Brake system for vehicles

In a brake by wire system, a linear differential pressure control valve is provided to control accurately wheel cylinder pressure. A brake fluid conduit extending from a reservoir is branched out into two conduits, each of which is transmitted to each of right and left wheel cylinders. A linear differential pressure control valve is disposed in the conduit and an another linear differential pressure control valve in each of the branched out conduits. Brake fluid sucked from the reservoir by a motor pump is discharged to each of the branched out conduits between the valve and the wheel cylinder. The two valves thus connected in series are operative step by step to control the wheel cylinder pressure in accordance with the current commanded in response to brake pedal depression. The former linear differential pressure control valve is energized at first at a normal braking operation and, therefor, the identical pressure is applied to both of the wheel cylinders so that the detected wheel cylinder pressure may be adjusted. Further, a hybrid system using the brake by wire and a conventional master cylinder is also provided.

CROSS REFERENCE TO THE RELATED APPLICATION
 The present application is based upon and claims the benefit of priority of
 the prior Japanese patent application No. Hei 10-38679 filed on Feb. 20,
 1998, the content of which is incorporated by reference.
 BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a brake system so called "brake by wire
 system" in which the state of a braking operation by a driver is converted
 to an electric signal for inducing a wheel braking force.
 2. Description of Related Art
 The brake by wire system is well known. In this system, an electric signal
 representing the state of brake pedal operation by a driver, i.e., the
 amount of brake pedal stroke or brake pedal depression pressure, is
 generated. A wheel braking force can be induced based on the electric
 signal. The conventional brake by wire system employs, as examples for
 inducing a wheel braking force, a method of directly pressing a brake pad
 against a brake disk separately in each wheel by using ultrasonic motors
 or a method of producing a wheel cylinder pressure by using a two-position
 valve and a pump, as disclosed in Japanese patent Laid-open publication
 No. Hei 9-188242, under the title of "brake fluid pressure control
 apparatus".
 SUMMARY OF THE INVENTION
 It is an object of the present invention, in a hydraulic type brake system
 for generating wheel cylinder pressure by using brake fluid, to provide a
 brake by wire type system that is simple in its construction but has a
 better characteristic and features on the control for producing braking
 force. To achieve this object, a linear differential pressure control
 valve and a pump are provided for generating and controlling a wheel
 cylinder pressure. Thus, a completely or partly shut-off control and a
 flow through control for the brake fluid in the hydraulic conduit in
 accordance with a commanded current can be flexibly and accurately
 accomplished. As a result, an adequate control responsive to the braking
 requirement by a driver can be easily realized.
 It is an another object of the invention to provide a brake by wire type
 system in which the detected brake fluid pressure difference between a
 pair of the left and right wheel cylinders for a vehicle can be easily
 compensated by means of mechanical or electronic adjustment. To achieve
 this object, in addition to first and second linear differential pressure
 control valves respectively disposed in each of the two conduits branched
 out from the brake fluid conduit communicating to a reservoir, a third
 linear differential pressure control valve is provided at the brake fluid
 conduit between the reservoir and the branched out portion of the conduit.
 While the first and second linear differential pressure control valves
 control each pressure of the left and right wheel cylinder, independently,
 the third linear differential pressure control valve controls commonly
 both pressures of the left and right wheel cylinders. To prevent from
 imposing unnecessary yawing moment to a vehicle, it is inevitable to keep
 the braking force difference between the left and right wheels at a normal
 braking operation at the minimum. The third linear differential pressure
 control valve serves to keep the difference between a pair of wheel
 cylinder pressures to a level less than the predetermined amount to be
 required for the prevention of the unnecessary yawing moment of the
 vehicle.
 It is a further object of the invention to provide a fail safe hybrid
 construction incorporating not only a brake by wire function but also a
 mechanical brake function directly responsive to the brake pedal operation
 by a driver. According to the above described invention, even if a
 malfunction takes place in the brake by wire function, the mechanical
 brake function for the front wheels or the front and rear wheels will
 serve. Furthermore, to facilitate the hybrid function, a servo function
 directly responsive to driver's brake pedal depression is provided under
 the help of the pump driven by a signal separately generated at the time
 of the brake pedal depression, even if an electronically control unit for
 the brake by wire function does not work. For this purpose, the brake
 system of this invention has a brake fluid conduit extending from a
 portion between a two-position valve and the third linear differential
 pressure control valve to a servo room of a master cylinder.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 The preferred embodiments of the present invention will be described
 hereinafter with reference to the accompanying drawings.
 (First Embodiment)
 FIGS. 1A, 1B and 1C are a schematic diagram of a brake system for
 rear-drive four wheel vehicles, a schematic diagram of an electronic
 control unit and a schematic diagram of a brake pedal, respectively,
 according to a first embodiment of the present invention. Each of Figs.
 hereinafter shows the valve member position of respective valves in the
 case that a brake pedal is not depressed. The brake fluid stored in a
 reservoir 2 under atmospheric pressure is transmitted hermetically to each
 of wheel cylinders 3, 4, 5 and 6 through a hydraulic unit 1. Wheel speed
 sensors 7, 8, 9 and 10 are equipped respectively in a rear right wheel RR,
 a rear left wheel RL, a front right wheel FR, and a front left wheel FL
 for detecting the velocity of each wheel.
 The hydraulic unit 1 is provided with a first brake conduit line 50 for the
 rear right wheel RR and the rear left wheel RL and a second brake conduit
 line 60 for the front right wheel FR and the front left wheel FL. As the
 structure of the second brake conduit line 60 is the same as that of the
 first brake conduit line 50, only the structure of the first brake conduit
 line 50 will be described in detail hereinafter.
