boom  4 . Further, in this example, the IMUs  32 C are mounted on the arm  5  and the bucket  6 ; however, an additional IMU  32 C may be mounted on the boom  4 . Accordingly, the movement determining unit  301  can determine the occurrence of lifting, based on the difference between output values of the respective IMUs  32 C mounted on the boom  4  and the bucket  6 , in addition to the difference between output values of the respective IMUs  32 C mounted on the arm  5  and the bucket  6 , thereby improving determination accuracy. Further, the IMU  32 C is not required to be mounted on the arm  5 , and the IMUs  32 C may be mounted on the boom  4  and the bucket  6 . In this case, the movement determining unit  301  may determine the occurrence of lifting, based on the difference between output values of the respective IMUs  32 C mounted on the boom  4  and the bucket  6 . 
     &lt;Method for Determining Occurrence of Vibration&gt; 
     The movement determining unit  301  can determine the occurrence of vibration when a sensor capable of detecting vibration, such as an acceleration sensor, an angular acceleration sensor, or an IMU, is mounted on the body (upper turning body  3 ). The above sensor is included in the various types of sensors  32 . More specifically, the movement determining unit  301  may determine that the body of the shovel has been vibrated, when there is vibration that is caused by a change in the moment of inertia of the attachment and that matches the natural frequency of the body of the shovel, based on the outputs of the various types of sensors  32 . 
     Further, as described above, vibration is generated while the attachment is being moved in the air. Therefore, the movement determining unit  301  may determine that the body of the shovel has been vibrated, when there is vibration that is caused by a change in the moment of inertia of the attachment during in-air movement of the attachment, and that matches the natural frequency of the body of the shovel, based on the output of the various types of sensors  32 . 
     [Detailed Configuration for Correcting Movement of Attachment] 
     Next, referring to  FIG.  27    through  FIG.  35   , a characteristic configuration of the shovel  100  according to the present embodiment, that is, an example configuration for correcting the movement of the attachment in order to minimize an unintended movement will be described. 
       FIG.  27    is a drawing illustrating a first example of the characteristic configuration of the shovel  100  according to the present embodiment. More specifically, the first example mainly illustrates a configuration of a hydraulic circuit that supplies hydraulic oil to the boom cylinder  7  of the shovel  100  according to the present embodiment. 
     In the present example, it is assumed that the boom  4 , namely the boom cylinder  7 , is operated by the lever  26 A. The same applies to  FIG.  28    through  FIG.  35   . Further, a pilot line  27  that applies a secondary-side pilot pressure from the lever  26 A to the port of the boom direction control valve  17 A, which supplies hydraulic oil to the boom cylinder  7  and is included in the control valve  17 , is referred to as a pilot line  27 A. 
     As illustrated in  FIG.  27   , bypass oil passages  271  and  272  for discharging hydraulic oil into a tank T is provided. The bypass oil passage  271  extends from the rod-side oil chamber of the boom cylinder  7 , and the bypass oil passage  272  extends from the bottom-side oil chamber of the boom cylinder  7 . 
     An electromagnetic relief valve  33  for discharging hydraulic oil of the rod-side oil chamber into the tank T is provided in the bypass oil passage  271 . 
     An electromagnetic relief valve  34  for discharging hydraulic oil of the bottom-side oil chamber into the tank T is provided in the bypass oil passage  272 . 
     Note that the bypass oil passages  271  and  272 , and the electromagnetic relief valves  33  and  34  may be provided inside of the control valve  17  or outside of the control valve  17 . 
     Further, the various types of sensors  32  include pressure sensors  32 D and  32 E that detect the rod pressure P R  and the bottom pressure P B  of the boom cylinder  7 . The outputs of the pressure sensors  32 D and  32 E are input into the controller  30 . 
     The controller  30 , which serves as the movement correcting unit  302 , can monitor the rod pressure P R  and the bottom pressure P B  based on output signals from the pressure sensors  32 D and  32 E. The movement correcting unit  302  outputs current command values to the electromagnetic relief valves  33  and  34  as appropriate, so as to forcibly discharge hydraulic oil of either the rod-side oil chamber or the bottom-side oil chamber of the boom cylinder  7  into the tank T, thereby reducing excessive pressure in the boom cylinder  7 . Accordingly, it is possible to minimize unintended movements such as dragging and lifting of the shovel  100 , by reducing excessive pressure generated in the boom cylinder  7 , using the correction method for correcting the movement of the boom cylinder  7  described with reference to  FIG.  9 A  through  FIG.  17 C . 
     Next,  FIG.  28    is a drawing illustrating a second example of the characteristic configuration of the shovel  100  according to the present embodiment. More specifically, the second example mainly illustrates a configuration of a hydraulic circuit that supplies hydraulic oil to the boom cylinder  7  of the shovel  100  according to the present embodiment. 
     As illustrated in  FIG.  28   , an electromagnetic proportional valve  36  is provided in the pilot line  27 A between the lever  26 A and the port of the boom direction control valve  17 A. 
     Further, similar to  FIG.  27   , the various types of sensors  32  include the pressure sensors  32 D and  32 E that detect the rod pressure P R  and the bottom pressure P B  of the boom cylinder  7 . The outputs of the pressure sensors  32 D and  32 E are input into the controller  30 . 
     The controller  30 , which serves as the movement correcting unit  302 , can monitor the rod pressure P R  and the bottom pressure P B  based on output signals from the pressure sensors  32 D and  32 E. The movement correcting unit  302  outputs a current command value to the electromagnetic proportional valve  36  as appropriate, so as to change a pilot pressure corresponding to the state of an operation with the lever  26 A and input the changed pilot pressure into the port of the boom direction control valve  17 A. Namely, the movement correcting unit  302  outputs a current command value to the electromagnetic proportional valve  36  as appropriate, so as to control the boom direction control valve  17 A. As a result, the movement correcting unit  302  can cause hydraulic oil of either the rod-side oil chamber or the bottom-side oil chamber of the boom cylinder  7  to be discharged into the tank T as appropriate, thereby reducing excessive pressure in the boom cylinder  7 . Accordingly, it is possible to minimize unintended movements such as dragging and lifting of the shovel  100 , by reducing excessive pressure generated in the boom cylinder  7 , using the correction method for correcting the movement of the boom cylinder  7  described with reference to  FIG.  9 A  through  FIG.  17 C . 
     In this example, a signal corresponding to the state of an operation performed by the operator with the lever  26 A, namely a signal corresponding to the operating state of the boom  4  is corrected and the corrected signal is input into the boom direction control valve  17 A. However, a signal different from the signal corresponding to the operating state of the boom  4  may be input into the boom direction control valve  17 A. For example, the electromagnetic proportional valve  36  may be provided in an oil passage that branches from the pilot line  25  located on an upstream side (on the pilot pump  15  side) relative to the lever  26 A, and that is connected to the port of the boom direction control valve  17 A. In this case, the movement correcting unit  302  may input the signal different from the signal corresponding to the operating state of the boom  4  into the boom direction control valve  17 A, such that the boom direction control valve  17 A can be controlled regardless of the state of an operation with the lever  26 A. Further, in normal state, the controller  30  may output a current command to the electromagnetic proportional valve  36 , based on a pressure signal corresponding to the state of an operation with the lever  26 A detected by the pressure sensor  29 . As a result, the boom direction control valve  17 A can be controlled in accordance with the state of the operation performed by the operator with the lever  26 A. 
     Next,  FIG.  29    is a drawing illustrating a third example of the characteristic configuration of the shovel  100  according to the present embodiment. More specifically, the third example mainly illustrates a configuration of a hydraulic circuit that supplies hydraulic oil to the boom cylinder  7  of the shovel  100  according to the present embodiment. 
     As illustrated in  FIG.  29   , similar to  FIG.  27   , the various types of sensors  32  include the pressure sensors  32 D and  32 E that detect the rod pressure P R  and the bottom pressure P B  of the boom cylinder  7 . The outputs of the pressure sensors  32 D and  32 E are input into the controller  30 . 
     The controller  30 , which serves as the movement correcting unit  302 , can monitor the rod pressure P R  and the bottom pressure P B  based on output signals from the pressure sensors  32 D and  32 E. The movement correcting unit  302  outputs, as appropriate, a current command value to the regulator  14 A that controls the inclination angle of the swash plate, so as to control the output and the flow rate of the main pump  14 . Namely, the movement correcting unit  302  outputs a current command value to the regulator  14 A as appropriate, so as to control the operation of the main pump  14 . As a result, the flow rate of hydraulic oil supplied to the boom cylinder  7  can be controlled, thereby reducing excessive pressure in the boom cylinder  7 . Accordingly, it is possible to minimize unintended movements such as dragging and lifting of the shovel  100 , by reducing excessive pressure generated in the boom cylinder  7 , using the correction method for correcting the movement of the boom cylinder  7  described with reference to  FIG.  9 A  through  FIG.  17 C . 
     Next,  FIG.  30    is a drawing illustrating a fourth example of the characteristic configuration of the shovel  100  according to the present embodiment. More specifically, the fourth example mainly illustrates a configuration of a hydraulic circuit that supplies hydraulic oil to the boom cylinder  7  of the shovel  100  according to the present embodiment. 
     As illustrated in  FIG.  30   , similar to  FIG.  27   , the various types of sensors  32  include the pressure sensors  32 D and  32 E that detect the rod pressure P R  and the bottom pressure P B  of the boom cylinder  7 . The outputs of the pressure sensors  32 D and  32 E are input into the controller  30 . 
     The controller  30 , which serves as the movement correcting unit  302 , can monitor the rod pressure P R  and the bottom pressure P B  based on output signals from the pressure sensors  32 D and  32 E. The movement correcting unit  302  outputs, as appropriate, a current command value to an engine control module (EMC)  11 A that controls the operating state of the engine  11 , so as to control the output of the engine  11 . Namely, the movement correcting unit  302  outputs a current command value to the EMC  11 A as appropriate, so as to control the output of the engine  11 . As a result, the output of the main pump  14  driven by the engine  11  is controlled, thereby controlling the flow rate of hydraulic oil supplied to the boom cylinder  7 . Namely, the movement correcting unit  302  can reduce excessive pressure in the boom cylinder  7 . Accordingly, it is possible to minimize unintended movements such as dragging and lifting of the shovel  100  by reducing excessive pressure generated in the boom cylinder  7 , using the correction method for correcting the movement of the boom cylinder  7  described with reference to  FIG.  9 A  through  FIG.  17 C . 
     Next,  FIG.  31    is a drawing illustrating a fifth example of the characteristic configuration of the shovel  100  according to the present embodiment. More specifically, the fifth example mainly illustrates a configuration of a hydraulic circuit that supplies hydraulic oil to the boom cylinder  7  of the shovel  100  according to the present embodiment. 
     In this example, it is assumed that pressure sensors similar to the pressure sensors  32 D and  32 E of  FIG.  27    through  FIG.  30    are included in the various types of sensors  32 . The same applies to  FIG.  32    through  FIG.  35   . 
     As illustrated in  FIG.  31   , in this example, the control valve  17  includes an electromagnetic selector valve  38 . 
     The electromagnetic selector valve  38  is provided such that hydraulic oil flows from an oil passage  311 , which connects the boom direction control valve  17 A and the bottom-side oil chamber of the boom cylinder  7 , to an oil passage  312 , which circulates hydraulic oil into the tank T. Accordingly, when in a communication state, the electromagnetic selector valve  38  can discharge hydraulic oil in the bottom-side oil chamber of the boom cylinder  7  into the tank T. 
     The controller  30 , which serves as the movement correcting unit  302 , can monitor the rod pressure P R  and the bottom pressure P B  based on output signals from the various types of sensors  32  (the pressure sensors that detect the pressure of the rod-side oil chamber and the pressure of the bottom-side oil chamber of the boom cylinder  7 ). The movement correcting unit  302  outputs, as appropriate, a current command value to the electromagnetic selector valve  38 , so as to control a communication state and a shutoff state of the electromagnetic selector valve  38 . Namely, the movement correcting unit  302  outputs a current command value to the electromagnetic selector valve  38  as appropriate, so as to cause hydraulic oil in the bottom-side oil chamber of the boom cylinder  7  to be discharged into the tank T via the electromagnetic selector valve  38 , thereby reducing excessive pressure (bottom pressure P B ) generated in the bottom-side oil chamber of the boom cylinder  7 . Accordingly, it is possible to minimize unintended movements such as dragging and lifting of the shovel  100  by reducing excessive pressure generated in the boom cylinder  7 , using the correction method for correcting the movement of the boom cylinder  7  described with reference to  FIG.  9 A  through  FIG.  17 C . 
     Further, an electromagnetic selector valve may be provided within the control valve  17  such that hydraulic oil flows from an oil passage, which connects the boom direction control valve  17 A and the rod-side oil chamber of the boom cylinder  7 , to the oil passage  312 , which circulates hydraulic oil into the tank T. In this case, the movement correcting unit  302  may also output a current command value to the electromagnetic selector valve as appropriate, so as to reduce excessive pressure generated in the rod-side oil chamber of the boom cylinder  7 . 
     Next,  FIG.  32    is a drawing illustrating a sixth example of the characteristic configuration of the shovel  100  according to the present embodiment. More specifically, the sixth example mainly illustrates a configuration of a hydraulic circuit that supplies hydraulic oil to a boom cylinder  7  of the shovel  100  according to the present embodiment. In  FIG.  32   , two boom cylinders  7  are illustrated. The two boom cylinders  7  have the same configuration in which the control valve  17  and a pressure holding circuit  40 , which will be described below, are provided between the main pump  14  and each of the boom cylinders  7 . Thus, one boom cylinder  7  (on the right in the figure) will be mainly described. 
     In this example, similar to  FIG.  27   , an electromagnetic relief valve  33  for discharging hydraulic oil in the rod-side oil chamber into the tank T is provided in an oil passage that branches from an oil passage between the control valve  17  and the rod-side oil chamber of a boom cylinder  7 . The same applies to  FIG.  33   . 
     As illustrated in  FIG.  32   , in this example, the shovel  100  includes the pressure holding circuit  40 . Even if a hydraulic hose is damaged, for example is ruptured, the pressure holding circuit  40  holds hydraulic oil of the bottom-side oil chamber of the boom cylinder  7  so as not to discharge the hydraulic oil. The same applies to  FIG.  33    through  FIG.  35   . 
     The pressure holding circuit  40  is provided in an oil passage that connects the control valve  17  to the bottom-side oil chamber of the boom cylinder  7 . The pressure holding circuit  40  mainly includes a holding valve  42  and a spool valve  44 . 
     Regardless of the state of the spool valve  44 , the holding valve  42  supplies hydraulic oil, received from the control valve  17  via an oil passage  321 , to the bottom-side oil chamber of the boom cylinder  7 . 
     Further, when the spool valve  44  is in a shutoff state (spool state on the left of the figure), the holding valve  42  holds hydraulic oil of the bottom-side oil chamber of the boom cylinder  7  such that the hydraulic oil is not discharged to the downstream side of the pressure holding circuit  40 . Conversely, when the spool valve  44  is in a communication state (spool state on the right of the figure), the holding valve  42  discharges hydraulic oil of the bottom-side oil chamber of the boom cylinder  7  to the downstream side of the pressure holding circuit  40  via an oil passage  322 . 
     The communication state and the shutoff state of the spool valve  44  are controlled in accordance with a pilot pressure that is input into the port of the spool valve  44  from a boom-lowering remote control valve  26 Aa. The pilot pressure input from the boom-lowering remote control valve  26 Aa corresponds to the state of a lowering operation of the boom  4  (a boom lowering operation) performed with the lever  26 A. More specifically, when a pilot pressure, indicating that the boom lowering operation is being performed, is input from the boom-lowering remote control valve  26 Aa, the spool valve  44  is put in a communication state (spool state on the right of the figure). Conversely, when a pilot pressure, indicating that the boom lowering operation is not performed, is input from the boom-lowering remote control valve  26 Aa, the spool valve  44  is put in a shutoff state (spool state on the left of the figure). Accordingly, even if a hydraulic hose located on the downstream side of the pressure holding circuit  40  is damaged, hydraulic oil (bottom pressure) of the bottom-side oil chamber of the boom cylinder  7  can be held, thereby preventing the falling of the boom  4  when the boom lowering operation is not performed. 
     Further, the pressure holding circuit  40  also includes an electromagnetic relief valve  46 . 
     The electromagnetic relief valve  46  is provided in an oil passage  324  that branches from an oil passage  323  and is connected to the tank T. The oil passage  323  is provided between the holding valve  42  of the holding circuit  40  and the bottom oil chamber of the boom cylinder  7 . Namely, the electromagnetic relief valve  46  releases hydraulic oil from the oil passage  323 , which is on the upstream side (the boom cylinder  7  side) relative to the holding valve  42 , into the tank T. Accordingly, regardless of the operating state of the pressure holding circuit  40 , and specifically, regardless of the communication state or the shutoff state of the spool valve  44 , the electromagnetic relief valve  46  can discharge hydraulic oil of the bottom-side oil chamber of the boom cylinder  7  into the tank T. Namely, the pressure holding circuit  40  can reduce excessive pressure by discharging hydraulic oil of the bottom-side oil chamber of the boom cylinder  7  regardless of whether the boom lowering operation is performed, while also preventing the falling of the boom  4 , using the function for holding hydraulic oil of the bottom-side oil chamber of the boom cylinder  7 . 
