Abstract:
Disclosed is a controller of a hybrid, construction machine wherein electric power is generated by utilizing the standby flow rate of first and second main pumps, and the standby flow rate is converted into energy. Pilot channels are connected to the upstream side of on/off valves which are closed when first and second main pumps ensure a standby flow rate, and a controller unit judges that the first and second main pumps are discharging at the standby low rate based on pressure signals from first and second pressure sensors, and brings first and second solenoid valves to an open position.

Description:
TECHNICAL FIELD 
     This invention relates to a controller of a hybrid construction machine using an electric motor as a drive source. 
     BACKGROUND 
     A hybrid structure in a construction machine such as a power shovel uses, for example, an excess output of an engine to rotate a generator for electric power generation. Then, the generated electric power is stored in a battery and the electric motor is driven by the electric power stored in the battery to actuate an actuator. Also, discharge energy from the actuator is used to rotate the generator for electric power generation. Then, similarly, the generated electric power is stored in the battery, and the electric motor is driven by the electric power of the battery for actuation of the actuator. 
     In a power shovel or the like, even when an actuator in a work mechanical system is stopped, the engine is maintained in a rotating state. In this event, since a pump rotates together with the engine, the pump discharges so-called standby flow rate. 
     [Patent Literature 1] JP-A 2002-275945 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     In the controllers in the related art as described above, since a so-called standby flow rate discharged from a pump when an actuator of a work mechanical system is stopped is simply sent back to a tank, most of the standby flow rate disadvantageously causes a loss of energy. 
     It is an object of the present invention to provide a controller of a hybrid construction machine which is adapted to use a standby flow rate of a main pump to enable a power generation function in order to achieve energy regeneration. 
     Solution to Problem 
     A first invention provides a controller of a hybrid construction machine which is equipped with a variable displacement type of a main pump, a circuit system connected to the main pump and including a plurality of operated valves, a neutral channel guiding discharge oil of the main pump toward a tank when all the operated valves provided in the circuit system are maintained in a neutral position, a throttle provided in a portion of the neutral channel downstream of a most-downstream operated valve of the operated valves for generating a pilot pressure, a pilot channel guiding a pressure generated between the most-downstream operated valve and the throttle, a regulator connected to the pilot channel and controlling a tilting angle of the main pump, and a pressure sensor detecting a pressure in the pilot channel. The controller of a hybrid construction machine comprises an on/off valve that is provided in a portion of the neutral channel between the most-downstream operated valve and a throttle for generating a pilot pressure, and is maintained in an open position under normal conditions and switched to a closed position when a pilot pressure in the pilot channel reaches a set pressure or higher and the main pump ensures a standby flow rate; a variable displacement type of a sub-pump connected to a discharge of the main pump; an electric motor for rotating the sub-pump; an assist hydraulic motor that rotates the electric motor; a solenoid valve that is provided in a connection process between the main pump and the assist hydraulic motor and performs closing/opening operation; and a controller unit. The pilot channel is connected to an upstream side of the on/off valve. The controller unit closes the on/off valve and switches the solenoid valve to an open position when determining, based on a pressure signal from the pressure sensor, that the main pump is discharging a standby flow rate. 
     A second invention provides the controller in which the main pump and the solenoid valve are connected to each other through a standby channel, and the standby channel is connected to a connection process between the main pump and a most-upstream operated valve of the operated valves. 
     A third invention provides the controller in which the sub-pump, the assist hydraulic motor and the electric motor rotate coaxially, and the electric motor has a function as a generator. 
     A fourth invention provides the controller in which oil discharged from or supplied to an actuator can be introduced into the assist hydraulic motor. 
     Advantageous Effects of Invention 
     According to the first invention, the standby flow rate uselessly discharged in the related art can be regenerated as energy of power generation, thus achieving energy conservation. 
     According to the second invention, the loss of pressure of the fluid guided to a standby channel can be reduced. 
     According to the third invention, the electric motor can be also used as a generator, thus simplifying the entire structure. 
     According to the fourth invention, since a part of the oil discharged from or supplied to an actuator can be introduced to the assist hydraulic motor, even while the actuator is operated, the power generation function can be fulfilled. 
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  illustrates a controller of a power shovel according to a first embodiment, which includes a variable displacement type of first and second main pumps MP 1 , MP 2  which drive an engine E. The first and second main pumps MP 1 , MP 2  rotate coaxially. Note that reference numeral  1  denotes a generator which is mounted on the engine E and uses the surplus power of the engine to implement the function of generating electric power. 
     The first main pump MP 1  is connected to a first circuit system S 1 . To the first circuit system S 1  are connected, in order of upstream toward downstream, a operated valve  2  for controlling a rotation motor RM, an operated valve  3  for controlling an arm cylinder (not shown), a boom-in-second-gear operated valve  4  for controlling a boom cylinder BC, an auxiliary operated valve  5  for controlling an auxiliary attachment (not shown), and a first travel-motor operated valve  6  for controlling a first travel motor for left traveling (not shown). 
