Abstract:
In a brake-by-wire vehicle brake system using a feedback control unit ( 12   a,    12   b   , 31, 36, 45 ) for producing a brake fluid pressure according to a brake input, a response restricting unit ( 26, 39, 48 ) restricts a response property of the feedback control unit when an extraneous brake fluid control operation is detected. Thereby, the control unit is prevented from excessively reacting to changes in the brake fluid pressure caused by the extraneous brake fluid control operation, and the vehicle brake system can provide a highly responsive property and a resistance to changes in the brake fluid pressure at the same time.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a vehicle brake system, and in particular to a vehicle brake system using an electric actuator for producing a brake fluid pressure. 
       BACKGROUND OF THE INVENTION 
       [0002]    In electric vehicles and hybrid vehicles, it is a common practice to use the electric motor which is connected to the drive axle of the vehicle as a generator that produces electric power and provides a braking force at the same time when decelerating the vehicle. This is called as regenerative braking. However, the regenerative braking is generally inadequate for providing all of the need for the braking force, and it is common to combine the more conventional hydraulic (friction) braking with the regenerative braking. JP 2008-143256A discloses an electronically controlled brake-by-wire system that controls the hydraulic brake and the regenerative brake in a coordinated manner. 
         [0003]    In a brake-by-wire system, a target brake force is set by an input amount given by a depression stroke of the brake pedal (pedal stroke), and the target braking force is distributed between the hydraulic braking and the regenerative braking. The hydraulic braking is based on the use of a motor actuated cylinder which is actuated by an electric motor, and supplies a brake fluid pressure for operating the wheel cylinders. 
         [0004]    The ABS (anti-lock brake system) for preventing the locking of wheels at the time of braking is widely used in the existing vehicles, and performs the functions thereof by controlling the brake fluid pressure that is supplied to the wheel cylinders. JP 2007-331538A discloses an ABS system, and such an ABS system can be combined with a brake-by-wire system as an extraneous system that involves an extraneous brake fluid control operation. The VSA (vehicle stability assist) system is another example of systems that are extraneous to the main vehicle brake system but performs the functions thereof by acting upon the brake fluid pressure that is distributed to different wheels. 
         [0005]    In the brake-by-wire system, the motor actuated cylinder is operated according to the target brake force or the target brake fluid pressure which is allocated to the hydraulic brake. The control of the brake fluid pressure is typically based either on the cylinder stroke of the motor actuated cylinder or the motor current of the electric motor of the motor actuated cylinder (or the force applied to the piston of the motor actuated cylinder). 
         [0006]    When the control process is based on the cylinder stroke, the target cylinder stroke of the motor actuated cylinder is determined according to the target brake fluid pressure by taking into account the loss property of the brake fluid path between the motor actuated cylinder and the wheel cylinder, the cylinder stroke is converted into rotational angle of the electric motor, and the electric motor is operated by a feedback control so as to cause the cylinder stroke to agree with the target cylinder stroke. 
         [0007]    When the control process is based on the motor current, the target motor torque is determined according to the target brake fluid pressure by taking into account the specifications or configurations of the motor actuated cylinder and the reduction gear unit of the electric motor, the motor torque is converted into motor current, and the electric motor is operated by a feedback control so as to cause the motor torque to agree with the target motor torque. 
         [0008]    When the control process is based on the cylinder stroke, the amount of the brake fluid that is required for producing the target brake fluid pressure is used as the target value. As this target value or the amount of the brake fluid gives the direct measure of the braking force, a high responsiveness can be achieved both in normal braking and in combined braking (combining both the regenerative braking and the hydraulic braking), and the braking force can be produced at a relatively high precision. However, as the computation of the target value based on the cylinder stroke is made on the basis of the fluid loss property, if any fluctuation in the fluid pressure occurs, the actual fluid pressure may excessively overshoot the target fluid pressure, and this may cause the deviation between the target fluid pressure and the actual fluid pressure to persist more than desired. Such fluctuations of fluid pressure may be caused by an extraneous brake fluid control operation performed by an extraneous system based on the use of the brake system such as a ABS, a traction control and other VSA systems. 
