Patent Publication Number: US-9422689-B2

Title: Shovel and method for controlling shovel

Description:
RELATED APPLICATIONS 
     This is a continuation of International Patent Application No. PCT/JP2012/067233, filed on Jul. 5, 2012 which is based on and claims the benefit of priority of Japanese Patent Application No. 2011-150372, filed on Jul. 6, 2011, the entire contents of each of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present invention is related to a shovel including a boom regenerative hydraulic motor and a method for controlling the shovel. 
     2. Description of Related Art 
     Until now, a hybrid type shovel including an electric motor generator for a boom, an electric motor generator for an engine, and an electric motor generator for a swing body is known. The boom-driving electric motor generator is rotationally driven by a boom regenerative hydraulic motor when lowering a boom. The electric motor generator for an engine is rotationally driven by an engine. The electric motor generator for a swing body is capable of a regenerating operation and a power running operation. 
     This hybrid type shovel shifts the electric motor generator for an engine to its power running operation when the electric motor generator for a boom or the electric motor generator for a swing body is in its regenerative operation. Thus, the hybrid type shovel can use regenerated electric power for driving the electric motor generator for an engine without charging a battery, and thus can make more efficient use of the regenerated electric power. 
     SUMMARY 
     A shovel according to an embodiment of the present invention is a shovel including hydraulic actuators including a boom cylinder. The shovel includes a hydraulic motor driven by hydraulic oil flowing out of the boom cylinder, a regenerating oil passage configured to supply the hydraulic oil flowing out of the boom cylinder to the hydraulic motor, a reusing oil passage configured to supply the hydraulic oil flowing out of the boom cylinder to another hydraulic actuator, and a reusing flow control valve configured to control a flow rate of hydraulic oil flowing in the reusing oil passage. 
     Also, a method for controlling a shovel according to an embodiment of the present invention is a method for controlling a shovel including hydraulic actuators including a boom cylinder. The method includes steps of driving a hydraulic motor by using hydraulic oil flowing out of the boom cylinder, supplying the hydraulic oil flowing out of the boom cylinder to the hydraulic motor, supplying the hydraulic oil flowing out of the boom cylinder to another hydraulic actuator through a reusing oil passage, and controlling a flow rate of hydraulic oil flowing in the reusing oil passage by using a reusing flow control valve. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a hybrid type shovel according to an embodiment of the present invention; 
         FIG. 2  is a diagram showing transition of operating states of the hybrid type shovel according to an embodiment of the present invention; 
         FIG. 3  is a block diagram showing a configuration example of a drive system of the hybrid type shovel according to an embodiment of the present invention; 
         FIG. 4  is a block diagram showing a configuration example of an electric energy storage system of the hybrid type shovel according to an embodiment of the present invention; 
         FIG. 5  is a diagram showing a configuration example of a hydraulic communication circuit in the hybrid type shovel according to an embodiment of the present invention; 
         FIG. 6  is a flowchart showing a flow of a communication circuit driving process; 
         FIG. 7  is a diagram showing a state of the communication circuit in an arm operation assisting process; 
         FIG. 8  is a diagram showing a state of the communication circuit in a boom regenerative electricity generating process; 
         FIG. 9  is a diagram showing a temporal transition of various physical quantities when a controller performs the arm operation assisting process or the boom regenerative electricity generating process in a dumping operation phase; and 
         FIG. 10  is a block diagram showing a configuration example of a drive system of another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The above known hybrid type shovel makes use of hydraulic oil flowing out of a boom cylinder for driving the boom regenerative hydraulic motor, and then does nothing other than draining the hydraulic oil to an oil tank. Thus, there is a room for improvement in making more efficient use of energy. 
     In view of the above, it is desirable to provide a shovel making more efficient use of hydraulic oil flowing out of a boom cylinder when lowering a boom, and a method for controlling the shovel. 
       FIG. 1  is a side view showing a hybrid type shovel to which an embodiment of the present invention is applied. 
     On a lower travel body  1  of the hybrid type shovel, an upper swing body  3  is mounted via a swing mechanism  2 . A boom  4  is attached to the upper swing body  3 . An arm  5  is attached to an end of the boom  4 . A bucket  6  is attached to an end of the arm  5 . The boom  4 , arm  5 , and bucket  6  are hydraulically driven by a boom cylinder  7 , an arm cylinder  8 , and a bucket cylinder  9 , respectively. On the upper swing body  3 , a cabin  10  is installed, and a drive source such as an engine or the like is mounted. 