     Each of the operated valves  2  to  6  is connected to the first main pump MP 1  via a neutral channel  7  and a parallel passage  8 . 
     A throttle  9  is disposed on the neutral channel  7  downstream of the first travel-motor operated valve  6  and generates a pilot pressure. The throttle  9  generates a higher pilot pressure on the upstream side of the throttle  9  with a higher rate of flow passing through the throttle  9 , and a lower pilot pressure with a lower rate of flow. 
     When all the operated valves  2  to  6  are in or near the neutral position, the neutral channel  7  guides all or part of the oil discharged from the first main pump MP 1  to a tank T. At this condition, the rate of flow passing through the throttle  9  is increased, so that a high pilot pressure is generated as described above. 
     On the other hand, when switching the operated valves  2  to  6  in a full stroke position, the neutral channel  7  is closed to block the flow of fluid. In this case, accordingly, the rate of flow passing through the throttle  9  is almost zero, which means that a pilot pressure of zero is kept. 
     However, depending on manipulated variables of the operated valves  2  to  6 , a portion of the pump discharge flow is directed to an actuator and another portion is directed from the neutral channel  7  to the tank. As a result, the throttle  9  generates a pilot pressure in accordance with the rate of flow passing through the neutral channel  7 . In other words, the throttle  9  generates a pilot pressure in accordance with the manipulated variables of the operated valves  2  to  6 . 
     An on/off valve  10  is mounted in the neutral channel  7  and between the most-downstream operated valve  6  and the throttle  9 . The on/off valve  10  has a solenoid  10   a  connected to a controller unit C. In other words, the on/off valve  10  is opened/closed in response to a command from the controller unit C. When being in a normal position, the on/off valve  10  is maintained in a full open state by a spring force of a spring  10   b . Upon excitation of the solenoid  10   a , the on/off valve  10  is switched against a spring force of the spring  10   b  and maintained in a closed state. 
     A pilot channel  11  is connected to a point of the neutral channel  7  between the operated valve  6  and the on/off valve  10 . The pilot channel  11  is connected to a regulator  12  which controls the tilting angle of the first main pump MP 1 . 
     The regulator  12  controls the discharge rate of the first main pump MP 1  in inverse proportion to the pilot pressure. Accordingly, when the operated valves  2  to  6  are fully stroked and then the flow rate in the neutral channel  7  changes to zero to reduce the pilot pressure to zero, the discharge rate of the first main pump MP 1  is maintained at maximum. 
     A first pressure sensor  13  is connected to the pilot channel  11  configured as described above, and detects a pressure signal which is then applied to the controller unit C. The pilot pressure in the pilot channel  11  varies in accordance with the manipulated variable of the operated valve. As a result, the pressure signal detected by the first pressure sensor  13  is proportional to the flowrate required by the first circuit system S 1 . 
     When the pressure signal from the first pressure sensor  13  reaches a set pressure, the controller unit C energizes the solenoid  10   a  to switch the on/off valve  10  to the closed position. Timing of such switching of the on/off valve  10  to the closed position is the time when the operated valves  2  to  6  are maintained around the neutral position and the pressure in the upstream side of the throttle  9  builds up to a set pressure. The controller unit C previously stores the set pressure. When the on/off valve  10  is switched to the closed position as described above, the pressure in the pilot channel  11  still acts on the regulator  12 , so that the first main pump MP 1  is maintained at a required tilting angle. As a result, the first main pump MP 1  is allowed to ensure a standby flow rate. 
     Upon switching of any of the operated valves  2  to  6 , a signal pressure of the pressure sensor  13  is reduced. Then, when the signal pressure is reduced to a preset pressure, the controller unit C de-energizes the solenoid  10   a  so that the on/off valve  10  returns to the open position by a spring force of the spring  10   b . Also, the controller unit C de-energizes the solenoid valve  58  to close the passages  55 ,  57 . 
     On the other hand, the second main pump MP 2  is connected to a second circuit system S 2 . To the second circuit system are connected, in order of upstream toward downstream, an operated valve  14  for controlling a second travel motor for right traveling (not shown), an operated valve  15  for controlling a bucket cylinder (not shown), an operated valve  16  for controlling the boom cylinder BC, and an arm-in-second-gear operated valve  17  for controlling the arm cylinder (not shown). Note that the operated valve  16  is provided with a sensor for detecting a manipulated direction and a manipulated variable of the operated valve  16 , and the manipulation signal is transmitted to the controller unit C. 
     Each of the operated valves  14  to  17  is connected to the second main pump MP 2  through the neutral channel  18 . The operated valve  15  and the operated valve  16  are connected to the second main pump MP 2  through a parallel passage  19 . 
