Abstract:
In a brake-by-wire vehicle brake system that uses a solenoid shut-off valve ( 24   a,    24   b ), the electric power supplied to the solenoid ( 33 ) of the solenoid shut-off valve is controlled so as to decelerate a movement of a plunger ( 31, 32 ) of the valve as the plunger reaches a point immediately adjacent to a terminal point thereof. Thereby, a highly responsive property can be achieved while the noises that are generated by the plunger striking a stopper ( 36 ) or a valve seat ( 34 ) can be minimized. When actuating the plunger in the valve closing direction, the solenoid may receive electric power at a first level, a second level lower than the first level and a third level intermediate between the first and second levels in that order.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a brake-by-wire vehicle brake system including a solenoid shut-off valve in a fluid path thereof, and in particular to a technology for reducing noises generated from such a solenoid shut-off valve. 
       BACKGROUND OF THE INVENTION 
       [0002]    In electric vehicles and hybrid vehicles, it is a common practice to use an 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. Such a brake-by-wire vehicle brake system that combines the hydraulic braking and the regenerative braking may also be incorporated with an ABS (anti-lock brake system) and a VSA (vehicle stability assist) system as disclosed in JP 2009-227023. In such a vehicle brake system, an ABS hydraulic unit and a VSA hydraulic unit are combined with a motor actuated cylinder which is typically provided with a simulator for producing a brake pedal reaction. 
         [0003]    A brake-by-wire system is typically provided with a conventional master cylinder which is directly connected to the brake pedal. In case of a failure in the brake system, the hydraulic pressure produced from the master cylinder is forwarded to the wheel cylinders. However, in normal operation, the motor actuated cylinder provides the necessary brake fluid pressure which is actuated according to the stroke of the brake pedal detected by a stroke sensor while the output line of the master cylinder is shut off by a solenoid shut-off valve. This solenoid shut-off valve typically consists of a normally open solenoid valve which shuts every time the brake pedal is operated. 
         [0004]    A normally open solenoid valve is normally held open by a return spring, and is closed by energizing the solenoid that displaces a plunger in a valve closing direction against the spring force of the return spring. The fully open position and fully closed position of the solenoid shut-off valve is defined by the plunger being engaged by corresponding mechanical stoppers. It is known that some impact noise is inevitable every time the plunger reaches the fully open position and the fully closed position under the spring force and the magnetic force, respectively. 
         [0005]    In particular, in a brake-by-wire brake system, the solenoid valve in the output line of the master cylinder closes and opens every time the brake pedal is depressed and released so that any slight noise caused by the operation of the solenoid valve could create a problem. The noises may be reduced by reducing the electric power supplied to the solenoid valve, but it impairs the responsiveness of the solenoid valve, and is therefore not acceptable as a solution to the problem in most applications. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    In view of such problems of the prior art, a primary object of the present invention is to provide a brake-by-wire vehicle brake system that can provide a highly responsive property while minimizing the noises that are generated by a solenoid valve provided in an output line of the master cylinder. 
         [0007]    According to the present invention, such an object can be accomplished by providing a vehicle brake system, comprising: a master cylinder configured to be actuated by a brake pedal and receive a supply of brake fluid from a reservoir; an input sensor for detecting an operation of the brake pedal; a motor actuated cylinder configured to be actuated by an electric motor according to a control signal and receive a supply of brake fluid from the reservoir; a solenoid shut-off valve provided in a fluid line between the master cylinder and the motor actuated cylinder, the solenoid shut-off valve comprising a plunger provided with a valve member, a valve seat provided in a housing of the solenoid valve and configured to cooperate with the valve member, a return spring urging the plunger in a valve opening direction, a solenoid for actuating the plunger in a valve closing direction when energized and a stopper for mechanically limiting a stroke of the plunger in the valve opening direction; and a control unit for providing the control signal to the motor actuated cylinder according to a detection signal from the input sensor and closing the solenoid shut-off valve when the master cylinder is actuated in response to the detection signal from the input sensor; wherein the control unit is configured to control electric power supplied to the solenoid to decelerate a movement of the plunger as the plunger reaches a point immediately adjacent to at least one of the stopper or the valve seat. 
