Abstract:
A double redundancy electro hydrostatic actuator system includes two hydraulic pumps; two fail safe valves connected with the two hydraulic pumps, respectively; one dual tandem hydraulic cylinder connected with the two fail safe valves and having a piston rod, wherein the piston rod is moved by switching supply and discharge of the fluid; two switching valves connected with the two fail safe valves; two accumulators connected with the two switching valves and the two hydraulic pumps, respectively; and two chambers connected with the two switching valves, respectively. Each of the two accumulators accumulates the fluid from a corresponding one of the two hydraulic pumps, and sends the fluid to a corresponding one of the two fail safe valves. The two chambers receive the fluid from the two fail safe valves, respectively.

Description:
INCORPORATION BY REFERENCE 
     This patent application claims priority on convention based on Japanese Patent Application No. 2007-335204 filed on Dec. 26, 2007. The disclosure thereof is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a double redundancy electro hydrostatic actuator system with a dual tandem hydraulic cylinder driven by using two systems of hydraulic circuits. 
     2. Description of Related Art 
     A double redundancy electro hydrostatic actuator system for controlling a dual tandem hydraulic cylinder by two systems of hydraulic circuits is known. Such a double redundancy hydrostatic actuator system is adopted in a wing of an airplane. That is, two systems of hydraulic circuits are provided to allow a dual tandem hydraulic cylinder to be operated, even when either of the two systems of hydraulic circuits does not operate. 
       FIG. 1  shows a conventional hydraulic actuator system. Two systems (A system and B system) of hydraulic pressure circuits are connected to the hydraulic cylinder  18 . Each system mainly includes a hydraulic source  2 , a reservoir  4 , a servo valve  6 , a relief valve  8 , a fail safe valve hydraulic source  10 , a fail safe valve reservoir  12 , a solenoid valve  14  and a fail safe valve  16 . The hydraulic source  2  supplies hydraulic fluid to a dual tandem hydraulic cylinder  18 . A wall  22  is provided for a main body of the hydraulic cylinder  18 . The wall  22  separates a space for hydraulic fluid supplied from the A system from a space for hydraulic fluid supplied from the B system. Flows of the hydraulic fluid from the A system and the B system are supplied to each other, thereby moving a piston rod  20  in the hydraulic cylinder  18 . 
     A fail safe valve  16  has a structure with a spool valve  27  taking any of three states and small and large pistons for switching the three states. A first one of the three stats is a state that the hydraulic fluid is supplied from the hydraulic source  2  to the hydraulic cylinder  18  or returned from the hydraulic cylinder  18 . A second one thereof is a state that the hydraulic source  2  stops the supply of hydraulic fluid to the hydraulic cylinder  18  when either the A system or the B system cannot operate due to a failure, and closes the hydraulic circuits between the fail safe valve  16  and the hydraulic cylinder  18  so that the hydraulic cylinder  18  may be moved by only a normally operating system. A third one thereof is a state that the hydraulic source  2  stops the supply of the hydraulic fluid to the hydraulic cylinder  18  and closes the hydraulic circuits between the fail safe valve  16  and the hydraulic cylinder  18  when both of the A system and the B system cannot operate due to a failure, and in addition, a flow of the hydraulic fluid is reduced by orifice. In the third state, the piston rod  20  performs a damping operation, since the flow of the hydraulic fluid is reduced even when external force is applied to the piston rod  20 . Switching of the fail safe valve  16  is performed among the three states by supplying the hydraulic fluid to the small and large pistons of the fail safe valve  16  such that the spool valve  27  is switched by the fail safe valve hydraulic source  10 . 
     Japanese Patent Application Publication (JP-P2001-295802A) discloses an electro hydrostatic actuator including a first position control system and a second position control system. The first position control system is a closed control system formed from a first operation section of the actuator, a position sensor for detecting the position of the first operation section, a controller, and an electric motor controlled by the controller to drive the hydraulic pump. The second position control system is a system which drives a second operation section for changing a displacement of the hydraulic pump in a direction of low displacement when a detection position signal outputted from the position sensor is coincident with a support position signal received by the controller. 
     SUMMARY 
     An object of the present invention is to provide a compact and light-weight electro hydrostatic actuator system in which a dual tandem hydraulic cylinder is controlled by two systems of hydraulic circuits. 