     Next, referring to  FIG. 2 , excavating/loading operation will be explained as an example of operations of the hybrid type shovel. First, as shown in a state CD 1 , an operator manipulates the shovel to swing the upper swing body  3 , to locate the bucket  6  above a position to be excavated, to open the arm  5 , and to open the bucket  6 . At the state, the operator manipulates the shovel to lower the boom  4 , and to lower the bucket  6  so that a tip of the bucket  6  is located at a desired height from an object to be excavated. Typically, when swinging the upper swing body  3  and when lowering the boom  4 , the operator visually confirms a position of the bucket  6 . Also, it is common that swinging the upper swing body  3  and lowering the boom  4  are performed simultaneously. The above operation is referred to as a boom lowering swinging operation, and this operation phase is referred to as a boom lowering swinging operation phase. 
     If the operator judges that a tip of the bucket  6  has reached a desired height, the operator manipulates the shovel to close the arm  5  until the arm  5  becomes nearly perpendicular to a ground surface as shown in a state CD 2 . In this way, a soil at a certain depth is excavated and scraped by the bucket  6  until the arm  5  becomes nearly perpendicular to the ground surface. Next, the operator manipulates the shovel to further close the arm  5  and the bucket  6  as shown in a state CD 3 , and then to close the bucket  6  until the bucket  6  becomes nearly perpendicular to the arm  5  as shown in a state CD 4 . That is, the operator manipulates the shovel to close the bucket  6  until an upper edge of the bucket  6  becomes nearly horizontal to scoop the scraped soil into the bucket  6 . The above operation is referred to as an excavating operation, and this operation phase is referred to as an excavating operation phase. 
     Next, if the operator judges that the bucket  6  has been closed until the bucket  6  becomes nearly perpendicular to the arm  5 , the operator manipulates the shovel to lift the boom  4  while closing the bucket  6  until a bottom of the bucket  6  reaches a desired height from the ground surface as shown in a state CD 5 . This operation is referred to as a boom lifting operation, and this operation phase is referred to as a boom lifting operation phase. Following this operation, or simultaneously, the operator manipulates the shovel to swing the upper swing body  3  to move the bucket  6  in a circular motion to a position for dumping as shown by an arrow AR 1 . This operation including the boom lifting operation is referred to as a boom lifting swinging operation, and this operation phase is referred to as a boom lifting swinging operation phase. 
     The reason why the operator manipulates the shovel to lift the boom  4  until the bottom of the bucket  6  reaches the desired height is that, for example, the bucket  6  collides with a truck bed of a dump truck unless the bucket  6  is lifted above the truck bed when dumping the soil onto the truck bed. 
     Next, if the operator judges that the boom lifting swinging operation has been completed, the operator manipulates the shovel to dump the soil in the bucket  6  by opening the arm  5  and bucket  6  while lowering the boom  4  as shown in a state CD 6 . This operation is referred to as a dumping operation, and this operation phase is referred to as a dumping operation phase. 
     Next, if the operator judges that the dumping operation has been completed, the operator manipulates the shovel to swing the upper swing body  3  in a direction indicated by an arrow AR 2  and to move the bucket  6  to a position immediately above the position to be excavated as shown in a state CD 7 . At this time, the operator manipulates the shovel to lower the boom  4  simultaneously with swinging the upper swing body  3  to lower the bucket  6  to a desired height from an object to be excavated. This operation is a part of the boom lowering swinging operation explained with the state CD 1 . Subsequently, the operator manipulates the shovel to lower the bucket  6  to the desired height as shown in the state CD 1  to perform the excavating operation and following operations again. 
     The above described “boom lowering swinging operation”, “excavating operation”, “boom lifting swinging operation”, and “dumping operation” constitute a cycle. The operator goes on with the excavating/loading operation while performing this cycle repeatedly. 
       FIG. 3  is a block diagram showing a configuration example of a drive system of a hybrid type shovel according to an embodiment of the present invention.  FIG. 3  indicates a mechanical drive system by a double line, a high pressure hydraulic line by a thick solid line, a pilot line by a dashed line, and an electric drive/control system by a thin solid line. 
     An engine  11  as a mechanical drive part and an electric motor generator  12  as an assist drive part are connected to two input shafts of a transmission  13 , respectively. An output shaft of the transmission  13  is connected to a main pump  14  and a pilot pump  15  as hydraulic pumps. The main pump  14  is connected to a control valve  17  via a high pressure hydraulic line  16 . 
     A regulator  14 A is configured to control a discharge rate of the main pump  14 . For example, the regulator  14 A controls a discharge rate of the main pump  14  by adjusting a swash plate tilt angle of the main pump  14  depending on a discharge pressure of the main pump  14 , a control signal from the controller  30 , or the like. 
