Vehicle braking system

Abstract

Vehicle braking system, comprising: a master cylinder (15) adapted to be actuated by a brake pedal (11) and to receive a supply of brake fluid from a reservoir (16); an input sensor (11a) for detecting an operation of the brake pedal (11); a motor-operated cylinder (13) adapted to be actuated by an electric motor (12) in accordance with a control signal and to receive a supply of brake fluid from the reservoir (16); a solenoid shut-off valve (24a, 24b) provided with a fluid line (22c, 22d) between the master cylinder (15) and the motor-operated cylinder (13), the solenoid shut-off valve (24a, 24b) comprising: a piston (32) provided with a valve element (31), a valve seat (34) provided in a housing of the solenoid shut-off valve (24a, 24b) and adapted to cooperate with the valve element (31), a return spring (35) urging the piston (32) in a valve-opening direction, a solenoid (33) for actuating the piston (32) in a valve-closing direction upon energization, and a stopper (36) for mechanically limiting a stroke of the piston (32) in the valve-opening direction; and a control unit (6) for providing the control signal to the motor-operated cylinder (13) according to a detection signal from the input sensor (11a) and for closing the solenoid shut-off valve (24a, 24b) when the motor-operated cylinder (13) is actuated in response to the detection signal from the input sensor (11a), characterized by that the control unit (6) is arranged to control electrical power supplied to the solenoid (33) in order to slow down a movement of the piston (32) when the piston (32) reaches a point which is immediately adjacent to at least one of the stopper (36) or the valve seat (34), and that the control unit (6) supplies electrical power to the solenoid (33) at a first level (D2), a second level (D3) which is lower than the first level (D2), and a third level (D1) which is between the first level (D2) and the second level (D3), in this order, when the piston (32) is actuated in the valve closing direction, so that the solenoid shut-off valve (24a, 24b) is held in a fully closed state by the solenoid (33) which is excited by the electrical power at the third level (D1) against a biasing force of the return spring (35).

Description

TECHNICAL FIELD

[0001] The present invention relates to a brake-by-wire vehicle braking system including a solenoid cut-off valve in a fluid path thereof, and more particularly to a technology for reducing noise generated from a solenoid cut-off valve. BACKGROUND OF THE INVENTION

[0002] In electric vehicles and hybrid vehicles, it is common practice to use an electric motor connected to the vehicle's drive axle as a generator, generating electrical power and simultaneously providing braking force as the vehicle decelerates. This is called regenerative braking. Such a brake-by-wire system, which combines hydraulic braking and regenerative braking, can also be incorporated into an ABS (anti-lock braking system) and a VSA (vehicle stability assist) system, as disclosed in JP 2009-227023. In such a vehicle braking system, an ABS hydraulic unit and a VSA hydraulic unit are combined with a motor-operated cylinder, which is typically provided with a simulator for generating a brake pedal response.

[0003] A vehicle braking system is typically provided with a conventional master cylinder directly connected to the brake pedal. In the event of a failure in the braking system, the hydraulic pressure generated by the master cylinder is directed to the wheel cylinders. However, during normal operation, the master cylinder provides the necessary brake fluid pressure, which is applied according to the brake pedal stroke detected by the stroke sensor, while the master cylinder's output line is shut off by a solenoid cut-off valve. This solenoid cut-off valve typically consists of a normally open solenoid cut-off valve that closes whenever the brake pedal is applied.

[0004] A normally open solenoid shutoff valve is normally held open by a return spring and is closed by the energization of the coil, which moves a piston in a valve-closing direction against the spring force of the return spring. The fully open and fully closed positions of the solenoid shutoff valve are defined by the piston engaging a corresponding mechanical stop. It is known that some impact noise is inevitable each time the piston reaches the fully open and fully closed positions under the spring force and solenoid force, respectively.

[0005] Specifically, in a brake-by-wire braking system, the solenoid isolation valve in the master cylinder output line opens and closes each time the brake pedal is depressed and released, so any slight noise caused by the solenoid isolation valve's operation could be a problem. The noise can be reduced by reducing the electrical power supplied to the solenoid isolation valve, but it impairs the responsiveness of the solenoid isolation valve, and therefore, it is not acceptable as a solution to the problem in most applications. SUMMARY OF THE INVENTION

[0006] In view of such problems in the prior art, the main object of the present invention is to provide a brake-by-wire vehicle braking system which can provide a high responsive property while minimizing the noise generated by a solenoid cut-off valve provided in an output line of the master cylinder.