     In an aspect of the present invention, a double redundancy electro hydrostatic actuator system includes two hydraulic pumps; two fail safe valves connected with the two hydraulic pumps, respectively; one dual tandem hydraulic cylinder connected with the two fail safe valves and having a piston rod, wherein the piston rod is moved by switching supply and discharge of the fluid; two switching valves connected with the two fail safe valves; two accumulators connected with the two switching valves and the two hydraulic pumps, respectively; and two chambers connected with the two switching valves, respectively. Each of the two accumulators accumulates fluid from a corresponding one of the two hydraulic pumps, and sends the fluid to a corresponding one of the two fail safe valves. The two chambers receive the fluid from the two fail safe valves, respectively. 
     In another aspect of the present invention, a method of controlling a double redundancy electro hydrostatic actuator system, is achieved by generating hydraulic of a predetermined pressure by two hydraulic pumps, respectively; by accumulating fluids from the two hydraulic pumps by two accumulators, respectively; by controlling two switching valves to transfer the hydraulic pressures from the two accumulators to two fail safe valves, respectively; by controlling the two fail safe valves to transfer the hydraulic pressures from the two switching valves to a dual tandem hydraulic cylinder; and by driving a piston rod of the hydraulic pressure actuator with the hydraulic pressures. 
     According to the present invention, a compact and light-weight double redundancy electro hydrostatic actuator system is provided in which the dual tandem hydraulic cylinder is controlled by two systems of hydraulic circuits. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a conventional hydraulic actuator system; 
         FIG. 2  shows a double redundancy electro hydrostatic actuator system according to a first embodiment of the present invention; 
         FIG. 3  shows a state of a fail safe valve when two systems of hydraulic circuits are normally operated; 
         FIG. 4  shows a state of the fail safe valve when one system of hydraulic circuit fails down; 
         FIG. 5  shows a state of the fail safe valve when both of two systems of hydraulic circuits fail down; and 
         FIG. 6  shows the electro hydrostatic actuator system according to a second embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, a double redundancy electro hydrostatic actuator system of the present invention will be described in detail with reference to the attached drawings. 
     First Embodiment 
       FIG. 2  shows a configuration of a fail safe valve system according to a first embodiment of the present invention. The fail safe valve of the electro hydrostatic actuator system includes a dual tandem hydraulic cylinder  76  and a hydraulic circuit of an A system and a hydraulic circuit of a B system. The hydraulic cylinder  76  is provided with a piston rod  84  and a wall  86 . The wall  86  divides a space inside the hydraulic cylinder  76  into a space for hydraulic fluid from the hydraulic circuit of the A system and a space for hydraulic fluid from the hydraulic circuit of the B system. The hydraulic cylinder  76  moves the piston rod  84  based on the hydraulic pressure supplied from the hydraulic circuit of the A system and the hydraulic pressure supplied from the hydraulic circuit of the B system. 
     The hydraulic circuit of the A system is provided with an electric motor  30 A, a variable displacement hydraulic pump  32 A, a fail safe valve  74 A, a solenoid valve  72 A, a pop-up chamber  78 A, a check valve  40 A, a check valve  41 A, a relief valve  44 A, a relief valve  45 A, a filter circuit  46 A, a relief valve  48 A, an accumulator  70 A, and a refill valve  42 A. Moreover, the hydraulic circuit of the A system is provided with a plurality of circuits which lead hydraulic fluid to transfer a hydraulic pressure. The plurality of circuits contains a hydraulic circuit  100 A between the accumulator  70 A and the solenoid valve  72 A, a circuit  101 A, a hydraulic circuit  102 A between the pop-up chamber  78 A and the solenoid valve  72 A, a circuit  103 A, a circuit  104 A, a circuit  116 A, a circuit  118 A and a hydraulic circuit  108 A. 
     The electric motor  30 A has a rotatable shaft and is connected to the hydraulic pump  32 A through the shaft. The motor  30 A generates rotation force based on the supplied electric current to rotate the shaft. The hydraulic pump  32 A discharges the hydraulic pressure which flows through the circuits  103 A and  104 A by using the rotation force transferred from the electric motor  30 A through the shaft. 