     The controller  30 , which serves as the movement correcting unit  302 , can monitor the rod pressure P R  and the bottom pressure P B  based on output signals from the various types of sensors  32  (the pressure sensors that detect the pressure of the rod-side oil chamber and the pressure of the bottom-side oil chamber of the boom cylinder  7 ). Further, the movement correcting unit  302  outputs, as appropriate, current command values to the electromagnetic relief valves  33  and  46 , so as to forcibly discharge hydraulic oil of either the rod-side oil chamber or the bottom-side oil chamber of the boom cylinder  7  into the tank T regardless of whether the boom lowering operation is performed. As a result, excessive pressure in the boom cylinder  7  can be reduced. Accordingly, it is possible to minimize unintended movements such as dragging and lifting of the shovel  100 , by reducing excessive pressure generated in the boom cylinder  7 , using the correction method for correcting the movement of the boom cylinder  7  described with reference to  FIG.  9 A  through  FIG.  17 C . 
     Next,  FIG.  33    is a drawing illustrating a seventh example of the characteristic configuration of the shovel  100  according to the present embodiment. More specifically, the seventh example mainly illustrates a configuration of a hydraulic circuit that supplies hydraulic oil to a boom cylinder  7  of the shovel  100  according to the present embodiment. 
     As illustrated in  FIG.  33   , in this example, an electromagnetic relief valve  50  is provided in an oil passage  332  that branches from an oil passage  331  and is connected to the tank T. The oil passage  331  is provided between the bottom oil chamber of the boom cylinder  7  and a pressure holding circuit  40 . Accordingly, regardless of the operating state of the pressure holding circuit  40 , and specifically, regardless of the communication state or the shutoff state of a spool valve  44 , the electromagnetic relief valve  50  can discharge hydraulic oil of the bottom-side oil chamber of the boom cylinder  7  into the tank T. Namely, the pressure holding circuit  40  can reduce excessive pressure by discharging hydraulic oil of the bottom-side oil chamber of the boom cylinder  7  regardless of whether the boom lowering operation is performed, while also preventing the falling of the boom  4  by the function for holding hydraulic oil of the bottom-side oil chamber of the boom cylinder  7 . 
     The controller  30 , which serves as the movement correcting unit  302 , can monitor the rod pressure P R  and the bottom pressure P B  based on output signals from the various types of sensors  32  (the pressure sensors that detect the pressure of the rod-side oil chamber and the pressure of the bottom-side oil chamber of the boom cylinder  7 ). Further, the movement correcting unit  302  outputs, as appropriate, current command values to the electromagnetic relief valves  33  and  50 , so as to forcibly discharge hydraulic oil of either the rod-side oil chamber or the bottom-side oil chamber of the boom cylinder  7  into the tank T regardless of whether the boom lowering operation is performed. As a result, excessive pressure in the boom cylinder  7  can be reduced. Accordingly, it is possible to minimize unintended movements such as dragging and lifting of the shovel  100 , by reducing excessive pressure generated in the boom cylinders  7 , using the correction method for correcting the movement of the boom cylinder  7  described with reference to  FIG.  9 A  through  FIG.  17 C . 
     Next,  FIG.  34    is a drawing illustrating an eighth example of the characteristic configuration of the shovel  100  according to the present embodiment. More specifically, the eighth example mainly illustrates a configuration of a hydraulic circuit that supplies hydraulic oil to a boom cylinder  7  of the shovel  100  according to the present embodiment. 
     As illustrated in  FIG.  34   , an electromagnetic selector valve  52  and a shuttle valve  54  are provided in a pilot circuit that applies a pilot pressure, corresponding to the state of the boom lowering operation, from the boom-lowering remote control valve  26 Aa to the spool valve  44  of the pressure holding circuit  40 . 
     The electromagnetic selector valve  52  is provided in an oil passage  341 . The oil passage  341  branches from a pilot line  25 A provided between the pilot pump  15  and the boom-lowering remote control valve  26 Aa, bypasses the boom-lowering remote control valve  26 Aa, and is connected to one input port of the shuttle valve  54 . The electromagnetic selector valve  52  switches between the communication state and the shutoff state of the oil passage  341 . 
     Note that, instead of the electromagnetic selector valve  52 , an electromagnetic proportional valve may be employed to switch between the communication state and the shutoff state of the oil passage  341 . 
     As described above, the oil passage  341  is connected to the one input port of the shuttle valve  54 , and a secondary-side oil passage  342  of the boom-lowering remote control valve  26 Aa is connected to the other input port of the shuttle valve  54 . Among the two input pilot pressures, the shuttle valve  54  outputs a higher pilot pressure to the spool valve  44 . Accordingly, even when the boom lowering operation is not performed, a pilot pressure similar to that when the boom lowering operation is performed can be input into the spool valve  44  via the electromagnetic selector valve  52  and the shuttle valve  54 . Namely, even when the boom lowering operation is not performed, hydraulic oil in the bottom-side oil chamber of a boom cylinder  7  can flow out to the downstream side of the pressure holding circuit  40 . 
     Further, in this example, electromagnetic relief valves  56  and  58  are provided inside of the control valve  17 . 
     Note that the electromagnetic relief valves  56  and  58  may be provided outside of the control valve  17 , as long as the electromagnetic relief valves  56  and  58  can branch from oil passages between the boom direction control valve  17 A and the pressure holding circuit  40 , and can discharge hydraulic oil into the tank T. 
     The electromagnetic relief valve  56  is provided in an oil passage  343 . The oil passage  343  branches from an oil passage between the rod-side oil chamber of the boom cylinder  7  and the boom direction control valve  17 A, and is connected to the tank T. Accordingly, the electromagnetic relief valve  56  can discharge hydraulic oil of the rod-side oil chamber of the boom cylinder  7  into the tank T. 
     The electromagnetic relief valve  58  is provided in an oil passage  344 . The oil passage  344  branches from an oil passage between the pressure holding circuit  40  and the boom direction control valve  17 A, and is connected to the tank T. Accordingly, the electromagnetic relief valve  58  can discharge hydraulic oil, flowing out from the bottom-side oil chamber of the boom cylinder  7  via the pressure holding circuit  40 , into the tank T. That is, even when the boom lowering operation is not performed, the above-described electromagnetic selector valve  52  and the shuttle valve  54  cause hydraulic oil of the bottom-side oil chamber of the boom cylinder  7  to be discharged into the tank T, thereby reducing excessive bottom pressure P B . 
     In this example, if the electromagnetic selector valve  38  of  FIG.  31    is provided within the control valve  17 , the electromagnetic relief valve  58  may be replaced with the electromagnetic selector valve  38 . Further, as described above with reference to  FIG.  31   , an electromagnetic selector valve may be provided within the control valve  17  such that hydraulic oil passes from the oil passage, which connects the boom direction control valve  17 A and the rod-side oil chamber of the boom cylinder  7 , to an oil passage, which circulates hydraulic oil into the tank T. In this case, the electromagnetic relief valve  56  may be replaced with the above-described electromagnetic selector valve. 
     The controller  30 , which serves as the movement correcting unit  302 , can monitor the rod pressure P R  and the bottom pressure P B  based on output signals from the various types of sensors  32  (the pressure sensors that detect the pressure of the rod-side oil chamber and the pressure of the bottom-side oil chamber of the boom cylinder  7 ). Further, the movement correcting unit  302  outputs, as appropriate, current command values to the electromagnetic selector valve  52  and the electromagnetic relief valves  56  and  58 , so as to forcibly discharge hydraulic oil of either the rod-side oil chamber or the bottom-side oil chamber of the boom cylinder  7  into the tank T regardless of whether the boom lowering operation is performed. As a result, excessive pressure in the boom cylinder  7  can be reduced. Accordingly, it is possible to minimize unintended movements such as dragging and lifting of the shovel  100 , by reducing excessive pressure generated in the boom cylinders  7 , using the correction method for correcting the movement of the boom cylinder  7  described with reference to FIG.  9 A through  FIG.  17 C . 
     Next,  FIG.  35    is a drawing illustrating a ninth example of the characteristic configuration of the shovel  100  according to the present embodiment. More specifically, the ninth example mainly illustrates a configuration of a hydraulic circuit that supplies hydraulic oil to a boom cylinder  7  of the shovel  100  according to the present embodiment. 
     As illustrated in  FIG.  35   , in this example, an electromagnetic proportional valve  60  and a shuttle valve  54 , which is similar to that of  FIG.  34   , are provided in a pilot circuit that applies a pilot pressure, corresponding to the state of the boom lowering operation, from the boom-lowering remote control valve  26 Aa to the spool valve  44  of the pressure holding circuit  40 . 
     The electromagnetic proportional valve  60  is provided in an oil passage  351 . The oil passage  351  branches from the pilot line  25 A provided between the pilot pump  15  and the boom-lowering remote control valve  26 Aa, bypasses the boom-lowering remote control valve  26 Aa, and is connected to one input port of the shuttle valve  54 . The electromagnetic proportional valve  60  controls the switching between the communication state and the shutoff state of the oil passage  341 , and also controls a pilot pressure input into the shuttle valve  54 . 
     Similar to  FIG.  34   , the oil passage  351  is connected to the one input port of the shuttle valve  54 , and a secondary-side oil passage  352  of the boom-lowering remote control valve  26 Aa is connected to the other input port of the shuttle valve  54 . Among the two input pilot pressures, the shuttle valve  54  outputs a higher pilot pressure to the spool valve  44 . Accordingly, even when the boom lowering operation is not performed, a pilot pressure similar to that when the boom lowering operation is performed can be input into the spool valve  44  via the electromagnetic selector valve  52  and the shuttle valve  54 . Namely, even when the boom lowering operation is not performed, hydraulic oil in the bottom-side oil chamber of a boom cylinder  7  can flow out to the downstream side of the pressure holding circuit  40 . 
     Further, in this example, the electromagnetic relief valve  56  is provided inside of the control valve  17 . 
     Note that the electromagnetic relief valve  56  may be provided outside of the control valve  17 , as long as the electromagnetic relief valve  56  can branch from an oil passage provided between the boom direction control valve  17 A and the pressure holding circuit  40 , and can discharge hydraulic oil into the tank T. 
     Similar to  FIG.  34   , the electromagnetic relief valve  56  is provided in an oil passage  353 . The oil passage  353  branches from an oil passage provided between the rod-side oil chamber of the boom cylinder  7  and the boom direction control valve  17 A, and is connected to the tank T. Accordingly, the electromagnetic relief valve  56  can discharge hydraulic oil of the rod-side oil chamber of the boom cylinder  7  into the tank T. 
     The controller  30 , which serves as the movement correcting unit  302 , can monitor the rod pressure P R  and the bottom pressure P B  based on output signals from the various types of sensors  32  (the pressure sensors that detect the pressure of the rod-side oil chamber and the pressure of the bottom-side oil chamber of the boom cylinder  7 ). Further, the movement correcting unit  302  outputs, as appropriate, a current command value to the electromagnetic relief valve  56 , so as to forcibly discharge hydraulic oil in the rod-side oil chamber of the boom cylinder  7  into the tank T, thereby reducing excessive pressure (rod pressure) in the rod-side oil chamber of the boom cylinder  7 . 
     Further, because the electromagnetic proportional valve  60  is employed, a pilot pressure, input into the shuttle valve  54  via the shuttle valve  54 , can be finely controlled. Therefore, the controller  30  can finely control the operating state of the electromagnetic proportional valve  60  by outputting a current command value to the electromagnetic proportional valve  60 . As a result, the controller  30  can finely adjust the flow rate of hydraulic oil flowing out from the bottom-side oil chamber of the boom cylinder  7  via the pressure holding circuit  40 . In other words, independently of the control valve  17 , the controller  30  can adjust the flow rate of hydraulic oil flowing out from the bottom-side oil chamber of the boom cylinder  7  via the control valve  17  during the boom lowering operation. Accordingly, regardless of whether the boom lowering operation is performed, the controller  30 , which serves as the movement correcting unit  302 , can cause hydraulic oil in the bottom-side oil chamber of the boom cylinder  7  to be discharged into the tank T as necessary by outputting a current command value to the electromagnetic proportional valve  6 . As a result, excessive pressure in the boom cylinder  7  can be reduced. 
     Accordingly, it is possible to minimize unintended movements such as dragging and lifting of the shovel  100 , by reducing excessive pressure generated in the boom cylinder  7 , using the correction method for correcting the movement of the boom cylinder  7  described with reference to  FIG.  9 A  through  FIG.  17 C . 
     [Details of Process for Correcting Movement of Attachment] 
     Next, referring to  FIG.  36   , a process for correcting the movement of the attachment (a movement correcting process) performed by the controller  30  (the movement determining unit  301  and the movement correcting unit  302 ) will be described. 
       FIG.  36    is a flowchart schematically illustrating an example of the movement correcting process performed by the controller  30 . This process is repeatedly performed at predetermined time intervals. 
     In step S 3600 , the movement determining unit  301  determines whether the shovel  100  is traveling, based on inputs from the pressure sensor  29  and the various types of sensors  32 . If the movement determining unit  30  determines that the shovel  100  is not traveling, the process proceeds to step S 3602 . If the movement determining unit  30  determines that the shovel  100  is traveling, the process ends. 
     In step S 3602 , the movement determining unit  301  determines whether the attachment is in operation, namely the movement determining unit  301  determines whether work (excavation work) using the attachment is being performed, based on inputs from the pressure sensor  29  and the various types of sensors  32 . If the movement determining unit  301  determines that the attachment is in operation, the process proceeds to step S 3604 . If the movement determining unit  301  determines that the attachment is not in operation, the process ends. 
     In step S 3604 , the movement determining unit  301  determines the occurrence of an unintended movement, based on inputs from the pressure sensor  29  and the various types of sensors  32 . At this time, the movement determining unit  301  uses the above-described determination methods to determine the occurrence of some or all of the unintended movements. If the movement determining unit  301  determines that an unintended movement has occurred, the process proceeds to step S 3606 . If the movement determining unit  301  determines that an unintended movement has not occurred, the process ends. 
     In step S 3606 , the movement correcting unit  302  acquires a target control value for the movement that is determined to have occurred (determined movement). For example, if the movement correcting unit  302  determines that vibration has occurred, the movement correcting unit  302  acquires the thrust limit F MAX  or the holding thrust F MIN , in accordance with the method described with reference to  FIG.  18   . If the movement correcting unit  302  determines that an unintended movement other than vibration, such as dragging or lifting, has occurred, the movement correcting unit  302  may acquire the thrust limit as a target control value by table reference, in accordance with the method described with reference to  FIG.  18    as well. 
     In step S 3608 , the movement correcting unit  302  outputs a control command to the control target, and corrects the movement of the attachment. As described above, examples of the control target include the electromagnetic relief valves  33  and  34 , the electromagnetic proportional valve  36 , the regulator  14 A, the EMC  11 A, the electromagnetic selector valve  38 , the electromagnetic relief valve  46 , the electromagnetic relief valve  50 , the electromagnetic selector valve  52 , the electromagnetic relief valves  56  and  58 , and the electromagnetic proportional valve  60 . 
     For example, in order to prevent a movement not intended by an operator of a shovel, the technique that corrects (minimizes) the movement of the attachment of the shovel is known (see Patent Document 1 above). 
     Patent Document 1 describes the technique that controls the pressure of a hydraulic cylinder, which drives the attachment of the shovel, not to exceed a predetermined maximum allowable pressure, thereby minimizing an unintended movement such as the dragging or lifting of the shovel. 
     However, the technique described in Patent Document 1 corrects the movement of the attachment of the shovel without determining whether an unintended movement has actually occurred. Thus, the operator&#39;s operability may be decreased. 
     In light of the above, in the present embodiment, the occurrence of an unintended movement is determined by the movement determining unit  301 . If the movement determining unit  301  determines that an unintended movement has occurred, the movement correcting unit  302  corrects the movement of the attachment. Accordingly, after the unintended movement is determined to have actually occurred, the movement of the attachment is corrected, thus preventing a decrease in the operator&#39;s operability while minimizing the unintended movement. 
     The following clauses are further disclosed with respect to the above-described embodiments and variations described below. 
     (1-1) A shovel includes: 
     a traveling body; 
     a turning body turnably mounted on the traveling body; 
     an attachment attached to the turning body; 
     a detector attached to the turning body or the attachment and configured to detect a relative position of a fixed reference object around the shovel with respect to one of the turning body and the attachment; and 
     a determining unit configured to determine whether a predetermined unintended movement occurs, based on a change in the detected relative position of the reference object around the shovel with respect to the one of the turning body and the attachment. 
     (1-2) The shovel according to (1-1), wherein the detector detects a relative position of a ground surface around the shovel with respect to the one of the turning body and the attachment. The ground surface serves as the reference object. 
     (1-3) The shovel according to (1-1) or (1-2), wherein the detector is attached to the turning body. 
     (1-4) The shovel according to (1-4), wherein the determining unit determines that unintended movement has occurred, when a relative position of the reference object with respect to the turning body is moved approximately parallel to a flat surface on which the shovel is located, the unintended movement being a dragging movement. 
