Patent Publication Number: US-11378101-B2

Title: Shovel

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation application of International Application No. PCT/JP2018/027975 filed on Jul. 25, 2018 and designated the U.S., which is based on and claims priority to Japanese Patent Application No. 2017-145751 filed with the Japanese Patent Office on Jul. 27, 2017, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a shovel. 
     2. Description of the Related Art 
     Conventionally, a shovel is known in which a hydraulic actuator is operated by switching to various work modes by changing an engine speed depending on work contents and controlling a discharge pressure and a discharge amount of a hydraulic pump. The work modes include an SP mode that is selected when the work amount is to be most prioritized, and an A mode that is selected when the shovel is to be operated at a low speed and a low noise while prioritizing fuel efficiency. 
     However, because the above-described shovel changes the maximum operating speed by switching the engine speed for each work mode, responsiveness and acceleration/deceleration characteristics in response to the operation of the operating device in the SP mode and the A mode are the same. 
     Hence, for example, even when an operator selects the A mode to move the shovel carefully for work requiring accuracy and safety, the same rapid movement as that of the SP mode is performed. This does not follow the operator&#39;s intention and is likely to make the operator feel tired. 
     SUMMARY OF THE INVENTION 
     One embodiment of the present disclosure is intended to provide a shovel capable of controlling the acceleration/deceleration characteristics depending on the work mode. 
     A shovel according to an embodiment of the present invention includes a lower traveling body, an upper turning body pivotally mounted on the lower traveling body, a hydraulic pump mounted on the upper turning body, a hydraulic actuator driven by hydraulic oil discharged from the hydraulic pump, an operating device used to operate the actuator, and a control device configured to control an acceleration/deceleration characteristics of the hydraulic actuator in response to an operation of the operating device depending on a work mode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a lateral view of a shovel according to an embodiment of the present invention; 
         FIG. 2  is a block diagram illustrating an example of a configuration of a driving system of a shovel in  FIG. 1 ; 
         FIG. 3  is a schematic diagram illustrating a first configuration example of a hydraulic circuit mounted on a shovel of  FIG. 1 ; 
         FIG. 4  is a diagram ( 1 ) illustrating a relationship between a lever operation amount and an opening area of a bleed valve depending on a work mode; 
         FIG. 5  is a diagram ( 2 ) illustrating a relationship between a lever operation amount and an opening area of a bleed valve depending on a work mode; 
         FIG. 6  is a diagram ( 3 ) illustrating a relationship between a lever operation amount and an opening area of a bleed valve depending on a work mode; 
         FIG. 7  is a diagram illustrating a relationship between a current value of a proportional valve and an opening area of a bleed valve; 
         FIG. 8  is a diagram illustrating a temporal transition of a cylinder pressure when a boom is operated; 
         FIG. 9  is a schematic diagram illustrating an modified embodiment of a first configuration of a hydraulic circuit mounted on a shovel of  FIG. 1 ; 
         FIG. 10  is a schematic diagram illustrating a second configuration example of a hydraulic circuit mounted on a shovel of  FIG. 1 ; 
         FIG. 11  is a diagram illustrating a relationship between a lever operation amount and a PT opening area of a control valve depending on a work mode; 
         FIG. 12  is a schematic diagram illustrating another example of a hydraulic circuit to be mounted on a shovel of  FIG. 1 ; and 
         FIG. 13  is a diagram illustrating an example of a configuration of an operation system including an electrical operating device. 
     
    
    
     EMBODIMENTS FOR CARRYING OUT THE INVENTION 
     Hereinafter, embodiments for carrying out the invention with reference to the drawings will be described. In each drawing, the same components are indicated by the same reference numerals and overlapping descriptions may be omitted. 
     First, an overall configuration of a shovel according to an embodiment of the present invention will be described with reference to  FIG. 1 .  FIG. 1  is a lateral view of a shovel (excavator) according to an embodiment of the present invention. 
     As illustrated in  FIG. 1 , an upper turning body  3  is pivotally mounted on a lower traveling body  1  of the shovel via a turning mechanism  2 . A boom  4  is attached to the upper turning body  3 . An arm  5  is attached to a distal end of the boom  4 , and a bucket  6  as an end attachment is attached to the distal end of the arm  5 . The boom  4 , the arm  5 , and the bucket  6  constitute an excavating attachment as an example of an attachment and are hydraulically driven by a boom cylinder  7 , an arm cylinder  8 , and a bucket cylinder  9 , respectively. The upper turning body  3  includes a cabin  10  that is an operator&#39;s cab, and a power source such as an engine  11  is mounted thereon. 
     A controller  30  is provided within the cabin  10 . The controller  30  serves as a main control unit for controlling the driving of the shovel. In this embodiment, the controller  30  is comprised of a computer including a CPU, RAM, ROM, and the like. Various functions of the controller  30  are implemented, for example, by executing a program stored in a ROM by a CPU. 
     Next, a configuration of the driving system of the shovel of  FIG. 1  will be described with reference to  FIG. 2 .  FIG. 2  is a block diagram illustrating an example of a configuration of a drive system of a shovel in  FIG. 1 . In  FIG. 2 , a mechanical power system, a high pressure hydraulic line, a pilot line, and an electrical control system are shown by double, solid, dashed, and dotted lines, respectively. 
     As illustrated in  FIG. 2 , the drive system of the shovel primarily includes an engine  11 , a regulator  13 , a main pump  14 , a pilot pump  15 , a control valve  17 , an operating device  26 , a discharge pressure sensor  28 , an operation pressure sensor  29 , a controller  30 , a proportional valve  31 , a work mode selection dial  32 , and the like. 
     The engine  11  is a drive source of the shovel. In the present embodiment, the engine  11  is, for example, a diesel engine that operates to maintain a predetermined rotational speed. An output shaft of the engine  11  is also coupled to an input shaft of the main pump  14  and the pilot pump  15 . 
     The main pump  14  supplies hydraulic oil to the control valve  17  via a high pressure hydraulic line. In the present embodiment, the main pump  14  is a swash plate variable displacement hydraulic pump. 
     The regulator  13  controls the discharge amount of the main pump  14 . In the present embodiment, the regulator  13  controls the discharge amount of the main pump  14  by adjusting a tilt angle of the swash plate of the main pump  14  in response to a control command from the controller  30 . 