     The relief valve  44 A connects the circuit  104 A with the circuit  103 A only when the hydraulic pressure of the circuit  104 A is higher than that of the circuit  103 A by a predetermined pressure. The relief valve  45 A connects the circuit  103 A with the circuit  104 A only when the hydraulic pressure of the circuit  103 A is higher than that of the circuit  104 A by a predetermined pressure. The filter circuit  46 A is interposed between the circuit  101 A and the hydraulic circuit  100 A between the accumulator  70 A and the solenoid valve  72 A. The filter circuit  46 A removes contaminants in the hydraulic operating fluid. The relief valve  48 A connects the circuit  101 A with the circuit  100 A only when the hydraulic pressure of the circuit  101 A is higher than that of the circuit  100 A by a predetermined pressure. The check valve  40 A connects the circuit  100 A with the circuit  104 A only when the hydraulic pressure of the circuit  100 A is higher than that of the circuit  104 A. The check valve  41 A connects the circuit  100 A with the circuit  103 A only when the hydraulic pressure of circuit  100 A is higher than that of the circuit  103 A. 
     The accumulator  70 A is connected with the hydraulic circuit  100 A and accumulates the inner leakage of the hydraulic pump  32 A. The pressure generated in the accumulator  70 A at this time is called a case drain pressure of the hydraulic pump  32 A. The pop-up chamber  78 A receives the hydraulic fluid from the hydraulic circuit  102 A between the pop-up chamber  78 A and the solenoid valve  72 A. The solenoid valve  72 A connects one of the hydraulic circuit  100 A between the accumulator  70 A and the solenoid valve  72 A and the hydraulic circuit  102 A between the pop-up chamber  78 A and the solenoid valve  72 A with the hydraulic circuit  108 A in response to a fail safe signal which is generated when a failure has occurred in the hydraulic circuit of the A system. 
     The fail safe valve  74 A is connected with the circuits  103 A,  104 A,  116 A, and  118 A, the hydraulic circuit  108 A and a hydraulic circuit  108 B. The fail safe valve  74 A is provided with a spool valve  97 A, a small piston  82 A, a large piston  80 A, and a spring  98 A. A first fail safe valve chamber  111 A is formed between the small piston  82 A and the large piston  80 A and a second fail safe valve chamber  112 A is formed on the large piston side. The first hydraulic chamber  111 A is connected with the hydraulic circuit  108 A, and the second hydraulic chamber  112 A is connected with the hydraulic circuit  108 B. The spool valve  97 A is arranged to internally contact a spool chamber and is inserted to be slidable into a direction L or R. The spool valve  97 A is driven and switched to one of a normal state  92 A, a bypass state  94 A and a damping state  96 A by the spool valve  97 A sliding into the direction L or R based on the hydraulic pressure of the hydraulic circuit  108 A and the hydraulic pressure of the hydraulic circuit  108 B. 
     The fail safe valve  74 A connects the circuit  103 A with the circuit  116 A and the circuit  104 B with the circuit  118 A, when being switched to the normal state  92 A. The fail safe valve  74 A closes the circuit  103 A and the circuit  104 A and connects the circuit  116 A and the circuit  118 A when being switched to the bypass state  94 A. The fail safe valve  74 A closes the circuits  103 A and  104 A and connects the circuit  116 A and the circuit  118 A through the orifice when being switched to the damping state  96 A. The spring  98 A applies external elastic force to the spool valve  97 A such that the spool valve  97 A moves to the direction L. 
     The small piston  82 A is arranged to internally contact the first hydraulic chamber  111 A to be slidable into the direction L or R. Also, the large piston  80 A is arranged to internally contact the second hydraulic chamber  112 A so as to be slidable into the direction L or R. The large piston  80 A moves to the direction R when the hydraulic pressure in the second hydraulic chamber  112 A is higher than the hydraulic pressure of the first hydraulic chamber  111 A. At this time, the large piston  80 A limits the movement of the small piston  82 A such that the small piston  82 A does not move freely from a predetermined position into the direction L. Thus, the position of the spool valve  97 A is limited to take the normal state  92 A or the bypass state  94 A and not to take the damping state  96 A. The small piston  82 A drives the spool valve  97 A into the direction R when the hydraulic pressure of the first hydraulic chamber  111 A is higher than the elastic force of the spring  98 A. The spring  98 A drives the spool valve  97 A into the direction L when the hydraulic pressure of the first hydraulic chamber  111 A is low and the hydraulic pressure of the second hydraulic chamber  112 A is high. At this time, since the movement of the spool valve  97 A is limited by the large piston  80 A, the spool valve  97 A is settled in a predetermined position, i.e., the bypass state. The spring  98 A drives the spool valve  97 A into the direction L when the hydraulic pressure of the first chamber  111 A is low and the hydraulic pressure of the second hydraulic chamber  112 A is low. At this time, the spool valve  97 A is settled in a predetermined position, i.e., the damping state  96 A. 