         [0009]    When the control process is based on the motor current, the motor torque that is required to produce the target fluid pressure is used as the target value so that the actual fluid pressure may be made to agree with the target fluid pressure without relying on an accurate estimation of the fluid loss property. However, the amount of the brake fluid that is required for producing the target fluid pressure is not considered. Therefore, as compared to the case where the control process is based on the cylinder stroke, the responsiveness is lower, and the coordinated control of the normal braking and the regenerative braking may be performed only with a reduced responsiveness. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    In view of such problems of the prior art, a primary object of the present invention is to provide a vehicle brake system that can provide a highly responsive property and a resistance to changes in the brake fluid pressure at the same time. 
         [0011]    A second object of the present invention is to provide a vehicle brake system which is highly robust against changes in the brake fluid pressure and abnormal conditions of sensors. 
         [0012]    According to the present invention, such objects can be accomplished by providing a vehicle brake system, comprising: an input amount sensor for detecting an input amount applied to a brake pedal; a motor actuated cylinder configured to be actuated by an electric motor for producing a brake fluid pressure in dependence on a control input thereof; a wheel cylinder that produces a braking force according to the brake fluid pressure supplied thereto by the motor actuated cylinder; a feedback control unit including a target setting unit for setting a target operation amount of the motor actuated cylinder according to the input amount applied to the brake pedal, an operation amount sensor for detecting an actual operation amount of the motor actuated cylinder and a feedback unit for providing the control input for the motor actuated cylinder so as to minimize a deviation between the target operation amount and the actual operation amount; and a response restricting unit for restricting a response property of the feedback control unit when a prescribed condition is met such as when an extraneous brake fluid control operation is detected. 
         [0013]    Thereby, the vehicle brake system demonstrates a highly responsive property under normal condition by performing a feedback control based on the operation amount of the motor actuated cylinder typically consisting of the cylinder stroke thereof, and a high resistance to changes in the fluid loss property or other fluctuations in the brake hydraulic system can be achieved when an extraneous brake fluid control operation typically consisting of a VSA or other vehicle motion control system is detected. 
         [0014]    According to a certain aspect of the present invention, the response restricting unit comprises a low pass filter for filtering out a high frequency component from a signal representing the input amount and a switching unit for selectively activating the low pass filter when an extraneous brake fluid control operation is detected. 
         [0015]    According to a specific aspect of the present invention, the operation amount of the motor actuated cylinder comprises a motor angle or a motor current. 
         [0016]    According to another aspect of the present invention, the feedback control unit comprises a first feedback control unit using a motor angle as the operation amount and a second feedback control unit using a motor current as the operation amount, and the response restricting unit comprise a switching unit for controlling the motor actuated cylinder by using the first feedback control unit when the prescribed condition is not met and the second feedback control unit when the prescribed condition is met. 
         [0017]    In this case, when the prescribed condition is met such as when an extraneous brake fluid control operation is detected, the feedback control based on the motor current is performed, and this allows the brake to be operated with a similar impression as that based on the motor angle while providing a reasonable robustness against fluctuations in the brake fluid system. 
         [0018]    The extraneous brake fluid control operation may comprise a reduction and/or an increase in the fluid pressure of the wheel cylinder. 
         [0019]    The vehicle brake system may further comprise a sensor abnormality detection circuit, the response restricting unit restricting a response property of the feedback control unit when an abnormal condition of a sensor is detected by the sensor abnormality detection circuit, so that the vehicle brake system may be made robust against errors in the sensors for the brake system. 
         [0020]    For an improved response and/or stability, the vehicle brake system may further comprise a brake fluid pressure sensor for detecting the brake fluid pressure and a brake fluid pressure compensating unit for compensating the input amount according to the detected brake fluid pressure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    Now the present invention is described in the following with reference to the appended drawings, in which: 
           [0022]      FIG. 1  is a schematic diagram showing a vehicle incorporated with a vehicle brake system embodying the present invention; 
           [0023]      FIG. 2  is a diagram showing the overall structure of the vehicle brake system according to the present invention; 
           [0024]      FIG. 3  is a block diagram of a control unit for the vehicle brake system given as a first embodiment of the present invention; and 
           [0025]      FIG. 4  is a view similar to  FIG. 3  showing a second embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]      FIG. 1  shows a brake system of an electric or hybrid vehicle embodying the present invention. This vehicle V comprises a pair of front wheels  2  located on the front side thereof and a pair of rear wheels  3  located on the rear side thereof. The front wheels  2  are connected to front axles  4  which are in turn connected to a motor/generator  5  in a torque transmitting relationship via a differential gear device (not shown in the drawing). 
