Patent Publication Number: US-9897119-B2

Title: Control system for hybrid construction machine

Description:
TECHNICAL FIELD 
     The present invention relates to a control system for a hybrid construction machine that includes a regeneration apparatus for performing energy regeneration using working fluid introduced from an actuator. 
     BACKGROUND ART 
     As a conventional hybrid construction machine, a known hybrid construction machine rotates a hydraulic motor using working oil introduced from a turning motor so as to perform energy regeneration. 
     JP2009-281525A discloses that an electromagnetic switching valve is switched to the opening position to perform turning regeneration and the degree of opening of a proportional electromagnetic throttle valve disposed in parallel to a relief valve is controlled to reduce the passage resistance of the relief valve when a pressure signal of a pressure sensor that detects a turning pressure during turning or a brake pressure during braking of the turning motor reaches a preliminarily set pressure. 
     SUMMARY OF INVENTION 
     The prior art described above controls the degree of opening of the proportional electromagnetic throttle valve to maintain the turning pressure of the turning motor. Thus, the regenerative control is complicated. In the case where the degree of opening of the proportional electromagnetic throttle valve becomes large, the turning pressure of the turning motor is reduced, the electromagnetic switching valve is switched to the closed position, and then the turning regeneration is stopped. Subsequently, when the turning motor is turning, the turning pressure is increased again, the electromagnetic switching valve is switched to the opening position, and then the turning regeneration is restarted. In this way, the electromagnetic switching valve may repeat opening and closing. In the case where this situation occurs, the pressure variation due to opening and closing of the electromagnetic switching valve may cause vibration. 
     To avoid this situation, the degree of opening of the proportional electromagnetic throttle valve may be controlled to be small. However, in this case, the regeneration amount becomes small. Thus, the efficiency is poor. 
     An object of the present invention is to provide a control system for a hybrid construction machine that allows efficient regeneration with a simple regenerative control. 
     According to one aspect of the present invention, a control system for a hybrid construction machine includes a fluid pressure pump as a driving source of a turning motor and a fluid pressure cylinder; a regenerative motor for regeneration configured to rotate by a working fluid introduced from a turning circuit for driving the turning motor and a working fluid introduced from the fluid pressure cylinder; a rotating electrical machine coupled to the regenerative motor; a pressure detector configured to detect a turning pressure during a turning operation or a brake pressure during a braking operation of the turning motor; a turning-regeneration-use switching valve configured to open when a pressure detected by the pressure detector reaches a preliminarily set turning-regeneration starting pressure, so as to introduce the working fluid from the turning circuit to the regenerative motor for turning regeneration; an operating-state detector configured to detect an operating state of the fluid pressure cylinder; and a cylinder-regeneration-use switching valve disposed in parallel to the turning-regeneration-use switching valve, the cylinder-regeneration-use switching valve being configured to open on the basis of a detection result of the operating-state detector, so as to introduce the working fluid from the fluid pressure cylinder to the regenerative motor for cylinder regeneration. When the turning regeneration is performed alone, the working fluid from the turning circuit is introduced into the regenerative motor without pressure reduction. When the turning regeneration and the cylinder regeneration are simultaneously performed, the working fluid from the turning circuit is reduced in pressure, joins the working fluid from the fluid pressure cylinder, and is introduced into the regenerative motor. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a circuit diagram illustrating a control system for a hybrid construction machine according to a first embodiment of the present invention. 
         FIG. 2  is a circuit diagram illustrating a control system for a hybrid construction machine according to a second embodiment of the present invention. 
         FIG. 3  is a circuit diagram illustrating a control system for a hybrid construction machine according to a third embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following describes control systems for a hybrid construction machine according to embodiments of the present invention with reference to the drawings. In the following embodiments, a description will be given of the case where the hybrid construction machine is a hydraulic shovel. 
     &lt;First Embodiment&gt; 
     A description will be given of a control system  100  for a hybrid construction machine according to a first embodiment of the present invention with reference to  FIG. 1 . 
     A hydraulic shovel includes first and second main pumps  71  and  72  as fluid pressure pumps driven by an engine  73 . The first and second main pumps  71  and  72  are variable-displacement type pumps with adjustable tilting angles, and rotate coaxially with each other. 
     The working oil (working fluid) to be discharged from the first main pump  71  is supplied to an operation valve  1 , an operation valve  2  for arm first speed, an operation valve  3  for boom second speed, an operation valve  4 , and an operation valve  5  in this order from the upstream side. The operation valve  1  controls a turning motor  76 . The operation valve  2  for arm first speed controls an arm cylinder (not illustrated). The operation valve  3  for boom second speed controls a boom cylinder  77 . The operation valve  4  controls an auxiliary attachment (not illustrated). The operation valve  5  controls a first travel motor (not illustrated) for left traveling. The respective operation valves  1  to  5  control the flows of the working oil to be introduced from the first main pump  71  into respective actuators, so as to control the operations of the respective actuators. The respective operation valves  1  to  5  are operated by the pilot pressure to be supplied in association with the manual operation of the operating lever by the operator of the hydraulic shovel. 
     The respective operation valves  1  to  5  are connected to the first main pump  71  through a neutral flow path  6  and a parallel flow path  7  in parallel to each other. On the downstream side of the operation valve  5  in the neutral flow path  6 , a pilot-pressure generating mechanism  8  for generating a pilot pressure is provided. The pilot-pressure generating mechanism  8  generates a high pilot pressure on the upstream side when the passing flow is large while generating a low pilot pressure on the upstream side when the passing flow is small. 