[0007] According to the present invention, this object is achieved by providing a vehicle braking system having the features of claim 1. The vehicle braking system comprises: a master cylinder configured to be actuated by a brake pedal and to receive a supply of brake fluid from a reservoir; an input sensor for detecting an actuation of the brake pedal; a motor-operated cylinder configured to be actuated by an electric motor according to a control signal and to receive a supply of brake fluid from the reservoir; a solenoid shut-off valve provided with a fluid line between the master cylinder and the motor-operated cylinder, the solenoid shut-off valve comprising: a piston provided with a valve element; a valve seat provided in a housing of the solenoid shut-off valve and configured toto cooperate with the valve element, a return spring that urges the piston in a valve-opening direction, a solenoid for actuating the piston in a valve-closing direction when energized, and a stopper for mechanically limiting a stroke of the piston in the valve-opening direction; and a control unit for providing the control signal to the motor-operated cylinder according to a detection signal from the input sensor and for closing the solenoid shut-off valve when the motor-operated cylinder is actuated in response to the detection signal from the input sensor. The control unit is configured to control electrical power supplied to the solenoid to decelerate movement of the piston when the piston reaches a point immediately adjacent to at least one of the stopper or the valve seat.and that the control unit supplies electrical power to the solenoid at a first level, a second level lower than the first level, and a third level intermediate between the first level and the second level, in this order, when the piston is actuated in the valve closing direction, so that the solenoid shut-off valve is held in a fully closed state by the solenoid energized by the electrical power at the third level against a biasing force of the return spring.

[0008] This allows the piston to initially actuate at a relatively high speed, but slows down as the piston approaches its endpoint of travel, whether the fully closed position or the fully opened position, minimizing the impact noise caused by the piston hitting the stopper or valve seat. Therefore, the solenoid shutoff valve can be actuated at a rapid rate without generating excessive noise.

[0009] Furthermore, the piston is initially allowed to actuate at a rapid rate and is decelerated during a time interval immediately preceding the fully closed position of the valve, allowing the valve to close at a rapid rate, but the piston is decelerated before the valve element impacts the valve seat. Consequently, a rapid response and quiet valve closing can be achieved simultaneously.

[0010] The power consumption of the solenoid can be made such that there is no difference from a conventional arrangement if a time average of the first level and the second level is equal to the third level.

[0011] According to another aspect of the present invention, the control unit supplies electrical power to the solenoid at a fourth level substantially lower than required to counteract the spring force of the return spring, a fifth level higher than the fourth level, and a zero level, in that order, when the piston is actuated in the valve opening direction.

[0012] Thereby, when the piston is actuated in the valve opening direction under the spring force of the return spring, the piston is initially allowed to move in the valve opening direction without substantial resistance, but is thereafter restrained by the energization of the coil at the fifth level for a time interval immediately preceding the fully open position of the valve, so that the speed of the piston at the time of impact with 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 depressed quickly, which means that powerful braking is required, the noise generated by the solenoid shutoff valve is insignificant. Based on this finding, the control unit cannot regulate the electrical power supplied to the solenoid to slow down piston movement if the brake pedal actuation speed is greater than a threshold. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The present invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a schematic diagram showing a vehicle equipped with a vehicle braking system embodying the present invention; Fig. Fig. 2 is a diagram showing the overall structure of the vehicle braking system according to the present invention; Fig. 3 is a vertical sectional view showing a solenoid cut-off valve used in the vehicle braking system; Fig. 4 is a flowchart showing a control process for adjusting the duty ratio of the electric power supplied to the solenoid cut-off valve at the time of activating the brake; Fig. 5 is a flowchart showing a control process for adjusting the duty ratio of the electric power supplied to the solenoid cut-off valve at the time of brake release; Fig. 6 is a timing chart showing the duty cycle of the electric power supplied to the solenoid shut-off valve and the displacement of the spool of the solenoid shut-off valve according to the present invention; and Fig. 7 a timing diagram similar to that in Fig. 6, which shows the operating mode of the prior art. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Fig. Figure 1 shows a braking system of an electric or hybrid vehicle embodying the present invention. This vehicle V includes a pair of front wheels 2 located at its front end and a pair of rear wheels 3 located at its rear end. The front wheels 2 are connected to front axles 4, which in turn are connected to a motor / generator 5 in a torque-transmitting relationship via a differential gear device (not shown in the drawing).