     A throttle  20  is provided in the neutral channel  18  downstream of the operated valve  17 . The throttle  20  is exactly identical in function with the throttle  9  in the first circuit system S 1 . 
     An on/off valve  21  is provided in the neutral channel  18  between the most downstream operated valve  17  and the throttle  20 . The on/off valve  21  is structured similarly to the on/off valve  10  in the first circuit system S 1 . Specifically, the on/off valve  21  has a solenoid  21   a  connected to the controller unit C, and opens/closes in response to an instruction from the controller unit C. When in the normal position, the on/off valve  21  is maintained in the full open state by a spring force of a spring  21   b . Upon energization of the solenoid  21   a , the on/off valve  21  is switched against the spring force of the spring and maintained in the closed position. 
     A pilot channel  22  is connected to a portion of the neutral channel  18  between the operated valve  17  and the on/off valve  21 , and also connected to a regulator  23  for controlling the tilting angle of the second main pump MP 2 . 
     The regulator  23  controls the discharge rate of the second main pump MP 2  in inverse proportion to the pilot pressure. Accordingly, when the operated valves  14  to  17  are fully stroked so that the flow rate in the neutral channel  18  changes to zero and the pilot pressure becomes zero, a maximum discharge rate of the second main pump MP 2  is maintained. 
     A second pressure sensor  24  is connected to the pilot channel  22  configured as described above, and detects a pressure signal which is then transmitted to the controller unit C. The pilot pressure in the pilot channel  22  varies in accordance with the manipulated variable of the operated valve. As a result, the pressure signal detected by the second pressure sensor  24  is proportional to the flowrate required by the second circuit system S 2 . 
     When the pressure signal from the second pressure sensor  24  reaches a set pressure, the controller unit C energizes the solenoid  21   a  to switch the on/off valve  21  to the closed position. Timing of such switching of the on/off valve  21  to the closed position is the time when the operated valves  14  to  17  are maintained around the neutral position and the pressure in the upstream side of the throttle  20  builds up to a set pressure. The controller unit C previously stores the set pressure. When the on/off valve  21  is switched to the closed position as described above, the pressure in the pilot channel  22  at this time acts on the regulator  23 , so that the second main pump MP 2  is maintained at a required tilting angle. As a result, the second main pump MP 2  is allowed to ensure a standby flow rate. 
     Upon switching of any of the operated valves  14  to  17 , a signal pressure of the pressure sensor  24  is reduced. Then, when the signal pressure is reduced to a preset pressure, the controller unit C de-energizes the solenoid  21   a  so that the on/off valve  21  returns to the open position by a spring force of the spring  21 . Also, the controller unit C de-energizes the solenoid valve  59  to close the passages  56 ,  57 . 
     A generator  1  provided in the engine E is connected to a battery charger  25 . The electric power generated by the generator  1  is supplied through the battery charger  25  to a battery  26 . 
     The battery charger  25  is adapted to charge the battery  26  even when it is connected to a usual household power source  27 . That is, the battery charger  25  is connectable to an independent power source other than the controller. 
     On the other hand, an actuator port of the rotation-motor operated valve  2  connected to the first circuit system S 1  is connected to passages  28 ,  29  which communicate with the rotation motor RM. Brake valves  30 ,  31  are respectively connected to the passages  28 ,  29 . When the rotation motor operated valve  2  is kept in its neutral position, the actuator port is closed, so that the rotation motor RM maintains its stop state. 
     Upon switching of the rotation-motor operated valve  2  from this position in either direction, one passage  28  of the passages  28 ,  29  is connected to the first main pump MP 1 , while the other passage  29  is connected to the tank. As a result, pressure oil is supplied through the passage  28  to rotate the rotation motor RM, while the return oil flows from the rotation motor RM through the passage  29  back to the tank. 
     On the other hand, when the rotation-motor operated valve  2  is switched in the direction opposite to the above-described direction, the pump discharge oil flows into the passage  29 , while the passage  28  is connected to the tank, so that the rotation motor RM rotates in the opposite direction. 
     In this manner, during the operation of the rotation motor RM, the brake valve  30  or  31  functions as a relief valve. Then, when the pressure in the passage  28 ,  29  becomes a set pressure or higher, the brake valve  30 ,  31  is opened to maintain the pressure in the passage  28 ,  29  at the set pressure. When the rotation-motor operated valve  2  is moved back to the neutral position while the rotation motor RM is rotating, the actuator port of the operated valve  2  is closed. Even when the actuator port of the operated valve  2  is closed in this manner, the rotation motor RM continues to rotate by its inertial energy. By rotating by its inertial energy, the rotation motor RM acts as a pump. At this stage, the passages  28 ,  29 , the rotation motor RM and the brake valve  30  or  31  form a closed circuit. The brake valve  30  or  31  converts the inertial energy to thermal energy. 