     When all of the operation valves  1  to  5  are in the neutral position or adjacent to the neutral position, the neutral flow path  6  introduces all or a part of the working oil discharged from the first main pump  71  into the tank. At this time, the flow passing through the pilot-pressure generating mechanism  8  is increased. Thus, a high pilot pressure is generated. 
     On the other hand, when the operation valves  1  to  5  are switched to the full-stroke states, the neutral flow path  6  is closed to stop the flow of the working oil. In this case, the flow passing through the pilot-pressure generating mechanism  8  becomes little and the pilot pressure remains zero. However, depending on the operating amounts of the operation valves  1  to  5 , a part of the working oil discharged from the first main pump  71  is introduced into the actuators while the rest is introduced into the tank from the neutral flow path  6 . Accordingly, the pilot-pressure generating mechanism  8  generates the pilot pressure corresponding to the flow of the working oil in the neutral flow path  6 . That is, the pilot-pressure generating mechanism  8  generates the pilot pressure corresponding to the operating amounts of the operation valves  1  to  5 . 
     The pilot-pressure generating mechanism  8  connects to a pilot flow path  9 . Into the pilot flow path  9 , the pilot pressure generated by the pilot-pressure generating mechanism  8  is introduced. The pilot flow path  9  is connected to a regulator  10  that controls the tilting angle of the first main pump  71 . The regulator  10  controls the tilting angle of the first main pump  71  in inverse proportion to the pilot pressure in the pilot flow path  9  so as to control the displacement per one rotation of the first main pump  71 . Accordingly, the operation valves  1  to  5  are switched to the full-stroke states to stop the flow in the neutral flow path  6 . Then, when the pilot pressure in the pilot flow path  9  becomes zero, the tilting angle of the first main pump  71  becomes maximum. Thus, the displacement per one rotation becomes maximum. 
     The pilot flow path  9  is provided with a first pressure sensor  11  that detects the pressure in the pilot flow path  9 . 
     The working oil to be discharged from the second main pump  72  is supplied to an operation valve  12 , an operation valve  13 , an operation valve  14  for boom first speed, and an operation valve  15  for arm second speed in this order from the upstream side. The operation valve  12  controls a second travel motor (not illustrated) for right traveling. The operation valve  13  controls a bucket cylinder (not illustrated). The operation valve  14  for boom first speed controls the boom cylinder  77 . The operation valve  15  for arm second speed controls an arm cylinder (not illustrated). The respective operation valves  12  to  15  control the flows of the working oil to be introduced from the second main pump  72  into respective actuators, so as to control the operations of the respective actuators. The respective operation valves  12  to  15  are operated by the pilot pressure to be supplied in association with the manual operation of the operating lever by the operator of the hydraulic shovel. 
     The respective operation valves  12  to  15  are connected to the second main pump  72  through a neutral flow path  16 . The operation valve  13  and the operation valve  14  are connected to the second main pump  72  through a parallel passage  17  in parallel to the neutral flow path  16 . On the downstream side of the operation valve  15  in the neutral flow path  16 , a pilot-pressure generating mechanism  18  for generating a pilot pressure is provided. The pilot-pressure generating mechanism  18  has the same function as that of the pilot-pressure generating mechanism  8  on the first main pump  71  side. 
     The pilot-pressure generating mechanism  18  connects to a pilot flow path  19 . Into the pilot flow path  19 , the pilot pressure generated by the pilot-pressure generating mechanism  18  is introduced. The pilot flow path  19  is connected to a regulator  20  that controls the tilting angle of the second main pump  72 . The regulator  20  controls the tilting angle of the second main pump  72  in inverse proportion to the pilot pressure in the pilot flow path  19  so as to control the displacement per one rotation of the second main pump  72 . Accordingly, the operation valves  12  to  15  are switched to the full-stroke states to stop the flow in the neutral flow path  16 . Then, when the pilot pressure in the pilot flow path  19  becomes zero, the tilting angle of the second main pump  72  becomes maximum. Thus, the displacement per one rotation becomes maximum. 
     The pilot flow path  19  is provided with a second pressure sensor  21  that detects the pressure of the pilot flow path  19 . 
     At the engine  73 , an electric generator  22  is provided. The electric generator  22  generates electric power using the redundant force of the engine  73 . The electric power generated by the electric generator  22  is charged to a battery  24  via a battery charger  23 . The battery charger  23  allows charging the electric power to the battery  24  also in the case where the battery charger  23  is coupled to an ordinary household power supply  25 . 
     Next, the turning motor  76  will be described. 
     The turning motor  76  is provided at a turning circuit  75  for driving the turning motor  76 . The turning circuit  75  includes a pair of supply/discharge passages  26  and  27  and relief valves  28  and  29 . The supply/discharge passages  26  and  27  connect the first main pump  71  with the turning motor  76 . The operation valve  1  is provided in the supply/discharge passages  26  and  27 . The relief valves  28  and  29  are connected to the respective supply/discharge passages  26  and  27  and open at the set pressures. 
     In the case where the operation valve  1  is in the neutral position (the state illustrated in  FIG. 1 ), the actuator port of the operation valve  1  is closed. Accordingly, the supply and discharge of the working oil to/from the turning motor  76  are cut off. Thus, the turning motor  76  remains in the stop state. 