 The first brake conduit line 50 is provided with a conduit 51 extending
 from the reservoir 2 and a conduit 52 extending to a wheel cylinder 3 for
 the rear right wheel RR and a conduit 53 to a wheel cylinder 4 for the
 rear left wheel RL which are constituted by branching out from the conduit
 51. Linear differential pressure control valves 20 and 21 having a
 flow-through position and a differential pressure producing position are
 disposed in the conduits 52 and 53, respectively. The linear differential
 pressure control valves 20 and 21 at the flow-through position allow the
 brake fluid to flow between the reservoir 2 and the respective wheel
 cylinders 3 and 4 almost without flow restriction. The flow-through
 position turns to the differential pressure producing position when
 energized to the linear differential pressure control valves 20 and 21.
 The lift length of the valve member (a length between the valve member and
 a valve seat when the valve member is away from the valve seat) can be
 linearly varied in accordance with the current value applied to a solenoid
 coil of the linear differential pressure control valve. At the
 differential pressure producing position, each of the conduits 52 and 53
 can be controlled to completely shut off or partly shut off the brake
 fluid flow according to the lift length of the valve member. For example,
 assuming that the lift length of the valve member from the valve seat is
 at an intermediate position, the respective brake fluid flow from the
 brake cylinders 3 and 4 to the reservoir 2 through the respective conduits
 52 and 53 is partly restricted according to the lift length thereof so
 that the brake fluid pressure at the side of the wheel cylinders 3 and 4
 (wheel cylinder pressure) may be maintained to a certain pressure higher
 than that at the side of the reservoir 2. Each of linear differential
 pressure control valves 20, 21, 22 and 23 as disclosed in the first
 embodiment of the present invention is constructed to allow the maximum
 200 kgf /cm.sup.2 pressure (which corresponds to the maximum holding
 pressure of the wheel cylinder). The maximum holding pressure can be
 determined by a value of the spring coefficiency of the spring biasing
 against the valve member of each linear differential pressure control
 valves 20, 21, 22 and 23. Higher is the co-efficiency of the spring,
 higher the maximum holding pressure is. 200 kgf/cm.sup.2 is the maximum
 pressure normally required for each wheel cylinder of various kinds of
 vehicles.
 A motor pump 30 comprises a motor 33 to be driven when energized and
 trochoid pumps 31 and 32 to be driven by the motor 33. Each of the
 trochoid pumps 31 and 32 which is provided to respond to each of the wheel
 cylinders 3 and 4 sucks the brake fluid from the reservoir 2 through a
 conduit 54 and discharges the same to respective conduits 52 and 53
 between the wheel cylinders 3 and 4 and the linear differential pressure
 control valves 20 and 21. The trochoid pumps 31 and 32, a kind of gear
 pumps, have an advantage that a fluid discharge pulsation is smaller and a
 driving noise is lower, compared with those of a piston pump. Though the
 embodiments of the present invention show, as an example, the trochoid
 pump, the other type pumps such as piston pumps, outer-contact gear pumps
 and vane pumps can be used instead.
 The construction of the second brake conduit line 60 is same as that of the
 first brake conduit line 50 and each component of the second brake conduit
 line 60 corresponds to that of the first brake conduit line 50 as shown
 below. Linear differential pressure control valves 22 and 23 correspond to
 the linear differential pressure control valves 20 and 21, respectively, a
 motor 43 and trochoid pumps 41 and 42 of a motor pump 40 to the motor 33
 and the trochoid pumps 31 and 32 of the motor pump 30, respectively, and
 conduits 61, 62, 63 and 64 to the conduits 51, 52, 53 and 54,
 respectively.
 Each of the linear differential pressure control valves 20, 21, 22 and 23
 and the motors 33 and 43 is controlled or driven by control signals from
 an electronic control unit 100 (hereinafter referred to as ECU 100). When
 the ECU 100 does not generate the control signals, each valve element of
 the linear differential pressure control valves 20, 21, 22 and 23 is at
 the flow-through position as shown in FIG. 1. The ECU 100 is provided with
 a ROM 101, a RAM 102, a CPU 103 and an I/O interface which are well known.
 The ECU 100 receives respective detected signals from the wheel speed
 sensors 7, 8, 9 and 10 that detect a wheel velocity of respective wheels,
 pressure sensors 11, 12, 13 and 14 that detect a pressure of respective
 wheel cylinders and a pedal stroke sensor 15 that detects a stroke length
 of a pedal 200 depressed by a driver. The stroke length of the pedal is a
 parameter showing a braking operation state of the vehicle required by the
 driver. A pedal depression pressure sensor that detects a pressure to the
 pedal by the driver can be used in place of the pedal stroke sensor. There
 is further provided with a bias mechanism 300 which reacts to give a bias
 against the stroke length or the depression pressure due to the pedal
 operation of the driver so that the driver may feel the reaction of the
 pedal depression operation.
 Secondly, a brake control process to be executed in the brake system shown
 in FIG. 1A will be described briefly with reference to a flow chart of
 FIG. 2. The process shown in the flow chart is executed separately for
 respective wheels at a certain time interval, for example, at 6 ms.