     On the other hand, upon switching of the operated valve  16  is switched from the neutral position in either direction, the pressure oil fluid flowing from the second main pump MP 2  is supplied through a passage  32  to a piston chamber  33  of the boom cylinder BC, and the return oil flows from a rod chamber  34  of the boom cylinder BC through a passage  35  to the tank, resulting in extension of the boom cylinder BC. 
     In contrary, upon switching of the operated valve  16  in the direction opposite to the above-described direction, a pressure oil flowing from the second main pump MP 2  is supplied through the passage  35  to the rod chamber  34  of the boom cylinder BC, while the return fluid flows from the piston chamber  33  through the passage  32  back to the tank, resulting in contraction of the boom cylinder BC. Note that the boom-in-second-gear operated valve  3  is switched in conjunction with the operated valve  16 . 
     A proportional solenoid valve  36 , the degree of opening of which is controlled by the controller unit C, is provided in the passage  32  connected between the piston chamber  33  of the boom cylinder BC and the operated valve  16  as described above. Note that the proportional solenoid valve  36  is kept in the full open position when it is in its normal state. 
     Next, a variable displacement sub-pump SP for assisting in the output of the first, second main pump MP 1 , MP 2  will be described. 
     The variable displacement sub-pump SP rotates by a drive force of an electric motor MG also serving as a generator, and a variable displacement assist hydraulic motor AM also rotates coaxially by the drive force of the electric motor MG. The electric motor MG is connected to an inverter I which is connected to the battery  26 . The inverter I is connected to the controller unit C. Thus, the controller unit C can control a rotational speed and the like of the electric motor MG. 
     Tilting angles of the sub pump SP and the assist hydraulic motor AM are controlled by tilt-angle control units  37 ,  38  which are controlled through output signals of the controller unit C. 
     The sub-pump SP is connected to a discharge passage  39 . The discharge passage  39  is divided into two channels, a first assist channel  40  that merges with the discharge side of the first main pump MP 1  and a second assist channel  41  that merges with the discharge side of the second main pump MP 2 . The first, second assist channels  40 ,  41  are respectively provided with first, second solenoid proportional throttling valves  42 ,  43  the degrees of openings of which are controlled by signals output from the controller unit C. 
     Note that reference numerals  44 ,  45  in  FIG. 1  denote check valves fitted in the first, second assist channels  40 ,  41 . The check valves  44 ,  45  permit the fluid to flow from the sub pump SP to the first, second main pumps MP 1 , MP 2  only. 
     On the other hand, the assist hydraulic motor AM is connected to a connection passage  46 . The connection passage  46  is connected through the guiding passage  47  and check valves  48 ,  49  to the passages  28 ,  29  which are connected to the rotation motor RM. In addition, a solenoid directional control valve  50 , the opening/closing of which is controlled by the controller unit C, is provided in the guiding passage  47 . A pressure sensor  51  is disposed between the solenoid directional control valve  50  and the check valves  48 ,  49  for detecting a pressure of the rotation motor RM in the turning operation or a pressure of it in the braking operation. A pressure signal of the pressure sensor  51  is applied to the controller unit C. 
     A pressure relief valve  52  is provided in the guiding passage  47  downstream from the solenoid directional control valve  50  for the flow from the rotation motor RM to the connection passage  46 . The pressure relief valve  52  maintains the pressure in the passages  28 ,  29  to prevent so called runaway of the rotation motor RM in the event of a failure occurring in the system of the passage  46 , for example, in the solenoid directional control valve  50  or the like. 
     Another guiding passage  53  is provided between the boom cylinder BC and the proportional solenoid valve  36  and communicates with the connection passage  46 . A solenoid on/off valve  54  controlled by the controller unit C is disposed in the guiding passage  53 . 
     The assist hydraulic motor AM arranged as described above is also connected to the first, second main pumps MP 1 , MP 2  over the following connection path. Specifically, the standby channels  55 ,  56  are respectively connected to the discharge sides of the first, second main pumps MP 1 . MP 2  and upstream sides of the most-upstream operated valves  2 ,  14 . The standby channels  55 ,  56  are connected through the merging passage  57  to the connection passage  46 . Then, the first, second solenoid valves  58 ,  59  are respectively provided in the standby channels  55 ,  56 . Each of the first, second solenoid valves  58 ,  59  is equipped with a spring  58   a ,  59   a  at one end and a solenoid  58   b ,  59   b  at the other end, and the solenoid  58   b ,  59   b  is connected to the controller unit C. The first, second solenoid valve  58 ,  59  is usually maintained in the closed position by the spring force of the spring  58   a,    59   a , and switched to the open position at the time when the solenoid  58   b ,  59   b  are energized by a signal from the controlled. 