     When the operation valve  1  is switched to the right-side position in  FIG. 1 , the supply/discharge passage  26  is connected to the first main pump  71  while the supply/discharge passage  27  is communicated with the tank. Accordingly, the working oil is supplied through the supply/discharge passage  26  to turn the turning motor  76  and the working oil returning from the turning motor  76  is discharged to the tank through the supply/discharge passage  27 . On the other hand, when the operation valve  1  is switched to the left-side position in  FIG. 1 , the supply/discharge passage  27  is connected to the first main pump  71  while the supply/discharge passage  26  is communicated with the tank. Accordingly, the turning motor  76  turns in the reverse direction. 
     In the case where the turning pressures in the supply/discharge passages  26  and  27  reach the respective set pressures of the relief valves  28  and  29  during the turning operation of the turning motor  76 , the relief valves  28  and  29  open to introduce the excess flow on the high pressure side into the low pressure side. 
     When the operation valve  1  is switched to the neutral position during the turning operation of the turning motor  76 , the actuator port of the operation valve  1  is closed. Accordingly, the supply/discharge passages  26  and  27 , the turning motor  76 , and the relief valves  28  and  29  constitute a closed circuit. Even when the actuator port of the operation valve  1  is closed, the turning motor  76  continues rotating due to the inertial energy to exert the pumping action. Accordingly, a high pressure is provided in one passage among the supply/discharge passages  26  and  27  at low pressure during the turning operation while a low pressure is provided in the other passage among the supply/discharge passages  26  and  27  at high pressure during the turning operation. Thus, a braking force acts on the turning motor  76  to perform a braking operation. At this time, in the case where the brake pressures in the supply/discharge passages  26  and  27  reach the respective set pressures of the relief valves  28  and  29 , the relief valves  28  and  29  open to introduce the brake flow on the high pressure side into the low pressure side. 
     In the case where the suction flow of the turning motor  76  becomes insufficient during the braking operation of the turning motor  76 , the working oil of the tank is suctioned through check valves  54  and  55  that accept flows of the working oil only from the tank to the respective supply/discharge passages  26  and  27 . 
     Next, the boom cylinder  77  will be described. 
     The operation valve  14  that controls the operation of the boom cylinder  77  is operated by the pilot pressure to be supplied to pilot chamber  96   a  or  96   b  from a pilot pump  94  through a pilot valve  95  in association with the manual operation of an operating lever  93  by the operator of the hydraulic shovel. The operation valve  3  for boom second speed is switched in conjunction with the operation valve  14 . 
     In the case where the pilot pressure is supplied to the pilot chamber  96   a , the operation valve  14  is switched to the right-side position in  FIG. 1 . Then, the working oil discharged from the second main pump  72  is supplied to a piston-side chamber  31  of the boom cylinder  77  through a supply/discharge passage  30 , and the working oil returning from a rod-side chamber  32  is discharged to the tank through a supply/discharge passage  33 . Thus, the boom cylinder  77  expands. On the other hand, in the case where the pilot pressure is supplied to the pilot chamber  96   b , the operation valve  14  is switched to the left-side position in  FIG. 1 . Then, the working oil discharged from the second main pump  72  is supplied to the rod-side chamber  32  of the boom cylinder  77  through the supply/discharge passage  33 , and the working oil returning from the piston-side chamber  31  is discharged to the tank through the supply/discharge passage  30 . Thus, the boom cylinder  77  contracts. In the case where the pilot pressure is not supplied to the pilot chamber  96   a  or  96   b , the operation valve  14  is switched to the neutral position (the state illustrated in  FIG. 1 ). Accordingly, the supply and discharge of the working oil to/from the boom cylinder  77  are cut off. Thus, the boom remains in the stopped state. 
     In the case where the operation valve  14  is switched to the neutral position to stop the movement of the boom, a force in the contracting direction acts on the boom cylinder  77  due to own weights of the bucket, the arm, the boom, and so on. Thus, the boom cylinder  77  maintains the load using the piston-side chamber  31  in the case where the operation valve  14  is in the neutral position. The piston-side chamber  31  becomes a load-side pressure chamber. 
     The control system  100  for the hybrid construction machine includes a regeneration apparatus that recovers the energy of the working oil from the turning circuit  75  and the boom cylinder  77  to perform energy regeneration. The following describes this regeneration apparatus. 
     The regenerative control of the regeneration apparatus is performed by a controller  90 . The controller  90  comprises a CPU, a ROM, and a RAM. The CPU executes a regenerative control. The ROM stores a control program, a setting value, and similar data required for the processing operation of the CPU. The RAM temporarily stores information detected by various sensors. 
     The supply/discharge passages  26  and  27  connected to the turning motor  76  connect to respective branch passages  57  and  58 . The branch passages  57  and  58  are joined together to be connected to a turning regeneration passage  45  for introducing the working oil from the turning circuit  75  into a regenerative motor  88  for regeneration. The branch passages  57  and  58  include respective check valves  46  and  47  that accept flows of the working oil only from the respective supply/discharge passages  26  and  27  to the turning regeneration passage  45 . The turning regeneration passage  45  is connected to the regenerative motor  88  through a junction regeneration passage  44 . 
     The regenerative motor  88  is a variable displacement-type motor with an adjustable tilting angle, and is coupled to an electric motor  91  as a rotating electrical machine that doubles as an electric generator so as to rotate coaxially with each other. In the case where the electric motor  91  functions as the electric generator, the electric power generated by the electric motor  91  is charged to the battery  24  via an inverter  92 . The regenerative motor  88  and the electric motor  91  may be directly coupled together or may be coupled together via a reducer. 