 A step 105 is provided in order to check whether a brake switch, that is
 well known as a stop lamp switch and not disclosed in this drawing, is
 switched on. The brake switch is switched on at the time when the pedal
 200 is substantially depressed by the driver and the vehicle is at the
 braking state thereby. If the answer is affirmative at the step 105, the
 process goes to a step 110 to drive the motors 33 and 43. At a step 120,
 the pedal stroke length PS is detected according to the detected signals
 from the pedal stroke sensor 15 and, at a step 130, each of the wheel
 cylinder pressures PW/C is detected by each of wheel cylinder pressure
 sensors 11, 12, 13 and 14 and, at a step 140, each wheel cylinder pressure
 to be applied to each of wheel cylinders 3, 4, 5 and 6 is determined based
 on the detected pedal stroke length PS and each of the detected wheel
 cylinder pressures PW/C. At a normal braking state, i.e., when a special
 braking control such as an anti-skid control is not performed, the aimed
 wheel cylinder pressures for the front and rear wheel cylinders are all
 same and can be determined in order to comply with, for an example, a well
 known model curb regarding the allocation of braking pressure, as
 described in a FIG. 3 which shows a wheel cylinder pressure in the
 horizontal line and a pedal stroke length in the vertical line. At a step
 150, each of driving patterns for actuating respective linear differential
 pressure control valves 20, 21, 22 and 23 is set according to the wheel
 cylinder pressure determined at the step 140 and the wheel cylinder
 pressures PW/C detected at the step 130. Assuming that the pedal stroke
 length is 50 mm and the detected wheel cylinder pressure PW/C is 25
 kgf/cm.sup.2, the linear differential pressure control valve is driven by
 a duty control so as to open the valve member by 25% from the flow-through
 position (75% pressure difference producing position) so that the pressure
 between the pressure (atmospheric pressure) of the reservoir 2 and each of
 wheel cylinder pressures may be maintained at its given pressure
 difference. The current value to be commanded to respective differential
 pressure control valves is controlled by the duty rated current. If there
 exists a difference between the detected wheel cylinder pressure PW/C at
 the step 130 and the set wheel cylinder pressure at the step 140, the
 commanded current value to the linear differential pressure control valve
 is controlled to eliminate the pressure difference.
 If, at the step 105, the answer becomes negative, a next step 160 is
 provided to switch off the motor and a subsequent step 170 to cut off the
 current supply to the linear differential pressure control valve to end up
 a whole control of the system.
 The brake system described in the first embodiment of the present invention
 of FIG. 1A is provided with two independent brake fluid conduit lines and
 four linear differential pressure control valves and four pumps in order
 to realize the brake by wire system. This system employs the linear
 differential pressure control valve in which the lifting value of the
 valve member can be variably controlled by the commanded current value so
 that not only the smooth adjustment between the braking requirement
 parameter of the driver(such as the brake pedal depression pressure or the
 pedal stroke length) and the detected value from each wheel cylinder
 pressure sensors 11, 12, 13 and 14, but also smooth control for decreasing
 or increasing the wheel cylinder pressure may be achieved..
 Each of the pumps 31, 32, 41 and 42 is respectively disposed for each of
 wheel cylinders 3, 4, 5 and 6. There are no common brake fluid conduits
 connecting the first and second brake fluid conduit lines and, even in
 each conduit line, no common conduit lines for increasing the pressure to
 be supplied to each of wheel cylinders, though there exists only a common
 conduit line to be used for releasing the pressure to be supplied to each
 wheel cylinder. Therefor, even if a malfunction such as a break down of
 the brake fluid conduit takes places in one of the independent brake fluid
 lines, the other brake fluid line can be effectively used to increase the
 wheel pressure. Furthermore, in case that one of the trochoid pumps in
 each brake conduit line happens to leak largely the brake fluid from its
 output side to its input side and is not effectively operated, the other
 trochoid pump is operative to discharge the brake fluid sufficiently
 enough to increase the cylinder pressure. This construction is very
 effective from a fail safe standpoint.
 (Second Embodiment)
 FIGS. 4A, 4B and 4C show a construction of a brake system according to a
 second embodiment of the present invention. The construction having the
 same function and effect as those of the construction described in FIGS.
 1A, 1B and 1C has the same reference number as that of FIGS. 1A, 1B and
 1C. and its explanation will be omitted.
 The brake system as described in FIG. 4A is provided with two linear
 differential pressure control valves 70 and 80, in addition to the brake
 system of FIG. 1A. Each of the linear differential pressure valves 70 and
 80 is disposed in each of the brake fluid conduits 51 and 52 extending
 from the reservoir 2 to the branched out portion. Assuming that the
 required maximum wheel cylinder pressure is 200 kgf/cm.sup.2, the
 allowable maximum holding pressure of each of the linear differential
 pressure control valves 20, 21, 22 and 23 as well as the linear
 differential pressure control valves described at the second embodiment of
 the present invention can be set at a value of 100 kgf/cm.sup.2.
 The spring coefficiency of the bias spring of the linear differential
 pressure control valves of the second embodiment can be a half of that of
 the linear differential pressure control valves of the first embodiment
 shown in FIG. 1A. The current value for energizing the solenoid of the
 second embodiment can be also a half, compared with that of the first
 embodiment. Therefor, the solenoid having a relatively low heat resistance
 characteristic can be employed and the size of the linear differential
 pressure control valves becomes compact.
 Each of the linear differential pressure control valves 70 and 80 is
 disposed in series with each of the linear differential pressure control
 valves 20 and 21 and each of the linear differential pressure control
 valves 22 and 23 in each of brake fluid conduit lines, respectively. Thus,
 the pressure of each wheel cylinder can be increased to 200 kgf/cm.sup.2
 by the series connection of each of the linear differential pressure
 control valves 70 and 80 and each of the linear differential pressure
 control valves 20, 21, 22 and 23. The series connection of the linear
 differential pressure control valves serves to suppress the current value
 to be applied to the linear differential pressure control valve and
 therefor, the heat resistance construction of the linear differential
 pressure control valve can be easily and compactly realized. This is an
 advantage, especially, in the brake by wire system, because it is
 necessary to supply the current to the linear differential pressure
 control valve during all the time when the driver keeps the depression
 operation of the pedal so that the relatively high heat resistance
 characteristic of the solenoid may be required.