     For the purpose of reducing the pressure loss in the fluid introduced into the standby channel  55 ,  56 , the standby channel  55 ,  56  is connected to a point on the discharge side of the first, second main pump MP 1 , MP 2  and upstream of the most-upstream operated valve  2 ,  14 . 
     Note that reference numeral  60  denotes a check valve provided in the merging passage  57  for directing the pressure oil flowing from the first, second solenoid valves  58 ,  59  and the standby cannels  55 ,  56  toward the connection passage  46 . 
     The operation according to the first embodiment will be described below. 
     When the operated valves  2  to  6 ,  14  to  17  in each of the first, second circuit systems S 1 , S 2  are kept in their neutral positions now, the total amount of oil discharged from the first, second main pump MP 1 , MP 2  is introduced from the neutral channel  7 ,  18  through the throttle  9 ,  20  to the tank. When the total amount of pump discharge fluid is directed through the throttle  9 ,  20  to the tank in this manner, the pressure in the upstream side of the throttle  9 ,  20  builds up, and the pressure at this time is directed through the pilot channel  11 ,  22  to the regulator  12 ,  23 . As a result, by action of the pilot pressure thus building up, the regulator  12 ,  23  reduces the tilting angle of the first, second main pump MP 1 , MP 2 , thus maintaining the standby flow rate. 
     Then, the pilot pressure in the pilot channel  11 ,  22  reaches a set pressure, the controller unit C detects the pressure by receiving a pressure signal from the first, second pressure sensor  13 ,  24 , and switches the on/off valve  10 ,  21  to the closed position. Even when the on/off valve  10 ,  21  is switched to the closed position, the pressure in the pilot channel  11 ,  22  acts on the regulator  12 ,  23 , so that the first, second main pump MP 1 , MP 2  discharge a standby flow. Also, at this time, the controller unit C energizes the solenoid  58   b ,  59   b  of the first, second solenoid valve  58 ,  59  so that the solenoid valve is switched from the closed position to the open position. 
     The standby flow discharged from the first, second main pump MP 1 , MP 2  is supplied to the assist hydraulic motor AM through the standby channel  55 ,  56 , the first, second solenoid valve  58 ,  59 , the merging passage  57  and the check valve  60 . 
     For introducing the standby flows of the first, second main pumps MP 1 , MP 2  to the assist hydraulic motor AM as described above, the controller unit C operates the tilting angle control unit  38  to maintain the tilting angle of the assist hydraulic motor AM to a pre-stored set tilting angle, and the tilting angle control unit  37  to set the tilting angle of the sub pump SP to zero, and maintains the electric motor MG in a regenerative state through the inverter I. 
     Accordingly, the electric motor/generator MG fulfills an electric generation function when rotated by a drive force of the assist hydraulic motor AM. That is, in the first embodiment, the electric motor MG is operated to exercise a function as a generator by use of the standby flows of the first, second main pumps MP 1 , MP 2 . The electric power thus generated is stored in the battery  26  and the electric power stored in the battery  26  can be used as a power source for the electric motor MG. 
     The above description has been given on the assumption that all the operated valves  2  to  6 ,  14  to  17  of both the first and second circuit systems S 1 , S 2  are maintained in the neutral position, but when the operated valves  2  to  6  or the operated valves  14  to  17  of either the first circuit system S 1  or the second circuit system S 2  are in the neutral position, the assist hydraulic motor AM is also rotated by the standby flow. In this case, the controller unit C switches either the solenoid valve  58  or  59  to the open position on the basis of a pressure signal from the corresponding pressure sensor  13  or  24 , and maintains the other solenoid valve  59  or  58  in the closed position. Accordingly, the pump standby flow of one of the first and second main pumps MP 1 , MP 2  is supplied to the assist hydraulic motor AM, and the torque of the assist hydraulic motor AM causes the electric motor MG to fulfill the power generation function. 
     Next, the use of an assist force of the sub-pump SP will be described. In the first embodiment, an assist flow for the sub-pump SP is pre-set. Within the range of the pre-set assist flow, the controller unit C determines how to most efficiently control the tilting angle of the sub-pump SP, the tilting angle of the assist hydraulic motor AM, the rotational speed of the electric motor MG and the like, and perform control on each of them. 
     When switching an operated valve in either the first circuit system S 1  or the second circuit system S 2 , if the on/off valves  10 ,  21  are in the closed position, the controller unit C switches the on/off valves  10 ,  21  to the open position. If the on/off valves  10 ,  21  are maintained in the open position, the pilot pressures in the pilot channels  11 ,  22  are reduced. Then, the signals representative of the reduced pilot pressures are transmitted to the controller unit C through the first, second sensors  13 ,  24 , and the controller unit C switches the first, second solenoid valves  58 ,  59  to the closed position illustrated in  FIG. 1 . As a result, the first, second main pumps MP 1 , MP 2  increases the discharge rate with a reduction in pilot pressure, and the total amount of discharge is supplied to the actuators connected to the first, second circuit systems S 1 , S 2 . 