     The turning regeneration passage  45  is provided with a switching valve  48  as a turning-regeneration-use switching valve controlled by switching control using a signal output from the controller  90 . Between the switching valve  48  and the check valves  46  and  47 , a pressure sensor  49  is provided as a pressure detector that detects the turning pressure during the turning operation or the brake pressure during the braking operation of the turning motor  76 . The pressure signal detected by the pressure sensor  49  is output to the controller  90 . 
     When the solenoid is not energized, the switching valve  48  is set to the closed position (the state illustrated in  FIG. 1 ) to block the turning regeneration passage  45 . When the solenoid is energized, the switching valve  48  is set to the opening position to open the turning regeneration passage  45 . In the case where the controller  90  determines that the pressure detected by the pressure sensor  49  reaches a turning-regeneration starting pressure that is preliminarily set, the controller  90  switches the switching valve  48  to the opening position. Accordingly, the working oil from the turning circuit  75  is introduced into the regenerative motor  88  so as to perform the turning regeneration. Thus, the switching valve  48  is a valve for performing the turning regeneration. 
     A description will be given of the path of the working oil from the turning circuit  75  to the regenerative motor  88 . For example, during the turning operation in which the turning motor  76  is turned by the working oil supplied through the supply/discharge passage  26 , the excess oil in the supply/discharge passage  26  flows in the turning regeneration passage  45  through the branch passage  57  and the check valve  46  and then introduced into the regenerative motor  88 . During the braking operation in which the operation valve  1  is switched to the neutral position when the turning motor  76  is turned by the working oil supplied through the supply/discharge passage  26 , the working oil discharged by the pumping action of the turning motor  76  flows in the turning regeneration passage  45  through the branch passage  58  and the check valve  47  and then introduced into the regenerative motor  88 . 
     In the case where the turning-regeneration starting pressure for switching the switching valve  48  to the opening position is set to a pressure lower than the set pressures of the relief valves  28  and  29 , the pressure of the turning circuit  75  might not remain at a pressure required for the turning operation or the braking operation of the turning motor  76  when the switching valve  48  is switched to the opening position. In the case where the turning-regeneration starting pressure is set to be similar to the set pressures of the relief valves  28  and  29 , most of the excess flow during the turning operation of the turning motor  76  or most of the brake flow during the braking operation might flow to the relief valves  28  and  29  when the switching valve  48  is switched to the opening position. Accordingly, the regeneration amount might become small. Therefore, the turning-regeneration starting pressure is set to a pressure slightly lower than the set pressures of the relief valves  28  and  29  in order not to affect the turning operation or the braking operation of the turning motor  76  and to ensure the regeneration amount. 
     On the downstream side of the switching valve  48  in the turning regeneration passage  45 , a pressure reducing valve  50  is disposed. The pressure reducing valve  50  is a fixed differential pressure valve that operates to have a fixed value of the differential pressure between the inlet and the outlet. 
     A bypass passage  56  that bypasses the pressure reducing valve  50  is connected to the turning regeneration passage  45 . In the bypass passage  56 , a bypass valve  51  that has a cut-off position and a communicating position is disposed. The bypass valve  51  is a pilot operated switching valve. The bypass valve  51  is set to a communicating position (the state illustrated in  FIG. 1 ) in the normal state in which the pilot pressure is not supplied to a pilot chamber  51   a . When the pilot pressure is supplied to the pilot chamber  96   b  of the operation valve  14 , the pilot pressure at the same pressure is simultaneously supplied to the pilot chamber  51   a . And then, the bypass valve  51  is set to the cut-off position. That is, the bypass valve  51  is set to the cut-off position by the pilot pressure for operating the operation valve  14  in the direction along which the piston-side chamber  31  of the boom cylinder  77  contracts. The bypass valve  51  is switched in conjunction with the contraction operation of the boom cylinder  77 . 
     In the supply/discharge passage  30  that connects the piston-side chamber  31  with the operation valve  14  of the boom cylinder  77 , an electromagnetic proportional throttle valve  34  is provided. The electromagnetic proportional throttle valve  34  controls the degree of opening by the output signal of the controller  90 . The electromagnetic proportional throttle valve  34  maintains at the full-open position in the normal state. 
     A boom regeneration passage  52  as a cylinder regeneration passage is connected to the supply/discharge passage  30 . The boom regeneration passage  52  branches from between the piston-side chamber  31  and the electromagnetic proportional throttle valve  34 . The boom regeneration passage  52  is a passage for introducing the working oil returning from the piston-side chamber  31  into the regenerative motor  88 . The turning regeneration passage  45  and the boom regeneration passage  52  are joined together and then connected to the junction regeneration passage  44 . 
     The boom regeneration passage  52  is provided with a switching valve  53  as a cylinder-regeneration-use switching valve controlled by switching control using a signal output from the controller  90 . When the solenoid is not energized, the switching valve  53  is set to the closed position (the state illustrated in  FIG. 1 ) to block the boom regeneration passage  52 . When the solenoid is energized, the switching valve  53  is set to the opening position to open the boom regeneration passage  52 . The switching valve  48  and the switching valve  53  are provided in parallel to each other. 