[0016] The motor / generator 5 functions both as an electric motor for propelling the vehicle and as a generator for providing regenerative braking. Specifically, the motor / generator 5 can receive electrical power from a battery 7 serving as an energy source (power source) via an inverter 10, and can also supply electrical energy (power) to the battery 7 (for charging) by converting kinetic energy into electrical energy (power) during regenerative braking by means of the motor / generator 5.

[0017] A control unit (ECU) 6, which is equipped with a CPU control circuit, performs various control operations for the vehicle V, including the distribution of braking force, as will be described below. The control unit 6 is electrically connected to the inverter 10. In the case of an electric vehicle, the Fig. 1 may be used 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, which is Fig. 1 is marked with a double-dot chain-dot line. The illustrated motor E is configured to drive the front wheels, but can also be configured to drive the rear wheels or all four wheels.

[0018] Each of the front and rear wheels 2, 3 is equipped with a conventional disc brake comprising a disc 2a, 3a integrally connected to the wheel 2, 3 and a brake caliper housed in a wheel cylinder 2b, 3b. The wheel cylinder 2b, 3b is connected to a brake fluid pressure generating unit 8 via a conventional brake pipe. The brake fluid pressure generating unit 8 consists of a hydraulic circuit configured to distribute hydraulic brake pressure to the various wheels and adjust the hydraulic brake pressure level for each wheel.

[0019] A wheel speed sensor 9 is provided in association with each wheel 2, 3, and a displacement sensor 11a is provided in association with a brake pedal 11 (operated by a vehicle driver) to detect a brake application amount or brake lowering stroke. The detection signals of the wheel speed sensors 9 and the displacement sensor 11a are forwarded to the control unit 6.

[0020] Upon detecting that an output signal of the displacement sensor 11a of the brake pedal 11 becomes greater than zero, the control unit 6 performs a braking control action. In the illustrated embodiment, the braking action is performed as a brake-by-wire system and includes regenerative cooperative control, which combines regenerative braking and hydraulic braking.

[0021] The braking system 1 of this vehicle V is described below with reference to Fig. 2. The braking system 1 is comprised of a brake-by-wire system that detects the brake operation amount (brake pedal stroke) of the brake pedal 11 by using an input sensor 11a (hereinafter referred to as a stroke sensor), and generates a brake fluid pressure according to the detected brake operation amount by using a motor-operated cylinder 13 (serving as a brake fluid pressure generating cylinder) configured to be driven by an electric motor 12 (hereinafter referred to as an electric servomotor).

[0022] As in Fig. 2, one end of a rod 14 is connected to the brake pedal 11, which in turn is pivotally connected to the vehicle body to convert the angular movement of the brake pedal 11 into a substantially linear movement of the rod 14, and the other end of the rod 14 engages a first piston 15a of a master cylinder 15 of a tandem type in the direction to force the piston 15a into the master cylinder 15. The master cylinder 15 additionally accommodates therein a second piston 15b on the side of the first piston 15a facing away from the rod 14, and the first and second pistons 15a and 15b are both elastically urged toward the rod 14 by respective springs. The brake pedal 11 is also urged by a spring (not shown in the drawing) so that the brake pedal 11 is in the Fig. 2 is held in the original position shown by a stopper not shown in the drawings when the brake is not applied.

[0023] The master cylinder 15 is provided with a reservoir 16 for receiving and supplying brake fluid in response to the displacement of the two pistons 15a and 15b. The pistons 15a and 15b are each equipped with a sealing element for closing the oil passages 16a and 16b, respectively, which connect the interior of the master cylinder 15 to the reservoir 16. Within the master cylinder 15, a first fluid chamber 17a is defined between the first and second pistons 15a and 15b, and a second fluid chamber 17b is defined on the side of the second piston 15b, which is opposite to the first piston 15a.

[0024] In addition to the electric servomotor 12, the motor-operated cylinder 13 is provided with a gear mechanism 18 connected to the electric servomotor 12, a threaded rod 19 connected to the gear mechanism 18 via a ball screw mechanism for angular movement, and a first piston 21a and a second piston 21b coaxial with the threaded rod 19 and connected to each other.

[0025] The second piston 21b is fixedly provided with a connecting member 20 projecting toward the first piston 21a, and the other end of the connecting member 20 is connected to the first piston 21a to allow relative axial movement with the first piston 21a to a certain extent. Further, the first and second pistons 21a and 21b are elastically urged toward the threaded rod 19 by respective springs 27a and 27b. Specifically, the spring 27a urges the first and second pistons 21a and 21b away from each other. Thereby, the first piston 21a is capable of advancing independently of the second piston 21b, but is capable of retracting the second piston 21b to the original position via the connecting member 20 when the first piston 21a retreats.