     The pilot pump  15  supplies hydraulic oil to various hydraulic control devices including the operating device  26  and the proportional valve  31  through the pilot line. In this embodiment, the pilot pump  15  is a fixed capacitive type hydraulic pump. 
     The control valve  17  is a hydraulic controller that controls the hydraulic system in the shovel. The control valve  17  includes control valves  171  to  176  and a bleed valve  177 . The control valve  17  may selectively supply the hydraulic oil discharged from the main pump  14  to one or more hydraulic actuators through the control valves  171  to  176 . The control valves  171  to  176  control the flow of hydraulic oil from the main pump  14  to the hydraulic actuator and the flow of hydraulic oil from the hydraulic actuator to the hydraulic oil tank. The hydraulic actuators include the boom cylinder  7 , the arm cylinder  8 , the bucket cylinder  9 , a left-side traveling hydraulic motor  1 A, a right-side traveling hydraulic motor  1 B, and a turning hydraulic motor  2 A. The bleed valve  177  controls the flow rate (hereinafter, referred to as a “bleed flow rate”) of the hydraulic oil discharged from the main pump  14  to the hydraulic oil tank without passing through the hydraulic actuator. The bleed valve  177  may be located outside the control valve  17 . 
     The operating device  26  is a device used by an operator for operation of the hydraulic actuator. In the present embodiment, the operating device  26  supplies the hydraulic oil discharged from the pilot pump  15  to the pilot ports of the control valves corresponding to the respective hydraulic actuators through the pilot lines. The pressure (pilot pressure) of the hydraulic oil supplied to each of the pilot ports is the pressure corresponding to a direction and an amount of operation of the levers or pedals (not illustrated) of the operating device  26  corresponding to each of the hydraulic actuators. 
     The discharge pressure sensor  28  detects the discharge pressure of the main pump  14 . In the present embodiment, the discharge pressure sensor  28  outputs the detected value to the controller  30 . 
     The operation pressure sensor  29  detects the operator&#39;s operation content using the operation device  26 . In the present embodiment, the operation pressure sensor  29  detects the operation direction and the amount of the operation of the lever or pedal of the operating device  26  corresponding to each of the hydraulic actuators in a form of pressure (operating pressure), and outputs the detected value to the controller  30 . The operation content of the operating device  26  may be detected using other sensors other than the operating pressure sensor. 
     The proportional valve  31  operates in response to a control command output by the controller  30 . In the present embodiment, the proportional valve  31  is a solenoid valve that adjusts a secondary pressure introduced from the pilot pump  15  to the pilot port of the bleed valve  177  within the control valve  17  in response to a current command output by the controller  30 . The proportional valve  31  operates, for example, to increase the secondary pressure introduced into the pilot port of the bleed valve  177  as the current command increases. 
     The work mode selection dial  32  is a dial for the operator to select the work mode, and enables the switching of multiple different work modes. Further, from the work mode selection dial  32 , data indicating a setting state of the engine speed and a setting state of the acceleration/deceleration characteristics depending on the work mode are always transmitted to the controller  30 . The work mode selection dial  32  allows switching of the work modes at multiple stages, including a POWER mode, a STD mode, an ECO mode, and an IDLE mode. The POWER mode is an example of the first mode, and the ECO mode is an example of the second mode.  FIG. 2  illustrates a state in which the POWER mode is selected by the work mode selection dial  32 . 
     The POWER mode is an operation mode selected when the workload is to be prioritized, using the highest engine RPM and the highest acceleration/deceleration characteristic. The STD mode is an operation mode selected to achieve both work and fuel efficiency while using the second highest engine RPM and the second highest acceleration/deceleration characteristic. The ECO mode is an operation mode selected to slow down the acceleration/deceleration characteristic of the hydraulic actuator corresponding to the lever operation, to improve accuracy of operation and safety, to operate the shovel with a low noise, to use the third highest engine RPM, and to use the third highest acceleration/deceleration characteristic. The IDLE mode is an operation mode selected when it is intended to idle the engine, utilizing the lowest engine speed and the lowest acceleration/deceleration characteristic. The engine  11  is constantly controlled by the engine speed of the work mode set by the work mode selection dial  32 . The opening of the bleed valve  177  is controlled based on the bleed valve opening characteristics of the work mode set by the work mode selection dial  32 . The opening characteristics of the bleed valve are described later. 
     In a configuration diagram of  FIG. 2 , the ECO mode is set to one of the modes selected by the work mode selection dial  32 . However, an ECO mode switch may be provided separately from the work mode selection dial  32 . In this case, the operation mode selection dial  32  may be used to adjust the engine RPM corresponding to each selected mode, and when the ECO mode switch is turned ON, the acceleration/deceleration characteristics corresponding to each mode of the operation mode selection dial  32  may be gradually changed. 
     Alternatively, the change of the work mode may be implemented by an audio input. In that case, the shovel includes a voice input device for inputting the operator&#39;s voice to the controller  30 . The controller  30  includes a voice identification unit that identifies the voice input by the voice input device. 
     As described above, the work mode is selected by a mode selection unit such as the work mode selection dial  32 , the ECO mode switch, and the voice identification unit. 
     Next, a configuration example of a hydraulic circuit mounted on a shovel will be described with reference to  FIG. 3 .  FIG. 3  is a schematic diagram illustrating an example of a configuration of a hydraulic circuit mounted on a shovel of  FIG. 1 .  FIG. 3 , similar to  FIG. 2 , illustrates a mechanical power system, a high pressure hydraulic line, a pilot line, and an electrical control system, respectively, by double, thick, dashed, and single dashed lines. 
     The hydraulic circuit of  FIG. 3  circulates the hydraulic oil from main pumps  14 L and  14 R driven by the engine  11  to the hydraulic oil tank through conduits  42 L and  42 R. The main pumps  14 L and  14 R correspond to the main pump  14  of  FIG. 2 . 
     The conduit  42 L is a high pressure hydraulic line connecting the control valves  171 ,  173 ,  175 L and  176 L disposed within the control valve  17  in parallel between the main pump  14 L and the hydraulic oil tank. The conduit  42 R is a high pressure hydraulic line connecting the control valves  172 ,  174 ,  175 R and  176 R disposed within the control valve  17  in parallel between the main pump  14 R and the hydraulic oil tank. 