     The operation valve  14  is provided with a sensor  97  that detects the operating direction and the operating amount of the operation valve  14 . The signal detected by the sensor  97  is output to the controller  90 . Detecting the operating direction and the operating amount of the operation valve  14  is equivalent to detecting the expansion/contraction direction and the expansion/contraction amount of the boom cylinder  77 . Accordingly, the sensor  97  functions as an operating-state detector that detects the operating state of the boom cylinder  77 . It should be noted that as the operating-state detector, instead of the sensor  97 , a sensor that detects the movement direction and the movement amount of the piston rod may be provided at the boom cylinder  77  or a sensor that detects the operating direction and the operating amount of the operating lever  93  may be provided at the operating lever  93 . 
     The controller  90  determines whether the operator intends to cause expansion or contraction of the boom cylinder  77  on the basis of the detection result of the sensor  97 . When the controller  90  determines the expansion operation of the boom cylinder  77 , the controller  90  maintains the electromagnetic proportional throttle valve  34  in the full-open position that is the normal state and maintains the switching valve  53  in the closed position. On the other hand, when the controller  90  determines the contraction operation of the boom cylinder  77 , the controller  90  computes the contraction speed required by the operator for the boom cylinder  77  on the basis of the operating amount of the operation valve  14  and closes the electromagnetic proportional throttle valve  34  while switching the switching valve  53  to the opening position. Accordingly, all of the working oil returning from the boom cylinder  77  is introduced to the regenerative motor  88  to perform boom regeneration. However, when the flow consumed in the regenerative motor  88  is less than the necessary flow to maintain the contraction speed required by the operator for the boom cylinder  77 . The boom cylinder  77  cannot maintain the contraction speed required by the operator. At this time, the controller  90  controls the degree of opening of the electromagnetic proportional throttle valve  34  to return a flow except the flow consumed by the regenerative motor  88  to the tank on the basis of the operating amount of the operation valve  14 , the tilting angle of the regenerative motor  88 , the rotational speed of the electric motor  91 , and similar parameter so as to maintain the contraction speed required by the operator for the boom cylinder  77 . 
     Next, a description will be given of a sub-pump  89  that assists the outputs of the first and second main pumps  71  and  72 . The sub-pump  89  is a variable-displacement type pump with an adjustable tilting angle, and is coupled to the regenerative motor  88  to rotate coaxially with each other. The sub-pump  89  rotates by the drive force of the electric motor  91 . The rotational speed of the electric motor  91  is controlled by the controller  90  through the inverter  92 . The tilting angles of the sub-pump  89  and the regenerative motor  88  are controlled by the controller  90  through respective tilting angle controllers  35  and  36 . 
     A discharge passage  37  is connected to the sub-pump  89 . The discharge passage  37  is formed to branch into a first assist flow path  38  and a second assist flow path  39 . The first assist flow path  38  joins the discharge side of the first main pump  71 . The second assist flow path  39  joins the discharge side of the second main pump  72 . The first and second assist flow paths  38  and  39  are provided with respective first and second electromagnetic proportional throttle valves  40  and  41  whose degrees of opening are controlled by the output signal of the controller  90 . Additionally, the first and second assist flow paths  38  and  39  are provided with respective check valves  42  and  43  on the downstream sides of the first and second electromagnetic proportional throttle valves  40  and  41 . The check valves  42  and  43  accept the flows of the working oil only from the sub-pump  89  to the respective first and second main pumps  71  and  72 . 
     When the sub-pump  89  is rotated by the drive force of the electric motor  91 , the sub-pump  89  assists the outputs of the first and second main pumps  71  and  72 . The controller  90  controls the degrees of opening of the first and second electromagnetic proportional throttle valves  40  and  41  on the basis of the respective pressure signals from the first and second pressure sensors  11  and  21  in order to prorate the working oil discharged from the sub-pump  89  so as to supply the working oil to the discharge sides of the first and second main pumps  71  and  72 . 
     When the working oil is supplied to the regenerative motor  88  through the junction regeneration passage  44  and then the regenerative motor  88  rotates, the rotational force of the regenerative motor  88  acts as an assist force on the electric motor  91  that rotates coaxially with the regenerative motor  88 . Accordingly, the power consumption of the electric motor  91  can be reduced corresponding to the rotational force of the regenerative motor  88 . 
     When the regenerative motor  88  is used as a driving source and the electric motor  91  is used as an electric generator, the tilting angle of the sub-pump  89  is set to zero to ensure an almost unloaded state. 
     The following describes the regenerative controls of the turning regeneration and the boom regeneration. 
     Firstly, a description will be given of the case where the turning regeneration is performed alone. 
     In the case where the controller  90  determines that the pressure detected by the pressure sensor  49  reaches the turning-regeneration starting pressure, the controller  90  switches the switching valve  48  to the opening position. Accordingly, the working oil from the turning circuit  75  is introduced into the regenerative motor  88 . Thus, the turning regeneration is performed. On the other hand, in the case where the controller  90  determines that the boom cylinder  77  is during the expansion operation or stopped on the basis of the detection result of the sensor  97 , the controller  90  set the switching valve  53  to the closed position. Accordingly, the working oil returning from the boom cylinder  77  is not introduced into the regenerative motor  88 . Thus, the boom regeneration is not performed. Here, during the expansion operation or during the stop of the boom cylinder  77 , the pilot pressure is not supplied to the pilot chamber  96   b  of the operation valve  14 . Accordingly, the pilot pressure is not supplied to the pilot chamber  51   a  of the bypass valve  51 . Thus, the bypass valve  51  is set to the communicating position. Accordingly, the working oil from the turning circuit  75  bypasses the pressure reducing valve  50  so as to be introduced into the regenerative motor  88  through the bypass valve  51 . 