     (1-5) The shovel according to (1-3) or (1-4), wherein the determining unit determines that the unintended movement has occurred, when a relative position of the reference object with respect to the turning body is moved approximately in a vertical direction, the unintended movement being a lifting movement. 
     (1-6) The shovel according to (1-1) or (1-2), wherein the detector is attached to the attachment, and detects a relative position of the reference object and a relative position of the turning body with respect to the attachment, and wherein the determining unit determines whether the unintended movement occurs, based on a change in the detected relative position of the reference object with respect to the attachment and a change in the detected relative position of the turning body with respect to the attachment. 
     (1-7) The shovel according to (1-1) through (1-6), further includes a movement correcting unit configured to correct the movement of the attachment when the determining unit determines that the unintended movement has occurred. 
     (1-8) The shovel according to (1-7), wherein the movement correcting unit corrects the movement of the attachment, when the determining unit determines that the unintended movement has occurred in a situation in which the traveling body is not operated and the attachment is being operated. 
     (2-1) A shovel includes: 
     a traveling body, 
     a turning body turnably mounted on the traveling body; 
     an attachment attached to the turning body; and 
     a determining unit configured to determine whether a predetermined unintended movement occurs. 
     (2-2) The shovel according to (2-1), wherein the unintended movement includes at least one of a movement in which the traveling body and the turning body are dragged forward or backward when viewed from the turning body, a movement in which front sides or rear sides of the traveling body and the turning body are lifted when viewed from the turning body, and a movement in which the traveling body and the turning body are vibrated due to the movement of the attachment, the unintended movement being determined to have occurred when the traveling body is not operated. 
     (2-3) The shovel according to (2-1) or (2-2), further comprising a sensor configured to detect a movement of the shovel, 
     wherein the determining unit determines whether the unintended movement occurs, based on an output of the sensor. 
     (2-4) The shovel according to (2-3), wherein the sensor is attached to the turning body, and configured to detect a movement of the turning body. 
     (2-5) The shovel according to (2-3), wherein the sensor is attached to the attachment, and configured to detect the movement of the attachment. 
     (2-6) The shovel according to (2-5), wherein the sensor includes a first sensor attached to a boom of the attachment and configured to detect a movement of the boom, and 
     the determining unit determines whether the unintended movement occurs, based on a change in an output of the first sensor. 
     (2-7) The shovel according to (2-5), wherein the sensor includes a second sensor attached to a bucket of the attachment and configured to detect a movement of the bucket, and also includes a third sensor attached to either a boom or an arm and configured to detect a movement of the boom or the arm, and 
     the determining unit determines whether the unintended movement occurs, based on a change in a relative relationship between an output of the second sensor and an output of the third sensor. 
     (2-8) The shovel according to (2-1) through (2-7), further comprising a movement correcting unit configured correct the movement of the attachment when the determining unit determines that the unintended movement has occurred. 
     (2-9) The shovel according to (2-8), wherein the movement correcting unit corrects the movement of the attachment, when the determining unit determines that the unintended movement has occurred in a situation in which the traveling body is not operated and the attachment is being operated. 
     (3-1) A shovel includes: 
     a traveling body; 
     a turning body turnably mounted on the traveling body; 
     an attachment attached to the turning body; 
     a hydraulic actuator configured to drive the attachment; and 
     a hydraulic control unit configured to control hydraulic pressure of the hydraulic actuator in relation to a movement of the attachment, the hydraulic control unit controlling the hydraulic pressure of the hydraulic actuator regardless of an operating state of the attachment. 
     (3-2) The shovel according to (3-1), further includes a control valve configured to control a movement of the hydraulic actuator in accordance with an operation by an operator, 
     wherein the hydraulic control unit controls the hydraulic pressure of the hydraulic actuator by discharging hydraulic oil from an oil passage between the control valve and the hydraulic actuator into a tank. 
     (3-3) The shovel according to (3-2), further includes a holding valve disposed in an oil passage between the control valve and the hydraulic actuator to hold hydraulic oil of the hydraulic actuator, 
     wherein the hydraulic control unit controls the hydraulic pressure of the hydraulic actuator by discharging hydraulic oil from an oil passage between the hydraulic actuator and the holding valve into the tank. 
     (3-4) The shovel according to (3-1), further includes a control valve configured to control a movement of the hydraulic actuator in accordance with an operation by an operator, 
     wherein the hydraulic control unit controls the hydraulic pressure of the hydraulic actuator by correcting a signal corresponding to the operating state of the attachment and inputting the corrected signal into the control valve, or by inputting a signal different from the signal corresponding to the operating state of the attachment into the control valve. 
     (3-5) The shovel according to (3-1), further includes a hydraulic pump configured to be driven by a predetermined power source to supply hydraulic oil to the hydraulic actuator, 
     wherein the hydraulic control unit controls the hydraulic pressure of the hydraulic actuator by controlling the hydraulic pump or the power source. 
     (3-6) The shovel according to (3-1), further includes: 
     a control valve configured to control a movement of the hydraulic actuator in accordance with an operation by an operator; 
     a holding valve disposed in an oil passage between the control valve and the hydraulic actuator to hold hydraulic oil of the hydraulic actuator, and 
     a releasing device configured to release the hydraulic oil of the hydraulic actuator held by the holding valve, in accordance with the operating state of the attachment, 
     wherein the hydraulic control unit controls the hydraulic pressure of the hydraulic actuator by controlling the releasing device so as to release the hydraulic oil held by the holding valve, regardless of the operating state of the attachment. 
     (3-7) The shovel according to any one of (3-1) to (3-6), further includes: 
     a determining unit configured to determine whether a predetermined unintended movement occurs, and 
     a movement correcting unit configured to use the hydraulic control unit to correct the movement of the attachment when the determining unit determines that the predetermined unintended movement has occurred. 
     (3-8) The shovel according to (3-7), wherein the movement correcting unit corrects the movement of the attachment, when the determining unit determines that the unintended movement has occurred in a situation in which the traveling body is not operated and the attachment is being operated. 
     [Variations and Modifications] 
     Although the embodiments have been specifically described, the present invention is not limited to the above-described embodiments. Variations, modifications, and substitutions may be made to the described subject matter without departing from the scope of the present invention. Further, any features described with reference to the above-described embodiments may be combined as appropriate, as long as no technical contradiction occurs. The same applies to the following variations. 
     &lt;First Variation&gt; 
     For example, in the above-described embodiments, the configurations (such as  FIG.  27    and  FIG.  31    through  FIG.  35   ) in which hydraulic oil in both the rod-side oil chamber and the bottom-side oil chamber of the boom cylinder  7  can be discharged into the tank T have been described; however, hydraulic oil in either the rod-side oil chamber or the bottom-side oil chamber may be discharged into the tank T. Specifically, if an oil chamber, whose pressure needs to be suppressed, is known in advance based on a determined unintended movement (for example, if an unintended movement is vibration, and a control target is fixed to the bottom-side oil chamber), a configuration in which hydraulic oil in only one of oil chambers is discharged into the tank T may be employed. 
     Further, in the above-described embodiments, the movement of the boom cylinder  7  (specifically, the pressure of the boom cylinder  7 ) of the attachment is mainly corrected. However, the movement of the arm cylinder  8  or the bucket cylinder  9  may be corrected, of course. In the following, a specific example in which the movement of the arm cylinder  8  is corrected will be described with reference to  FIG.  37    and  FIG.  38   . 
       FIG.  37    and  FIG.  38    are drawings illustrating a first variation of the shovel  100 . More specifically,  FIG.  37    depicts waveforms related to the dragging of the shovel  100 .  FIG.  37    depicts, from top to bottom, the speed v of the lower traveling body  1  along a straight line L 1  corresponding to the extending direction of the attachment, the acceleration α of the lower traveling body  1  along the straight line L 1 , a moment τ about the movement axis of the attachment (for example, a moment τ 2  about the movement axis of the arm  5  illustrated in  FIG.  38   ), and a force F 3  exerted by the attachment on the body of the shovel  100  along the straight line L 1 .  FIG.  38    is a drawing illustrating an example of a mechanical model of the shovel  100  performing excavation work, in which forces exerted on the shovel  100  during the excavation work are depicted. 
     In  FIG.  37   , dash-dot lines indicate waveforms for a comparative example in which the movement of the attachment is not corrected. 
     First, the comparative example in which the movement of the attachment is not corrected will be described. 
     As illustrated in  FIG.  37   , before a time t 0 , no dragging occurs, the lower traveling body  1  is stationary on the ground, and the speed v is zero. 
     At the time t 0 , when the operator tilts the levers  26 A and  26 B, the moment τ 2  (or a moment τ 1  or τ 3  about the movement axis of another part of the attachment) increases. Accordingly, the force F 3  exerted on the body of the shovel  100  along the straight line L 1  increases. Then, at a time t 1 , the force F 3  exceeds the maximum static friction force μN. As a result, the lower traveling body  1  starts to be dragged on the ground (starts to slide), and the speed v increases as indicated by the dash-dot line. 
     Next, the first variation in which the movement of the attachment is corrected will be described. 
     As illustrated in  FIG.  37   , at the time t 1 , when the lower traveling body  1  starts to slide, the acceleration a starts to increase. In other words, the dragging of the lower traveling body  1  appears as an increase in the acceleration α. Therefore, the movement determining unit  301  determines that the dragging of the lower traveling body  1  has occurred, based on the acceleration α detected by the above-described acceleration sensor  32 A. For example, when the acceleration α detected by the acceleration sensor  32 A exceeds a predetermined threshold value αTH, the movement determining unit  301  determines that dragging has occurred. When the movement determining unit  301  determines that dragging has occurred, the control that corrects the movement of the attachment by the movement correcting unit  302  is enabled (see  FIG.  36   .) 
     Specifically, at a time t 2 , the acceleration α exceeds the predetermined threshold value αTH. Thus, the correction control by the movement correcting unit  302  is enabled at the time t 2 . The correction control is enabled for a correction period of time T. In the correction period of time T, the movement correcting unit  302  decreases the moment τ 2  about the movement axis of the arm  5 , regardless of the state of an operation performed by the operator. When the moment τ 2  decreases, the force F 3  exerted by the attachment on the body of the shovel  100  decreases. Then, when the force F 3  drops below a kinetic friction force μ′N, the dragging starts to decrease. 
     After the correction period of time T has passed, the correction control for the movement of the attachment (arm  5 ) is disabled, and the moment τ 2  is returned to the moment before correction, which changes in accordance with the state of an operation performed by the operator. The correction period of time T may be approximately 1 millisecond to 2 seconds. Preferably, the correction period of time T may be approximately 10 milliseconds to 200 milliseconds, considering the results of simulation conducted by the inventors. 
     The force F also increases to the original level after the correction control is disabled. However, because the lower traveling body  1  is stationary on the ground, the lower traveling body  1  will not be dragged unless the force F exceeds the maximum static friction force ρN again. 
     For example, in the case of excavation work illustrated in  FIG.  38   , when the arm  5  is pulled (closed), with a large amount of sediment being loaded in the bucket  6 , the force F 3  is exerted, and the lower traveling body  1  starts to be dragged forward. Then, in accordance with the determination result by the movement determining unit  301 , the movement correcting unit  302  instantly reduces the pressure of the arm cylinder  8  so as to control the thrust of the arm cylinder  8 , thereby decreasing the pulling force of the arm  5 , that is, the moment τ 2 . As a result, the force F 3  exerted by the attachment on the body (the upper turning body  3 ) decreases, and drops below the kinetic friction force μ′N. Thus, the dragging of the shovel  100  stops. After the dragging of the shovel  100  stops, the correction control by the movement correcting unit  302  is disabled, and the moment τ 2  acting on the arm  5  is returned to the moment before correction, which changes in accordance with the state of an operation performed by the operator. At this time, because the maximum static friction force μN is not exceeded (force F 3 &gt;μ′N), dragging does not occur. By repeating the above process periodically at very short time intervals, it is possible to minimize the dragging of the shovel  100 , without requesting the operator to change the operation amount of the operation lever and without deteriorating the operator&#39;s operability. 
     As described above, the movement of a cylinder other than the boom cylinder  7  of the attachment may be corrected to minimize an unintended movement. 
     &lt;Second Variation&gt; 
     In the above-described embodiments and variation, the movement of the attachment is corrected by suppressing the pressure of the boom cylinder  7  so as to control the thrust of the boom cylinder  7 . However, the movement of the attachment may be corrected according to another aspect. In the following, a method for correcting the movement of the attachment by changing the position of at least one part of the attachment will be described with reference to  FIG.  39   . 
       FIG.  39    is a drawing illustrating a second variation of the shovel  100 . More specifically,  FIG.  39    is a drawing illustrating a method for correcting the movement of the attachment according to another aspect. In  FIG.  39   , a side view of the shovel  100  performing excavation work is depicted. The state of the attachment before correction is indicated by a continuous line, and the state of the attachment after correction is indicated by a dash-dot line. 
     For example, it is assumed that a large amount of sediment is placed in the bucket  6 , and the shovel  100  is holding the bucket  6  (namely, closing the arm  5  and the bucket  6 ). In this case, a moment T is generated, with the bucket  6  being the center and the bottom  3 A of the boom  4  being a point of action. A component of the moment T parallel to the ground surface acts as the force F 3  that drags the lower traveling body  1 . 
     When the movement of the attachment is corrected by the movement correcting unit  302 , and the orientation of the attachment is changed, the direction of the moment (force) acting on the bottom  3 A is changed from T to Ta. As an example, in  FIG.  39   , the movement correcting unit  302  changes the position of the boom  4  from the continuous line to the dash-dot line  4   a . A component (a force that drags the lower traveling body  1 ) Fa of the corrected moment Ta parallel to the ground surface becomes smaller than the force F 3  before correction. Accordingly, the dragging of the shovel  100  is minimized. Specifically, the movement correcting unit  302  moves the arm cylinder  8  in a contraction direction (a direction in which the arm  5  is lowered), regardless of the state of an operation performed by the operator. In this manner, the movement of the attachment is corrected. More specifically, for example, the movement correcting unit  302  may output a current command value to the electromagnetic proportional valve of  FIG.  28   , so as to move the arm cylinder  8  in the contraction direction. 
     Further, when the direction of the moment is changed from T to Ta, a component perpendicular to the ground surface, namely, a force that pushes the lower traveling body  1  to the ground increases. As a result, a normal force N increases as compared to that before correction, the kinetic friction force μ′N increases, and further, dragging is minimized. 
     In the example of  FIG.  39   , the dragging of the body of the shovel  100  is minimized by two actions of reducing the force F 3 , which affects the dragging movement, and of increasing the normal force N. However, it is also effective to use only one of the actions. 
     As described above, the movement of the attachment may be corrected to minimize an unintended movement by finely adjusting the orientation of the attachment of the shovel  100 . 
     &lt;Third Variation&gt; 
     In the above-described embodiments and variations, the movement of the attachment is corrected when an unintended movement is determined to have occurred. However, regardless of the occurrence of an unintended movement, the movement of the attachment may be corrected. In the following, a method for correcting the movement of the attachment regardless of the occurrence of an unintended movement will be described with reference to  FIG.  40   . 
       FIG.  40    is a drawing illustrating a third variation of the shovel  100 . Specifically,  FIG.  40    is a flowchart schematically illustrating an example of a process performed by the movement correcting unit  302  to minimize vibration. For example, this process is repeatedly performed at predetermined time intervals while the shovel  100  is in operation. 
     In step S 4000 , the movement determining unit  301  determines whether the attachment is being moved in the air. When the movement determining unit  301  determines that the attachment is moved in the air, the process proceeds to step S 4002 . When the movement determining unit  301  determines that the attachment is not moved in the air, the process ends. 
     In step S 4002 , the movement correcting unit  302  monitors the state of the attachment (such as a boom angle θ 1 , an arm angle θ 2 , and a bucket angle θ 3 ). 
     In step S 4004 , the movement correcting unit  302  determines the thrust limit F MAX  based on the state of the attachment (see  FIG.  18   ). 
     In step S 4006 , the movement correcting unit  302  determines the holding thrust F MIN  based on the state of the attachment (see  FIG.  18   ). 