 The maximum 200 kgf/cm.sup.2 differential pressure control by the system
 incorporating only one differential pressure control valve for a wheel
 cylinder as described in the FIG. 1 will result in a rough break down of
 control and thus, the control at the normal braking state covering 10 to
 50 kgf/cm.sup.2 which happens most frequently among the braking operations
 is relatively rough so that a control flexibility and a driver's pedal
 feeling may be adversely affected. To solve this drawback, it can be
 considered to adopt the linear differential pressure control valve having
 the allowable maximum holding pressure, 200 kgf/cm.sup.2, but having a
 very fine break down control characteristic. However, this also has
 disadvantages that the cost of the valve will increase and the control
 becomes complicated.
 The adoption of the linear differential pressure control valve having the
 maximum holding pressure 100 kgf/cm.sup.2, as described in the second
 embodiment of the present invention, has a merit that the more fine break
 down control is available and the increased flexibility of control will
 improve the driver's braking feeling, in the case that a similar control
 as that of the linear differential pressure control valve having the
 maximum holding pressure 200 kgf/cm.sup.2 is employed. Furthermore, the
 control by the linear differential pressure control valves in series
 connected for applying the pressure to the wheel cylinder has an another
 merit that the more and more fine break down control can be realized.
 The brake system mentioned above is controlled basically according to a
 similar flow chart as described in the FIG. 2. However, if the control of
 the linear differential control valves 70 and 80 and the control of the
 linear differential pressure control valves 20, 21, 22 and 23 are carried
 out respectively in the following ways, there exists an advantage. At a
 normal braking state, i.e., the wheel cylinder pressure is less than 50
 kgf/cm.sup.2 at the state of no urgent braking or no anti-skid control due
 to the wheel slip, only the linear differential pressure control valves 70
 and 80 are actuated to apply the pressure to each of the wheel cylinders
 and each of the linear differential pressure control valves 20, 21, 23 and
 24 is kept at the flow-through position without the actuation thereto. The
 same value of the pressures will be applied to respective right and left
 wheel cylinders ( for example, the wheel cylinders 3 and 4) due to the
 mechanical construction, not due to the adjustment by a soft program which
 might be required in order to carry out the same pressure control for both
 of right and left wheel cylinders in the case of the brake system
 disclosed in the FIG. 1A. The same pressure compensation to the right and
 left wheel cylinders that is important to eliminate unnecessary yaw moment
 for the vehicle can be easily achieved.
 When the pressure is applied to the right and left wheel cylinders
 separately for rear and front wheels by actuating the linear differential
 pressure control valves 70 and 80, the detected pressures of the pressure
 sensors 11 and 12 should be same and those of the pressure sensors 13 and
 14 should be same. If different, there are fluctuations of characteristic
 between the pressure sensors 11 and 12 or between the pressure sensors 13
 and 14. In this case, the fluctuation error of each pressure sensors 11,
 12, 13 and 14 can be adjusted to be eliminated for the subsequent control.
 When the pedal stroke length become larger to the extent of exceeding the
 normal braking region or when the pressure to be controlled by each of the
 linear differential pressure control valves 70 and 80 reaches near the
 allowable maximum holding pressure( for example, 100 kgf/cm.sup.2 ), the
 linear differential pressure control valves 20, 21, 22 and 23 may be
 actuated, in addition to the linear differential pressure control valves
 70 and 80, in order to increase the wheel cylinder pressure.
 FIGS. 5A, 5B, 5C and 5D and FIGS. 6A, 6B, 6C and 6D describe timing charts
 for controlling the pressure to the wheel cylinders 3 and 4 in the first
 brake fluid conduit line at the second embodiment of the present
 invention. The FIG. 5A shows a transition of the pressure of each of the
 wheel cylinders 3 and 4 and FIGS. 5B, 5C and 5D a commanded current value
 to each of the linear differential pressure control valves 20, 21 and 70
 at the normal braking operation and the anti-skid control when the wheel
 cylinder pressure is less than 70 kgf/cm.sup.2.
 At a time t.sub.0, a driver begins to depress a brake pedal and the pump
 motor 30 starts driving and, till the time t.sub.1, the linear
 differential pressure control valve 70 is energized at a 100% duty rate
 current so that the same pressure may be applied to respective wheel
 cylinders 3 and 4.
 At a time t.sub.1, if an excessive slip takes place only on a right wheel,
 the pressure of the wheel cylinder 3 decreases. At this time, the current
 supply to the linear differential pressure control valve 70 is cut off
 and, on the other hand, a current at 50% duty rate is supplied to the
 linear differential pressure control valve 21 corresponding to a rear left
 wheel which is not slipped excessively. The 50% duty rate current means
 the case that, for example, a pedal stroke sensor 15 detects that the
 driver recognizes the wheel slipping and releases a brake pedal
 depression. After the time t.sub.1, a solid line shows the pressure of the
 wheel cylinder 4 and a dotted line the pressure of the wheel cylinder 3.
 At a time t.sub.2, if the slip of the right wheel is restrained, a control
 for increasing the pressure to the wheel cylinder 3 is carried out. For
 this purpose, the current is supplied at a 100% duty rate to the linear
 differential pressure control valve 20 in order to close substantially the
 valve member. At this time, the pressure of the wheel cylinder 3 increases
 rapidly to induce effectively a wheel braking force. If the driver dose
 not depress more the pedal and the pedal position is kept as it is during
 the time from t.sub.2 to t.sub.3, the linear differential pressure control
 valve 21 is controlled at a 30% duty rate current which is smaller than
 the current controlled during the time from t.sub.1 to t.sub.2 in order
 not to convert the level of pressure of the wheel cylinder 4.
 After the time t.sub.3, if an excessive slip on the left wheel takes place,
 the current supply to the linear differential pressure control valve 20 is
 cut off to decrease the pressure of the wheel cylinder 4 and the linear
 differential pressure control valve 20 is controlled at a 30% duty rate
 current to keep the pressure of the wheel cylinder substantially at the
 same level. As mentioned above, only the linear differential pressure
 control valve 70 is used for the brake fluid control before the time t,
 when the anti-skid control starts and, after the time t.sub.1, the linear
 differential pressure control valves 20 and 21 are used without using the
 linear differential pressure control valve 70.