     The control valve  171  is a spool valve that supplies the hydraulic oil discharged from the main pump  14 L to the left-side traveling hydraulic motor  1 A and switches the flow of hydraulic oil in order to discharge the hydraulic oil discharged from the left-side traveling hydraulic motor  1 A to the hydraulic oil tank. 
     The control valve  172  is a spool valve that supplies the hydraulic oil discharged from the main pump  14 R to the right-side traveling hydraulic motor  1 B and switches the flow of the hydraulic oil in order to discharge the hydraulic oil discharged from the right-side traveling hydraulic motor  1 B to the hydraulic oil tank. 
     The control valve  173  is a spool valve that supplies the hydraulic oil discharged from the main pump  14 L to the turning hydraulic motor  2 A and switches the flow of the hydraulic oil in order to discharge the hydraulic oil discharged from the turning hydraulic motor  2 A to the hydraulic oil tank. 
     The control valve  174  is a spool valve to supply the hydraulic oil discharged from the main pump  14 R to the bucket cylinder  9  and to discharge the hydraulic oil from the bucket cylinder  9  to the hydraulic oil tank. 
     The control valves  175 L and  175 R are spool valves that supply the hydraulic oil discharged from the main pumps  14 L and  14 R to the boom cylinder  7  and that switch the flow of the hydraulic oil in order to discharge the hydraulic oil in the boom cylinder  7  to the hydraulic oil tank. 
     The control valves  176 L and  176 R are spool valves that supply the hydraulic oil discharged from the main pumps  14 L and  14 R to the arm cylinder  8  and that switch the flow of the hydraulic oil in order to discharge the hydraulic oil in the arm cylinder  8  to the hydraulic oil tank. 
     The bleed valve  177 L is a spool valve that controls the bleed flow rate with respect to the hydraulic oil discharged from the main pump  14 L. The bleed valve  177 R is a spool valve that controls the bleed flow rate with respect to the hydraulic oil discharged from the main pump  14 R. The bleed valves  177 L and  177 R correspond to the bleed valves  177  of  FIG. 2 . 
     The bleed valves  177 L and  177 R have, for example, a first valve position with a minimum opening area (0% opening) and a second valve position with a maximum opening area (100% opening). The bleed valves  177 L and  177 R can be moved steplessly between the first and second valve positions. 
     Regulators  13 L and  13 R control the discharge amount of the main pumps  14 L and  14 R by adjusting the tilt angle of the swash plate of the main pumps  14 L and  14 R. The regulators  13 L and  13 R correspond to the regulator  13  in  FIG. 2 . The controller  30  adjusts the tilting angle of the swash plate of the main pumps  14 L and  14 R with the regulators  13 L and  13 R in response to an increase in the discharge pressure of the main pumps  14 L and  14 R to decrease the discharge amount. This is intended cause an absorbed horsepower of the main pump  14 , which is expressed as the product of the discharge pressure and the discharge amount, not to exceed the output horsepower of the engine  11 . 
     The arm operation lever  26 A is an example of the operating device  26  and is used to operate the arm  5 . The arm operation lever  26 A utilizes the hydraulic oil discharged from the pilot pump  15  to introduce the control pressure depending on the lever operation amount into the pilot ports of the control valves  176 L and  176 R. Specifically, the arm operation lever  26 A introduces the hydraulic oil to the right pilot port of the control valve  176 L and introduces the hydraulic oil to the left pilot port of the control valve  176 R when operated in the arm closing direction. The arm operation lever  26 A, when operated in the arm opening direction, introduces the hydraulic oil to the left pilot port of the control valve  176 L and introduces the hydraulic oil to the right pilot port of the control valve  176 R. 
     The boom operation lever  26 B is an example of the operating device  26  and is used to operate the boom  4 . The boom operation lever  26 B utilizes the hydraulic oil discharged from the pilot pump  15  to introduce the control pressure depending on the amount of lever operation into the pilot ports of the control valves  175 L and  175 R. Specifically, the boom operating lever  26 B introduces hydraulic oil to the right pilot port of the control valve  175 L and introduces the hydraulic oil to the left pilot port of the control valve  175 R when being operated in the boom raising direction. The boom operation lever  26 B, when being operated in the boom lowering direction, introduces the hydraulic oil to the left pilot port of the control valve  175 L and introduces the hydraulic oil to the right pilot port of the control valve  175 R. 
     The discharge pressure sensors  28 L and  28 R are examples of the discharge pressure sensors  28 , detect the discharge pressure of the main pumps  14 L and  14 R, and output the detected value to the controller  30 . 
     The operation pressure sensors  29 A and  29 B are examples of the operation pressure sensor  29  that detects the operator&#39;s operation contents to the arm operation lever  26 A and the boom operation lever  26 B in a form of pressure and that outputs the detected value to the controller  30 . The operation contents are, for example, a lever operation direction, a lever operation amount (lever operation angle), and the like. 
     The right and left travelling levers (or pedals), the bucket operation lever, and the turning operation lever (neither of which is illustrated in the drawings) are operating devices for controlling the travel of the lower traveling body  1 , opening and closing of the bucket  6 , and the turn of the upper turning body  3 , respectively. These operating devices, like the arm operation levers  26 A and the boom operation levers  26 B, utilize the hydraulic oil discharged from the pilot pump  15  to introduce a control pressure depending on the lever operation amount (or pedal operation amount) into either the left or right pilot port of the control valve corresponding to each of the hydraulic actuators. The operator&#39;s operating contents for each of these operating devices, as well as the operation pressure sensors  29 A and  29 B, are detected by the corresponding operation pressure sensors in a form of pressure, and a detected value is output to the controller  30 . 
     The controller  30  receives an output, such as one from the operation pressure sensors  29 A and  29 B, and outputs a control command to the regulators  13 L and  13 R as needed to change the discharge amount of the main pumps  14 L and  14 R. If necessary, a current command is output to the proportional valves  31 L 1  and  31 R 1  to change the opening area of the bleed valves  177 L and  177 R. 
     The proportional valves  31 L 1  and  31 R 1  adjust the secondary pressure introduced from the pilot pump  15  to the pilot ports of the bleed valves  177 L and  177 R in response to a current command output from the controller  30 . The proportional valves  31 L 1 ,  31 R 1  correspond to the proportional valves  31  in  FIG. 2 . 
     The proportional valve  31 L 1  can adjust the secondary pressure so that the bleed valve  177 L stops at any position between the first and second valve positions. The proportional valve  31 R 1  can adjust the secondary pressure so that the bleed valve  177 R stops at any position between the first valve position and the second valve position. 