     The control valve  17  is configured to control a hydraulic system mounted on the hybrid type shovel. The hydraulic motors  1 A (for right) and  1 B (for left) for the lower travel body  1 , the boom cylinder  7 , the arm cylinder  8 , and the bucket cylinder  9  are connected to the control valve  17  via high pressure hydraulic lines. Hereinafter, the hydraulic motors  1 A (for right) and  1 B (for left) for the lower travel body  1 , the boom cylinder  7 , the arm cylinder  8 , and the bucket cylinder  9  are referred to collectively as hydraulic actuators. 
     The electric motor generator  12  is connected to an electric energy storage system  120  including a capacitor as an electric energy storage device via an inverter  18 A. The electric energy storage system  120  is connected to a swing-body-driving electric motor  21  as an electrically-driven work element via an inverter  20 . A rotary shaft  21 A of the swing-body-driving electric motor  21  is connected to a resolver  22 , a mechanical brake  23 , and a swing-body-driving transmission  24 . The pilot pump  15  is connected to a manipulation device  26  via a pilot line  25 . The swing-body-driving electric motor  21 , the inverter  20 , the resolver  22 , the mechanical brake  23 , and the swing-body-driving transmission  24  constitute a first load drive system. 
     The manipulation device  26  includes a lever  26 A, a lever  26 B, and a pedal  26 C. Each of the lever  26 A, the lever  26 B, and the pedal  26 C is connected to the control valve  17  and the pressure sensor  29  via hydraulic lines  27  and  28 , respectively. The pressure sensor  29  is configured to function as an operating condition detecting part to detect each operating condition of the hydraulic actuators. The pressure sensor  29  is connected to the controller  30  that performs drive control of an electric system. 
     Also, in this embodiment, a boom-regenerating electric generator  300  for obtaining boom regenerative electric power is connected to the electric energy storage system  120  via an inverter  18 C. The electric generator  300  is driven by a hydraulic motor  310  driven by hydraulic oil flowing out of the boom cylinder  7 . The electric generator  300  converts potential energy of the boom  4  (hydraulic energy of the hydraulic oil flowing out of the boom cylinder  7 ) into electric energy by using pressure of the hydraulic oil flowing out of the boom cylinder  7  when the boom  4  descends under its own weight.  FIG. 3  shows that the hydraulic motor  310  and the electric generator  300  are positioned away from each other for the purpose of illustration. However, in practice, a rotary shaft of the electric generator  300  is mechanically connected to a rotary shaft of the hydraulic motor  310 . That is, the hydraulic motor  310  is configured to be rotated by the hydraulic oil flowing out of the boom cylinder  7  when the boom  4  descends, and installed to convert the hydraulic energy of the hydraulic oil into rotational force when the boom  4  descends under its own weight. 
     The electric power generated by the electric generator  300  is supplied as regenerative electric power to the electric energy storage system  120  via the inverter  18 C. The electric generator  300  and the inverter  18 C constitute a second load drive system. 
     In this embodiment, a boom cylinder pressure sensor S 1  is attached to the boom cylinder  7 , and an arm cylinder pressure sensor S 2  is attached to the arm cylinder  8 . The boom cylinder pressure sensor S 1  detects hydraulic oil pressure in a bottom-side oil chamber of the boom cylinder  7 . The arm cylinder pressure sensor S 2  detects hydraulic oil pressure in a rod-side oil chamber of the arm cylinder  8 . Each of the boom cylinder pressure sensor S 1  and the arm cylinder pressure sensor S 2  is an example of a hydraulic actuator pressure detecting part, and outputs a detected pressure value to the controller  30 . 
     A communication circuit  320  is a hydraulic circuit configured to control a supply destination of the hydraulic oil flowing out of the boom cylinder  7 . For example, the communication circuit  320  supplies all or a part of the hydraulic oil flowing out of the boom cylinder  7  to the arm cylinder  8  in response to the control signal from the controller  30 . Also, the communication circuit  320  may supply all of the hydraulic oil flowing out of the boom cylinder  7  to the hydraulic motor  310 . Alternatively, the communication circuit  320  may supply a part of the hydraulic oil flowing out of the boom cylinder  7  to the arm cylinder  8  and may supply the remaining part to the hydraulic motor  310 . Operations of the communication circuit  320  will be explained below. 
       FIG. 4  is a block diagram showing a configuration example of the electric energy storage system  120 . The electric energy storage system  120  includes a capacitor  19 , a step-up/step-down voltage converter  100 , and a DC bus  110 . The capacitor  19  is provided with a capacitor voltage detecting part  112  for detecting a capacitor voltage value and a capacitor current detecting part  113  for detecting a capacitor current value. The capacitor voltage value detected by the capacitor voltage detecting part  112  and the capacitor current value detected by the capacitor current detecting part  113  are supplied to the controller  30 . 