     In step S 4008 , based on the thrust limit F MAX  and the holding thrust F MIN , the movement correcting unit  302  determines the upper limit P MAX  of the bottom pressure of a control target cylinder (for example, the boom cylinder  7 ) (see  FIG.  30   ). 
     In this manner, the movement correcting unit  302  may control the thrust of the cylinder, regardless of the occurrence of vibration, so as to minimize vibration. Further, for other unintended movements such as dragging and lifting, the movement correcting unit  302  may perform control in accordance with a target control value obtained by the above-described correction method (see  FIG.  9 A  through  FIG.  18   ), regardless of the occurrence of an unintended movement. 
     &lt;Fourth Variation&gt; 
     In the above-described embodiments and variations, in order to minimize an unintended movement, hydraulic oil in either the rod-side oil chamber or the bottom-side oil chamber of a control target cylinder (for example, the boom cylinder  7 ) is discharged into the tank; however, the hydraulic oil may be regenerated. In the following, a method for minimizing an unintended movement (such as dragging or lifting) by regenerating and supplying hydraulic oil between the rod-side oil chamber and the bottom-side oil chamber of a control target cylinder will be described. 
       FIG.  41    is a drawing illustrating an example configuration of a drive system mounted on a shovel according to a fourth variation. In  FIG.  41   , a mechanical power system is indicated by a double line, a hydraulic oil line is indicated by a thick continuous line, a pilot line is indicated by a dashed line, and an electric control system is indicated by a dash-dot line. 
     As described above (see  FIG.  2   ), a main pump  14  and a control valve  17  are connected to the output shaft of the engine  11 . The main pump  14  is, for example, a variable displacement hydraulic pump whose discharge flow rate per pump revolution is controlled by a regulator  14 A. The pilot pump  15  is a fixed displacement hydraulic pump. The control valve  17  is connected to the main pump  14  via a hydraulic oil line  16 . An operation device  26  is connected to the pilot pump  15  via a pilot line  25 . 
     As described above, the control valve  17  is a valve unit including a plurality of valves, and controls a hydraulic system of the shovel. The control valve  17  is connected to hydraulic actuators such as a traveling hydraulic motor  1 L, a traveling hydraulic motor  1 R, a boom cylinder  7 , an arm cylinder  8 , a bucket cylinder  9 , and a turning hydraulic motor  21  via hydraulic oil lines. 
     As described above, the operation device  26  is a device for operating the hydraulic actuators, and includes an operation lever and an operation pedal. The operation apparatus  26  is connected to the control valve  17  via a pilot line  27 , and is connected to a pressure sensor  29  via a pilot line  28 . 
     As described above, the pressure sensor  29  detects a pilot pressure generated by the operation device  26 , and transmits information related to the detected pilot pressure to the controller  30 . The pressure sensor  29  includes an arm pressure sensor that detects an operating state of an arm operation lever, and a boom pressure sensor that detects an operating state of a boom operation lever. 
     As described above, the controller  30  is a main controller that controls the driving of the shovel. In the fourth variation, the controller  30  is configured mainly by an arithmetic processing unit including a central processing unit (CPU) and an internal memory, and implements various functions by causing the CPU to execute a drive control program stored in the internal memory. 
     A cylinder pressure sensor  32 E is an example of the above-described various types of sensors  32 . Namely, the cylinder pressure sensor  32 E is included in the various types of sensors  32 . The cylinder pressure sensor  32 E is a sensor that detects the pressure of hydraulic oil in an oil chamber of a hydraulic cylinder, and outputs a detection value to the controller  30 . The cylinder pressure sensor  32 E includes an arm rod pressure sensor, a boom rod pressure sensor, an arm bottom pressure sensor, and a boom bottom pressure sensor. The arm rod pressure sensor detects an arm rod pressure. The arm rod pressure is the pressure of hydraulic oil in a rod-side oil chamber  8 R of the arm cylinder  8 . The boom rod pressure sensor detects a boom rod pressure. The boom rod pressure is the pressure of hydraulic oil in a rod-side oil chamber  7 R of the boom cylinder  7 . The arm bottom pressure sensor detects an arm bottom pressure. The arm bottom pressure is the pressure of hydraulic oil in a bottom-side oil chamber  8 B of the arm cylinder  8 . The boom bottom pressure sensor detects a boom bottom pressure. The boom bottom pressure is the pressure of hydraulic oil in a bottom-side oil chamber  7 B of the boom cylinder  7 . 
     An orientation sensor  32 G is an example of above-described various types of sensors  32 . Namely, the orientation sensor  32 G is included in the various types of sensors  32 . The orientation sensor  32 G is a sensor that detects the orientation of the shovel, and outputs a detection value to the controller  30 . The orientation sensor  32 G includes an arm angle sensor, a boom angle sensor, a bucket angle sensor, a turning angle sensor, and an inclination angle sensor. The arm angle sensor detects the opening and closing angle of the arm  5  relative to the boom  4  (hereinafter referred to as an “arm angle”). The boom angle sensor detects the raising and lowering angle of the boom  4  relative to the upper turning body  3  (hereinafter referred to as a “boom angle”). The bucket angle sensor detects the opening and closing angle of the bucket  6  relative to the arm  5  (hereinafter referred to as a “bucket angle”). Each of the arm angle sensor, the boom angle sensor, and the bucket angle sensor is configured by a combination of an acceleration sensor and a gyro sensor. Each of the arm angle sensor, the boom angle sensor, and the bucket angle sensor may be configured by a potentiometer, a stroke sensor, a rotary encoder, or the like. The turning angle sensor detects the turning angle of the upper turning body  3  relative to the lower traveling body  1 . The inclination angle sensor detects a body inclination angle that is the angle of the ground surface contacted by the shovel relative to a horizontal plane. 
     A display device DD is a device for displaying various types of information, and is, for example, a liquid crystal display installed in a cabin of the shovel. The display device DD displays various types of information in accordance with a control signal from the controller  30 . 
     A voice output device AD is a device for outputting various types of information by voice, and is, for example, a loudspeaker installed in the cabin of the shovel. The voice output device AD outputs various types of information by voice in accordance with a control signal from the controller  30 . 
     A regeneration valve V 1  is provided in a first oil passage C 1  that connects a rod-side oil chamber and a bottom-side oil chamber of a hydraulic cylinder. Namely, the regeneration valve V 1  is provided between the hydraulic cylinder and a flow rate control valve that adjusts the flow rate of hydraulic oil into the hydraulic cylinder. The regeneration valve V 1  is, for example, an electromagnetic proportional valve, and controls the flow area of the first oil passage C 1  in accordance with a control current from the controller  30 . The regeneration valve V 1  includes a boom regeneration valve and an arm regeneration valve. In the fourth variation, the regeneration valve V 1  is a boom regeneration valve provided in the first oil passage C 1  that connects the rod-side oil chamber  7 R and the bottom-side oil chamber  7 B of the boom cylinder  7 . The regeneration valve V 1  allows the bidirectional flow of hydraulic oil between the rod-side oil chamber  7 R and the bottom-side oil chamber  7 B. Namely, the regeneration valve V 1  does not include a check valve. However, the regeneration valve V 1  may have a first valve position, a second valve position, and a third valve position. The first valve position includes an oil passage in which a check valve is disposed to allow the flow of hydraulic oil only from the rod-side oil chamber  7 R to the bottom-side oil chamber  7 B. The second valve position includes an oil passage in which a check valve is disposed to allow the flow of hydraulic oil only from the bottom-side oil chamber  7 B to the rod-side oil chamber  7 R. The third valve position blocks the flow of hydraulic oil between the rod-side oil chamber  7 R and the bottom-side oil chamber  7 B. Alternatively, the regeneration valve V 1  may be configured by a first proportional valve and a second proportional valve. The first proportional valve includes a valve position corresponding to the first valve position and a valve position corresponding to the third valve position. The second proportional valve includes a valve position corresponding to the second valve position and a valve position corresponding to the third valve position. Further, the regeneration valve V 1  is provided outside of the control valve  17 . Therefore, the regeneration valve V 1  is controlled independently of spool valves within the control valve  17 . 
     The controller  30  uses various types of functional elements to perform calculation by obtaining the outputs of the pressure sensor  29 , the cylinder pressure sensor  32 F, and the orientation sensor  32 G. The various types of functional elements include an excavation operation detecting unit  302 A, an orientation detecting unit  302 B, a maximum allowable pressure calculating unit  302 C, and a regeneration valve control unit  302 D, which are detailed functional elements of the above-described movement correcting unit  302 . The various types of functional elements may be configured by software or may be configured by hardware. Further, the controller  30  outputs calculation results to the display device DD, the voice output device AD, the regeneration valve V 1 , and the like. 
     The excavation operation detecting unit  302 A is a functional element that detects whether an excavation operation is performed. In the fourth variation, the excavation operation detecting unit  302 A detects whether an arm excavation operation including an arm closing operation is performed. Specifically, the excavation operation detecting unit  302 A detects that an arm excavation operation has been performed, when an arm closing operation is detected, the boom rod pressure is a predetermined value or more, and a difference between the arm bottom pressure and the arm rod pressure is a predetermined value or more. The arm excavation operation includes a single operation of an arm closing operation only, a complex operation that is a combination of an arm closing operation and a boom lowering operation, and a complex operation that is a combination of an arm closing operation and a bucket closing operation. 
     The excavation operation detecting unit  302 A may detect whether a boom complex excavation operation including a boom raising operation is performed. Specifically, the excavation operation detecting unit  302 A detects that a boom complex excavation operation has been performed, when a boom raising operation is detected, the boom rod pressure is a predetermined value or more, and a difference between the arm bottom pressure and the arm rod pressure is a predetermined value or more. Furthermore, the excavation operation detecting unit  302 A may detect a boom complex excavation operation, on the condition that an arm closing operation has been additionally detected. 
     The excavation operation detecting unit  302 A may detect whether an excavation operation is performed, based on the outputs of other sensors such as the orientation sensor  32 G in addition to or in place of the outputs of the pressure sensor  29  and the cylinder pressure sensor  32 F. 
     The orientation detecting unit  302 B is a functional element that detects the orientation of the shovel. In the fourth variation, the orientation detecting unit  302  detects a boom angle, an arm angle, a bucket angle, a body inclination angle, and a turning angle, as the orientation of the shovel. 
     The maximum allowable pressure calculating unit  302 C is a functional element that calculates the maximum allowable pressure of hydraulic oil in a hydraulic cylinder during excavation work. The maximum allowable pressure changes in accordance with the orientation of the shovel. If hydraulic oil in a hydraulic cylinder exceeds the maximum allowable pressure during excavation work, an unintended movement of the shovel may occur. The unintended movement includes the lifting or dragging of the body of the shovel. In the fourth variation, the maximum allowable pressure calculating unit  302 C calculates the maximum allowable boom rod pressure during excavation work. If the boom rod pressure exceeds the maximum allowable boom rod pressure, the body of the shovel may be lifted. The maximum allowable pressure calculating unit  302 C may calculate the maximum allowable arm bottom pressure during excavation work. If the arm bottom pressure exceeds maximum allowable arm bottom pressure, the body of the shovel may be dragged toward an excavation point. 
     The regeneration valve control unit  302 D is a functional element that controls the regeneration valve V 1  in order to prevent an unintended movement of the body of the shovel during excavation work. In the fourth variation, the regeneration valve control unit  302 D controls the opening area of the regeneration valve V 1  not to exceed the maximum allowable boom rod pressure, in order to prevent the lifting of the body of the shovel. Specifically, when a predetermined condition (hereinafter referred to as a “control start condition”) on the stability of the body of the shovel is determined to be satisfied, the regeneration valve control unit  302 D controls the regeneration valve V 1  to prevent an unintended movement of the body of the shovel. 
     More specifically, when the arm excavation operation that is a single operation of an arm closing operation only is performed, and the boom rod pressure increases and reaches a given pressure that is less than or equal to the maximum allowable boom rod pressure, the regeneration valve control unit  302 D determines that the control start condition is satisfied. Then, the regeneration valve control unit  302 D opens the regeneration valve V 1  and increases the opening area of the regeneration valve V 1 . As a result, hydraulic oil flows from the rod-side oil chamber  7 R to the bottom-side oil chamber  7 B, and thus, the boom rod pressure decreases. At this time, the volume of hydraulic oil in the bottom-side oil chamber  7 B increases, and the boom cylinder  7  extends. In this manner, the regeneration valve control unit  302 D reduces the boom rod pressure such that the boom rod pressure does not exceed the maximum allowable boom rod pressure, thereby preventing the lifting of the body of the shovel. 
     Further, when the regeneration valve V 1  has opened, the regeneration valve control unit  302 D may output a control signal to one or both of the display device DD and the voice output device AD. This is to cause the display device DD to display a text message indicating that the regeneration valve V 1  has opened, or to cause the voice output device AD to output a voice message or alarm sound indicating that the regeneration valve V 1  has opened. 
     Next, referring to  FIG.  42   , a method for detecting the orientation of the shovel by the orientation detecting unit  302 B, and a method for calculating the maximum allowable pressure by the pressure calculating unit  302 C will be described.  FIG.  42    is a drawing illustrating the relationship between forces that act on the shovel when excavation is performed. 
     First, parameters related to control for preventing the lifting of the body of the shovel during excavation work will be described. 
     In  FIG.  42   , a point P 1  indicates a joint between the upper turning body  3  and the boom  4 , and a point P 2  indicates a joint between the upper turning body  3  and the cylinder of the boom cylinder  7 . Further, a point P 3  indicates a joint between a rod  7 C of the boom cylinder  7  and the boom  4 , and a point P 4  indicates a joint between the boom  4  and the cylinder of the arm cylinder  8 . Further, a point P 5  indicates a joint between a rod  8 C of the arm cylinder  8  and the arm  5 , and a point P 6  indicates a joint between the boom  4  and the arm  5 . Further, a point P 7  indicates a joint between the arm  5  and the bucket  6 , and a point P 8  indicates the tip of the bucket  6 . For clarification of explanation, the bucket cylinder  9  is not depicted in  FIG.  42   . 
     Further, in  FIG.  42   , the angle between a straight line that connects the point P 1  to the point P 3  and a horizontal line is represented as a boom angle θ 1 . The angle between a straight line that connects the point P 3  to the point P 6  and a straight line that connects the point P 6  to the point P 7  is represented as an arm angle θ 2 . The angle between the straight line that connects the point P 6  to the point P 7  and a straight line that connects the point P 7  to the point P 8  is represented as a bucket angle θ 3 . 
     Further, in  FIG.  42   , a distance D 1  indicates a horizontal distance between a center of rotation RC and the center of gravity GC of the shovel, that is, a distance between the line of action of gravity M·g, which is the product of the mass M of the shovel and gravitational acceleration g, and the center of rotation RC, at the time of the occurrence of lifting. The product of the distance D 1  and the magnitude of the gravity M·g represents the magnitude of a first moment of force about the center of rotation RC. Note that the symbol “·” represents “×” (a multiplication sign). 
     Further, in  FIG.  42   , a distance D 2  indicates a horizontal distance between the center of rotation RC and the point P 8 , that is, a distance between the line of action of a vertical component F R1  of an excavation reaction force F R  and the center of rotation RC. The product of the distance D 2  and the magnitude of the vertical component FR 1  represents the magnitude of a second moment of force about the center of rotation RC. An excavation angle θ is formed by the excavation reaction force F R  and the vertical axis, and the vertical component F R1  of the excavation reaction force F R  is expressed by F R1 =FR·cos θ. Furthermore, the excavation angle θ is calculated based on the boom angle θ 1 , the arm angle θ 2 , and the bucket angle θ 3 . 
     Further, in  FIG.  42   , a distance D 3  indicates a distance between a straight line, connecting the point P 2  to the point P 3 , and the center of rotation RC, that is, a distance between the line of action of a force F B , pulling the rod  7 C out of the boom cylinder  7 , and the center of rotation RC. The product of the distance D 3  and the magnitude of the force F B  represents the magnitude of a third moment of force about the center of rotation RC. 
     Further, in  FIG.  42   , a distance D 4  indicates a distance between the line of action of the excavation reaction force F R  and the point P 6 . The product of the distance D 4  and the magnitude of the excavation reaction force F R  represents the magnitude of a first moment of force about the point P 6 . 
     Further, in  FIG.  42   , a distance D 5  indicates a distance between a straight line, connecting the point P 4  to the point P 5 , and the point P 6 , that is, a distance between the line of action of an arm thrust F A , which closes the arm  5 , and the point P 6 . The product of the distance D 5  and the magnitude of the arm thrust F A  represents a second moment of force about the point P 6 . 
     It is assumed that the magnitude of a moment of force that causes the shovel to lift about the center of rotation RC by the vertical component F R1  of the excavation reaction force F R  and the magnitude of a moment of force that causes the shovel to lift about the center of rotation RC by the force F B  that pulls the rod  7 C out of the boom cylinder  7  are interchangeable with each other. In this case, the relationship between the magnitude of the second moment of force about the center of rotation RC and the magnitude of the third moment of force about the center of rotation RC is expressed by the following equation (31):
 