     Next, a negative controlling control (hereinafter, referred to as “negative control”) employed in the hydraulic circuit of  FIG. 3  will be described. 
     The conduits  42 L and  42 R include negative control throttles  18 L and  18 R arranged between each of the downstream bleed valves  177 L and  177 R and the hydraulic oil tank. The flow of hydraulic oil through the bleed valves  177 L and  177 R to the hydraulic oil tank 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”) for controlling the regulators  13 L and  13 R. Negative control pressure sensors  19 L and  19 R are sensors for detecting a negative control pressure and output detected values to the controller  30 . 
     In the present embodiment, the negative control throttles  18 L and  18 R are variable apertures in which the opening area varies. The negative control throttles  18 L and  18 R, however, may be fixed apertures. 
     The controller  30  controls the discharge amount of the main pumps  14 L and  14 R by adjusting the tilting angle of the swash plate of the main pumps  14 L and  14 R depending on the negative control pressure. Hereinafter, the relationship between the negative control pressure and the discharge amount of the main pumps  14 L and the  14 R is referred to as “negative control characteristics.” The negative control characteristics may be stored, for example, as a look-up table in a ROM or the like, or may be represented by a predetermined calculation expression. For example, the controller  30  refers to a table representing predetermined negative control characteristics, and the larger the negative control pressure, the smaller the discharge amount of the main pumps  14 L and the  14 R, and the smaller the negative control pressure, the larger the discharge amount of the main pumps  14 L and the  14 R. 
     Specifically, when none of the hydraulic actuators is operated as illustrated in  FIG. 3 , the hydraulic oil discharged from the main pumps  14 L and  14 R passes through the bleed valves  177 L and  177 R to the negative control throttles  18 L and  18 R. The flow of hydraulic oil through the bleed valves  177 L and  177 R increases the negative control pressure generated upstream of the negative control throttles  18 L and  18 R. As a result, the controller  30  reduces the discharge amount of the main pumps  14 L and  14 R to a predetermined allowable minimum discharge amount and reduces the pressure loss (pumping loss) when the discharged hydraulic oil passes through the conduits  42 L and  42 R. This predetermined minimum allowable discharge rate in a standby state is an example of the bleed flow rate, hereinafter referred to as a “standby flow rate.” 
     On the other hand, when any of the hydraulic actuators is operated, the hydraulic oil discharged from the main pumps  14 L and  14 R flows through a control valve corresponding to the hydraulic actuator of an operation object and flows into the hydraulic actuator of the operation object. Therefore, the bleed flow rate through the bleed valves  177 L and  177 R to the negative control throttles  18 L and  18 R is decreased, and the negative control pressure generated upstream of the negative control throttle  18 L and  18 R is reduced. As a result, the controller  30  increases the discharge rate of the main pumps  14 L and  14 R, while supplying sufficient hydraulic oil to the hydraulic actuators to be operated, and ensures that the hydraulic actuators to be operated are driven. Hereinafter, the flow rate of hydraulic oil flowing into the hydraulic actuator is referred to as an “actuator flow rate.” In this case, the flow rate of the hydraulic oil discharged from the main pumps  14 L and  14 R is equivalent to the sum of the actuator flow rate and the bleed flow rate. 
     With the configuration described above, the hydraulic circuit of  FIG. 3  can reliably supply a sufficient amount of hydraulic fluid from the main pumps  14 L and  14 R to the hydraulic actuator to be operated when the hydraulic actuator is operated. In the standby state, waste of hydraulic energy can be reduced. This is because the bleed flow rate can be reduced to the standby flow rate. 
     In the meantime, in the shovel, by gradually changing the responsiveness and acceleration/deceleration characteristics to the lever operation (or pedal operation) of the operating device  26  depending on the work contents, the operability of the shovel by the operator, the work efficiency of the shovel may be improved; the fatigue of the operator may be reduced; and the safety may be improved. For example, if a hydraulic actuator (boom, arm, bucket, etc.) moves swiftly in response to the lever operation during finishing work such as lever preparation work, a finishing surface may be damaged. In this case, fatigue accumulates in the operator if the lever is operated carefully. Thus, in operations requiring accuracy and safety, it is preferable to have lower responsiveness and/or acceleration/deceleration characteristics to the lever operation (or pedal operation) of the operating device  26 . Because the shovel can be moved cautiously (slowly), the hydraulic actuator (boom, arm, bucket, etc.) can be prevented from moving quickly in response to the lever operation. On the other hand, when it is desired to prioritize the amount of work, such as roughing excavation, the responsiveness to the lever operation (or pedal operation) of the operating device  26  and the acceleration/deceleration characteristics are preferably made higher. This is because the shovel can be moved at a high speed. 
     Conventionally, however, shovels having engine speed adjustment dials for adjusting the engine  11  speed depending on the nature of the work are known, but do not control the responsiveness or acceleration/deceleration characteristics to the lever operation (or pedal operation) of the operating device  26 . 
     Accordingly, in the present embodiment, the acceleration/deceleration characteristic control unit  300  of the controller  30  controls the acceleration/deceleration characteristics of the hydraulic actuator in response to the lever operation (or pedal operation) of the operating device  26  depending on the work mode selected by the work mode selection dial  32 . Further, when the ECO mode switch is provided separately from the work mode selection dial  32 , the ECO mode switch may be turned ON to relax the acceleration/deceleration characteristics. When a voice input device and a voice identification unit are provided, the acceleration/deceleration characteristic control unit  300  may control the acceleration/deceleration characteristics of the hydraulic actuator in response to the lever operation (or pedal operation) of the operating device  26  depending on the operation mode input from the voice input device and identified by the voice identification unit. This can improve the work efficiency of operators, reduce the fatigue of operators, and improve the safety. 
       FIGS. 4 to 6  are diagrams illustrating a relationship between a lever operation amount depending on a work mode and an opening area of a bleed valve.  FIG. 7  is a diagram illustrating a relationship between a current value of a proportional valve and an opening area of a bleed valve. The relationship between the lever operation amount and the opening area of the bleed valve (hereinafter referred to as “bleed valve opening characteristics”) and the relationship between the current value of the proportional valve and the opening area of the bleed valve (hereinafter referred to as “proportional valve characteristics”) may be stored in the ROM as a reference table, for example, or may be expressed by a predetermined calculation formula. Further, as will be discussed later in  FIG. 11 , the bleed valve opening characteristics may be determined based on the calculated results obtained by the lever operation amount and the control valve opening characteristics. 