     The step-up/step-down voltage converter  100  is configured to switch between a step-up operation and a step-down operation depending on operating conditions of the electric motor generator  12 , the swing-body-driving electric motor  21 , and the electric generator  300  so that a DC bus voltage value falls within a certain range. The DC bus  110  is arranged between the step-up/step-down voltage converter  100  and the inverters  18 A,  18 C, and  20 . The DC bus  110  allows electric power to be exchanged among the capacitor  19 , the electric motor generator  12 , the swing-body-driving electric motor  21 , and the electric generator  300 . 
     Here again, referring to  FIG. 3 , the controller  30  will be explained in detail. The controller  30  is a control device as a main controlling part configured to perform drive control of the hybrid type shovel. The controller  30  includes a processing unit including a Central Processing Unit (CPU) and an internal memory. The CPU executes a drive control program stored in the internal memory. 
     The controller  30  translates a signal supplied from the pressure sensor  29  into a swing speed command, and performs a drive control of the swing-body-driving electric motor  21 . In this case, the signal supplied from the pressure sensor  29  corresponds to a signal representing an amount of manipulation when the manipulation device  26  (a swing manipulating lever) is manipulated to swing the swing mechanism  2 . 
     Also, the controller  30  performs charge/discharge control of the capacitor  19  by performing the drive control of the step-up/step-down voltage converter  100  as a step-up/step-down voltage controlling part as well as performs operation control of the electric motor generator  12  (a switchover between an electrically driven (assist) operation and an electricity generating operation). Specifically, the controller  30  performs switchover control between the step-up operation and the step-down operation of the step-up/step-down voltage converter  100  based on a charging condition of the capacitor  19 , an operating condition (whether it is in the electrically driven (assist) operation or in the electricity generating operation) of the electric motor generator  12 , an operating condition (whether it is in the power running operation or in the regenerating operation) of the swing-body-driving electric motor  21 , and an operating condition of the electric generator  300 . In this way, the controller  30  performs the charge/discharge control of the capacitor  19 . 
     The switchover control between the step-up operation and the step-down operation of the step-up/step-down voltage converter  100  is performed based on a DC bus voltage value detected by a DC bus voltage detecting part  111 , a capacitor voltage value detected by the capacitor voltage detecting part  112 , and a capacitor current value detected by the capacitor current detecting part  113 . 
     In the above configuration, the electric power generated by the electric motor generator  12  as an assist motor is supplied to the DC bus  110  of the electric energy storage system  120  via the inverter  18 A, and supplied to the capacitor  19  via the step-up/step-down voltage converter  100 . Also, the regenerative electric power regenerated through the regenerative operation of the swing-body-driving electric motor  21  is supplied to the DC bus  110  of the electric energy storage system  120  via the inverter  20 , and supplied to the capacitor  19  via the step-up/step-down voltage converter  100 . Also, the electric power generated by the boom-regenerating electric generator  300  is supplied to the DC bus  110  of the electric energy storage system  120  via the inverter  18 C, and supplied to the capacitor  19  via the step-up/step-down voltage converter  100 . The electric power generated by the electric motor generator  12  or the electric generator  300  may be supplied directly to the swing-body-driving electric motor  21  via the inverter  20 . Also, the electric power generated by the swing-body-driving electric motor  21  or the electric generator  300  may be supplied directly to the electric motor generator  12  via the inverter  18 A. 
     The capacitor  19  may be any of rechargeable electric energy storage devices that allow the electric power to be exchanged with the DC bus  110  via the step-up/step-down voltage converter  100 . In this regard,  FIG. 4  shows the capacitor  19  as an electric energy storage device. However, instead of the capacitor  19 , a rechargeable secondary battery such as a lithium-ion battery, a lithium-ion capacitor, or other forms of electric source that allow electric power to be exchanged may be used as an electric energy storage device. 
     In addition to the above functions, the controller  30  also performs drive control of the communication circuit  320  depending on operating conditions of the hydraulic actuators and pressure conditions of the hydraulic oil in the hydraulic actuators. 
     Here, referring to  FIG. 5 , the communication circuit  320  will be explained in detail.  FIG. 5  is a diagram showing a configuration example of the communication circuit  320 . In this embodiment, the communication circuit  320  is arranged to connect the bottom side oil chamber of the boom cylinder  7 , the rod side oil chamber of the arm cylinder  8 , the control valve  17 , and the hydraulic motor  310 . 
     The communication circuit  320  includes a reusing flow control valve  321 , a regenerating flow control valve  322 , an electromagnetic valve  323 , and a check valve  324 . 