 F   R1   ·D 2= F   R ·cos θ· D 2= F   B   ·D 3  (31)
 
     Furthermore, the magnitude of a moment of force that closes the arm  5  about the point P 6  by the arm thrust F A  and the magnitude of a moment of force that opens the arm  5  about the point P 6  by the excavation reaction force F R  are considered to be balanced. In this case, the relationship between the magnitude of the first moment of force about the point P 6  and the magnitude of the second moment of force about the point P 6  is expressed by the following equation (32) and equation (32)′.
 
 F   A   ·D 5= F   R   ·D 4  (32)
 
 F   R   =F   A   ·D 5/ D 4  (32)′
 
     In the above equation (32)′, the symbol “/” represents “÷” (a division sign). 
     Further, from the equation (32) and the equation (32)′, the force F B  that pulls the rod  7 C out of the boom cylinder  7  is expressed by the following equation (33).
 
 F   B   =F   A   ·D 2· D 5·cos θ/( D 3· D 4)  (33)
 
     Further, the force F B  that pulls the rod  7 C out of the boom cylinder  7  is expressed by F B =P B ·A B −P B2 ·A B2 , where the annular pressure receiving area of a piston that faces the rod-side oil chamber  7 R of the boom cylinder  7  is represented as an area A B  as illustrated in the X-X cross-sectional view of  FIG.  42   , the pressure of hydraulic oil in the rod-side oil chamber  7 R is represented as a boom rod pressure P B , the circular pressure receiving area of the piston that faces the bottom-side oil chamber  7 B of the boom cylinder  7  is represented as an area A B2 , and the pressure of hydraulic oil in the bottom-side oil chamber  7 B is represented as a boom bottom pressure P B2 . Accordingly, the equation (33) is expressed by the following equation (34) and equation (34)′.
 
 P   B   =F   A   ·D 2· D 5·cos θ/( A   B   ·D 3· D 4)  (34)
 
 F   A   =P   B   ·A   B   ·D 3· D 4/( D 2· D 5·cos θ)  (34)′
 
     Further, the force F B , pulling the rod  7 C out of the boom cylinder  7  when the body of the shovel is lifted, is represented as a force F BMAX . The magnitude of the first moment of force about the center of rotation RC that prevents the lifting of the body of the shovel by the gravity M·g, and the magnitude of the third moment of force about the center of rotation RC that lifts the body of the shovel by the force F BMAX , are considered to be balanced. In this case, the relationship between the magnitude of the first moment of force and the magnitude of the third moment of force is expressed by the following equation (35).
 
 M·g·D 1= F   BMAX   ·D 3  (35)
 
 F   A   =P   B   ·A   B   ·D 3· D 4/( D 2· D 5·cos θ)  (34)′
 
     Furthermore, the boom rod pressure P B  at this point is represented as a maximum allowable boom rod pressure (hereinafter referred to as a “first maximum allowable pressure”) P BMAX  used to prevent the lifting of the body. The first maximum allowable pressure P BMAX  is expressed by the following equation (36).
 
 P   BMAX   =M·g·D 1/( A   B   ·D 3)  (36)
 
     Further, the distance D 1  is a constant, and similar to the excavation angle θ, the distances D 2  through D 5  are values determined according to the orientation of the excavation attachment, that is, the boom angle θ 1 , the arm angle θ 2 , and the bucket angle θ 3 . Specifically, the distance D 2  is determined according to the boom angle θ 1 , the arm angle θ 2 , and the bucket angle θ 3 , the distance D 3  is determined according to the boom angle θ 1 , the distance D 4  is determined according to the bucket angle θ 3 , and the distance D 5  is determined according to the arm angle θ 2 . 
     Accordingly, the maximum allowable pressure calculating unit  302 C can calculate the first maximum allowable pressure P BMAX  by using the boom angle θ 1  detected by the orientation detecting unit  302 B and the equation (36). 
     Further, the regeneration valve control unit  302 D can prevent the lifting of the body of the shovel by maintaining the boom rod pressure P B  at a given pressure that is less than or equal to the first maximum allowable pressure P BMAX . Specifically, when the boom rod pressure P B  reaches the given pressure, the regeneration valve control unit  302 D decreases the boom rod pressure P B  by increasing the flow rate of hydraulic oil flowing from the rod-side oil chamber  7 R into the bottom-side oil chamber  7 B. This is because a decrease in the boom rod pressure P B  results in a decrease in the arm thrust F A  as indicated by the equation (34)′, and further results in a decrease in the excavation reaction force F R  as indicated by the equation (32)′, and also a decrease in the vertical component F R1 . 
     Further, the position of the center of rotation RC is determined based on the output of the turning angle sensor. For example, when the turning angle between the lower traveling body  1  and the upper turning body  3  is zero degrees, the rear end of a part of the lower traveling body  1  that comes into contact with the ground surface serves as the center of rotation RC. When the turning angle between the lower traveling body  1  and the upper turning body  3  is 180 degrees, the front end of a part of the lower traveling body  1  that comes into contact with the ground surface serves as the center of rotation RC. Further, when the turning angle between the lower traveling body  1  and the upper turning body  3  is 90 degrees or 270 degrees, the side end of a part of the lower-part traveling body  1  that comes into contact with the ground surface serves as the center of rotation RC. 
     Next, parameters related to control for preventing the dragging of the body of the shovel toward an excavation point will be described. 
     The relationship between forces that move the body of the shovel in the horizontal direction during excavation work is expressed by the following inequality (37):
 
μ· N≥F   R2   (37)
 
     In the above inequality, μ represents a static friction coefficient of the ground surface contacted by the shovel, N represents a normal force against the gravity M·g of the shovel, and F R2  represents a horizontal component of the excavation reaction force F R  that drags the shovel toward an excavation point. Furthermore, μ·N represents a maximum static friction force that causes the shovel to be stationary. When the horizontal component F R2  of the excavation reaction force F R  exceeds the maximum static friction force μ·N, the shovel is dragged toward the excavation point. The static friction coefficient μ may be a value preliminarily stored in the ROM or the like or dynamically calculated based on various types of information. In the fourth variation, the static friction coefficient μ is preliminarily stored and is selected by an operator via an input device (not illustrated). The operator selects a desired friction condition (a static friction coefficient) from multiple levels of friction conditions (static friction coefficients) in accordance with the ground surface that the shovel contacts. 
     The horizontal component F R2  of the excavation reaction force F R  is expressed by F R2 =F R  sine, and the excavation reaction force F R  is expressed by F R =F A ·D 5 /D 4  from the equation (32)′. Accordingly, the inequality (37) is expressed by the following inequality (38).
 
μ· M·g≥F   A   ·D 5·sin θ/ D 4  (38)
 
     Further, the arm thrust F A  is expressed by F A =P A ·A A −P A2 ·A A2 , where the circular pressure receiving area of a piston that faces the bottom-side oil chamber  8 B of the arm cylinder  8  is represented as an area A A  as illustrated in the Y-Y cross-sectional view of  FIG.  42   , the pressure of hydraulic oil in the bottom-side oil chamber  8 B is represented as an arm bottom pressure PA, the circular pressure receiving area of the piston that faces the rod-side oil chamber  8 R of the arm cylinder  8  is represented as an area A A2 , and the pressure of hydraulic oil in the rod-side oil chamber  8 R is represented as an arm rod pressure P A2 . However, because P A  is much greater than P A2 , the arm thrust FA is expressed by F A =P A ·A A . Accordingly, the inequality (38) is expressed by the following inequality (39).
 