     The acceleration/deceleration characteristic control unit  300  controls the opening area of the bleed valve  177  by changing the bleed valve opening characteristics depending on the work mode selected by the work mode selection dial  32 . For example, as illustrated in  FIGS. 4 to 6 , the acceleration/deceleration characteristic control section  300  makes the opening area of the bleed valve  177  in the “ECO mode” setting larger than the opening area of the bleed valve  177  in the “STD mode” setting when the lever operation amount is the same. This is for increasing the bleed flow rate and reducing the actuator flow rate. This can slow down the responsiveness of the operating device  26  to the lever operation and reduce the acceleration/deceleration characteristics. Meanwhile, when the lever operation amount is the same, the acceleration/deceleration characteristic control unit  300  makes the opening area of the bleed valve  177  in the “POWER mode” setting smaller than the opening area of the bleed valve  177  in the “STD mode” setting. This is for reducing the bleed flow rate and increasing the actuator flow rate. This allows the acceleration/deceleration characteristics to be increased by increasing the responsiveness of the control device  26  in response to the lever operation. The bleed valve opening characteristic may be different for each operation mode in a portion of the operation area of the lever operation amount, for example, as illustrated in  FIG. 4 , and may be different for each operation mode in a part of the operation area of the lever operation amount, for example, as illustrated in  FIGS. 5 and 6 . The bleed opening characteristics are set so that the opening area changes rapidly with respect to the amount of change in lever operation in the area where the lever operation amount is small. On the other hand, in the area where the lever operation amount is large, the opening area is set to change gradually in response to the amount of change in lever operation. 
     More specifically, the acceleration/deceleration characteristic control unit  300  increases or decreases the opening area of the bleed valve  177  by outputting a control command corresponding to the work mode selected by the work mode selection dial  32  to the proportional valve  31 . For example, if the “ECO mode” is selected, the opening area of the bleed valve  177  is increased as illustrated in  FIG. 7  by reducing the current command to the proportional valve  31  to reduce the secondary pressure of the proportional valve  31 , compared to the case where the “STD mode” is selected. This is for increasing the bleed flow rate and reducing the actuator flow rate. On the other hand, when the “POWER mode” is selected, the opening area of the bleed valve  177  is reduced as illustrated in  FIG. 7  by increasing the secondary pressure of the proportional valve  31  by increasing the current command to the proportional valve  31  rather than when the “STD mode” is selected. This is for reducing the bleed flow rate and increasing the actuator flow rate. 
     Next, the process of controlling the acceleration/deceleration characteristics of the hydraulic actuators by changing the opening area of the bleed valves  177 L and  177 R will be described. The acceleration/deceleration characteristic control unit  300  repeatedly performs this process at a predetermined control cycle while the shovel is in operation. 
     First, the acceleration/deceleration characteristic control unit  300  acquires the work mode selected by the work mode selection dial  32  and selects the bleed valve opening characteristic corresponding to the acquired work mode. 
     Subsequently, the acceleration/deceleration characteristic control unit  300  determines the target current value of the proportional valves  31 L 1  and  31 R 1  based on the selected bleed valve opening characteristic and the proportional valve characteristic. In the present embodiment, the acceleration/deceleration characteristic control unit  300  refers to a table regarding the bleed valve opening characteristics and the proportional valve characteristics to determine the target current value of the proportional valves  31 L 1  and  31 R 1  that becomes the bleed valve opening area corresponding to the lever operation amount. That is, the target current value varies depending on the work mode. 
     Thereafter, the acceleration/deceleration characteristic control unit  300  outputs a current command corresponding to the target current value to the proportional valves  31 L 1  and  31 R 1 . The proportional valves  31 L 1  and  31 R 1  increase the secondary pressure acting on the pilot port of the bleed valves  177 L and  177 R, when receiving a current command corresponding to a target current value determined, for example, referring to a table for “POWER mode” settings. This reduces the opening area of the bleed valves  177 L and  177 R, reduces the bleed flow rate, and increases the actuator flow rate. As a result, the acceleration/deceleration characteristics can be increased by increasing the responsiveness of the operating device  26  to the lever operation. On the other hand, the proportional valves  31 L 1  and  31 R 1  reduce the secondary pressure acting on the pilot ports of the bleed valves  177 L and  177 R, when receiving a current command corresponding to a target current value determined, for example, referring to a table regarding the “ECO mode” setting. This increases the opening area of the bleed valves  177 L and  177 R, increases the bleed flow rate, and decreases the actuator flow rate. As a result, the acceleration/deceleration characteristics can be reduced by slowing down the responsiveness of the operating device  26  to the lever operation. 
       FIG. 8  is a diagram illustrating a temporal transition of the cylinder pressure when the boom  4  is operated.  FIG. 8  illustrates the temporal transition of the cylinder pressure of the boom cylinder  7  in the “ECO mode” setting and the “POWER mode” setting when the boom operation lever  26 B is operated by the operator at time t1. 
     As illustrated in  FIG. 8 , in the “ECO mode” setting, the period of time until the cylinder pressure of the boom cylinder  7  reaches the target cylinder pressure is longer than the period of time until the cylinder pressure of the boom cylinder  7  reaches the target cylinder pressure in the “POWER mode” setting. That is, in the “ECO mode” setting, the responsiveness in response to the operation of the boom operation lever  26 B is slower than the responsiveness in the “POWER mode” setting, and the acceleration/deceleration characteristics are reduced. This allows the hydraulic actuator to be driven without damaging the finishing surface by slowly moving the hydraulic actuator (boom, arm, bucket, and the like) in response to the lever operation when the finishing operation is performed, for example, as in grand leveling work. As a result, even when caution is required, it is possible to improve the operability of the shovel by the operator, to reduce the fatigue of the operator, and further to improve safety. 
     In the above-described process of controlling the acceleration/deceleration characteristics, the case of increasing or decreasing only the acceleration/deceleration characteristics depending on the selected work mode has been described. However, in addition to the acceleration/deceleration characteristics, the number of revolutions of the engine  11  driving the main pumps  14 L and  14 R may be increased or decreased. For example, when the “ECO mode” is selected, the RPM of the engine  11  may be decreased, and when the “POWER mode” is selected, the RPM of the engine  11  may be increased. 