     Thus, in the case where the turning regeneration is performed alone, the bypass valve  51  is set to the communicating position. Then, the working oil from the turning circuit  75  is introduced to the regenerative motor  88  without pressure reduction by the pressure reducing valve  50 . Accordingly, efficient regeneration is performed. 
     Here, in the case where the turning regeneration is performed alone, the working oil from the turning circuit  75  is introduced into the regenerative motor  88  without pressure reduction by the pressure reducing valve  50 . Accordingly, the pressure of the turning circuit  75  is likely to be reduced. In the case where the pressure of the turning circuit  75  is decreased compared with the turning-regeneration starting pressure, the switching valve  48  is switched to the closed position and then the turning regeneration is stopped. Subsequently, when the turning motor  76  is during the turning operation, the pressure of the turning circuit  75  is increased again, the switching valve  48  is switched to the opening position at the turning-regeneration starting pressure, and then the turning regeneration is restarted. In this way, the switching valve  48  may repeat opening and closing. In the case where this situation occurs, the pressure variation due to opening and closing of the switching valve  48  may cause vibration. 
     Therefore, in the case where the turning regeneration is performed alone, the controller  90  controls the tilting angle and the rotational speed of the regenerative motor  88  such that the pressure detected by the pressure sensor  49  is not decreased compared with the turning-regeneration starting pressure so as to control the regeneration flow to be introduced into the regenerative motor  88 . Specifically, the controller  90  computes a theoretical turning-regeneration flow from the pressure detected by the pressure sensor  49  so as to control the tilting angle and the rotational speed of the regenerative motor  88  such that the regeneration flow to be introduced into the regenerative motor  88  does not exceed the theoretical turning-regeneration flow. The theoretical turning-regeneration flow is computed using a map that specifies the relationship between the pressure detected by the pressure sensor  49  and the relief flow flowing through the relief valves  28  and  29 . That is, the controller  90  refers to the map to compute the relief flow (theoretical turning-regeneration flow) flowing through the relief valves  28  and  29  from the pressure detected by the pressure sensor  49  so as to control the regeneration flow to be introduced into the regenerative motor  88  not to exceed the relief flow. Accordingly, even in the case where the turning regeneration is performed alone and the working oil from the turning circuit  75  is introduced into the regenerative motor  88  without pressure reduction by the pressure reducing valve  50 , the pressure of the turning circuit  75  can be maintained at a pressure that does not affect the turning operation or the braking operation of the turning motor  76 . 
     Next, a description will be given of the case where the turning regeneration and the boom regeneration are simultaneously performed. 
     In the case where the controller  90  determines that the pressure detected by the pressure sensor  49  reaches the turning-regeneration starting pressure, the controller  90  switches the switching valve  48  to the opening position. Accordingly, the working oil from the turning circuit  75  is introduced into the regenerative motor  88 . Thus, the turning regeneration is performed. On the other hand, in the case where the controller  90  determines that the boom cylinder  77  is during the contraction operation on the basis of the detection result of the sensor  97 , the controller  90  switches the switching valve  53  to the opening position. Accordingly, the working oil returning from the boom cylinder  77  is introduced into the regenerative motor  88 . Thus, the boom regeneration is performed. Here, during the contraction operation of the boom cylinder  77 , the pilot pressure is supplied to the pilot chamber  96   b  of the operation valve  14  and the pilot pressure is simultaneously supplied to the pilot chamber  51   a  of the bypass valve  51 . Therefore, the bypass valve  51  is set to the cut-off position. Accordingly, the working oil from the turning circuit  75  is introduced into the regenerative motor  88  through the pressure reducing valve  50 . 
     Thus, in the case where the turning regeneration and the boom regeneration are simultaneously performed, the bypass valve  51  is set to the cut-off position and then the working oil from the turning circuit  75  is reduced in pressure by the pressure reducing valve  50  so as to be introduced into the regenerative motor  88 . Accordingly, the working oil from the turning circuit  75  is reduced in pressure and then joins the working oil returning from the boom cylinder  77 , so as to be introduced into the regenerative motor  88 . 
     The pressure of the working oil returning from the boom cylinder  77  is smaller than the pressure of the working oil from the turning circuit  75 . The pressure reducing valve  50  functions to reduce the differential pressure between the working oil returning from the boom cylinder  77  and the working oil from the turning circuit  75 . That is, the reduction in pressure of the working oil from the turning circuit  75  by the pressure reducing valve  50  ensures stably joining the working oil from the turning circuit  75  and the working oil returning from the boom cylinder  77  in the junction regeneration passage  44 . 
     As described above, during the turning regeneration, the pressure variation due to opening and closing of the switching valve  48  may cause vibration. However, in the case where the turning regeneration and the boom regeneration are simultaneously performed, the working oil from the turning circuit  75  is reduced in pressure by the pressure reducing valve  50 . Accordingly, the pressure of the turning circuit  75  is a pressure obtained by adding the pressure loss of the pressure reducing valve  50  to the pressure of the regenerative motor  88 . This prevents reduction in pressure of the turning circuit  75 , thus preventing the occurrence of vibration due to pressure reduction in the turning circuit  75 . 
     According to the first embodiment described above, it is possible to obtain the following effects. 