[0026] The motor-driven cylinder 13 is provided with oil passages 16b and 16c, which in turn communicate with the reservoir 16 via a communication passage 16a. The pistons 21a and 21b are equipped with sealing elements known per se to close the oil passages 16b and 16c when needed. In the motor-driven cylinder 13, a first fluid pressure generating chamber 23a is defined between the first and second pistons 21a and 21b, and a second fluid pressure generating chamber 23b is defined on the side of the second piston 21b facing away from the first piston 21a.

[0027] The first fluid chamber 17a of the master cylinder 15 is connected to the first fluid pressure generating chamber 23a of the motor-operated cylinder 13 via a fluid line 22c provided with a normally open solenoid shut-off valve (or normally open solenoid valve) 24a, and the second fluid chamber 17b of the master cylinder 15 is connected to the second fluid pressure generating chamber 23b of the motor-operated cylinder 13 via a fluid line 22d provided with a normally open solenoid shut-off valve (or normally open solenoid valve) 24b.A master cylinder side brake pressure sensor 25a is provided on the fluid line 22c between the first fluid chamber 17a and the solenoid cut-off valve 24a upstream of the normally open solenoid cut-off valve 24a, and a motor-operated cylinder side brake pressure sensor 25b is provided on the fluid line 22d between the solenoid cut-off valve 24b and the second fluid pressure generating chamber 23b downstream of the normally open solenoid cut-off valve 24b.

[0028] As in Fig. As shown in Figure 3, each of the solenoid shut-off valves 24a and 24b comprises a piston 32 having a valve element 31 formed at one end thereof, a solenoid 33 provided around the piston, and a return spring 35 urging the valve element 31 away from a valve seat 34 formed in a housing of the solenoid shut-off valve. When the solenoid 33 is not energized, the other end of the piston 32 abuts against a stopper 36 fixedly attached to the housing under the spring force of the return spring 35, so that the valve element 31 is kept away from the valve seat 34, as shown in Figure 3. Fig. 3. When an electric power greater than a prescribed value is supplied to the solenoid 33, the magnetic force caused by the solenoid displaces the piston 32 in the axial direction (downward, in Fig. 3) against the spring force of the return spring 35, which causes the valve element 31 to engage with the valve seat 34 and close the corresponding fluid line 22d, 22d.

[0029] Referring again to Fig. 2, a cylinder-like simulator 28 is provided on the line between the second fluid chamber 17b and the solenoid shutoff valve 24b via a normally closed solenoid shutoff valve 24c. The simulator 28 is provided with a cylinder having an internal space separated from a piston 28a. A fluid receiving chamber 28b is defined on the side of the piston 28a facing the solenoid shutoff valve 24b, and a coil spring 28c is disposed between the opposite sides of the piston 28a and the opposite axial end of the cylinder of the simulator 28.When the brake pedal 11 is lowered to cause the brake fluid in the second fluid chamber 17b to flow into the fluid receiving chamber 18b while the solenoid cut-off valves 24a and 24b are closed and the solenoid cut-off valve 24c is open, the biasing force of the coil spring 28c is transmitted to the brake pedal 11, so that the vehicle driver is caused to experience a brake pedal reaction from the brake pedal 11 in a manner similar to the case of a conventional brake system in which the master cylinder and the wheel cylinders are directly connected to each other.

[0030] The first fluid pressure generating chamber 23a and the second fluid pressure generating chamber 23b of the motor-operated cylinder 13 are connected to a plurality (four, in the illustrated embodiment) of wheel cylinders 2b, 3b via fluid lines 22e and 22f, which are provided with a VSA system 26, which may consist of a conventional vehicle behavior stabilization control system configured to control an ABS for preventing wheel lock at the time of braking, a TCS (Traction Control System) for preventing wheel spin at the time of acceleration, and a side slip control for controlling the side slip of the vehicle in a coordinated manner at the time of cornering. For details of such systems, reference should be made to various prior patent applications on such matters.The VSA system 26 includes brake actuators, which include various hydraulic devices responsible for controlling a first system for the wheel cylinders 2b of the front wheels and a second system for the wheel cylinders 3b of the rear wheels, and a VSA control unit 26a for controlling the various hydraulic devices. The VSA system of the described embodiment is provided with various control functions, but may include only parts of such control functions and / or may include other control functions without departing from the spirit of the present invention.