     Next, an alternative embodiment of the first configuration of the hydraulic circuit mounted on the shovel of  FIG. 1  will be described with reference to  FIG. 9 .  FIG. 9  is a schematic diagram illustrating a modification of a first configuration example of a hydraulic circuit mounted on a shovel of  FIG. 1 . In  FIG. 9 , similar to  FIG. 2 , the mechanical power system, the high pressure hydraulic line, the pilot line, and the electrical control system are illustrated by double, solid, dashed, and dashed-dotted lines, respectively. 
     The hydraulic circuit illustrated in  FIG. 9  differs from the hydraulic circuit of the first embodiment illustrated in  FIG. 3  in that the bleed valve  177 L and the negative control throttle  18 L are provided upstream of the conduit  42 L and the bleed valve  177 R and the negative control throttle  18 R are provided upstream of the conduit  42 R. Specifically, in the hydraulic circuit illustrated in  FIG. 9 , the bleed valve  177 L and the negative control throttle  18 L are provided in a conduit branching off from a position upstream of the control valve  171  provided at the upstream side of the conduit  42 L, for example, between the main pump  14 L and the discharge pressure sensor  28 L. The bleed valve  177 R and the negative contour throttle  18 R are provided in a conduit branches off from the position of the upstream side of the control valve  172  provided at the upstream side of the conduit  42 R, for example, between the main pump  14 R and the discharge pressure sensor  28 R. The other configuration is similar to the hydraulic circuit of the first example illustrated in  FIG. 3 , and thus the description thereof will not be repeated. Additionally, the conduits  42 L and  42 R between the control valves may branch off to discharge the hydraulic oil to the hydraulic oil tank via the bleed valves  177 L,  177 R and the negative control throttles  18 L,  18 R. 
     Referring now to  FIGS. 10 and 11 , another configuration example of a hydraulic circuit mounted on a shovel of  FIG. 1  will be described  FIG. 10  is a schematic diagram illustrating a second configuration example of a hydraulic circuit mounted on a shovel of  FIG. 1 . The hydraulic circuit illustrated in  FIG. 10  includes bleed, valves  179 L and  179 R. The hydraulic circuit illustrated in  FIG. 10  includes the pressure reducing valves  33 L 1 ,  33 R 1 ,  33 L 2 , and  33 R 2  and does not include the proportional valves  31 L 1  and  31 R 1  illustrated in the hydraulic circuit of the first configuration example. The pressure reducing valves  33 L 1 ,  33 R 1 ,  33 L 2 , and  33 R 2  serve in the same way as the proportional valves  31 L 1  and  31 R 1  do as illustrated in  FIG. 3 . 
     Hereinafter, different points from the hydraulic circuit of the first configuration example will be described. 
     The controller  30  receives outputs from the operation pressure sensors  29 A and  29 B and the like, outputs a control command to the regulators  13 L and  13 R as needed, and changes the discharge amount of the main pumps  14 L and  14 R. The controller  30  also outputs a current command SL and SR to the pressure reducing valves  33 L 1  and  33 R 1  to depressurize the secondary pressure PPL and PPR, which are pilot port pressures, introduced to the pilot ports of the control valves  175 L and  175 R depending on the amount of operation of the boom operation lever  26 B. The controller  30  also outputs a current command to the pressure reducing valves  33 L 2  and  33 R 2  to depressurize the secondary pressure PPL and PPR, which are the pilot port pressures, introduced to the pilot ports of the control valves  176 L and  176 R depending on the amount of operation of the arm operation lever  26 A. 
     In the second configuration example, the acceleration/deceleration characteristic control unit  300  of the controller  30  controls the acceleration/deceleration characteristic of the hydraulic actuator by changing the pilot pressure of the pressure reducing valves  33 L 1  and  33 R 1  as discussed above, in response to the lever operation (or pedal operation) of the operating device  26  depending on the work mode selected by the work mode selection dial  32 , similar to the first configuration example. This can improve the work efficiency of operators, reduce the fatigue of operators, and improve safety. 
       FIG. 11  is a diagram illustrating a relationship between a lever operation amount depending on a work mode and a PT opening area of a control valve. The PT opening area of the control valve means an opening area between a port communicating with the main pumps  14 L and  14 R in the control valves  175 L and  1758  and a port communicating with the hydraulic oil tank T in the control valves  175 L and  175 R. The control valves  175 L and  175 R in  FIG. 10  are expressed as a hydraulic circuit, but each of the control valves  175 L and  175 R includes a spool valve, and the spool valve creates a left side circuit state (all closed state), a right side circuit (all opened state) and a middle side circuit state (partially opened state). In the partially opened state, part of hydraulic oil supplied from the main pumps  14 L and  14 R goes to the actuator  7  and the rest of the hydraulic oil supplied from the main pump  14 L and  14 R goes to the tanks T. Thus, the spool valve creates the PT opening area discussed above. Because one skilled in the art would understand the function of the spool valve in the control valves  175 L, and  175 R, the specific structure is omitted and expresses as a hydraulic circuit in  FIG. 10 . The relationship between the lever operation amount and the PT opening area of the control valve (hereinafter referred to as “control valve opening characteristics”) and the relationship between a current value of the pressure reducing valve and the PT opening area of the control valve (hereinafter referred to as “pressure reducing valve characteristics”) may be stored in the ROM as a reference table, for example, or may be expressed by a predetermined calculation formula. 
     The acceleration/deceleration characteristic control unit  300  controls the PT opening area of the control valve by changing the control valve opening characteristic depending on the work mode selected by the work mode selection dial  32 . For example, as illustrated in  FIG. 11 , the acceleration/deceleration characteristic control unit  300  makes the PT opening area of the control valves  175 L and  175 R in the “ECO mode” setting larger than the PT opening area of the control valves  175 L and  175 R in the “STD mode” setting when the lever operation amount is the same. This is because in the “ECO mode,” the flow rate of the hydraulic oil flowing into the hydraulic oil tank is increased to reduce the flow rate of the hydraulic oil flowing into the boom cylinder  7 . This can slow down the responsiveness of the operating device  26  in response to the lever operation and reduce the acceleration/deceleration characteristics. Meanwhile, when the lever operation amount is the same, the acceleration/deceleration characteristic control unit  300  makes the PT opening area of the control valves  175 L and  175 R in the “POWER mode” setting smaller than the PT opening area of the control valves  175 L and  175 R in the “STD mode” setting. This is because in the “POWER mode,” the flow rate of the hydraulic oil flowing into the hydraulic oil tank is reduced to increase the flow rate of the hydraulic oil flowing into the boom cylinder  7 . This allows the acceleration/deceleration characteristics to be increased by increasing the responsiveness of the operating device  26  in response to the lever operation. As illustrated in  FIG. 11 , the control valve opening characteristics may differ for each operation mode in a part of the operational range of the lever operation amount, or may differ for each operation mode in all the operation range of the lever operation amount, similar to the bleed valve opening characteristics in the first configuration example. 