     The fail safe valve  74 A is switched to one of the normal state  92 A, the bypass state  94 A and the damping state  96 A by the spool valve  97 A sliding into the direction L or R. That is, the state is switched between the normal state  92 A and the bypass state  94 A and between the bypass state  94 A and the damping state  96 A by the spool valve  97 A moving to the direction L or R. The fail safe valve  74 A connects the circuit  103 A with the circuit  116 A and the circuit  104 A with the circuit  118 A, when being switched to the normal state  92 A. The fail safe valve  74 A closes the circuit  103 A and the circuit  104 A and connects the circuit  118 A and the circuit  116 A when being switched to the bypass state  94 A. The fail safe valve  74 A closes the circuit  103 A and the circuit  104 A and connects the circuits  118 A and the circuit  116 A through the orifice when being switched to the damping state  96 A. 
     The solenoid valve  72 A is provided with a feed circuit  88 A and a return circuit  90 A, as shown in  FIG. 3 . The solenoid valve  72 A switches the state in response to a failure signal which is generated when a failure has occurred in the hydraulic circuit of the A system. The solenoid valve  72 A closes the hydraulic circuit  102 A between the pop-up chamber  78 A and the solenoid valve  72 A in an open state when the failure signal is not supplied, and connects the hydraulic circuit  100 A with the hydraulic circuit  108 A through the feed circuit  88 A. The solenoid valve  72 A closes the hydraulic circuit  100 A when the failure signal is supplied and connects the hydraulic circuit  108 A with the hydraulic circuit  102 A through the return circuit  90 A. 
     In the above description, the A system is described mainly. However, the same things can be applied to the B system. 
     Next, states of the fail safe valves of the two systems will be described with reference to  FIGS. 3 to 5 . 
       FIG. 3  shows a state of the fail safe valve when the two systems of hydraulic circuits normally operate. Operations of the fail safe valves  74 A and  74 B when both the A system and the B system normally operate will be described. First, the A system will be described. A case drain pressure of the hydraulic pump  32 A is accumulated in the accumulator  70 A in the A system, and the hydraulic pressure is transferred from the accumulator  70 A to the fail safe valve  74 A through the hydraulic circuit  100 A, the feed circuit  88 A of the solenoid valve  72 A, and the hydraulic circuit  108 A. The hydraulic circuit  108 A is connected to the hydraulic chamber  112 B which houses the large piston  80 B of the fail safe valve  74 B in the B system. Meanwhile, the hydraulic circuit  108 A is connected to the first chamber  111 A which houses the small piston  82 A of the fail safe valve  74 A in the A system. Since the hydraulic fluid has a pressure, the small piston  82 A in the A system is pushed in a direction of R in  FIG. 3  and the large piston  80 B in the B system is pushed in a direction of L in  FIG. 3 . Thus, the spool valve  97 A is pushed toward the small piston  82 A by the spring  98 A. In the state shown in  FIG. 3 , the small piston  82 A pushes the spool valve  97 A in the direction of R in  FIG. 3  to set the normal state  92 A in which the hydraulic circuit  116 A and the hydraulic circuit  118 A are connected with the hydraulic circuits  103 A and  104 A. That is, the hydraulic pressure is transferred between the hydraulic pump  32 A and the hydraulic cylinder  76 A. 