     The reusing flow control valve  321  controls flow rate of hydraulic oil flowing in a reusing oil passage C 3  that connects a boom cylinder bottom side oil passage C 1  (highlighted by a thick line) and an arm cylinder rod side oil passage C 2  (equally highlighted by a thick line). In this embodiment, the reusing flow control valve  321  is, for example, an electromagnetic spool valve with  3  ports and  2  positions. The boom cylinder bottom side oil passage C 1  connects the bottom side oil chamber of the boom cylinder  7  and a boom-driving flow control valve  17 B of the control valve  17 . Also, the arm cylinder rod side oil passage C 2  connects the rod side oil chamber of the arm cylinder  8  and an arm-driving flow control valve  17 A of the control valve  17 . 
     In this embodiment, one end of the reusing oil passage C 3  is connected to the arm cylinder rod side oil passage C 2 . The reusing oil passage C 3  may be connected to an oil passage that connects the bottom side oil chamber of the arm cylinder  8  and the arm-driving flow control valve  17 A of the control valve  17 . In this case, hydraulic oil flowing out of the bottom side oil chamber of the boom cylinder  7  can flow into the bottom side oil chamber of the arm cylinder  8 , and thus can be used for an arm closing operation. Also, the reusing oil passage C 3  may be connected to an oil passage that connects the main pumps  14 L,  14 R and the control valve  17 , i.e., may be connected to upstream of the control valve  17 . In this case, hydraulic oil flowing out of the bottom side oil chamber of the boom cylinder  7  can be used for hydraulic actuators other than the arm cylinder  8 . 
     The regenerating flow control valve  322  controls a flow rate of hydraulic oil flowing in a regenerating oil passage C 4  that connects the boom cylinder bottom side oil passage C 1  and the hydraulic motor  310 . In this embodiment, the regenerating flow control valve  322  is, for example, a spool valve with  2  ports and  2  positions. 
     The electromagnetic valve  323  controls the regenerating flow control valve  322 . In this embodiment, for example, the electromagnetic valve  323  selectively exerts a control pressure generated by a pilot pump on a pilot port of the regenerating flow control valve  322 . 
     The check valve  324  is arranged in the reusing oil passage C 3 , and prevents hydraulic oil from flowing from the arm cylinder rod side oil passage C 2  to the boom cylinder bottom side oil passage C 1 . 
     Here, referring to  FIG. 6 , a process will be explained in which the controller  30  controls a flow of hydraulic oil in the communication circuit  320  (hereinafter referred to as “communication circuit driving process”).  FIG. 6  is a flowchart showing a flow of the communication circuit driving process. The controller  30  performs this communication circuit driving process repeatedly at predetermined control periods during operation of the shovel. 
     First, the controller  30  detects amounts of manipulation of a boom manipulating lever and an arm manipulating lever based on outputs of the pressure sensor  29 . Then, the controller  30  determines whether it is in the dumping operation phase, i.e., whether a boom lowering operation and an arm opening operation are being performed simultaneously (step ST 1 ). To determine whether it is in the dumping operation phase, the controller  30  may determine whether a boom lowering operation, an arm opening operation, and a bucket opening operation are being performed simultaneously. Also, the controller  30  may determine whether it is in the dumping operation phase based on outputs of angle sensors (not shown) or displacement sensors (not shown). The angle sensors detect pivot angles of the boom  4 , the arm  5 , and the bucket  6 . The displacement sensors detect displacements of the boom cylinder  7 , the arm cylinder  8 , and the bucket cylinder  9 . 
     If the controller  30  determines that it is not in the dumping operation phase, i.e., that the boom lowering operation and the arm opening operation are not being performed simultaneously (NO in step ST 1 ), the controller  30  keeps on monitoring the outputs of the pressure sensor  29  until the controller  30  determines that it is in the dumping operation phase. 
     If the controller  30  determines that it is in the dumping operation phase, i.e., that the boom lowering operation and the arm opening operation are being performed simultaneously (YES in step ST 1 ), the controller  30  compares a pressure P 1  detected by the boom cylinder pressure sensor S 1  and a pressure P 2  detected by the arm cylinder pressure sensor S 2  (step ST 2 ). 
     If the detected pressure P 1  is greater than the detected pressure P 2 , i.e., if the pressure of the hydraulic oil in the bottom side oil chamber of the boom cylinder  7  is greater than the pressure of the hydraulic oil in the rod side oil chamber of the arm cylinder  8  (YES in step ST 2 ), the controller  30  performs an arm operation assisting process (step ST 3 ). 
     Specifically, the controller  30  outputs a predetermined control signal to the reusing flow control valve  321  and the electromagnetic valve  323  in the communication circuit  320 . Then, the controller  30  causes the hydraulic oil flowing out of the bottom side oil chamber of the boom cylinder  7  to flow into the rod side oil chamber of the arm cylinder  8 . 