     More specifically, the acceleration/deceleration characteristic control unit  300  increases or decreases the PT opening area of the control valves  175 L and  175 R by outputting, for example, a control command corresponding to the work mode selected by the work mode selection dial  32  to the pressure reduction valves  33 L 1  and  33 R 1 . For example, when the “ECO mode” is selected, the PT opening area of the control valves  175 L and  175 R is increased by decreasing the current command for the pressure reducing valves  33 L 1  and  33 R 1  and reducing the secondary pressure of the pressure decreasing valves  33 L 1  and  33 R 1 , compared to the case where the “STD mode” is selected. On the other hand, when the “POWER mode” is selected, the PT opening area of the control valves  175 L and  175 R is decreased by increasing the current command for the pressure reducing valves  33 L 1  and  33 R 1  and increasing the secondary pressure of the pressure reducing valves  33 L 1  and  33 R 1 , rather than when the “STD mode” is selected. 
     The acceleration/deceleration characteristic control unit  300  increases or decreases the PT opening area of the control valves  176 L and  176 R by outputting, for example, a control command corresponding to the work mode selected by the work mode selection dial  32  to the pressure reduction valves  33 L 2  and  33 R 2 . For example, when the “ECO mode” is selected, the PT opening area of the control valves  176 L and  176 R is increased by decreasing the current command for the pressure reducing valves  33 L 2  and  33 R 2  and decreasing the secondary pressure of the pressure reducing valves  33 L 2  and  33 R 2 , compared to the case where the “STD mode” is selected. On the other hand, in the case of the “POWER mode,” the PT opening area of the control valves  176 L and  176 R is decreased by increasing the current command for the pressure reduction valves  33 L 2  and  33 R 2  and increasing the secondary pressure of the pressure reduction valves  33 L 2  and  33 R 2 , rather than in the case of the “STD mode.” 
     Next, the process of controlling the acceleration/deceleration characteristics of the hydraulic actuator by adjusting the pilot pressure acting on the control valves  175 L and  175 R by the acceleration/deceleration characteristic control unit  300  will be described. The acceleration/deceleration characteristic control unit  300  repeatedly performs this process at a predetermined control cycle while the shovel is in operation. 
     First, the acceleration/deceleration characteristic control unit  300  acquires the work mode selected by the work mode selection dial  32  and selects the control valve opening characteristic corresponding to the acquired work mode. 
     Subsequently, the acceleration/deceleration characteristic control unit  300  determines the target current values of the pressure reducing valves  33 L 1  and  33 R 1  based on the selected control valve opening characteristic and the pressure reducing valve characteristic. In the present embodiment, the acceleration/deceleration characteristic control section  300  refers to a table regarding the control valve opening characteristics and the pressure reducing valve characteristics, and determines the target current value of the pressure reducing valves  33 L 1  and  33 R 1  that are the PT opening area of the control valve corresponding to the lever operation amount. That is, the target current value varies depending on the work mode. 
     Thereafter, the acceleration/deceleration characteristic control unit  300  outputs a current command corresponding to the target current value to the pressure reducing valves  33 L 1  and  33 R 1 . The pressure reducing valves  33 L 1  and  33 R 1  reduce the secondary pressure acting on the pilot ports of the control valves  175 L and  175 R when receiving a current command corresponding to a target current value determined with reference to a table regarding the “ECO mode” setting. This increases the PT opening area of the control valves  175 L and  175 R, increases the flow rate of the hydraulic oil flowing into the hydraulic oil tank, and decreases the flow rate of the hydraulic oil flowing into the boom cylinder  7 . As a result, the acceleration/deceleration characteristics can be decreased by slowing down the responsiveness of the operating device  26  in response to the lever operation. On the other hand, the pressure reducing valves  33 L 1  and  33 R 1  increase the secondary pressure acting on the pilot ports of the control valves  175 L and  175 R when receiving a current command corresponding to a target current value determined with reference to a table regarding the “POWER mode” setting. Accordingly, because the opening area of the pressure reducing valves  33 L 1  and  33 R 1  is decreased, the flow rate of the hydraulic oil flowing into the hydraulic oil tank is decreased, and the flow rate of the hydraulic oil flowing into the boom cylinder  7  is increased. As a result, the acceleration and deceleration characteristics can be increased by increasing the responsiveness of the control device  26  in response to the lever operation. 
     In the above-described process of controlling the acceleration/deceleration characteristics, the case of increasing or decreasing only the acceleration/deceleration characteristic depending on the selected work mode has been described. However, in addition to the acceleration/deceleration characteristics, the number of revolutions of the engine  11  driving the main pumps  14 L and  14 R may be increased or decreased. For example, when the “ECO mode” is selected, the RPM of the engine  11  may be reduced, and when the “POWER mode” is selected, the RPM of the engine  11  may be increased. Here, the bleed valves  177 L and  177 R are determined to have the bleed valve opening characteristics based on the calculation results obtained by the lever operation amount and the control valve opening characteristics. As a result, the operation of each hydraulic actuator corresponding to the acceleration/deceleration characteristic determined in the work mode and the amount of lever operation can be implemented, and good operability can be obtained. 
     Also, the lever operation amount and the control valve opening characteristics can be applied to various patterns, as well as the lever operation amount and bleed valve opening characteristics illustrated in  FIGS. 3 to 6 , without being limited to the characteristics illustrated in  FIG. 11 . 
     Despite the above description of the embodiments of the present invention, the above description is not intended to limit the content of the invention, and various alternations and modifications can be made within the scope of the present invention. 