 P   A   ≤μ·M·g·D 4/( A   A   ·D 5·sin θ)  (39)
 
     When the right side and the left side of the inequality (39) are equal, the arm bottom pressure P A  corresponds to a maximum allowable arm bottom pressure that can avoid the body being dragged toward an excavation point, that is, a maximum allowable arm bottom pressure (hereinafter referred to as a “second maximum allowable pressure”) P AMAX  used to prevent the body from being dragged toward an excavation point. 
     Based on the above-described relationships, the maximum allowable pressure calculating unit  302 C uses the boom angle θ 1 , the arm angle θ 2 , and the bucket angle θ 3  detected by the orientation detecting unit  302 B and the inequality (39) to calculate the second maximum allowable pressure P AMAX . 
     Further, the regeneration valve control unit  302 D can prevent the body of the shovel from being dragged toward an excavation point by maintaining the arm bottom pressure P A  at a given pressure that is less than or equal to the second maximum allowable pressure P AMAX . Specifically, when the arm bottom pressure P A  reaches the given pressure, the regeneration valve control unit  302 D decreases the arm bottom pressure P A  by decreasing the flow rate of hydraulic oil flowing from a first pump  14 L into the bottom-side oil chamber  8 B. In a case where a regeneration valve is provided in an oil passage that connects the rod-side oil chamber  8 R to the bottom-side oil chamber  8 B, the regeneration valve control unit  302 D may decrease the arm bottom pressure P A  by increasing the flow rate of hydraulic oil flowing from the bottom-side oil chamber  8 B into the rod-side oil chamber  8 R, when the arm bottom pressure P A  reaches the given pressure. This is because a decrease in arm bottom pressure P A  results in a decrease in the arm thrust F A , and further results in a decrease in the horizontal component F R2  of the excavation reaction force F R . 
     Next, referring to  FIG.  43   , an example configuration of a hydraulic circuit installed in the shovel of  FIG.  1    will be described.  FIG.  43    is a drawing illustrating an example configuration of a hydraulic circuit installed in the shovel. In the example of  FIG.  43   , the drive system includes the first pump  14 L, a second pump  14 R, the control valve  17 , and hydraulic actuators. The hydraulic actuators include the boom cylinder  7 , the arm cylinder  8 , the bucket cylinder  9 , and the turning hydraulic motor  21 . In addition, the hydraulic actuators may include the traveling hydraulic motors  1 L and  1 R. 
     The turning hydraulic motor  21  is a hydraulic motor that turns the upper turning body  3 . Ports  21 L and  21 R are connected to a hydraulic oil tank T via respective relief valves  22 L and  22 R, and are also connected to the hydraulic oil tank T via respective check valves  23 L and  23 R. 
     The first pump  14  sucks hydraulic oil from the hydraulic oil tank T and discharges the hydraulic oil. The first pump  14 L is connected to a regulator  14 AL. The regulator  14 AL changes the inclination angle of a swash plate of the first pump  14 L in accordance with a command from the controller  30 , and controls a displacement volume (discharge flow rate per pump revolution). The same applies to a regulator  14 AR for the second pump  14 R. The first pump  14 L and the second pump  14 R correspond to the main pump  14  of  FIG.  41   , and the regulators  14 AL and  14 AR correspond to the regulator  14 A of  FIG.  41   . 
     The first pump  14 L and the second pump  14 R circulate hydraulic oil into the hydraulic oil tank T through center bypass pipelines  400 L and  400 R, parallel pipelines  420 L and  420 R, and return pipelines  430 L,  430 R, and  430 C. 
     The center bypass pipeline  400 L is a hydraulic oil line that passes through flow rate control valves  170 ,  172 L, and  173 L provided within the control valve  17 . The center bypass pipeline  400 R is a hydraulic oil line that passes through flow rate control valves  171 ,  172 R, and  173 R provided within the control valve  17 . 
     The parallel pipeline  420 L is a hydraulic oil line that extends parallel to the center bypass pipeline  400 L. When the flow of hydraulic oil passing through the center bypass pipeline  400 L is limited or blocked by the flow rate control valve  170  or the flow rate control valve  172 L, the parallel pipeline  420 L supplies hydraulic oil to a further downstream flow rate control valve. The parallel pipeline  420 R is a hydraulic oil line that extends parallel to the center bypass pipeline  400 R. When the flow of hydraulic oil passing through the center bypass pipeline  400 R is limited or blocked by the flow rate control valve  171  or the flow rate control valve  172 R, the parallel pipeline  420  supplies hydraulic oil to a further downstream flow rate control valve. 
     The return pipeline  430 L is a hydraulic oil line that extends parallel to the center bypass pipeline  400 L. The return pipeline  430 L causes hydraulic oil, passing through the flow rate control valves  170 ,  172 L, and  173 L from the hydraulic actuators, to be distributed to the return pipeline  430 C. The return pipeline  430 R is a hydraulic oil line that extends parallel to the center bypass pipeline  400 R. The return pipeline  430 R causes hydraulic oil, passing through the flow rate control valves  171 ,  172 R, and  173 R from the hydraulic actuators, to be distributed to the return pipeline  430 C. 
     The center bypass pipelines  400 L and  400 R include negative control throttles  18 L and  18 R and relief valves  19 L and  19 R between the most downstream flow rate control valves  173 L and  173 R and the hydraulic oil tank T. The flow of hydraulic oil discharged from the first pump  14 L and the second pump  14 R is limited by the negative control throttles  18 L and  18 R. The negative control throttles  18 L and  18 R generate a control pressure (hereinafter referred to as a “negative control pressure”) so as to control the regulators  14 AL and  14 AR. The relief valves  19 L and  19 R are opened to discharge hydraulic oil in the center bypass pipelines  400 L and  400 R into the hydraulic oil tank T, when the negative control pressure reaches a predetermined relief pressure. 
     A spring-type check valve  20  is provided at the most downflow part of the return pipeline  430 C. The spring-type check valve  20  functions to increase the pressure of hydraulic oil in a pipeline  440  that connects the turning hydraulic motor  21  and the return pipeline  430 C. With this configuration, hydraulic oil can be securely supplied to the suction-side ports of the turning hydraulic motor  21  during turning deceleration, thereby preventing cavitation. 
     The control valve  17  is a hydraulic control unit that controls a hydraulic drive system in the shovel. In the fourth variation, the control valve  17  is a cast component including the flow rate control valves  170 ,  171 ,  172 L,  172 R,  173 L, and  173 R, the center bypass pipelines  400 L and  400 R, the parallel pipelines  420 L and  420 R, and the return pipelines  430 L and  430 R. 
     The flow rate control valves  170 ,  171 ,  172 L,  172 R,  173 L, and  173 R are valves that control the direction and the flow rate of hydraulic oil flowing into and out of the hydraulic actuators. In the example of  FIG.  43   , each of the flow rate control valves  170 ,  171 ,  172 L,  172 R,  173 L, and  173 R is a three-port, three-position spool valve that operates with a pilot pressure generated by the operation device  26 . The pilot pressure is supplied to either a right or a left pilot port of each of the flow rate control valves  170 ,  171 ,  172 L,  172 R,  173 L, and  173 R. The pilot pressure is generated in accordance with the amount of operation, and is supplied to a pilot port corresponding to the direction of operation (the angle of operation). 
     Specifically, the flow rate control valve  170  is a spool valve that controls the direction and the flow rate of hydraulic oil flowing into and out of the turning hydraulic motor  21 . The flow rate control valve  171  is a spool valve that controls the direction and the flow rate of hydraulic oil flowing into and out of the bucket cylinder  9 . 
     The flow rate control valves  172 L and  172 R are spool valves that control the direction and the flow rate of hydraulic oil flowing into and out of the boom cylinder  7 . The flow rate control valves  173 L and  173 R are spool valves that control the direction and the flow rate of hydraulic oil flowing into and out of the arm cylinder  8 . 
     The regeneration valve V 1  is a valve that controls the flow rate by adjusting the size of the opening in accordance with a command from the controller  30 , and is provided in the first oil passage C 1 . The first oil passage C 1  connects a second oil passage C 2  to a third oil passage C 3 . The second oil passage C 2  connects the rod-side oil chamber  7 R of the boom cylinder  7  to the flow rate control valves  172 L and  172 R. The third oil passage C 3  connects the bottom-side oil chamber  7 B of the boom cylinder  7  to the flow rate control valves  172 L and  172 R. In the example of  FIG.  43   , the regeneration valve V 1  is a boom regeneration valve disposed outside of the control valve  17 , and is also a one-port, two-position electromagnetic proportional valve that switches between communication and shutoff of the second oil passage C 2  and the third oil passage C 3 . Specifically, when the regeneration valve V 1  is at the first valve position, the regeneration valve V 1  opens at the maximum level, and causes the second oil passage C 2  to communicate with the third oil passage C 3 . When the regeneration valve V 1  is at the second valve position, the regeneration valve V 1  shuts off the communication between the second oil passage C 2  and the third oil passage C 3 . Further, the regeneration valve V 1  can remain at any position between the first valve position and the second valve position. The opening area of the regeneration valve V 1  increases as the regeneration valve V 1  approaches the first valve position. Similar to the flow rate control valve, the regeneration valve V 1  may be provided inside of the control valve  17 . In this case, the first oil passage C 1  is also provided inside of the control valve  17 . 
     The controller  30  outputs a command to the regeneration valve V 1  in response to detecting that the boom rod pressure has reached a predetermined pressure, for example. In response to receiving the command, the regeneration valve V 1  changes its position from the second valve position toward the first valve position, and causes the second oil passage C 2  to communicate with the third oil passage C 3 . 
     In the example of  FIG.  43   , the regeneration valve V 1  further includes an arm regeneration valve V 1   a . The arm regeneration valve V 1   a  is an electromagnetic proportional valve that is provided in a first oil passage C 1   a  connecting the rod-side oil chamber  8 R and the bottom-side oil chamber  8 B of the arm cylinder  8 . The arm regeneration valve V 1   a  controls the flow area of the first oil passage C 1   a  in accordance with a control current from the controller  30 , for example. The arm regeneration valve V 1   a  allows the bidirectional flow of hydraulic oil between the rod-side oil chamber  8 R and the bottom-side oil chamber  8 B. Namely, the regeneration valve V 1  does not include a check valve. Further, the arm regeneration valve V 1   a  is provided outside of the control valve  17 . Therefore, the arm regeneration valve V 1   a  is controlled independently of the spool valves within the control valve  17 . 
     Specifically, the first oil passage C 1   a  connects a second oil passage C 2   a  to a third oil passage C 3   a . The second oil passage C 2   a  connects the rod-side oil chamber  8 R of the arm cylinder  8  to the flow rate control valves  173 L and  173 R. The third oil passage C 3   a  connects the bottom-side oil chamber  8 B of the arm cylinder  8  to the flow rate control valves  173 L and  173 R. In the example of  FIG.  43   , the arm regeneration valve V 1   a  is a one-port, two-position electromagnetic proportional valve that is capable of switching between communication and shutoff of the second oil passage C 2   a  and the third oil passage C 3   a . Specifically, when the arm regeneration valve V 1   a  is at the first valve position, the arm regeneration valve V 1   a  opens at the maximum level, and causes the second oil passage C 2   a  to communicate with the third oil passage C 3   a . When the arm regeneration valve V 1   a  is at the second valve position, the arm regeneration valve V 1   a  shuts off the communication between the second oil passage C 2   a  and the third oil passage C 3   a . Further, the arm regeneration valve V 1   a  can remain at any position between the first valve position and the second valve position. The opening area of the arm regeneration valve V 1   a  increases as the arm regeneration valve V 1   a  approaches the first valve position. Similar to the flow rate control valve, the arm regeneration valve V 1   a  may be provided inside of the control valve  17 . In this case, the first oil passage C 1   a  is also provided inside of the control valve  17 . 
     Next, referring to  FIG.  44   , a process performed by the controller  30  to support excavation work while preventing the body of the shovel from being lifted (hereinafter referred to as a “first support process”) will be described.  FIG.  44    is a flowchart illustrating a flow of the first support process. The controller  30  repeatedly performs the first support process at predetermined intervals. 
     First, the excavation operation detecting unit  302 A of the controller  30  determines whether an excavation operation is being performed (step S 1 ). 
     For example, the excavation operation detecting unit  302 A of the controller  30  detects whether an arm closing operation is being performed based on the output of the pressure sensor  29 . If it is determined that the arm closing operation is being performed, the excavation operation detecting unit  302 A calculates a difference between the arm bottom pressure and the arm rod pressure. If the calculated difference is a predetermined value or more, the excavation operation detecting unit  302 A determines that the excavation operation is being performed (the arm excavation operation is being performed). 
     Alternatively, the controller  30  detects whether a boom raising operation is being performed based on the output of the pressure sensor  29 . If it is determined that the boom raising operation is being performed, the excavation operation detecting unit  302 A acquires the boom rod pressure. Further, the excavation operation detecting unit  302 A calculates a difference between the arm bottom pressure and the arm rod pressure. If the acquired boom rod pressure is a predetermined value or more, and also the calculated difference is a predetermined value or more, the excavation operation detecting unit  302 A determines that the excavation operation is being performed (the boom raising operation is being performed). 
     If the excavation operation detecting unit  302 A determines that the excavation operation is not performed (no in step S 1 ), the excavation operation detecting unit  302 A ends the current first support process. 
     Conversely, if the excavation operation detecting unit  302 A determines that the excavation operation is being performed (yes in step S 1 ), the orientation detecting unit  302 B detects the orientation of the shovel (step S 2 ). Specifically, the orientation detecting unit  302 B detects the boom angle θ 1 , the arm angle θ 2 , and the bucket angle θ 3  based on the outputs of the arm angle sensor, the boom angle sensor, and the bucket angle sensor. Accordingly, the maximum allowable pressure calculating unit  302 C of the controller  30  can obtain the distance between the line of action of a force exerted on the excavation attachment and a predetermined center of rotation. 
     Next, the maximum allowable pressure calculating unit  302 C calculates the first maximum allowable pressure P BMAX , based on detected values of the orientation detecting unit  302 B (step S 3 ). Specifically, the maximum allowable pressure calculating unit  302 C uses the above-described equation (36) to calculate the first maximum allowable pressure P BMAX . 
     Next, the maximum allowable pressure calculating unit  302 C sets a given pressure that is less than or equal to the calculated first maximum allowable pressure P BMAX  as a target boom rod pressure P BT  (step S 4 ). Specifically, the maximum allowable pressure calculating unit  302 C sets a value obtained by subtracting a predetermined value from the first maximum allowable pressure P BMAX  as the target boom cylinder pressure P BT . 
     Next, the regeneration valve control unit  302 D of the controller  30  determines whether a control start condition, which is a predetermined condition on the stability of the body of the shovel, is satisfied (step S 5 ). For example, the regeneration valve control unit  302 D determines that the control start condition is satisfied when the boom rod pressure P B  has reached the target boom cylinder pressure P BT . This is because it can be determined that the body of the shovel would be lifted if the boom rod pressure P B  continued to rise. 
     If it is determined that the control start condition is satisfied (yes in step S 5 ), for example, if the boom rod pressure P B  has reached the target boom cylinder pressure P BT , the regeneration valve control unit  302 D controls the regeneration valve V 1  (boom regeneration valve) to reduce the boom rod pressure P B  (step S 6 ). Specifically, the regeneration valve control unit  302 D supplies a control current to the regeneration valve V 1 , so as to increase the opening area of the regeneration valve V 1 . This is to increase the flow area of the first oil passage C 1 . By causing hydraulic oil to flow from the rod-side oil chamber  7 R into the bottom-side oil chamber  7 B, the regeneration valve control unit  302 D reduces the boom rod pressure P B . At this time, the regeneration valve control unit  302 D may perform feedback control of the boom rod pressure P B  based on the output of the boom rod pressure sensor. As a result, the boom cylinder  7  extends, thus resulting in a decrease in the vertical component F R1  of the excavation reaction force F R . Accordingly, the body of the shovel is prevented from being lifted. 
     In step S 5 , if it is determined that the control start condition is not satisfied (no in step S 5 ), for example, if the boom rod pressure P B  remains below the target boom cylinder pressure P BT , the regeneration valve control unit  302 D ends the current first support process, without reducing the boom rod pressure P B . This is because there is no possibility that the body of the shovel may be lifted. 