     In the regenerative control of this embodiment, in the case where the turning regeneration is performed alone, the working oil from the turning circuit  75  is introduced into the regenerative motor  88  without pressure reduction by the pressure reducing valve  50 . In the case where the turning regeneration and the boom regeneration are simultaneously performed, the working oil from the turning circuit  75  is reduced in pressure by the pressure reducing valve  50  and then introduced into the regenerative motor  88 . Thus, this control is simple. Additionally, in the case where the turning regeneration is performed alone, the working oil from the turning circuit  75  is introduced into the regenerative motor  88  without pressure reduction. Thus, efficient regeneration is performed. This allows efficient regeneration with the simple regenerative control. 
     The following describes modifications of the first embodiment. 
     In the first embodiment, the case where the bypass valve  51  is the pilot operated switching valve has been described. Instead, the bypass valve  51  may be constituted by an electromagnetic valve. In this case, the bypass valve  51  is set to the cut-off position by a signal output from the controller  90  on the basis of the detection result of the sensor  97 . Specifically, in the case where the controller  90  determines that the boom cylinder  77  is during the contraction operation on the basis of the detection result of the sensor  97 , the controller  90  switches the bypass valve  51  to the cut-off position. 
     In the first embodiment, as an example for performing the regeneration using the working oil returning from the fluid pressure cylinder, the case using the working oil returning from the boom cylinder  77  has been described. However, instead of the boom cylinder  77 , the working oil returning from the arm cylinder for driving the arm or the bucket cylinder for driving the bucket may be used to perform regeneration. In the case where the operation valves  2  and  13  are in the neutral positions, the arm cylinder and the bucket cylinder are frequently in the state where the load is maintained in the rod-side chamber. Therefore, the rod-side chamber may be set as the load-side pressure chamber. 
     &lt;Second Embodiment&gt; 
     A description will be given of a control system  200  for a hybrid construction machine according to a second embodiment of the present invention with reference to  FIG. 2 . Hereinafter, the difference from the first embodiment described above will be mainly described. Like reference numerals designate configurations with functions corresponding or identical to those in the first embodiment, and therefore such configurations will not be further elaborated here. 
     In the control system  200  for the hybrid construction machine, the turning regeneration passage  45  is provided with a switching valve  201  as a turning-regeneration-use switching valve with the functions of the switching valve  48  and the bypass valve  51  in the first embodiment described above. 
     The switching valve  201  is an electromagnetic valve that has three positions of a cut-off position A, a first communicating position B, and a second communicating position C. The switching valve  201  is switched in position by the output signal of the controller  90 . The switching valve  201  includes three ports of an inlet port  201   a , an outlet port  201   b , and a bypass port  201   c . The pressure of the turning circuit  75  is introduced into the inlet port  201   a . The outlet port  201   b  is communicated with the pressure reducing valve  50 . The bypass port  201   c  is communicated with the bypass passage  56 . The bypass passage  56  connects the bypass port  201   c  of the switching valve  201  with the downstream side of the pressure reducing valve  50  in the turning regeneration passage  45 . 
     In the cut-off position A of the switching valve  201 , the respective communications of the outlet port  201   b  and the bypass port  201   c  with the inlet port  201   a  are cut off. In the first communicating position B, the outlet port  201   b  and the bypass port  201   c  are each communicated with the inlet port  201   a . In the second communicating position C, the outlet port  201   b  is communicated with the inlet port  201   a  while the communication of the bypass port  201   c  with the inlet port  201   a  is cut off. 
     In the case where the controller  90  determines that the pressure detected by the pressure sensor  49  is less than the turning-regeneration starting pressure, the controller  90  sets the switching valve  201  to the cut-off position A. In the cut-off position A, the working oil from the turning circuit  75  is not introduced into the regenerative motor  88 . Thus, the turning regeneration is not performed. 
     In the case where the controller  90  determines that the pressure detected by the pressure sensor  49  reaches the turning-regeneration starting pressure and that the boom cylinder  77  is during the expansion operation or stopped on the basis of the detection result of the sensor  97 , the controller  90  sets the switching valve  201  to the first communicating position B and sets the switching valve  53  to the closed position. That is, in the case where the pressure detected by the pressure sensor  49  reaches the turning-regeneration starting pressure and the switching valve  53  is in the closed position, the switching valve  201  is set to the first communicating position B. Accordingly, only the working oil from the turning circuit  75  is introduced into the regenerative motor  88 . Thus, the turning regeneration is performed alone. At this time, the bypass passage  56  is opened by the switching valve  201 . Accordingly, the working oil from the turning circuit  75  is introduced into the regenerative motor  88  while bypassing the pressure reducing valve  50 . Thus, in the case where the turning regeneration is performed alone, the working oil from the turning circuit  75  is introduced into the regenerative motor  88  without pressure reduction by the pressure reducing valve  50 . 
     In the case where the controller  90  determines that the pressure detected by the pressure sensor  49  reaches the turning-regeneration starting pressure and that the boom cylinder  77  is during the contraction operation on the basis of the detection result of the sensor  97 , the controller  90  sets the switching valve  201  to the second communicating position C and sets the switching valve  53  to the opening position. That is, in the case where the pressure detected by the pressure sensor  49  reaches the turning-regeneration starting pressure and the switching valve  53  is in the opening position, the switching valve  201  is set to the second communicating position C. Accordingly, the working oil from the turning circuit  75  and the working oil returning from the boom cylinder  77  are introduced into the regenerative motor  88 . Thus, the turning regeneration and the boom regeneration are simultaneously performed. At this time, the turning regeneration passage  45  is opened by the switching valve  201  while the bypass passage  56  is cut off. Accordingly, the working oil from the turning circuit  75  is introduced into the regenerative motor  88  through the pressure reducing valve  50 . Thus, in the case where the turning regeneration and the boom regeneration are simultaneously performed, the working oil from the turning circuit  75  is reduced in pressure by the pressure reducing valve  50  and then introduced into the regenerative motor  88 . 