[0031] The overall control of the brake fluid pressure generating unit 8 is performed by the control unit 6. The control unit 6 receives various detection signals from the stroke sensor 11a, the brake pressure sensors 25a and 25b, and other sensors (not shown in the drawings) for detecting the behavior of the vehicle. According to the detection signal from the stroke sensor 11a and the operating state of the vehicle, which can be detected from the detection signals of the various sensors, the control unit 6 controls the brake fluid pressure generated by the motor-operated cylinder 13.In the case of a hybrid vehicle (or electric vehicle), as in the illustrated embodiment, the control unit 6 is configured to control the braking force allocation or the brake fluid pressure generated by the motor-operated cylinder 13 when the motor / generator provides regenerative braking, depending on the amount or magnitude of regenerative braking.

[0032] The control operation mode during normal braking is described below. Fig. Figure 2 shows the system state when the brake pedal 11 is not depressed. The detection value of the stroke sensor 11a is at an initial value (=0), and the control unit 6 does not generate a brake fluid pressure generation signal. At this time, the threaded rod 19 of the motor-operated cylinder 13 is in the most retracted position, and the two pistons 21a and 21b of the motor-operated cylinder 13 are also in their respective most retracted positions under the spring force of the return springs 27a and 27b, so that no brake fluid pressure is generated in either of the fluid pressure generation chambers 23a and 23b.

[0033] When the brake pedal 11 is depressed to a certain degree and the detection value of the stroke sensor 11a has become greater than zero, brake-by-wire control is performed such that the two solenoid cutoff valves 24a and 24b are closed to prevent the fluid pressure generated by the master cylinder 15 from being transmitted to the motor-operated cylinder 13 and the solenoid cutoff valve 24c is opened to cause the fluid pressure generated by the master cylinder 15 to be transmitted to the simulator 28. According to the input detection value (brake operation amount) detected by the stroke sensor 11a, the control unit 6 detects a target fluid pressure taking regenerative braking into account and transmits a corresponding motor drive command value (operation value) to the electric servomotor 12.This, in turn, causes the threaded rod 19 and consequently the first piston 21 to be ejected according to this command value, and a brake fluid pressure corresponding to the input or lowering stroke (brake application value) of the brake pedal 11 is generated in the first fluid pressure generating chamber 23a. At the same time, the second piston 21b is displaced forward under the pressure in the first fluid pressure generating chamber 23a against the biasing force of the return spring 27b, and the corresponding brake fluid pressure is generated in the second fluid pressure generating chamber 23b.

[0034] When the vehicle operator has depressed the brake pedal 11 in the return direction according to the return stroke of the brake pedal (or released the brake pedal) detected by the stroke sensor 11a, the electric servomotor 12 returns the threaded rod 19 and consequently the first piston 21a toward the home position, so that the brake fluid pressure is reduced by an amount corresponding to the return stroke or the current depression of the brake pedal 11. When the brake pedal 11 is completely returned to the home position by the return spring (not shown in the drawing), the control unit 6 opens the solenoid shutoff valves 24a and 24b. This allows the brake fluid in the wheel cylinders 2b and 3b to return to the reservoir 16 via the motor-operated cylinder 13, and the braking force is eliminated.When the detected value of the stroke sensor 11a returns to the initial value, the first piston 21a is caused to return to the initial position, and this in turn causes the second piston 21b to return to the initial position due to the force transmitted via the connecting member 20.

[0035] When normal braking control is performed, the brake fluid pressure generated by the motor-operated cylinder 13 is supplied to the wheel cylinders 2b and 3b of the front and rear wheels via the VSA system 26. When the VSA system 26 performs brake force distribution control, the braking force of each wheel is individually controlled as required by the VSA system 26. When the VSA system 26 is not operating, the VSA system 26 allows the brake fluid supplied by the motor-operated cylinder 13 to be supplied directly to the wheel cylinders 2b and 3b of the front and rear wheels.

[0036] When regenerative braking is performed, the control unit 6 causes the motor / generator 5 to operate as a generator, so that the amount of regenerative braking is generated depending on the brake application amount determined by the stroke of the brake pedal 11. When the vehicle deceleration required by the brake application amount cannot be generated by regenerative braking alone, the electric servomotor 12 drives the motor-operated cylinder 13, and coordinated combined braking, which includes regenerative braking and hydraulic braking, is performed. In this embodiment, the target brake fluid pressure can be determined by subtracting the regenerative braking from the total required braking force determined by the brake application amount or the input amount.Alternatively, the amount of actuation of the motor-operated cylinder can be selected to generate a hydraulic braking force that is related to a specific ratio to the total required braking force. According to the present invention, this control action can be performed in a manner known per se, provided that the operation of the motor-operated cylinder 13 is carried out in conjunction with the lowering stroke of the brake pedal 11.