         [0027]    The motor/generator  5  operates both as an electric motor for propelling the vehicle and a generator for providing a regenerative braking. More specifically, the motor/generator  5  can receive electric power from a rechargeable battery  7  serving as a power source via an inverter  10 , and can also supply electric power to (recharge) the battery  7  by converting the kinetic energy into electric power by the regenerative braking. 
         [0028]    A control unit (ECU)  6  incorporated with a CPU control circuit performs various control actions for the vehicle V including the distribution of braking force as will be described hereinafter. The control unit  6  is electrically connected to the inverter  10 . In the case of an electric vehicle, the structure illustrated in  FIG. 1  may be applied as it is, or, alternatively, an additional motor/generator for the rear wheels  3  may be included in the vehicle  1 . In the case of a hybrid vehicle, the front axles  4  are additionally connected to the output shaft of an internal combustion engine E indicated by the double-dot chain dot lines in  FIG. 1 . The illustrated engine E is configured to drive the front wheels, but may also be configured to drive the rear wheels or all of the four wheels. 
         [0029]    Each of the front and rear wheels  2 ,  3  is provided with a per se known disk brake including a disk  2   a,    3   a  integrally attached to the wheel  2 ,  3  and a caliper incorporated with a wheel cylinder  2   b,    3   b.  The wheel cylinder  2   b,    3   b  is connected to a brake fluid pressure generating unit  8  via a per se known brake tubing. The brake fluid pressure generating unit  8  consists of a hydraulic circuit configured to distribute hydraulic brake pressure to the different wheels and adjust the hydraulic brake pressure level for each wheel. 
         [0030]    A wheel speed sensor  9  is provided in association with each wheel  2 ,  3 , and a displacement sensor  11   a  is provided in association with a brake pedal  11  (that is operated by a vehicle operator) to detect a brake operation amount or a brake depression stroke. The detection signals of the wheel speed sensors  9  and the displacement sensor  11   a  are forwarded to the control unit  6 . 
         [0031]    Upon detecting an output signal of the displacement sensor  11  a of the brake pedal  11  becoming greater than zero, the control unit  6  performs a control action for braking. In the illustrated embodiment, the braking action is performed as that of a brake-by-wire system, and includes a regenerative cooperative control combining a regenerative braking and a hydraulic braking. 
         [0032]    The brake system  1  of this vehicle V is described in the following with reference to  FIG. 2 . The brake system  1  consists of a brake-by-wire system that detects the brake operation amount (brake pedal stroke) of the brake pedal  11  by using the stroke sensor  11   a  (serving as an input amount sensor) and produces a brake fluid pressure according to the detected brake operation amount by using a motor actuated cylinder  13  (serving as a brake fluid pressure generating cylinder) configured to be actuated by an electric servo motor  12 . 
         [0033]    As shown in  FIG. 2 , an end of a rod  14  is connected to the brake pedal  11  which is in turn pivotally connected to the vehicle body so as to convert the angular movement of the brake pedal  11  into a substantially linear motion of the rod  14 , and the other end of the rod  14  engages a first piston  15   a  of a master cylinder  15  of a tandem type in the direction to force the piston  15   a  into the master cylinder  15 . The master cylinder  15  additionally receives therein a second piston  15   b  on the side of the first piston  15   a  facing away from the rod  14 , and the first and second pistons  15   a  and  15   b  are both resiliently urged toward the rod  14  by respective springs. The brake pedal  11  is also urged by a spring (not shown in the drawing) such that the brake pedal  11  is held at the initial position shown in  FIG. 2  by a stopper not shown in the drawing when the brake is not being operated. 
         [0034]    The master cylinder  15  is provided with a reservoir tank  16  for receiving and feeding the brake fluid depending on the displacement of the two pistons  15   a  and  15   b . The pistons  15   a  and  15   b  are each fitted with seal members to shut oil passages  16   a  and  16   b  communicating the interior of the master cylinder  15  with the reservoir tank  16 , respectively. Inside the master cylinder  15 , a first fluid chamber  17   a  is defined between the first and second pistons  15   a  and  15 , and a second fluid chamber  17   b  is defined on the side of the second piston  15   b  facing away from the first piston  15   a.    