 FIG. 6 is a timing chart showing a case at an urgent braking operation that
 a driver depresses the brake pedal strongly and rapidly and subsequently,
 an anti-skid control is carried out. During the time from t.sub.0 to
 t.sub.1, the contents of control are similar as described in the FIG. 5.
 When the ECU 100 determines, based on the detected signal from the pedal
 stroke sensor 15, at the time t.sub.0 ' that the variation of a pedal
 stroke length per a unit time exceeds a predetermined amount and judges as
 an urgent braking, the current will be supplied to the linear differential
 pressure control valves 20 and 21, too, at the time t1. In this case, as
 the current supply to the linear differential pressure control valves 20
 and 21 is carried out at the time when the wheel cylinder pressure is
 about 90 kgf/cm.sup.2 which is below the maximum holding pressure (100
 kgf/cm.sup.2 ) of the linear differential pressure control valve 70, the
 pressure change of the wheel cylinder is very smooth. During the time from
 t.sub.1 to t.sub.2, all the linear differential pressure control valves
 are controlled at a 100% duty rate current to response to the urgent
 braking.
 Assuming that a front right wheel slips excessively at the time t.sub.2,
 the current supply to the linear differential pressure control valve 70 is
 cut off so that the pressure of the wheel cylinder may decrease till the
 maximum holding pressure of each linear differential pressure control
 valves 20 and 21. If the slip of the front right wheel can not be
 restrained even at the time t.sub.3, the current supply to the linear
 differential pressure control valve 20 is cut off and the current supply
 to the linear differential pressure control valve 21 is kept at the 100%
 duty rate so that the pressure of the wheel cylinder 3 further decreases.
 After the time t.sub.3, a solid line shows the pressure of the wheel
 cylinder 4 and a dotted line the pressure of the wheel cylinder 3.
 At the time t.sub.4, if the slip of the front right wheel has been
 restrained, the pressure increase to the wheel cylinder 3 commences by
 supplying a 100% duty rate current to the linear differential pressure
 control valve 70. Then, the pressure to the wheel cylinder 4 will increase
 to the value more than 100 kgf/cm.sup.2 due to the pressure held by both
 of the linear differential pressure control valves 21 and 70.
 When the pressure of the wheel cylinder 3 comes near the maximum holding
 pressure of the differential pressure control valve 70 (about 90
 kgf/cm.sup.2, same as the pressure at the time t.sub.1) and unless the
 excessive slip on the front right wheel takes place, a 100% duty rate
 current is supplied to the linear differential pressure control valve 20,
 too. If it is detected that the vehicle came to stop or the depression of
 brake pedal by the driver was cancelled, the current supply to each of the
 linear differential pressure control valves 20, 21 and 70 and to the pump
 cease.
 (Third Embodiment)
 FIGS. 7A and 7B show a construction of a brake system according to a third
 embodiment of the present invention. The construction having the same
 function and effect as those of the construction described in FIGS. 1A and
 1B and FIGS. 4A and 4B has the same reference number as that of FIGS. 1A,
 1B, 4A and 4B and its explanation will be omitted. In the FIG. 7A, the
 first and second brake fluid conduit lines 50 and 60 are provided with the
 same construction as described in the FIG. 4A, except a motor pump 240
 having a motor 245 for driving all of four trochoid pumps 241, 242, 243
 and 244, instead of the motors 33 and 43 for respectively driving the
 pumps 31 and 32 and the pumps 41 and 42. At the side of the first brake
 fluid conduit line 50, only a brake by wire system is constituted, but at
 the side of the second brake fluid conduit line 60, not only the brake by
 wire system but also a conventional mechanical brake system are
 constituted as a hybrid system. The brake pedal 200 is connected with a
 brake booster 201 which boosts the pedal depression force in use of an
 engine intake manifold vacuum pressure. A rod extending from the brake
 booster 201 is connected to a single master cylinder 400 which produces a
 master cylinder pressure in accordance with the brake pedal depression by
 a driver. The axial length(longitudinal direction in the drawing) of the
 master cylinder 400 may be shorter than that of the conventional tandem
 master cylinder for vehicles. The size of the brake booster 201 may be
 more compact than the conventional one. A two-position valve 212, which is
 normally at a shut-off position, is disposed in a brake fluid conduit
 between the reservoir 2 and an intersection of the fluid conduit 61
 extending to the linear differential pressure control valve 80 and the
 fluid conduit 64 transmitting to the suction side of the pumps 243 and
 244. An another two-position valve 213, which is normally at a
 flow-through position, is disposed in the brake fluid conduit 261
 extending from the master cylinder 400 to a portion of the fluid conduit
 61 which is just before the linear differential pressure control valve 80.
 The master cylinder pressure produced by the master cylinder 400 is
 transmitted to the wheel cylinders 5 and 6 through the two-position valve
 213, the linear differential pressure control valve 80 and the linear
 differential pressure control valves 22 and 23.
 The hybrid brake system has a merit from a fail safe standpoint. Even if a
 malfunction takes place in the brake by wire system (failures of ECU or
 actuators) for the rear wheel, the pressure to the front right and left
 wheel cylinders 5 and 6 can be induced mechanically in responsive to the
 brake pedal depression. As an another merit, the driver can enjoy a
 natural brake pedal feeling because of the application of the master
 cylinder 400. Furthermore, at an urgent braking operation, a larger wheel
 cylinder pressure than the master cylinder pressure induced by the
 driver's brake pedal depression can be obtained by supplying the current
 to the linear differential pressure control valve 80 and driving the pumps
 243 and 244, because the linear differential pressure control valve 80 is
 controlled to produce a pressure difference between the master cylinder
 pressure and the wheel cylinder pressure. If the two-position valve 213 is
 switched to the shut-off position and the two-position 212 to the
 flow-through position, the excessive pressure to the wheel cylinders 5 and
 6 can be prevented by controlling the linear differential pressure control
 valves 80, 20 and 23 and driving the pumps 243 and 244, as described
 before.