         [0008]    Thereby, the plunger is initially actuated at a relatively high speed but is decelerated as the plunger approaches a terminal point of the movement thereof, be it the position for fully closing the valve or the position for fully opening the valve, so that the impact noises that may be caused by the plunger striking the stopper or the valve seat can be minimized. Therefore, the solenoid shut-off valve can be actuated at a rapid rate without causing excessive noises. 
         [0009]    According to a more specific aspect of the present invention, the control unit supplies electric power to the solenoid at a first level, a second level lower than the first level and a third level intermediate between the first and second levels in that order when actuating the plunger in the valve closing direction. Thereby, the plunger is initially allowed to be actuated at a rapid rate, and is decelerated during a time interval immediately preceding the fully closed position of the valve so that the valve can be closed at a rapid rate, but the plunger is decelerated before the valve member strikes the valve seat. As a result, a rapid response and a quiet valve closure can be achieved at the same time. 
         [0010]    The power consumption of the solenoid can be made no different from that of the conventional arrangement if a time average of the first level and the second level is substantially equal to the third level. 
         [0011]    According to another aspect of the present invention, the control unit supplies electric power to the solenoid at a fourth level which is significantly less than required to oppose the spring force of the return spring, a fifth level which is higher than the fourth level and at a zero level in that order when actuating the plunger in the valve opening direction. 
         [0012]    Thereby, when actuating the plunger in the valve opening direction under the spring force of the return spring, the plunger is initially allowed to move in the valve opening direction without any significant resistance, but is thereafter resisted by the energization of the solenoid at the fifth level for a time interval immediately preceding the fully open position of the valve so that the speed of the plunger at the time of striking the stopper can be minimized, and the quick response and a quiet valve opening movement can be achieved at the same time. 
         [0013]    When the brake pedal is rapidly depressed, as it means that a powerful braking is required, and the noises that may be produced from the solenoid shut-off valve is of no importance. Based on such a recognition, the control unit may not control the electric power supplied to the solenoid to decelerate a movement of the plunger when an operation speed of the brake pedal is greater than a threshold value. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Now the present invention is described in the following with reference to the appended drawings, in which: 
           [0015]      FIG. 1  is a schematic diagram showing a vehicle incorporated with a vehicle brake system embodying the present invention; 
           [0016]      FIG. 2  is a diagram showing the overall structure of the vehicle brake system according to the present invention; 
           [0017]      FIG. 3  is a vertical sectional view showing a solenoid valve used in the vehicle brake system; 
           [0018]      FIG. 4  is a flowchart showing a control process for setting the duty ratio of the electric power supplied to the solenoid valve at the time of activating the brake; 
           [0019]      FIG. 5  is a flowchart showing a control process for setting the duty ratio of the electric power supplied to the solenoid valve at the time of releasing the brake; 
           [0020]      FIG. 6  is a time chart showing the duty ratio of the electric power supplied to the solenoid valve and the displacement of the plunger of the solenoid valve according to the present invention; and 
           [0021]      FIG. 7  is a time chart similar to that of  FIG. 6  showing the mode of operation of the prior art. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]      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). 
         [0023]    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. 
         [0024]    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. 
         [0025]    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. 
         [0026]    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 . 
         [0027]    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. 
         [0028]    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 . 
         [0029]    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. 
         [0030]    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  22   a  and  22   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.    
         [0031]    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. 
         [0032]    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. 
         [0033]    The motor actuated cylinder  13  is provided with oil passages  16   b  and  16   c  which in turn communicate with the reservoir tank  16  via a communication passage  16   a , and the pistons  21   a  and  21   b  are fitted with per se known seal members to shut the oil passages  16   b  and  16   c  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.    
         [0034]    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 fluid line  22   c  provided with 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 fluid line  22   d  provided with a normally open solenoid valve  24   b . A master cylinder side brake pressure sensor  25   a  is provided on the fluid line  22   c  between the first fluid chamber  17   a  and the solenoid valve  24   a  upstream of the normally open solenoid valve  24   a , and a motor actuated cylinder side brake pressure sensor  25   b  is provided on the fluid line  22   d  between the solenoid valve  24   b  and the second fluid pressure generating chamber  23   b  downstream of the normally open solenoid valve  24   b.    