         [0035]    In addition to the electric servo motor  12 , the motor actuated cylinder  13  is provided with a gear mechanism  18  connected to the electric servo motor  12 , a screw rod  19  which is connected to the gear mechanism  18  via a ball screw mechanism for an axial movement, and a first piston  21   a  and a second piston  21   b  which are connected to the screw rod  19  coaxially and in tandem. 
         [0036]    The second piston  21   b  is fixedly provided with a connecting member  20  projecting toward the first piston  21   a,  and the other end of the connecting member  20  is connected to the first piston  21   a  so as to allow a relative axial movement with the first piston  21   a  to a certain extent. Further, the first and second pistons  21   a  and  21   b  are both resiliently urged toward the screw rod  19  by respective springs  27   a  and  27   b.  In particular, the spring  27   a  urges the first and second pistons  21   a  and  21   b  away from each other. Thereby, the first piston  21   a  is able to advance (move toward the second piston  21   a ) independently from the second piston  21   b,  but is able to pull the second piston  21   b  back to the initial position via the connecting member  20  when the first piston  21   a  retreats. 
         [0037]    The motor actuated cylinder  13  is provided with oil passages  22   a  and  22   b  which in turn communicate with the reservoir tank  16  via a communication passage  22 , and the pistons  21   a  and  21   b  are fitted with per se known seal members to shut the oil passages  22   a  and  22   b  as required. In the motor actuated cylinder  13 , a first fluid pressure generating chamber  23   a  is defined between the first and second pistons  21   a  and  21   b,  and a second fluid pressure generating chamber  23   b  is defined on the side of the second piston  21   b  facing away from the first piston  21   a.    
         [0038]    The first fluid chamber  17   a  of the master cylinder  15  is communicated with the first fluid pressure generating chamber  23   a  of the motor actuated cylinder  13  via a normally open solenoid valve  24   a,  and the second fluid chamber  17   b  of the master cylinder  15  is communicated with the second fluid pressure generating chamber  23   b  of the motor actuated cylinder  13  via a normally open solenoid valve  24   b  by using appropriate tubing. A master cylinder side brake pressure sensor  25   a  is provided on the line between the first fluid chamber  17   a  and the solenoid valve  24   a,  and a motor actuated cylinder side brake pressure sensor  25   b  is provided on the line between the solenoid valve  24   b  and the second fluid pressure generating chamber  23   b.    
         [0039]    A cylinder type simulator  28  is provided on the line between the second fluid chamber  17   b  and the solenoid valve  24   b  via a normally closed solenoid valve  24   c.  The simulator  28  is provided with a cylinder having an interior separated by a piston  28   a.  A fluid receiving chamber  28   b  is defined on the side of the piston  28   a  facing the solenoid valve  24   b,  and a compression coil spring  28   c  is interposed between the opposite side of the piston  28   a  and the opposing axial end of the cylinder of the simulator  28 . When the brake pedal  11  is depressed to cause the brake fluid in the second fluid chamber  17   b  to flow into the fluid receiving chamber  18   b  while the solenoid valves  24   a  and  24   b  are closed and the solenoid valve  24   c  is open, the biasing force of the compression coil spring  28   c  is transmitted to the brake pedal  11  so that the vehicle operator is caused to experience a brake pedal reaction from the brake pedal  11  in a similar manner as in the case with the conventional brake system in which the master cylinder and the wheel cylinder are directly connected to each other. 
         [0040]    The first fluid pressure generating chamber  23   a  and second fluid pressure generating chamber  23   b  of the motor actuated cylinder  13  are communicated with a plurality (four, in the illustrated embodiment) of wheel cylinders  2   b,    3   b  via a VSA system  26  which may consist of a per se known vehicle behavior stabilization control system configured to control an ABS for preventing the locking of wheels at the time of braking, a TCS (traction control system) for preventing the slipping of wheels at the time of acceleration and a side slip control for controlling the side slip of the vehicle at the time of cornering in a coordinated manner. For details of such systems, reference should be made to various prior patent publications on such subject matters. The VSA system  26  includes brake actuators including various hydraulic devices responsible for the control of a first system for the wheel cylinders  2   b  of the front wheels and a second system for the wheel cylinder  3   b  of the rear wheels, and a VSA control unit  26   a  for controlling the various hydraulic devices. The VSA system of the illustrated embodiment is provided with various control functions, but may include only part of such control functions and/or may include other control functions without departing from the spirit of the present invention. 