 (Fourth Embodiment)
 FIGS. 8A and 8B show a construction of a brake system according to a fourth
 embodiment of the present invention. The construction having the same
 function and effect as those of the construction described in FIGS. 7A and
 7B has the same reference number as that of FIGS. 7A and 7B and its
 explanation will be omitted. In addition to the two-position valves 212
 and 213, the linear differential pressure control valves 70, 80, 20, 21,
 22 and 23, and the motor pump 240, as mentioned in the FIG. 7A, this
 system is provided with two-position valves 210 and 211 for rear wheels,
 too, which correspond to the two-position valves 212 and 213 for front
 wheels. The two-position valve 211, which is normally at a shut-off
 position, is disposed in the brake fluid conduit 51 between the reservoir
 2 and an intersection of the fluid conduit 51 extending to the linear
 differential pressure control valve 70 and the fluid conduit 54
 transmitting to the suction side of the pumps 241 and 242. The
 two-position valve 210, which is normally at a flow-through position, is
 disposed in the brake fluid conduit 262 extending from the master cylinder
 203 to a portion of the fluid conduit 51 which is just before the linear
 differential pressure control valve 70. The master cylinder 203 is a
 tandem master cylinder having first and second fluid pressure rooms. Each
 of the first and second fluid pressure rooms is respectively connected
 with the fluid conduits 261 and 262, each of which extends to each of the
 front wheel cylinders 5 and 6 and the rear wheel cylinders 3 and 4.
 If the stroke sensor 15 detects the depression of the brake pedal, the
 two-position valves 210 and 213 are switched to the shut-off position and
 the two-position valves 211 and 212 to the flow-through position. At the
 same time, the motor 245 is energized and the trochoid pumps 241, 242, 243
 and 244 are driven. Thus, the fluid communication between the tandem
 cylinder 203 and the hydraulic unit 1 are completely shut off by the
 two-position valves 210 and 213 and each pressure of the wheel cylinders
 3, 4, 5 and 6 can be electrically controlled, as shown in the FIGS. 2 and
 3. This system constitutes not only a brake by wire system but also a
 conventional mechanical brake system. If a malfunction takes places in the
 brake by wire system, the pressure to the wheel cylinders 3, 4, 5 and 6
 will be applied from the master cylinder 203 in response to the brake
 pedal depression by the driver, which brings an advantage from a fail safe
 standpoint. The adoption of a malfunction detecting system will make it
 available that if a malfunction such as a broken wire is detected, the
 control of the linear differential pressure control valves 70 and 80 and
 the other components will be prohibited.
 (Fifth Embodiment)
 FIGS. 9A and 9B show a construction of a brake system according to a fifth
 embodiment of the present invention. The construction having the same
 function and effect as those of the construction described in FIGS. 8A and
 8B has the same reference number as that of FIGS. 8A and 8B and its
 explanation will be omitted.
 In the fourth embodiment shown in the FIG. 8A, the two-position valve 211
 is disposed in the fluid conduit between the reservoir 2 and the
 intersection of the fluid conduit 51 extending to the linear differential
 pressure control valve 70 and the fluid conduit 54 transmitting to the
 suction side of the pumps 241 and 242 and, further, the two-position valve
 212 is disposed in the fluid conduit between the reservoir 2 and the
 intersection of the fluid conduit 61 extending to the linear differential
 pressure control valve 80 and the fluid conduit 64 transmitting to the
 suction side of the pumps 243 and 244. However, in the fifth embodiment
 shown in FIG. 9A, the two-position valves 301 and 302 are disposed
 respectively in the conduits 51 and 61, in place of the two position
 valves 211 and 212. The fluid pressure rooms 203e and 203f of the master
 cylinder 203 are respectively connected to the conduits 261 and 262. There
 is provided with a fluid conduit 270 which connects a servo room 203b to
 the intersection of the conduit 261 and the conduit 61 between the
 two-position valve 212 and the linear differential pressure control valve
 80. A non-return valve 303 is disposed in the conduit 270 in order to
 prevent the reverse flow of brake fluid from the servo room 203b when a
 driver depresses a brake pedal.
 A piston 203a in the master cylinder 203 is provided with a conduit 203c in
 order to flow through the fluid from the reservoir 2 to the servo room
 203c. The flow through or the shut off control of the fluid between the
 reservoir 2 and the servo room 203c will be performed by an end portion of
 a bush rod 200a connected with the brake pedal 200. A groove provided at
 an outer circumference of the piston 203a communicating with the conduit
 203c is opened to an inlet portion of the reservoir 2 all over the
 distance where the piston 203a moves according to the depression of the
 bush rod 200a so as to allow the conduit 203c to connect to the reservoir
 2. The outer surface of the piston 203a is provided with sealing material
 not shown in this drawing in order to seal the space between the groove of
 the piston 203a and the servo room 203b and between the groove of the
 piston 203a and the fluid pressure room 203e. The outer surface of an
 intermediate piston 203d is also provided with sealing material not shown
 in this drawing in order to seal the space between the fluid pressure
 rooms 203e and 203f. The servo room 203b serves to reduce a reaction force
 of the brake pedal 200 by the application of the fluid pressure. In
 another word, a master cylinder pressure which is higher than the master
 cylinder pressure to be induced only by the driver's brake pedal
 depression can be obtained in cooperation with the fluid pressure room
 203e.