     When the discharge rate from the first main pump MP 1  or the second main pump MP 2  is increased as described above, the controller unit C maintains the electric motor MG in the state of rotation at all times. The drive source of the electric motor MG is the electric power stored in the battery  26 . In this regard, part of this electric power has been stored by use of the standby flow of the first, second main pump MP 1 , MP 2  as described earlier, thus enhanced energy efficiency. 
     If the sub-pump SP is rotated by the drive force of the electric motor MG, the sub-pump SP discharges an assist flow. The controller unit C controls the degrees of openings of the first, second proportional solenoid throttling valves  42 ,  43  in response to the pressure signals from the first, second pressure sensors  13 ,  24 , to proportionally divide the discharge flow of the sub-pump SP for delivery to the first, second circuit systems S 1 , S 2 . 
     On the other hand, if the rotation-motor operated valve  2  is switched, for example, in one of the opposite directions in order to drive the rotation motor RM connected to the first circuit system S 1 , the passage  28  communicates with the first main pump MP 1 , while the other passage  29  communicates with the tank, thus rotating the rotation motor RM. The turning pressure at this time is maintained at a set pressure of the brake valve  30 . If the operated valve  2  is switched in the other direction, the passage  29  communicates with the first main pump MP 1 , while the passage  28  communicates with the tank, thus rotating the rotation motor RM. The turning pressure at this time is also maintained at a set pressure of the brake valve  31 . 
     If the rotation-motor operated valve  2  is switched to the neutral position during the rotation operation of the rotation motor RM, a closed circuit is constituted between the passages  28 ,  29  as described earlier, and the brake valve  30  or  31  keeps the brake pressure in the closed circuit for conversion of inertial energy to thermal energy. 
     The pressure sensor  51  detects the turning pressure or the brake pressure and applies a signal indicative of the detected pressure to the controller unit C. When the detected pressure is lower than the set pressure of brake valve  30 ,  31  within a range of it having no influence on the turning operation of the rotation motor RM or the braking operation, the controller unit C switches the solenoid directional control valve  50  from the closed position to the open position. By thus switching the solenoid directional control valve  50  to the open position, the pressure oil introduced into the rotation motor RM flows into the guiding passage  47  and then through the pressure relief valve  52  and the connection passage  46  into the assist hydraulic motor AM. 
     At this stage, the controller unit C controls the tilting angle of the assist hydraulic motor AM in response to the pressure signal from the pressure sensor  51  as follows. 
     Specifically, if the pressure in the passage  28  or  29  is not maintained at a level required for the turning operation or the braking operation, the rotation motor RM cannot be rotated or braked 
     For this reason, in order to maintain the pressure in the passage  28  or  29  to be equal to the turning pressure or the brake pressure, the controller unit C controls the load on the rotation motor RM while controlling the tilting angle of the assist hydraulic motor AM. Specifically, the controller unit C controls the tilting angle of the assist hydraulic motor AM such that the pressure detected by the pressure sensor  51  becomes approximately equal to the turning pressure of the rotation motor RM or the brake pressure. 
     If the assist hydraulic motor AM obtains a torque as described above, then the torque acts on the electric motor MG which rotates coaxially with the assist hydraulic motor AM. In this regard, the torque of the assist hydraulic motor AM acts as an assist force intended to the electric motor MG. This makes it possible to reduce the power consumption of the electric motor MG by an amount of power corresponding to the torque of the assist hydraulic motor AM. 
     The torque of the assist hydraulic motor AM may be used to assist the torque of the sub-pump SP. In this event, the assist hydraulic motor AM and the sub-pump SP are combined with each other to fulfill the pressure conversion function. 
     That is, the pressure flowing into the connection passage  46  is often lower than the pump discharge pressure. For the purpose of using the low pressure to maintain a high discharge pressure of the sub-pump SP, the assist hydraulic motor AM and the sub-pump SP are adapted to fulfill the booster function. 
     Specifically, the output of the assist hydraulic motor AM depends on a product of a displacement volume Q 1  per rotation and the pressure P 1  at this time. Likewise, the output of the sub-pump SP depends on a product of a displacement volume Q 2  per rotation and the discharge pressure P 2 . In the embodiment, since the assist hydraulic motor AM and the sub-pump SP rotate coaxially, equation Q 1 ×P 1 =Q 2 ×P 2  must be established. For this purpose, for example, assuming that the displacement volume Q 1  of the assist hydraulic motor AM is three times as high as the displacement volume Q 2  of the sub-pump SP, that is, Q 1 =3Q 2 , the equation Q 1 ×P 1 =Q 2 xP 2  results in 3Q 2 ×P 1 =Q 2 ×P 2 . Dividing both sides of this equation by Q 2  gives 3P 1 =P 2 . 