         [0041]    The overall control of the brake fluid pressure generating unit  8  is performed by the control unit  6 . The control unit  6  receives the various detection signals from the stroke sensor  11   a,  the brake pressure sensors  25   a  and  25   b  and other sensors (not shown in the drawings) for detecting the behavior of the vehicle. Based on the detection signal from the stroke sensor  11   a , and the operating condition of the vehicle that can be determined from the detection signals of the various sensors, the control unit  6  controls the brake fluid pressure generated by the motor actuated cylinder  13 . In the case of a hybrid vehicle (or electric vehicle) as is the case with the illustrated embodiment, as the motor/generator provides a regenerative braking, the control unit  6  is configured to control the brake force allocation or control the brake fluid pressure produced by the motor actuated cylinder  13  depending on the extent or magnitude of the regenerative braking. 
         [0042]    The mode of control operation during a normal braking is described in the following.  FIG. 2  shows the state of the system when the brake pedal  11  is not being operated. The detection value of the stroke sensor  11   a  is at an initial value (=0), and the control unit  6  does not produce any brake fluid pressure generation signal. At this time, the screw rod  19  of the motor actuated cylinder  13  is in the most retracted position and the two pistons  21   a  and  21   b  of the motor actuated cylinder  13  are also in the respective most retracted positions under the spring force of the return springs  27   a  and  27   b  so that no brake fluid pressure is produced in either of the fluid pressure generating chambers  23   a  and  23   b.    
         [0043]    When the brake pedal  11  is depressed to a certain extent, and the detection value of the stroke sensor  11   a  has become greater than zero, a brake-by-wire control is performed in such a manner that the two solenoid valves  24   a  and  24   b  are closed to prevent the fluid pressure generated by the master cylinder  15  to be transmitted to the motor actuated cylinder  13  and the solenoid  24   c  is opened to cause the fluid pressure generated by the master cylinder  15  to be transmitted to the simulator  28 . Based on the input amount detection value (brake operation amount) detected by the stroke sensor  11   a , the control unit  6  determines a target fluid pressure that takes into account the regenerative braking, and forwards a corresponding motor drive command value (operation amount) to the electric servo motor  12 . This in turn causes the screw rod  19  and hence the first piston  21   a  to be pushed out according to this command value, and a brake fluid pressure corresponding to the input or the depression stroke (brake operation amount) of the brake pedal  11  is generated in the first fluid pressure generating chamber  23   a.  At the same time, the second piston  21   b  is displaced forward under the pressure in the first fluid pressure generating chamber  23   a  against the biasing force of the return spring  27   b,  and the corresponding brake fluid pressure is generated in the second fluid pressure generating chambers  23   b.    
         [0044]    When the vehicle operator has displaced the brake pedal  11  in the returning direction (or has released the brake pedal), according to the returning stroke of the brake pedal detected by the stroke sensor  11   a,  the electric servo motor  12  returns the screw rod  19  and hence the first piston  21   a  towards the initial position such that the brake fluid pressure is diminished by an amount corresponding to the returning stroke or the current depression of the brake pedal  11 . When the brake pedal  11  is fully returned to the initial position by the return spring not shown in the drawing, the control unit  6  opens the solenoid valves  24   a  and  24   b.  As a result, the brake fluid in the wheel cylinders  2   b  and  3   b  is allowed to return to the reservoir tank  16  via the motor actuated cylinder  13  and the braking force is eliminated. As the detected value of the stroke sensor  11   a  returns to the initial value, the first piston  21   a  is caused to return to the initial position, and this in turn causes the second piston  21   b  to return to the initial position owing to the force transmitted via the connecting member  20 . 
         [0045]    When the normal braking control is performed, the brake fluid pressure generated by the motor actuated cylinder  13  is supplied to the wheel cylinders  2   b  and  3   b  of the front and rear wheels via the VSA system  26 . When the VSA system  26  performs the braking force distributing control, the braking force of each wheel is individually controlled as commanded by the VSA system  26 . When the VSA system  26  is not in operation, the VSA system  26  allows the brake fluid supplied by the motor actuated cylinder  13  to be directly supplied to the wheel cylinders  2   b  and  3   b  of the front and rear wheels. 