     For example, the shovel that supports a complex excavation operation while preventing the lifting of the body of the shovel is known (see Patent Document 1 described above). The shovel includes an electromagnetic proportional valve placed in a pilot line between a boom selector valve and a boom operation lever. The boom selector valve is a spool valve that controls the flow rate of the hydraulic oil flowing into and out of the boom cylinder. The electromagnetic proportional valve controls a pilot pressure, acting on a boom-raising pilot port of the boom selector valve, in accordance with a control current from the controller. Specifically, the electromagnetic proportional valve has a configuration in which the secondary-side pressure, acting on the boom-raising pilot port, becomes greater than the primary-side pressure as the control current from the controller increases. 
     In the shovel described in Patent Document 1, if the pressure of hydraulic oil reaches a predetermined threshold while a complex excavation operation that is a combination of a boom raising operation and an arm closing operation is being performed, a control current is supplied to the electromagnetic proportional valve so as to increase the pilot pressure acting on the boom-raising pilot port. By increasing the amount of hydraulic oil flowing from the rod-side oil chamber of the boom cylinder into the hydraulic oil tank, it is possible to reduce the pressure of the hydraulic oil in the rod-side oil chamber. As a result, the raising speed of the boom increases, and the vertical component of the excavation reaction force decrease. Thus, the body of the shovel is prevented from being lifted. Furthermore, by similar control, the body of the shovel is also prevented from being dragged toward an excavation point during excavation work. 
     However, the shovel in Patent Document 1 forcibly increases the raising speed of the boom  4  by increasing the pilot pressure, acting on the boom-raising pilot port during the complex excavation operation, so as to prevent the lifting of the body of the shovel. Therefore, the operator may feel discomfort depending on the raising speed of the boom  4 . 
     Conversely, with the above-described configuration according to the fourth variation, it is possible for the controller  30  to prevent the body of the shovel from being lifted during complex excavation work without affecting a pilot pressure. Therefore, it is possible for the shovel to perform excavation work that makes efficient use of its body weight at a point immediately before the body of the shovel is lifted, while also causing less discomfort to the operator. Furthermore, work efficiency can be improved by eliminating the need to perform an operation for returning the lifted shovel to its original orientation, thereby also decreasing fuel consumption, preventing a failure of the body, and reducing the operator&#39;s operation burden. 
     Further, the controller  30  automatically controls the opening area of the regeneration valve V 1  to reduce the boom rod pressure P B . Namely, the controller  30  reduces the boom rod pressure P B , independently of the operation of the boom operation lever by the operator. Therefore, it is not necessary for the operator to finely adjust the boom operation lever to prevent the lifting of the body of the shovel. 
     Further, the controller  30  moves hydraulic oil between the rod-side oil chamber  7 R and the bottom-side oil chamber  7 B. Therefore, it is possible to reduce the amount of hydraulic oil discharged into the hydraulic oil tank T in a useless manner, as compared to a configuration in which hydraulic oil is discharged from the rod-side oil chamber  7 R into the hydraulic oil tank T via, for example, a relief valve. 
     Further, even if the regeneration valve V 1  is left open due to an abnormal control current while the shovel is not in operation, the contraction of the boom cylinder  7  stops at the time when a force that contracts the boom cylinder  7  by the body weight of the attachment is balanced with a force that extends the boom cylinder  7 . This is because hydraulic oil does not flow into anywhere other than the rod-side oil chamber  7 R and the bottom-side oil chamber  7 B. Therefore, excessive contraction of the boom cylinder  7  can be prevented, unlike a case in which an electromagnetic relief valve, provided in an oil passage that connects the bottom-side oil chamber  7 B to the hydraulic oil tank T, is left open. 
     Next, referring to  FIG.  45   , changes in physical quantities over time during arm excavation work will be described.  FIG.  45    is a drawing illustrating changes in the arm bottom pressure P A , the boom rod pressure P B , the body inclination angle, and the stroke amount of the boom cylinder over time. Each continuous line in  FIG.  45    indicates changes when the first support process is performed, and each dotted line in  FIG.  45    indicates changes when the first support process is not performed. In the example of  FIG.  45   , the operator is performing arm excavation work by performing an arm closing operation only. 
     At a time t 1 , the bucket  6  comes into contact with the ground surface. At a time t 2 , the arm bottom pressure P A  relatively rapidly increases. This is because the excavation load rapidly increases as excavation work progresses. 
     Thereafter, at a time t 3  a little later than the rapid increase in the arm bottom pressure P A , the boom rod pressure P B  relatively rapidly increases, similar to the arm bottom pressure P A . 
     Thereafter, at a time t 4 , upon the boom rod pressure P B  reaching the target boom rod pressure P BT , the controller  30  supplies a control current to the regeneration valve V 1  so as to increase the opening area of the regeneration valve V 1  when the first support process is used. Accordingly, the boom rod pressure P B  is maintained at the target boom rod pressure P BT , as indicated by the continuous line. At this time, the boom rod pressure P B  is maintained at the target boom rod pressure P BT  by increasing or decreasing the opening area of the regeneration valve V 1  in accordance with the change in the boom rod pressure P B . Specifically, the controller  30  increases the opening area of the regeneration valve V 1  when the boom rod pressure P B  exceeds the target boom rod pressure P BT , and decreases the opening area of the regeneration valve V 1  when the boom rod pressure P B  drops below the target boom rod pressure P BT . 
     Accordingly, the stroke amount of the boom cylinder starts to increase at the time t 4 , and relatively gradually increases thereafter. Namely, the boom  4  is gradually raised. When the arm  5  is closed, the excavation reaction force increases, and as a result, the boom rod pressure P B  exceeds the target boom rod pressure P BT . Each time the boom rod pressure P B  exceeds the target boom rod pressure P BT , the opening area of the regeneration valve V 1  increases, thereby causing hydraulic oil to flow from the rod-side oil chamber  7 R into the bottom-side oil chamber  7 B. 
     Accordingly, the body inclination angle is maintained approximately the same and does not change largely. Namely, the body of the shovel is not lifted. 
     If the first support process is not used, the opening area of the regeneration valve V 1  would not be increased even when the boom rod pressure P B  reaches the target boom rod pressure P BT . As a result, as indicated by the dotted line, the boom rod pressure P B  would exceed the target boom rod pressure P BT  and would continue to increase until the body of the shovel is lifted at a time t 5 . Once the shovel is lifted, a further increase in the boom rod pressure P B  is reduced. This is because a further increase in excavation load is reduced by the lifting of the body. 
     Further, the stroke amount of the boom cylinder would be maintained the same even after the time t 4 , as indicated by the dotted line. Namely, the boom cylinder  7  would not be extended. In addition, as indicated by the dotted line, the body inclination angle would start to increase at the time t 5  and would relatively gradually increase thereafter because of the lifting of the shovel. 
     Conversely, the controller  30  according to the fourth variation opens the regeneration valve V 1  when the boom rod pressure P B  reaches the target boom rod pressure P BT . Accordingly, it is possible to prevent the body of the shovel from being lifted. 
     Further, the controller  30  can control the regeneration valve V 1  independently of the operation related to the boom cylinder  7 . Specifically, even when the operator is not operating the boom operation lever during arm excavation work, the controller  30  can open the regeneration valve V 1  as necessary, so as to extend the boom cylinder and decrease the boom rod pressure. Thus, it is possible to prevent the body of the shovel from being lifted. 
     Next, referring to  FIG.  46   , a configuration example of another hydraulic circuit installed in the shovel of  FIG.  1    will be described.  FIG.  46    is a drawing illustrating a configuration example of another hydraulic circuit installed in the shovel of  FIG.  1   . The hydraulic circuit of  FIG.  46    differs from the hydraulic circuit of  FIG.  43   , mainly in that the control valve  17  includes variable load check valves  510 ,  520 , and  530 , a converging valve  550 , and unified bleed-off valves  560 L and  560 R; however, the hydraulic circuit of  FIG.  46    is the same as the hydraulic circuit of  FIG.  43    in other respects. Therefore, a description of common elements will not be provided, and only differences will be described. 
     The variable load check valves  510 ,  520 , and  530  operate in accordance with commands from the controller  30 . In the example of  FIG.  46   , the variable load check valves  510 ,  520 , and  530  are one-port, two-position electromagnetic valves that are capable of switching communication and shutoff between the flow rate control valves  171  through  173  and one or both of the first pump  14 L and the second pump  14 R. Note that the variable load check valves  510 ,  520 , and  530  include check valves that blocks the flow of hydraulic oil returning to the pump side. Specifically, when the variable load check valve  510  is at a first position, the variable load check valve  510  causes the flow rate control valve  173  to communicate with one or both of the first pump  14 L and the second pump  14 R. When the variable load check valve  510  is at a second position, the variable load check valve  510  shuts off the communication therebetween. The same applies to the variable load check valve  520  and the variable load check valve  530 . 
     The converging valve  550  switches converging and non-converging of hydraulic oil discharged from the first pump  14 L (hereinafter referred to as a “first hydraulic oil”) and hydraulic oil discharged from the second pump  14 R (hereinafter referred to as a “second hydraulic oil”). In the example of  FIG.  46   , the converging valve  550  is a one-port, two-position electromagnetic valve that operates in accordance with a command from the controller  30 . Specifically, when the converging valve  550  is at a first position, the converging valve  550  causes coversing of the first hydraulic oil with the second hydraulic oil. When the converging valve  550  is at a second position, the converging valve  550  does not cause coversing of the first hydraulic oil with the second hydraulic oil. 
     The unified bleed-off valves  560 L and  560 R operate in accordance with commands from the controller  30 . In the example of  FIG.  46   , the unified bleed-off valve  560 L is a one-port, two-position electromagnetic valve that is capable of controlling the amount of the first hydraulic oil discharged into the hydraulic oil tank T. The same applies to the unified bleed-off valve  560 R. With the above configuration, the unified bleed-off valves  560 L and  560 R enable a combined opening of related flow rate control valves of the flow rate control valves  170  through  173 . Specifically, when the converging valve  550  is at the second position, the unified bleed-off valve  560 L enables a combined opening of the flow rate control valve  170  and the flow rate control valve  173 , and the unified bleed-off valve  560 R enables a combined opening of the flow rate control valve  171  and the flow rate control valve  172 . When the unified bleed-off valve  560 L is at a first position, the unified bleed-off valve  560 L serves as a variable throttle valve that controls the area of the combined opening of the flow rate control valve  170  and the flow rate control valve  173 . When the unified bleed-off valve  560 L is at a second position, the unified bleed-off valve  560 L blocks the combined opening of the flow rate control valve  170  and the flow rate control valve  173 . The same applies to the unified bleed-off valve  560 R. 
     Each of the variable load check valves  510 ,  520 , and  530 , the converging valve  550 , and the unified bleed-off valves  560 L and  560 R may be a spool valve driven by a pilot pressure. 
     Next, referring to  FIG.  47   , a process performed by the controller  30  to support arm excavation work while preventing the body of the shovel from being dragged toward an excavation point (hereinafter referred to as a “second support process”) will be described.  FIG.  47    is a flowchart illustrating a flow of the second support process. The controller  30  repeatedly performs the second support process at predetermined intervals. 
     First, the excavation operation detecting unit  302 A of the controller  30  determines whether an arm excavation operation including an arm closing operation is being performed (step S 11 ). Specifically, the excavation operation detecting unit  302 A detects whether an arm closing operation is being performed based on the output of the pressure sensor  29 . If it is determined that the arm closing operation is being performed, the excavation operation detecting unit  302 A calculates a difference between the arm bottom pressure and the arm rod pressure. If the calculated difference is a predetermined value or more, the excavation operation detecting unit  302 A determines that the arm excavation operation is being performed. 
     If the excavation operation detecting unit  302 A determines that the arm excavation operation is not being performed (no in step S 11 ), the excavation operation detecting unit  302 A ends the current second support process. 
     Conversely, if the excavation operation detecting unit  302 A determines that the arm excavation operation is being performed (yes in step S 11 ), the orientation detecting unit  302 B detects the orientation of the shovel (step S 12 ). 
     Next, the maximum allowable pressure calculating unit  302 C calculates the second maximum allowable pressure, based on the output of the orientation detecting unit  302 B (step S 13 ). Specifically, the maximum allowable pressure calculating unit  302 C uses the above-described inequality (39) to calculate the second maximum allowable pressure P AMAX . 
     Next, the maximum allowable pressure calculating unit  302 C sets a given pressure that is less than or equal to the calculated second maximum allowable pressure P AMAX  as a target arm bottom pressure P AT  (step S 14 ). Specifically, the maximum allowable pressure calculating unit  302 C sets the second maximum allowable pressure P AMAX  as the target arm bottom pressure P AT . 
     Next, the regeneration valve control unit  302 D of the controller  30  determines whether a control start condition, which is a predetermined condition on the stability of the body of the shovel, is satisfied (step S 15 ). For example, the regeneration valve control unit  302 D determines that the control start condition is satisfied when the arm bottom pressure P A  has reached the target arm bottom pressure P AT . This is because it can be determined that the body of the shovel would be dragged toward the excavation point if the arm bottom pressure P A  continued to rise. 
     If it is determined that the control start condition is satisfied (yes in step S 15 ), for example, if the arm bottom pressure P A  has reached the target arm bottom pressure P AT , the regeneration valve control unit  302 D controls the arm regeneration valve V 1   a  to reduce the difference between the arm bottom pressure P A  and the arm rod pressure P A2  (step S 16 ). Specifically, the regeneration valve control unit  302 D supplies a control current to the arm regeneration valve V 1   a , so as to open the arm regeneration valve V 1   a  and increase the opening area. This is to increase the flow area of the first oil passage C 1   a . If the opening area of a cylinder/tank (CT) port of the flow rate control valve  173  is large, the regeneration valve control unit  302 D causes hydraulic oil to flow out of the bottom-side oil chamber  8 B into the tank, so as to reduce the arm bottom pressure P A . As a result, the extension of the arm cylinder  8  is suppressed, thereby decreasing or eliminating the horizontal component F R2  of the excavation reaction force F R . Accordingly, the body of the shovel is prevented from being dragged toward the excavation point. 
     Further, even if the opening area of the CT port of the flow rate control valve  173  is small, the regeneration valve control unit  302 D increases the arm rod pressure P A2  and decreases the difference between the arm bottom pressure P A  and the arm rod pressure P A2  by causing hydraulic oil to flow into the rod-side oil chamber  8 R. As a result, the extension of the arm cylinder  8  is suppressed, thereby decreasing or eliminating the horizontal component F R2  of the excavation reaction force F R . Accordingly, the body of the shovel is prevented from being dragged toward the excavation point. 
     In the above manner, hydraulic oil discharged from the arm cylinder  8  is supplied to an oil chamber located on the side opposite to the discharge side of the arm cylinder  8  or is discharged into the tank, in accordance with the size of the opening of the cylinder/tank port of the flow rate control valve  173 . As a result, the extension of the arm cylinder  8  is suppressed or stopped, thereby preventing the body of the shovel from being dragged toward the excavation point. 
     If it is determined that the control start condition is not satisfied (no in step S 15 ), for example, if the arm bottom pressure P A  remains below the target arm bottom pressure P AT , the regeneration valve control unit  302 D ends the current second support process, without reducing the arm bottom pressure PA. This is because there is no possibility that the body of the shovel may be dragged. 