         [0035]    As shown in  FIG. 3 , each of the solenoid valves  24   a  and  24   b  comprises a plunger  32  having a valve member  31  formed at one end thereof, a solenoid  33  provided around the plunger  32  and a return spring  35  urging the valve member  31  away from a valve seat  34  formed in a housing of the solenoid valve. When the solenoid  33  is not energized, the other end of the plunger  32  abuts a stopper  36  fixedly attached to the housing under the spring force of the return spring  35  so that the valve member  31  is held spaced from the valve seat  34  as shown in  FIG. 3 . When an electric power greater than a prescribed value is supplied to the solenoid  33 , the magnetic force created by the solenoid displaces the plunger  32  in the axial direction (downward in  FIG. 3 ) against the spring force of the return spring  35 , causing the valve member  31  to engage the valve seat  34  and shutting the corresponding fluid line  22   d ,  22   d.    
         [0036]    Referring to  FIG. 2  once again, 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. 
         [0037]    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 fluid lines  22   e  and  22   f  which provided with 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. 
         [0038]    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. 
         [0039]    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.    
         [0040]    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.    
         [0041]    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 . 
         [0042]    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. 
         [0043]    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 . 
         [0044]    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. 
         [0045]    The control process for the solenoid valves  24   a  and  24   b  by the control unit  6  is described in the following with reference to  FIGS. 4 to 6 . As the solenoid valves  24   a  and  24   b  are identical to each other in structure, and are operated in a similar manner, the solenoid valves  24   a  and  24   b  are collectively referred to as the solenoid valve  24  in the following description for the convenience of description. 
         [0046]    When the brake pedal  11  is operated and the detection value of the stroke sensor  11   a  increases from the initial value, the control unit  6  operates the solenoid valve  24  at a required duty ratio based on PWM control so that a brake fluid pressure corresponding to the pedal stroke may be produced. In particular, a drive flag of the solenoid valve  24  is set as “1”, and the duty ratio D for the solenoid valve  24  is set by executing an activation duty ratio setting process shown in  FIG. 4 . 
         [0047]    When the drive flag of the solenoid valve  24  is set as “1”, the control unit  6  computes a pedal stroke speed Vp from the detected value of the stroke sensor  11   a , and determines if the computed pedal stroke speed Vp is equal to or smaller than a prescribed threshold value Vpth in step ST 1 . If the computed pedal stroke speed Vp is greater than the prescribed threshold value Vpth or the determination result of step ST 1  is No, the control unit  6  sets the duty ratio D for the solenoid valve  24  as a normal duty ratio D 1  in step ST 8 , and terminates the activation duty ratio setting process. When the stroke speed of the brake pedal is high, it means that a powerful braking is desired, and it is appropriate to shut the solenoid valves  24   a  and  24   b  as quickly as possible while the noises generated by the solenoid valves  24   a  and  24   b  are of a small concern, 
         [0048]    When the computed pedal stroke speed Vp is equal to or smaller than the prescribed threshold value Vpth or the determination result of step ST 1  is Yes, the control unit  6  initializes a timer (set the zero value thereon), and sets the duty ratio D for the solenoid valve  24  as an activation duty ratio D 2  which is greater than the normal duty ratio D 1  in step ST 2 . Then, the control unit  6  adds value “1” to the timer in step ST 3 , and determines if the time set on the timer has exceeded a first prescribed time period T 1  in step ST 4 . The first prescribed time period T 1  is slightly shorter than the time period required for the plunger  32  to reach the fully closed position from the start of supplying electric power to the solenoid  33  when the electric current of the activation duty ratio D 2  is supplied to the solenoid  33 . 