     The second embodiment described above provides operations and effects similar to those in the first embodiment, and eliminates a need for the bypass valve  51  that is necessary in the first embodiment, thus reducing the cost. 
     &lt;Third Embodiment&gt; 
     A description will be given of a control system  300  for a hybrid construction machine according to a third embodiment with reference to  FIG. 3 . Hereinafter, the difference from the first embodiment described above will be mainly described. Like reference numerals designate configurations with functions corresponding or identical to those in the first embodiment, and therefore such configurations will not be further elaborated here. 
     In the control system  300  for the hybrid construction machine, the turning regeneration passage  45  is provided with a switching valve  301  as a turning-regeneration-use switching valve with the functions of the switching valve  48 , the pressure reducing valve  50 , and the bypass valve  51  in the first embodiment described above. 
     The switching valve  301  is an electromagnetic valve that has three positions of a cut-off position A, a first communicating position B, and a second communicating position C. The switching valve  301  is switched in position by the output signal of the controller  90 . The switching valve  301  cuts off the turning regeneration passage  45  in the cut-off position A, introduces the working oil from the turning circuit  75  into the regenerative motor  88  without pressure reduction in the first communicating position B, and reduces the pressure of the working oil from the turning circuit  75  by throttling so as to introduce the working oil into the regenerative motor  88  in the second communicating position C. 
     In the case where the controller  90  determines that the pressure detected by the pressure sensor  49  is less than the turning-regeneration starting pressure, the controller  90  sets the switching valve  301  to the cut-off position A. In the cut-off position A, the working oil from the turning circuit  75  is not introduced into the regenerative motor  88 . Thus, the turning regeneration is not performed. 
     In the case where the controller  90  determines that the pressure detected by the pressure sensor  49  reaches the turning-regeneration starting pressure and that the boom cylinder  77  is during the expansion operation or stopped on the basis of the detection result of the sensor  97 , the controller  90  sets the switching valve  301  to the first communicating position B and sets the switching valve  53  to the closed position. That is, in the case where the pressure detected by the pressure sensor  49  reaches the turning-regeneration starting pressure and the switching valve  53  is in the closed position, the switching valve  301  is set to the first communicating position B. Accordingly, only the working oil from the turning circuit  75  is introduced into the regenerative motor  88 . Thus, the turning regeneration is performed alone. At this time, the working oil from the turning circuit  75  is introduced into the regenerative motor  88  without pressure reduction by the switching valve  301 . Thus, in the case where the turning regeneration is performed alone, the working oil from the turning circuit  75  is introduced into the regenerative motor  88  without pressure reduction. 
     In the case where the controller  90  determines that the pressure detected by the pressure sensor  49  reaches the turning-regeneration starting pressure and that the boom cylinder  77  is during the contraction operation on the basis of the detection result of the sensor  97 , the controller  90  sets the switching valve  301  to the second communicating position C and sets the switching valve  53  to the opening position. That is, in the case where the pressure detected by the pressure sensor  49  reaches the turning-regeneration starting pressure and the switching valve  53  is in the opening position, the switching valve  301  is set to the second communicating position C. Accordingly, the working oil from the turning circuit  75  and the working oil returning from the boom cylinder  77  are introduced into the regenerative motor  88 . Thus, the turning regeneration and the boom regeneration are simultaneously performed. At this time, the working oil from the turning circuit  75  is throttled by the switching valve  301  and then introduced into the regenerative motor  88 . Thus, in the case where the turning regeneration and the boom regeneration are simultaneously performed, the working oil from the turning circuit  75  is reduced in pressure by throttling and then introduced into the regenerative motor  88 . 
     The third embodiment described above provides operations and effects similar to those in the first embodiment, and eliminates a need for the pressure reducing valve  50 , the bypass passage  56 , and the bypass valve  51  that are necessary in the first embodiment, thus reducing the cost. 
     The following describes modifications of the third embodiment. 
     The switching valve  301  may be constituted by an electromagnetic proportional throttle valve whose degree of opening is controlled by the output signal of the controller  90 . In this case, the controller  90  sets the opening area of the throttling of the switching valve  301  to maximum in the case where the turning regeneration is performed alone. On the other hand, in the case where the turning regeneration and the boom regeneration are simultaneously performed, the controller  90  adjusts the opening area of the throttling of the switching valve  301  such that the differential pressure between the inlet and the outlet of the switching valve  301  becomes constant irrespective of the passing flow. Specifically, the controller  90  computes a theoretical turning-regeneration flow from the pressure detected by the pressure sensor  49  and adjusts the opening area of the throttling on the basis of the theoretical turning-regeneration flow. It should be noted that the pilot pressure may be controlled by the output signal of the controller  90  so as to control the opening area of the throttling using this pilot pressure in the case where the switching valve  301  is thus configured. 
     Embodiments of this invention were described above, but the above embodiments are merely examples of applications of this invention, and the technical scope of this invention is not limited to the specific constitutions of the above embodiments. 
     This application claims priority based on Japanese Patent Application No.2012-245559 filed with the Japan Patent Office on Nov. 7, 2012, the entire contents of which are incorporated into this specification.