[0037] The timing of closing the solenoid shut-off valve 24c may be selected as the time at which the fluid pressure of the second fluid chamber 17b has dropped to a sufficiently low level to cause the piston 28a to move to the position shown in Fig. 2 under the biasing force of the coil spring 28c. For example, this timing may be selected as the time when a predetermined period of time has elapsed since the two solenoid shutoff valves 24a and 24b are closed. It is also possible to select the timing when the detection value of the brake pressure sensor 25b on the motor-operated cylinder 13 side has dropped below a predetermined value, such as zero.

[0038] The control operation for the solenoid shut-off valves 24a and 24b by the control unit 6 will be described below with reference to the Fig. 4 to 6. Since the solenoid shutoff valves 24a and 24b have an identical structure to each other and are operated in the same manner, the solenoid shutoff valves 24a and 24b are collectively referred to as the solenoid shutoff valve 24 in the following description for convenience of description.

[0039] When the brake pedal 11 is depressed and the detection value of the stroke sensor 11a increases from the initial value, the control unit 6 operates the solenoid shutoff valve 24 with a required duty ratio based on PWR control, so that a brake fluid pressure corresponding to the pedal stroke can be produced. Specifically, a drive flag of the solenoid shutoff valve 24 is set to "1", and the duty ratio D of the solenoid shutoff valve 24 is determined by executing a Fig. 4 shown activation duty cycle setting procedure.

[0040] When the drive flag of the solenoid shut-off valve 24 is set to "1," the control unit 6 calculates a pedal stroke speed Vp from the detected value of the stroke sensor 11a, and determines in step ST1 whether the calculated pedal stroke speed Vp is equal to or less than a predetermined threshold value Vpth. If the calculated pedal stroke speed Vp is greater than the predetermined threshold value Vpth or the determination result of step ST1 is NO, the control unit 6 sets the duty ratio D of the solenoid shut-off valve 24 as a normal duty ratio D1 in step ST8, and ends the activation duty ratio setting process.If the stroke speed of the brake pedal is high, this means that powerful braking is desired and it is appropriate to close the solenoid shut-off valves 24a and 24b as quickly as possible, while the noise generated by the solenoid shut-off valves 24a and 24b is of little importance.

[0041] If the calculated pedal stroke speed Vp is equal to or less than the predetermined threshold value Vpth or the determination result of step ST1 is Yes, the control unit 6 initializes a timer (sets the zero value thereto) in step ST2, and sets the duty ratio D for the solenoid shutoff valve 24 as an activation duty ratio D2, which is larger than the normal duty ratio D1. Then, the control unit 6 adds the value "1" to the timer in step ST3, and determines in step ST4 whether the time set on the timer has exceeded a first prescribed time period T1. The first prescribed time period T1 is slightly shorter than the time required for the spool 32 to reach the fully closed position from the start of the supply of electric power to the solenoid 33 when the electric current of the activation duty ratio D2 is supplied to the solenoid 33.

[0042] If the time set on the timer has not exceeded the first prescribed time T1 in step ST4 or the determination result of step ST4 is No, steps ST3 and ST4 are repeated. Once the time set on the timer has exceeded the first prescribed time T1 or once the determination result of step ST4 has changed to Yes, the control unit 6 sets the duty ratio D for the solenoid shut-off valve 24 as an activation-deceleration duty ratio D1 in step ST5. The activation-deceleration duty ratio D3 is smaller than the normal duty ratio D1, and thus smaller than the activation duty ratio D2. In the illustrated embodiment, the activation-deceleration duty ratio D3 is selected as a value that balances the spring force of the return spring 35 when the piston 32 is in the fully closed position.

[0043] Thereafter, the control unit 6 determines in step ST6 whether the time on the timer has exceeded a second prescribed time T2. The second prescribed time T2 is selected as a value that reduces the displacement speed of the piston 32 to a desired extent when the electric current with the activation duty cycle D2 is supplied to the solenoid 33 for the first prescribed time T1, followed by the supply of electric current with the activation deceleration duty cycle D3 to the solenoid 33 for the second prescribed time T2. If the time on the timer has not exceeded the second prescribed time T2 and therefore the determination result of step ST6 is No, the value "1" is added to the timer in step ST7, and the program flow returns to step ST6.