         [0049]    If the time set on the timer has not exceeded the first prescribed time period T 1  in step ST 4  or the determination result of step ST 4  is No, steps ST 3  and ST 4  are repeated. Once the time set on the timer has exceeded the first prescribed time period T 1  or once the determination result of step ST 4  has changed to Yes, the control unit  6  sets the duty ratio D for the solenoid valve  24  as an activation deceleration duty ratio D 3  in step ST 5 . The activation deceleration duty ratio D 3  is smaller than the normal duty ratio D 1 , and is hence smaller than the activation duty ratio D 2 . In the illustrated embodiment, the activation deceleration duty ratio D 3  is selected as a value that balances with the spring force of the return spring  35  when the plunger  32  is at the fully closed position. 
         [0050]    Thereafter, the control unit  6  determines if the time on the timer has exceeded a second prescribed time period T 2  in step ST 6 . The second prescribed time period T 2  is selected as a value that reduces the displacement speed of the plunger  32  to a desired extent when the electric current of the actuation duty ratio D 2  is supplied to the solenoid  33  for the first prescribed time period T 1 , and it is followed by the supply of electric current of the activation deceleration duty ratio D 3  to the solenoid  33  for the second prescribed time period T 2 . If the time on the timer has not exceeded the second prescribed time period T 2  and, therefore, the determination result of step ST 6  is No, the value “1” is added to the timer in step ST 7 , and the program flow returns to step ST 6 . 
         [0051]    Once the second prescribed time period T 2  has elapsed in step ST 6 , and, therefore, the determination result of step ST 6  has changed to Yes, the control unit  6  sets the duty ratio for the solenoid valve  24  to the normal duty ratio D 1  in step ST 8 , and terminates the activation duty ratio setting process. Thereafter, the solenoid valve  24  is held in the fully closed state by the solenoid  33  energized by the electric current of the normal duty ratio D 1  against the biasing force of the return spring  35 . As can be readily appreciated, the normal duty ratio D 1  is selected as a level which is adequate to hold the valve in the fully closed position against the biasing force of the return spring  35 . 
         [0052]    When the brake pedal  11  is released, and the detection value of the stroke sensor  11   a  has returned to the initial value, the control unit  6  sets the drive flag for the solenoid valve  24  to “0”, and sets the duty ratio D for the solenoid valve  24  by executing a deactivation duty ratio setting process shown in  FIG. 5 . 
         [0053]    When the drive flag for the solenoid valve  24  is set to “0”, the control unit  6  initializes a timer (set the zero value thereto), and sets the duty ratio D for the solenoid valve  24  as a de-activation start duty ratio D 4  in step ST 11 . The de-activation start duty ratio D 4  is selected such that the plunger  32  can be displaced from the fully closed position to the fully open position under the spring force of the return spring  35 , and may consist of zero as is the case with the illustrated embodiment. Then, the control unit  6  adds value “1” to the timer in step ST 12 , and determines if the time set on the timer has exceeded a third prescribed time period T 3  in step ST 13 . The third prescribed time period T 3  is slightly shorter than the time period required for the plunger  32  to reach the fully closed position from the fully open position (under the spring force of the return spring  35 ) when the electric current of the de-activation start duty ratio D 4  is supplied to the solenoid  33 . 
         [0054]    If the time set on the timer has not exceeded the third prescribed time period T 3  in step ST 13  or the determination result of step ST 13  is No, steps ST 12  and ST 13  are repeated. Once the time set on the timer has exceeded the third prescribed time period T 3  in step ST 13  or once the determination result of step ST 13  has changed to Yes, the control unit  6  sets the duty ratio D for the solenoid valve  24  as a de-activation deceleration duty ratio D 5  in step ST 14 . The de-activation deceleration duty ratio D 5  is greater than the de-activation start duty ratio D 4 . In the illustrated embodiment, the de-activation deceleration duty ratio D 5  is selected as a value that balances with the spring force of the return spring  35  when the plunger  32  is at the fully open position. 