     With the above configuration, it is possible for the controller  30  to prevent the body of the shovel from being dragged toward an excavation point during arm excavation work without affecting a pilot pressure. Therefore, it is possible for the shovel to perform arm excavation work that makes efficient use of its body weight at a point immediately before the body of the shovel is dragged. Furthermore, work efficiency can be improved by eliminating the need to perform an operation for returning the dragged shovel to its original orientation, thereby also decreasing fuel consumption, preventing a failure of the body, and reducing the operator&#39;s operation burden. 
     Further, the controller  30  moves hydraulic oil between the rod-side oil chamber  8 R and the bottom-side oil chamber  8 B. Therefore, it is possible to reduce a pressure loss occurring in a pipeline or the like, as compared to a configuration in which hydraulic oil is discharged from the bottom-side oil chamber  8 B into the hydraulic oil tank T via, for example, a relief valve. Further, even if the arm regeneration valve V 1   a  is left open, the extension and contraction of the arm cylinder  8  stops at the time when a force that extends the arm cylinder  8  is balanced with a force that contracts the arm cylinder  8 . Thus, the arm cylinder  8  is not excessively extended or contracted. 
     Next, referring to  FIG.  48   , a process performed by the controller  30  of the shovel having the hydraulic circuit of  FIG.  46    to support excavation work, while preventing the body of the shovel from being dragged toward an excavation point (hereinafter referred to as a “third support process”) will be described.  FIG.  48    is a flowchart illustrating a flow of the third support process. The controller  30  repeatedly performs the third support process at predetermined intervals. 
     First, the excavation operation detecting unit  302 A of the controller  30  determines whether a complex excavation operation including a boom raising operation and an arm closing operation is being performed (step S 21 ). Specifically, the excavation operation detecting unit  302 A detects whether a boom raising operation is being performed based on the output of the pressure sensor  29 . If it is determined that the boom raising operation is being performed, the excavation operation detecting unit  302 A obtains the boom rod pressure. Further, the excavation operation detecting unit  302 A calculates a difference between the arm bottom pressure and the arm rod pressure. Then, if the obtained boom rod pressure is a predetermined value or more and the calculated difference is a predetermined value or more, the excavation operation detecting unit  302 A determines that the complex excavation operation is being performed. 
     If the excavation operation detecting unit  302 A determines that the complex excavation operation is not being performed (no in step S 21 ), the excavation operation detecting unit  302 A ends the this time third support process. 
     Conversely, if the excavation operation detecting unit  302 A determines that the complex excavation operation is being performed (yes in step S 21 ), the orientation detecting unit  302 B detects the orientation of the shovel (step S 22 ). 
     Next, the maximum allowable pressure calculating unit  302 C calculates the first maximum allowable pressure and the second maximum allowable pressure, based on detected values of the orientation detecting unit  302 B (step S 23 ). Specifically, the maximum allowable pressure calculating unit  302 C uses the above-described equation (36) to calculate the first maximum allowable pressure P BMAX  and uses the above-described inequality (39) to calculate the second maximum allowable pressure P AMAX . 
     Next, the maximum allowable pressure calculating unit  302 C sets a given pressure that is less than or equal to the calculated first maximum allowable pressure P BMAX  as a target boom rod pressure P BT  (step S 24 ). 
     Next, the regeneration valve control unit  302 D of the controller  30  determines whether a control start condition, which is a predetermined condition on the stability of the body of the shovel, is satisfied (step S 25 ). For example, the regeneration valve control unit  302 D determines that the control start condition is satisfied when the boom rod pressure P B  has reached the target boom rod pressure P BT . In this step, whether the control start condition is satisfied is determined based on the boom rod pressure P B . However, whether the control start condition is satisfied may be determined based on whether the magnitude of the vertical component of the excavation reaction force satisfies a predetermined condition. In this manner, determination in preventing lifting may be made based on parameters contributing to the vertical component. 
     If it is determined that the control start condition is satisfied (yes in step S 25 ), for example, if the boom rod pressure P B  has reached the target boom rod pressure P BT , the regeneration valve control unit  302 D controls the regeneration valve V 1  (boom regeneration valve) to reduce the boom rod pressure P B  (step S 26 ). Specifically, the regeneration valve control unit  302 D supplies a control current to the regeneration valve V 1 , so as to open the regeneration valve V 1  and increase the opening area. This is to increase the flow area of the first oil passage C 1 . By causing hydraulic oil to flow out of the rod-side oil chamber  7 R, the regeneration valve control unit  302 D reduces the boom rod pressure P B . As a result, the boom cylinder  7  extends, thereby decreasing the vertical component F R1  of the excavation reaction force F R . Accordingly, the body of the shovel is prevented from being lifted. 
     Thereafter, the regeneration valve control unit  302 D of the controller  30  continues to monitor the boom rod pressure P B . If the boom rod pressure P B  further increases regardless of the increased opening area of the regeneration valve V 1 , and has reached the first maximum allowable pressure P BMAX  (yes in step S 27 ), the regeneration valve control unit  302 D controls the arm regeneration valve V 1   a  to reduce the arm bottom pressure P A  (step S 28 ). Specifically, the regeneration valve control unit  302 D supplies a control current to the arm regeneration valve V 1   a , so as to open the arm regeneration valve V 1   a  and increase the opening area. This is to increase the flow area of the first oil passage C 1   a . By causing hydraulic oil to flow out of the bottom-side oil chamber  8 B, the regeneration valve control unit  302 D reduces the arm bottom pressure P A . As a result, the extension of the arm cylinder  8  is suppressed or stopped, thereby decreasing or eliminating the vertical component F R1  of the excavation reaction force F R . Accordingly, the body of the shovel is prevented from being lifted. 
     In step S 25 , if it is determined that the control start condition is not satisfied (no in step S 25 ), for example, if the boom rod pressure P B  remains below the target boom rod pressure P BT , the controller  30  causes the process to proceed to step S 29 , without reducing the boom rod pressure P B . This is because there is no possibility that the body of the shovel may be lifted. 
     Similarly, in step S 27 , if the boom rod pressure P B  remains below the first maximum allowable pressure P BMAX  (no in step S 27 ), the controller  30  causes the process to proceed to step S 29 , without reducing the arm bottom pressure P A . This is because there is no possibility that the body of the shovel may be lifted. 
     Next, in step S 29 , the maximum allowable pressure calculating unit  302 C sets a given pressure that is less than or equal to the calculated second maximum allowable pressure P AMAX  as a target arm bottom pressure P AT . Specifically, the maximum allowable pressure calculating unit  302 C sets the second maximum allowable pressure P AMAX  as the target arm bottom pressure P AT . 
     Thereafter, the regeneration valve control unit  302 D of the controller  30  determines whether an additional control start condition is satisfied (step S 30 ). For example, the regeneration valve control unit  302 D determines that the additional control start condition is satisfied when the arm bottom pressure P A  has reached the target arm bottom pressure P AT . 
     If it is determined that the additional control start condition is satisfied (yes in step S 30 ), for example, if the arm bottom pressure P A  has reached the target arm bottom pressure P AT , the regeneration valve control unit  302 D controls the arm regeneration valve V 1   a  to reduce the difference between the arm bottom pressure P A  and the arm rod pressure P A2 , thereby reducing the arm thrust F A  (step S 31 ). Specifically, the regeneration valve control unit  302 D supplies a control current to the arm regeneration valve V 1   a , so as to open the arm regeneration valve V 1   a  and increase the opening area. This is to increase the flow area of the first oil passage C 1   a . By causing hydraulic oil to flow out of the bottom-side oil chamber  8 B, the regeneration valve control unit  302 D reduces the arm bottom pressure P A . As a result, the extension of the arm cylinder  8  is suppressed or stopped, thereby decreasing or eliminating the horizontal component F R2  of the excavation reaction force F R . Accordingly, the body of the shovel is prevented from being dragged toward an excavation point. 
     Further, if the arm rod pressure P A2  has reached the target arm rod pressure P AT  at the time of the contraction of the arm cylinder  8 , the regeneration valve control unit  302 D controls the arm regeneration valve V 1  to reduce the difference between the arm bottom pressure P A  and the arm rod pressure P A2 , thereby reducing the arm thrust F A . In this case, it is possible to prevent the shovel from being dragged even when the arm  5  is rotated in the opening direction. In this step, whether the control start condition is satisfied is determined based on the arm rod pressure P A2  or the arm bottom pressure P A . However, whether the control start condition is satisfied may be determined based on whether the magnitude of the horizontal component of the excavation reaction force satisfies a predetermined condition. In this manner, determination in preventing dragging may be made based on parameters contributing to the horizontal component. 
     In step S 30 , if it is determined that the additional control start condition is not satisfied (no in step S 30 ), for example, if the arm bottom pressure P A  remains below the target arm bottom pressure P AT , the controller  30  ends the current third support process, without reducing the arm bottom pressure P A . This is because there is no possibility that the body of the shovel may be dragged. 
     A series of steps S 24  through S 28  for preventing the lifting of the shovel and a series of steps S 29  through S 31  for preventing the dragging of the shovel are performed in any order. Therefore, the two series of steps may be performed concurrently. Alternatively, the series of steps for preventing the dragging of the shovel may be performed prior to the series of steps for preventing the lifting of the shovel. 
     With the above configuration, it is possible for the controller  30  to prevent the body of the shovel from being lifted or dragged toward an excavation point during complex excavation operation without affecting a pilot pressure. Therefore, it is possible for the shovel to perform complex excavation operation that makes efficient use of its body weight at a point immediately before the body of the shovel is lifted or dragged. Furthermore, work efficiency can be improved by eliminating the need to perform an operation for returning the lifted or dragged shovel to its original orientation, thereby also decreasing fuel consumption, preventing a failure of the body, and reducing the operator&#39;s operation burden. 
     In the above-described fourth variation, the maximum allowable pressure calculating unit  302 C and the regeneration valve control unit  302 D perform calculation based on the assumption that the ground surface contacted by the shovel is a flat surface; however, the fourth variation is not limited thereto. In the above-described fourth variation, even if the ground surface contacted by the shovel is an inclined surface, calculation may be properly performed by additionally taking into account the output of the inclination angle sensor. 
     Further, in the above-described fourth variation, the controller  30  may be configured to prevent the lifting of the body of the shovel during a bucket closing operation. In this case, the controller  30  opens the regeneration valve V 1  when the boom rod pressure has exceeded the target boom rod pressure P BT . 
     Further, the controller  30  may be configured to prevent the lifting of the body of the shovel during a complex excavation operation including a bucket closing operation and a boom raising operation. In this case, the controller  30  opens the regeneration valve V 1  when the boom rod pressure has exceeded the target boom rod pressure P BT . Further, the controller  30  opens a bucket regeneration valve provided in a first oil passage that connects the rod-side oil chamber to the bottom-side oil chamber of the bucket cylinder  9  when the boom rod pressure has reached the first maximum allowable pressure P BMAX . In this manner, the controller  30  may prevent the lifting of the body of the shovel during a complex excavation operation including a bucket closing operation and a boom raising operation. Similarly, the controller  30  may use the bucket regeneration valve to prevent the dragging of the body of the shovel. 
     Further, in the above-described fourth variation, the regeneration valve V 1  is used to cause hydraulic oil to flow out of the rod-side oil chamber  7 R, but may be used to cause hydraulic oil to flow out of the bottom-side oil chamber  7 B. Further, the arm regeneration valve V 1   a  is used to cause hydraulic oil to flow out of the bottom-side oil chamber  8 B, but may be used to cause hydraulic oil to flow out of the rod-side oil chamber  8 R. In other words, the controller  30  may open the arm regeneration valve V 1   a , and cause hydraulic oil to flow from the rod-side oil chamber  8 R into the bottom-side oil chamber  8 B of the arm cylinder  8  or to flow from the bottom-side oil chamber  8 B into the rod-side oil chamber  8 R in accordance with the weight of the attachment. The same applies to the bucket regeneration valve. 
     Further, in the above-described fourth variation, hydraulic cylinders such as the boom cylinder  7  and the arm cylinder  8  are moved by hydraulic oil that is discharged by the engine-driven main pump  14 ; however, the hydraulic cylinders may be moved by hydraulic oil that is discharged by a hydraulic pump driven by an electric motor. 
     Further, in the above-described fourth variation, regardless of whether an unintended movement such as the dragging or lifting of the body of the shovel has occurred, the controller  30  performs control that minimizes the dragging or lifting of the body of the shovel. However, the controller  30  may, of course, determine the occurrence of an unintended movement. Namely, the controller  30  may perform control that minimizes the dragging or lifting of the body of the shovel when the occurrence of the dragging or lifting of the body of the shovel is determined by the determination methods (see  FIG.  19 A  through  FIG.  26 B ). 
     Further, the above-described configuration according to the fourth variation may be installed in any other construction machine such as a forklift or a loader that use hydraulic cylinders for raising and lowering operations. 
     The following clauses are further disclosed with respect to the above-described fourth variation. 
     (1) A shovel includes: 
     a traveling body; 
     a turning body turnably mounted on the traveling body; 
     an attachment attached to the turning body; 
     a hydraulic actuator configured to drive a work element constituting the attachment; 
     a first oil passage that connects a rod-side oil chamber to a bottom-side oil chamber of a hydraulic cylinder, the hydraulic cylinder serving as the hydraulic actuator, 
     a regeneration valve disposed in the first oil passage; and 
     a control unit configured to control the regeneration valve, based on whether a predetermined condition on stability of a body of the shovel is satisfied. 
     (2) The shovel according to (1), further includes: 
     a flow rate control valve configured to control a flow rate of hydraulic oil that flows into and out of the hydraulic cylinder; 
     a second oil passage that connects the rod-side oil chamber of the hydraulic cylinder to the flow rate control valve; and 
     a third oil passage that connects the bottom-side oil chamber of the hydraulic cylinder to the flow rate control valve, 
     wherein the first oil passage connects the second oil passage to the third oil passage. 
     (3) The shovel according to (1) or (2), wherein the hydraulic cylinder is a boom cylinder, and the control unit opens the regeneration valve so as to cause hydraulic oil to flow from the rod-side oil chamber into the bottom-side oil chamber of the boom cylinder. 
     (4) The shovel according to any one of (1) to (3), wherein the control unit controls the regeneration valve independently of an operation related to the hydraulic cylinder. 
     (5) The shovel according to (1) or (2), wherein the hydraulic cylinder is an arm cylinder, and the control unit opens the regeneration valve so as to cause hydraulic oil to flow from the rod-side oil chamber into the bottom-side oil chamber of the arm cylinder or from the bottom-side oil chamber into the rod-side oil chamber of the arm cylinder in accordance with weight of the attachment. 
     (6) The shovel according to (1), wherein the regeneration valve is disposed between a flow rate control valve and the hydraulic cylinder, the flow rate control valve being configured to control a flow rate of hydraulic oil that flows into and out of the hydraulic cylinder. 
     (7) The shovel according to (2), wherein hydraulic oil discharged from an oil chamber on one side of the hydraulic cylinder is supplied to an oil chamber located on another side opposite to the one side of the hydraulic cylinder, or is discharged into a tank, in accordance with a size of an opening of a cylinder/tank port of the flow rate control valve. 
     Further, the present invention is not limited to these embodiments, but various variations and modifications may be made to the described subject matter without departing from the scope of the present invention.