 The operation of this system will be explained in the case of the brake
 control for the front wheel. If the function of the ECU 100 is normal and
 the stroke sensor 15 detects the depression of the brake pedal, the
 two-position valve 213 is switched to the shut-off position and the
 two-position valve 302 to the flow-through position and the linear
 differential pressure control valve 80 is ready to be energized to control
 the differential pressure. At the same time, the motor 245 is energized
 and the trochoid pumps 241, 242, 243 and 244 are driven. Thus, the fluid
 communication between the fluid pressure rooms 203e and 203f of the tandem
 cylinder 203 and the hydraulic unit 1 are completely shut off by the
 two-position valves 210 and 213 and each pressure of the wheel cylinders 5
 and 6 can be electrically controlled by the commanded current to the
 linear differential pressure control valve 80, as described in the second
 embodiment of the present invention.
 Next, if the ECU 100 encounters a malfunction (such as a failure of CPU or
 sensors and the processes by the ECU are inhibited), but a brake operation
 is required, the motor 245 can be driven, not through the ECU but
 directly, by an electric signal of the stop lamp(well known and not shown
 in this drawing) or the stroke sensor 15 which is generated when the brake
 pedal is substantially depressed. At this time, as the current supply to
 respective control valves can not be controlled because of the malfunction
 of the ECU and the inhibition of its processes, the two-position valve 302
 is at the shut-off position, the two-position valve 213 at the
 flow-through position and the linear differential control valves 80, 22
 and 23 at the flow-through position. Therefor, the brake fluid discharged
 from the trochoid pumps 243 and 244 is supplied to the servo room 203b of
 the master cylinder 203.
 As the area (S) of the end surface of the piston 203a is larger than the
 area (S1) of the end surface of the bush rod 200a, assuming that the brake
 fluid pressure in the servo room 203b is P, the surface pressure (SP)
 applied to the end surface of the piston 203a is larger than the surface
 pressure (SP1) to the end surface of the bush rod 200a. This means a servo
 mechanism which serves to apply to the piston 203a a higher pressure than
 the depression pressure by the driver (pressure ratio; S/S1). As described
 above, if the motor can be driven at the time of the brake requirement,
 even when the ECU encounters the malfunction, the higher pressure than the
 driver's depression pressure can be produced as a master cylinder
 pressure.
 FIG. 10A shows a relay circuit 100P for driving the motor 245, when the ECU
 encounters a malfunction. As shown in this drawing, a relay switch SW can
 be automatically switched on by a signal representing that the ECU
 encounters an malfunction. Then, the motor can be driven in responsive to
 only the on or off state of the stop lamp switch STP.
 FIG. 10B shows an alternative relay circuit 100Q for driving the motor 245
 which is constituted in the ECU. In the case that the ECU is normally
 functioned, a IC 352 generates a OK signal based on normal pulse signals
 from a CPU 351 upon receipt of a signal representing that the stop lamp
 switch STP is switched on. The OK signal is transmitted to a motor driving
 IC 353 so that the motor driving IC 353 may serve to supply a current to
 the motor 245 in accordance with a control signal generated from the CPU
 351. On the other hand, in the case that the ECU is not functioned
 normally, the IC 352 generates an inhibition signal based on abnormal
 pulse signals from the CPU 351 upon receipt of a signal representing that
 the stop lamp switch STP is switched on. The inhibition signal is
 transmitted to a motor driving IC 353 so that the motor driving IC 353 may
 serve to supply a current to the motor 245 in accordance with signals for
 driving the motor generated from the IC 352, inhibiting a control signal
 generated from the CPU 351. As mentioned above, in the case of the
 malfunction of the ECU, the motor 245 can be driven without using the
 control signals from the ECU only by the signal of the stop lamp switch
 STP.
 This brake system also has an advantage, same as in the first to fourth
 embodiments, that, as an identical pressure may be applied to each of the
 wheel cylinders 5 and 6, the adjustment operation regarding the
 fluctuation of the wheel cylinder pressure sensors can be very easily
 performed.
 FIG. 11 shows a flow chart for adjusting the fluctuation of the pressure
 sensors. At a step 500, it is judged whether or not a brake demand by a
 driver exists. This will be judged by a signal from the stroke sensor 15.
 If the answer is affirmative, the process goes to a step 510 where an
 aimed wheel cylinder pressure is determined according to an amount of the
 brake demand by the driver such as a brake stroke length. If the answer is
 negative, the process is ended. At a step 520, whether the existing wheel
 pressure is more than 100 kgf/cm.sup.2 is judged. A value of 90
 kgf/cm.sup.2 may be used in consideration of the maximum holding pressure
 of the linear differential pressure control valve, as explained in the
 FIG. 6A. If the answer is negative, the process goes to a step 530 where
 the linear differential pressure control valve 80 is controlled to its
 differential pressure producing position and the motor 245 is driven so
 that each pressure of the wheel cylinders 5 and 6 may be increased. As
 both of the linear differential pressure control valves 22 and 23 are kept
 at their shut-off positions, each pressure of the wheel cylinders 5 and 6
 becomes identical. At this time, if the detected values of the pressure
 sensors 13 and 14 are different each other, there exists a fluctuation
 error of the detected values of the pressure sensors 13 and 14 and,
 therefor, the detected values may be adjusted to eliminate the fluctuation
 error. If the answer is affirmative at the step 520, the process goes to a
 step 550. Though the step 550 is not the process for adjusting the
 fluctuation errors of the detected values of the pressure sensors, this
 process may be used to confirm whether the adjustment has been complete.