     Accordingly, if the tilting angle of the sub-pump SP is changed to control the displacement volume Q 2 , a predetermined discharge pressure of the sub-pump SP can be maintained using the output of the assist hydraulic motor AM. In other words, the hydraulic pressure from the rotation motor RM can be built up and then discharged from the sub-pump SP. 
     In this regard, the tilting angle of the assist hydraulic motor AM is controlled such that the pressure in the passage  28 ,  29  is maintained to be equal to the turning pressure or the brake pressure as described earlier. For this reason, in the case of using the pressure oil from the rotation motor RM, the tilting angle of the assist hydraulic motor AM is logically determined. After the tilting angle of the assist hydraulic motor AM has been determined in this manner, the tilting angle of the sub-pump SP is controlled in order to fulfill the aforementioned pressure conversion function. 
     If the pressure in the system of the passage  46  is reduced below the turning pressure or the brake pressure for any reasons, the controller unit C closes the solenoid directional control valve  50  on the basis of the pressure signal sent from the pressure sensor  51  such that the rotation motor RM is not affected. 
     When a pressure-oil leak occurs in the connection passage  46 , the pressure relief valve  52  operates to prevent the pressure in the passage  28 ,  29  from reducing more than necessary, thus preventing runaway of the rotation motor RM. 
     Next, a description will be given of control for the boom cylinder BC. 
     Upon switching of the operated valve  16  in order to actuate the boom cylinder BC, a sensor (not shown) provided in the operated valve  16  detects the manipulated direction and the manipulated variable of the operated valve  16 , and sends the manipulation signal to the controller unit C. 
     The controller unit C determines in response to the manipulation signal of the sensor whether the operator is about to move up or down the boom cylinder BC. If the controller unit C receives a signal representative of moving-up of the boom cylinder BC, the controller unit C maintains the proportional solenoid valve  36  in the normal state. In other words, the proportional solenoid valve  36  is kept in the full-open position. At this time, the controller unit C keeps the solenoid on/off valve  54  in the closed position which is not shown and controls the rotational speed of the electric motor MG and the tilting angle of the sub-pump SP. 
     On the other hand, if the controller unit C receives the signal representative of moving-down of the boom cylinder BC from the sensor, the controller unit C calculates a moving-down speed of the boom cylinder BC desired by the operator in accordance with the manipulated variable of the operated valve  16 , and closes the proportional solenoid valve  36  and switches the solenoid on/off valve  54  to the open position. 
     By closing the proportional solenoid valve  36  and switching the solenoid on/off valve  54  to the open position as described above, the total amount of oil returning from the boom cylinder BC is supplied to the assist hydraulic motor AM. However, if the flow rate consumed by the assist hydraulic motor AM is lower than the flow rate required for maintaining the moving-down speed desired by the operator, the boom cylinder BC cannot maintains the moving-down speed desired by the operator. In this event, the controller unit C controls, based on the manipulated variable of the operated valve  16 , the tilting angle of the assist hydraulic motor AM, the rotational speed of the electric motor MG and the like, the degree of opening of the proportional solenoid valve  36  to direct a greater flow rate than that consumed by the assist hydraulic motor AM back to the tank, thus maintaining the moving-down speed of the boom cylinder BC desired by the operator. 
     On the other hand, upon the pressure oil being supplied to the assist hydraulic motor AM, the assist hydraulic motor AM rotates and this torque acts on the electric motor MG which rotates coaxially. The torque of the assist hydraulic motor AM acts as an assist force intended to the electric motor MG. Thus, the power consumption can be reduced by an amount of power corresponding to the torque of the assist hydraulic motor AM. 
     In this regard, the sub-pump SP can be rotated using only a torque of the assist hydraulic motor AM without a power supply to the electric motor MG. In this case, the assist hydraulic motor AM and the sub-pump SP fulfill the pressure conversion function as in the aforementioned case. 
     Next, the simultaneous actuation of the rotation motor RM for the turning operation and the boom cylinder BC for the moving-down operation will be described. 
     When the boom cylinder BC is moved down during the rotation of the rotation motor RM, the pressure oil from the rotation motor RM and the return oil from the boom cylinder BC join in the connection passage  46  and flow into the assist hydraulic motor AM. 
     In this regard, if the pressure in the connection passage  46  rises, the pressure in the guiding passage  47  also rises with this pressure rise. Even if the pressure in the guiding passage  47  exceeds the turning pressure or the brake pressure of the rotation motor RM, it has no influence on the rotation motor RM because the check valves  48 ,  49  are provided. 
     If the pressure in the connection passage  46  reduces lower than the turning pressure or the brake pressure as described earlier, the controller unit C closes the solenoid directional control valve  50  on the basis of a pressure signal from the pressure sensor  51 . 