     For example, in  FIGS. 3, 9 and 10 , the respective control valves  171 ,  173 ,  175 L and  176 L, which control the flow of hydraulic oil from the main pump  14 L to the hydraulic actuator, are connected in parallel with each other between the main pump  14 L and the hydraulic oil tank. However, the control valves  171 ,  173 ,  175 L and  176 L may be each connected in series between the main pump  14 L and the hydraulic oil tank. In this case, the conduit  42 L can supply the hydraulic oil to adjacent control valves located downstream, without being interrupted by a spool, even if the spool including each control valve has been switched to any valve position. 
     Similarly, the respective control valves  172 ,  174 ,  175 R and  176 R, which control the flow of hydraulic oil from the main pump  14 R to the hydraulic actuator, are connected in parallel with each other between the main pump  14 R and the hydraulic oil tank. However, each of the control valves  172 ,  174 ,  175 R and  176 R may be connected in series between the main pump  14 R and the hydraulic oil tank. In this case, the conduit  42 R can supply the hydraulic oil to adjacent control valves positioned downstream without being interrupted by a spool, even if the spools that include each control valve have been switched to any valve position. 
     Alternatively, the control valves  171 ,  173 ,  175 L, and  176 L may be each connected in series between the main pump  14 L and the hydraulic oil tank, and the control valves  172 ,  174 ,  175 R, and  176 R may be each connected in series between the main pump  14 R and the hydraulic oil tank, for example having center bypass conduits  40 L,  40 R, and parallel conduits  42 L,  42 R, as illustrated in  FIG. 12 .  FIG. 12  is a schematic diagram illustrating another example of a hydraulic circuit mounted on a shovel of  FIG. 1 . In  FIG. 12 , similar to  FIG. 2 , the mechanical power system, the high pressure hydraulic line, the pilot line, and the electrical control system are illustrated by double, solid, dashed, and dashed and dotted lines, respectively. 
     The hydraulic system illustrated in  FIG. 12  circulates the hydraulic oil from the main pumps  14 L,  14 R driven by the engine  11  to the hydraulic oil tank via center bypass conduits  40 L,  40 R, and parallel conduits  42 L,  42 R. 
     The center bypass conduit  40 L is a high pressure hydraulic line passing through control valves  171 ,  173 ,  175 L and  176 L disposed within the control valve  17 . 
     The center bypass conduit  40 R is a high pressure hydraulic line passing through control valves  172 ,  174 ,  175 R and  176 R disposed within the control valve  17 . 
     The control valve  178 L is a spool valve that controls the flow rate of the hydraulic oil flowing from the rod side oil chamber of the arm cylinder  8  to the hydraulic oil tank. The control valve  178 R is a spool valve that controls the flow rate of the hydraulic oil flowing from the bottom side oil chamber of the boom cylinder  7  to the hydraulic oil tank. The control valves  178 L and  178 R have a first valve position with a minimum opening area (0% opening) and a second valve position with a maximum opening area (100% opening). The control valves  178 L,  178 R are movable between the first and second valve positions in a stepless manner. The control valves  178 L and  178 R are controlled by the pressure control valves  31 L and  31 R, respectively. 
     The parallel conduit  42 L is a high pressure hydraulic line parallel to the center bypass conduit  40 L. The parallel conduit  42 L supplies the hydraulic oil to the lower control valve when the flow of hydraulic oil passing through the center bypass conduit  40 L is restricted or interrupted by either the control valves  171 ,  173 ,  175 L. 
     The parallel conduit  42 R is a high pressure hydraulic line parallel to the center bypass conduit  40 R. The parallel conduit  42 R supplies hydraulic oil to the downstream control valve when the flow of hydraulic oil through the center bypass conduit  40 R is restricted or interrupted by either of the control valves  172 ,  174 , and  175 R. 
     In the embodiments described above, a hydraulic actuator is employed as the actuator  26 , although an electric actuator may be employed.  FIG. 13  illustrates an example of a configuration of an operation system including an electrical actuator. Specifically, the operation system shown in  FIG. 13  is an example of a boom operation system. The boom operation system mainly includes a pilot pressure operated control valve  17 , a boom operation lever  26 B as an electric operation lever, a controller  30 , a solenoid valve  60  for a boom up operation, and a solenoid valve  62  for a boom down operation. The operating system of  FIG. 13  may be also applied to an arm operating system, a bucket operating system and the like. 
     The pilot pressure operated control valve  17  includes control valves  175 L and  175 R for the boom cylinder  7 , as illustrated in  FIG. 3 . The solenoid valve  60  is configured to adjust the flow path area of the oil passage that drives the pilot pump  15  and the right-side (raising-side) pilot port of the control valve  175 L and the left-side (raising-side) pilot port of the control valve  175 R. The solenoid valve  62  is configured to adjust the flow path area of the oil passage for the pilot pump  15  and the right-side (lowering-side) pilot port of the control valve  175 R. 
     When manual operation is performed, the controller  30  generates a boom-up operation signal (electrical signal) or a boom-down operation signal (electrical signal) in response to an operation signal (electrical signal) output by the operation signal generator of the boom operation lever  26 B. The operation signal output from the operation signal generator of the boom operation lever  26 B is an electrical signal that varies depending on the operation amount and the direction of the boom operation lever  26 B. 
     Specifically, when the boom operation lever  26 B is operated in the boom raising direction, the controller  30  outputs a boom-up operation signal (an electrical signal) depending on the amount of lever operation to the solenoid valve  60 . The solenoid valve  60  adjusts the flow passage area in response to the boom-up operation signal (electrical signal) and controls the pilot pressure acting on the right-side (raising-side) pilot port of the control valve  175 L and the left-side (raising-side) pilot port of the control valve  175 R. Similarly, when the boom operation lever  26 B is operated in the boom down direction, the controller  30  outputs a boom-down operation signal (electrical signal) corresponding to the lever operation amount to the solenoid valve  62 . The solenoid valve  62  adjusts the flow passage area in response to a boom-down operation signal (electrical signal) to control the pilot pressure acting on the right-side (lowering-side) pilot port of the control valve  175 R. 
     When automatic control is performed, the controller  30  generates a boom-up operation signal (electrical signal) or a boom-down operation signal (electrical signal) in response to the correction operation signal (electrical signal) instead of the operation signal output by the operation signal generator of the boom operation lever  26 B. The correction operation signal may be an electrical signal generated by the controller  30  or an electrical signal generated by an external controller other than the controller  30 . 
     As discussed above, embodiments of the present invention can provide a shovel capable of controlling acceleration/deceleration characteristics depending on a work mode.