     Also, the controller  30  controls a discharge rate of the main pump  14 R by outputting a predetermined control signal to a regulator  14 RA. Then, the controller  30  allows hydraulic oil to be supplied to the rod side oil chamber of the arm cylinder  8  at a desired flow rate by using the hydraulic oil flowing out of the bottom side oil chamber of the boom cylinder  7  and hydraulic oil discharged from the main pump  14 R. Specifically, the controller  30  determines a flow rate of hydraulic oil to be discharged from the main pump  14 R based on the pressure P 1  detected by the boom cylinder pressure sensor S 1  and the pressure P 2  detected by the arm cylinder pressure sensor S 2 . 
     In this way, the controller  30  allows hydraulic energy of the hydraulic oil flowing out of the boom cylinder  7  in the dumping operation phase to be used for the arm opening operation without converting the hydraulic energy into electric energy. As a result, the controller  30  can make more efficient use of the hydraulic oil that had been drained to the oil tank after rotating the hydraulic motor  310  as before. 
     In contrast, if the detected pressure P 1  is lower than or equal to the detected pressure P 2 , i.e., if the pressure of the hydraulic oil in the bottom side oil chamber of the boom cylinder  7  is lower than or equal to the pressure of the hydraulic oil in the rod side oil chamber of the arm cylinder  8  (NO in step ST 2 ), the controller  30  performs a boom regenerative electricity generating process (step ST 4 ). 
     Specifically, the controller  30  outputs a predetermined control signal to the reusing flow control valve  321  and the electromagnetic valve  323  in the communication circuit  320 . Then, the controller  30  causes the hydraulic oil flowing out of the bottom side oil chamber of the boom cylinder  7  to flow into the hydraulic motor  310 , and causes the electric generator  300  to generate electricity. 
     This is because the pressure of the hydraulic oil in the rod side oil chamber of the arm cylinder  8  is greater than the pressure of the hydraulic oil in the bottom side oil chamber of the boom cylinder  7 , and because it is impossible to cause the hydraulic oil flowing out of the bottom side oil chamber of the boom cylinder  7  to flow into the rod side oil chamber of the arm cylinder  8 . 
     The controller  30  may supply a part of the hydraulic oil flowing out of the boom cylinder  7  to the arm cylinder  8 , and may cause the remaining part of the hydraulic oil flowing out of the boom cylinder  7  to flow into the hydraulic motor  310 . This is to make best use of the hydraulic energy of the hydraulic oil flowing out of the boom cylinder  7  even if a flow rate of the hydraulic oil flowing out of the boom cylinder  7  is greater than a flow rate of hydraulic oil required for the arm opening operation in the arm operation assisting process. 
     Also, even if the boom lowering operation and the arm opening operation or a bucket opening operation are not being performed simultaneously, the controller  30  performs the boom regenerative electricity generating process if the boom lowering operation is being performed. This is to make best use of the hydraulic energy of the hydraulic oil flowing out of the boom cylinder  7 . 
     Also, in this embodiment, the controller  30  allows the hydraulic oil flowing out of the boom cylinder  7  to be used for the arm opening operation. However, the hydraulic oil may be used for an arm closing operation, a bucket closing operation, a bucket opening operation, or a traveling of the lower travel body  1 . 
     Here, referring to  FIGS. 7 and 8 , there will be explained in detail an operation of the communication circuit  320  in the arm operation assisting process and the boom regenerative electricity generating process.  FIG. 7  shows a state of the communication circuit  320  in the arm operation assisting process.  FIG. 8  shows a state of the communication circuit  320  in the boom regenerative electricity generating process. Also, thick solid lines in  FIGS. 7 and 8  indicate that there is a flow of hydraulic oil. 
       FIG. 7  shows a state where hydraulic oil discharged from the main pump  14 L flows into the rod side oil chamber of the boom cylinder  7 , hydraulic oil discharged from the main pump  14 R flows into the rod side oil chamber of the arm cylinder  8 , and a boom lowering operation and an arm opening operation are being performed simultaneously. In  FIG. 7 , a pressure P 1  detected by the boom cylinder pressure sensor S 1  is greater than a pressure P 2  detected by the arm cylinder pressure sensor S 2 . 
     In the state like this, the reusing flow control valve  321  switches its valve position to a first valve position  321 A in response to a control signal from the controller  30 . As a result, a flow of hydraulic oil from the boom cylinder  7  to the control valve  17  is closed off. Hydraulic oil flowing out of the boom cylinder  7  reaches the arm cylinder rod side oil passage C 2  through the reusing oil passage C 3 , joins together with hydraulic oil discharged from the main pump  14 R, and flows into the rod side oil chamber of the arm cylinder  8 . 