[0044] Once the second prescribed time period T2 has elapsed in step ST6, and the determination result in step ST6 has therefore changed to Yes, the control unit 6 sets the duty ratio for the solenoid shutoff valve 24 to the normal duty ratio D1 in step ST8 and terminates the activation duty ratio setting process. Thereafter, the solenoid shutoff valve 24 is held in the fully closed state by the solenoid 33, which is energized by the electric current with the normal duty ratio D1, against the return force of the return spring 35. As can be readily appreciated, the normal duty ratio D1 is selected as a level sufficient to maintain the valve in the fully closed position against the return force of the return spring 35.

[0045] When the brake pedal 11 is released and the detection value of the stroke sensor 11a has returned to the initial value, the control unit 6 sets the drive flag for the solenoid shut-off valve 24 to “0”, and sets the duty ratio D for the solenoid shut-off valve 24 by executing a Fig. 5 shown deactivation duty cycle setting procedure.

[0046] When the drive flag of the solenoid shutoff valve 24 is set to "0," the control unit 6 initializes a timer (sets the zero value thereto) in step ST11, and sets the duty ratio D for the solenoid shutoff valve 24 as a deactivation start duty ratio D4. The deactivation start duty ratio D4 is selected such that the spool 32 can be displaced from the fully closed position to the fully opened position under the spring force of the return spring 35, and may be zero, as in the case of the present embodiment. Then, the control unit 6 adds the value "1" to the timer in step ST12, and determines in step ST13 whether the time set on the timer has exceeded a third prescribed time period T3.The third prescribed time period T3 is slightly shorter than the time period required for the piston 32 to reach the fully closed position from the fully opened position (under the spring force of the return spring 35) when the electric current having the deactivation start duty cycle D4 is supplied to the solenoid 33.

[0047] If the time set on the timer has not exceeded a third prescribed time period T3 in step ST13 or the determination result of step ST3 is No, steps ST12 and ST13 are repeated. Once the time set on the timer has exceeded the third prescribed time period T3 in step ST13 or once the determination result in step ST13 has changed to Yes, the control unit 6 sets the duty ratio D for the solenoid shut-off valve 24 as a deactivation deceleration duty ratio D5 in step ST14. The deactivation deceleration duty ratio D5 is greater than the deactivation start duty ratio D4. In the illustrated embodiment, the deactivation deceleration duty ratio D5 is selected as a value that balances the spring force of the return spring 35 when the piston 32 is in the fully open position.

[0048] Next, the control unit 6 determines in step ST15 whether the time on the timer has exceeded a fourth prescribed time T4. If the time on the timer has not exceeded the fourth prescribed time T4 or the determination result of step ST15 is No, the value "1" is added to the timer in step ST16, and the program flow returns to step ST15. If the fourth prescribed time T4 has elapsed and therefore the determination result of step ST15 has changed to Yes, the control unit 6 sets the duty ratio D for the solenoid shut-off valve 24 to zero in step ST17 and ends the deactivation duty ratio setting process. Consequently, the solenoid shut-off valve 24 is held at the fully open position under the spring force of the return spring 35.

[0049] The activation duty cycle D2, the activation deceleration duty cycle D3, the first prescribed time period T1 and the second prescribed time period T2 can be selected such that the following relationship holds. (D2−D1)×T1=(D1−D3)×(T2−T1)

[0050] In other words, the time average value of the electric current during the time interval from T0 to T2, which includes the interval of the activation-deceleration duty cycle D3 and the interval of the activation duty cycle D2, is equal to the normal duty cycle D1.

[0051] By controlling the solenoid shut-off valves 24a and 24b in this way, the piston 32 moves as shown in Fig. 6 when the pedal stroke speed Vp is equal to or less than the threshold value VPth. Specifically, when the brake pedal 11 is operated and the drive flag is set to "1", the piston 32 is initially driven by the activation duty ratio D2, which is larger than the normal duty ratio D1, and the duty ratio is changed to the deactivation deceleration duty ratio D3, which is smaller than the normal duty ratio D1, immediately before the piston 32 reaches the fully closed position. Therefore, the piston 32 is decelerated as it approaches the fully closed position.

[0052] When the brake pedal 11 is released and the drive flag is set to "0," the piston 32 is initially driven toward the fully closed position under the spring force of the return spring 35 while the duty cycle is at the deactivation duty cycle D4, which may be zero. The duty cycle is changed to the deactivation deceleration duty cycle D5 as the piston approaches its fully closed position. After the piston is appropriately decelerated by the deactivation deceleration duty cycle D5, the supply of electric current to the solenoid 33 is stopped, and the piston 32 is held at the fully open position under the spring force of the return spring 35.