         [0055]    Thereafter, the control unit  6  determines if the time on the timer has exceeded a fourth prescribed time period T 4  in step ST 15 . If the time on the timer has not exceeded the fourth prescribed time period T 4  or the determination result of step ST 15  is No, the value “1” is added to the timer in step ST 16 , and the program flow returns to step  15 . If the fourth prescribed time period T 4  has elapsed, and, therefore, the determination result of ST 15  has changed to Yes, the control unit  6  sets the duty ratio D for the solenoid valve  24  to zero in step ST 17 , and terminates the deactivation duty ratio setting process. As a result, the solenoid valve  24  is held at the fully opening position under the spring force of the return spring  35 . 
         [0056]    The activation duty ratio D 2 , the activation deceleration duty ratio D 3 , the first prescribed time period T 1  and the second prescribed time period T 2  may be selected such that the following relationship holds. 
         [0000]      ( D 2− D 1)× T 1=( D 1− D 3)×( T 2− T 1)  (1)
 
         [0000]    In other words, the time average of the electric current during the time interval of T 0  to T 2  including the interval of the activation deceleration duty ratio D 3  and the interval of the activation duty ratio D 2  is equal to the normal duty ratio D 1 . 
         [0057]    By controlling the solenoid valves  24   a  and  24   b  in this manner, the plunger  32  displaces as illustrated in  FIG. 6  when the pedal stroke speed Vp is equal to or less than the threshold value VPth. More specifically, when the brake pedal  11  is operated, and the drive flag is set to “1”, the plunger  32  is initially driven by the activation duty ratio D 2  which is greater than the normal duty ratio D 1 , and the duty ratio is changed to the de-activation deceleration duty ratio D 3  which is smaller than the normal duty ratio D 1  immediately before the plunger  32  reaches the fully closed position. Therefore, the plunger  32  is decelerated as it approaches the fully closed position. 
         [0058]    When the brake pedal  11  is released, and the drive flag is set to “0”, the plunger  32  is initially driven toward the fully open position under the spring force of the return spring  35  while the duty ratio is at the de-activation duty ratio D 4  which may be zero, and the duty ratio is changed to the de-activation deceleration duty ratio D 5  as the plunger approaches the fully open position. After the plunger  32  is adequately decelerated by the de-activation deceleration duty ratio D 5 , the supply of electric current to the solenoid  33  is terminated, and the plunger  32  is held at the fully open position under the spring force of the return spring  35 . 
         [0059]    On the other hand, according to the conventional arrangement shown in FIG.  7 , the solenoid valves  24   a  and  24   b  are driven by a fixed duty ratio. In this case, the speed of the plunger  32  is at the maximum level when arriving at the fully closed position and the fully open position so that the noises that are generated when the valve member  31  strikes the valve seat  34  and when the plunger  32  abuts the stopper  36  are significant. However, according to the present invention, the speed of the plunger is controlled when the valve member  31  strikes the valve seat  34  and when the plunger  32  abuts the stopper  36  so that the generation of noises can be minimized. 
         [0060]    Even though the speed at which the plunger  32  strikes the stopper  36  is reduced, the initial speed of actuating the plunger  32  is increased because the activation duty ratio D 2  is greater than the normal duty ratio D 1 , and the overall time period required for the plunger  32  to travel from the fully open position to the fully closed position can be minimized. 
         [0061]    By performing the process for decelerating the plunger in a late part of each stroke of the plunger (steps ST 2  to ST 7 ) only when the pedal stroke speed Vp is equal to or smaller than the prescribed threshold value Vpth, the rapid and accurate operation of the brake system can be performed in response to a rapid depression of the brake pedal which typically indicates the need for a powerful braking. 
         [0062]    Because the normal duty ratio D 1  is smaller than the activation duty ratio D 2 , the increase in the initial speed of the plunger  32  and the minimization of power consumption of the solenoid in the fully closed position can be achieved at the same time. 
         [0063]    The power consumption of the solenoid can be made no different from that of the conventional arrangement if the time average of the electric current during the time interval of T 0  to T 2  including the interval of the activation deceleration duty ratio D 3  and the interval of the activation duty ratio D 2  is equal to the normal duty ratio D 1 . Thereby, the load of the solenoid is no greater than the conventional arrangement, and the existing design can be used without causing any undue stress to the solenoid valve. 
         [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]    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. 
         [0066]    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.