         [0046]    When the regenerative braking is being performed, the control unit  6  causes the motor/generator  5  to operate as a generator such that the amount of the regenerative braking is produced in dependence on the brake operating amount given by the stroke of the brake pedal  11 . If the vehicle deceleration commanded by the brake operating amounted (or by the vehicle operator) cannot be produced by the regenerative braking alone, the electric servo motor  12  actuates the motor actuated cylinder  13 , and the coordinated combined braking involving both the regenerative braking and the hydraulic braking is performed. In this embodiment, the target brake liquid pressure can be determined by subtracting the regenerative braking force from the total required braking force which is determined by the brake operating amount or the input amount. Alternatively, the operation amount of the motor actuated cylinder may be selected such that a hydraulic braking force corresponding to a certain ratio to the entire required braking force is produced. According to the present invention, this control action may be performed in a per se known manner as long as the operation of the motor actuated cylinder  13  is performed in association with the depression stroke of the brake pedal  11 . 
         [0047]    The timing of closing the solenoid valve  24   c  may be selected as the time point at which the fluid pressure of the second fluid chamber  17   b  has dropped to an adequately low level to cause the piston  28   a  to return to the initial position illustrated in  FIG. 2  under the biasing force of the compression coil spring  28   c.  For instance, this timing may be selected as the time point when a prescribed time period has elapsed since the two solenoid valves  24   a  and  24   b  are closed. It is also possible to select the timing when the detection value of the brake pressure sensor  25   b  on the side of the motor actuated cylinder  13  has dropped below a prescribed value such as zero. 
         [0048]    As shown in  FIG. 3 , the control unit  6  includes a fluid pressure adjust control circuit  6   a  as a main part thereof. The structure of the fluid pressure adjust control circuit  6   a  is described in the following with reference to  FIG. 3 . In the fluid pressure adjust control circuit  6   a,  the brake input amount (displacement) given by the detection signal of the stroke sensor  11   a  is forwarded to a brake force standard value setting circuit  31  that provides a standard value B 0  representing a target fluid pressure corresponding to the given brake input amount (displacement) by using a map or a mathematical function. The input of the brake force standard value setting circuit  31  may not necessarily consist of the brake pedal stroke, but may also consist of a detectable input amount (such as the fluid pressure given by the brake pressure sensor  25   a  and the pressure that is applied to the brake pedal  11 ), or the required braking force given in relation with the regenerative braking force. 
         [0049]    The standard value B 0  obtained by the brake force standard value setting circuit  31  is forwarded to an adder  32  whose output is connected to a target value setting circuit  33  serving as a means for setting the target operating amount. The target value setting circuit  33  gives a target value Sm or the target operating amount of the electric servo motor  12  for the given input. The target value Sm obtained by the target value setting circuit  33  is forwarded to a motor angle converting unit  34  which converts the target value Sm into a corresponding target motor angle θt. In the circuit shown in  FIG. 3 , the target value Sm corresponds to the target stroke of the motor actuated cylinder  13 , and the target motor angle θt corresponds to the motor angle of the electric servo motor  12  that produces the target stroke of the motor actuated cylinder  13 . 
         [0050]    The target motor angle θt obtained by the motor angle converting unit  34  is forwarded to a subtractor  35  via a low pass filter switching circuit  39 , and the output value of the subtractor  35  is forwarded to a motor angle feedback circuit  36 . A motor angle control amount given by the output of the motor angle feedback circuit  36  is used for controlling the rotational angle of the electric servo motor  12  via a motor drive circuit  40 , and hence the stroke of the motor actuated cylinder  13  so that the brake fluid pressure corresponding to a brake control amount Bs is produced. 