     Also, the electromagnetic valve  323  switches a valve position of the regenerating flow control valve  322  to a first valve position  322 A in response to a control signal from the controller  30 . As a result, a flow of hydraulic oil from the boom cylinder  7  to the hydraulic motor  310  is closed off, and all of the hydraulic oil flowing out of the boom cylinder  7  flow into the rod side oil chamber of the arm cylinder  8 . 
     Also, the controller  30  outputs a control signal to the regulator  14 RA, decreases a discharge rate of the main pump  14 R, and decreases a flow rate of hydraulic oil flowing from the main pump  14 R to the rod side oil chamber of the arm cylinder  8 . Also, the controller  30  may decrease or eliminate a flow rate of the hydraulic oil flowing from the main pump  14 R to the rod side oil chamber of the arm cylinder  8  by controlling the arm-driving flow control valve  17 A. In the case where the controller  30  has eliminated the flow rate of the hydraulic oil flowing from the main pump  14 R to the rod side oil chamber of the arm cylinder  8 , only the hydraulic oil flowing out of the bottom side oil chamber of the boom cylinder  7  is supplied to the rod side oil chamber of the arm cylinder  8 . 
     In this way, the communication circuit  320  causes all of the hydraulic oil flowing out of the boom cylinder  7  to flow into the rod side oil chamber of the arm cylinder  8  if a boom lowering operation and an arm opening operation are performed simultaneously and if the detected pressure P 1  is greater than the detected pressure P 2 . 
     Also,  FIG. 8  shows a state where hydraulic oil discharged from the main pump  14 L flows into the rod side oil chamber of the boom cylinder  7 , and only a boom lowering operation is being performed. 
     In a state like this, the reusing flow control valve  321  switches its valve position to a second valve position  321 B in response to a control signal from the controller  30 . As a result, a flow of hydraulic oil from the boom cylinder  7  to the arm cylinder  8  is closed off. A part of the hydraulic oil flowing out of the boom cylinder  7  reaches the control valve  17  through the boom cylinder bottom side oil passage Cl, and then is drained to the oil tank through the control valve  17 . 
     Also, the electromagnetic valve  323  switches a valve position of the regenerating flow control valve  322  to a second valve position  322 B in response to a control signal from the controller  30 . As a result, a remaining part of the hydraulic oil flowing out of the boom cylinder  7  flows into the hydraulic motor  310 , rotates the hydraulic motor  310  and the electric generator  300 , and then is drained to the oil tank. 
     In this way, if the boom lowering operation is being singularly performed, the communication circuit  320  causes a part of the hydraulic oil flowing out of the boom cylinder  7  to flow into the hydraulic motor  310 , and causes the electric generator  300  to generate electricity. The controller  30  may cause all of the hydraulic oil flowing out of the boom cylinder  7  to flow into the hydraulic motor  310 . 
     Next, referring to  FIG. 9 , temporal changes will be explained in each of a pilot pressure (see an upper graph of  FIG. 9 ), a cylinder displacement (see a central graph of  FIG. 9 ), and a cylinder pressure (see a lower graph of  FIG. 9 ) when the controller  30  performs the arm operation assisting process or the boom regenerative electricity generating process in the dumping operation phase. Trends indicated by solid lines in each of the upper graph, the central graph, and the lower graph of  FIG. 9  represent changes in a pilot pressure of the boom manipulating lever, a displacement of the boom cylinder  7 , and a pressure of the hydraulic oil in the bottom side oil chamber of the boom cylinder  7  (a pressure P 1  detected by the boom cylinder pressure sensor S 1 ), respectively. Also, trends indicated by dashed lines in each of the upper graph, the central graph, and the lower graph of  FIG. 9  represent changes in a pilot pressure of the arm manipulating lever, a displacement of the arm cylinder  8 , and a pressure of the hydraulic oil in the rod side oil chamber of the arm cylinder  8  (a pressure P 2  detected by the arm cylinder pressure sensor S 2 ), respectively. 
     In a time point t 0 , if the boom manipulating lever is manipulated in a lowering direction and if a pilot pressure in the lowering direction of the boom manipulating lever increases, the controller  30  performs the boom regenerative electricity generating process and puts the communication circuit  320  into the state in  FIG. 8 . This is because the hydraulic energy of the hydraulic oil flowing out of the boom cylinder  7  due to the boom lowering operation becomes available, and because it is impossible to perform the arm operation assisting process due to the fact that the detected pressure P 1  is lower than or equal to the detected pressure P 2 . At this point in time, the arm manipulating lever has already been manipulated in an opening direction, and the pilot pressure in the opening direction of the arm manipulating lever has already become greater than or equal to a predetermined level. 