[0053] On the other hand, the solenoid shut-off valves 24a and 24b are operated according to the Fig.7, the piston 32 is driven with a fixed duty cycle. In this case, the speed of the piston 32 is at its maximum when reaching the fully closed position and the fully open position, so that the noise generated when the valve element 31 impacts the valve seat 34 and when the piston 32 abuts the stopper 36 is considerable. However, according to the present invention, the speed of the piston is controlled when the valve element 31 impacts the valve seat 34 and when the piston 32 abuts the stopper 36, so that the generation of noise can be minimized.

[0054] Although the speed at which the piston 32 hits the stopper 36 is reduced, the initial speed of actuating the piston 32 is increased because the activation duty cycle D2 is larger than the normal duty cycle D1, and the total time required for the piston 32 to move from the fully open position to the fully closed position can be minimized.

[0055] By performing the piston deceleration operation in a late part of each piston stroke (steps ST2 to ST7) only when the pedal stroke speed Vp is equal to or less than the prescribed threshold 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 powerful braking.

[0056] Since the normal duty cycle D1 is smaller than the activation duty cycle D2, the increase in initial speed of the piston 32 and the minimization of the power consumption of the solenoid in the fully closed position can be achieved at the same time.

[0057] The power consumption of the solenoid may be designed such that there is no difference from the conventional arrangement when the time average value of the electric current during the time interval from T0 to T2, which includes the interval of the activation-deceleration duty cycle D3 and the interval of the activation duty cycle D2, is equal to the normal duty cycle D1.

[0058] As a result, the load on the solenoid is no greater than the conventional arrangement, and the existing design can be used without subjecting the solenoid shut-off valve to undue stress.

[0059] The present invention is most advantageously used in brake-by-wire systems that combine hydraulic braking and regenerative braking in an intelligent manner, but also in more conventional brake-by-wire systems that use only hydraulic brakes.

Claims

[1] Vehicle braking system, comprising: a master cylinder (15) adapted to be actuated by a brake pedal (11) and to receive a supply of brake fluid from a reservoir (16); an input sensor (11a) for detecting an operation of the brake pedal (11); a motor-operated cylinder (13) adapted to be actuated by an electric motor (12) in accordance with a control signal and to receive a supply of brake fluid from the reservoir (16); a solenoid shut-off valve (24a, 24b) provided with a fluid line (22c, 22d) between the master cylinder (15) and the motor-operated cylinder (13), the solenoid shut-off valve (24a, 24b) comprising: a piston (32) provided with a valve element (31), a valve seat (34) provided in a housing of the solenoid shut-off valve (24a, 24b) and adapted to cooperate with the valve element (31), a return spring (35) urging the piston (32) in a valve-opening direction, a solenoid (33) for actuating the piston (32) in a valve-closing direction upon energization, and a stopper (36) for mechanically limiting a stroke of the piston (32) in the valve-opening direction; and a control unit (6) for providing the control signal to the motor-operated cylinder (13) according to a detection signal from the input sensor (11a) and for closing the solenoid shut-off valve (24a, 24b) when the motor-operated cylinder (13) is actuated in response to the detection signal from the input sensor (11a), characterized by , that the control unit (6) is arranged to control electrical power supplied to the solenoid (33) in order to slow down a movement of the piston (32) when the piston (32) reaches a point which is immediately adjacent to at least one of the stopper (36) or the valve seat (34), and that the control unit (6) supplies electrical power to the solenoid (33) at a first level (D2), a second level (D3) which is lower than the first level (D2), and a third level (D1) which is between the first level (D2) and the second level (D3), in this order, when the piston (32) is actuated in the valve closing direction, so that the solenoid shut-off valve (24a, 24b) is held in a fully closed state by the solenoid (33) which is excited by the electrical power at the third level (D1) against a biasing force of the return spring (35). [2] A vehicle braking system according to claim 1, wherein a time average of the first level (D2) and the second level (D3) is equal to the third level (D1). [3] A vehicle braking system according to claim 1, wherein the control unit (6) supplies electrical power to the solenoid (33) at a fourth level (D4) which is substantially lower than required to counteract the spring force of the return spring (35), a fifth level (D5) which is higher than the fourth level (D4) and at a zero level in this order when the piston (32) is actuated in the valve opening direction. [4] A vehicle braking system according to claim 1, wherein the control unit (6) does not control the electric power supplied to the solenoid (33) to decelerate a movement of the piston (32) when an operating speed (Vp) of the brake pedal (11) is greater than a threshold value (Vpth).

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