         [0051]    The standard value B 0  produced from the brake force standard value setting circuit  31  is also forwarded to a subtractor  37  which also receives the detection signal (actual fluid pressure B) from a brake pressure sensor  25   b  for detecting the brake fluid pressure generated by the motor actuated cylinder  13  as a feedback value. The output of the subtractor  37  is forwarded to a fluid pressure compensation circuit  38 , and the output of the fluid pressure compensation circuit  38  or a compensation value ΔB (=B 0 −B) is forwarded to the other input of the adder  32 . The adder  32  thus adds the compensation value ΔB to the standard value B 0 , and forwards the sum (B 0 +ΔB) to the target value setting circuit  33 . Thereby, the actual fluid pressure B is properly reflected in the target value Sm obtained by the target value setting circuit  33 . 
         [0052]    The motor angle of the electric servo motor  12  is detected by a rotational angle sensor (such as a rotary encoder)  12   a,  and the actual motor angle θm is forwarded to the subtractor  35  as a feedback value. Therefore, the motor angle feedback circuit  36  receives the output (θt−θm) of the subtractor  35 , and determines the motor angle control value θ according to the difference (θt−θm) between the target motor angle θt and the actual motor angle θm. The motor angle control value θ is forwarded to the motor drive circuit  40  so that the electric servo motor  12  is controlled by the motor drive circuit  40  according to the motor angle control value θ. In this manner, the stroke of the motor actuated cylinder  13  is controlled as a motor angle feedback control of the electric servo motor  12 . 
         [0053]    In the arrangement shown in  FIG. 3 , the low pass filter switching circuit  39  receives a VSA operation signal when the VSA system  26  is activated. The low pass filter switching circuit  39  performs a filtering operation with a prescribed cut off frequency when a VSA signal is being supplied thereto, and does not perform any filtering operation when a VSA signal is not supplied thereto by allowing the signal pass therethrough without any filtering action. 
         [0054]    When the VSA system  26  is in operation, an assisting brake fluid pressure is generated in the part of the brake fluid hydraulic system downstream to the motor actuated cylinder  13 , separately from the brake fluid pressure generated by the motor actuated cylinder  13 . More specifically, referring to  FIG. 2 , when the VSA system  26  is in operation, the brake fluid pressure may be reduced by releasing the brake fluid to a low pressure reservoir  26   c  via a normally closed “out valve” (depressurizing valve)  26   b , or may be increased by forwarding the brake fluid pressurized by a motor pump  26   d  to the wheel cylinders  2   b,    3   b  via a normally open “in valve” (pressurizing valve)  26   e.  At such a time, owing to the movement of the brake fluid in the line leading to the wheel cylinders  2   b,    3   b  and the operation of a regulator valve  26   f  used by the VSA system  26 , the actual fluid pressure B detected by the brake pressure sensor  25   b  may vary. This affects the brake fluid pressure generated for the given stroke of the motor actuated cylinder  13 , and this in turn causes the motor angle control amount θ produced by the motor angle feedback circuit  36  to deviate from the original value. If this deviation is excessively compensated by the motor actuated cylinder  13 , the responsiveness of the fluid pressure control by the VSA system  26  may be adversely affected. 
         [0055]    According to the illustrated embodiment, when the VSA system  26  is in operation, the output signal of the motor angle converting unit  34  is passed through the low pass filter switching circuit  39  so that the input signal to the subtractor  35  is given with a certain phase delay determined by the cut-off frequency thereof. Thereby, the motor angle control is restricted, and the piston stroke of the motor actuated cylinder  13  is hence favorably controlled so that the fluid pressure responsiveness of the VSA system  26  is ensured. 
         [0056]    When the VSA system is not in operation, the low pass filter switching circuit  39  is disabled, and the output signal of the motor angle converting unit  34  is directly forwarded to the subtractor  35  so that the motor angle control is performed in a highly responsive manner, and the piston stroke of the motor actuated cylinder  13  is allowed to change at the original brisk rate. 
         [0057]    A second embodiment of the present invention is described in the following with reference to  FIG. 4 . In  FIG. 4 , the parts corresponding to those shown in FIG.  3  are denoted with like numerals without repeating the description of such parts. The control unit  6  of the second embodiment includes a fluid pressure adjust control circuit  6   a  similar to that of the previous embodiment and a torque control circuit  6   b  which is connected in parallel with the fluid pressure adjust control circuit  6   a.  In this case, the low pass filter switching circuit  39  connected to the VSA system  26  is absent, and the output of the motor angle converting unit  34  is directly connected to the subtractor  35 . 