     Accordingly, when the turning operation of the rotation motor RM and the moving-down operation of the boom cylinder BC are simultaneously performed as described above, the tilting angle of the assist hydraulic motor AM may be determined with reference to the required moving-down speed of the boom cylinder BC irrespective of the turning pressure or the brake pressure. 
     At all events, the output of the assist hydraulic motor AM can be used to assist the output of the sub-pump SP, and also the flow rate discharged from the sub-pump SP can be proportionally divided at the first, second proportional solenoid throttling valves  42 ,  43  for delivery to the first, second circuit systems S 1 , S 2 . 
     On the other hand, for use of the assist hydraulic motor AM as a drive source and the electric motor MG as a generator, the tilting angle of the sub-pump SP is changed to zero such that the sub-pump SP is put under approximately no-load conditions, and the assist hydraulic motor AM is maintained to produce an output required for rotating the electric motor MG. By doing so, the output of the assist hydraulic motor AM can be used to allow the electric motor MG to fulfill the generator function. 
     In the embodiment, the output of the engine E can be used to allow the generator  1  to generate electric power or the assist hydraulic motor AM can be used to allow the electric motor MG to generate electric power. Then, the electric power thus generated is stored in the battery  26 . In the embodiment, since the household power source  27  may be used to accumulate electric power in the battery  26 , the electric power of the electric motor MG can be utilized for various components. 
     Since the check valves  44 ,  45  are provided and the solenoid directional control valve  50  and the solenoid on/off valve  54  or the first, second solenoid valves  58 ,  59  are provided, for example, when a failure occurs in the system of the sub-pump SP and the assist hydraulic motor AM, the system of the first, second main pumps MP 1 , MP 2  can be hydraulically disconnected from the system of the sub-pump SP and the assist hydraulic motor AM. In particular, the solenoid directional control valve  50 , the solenoid on/off valve  54  and the first, second solenoid valves  58 ,  59 , which are in the normal conditions, are maintained in the closed position by a spring force of the springs as illustrated in the drawings, and also the proportional solenoid valve  36  is kept in the normal position which is the full open position. For this reason, even if a failure occurs in the electric system, the system of the first, second main pumps MP 1 , MP 2  can be hydraulically disconnected from the system of the sub-pump SP and the assist hydraulic motor AM as described above. 
       FIG. 2  illustrates a second embodiment employing a solenoid valve  61  which is formed by combining together the first, second solenoid valves  58 ,  59  described in the first embodiment. Specifically, the standby channels  55 ,  56 , which are respectively connected to the first, second main pumps MP 1 , MP 2 , are connected to one solenoid valve  61 . The solenoid valve  61  has a spring  61   a  mounted at one end and a solenoid  61   b  mounted at the other end. The solenoid  61   b  is connected to the controller unit C. The solenoid valve  61  maintains in the closed position as illustrated in  FIG. 2  under normal conditions by a spring force of the spring  61   a  so as to block the communication between the two standby channels  55 ,  56  and the merging passage  57 . 
     The solenoid  61   b  is energized by a signal from the controller unit C, so that the solenoid valve  61  is switched from the closed position to the open position. Timing of this switching is the time when pressure signals of the respective pressure sensors  13 ,  24  builds up, so that the on/off valves  10 ,  21  are closed. If the solenoid valve  61  is switched from the closed position to the open position in this manner, both the standby channels  55 ,  56  simultaneously communicate with the merging passage  57 . 
     In the second embodiment configured as described above, only when all the operated valves  2  to  6  and  14  to  17  of both the circuit systems S 1 , S 2  are maintained in the neutral position, the standby flow of the first, second main pumps MP 1 , MP 2  can be used to rotate the assist hydraulic motor AM, so that the electric motor MG can fulfill the power generation function. 
     The other structures and operations are similar to those in the first embodiment. 
     The on/off valves  10 ,  21  described in the first, second embodiments are on/off controlled, but may be adapted to be varied in the degree of opening in accordance with a control signal of the controller unit C. 
     The on/off valves  10 ,  21  are designed to close/open in response to a control signal from the controller unit C, but may be subjected to the opening/closing control using the pressure in the neutral channels  7 ,  18  as pilot pressure. 
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a circuit diagram illustrating a first embodiment. 
       FIG. 2  is a circuit diagram illustrating a second embodiment. 
     REFERENCE SIGNS LIST 
     
         
         MP 1  First main pump 
         MP 2  Second main pump 
         S 1  First circuit system 
         S 2  Second circuit system 
           2 - 6  Operated valve 
           10 ,  21  On/off valve 
           11 ,  22  Pilot channel 
           12 ,  23  Regulator 
           13  First pressure sensor 
         C Controller unit 
           14 - 17  Operated valve 
           24  Second pressure sensor 
         SP Sub-pump 
         AM Assist hydraulic motor 
         MG Electric motor/generator 
           58  First solenoid valve 
           59  Second solenoid valve 
           61  Solenoid valve