     Next, the B system will be described. The case drain pressure of the variable capacitance hydraulic pump  32 B is accumulated in the accumulator  70 B in the B system, and the hydraulic pressure is transferred from the accumulator  70 B to the fail safe valve  74 B through the hydraulic circuit  100 B, the feed circuit  88 B of the solenoid valve  72 B, and the hydraulic circuit  108 B. The hydraulic circuit  108 B is connected to the second chamber  112 A which houses the large piston  80 A of the fail safe valve  74 A in the A system. Also, the hydraulic circuit  108 B is connected to the first chamber  111 B which houses the small piston  82 B of the fail safe valve  74 B in the B system. Since the hydraulic fluid has a pressure, the small piston  82 B in the B system is pushed in the direction of L in  FIG. 3  and the large piston  80 A in the A system is pushed in the direction of R in  FIG. 3 . The spool valve  97 B can take either of the normal state  92 B, the bypass state  94 B and the damping state  96 B and is pushed toward the small piston  82 B by the spring  98 B. In the state shown in  FIG. 3 , the small piston  82 B pushes the spool valve  97 B in the direction of L in  FIG. 3  to set the normal state  92 B in which the hydraulic circuit  116 B and the hydraulic circuit  118 B are connected to the spool valve  97 B. Namely, the hydraulic is transferred between the hydraulic pump  32 B and the hydraulic cylinder  76 . 
       FIG. 4  shows states of the fail safe valves  74 A and  74 B when either of two systems of hydraulic circuits fails down. A case will be described where the failure has occurred at any point in the B system. First, the A system will be described. Since the A system is in the normal state, the hydraulic pressure accumulated in the accumulator  70 A in the A system is transferred from the hydraulic circuit  100 A to the fail safe valves  74 A and  74 B through the feed circuit  88 A of the solenoid valve  72 A. The hydraulic circuit  108 A is connected to the second chamber  112 B which houses the large piston  80 B of the fail safe valve  74 B in the B system. Also, the hydraulic circuit  108 A is connected to the first chamber  111 A which houses the small piston  82 A of the fail safe valve  74 A in the A system. Since the hydraulic fluid has a pressure, the small piston  82 A in the A system is pushed in the direction of R in  FIG. 4  and the large piston  80 B in the B system is pushed in the direction of L in  FIG. 4 . The spool valve  97 A can takes either of the normal state  92 A, the bypass state  94 A and the damping state  96 A. In this case, the spool valve  97 A can be set to the normal state  92 A and is pushed toward the small piston  82 A. In the state shown in  FIG. 4 , the small piston  82 A pushes the spool valve  97 A in the direction of R in  FIG. 4  to set the normal state  92 A in which the hydraulic circuit  116 A and the hydraulic circuit  118 A are connected to the hydraulic pump  32 A. That is, the hydraulic pressure is transferred between the hydraulic pump  32 A and the hydraulic cylinder  76 A. 
     Next, the B system will be described. A fail signal is given to the solenoid valve  72 B in the B system, the supply of the hydraulic pressure from the accumulator  70 B is stopped and a return circuit  90 B is connected to the pop-up chamber  78 B. The pop-up chamber  78 B serves to receive and absorb the hydraulic pressure. Accordingly, by returning the hydraulic fluid from the hydraulic circuit  108 B connected to the solenoid valve  72 B in the B system, the pop-up chamber  78 B receives the hydraulic pressure. Since the piston of the pop-up chamber  78 B moves in the direction of H in  FIG. 4 , the returned hydraulic fluid is received in the pop-up chamber  78 B. As a result, the large piston  80 A in the A system moves in a direction of L in  FIG. 4  due to the hydraulic pressure of the hydraulic circuit  108 A, and the small piston  82 B in the B system moves in the direction of R in  FIG. 4  due to force of the spring  98 B such that the spool valve  97 B moves in the direction of R. Thus, the small piston  82 B contacts the large piston  80 B in the B system. Through limitation of the movement of the small piston  82 B in the B system, the spool valve  97 B is set to the bypass state  94 B and the hydraulic circuit  116 B and the hydraulic circuit  118 B are connected. In the bypass state  94 B, the spool valve  97 B stops the supply of the hydraulic pressure from the hydraulic pump  32 B to the hydraulic cylinder  76  and allows movement of the hydraulic fluid remaining in the hydraulic cylinder  76 , the hydraulic circuit  116 B and the hydraulic circuit  118 B. Consequently, when the hydraulic cylinder  76  is to be operated by the A system, the piston rod  84  can be operated. That is, the B system can be separated. 