         [0058]    The torque control circuit  6   b  includes an adder  41  having a first input receiving the standard value B 0 , a torque converting unit  42 , a current converting unit  43 , a subtractor  44  and a motor current feedback circuit  45  which are connected in series in this order. The standard value B 0  is also supplied to an input of another subtractor  46  whose other input receives the actual brake fluid pressure B as a feedback value, and the output of the subtractor  46  is forwarded to a liquid pressure compensating circuit  47 . The compensation value ΔB (=B 0 −B) produced from the liquid pressure compensating circuit  47  is forwarded to the other input of the adder  41  which forwards the sum (B 0 +ΔB) of the standard value B 0  and the compensation value ΔB to the torque converting unit  42 . Thereby, the actual fluid pressure B is reflected in the target torque Tt obtained by the torque converting unit  42 . 
         [0059]    The target torque Tt obtained by the torque converting unit  42  is converted into a target electric current It corresponding to the target torque Tt by the current converting unit  43 , and the target electric current It is forwarded to the input of the subtractor  44 . The motor current of the electric servo motor  12  is detected by a current sensor  12   b,  and the actual motor current Im detected by the current sensor  12   b  is forwarded to the other input of the subtractor  44  as a feedback value. Thus, the motor current feedback circuit  45  receives the output value (It−Im) of the subtractor  44  as a control input, and provides the motor current control amount I according to the difference (It−Im) between the target motor current It and the actual motor current Im. 
         [0060]    The motor angle control amount θ produced from the motor angle feedback circuit  36  and the motor current control amount I produced from the motor current feedback circuit  45  are forwarded to a pair of selection terminals of a switching unit  48  consisting of a two position selector switch. The switching unit  48  is configured to be operated by the VSA operation signal supplied from the VSA system  26  when the VSA system  25  is put into operation. 
         [0061]    When the VSA system  25  is not in operation, the switching unit  48  is in the position to cause the motor angle control amount θ to be supplied to the motor drive circuit  40  so that the motor angle feedback control is performed as in the first embodiment. On the other hand, when the VSA system  25  is in operation, the switching unit  48  is switched over as indicated by the imaginary line in  FIG. 4  so that the motor current control amount I is supplied to the motor drive circuit  40 . Therefore, in the latter case, the electric servo motor  12  is controlled by a motor current feedback control or a motor torque feedback control. 
         [0062]    On the other hand, when the VSA system is not in operation, the control unit  6  selects the motor angle feedback control which is based on the displacement of the piston stroke of the motor actuated cylinder  13 . In this case, because the VSA system  26  is not in operation and does not cause any changes in the loss property of the hydraulic system for the brake system, the brake system may be operated with a high responsiveness based on the motor angular position which allows the actual brake fluid pressure to accurately track the target brake fluid pressure. 
         [0063]    Optionally, the fluid pressure adjust control circuit  6   a  may be provided with a sensor abnormality detection circuit  49  to detect an event where the rotational angle sensor  12   a  produces an abnormal detection value due to noises or a fault in the sensor (as indicated by the imaginary line in  FIG. 4 ). In the event of detecting an abnormal detection value, the sensor abnormality detection circuit  49  forwards a sensor abnormal detection signal to the switching unit  48  so that the motor current feedback control may be performed as in the case where the VSA system  26  is in operation. Thereby, in the event of detecting an abnormal motor angle due to noises or sensor failures, the brake system is allowed to operate in an adequately stable manner. 
         [0064]    Although the present invention has been described in terms of a preferred embodiment thereof, it is obvious to a person skilled in the art that various alterations and modifications are possible without departing from the scope of the present invention which is set forth in the appended claims. 
         [0065]    For instance, the first embodiment may be modified by replacing the fluid pressure adjust control circuit  6   a  with the torque control circuit  6   b  and adding the low pass filter switching circuit  39  to the output of the motor angle converting unit  34  of the torque control circuit  6   b.    
         [0066]    The present invention is most advantageously used in brake-by-wire brake systems which combine the hydraulic braking and the regenerative braking in an intelligent manner, but also to more conventional brake-by-wire systems using only the hydraulic braking. 
         [0067]    The contents of the original Japanese patent application on which the Paris Convention priority claim is made for the present application as well as the contents of the prior art references mentioned in this application are incorporated in this application by reference.