     By the above manipulation, the boom cylinder  7  is slowly displaced toward a contraction side and operates to lower the boom  4 , and the arm cylinder  8  is displaced toward a contraction side and operates to open the arm  5 . The controller  30  may determine a start timing of the arm operation assisting process or the boom regenerative electricity generating process based on such displacements of the boom cylinder  7  and the arm cylinder  8 . 
     Subsequently, if the detected pressure P 1  becomes greater than the detected pressure P 2  at a time point t 1 , the controller  30  stops the boom regenerative electricity generating process. Then, the controller  30  performs the arm operation assisting process and puts the communication circuit  320  into the state in  FIG. 7 . This is because it has become possible to cause the hydraulic oil flowing out of the boom cylinder  7  to flow into the arm cylinder  8  due to the fact that the detected pressure P 1  has become greater than the detected pressure P 2 . 
     Even if the controller  30  performs the arm operation assisting process, the controller  30  may keep on performing the boom regenerative electricity generating process by using a part of the hydraulic oil flowing out of the boom cylinder  7 . In that case, the reusing flow control valve  321  is set to the first valve position  321 A, and the regenerating flow control valve  322  is set to the second valve position  322 B. 
     Subsequently, if the detected pressure P 1  becomes lower than the detected pressure P 2  again at a time point t 2 , the controller  30  stops the arm operation assisting process. Then, the controller  30  performs the boom regenerative electricity generating process and puts the communication circuit  320  into the state in  FIG. 8  again. This is because it is impossible to perform the arm operation assisting process due to the fact that the detected pressure P 1  has become lower than or equal to the detected pressure P 2 . 
     By the above configuration, the hybrid type shovel according to this embodiment can make use of the hydraulic energy of the hydraulic oil flowing out of the boom cylinder  7  during a boom lowering operation for operations of other hydraulic actuators without converting it into electric energy. Thus, it is possible to make more efficient use of the hydraulic oil flowing out of the boom cylinder  7  during a boom lowering operation. 
     Also, the hybrid type shovel according to this embodiment confirms that the pressure of the hydraulic oil in the boom cylinder  7  is greater than the pressure of the hydraulic oil in other hydraulic actuator as a prospective supply destination of the hydraulic oil. On that basis, the hybrid type shovel according to this embodiment causes the hydraulic oil flowing out of the boom cylinder  7  to flow into the other hydraulic actuator as the prospective supply destination. In contrast, if the pressure of the hydraulic oil in the boom cylinder  7  is lower than the pressure of the hydraulic oil in the other hydraulic actuator as the prospective supply destination of the hydraulic oil, the hybrid type shovel according to this embodiment closes off an oil passage between the boom cylinder  7  and the other hydraulic actuator as the prospective supply destination. Thus, it is possible to cause the hydraulic oil flowing out of the boom cylinder  7  to reliably flow into the other hydraulic actuator as the prospective supply destination. 
     Also, the hybrid type shovel according to this embodiment confirms that the other hydraulic actuator as the prospective supply destination of the hydraulic oil flowing out of the boom cylinder  7  is in operation. On that basis, the hybrid type shovel according to this embodiment causes the hydraulic oil flowing out of the boom cylinder  7  to flow into the other hydraulic actuator as the prospective supply destination. In contrast, if the other hydraulic actuator as the prospective supply destination is not in operation, the hybrid type shovel according to this embodiment causes the hydraulic oil flowing out of the boom cylinder  7  to flow into the hydraulic motor  310 , and causes the electric generator  300  to generate electricity. Thus, the hybrid type shovel according to this embodiment can make efficient and reliable use of the hydraulic oil flowing out of the boom cylinder  7  depending on operating conditions of the other hydraulic actuator as the prospective supply destination. 
     Next, referring to  FIG. 10 , a shovel according to another embodiment of the present invention will be explained. 
       FIG. 10  is a block diagram showing a configuration example of the shovel according to another embodiment of the present invention. As in  FIG. 3 ,  FIG. 10  indicates a mechanical drive system by a double line, a high pressure hydraulic line by a thick solid line, a pilot line by a dashed line, and an electric drive/control system by a thin solid line. 
     The shovel according to this embodiment is different from the hybrid type shovel according to the foregoing embodiment in that it includes a swing-body-driving hydraulic motor  40  instead of the first load drive system as an electrically-driven swing mechanism. However, the shovel is the same as the hybrid type shovel in other aspects. By this configuration, the shovel according to this embodiment can achieve the same effect as the hybrid type shovel according to the foregoing embodiment. 
     While certain preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and various changes and substitutions may be made without departing from the scope of the present invention. 
     For example, in the above embodiments, the reusing flow control valve  321  and the regenerating flow control valve  322  are configured as two individually independent spool valves. However, they may be configured as a single spool valve.