       FIG. 5  shows states of the fail safe valves when both of the two systems of hydraulic circuits fail down. The fail signal is supplied to each of the solenoid valves  72 A and  72 B in the A system and the B system. The supply of the hydraulic pressures from the accumulators  70 A and  70 B in the A system and the B system is stopped and the return circuits  90 A and  90 B are connected to the pop-up chambers  78 A and  78 B. The pop-up chambers  78 A and  78 B serve to receive and absorb the hydraulic pressures. Accordingly, the hydraulic fluid is returned from the hydraulic circuit  108 A connected to the solenoid valves  72 A in the A system, and the pop-up chamber  78 A in the A system receives the hydraulic pressure. The hydraulic pressure is returned from the hydraulic circuit  108 B connected to the solenoid valve  72 B in the B system and the pop-up chamber  78 B in the B system receives the hydraulic pressure. 
     As a result, both the large piston  80 A and the small piston  82 A of the fail safe valve  74 A in the A system moves in a direction of L in  FIG. 5  due to the force of the spring  98 A. The large piston  80 B and the small piston  82 B in the fail safe valve  74 B of the B system move in a direction of R in  FIG. 5 . Through the movement of the small piston  82 A in the A system, the spool valve  97 A is set to the damping state  96 A and the hydraulic circuit  116 A and the hydraulic circuit  118 A are connected through an orifice. Furthermore, through the movement of the small piston  82 B in the B system, the spool valve  97 B is switched to the damping state  96 B and the hydraulic circuit  116 B and the hydraulic circuit  118 B are connected to each other through an orifice. In the damping state  96 A, the spool valve  97 A stops the supply of the hydraulic pressure from the hydraulic pump  32 A to the hydraulic cylinder  76 A and allows movement of the hydraulic fluid remaining in the hydraulic cylinder  76 , the hydraulic circuit  116 A and the hydraulic circuit  118 A. However, due to a configuration of reducing the flow of the hydraulic fluid by the orifice, even when an external force is applied to the piston rod  84 , the piston rod  84  does not smoothly operate and thus a damping operation is performed against the external force. 
     As described above, the hydraulic sources for operating the fail safe valves  74 A and  74 B are ensured by the accumulators  70 A and  70 B. Accordingly, to operate the fail safe valves  74 A and  74 B, a hydraulic pump or a hydraulic circuit having some distances is not required. The accumulators  70 A and  70 B or the pop-up chambers  78 A and  78 B are lighter than the hydraulic pump or the hydraulic piping. Therefore, according to the present invention, a light-weight double redundancy electro hydrostatic actuator system as a whole can be built. 
     Second Embodiment 
       FIG. 6  shows the configuration of the double redundancy electro hydrostatic actuator system according to a second embodiment of the present invention. The system includes two systems (A system and B system) of hydraulic circuits to the dual tandem hydraulic cylinder  76 . The same and similar components are assigned with the same and similar reference numerals and the detailed description of them is omitted. 
     In the A system, the electric motor  30 A is connected to a variable displacement hydraulic pump  32 A which is the hydraulic source for working the hydraulic cylinder  76 . The hydraulic pressure of the hydraulic pump  32 A on the high pressure side is accumulated in the accumulator  70 A through the shuttle valve  174 A. The accumulator  70 A is connected to the solenoid valve  72 A. The pop-up chamber  78 A is attached to the solenoid valve  72 A. The solenoid valve  72 A is connected to the fail safe valves  74 A and  74 B through the hydraulic circuit  108 A. Furthermore, the fail safe valve  74 A is connected to the hydraulic cylinder  76  through the hydraulic circuits  116 A and  118 A. The hydraulic pressure accumulated in the accumulator  70 A is transferred to the fail safe valve  74 A through the solenoid valve  72 A. As described in the first embodiment, when the fail signal is supplied to the solenoid valve  72 A, the solenoid valve  72 A operates to stop the supply of the hydraulic pressure from the accumulator  70 A, and the hydraulic fluid is returned to the pop-up chamber  78 A and the fail safe valve  74 A operates. An operation of the fail safe valve  74 A is the same as that in first embodiment. The B system operates in the same manner as the A system. 
     The hydraulic pressure accumulated in the accumulator  70 A can be used as the case drain pressure of the hydraulic pump  32 A. However, when the hydraulic pressure accumulated in the accumulator  70 A is directly supplied from the hydraulic pump  32 A as the case drain pressure, a pressure exceeding a pressure resistance of the pump case of the hydraulic pump  32 A is applied to the pump case, thereby possibly destroying the hydraulic pump  32 A. Thus, the hydraulic pressure accumulated in the accumulator  70 A is transferred to a boot strap reservoir  176 A to feed the reduced pressure to the hydraulic pump  32 A.