 When the aimed wheel cylinder pressure is more than 100 kgf/cm.sup.2, each
 of the linear differential pressure control valves 22 and 23 is energized
 in addition to the current supply to the linear differential pressure
 control valve 80 and the motor is driven so that each of the wheel
 cylinder pressure may be independently controlled to achieve the aimed
 pressure. At this time, unless each value of the duty rate current to each
 of the linear differential pressure control valves 22 and 23 is same, it
 may be presumed that the error adjustment is not complete.
 As only the linear differential pressure control valve 80 is energized
 always at an early stage of braking operation and the detected values of
 the pressure sensors 5 and 6 can be adjusted in order to obtain the same
 values, an accurate independent brake control can be secured for each of
 the wheel cylinder pressures.
 At the time of the malfunction of the ECU 100, each of the valves in the
 brake conduit line 50 for the rear wheels keeps its valve member position
 as shown in the drawing which is the same in the brake conduit line 60 for
 the front wheels. However, the pressure of the second master cylinder room
 203f, which is the same pressure of the first master cylinder room 203e,
 directly induced by the driver's brake pedal operation and further
 enhanced by the servo function of the servo room 203b will be applied to
 the wheel cylinders 3 and 4 to obtain a sufficient braking force to each
 of the rear wheels.
 In the case that the ECU 100, especially the CPU 103, is normal, the brake
 by wire system can work, as described in the second embodiment of the
 present invention. In the case that the ECU 100 or the CPU is abnormal,
 the master cylinder pressure higher than the pressure responsive to the
 driver's direct depression force is applied to the wheel cylinders even
 without a conventional brake booster, but with a help of the pump motor
 240 driven separately. Even when both of the ECU and the pump motor 240
 encounter the malfunction, the master cylinder pressure responsive to the
 driver's direct depression force may be applied to the wheel cylinders.
 As mentioned above, when the ECU 100 is normal and the braking operation is
 required, each of the two-position valves 301 and 302 turns to the
 flow-through position. However, if only the two-position valve 301 turns
 to the flow-through position and the two-position valve 302 is kept at the
 shut-off position, the brake fluid discharged from the pump and passed
 through the linear differential pressure control valves 22, 23 and 80 can
 not be returned, but transmitted to the servo room 203b. While the brake
 fluid discharged from the pump is mainly used to supply to the wheel
 cylinders, only the excessive brake fluid is transmitted. This will serve
 to produce a servo function for the driver's depression force which gives
 an enhanced force against both of the master cylinder pressure rooms 203e
 and 203f. Thus, the brake pedal depression feeling will be improved.
 Sixth Embodiment
 FIG. 12 shows a brake system according to a sixth embodiment of the present
 invention. This system is provided with a X type brake fluid conduit
 arrangement constituted by a first conduit line 50A connecting a front
 left wheel cylinder 6 and a rear right wheel cylinder 3 to one pressure
 room of the master cylinder 203 and a second conduit line 60A connecting a
 front right wheel cylinder 5 and a rear left wheel cylinder 4 to the other
 pressure room of the master cylinder 203. As the construction of the
 second conduit line 60A is same as that of the first conduit line 50A, the
 detail explanation of the brake system of this embodiment will be made
 hereinafter with respect to the first conduit line 50A.
 The first conduit line 50A comprises a fluid conduit 401 extending from the
 master cylinder 203 and two fluid conduits 402 and 403 which are branched
 out from the fluid conduit 401, as shown in the FIG. 12. One of the
 branched out fluid conduit 402 is transmitted to the front left wheel
 cylinder 6 and the other of the branched out fluid conduit 403 to the rear
 right wheel cylinder 3. A linear differential pressure control valve 411
 having a flow-through position and a differential pressure producing
 position is disposed in the conduit 402. A fluid conduit 404 extending
 from the reservoir 2 is connected with the conduit 402 between the linear
 differential pressure control valve 411 and the wheel cylinder 6. A pump
 431 is disposed in the conduit 404 in order to suck the brake fluid from
 the reservoir 2 and discharge the same to the conduit 402 between the
 linear differential pressure control valve 411 and the wheel cylinder 6. A
 two-position valve 413 having a flow-through position and a shut-off
 position is disposed in the conduit 404 at a down stream of the discharge
 side of the pump 431. There is provided with a fluid conduit 405
 connecting the conduit 404 between the two-position valve 413 and the
 discharge side of the pump 431 to the suction side of the pump 431 of the
 conduit 404. A linear differential pressure control valve 414 having a
 flow-through position and a differential pressure producing position is
 disposed in the conduit 405.
 The fluid conduit 403 is provided with a two-position valve 412 having a
 flow-through position and a shut-off position. A fluid conduit 406 is
 connected with the conduit 403 between the two-position valve 412 and the
 wheel cylinder 5 and is provided with a reservoir 421 and a two-position
 valve 415 having a flow-through position and a shut-off position so as to
 allow the brake fluid to run to the reservoir 421 for releasing the wheel
 cylinder pressure at the time of an anti-skid control or the like. Each
 valve member position of the valves shown in the drawing is at the time
 when the valve is not energized.
 At a normal operation of this system, the linear differential pressure
 control valve 411 is controlled at the differential pressure producing
 position, the two-position valve 413 at the flow-through position and the
 linear differential pressure control valve 414 at the differential
 pressure producing position, while the pump 431 is driven. Then, the pump
 431 discharges the brake fluid sucked from the reservoir 2 to the conduit
 402 and the linear differential pressure control valve 411 is actuated in
 order to control the pressure between the master cylinder 203 and the
 wheel cylinder 4 to a predetermined differential pressure so that an aimed
 braking force may be applied to the front left wheel. On the other hand,
 as the two-position valve 412 in the conduit 403 is kept at the
 flow-through position, a braking force is applied to the rear right wheel
 with the pressure same as the master cylinder pressure.
 At an anti- skid control operation of this system, the front left wheel
 cylinder pressure can be decreased or increased by changing at the duty
 control each of the valve member positions of the two-position valve 413
 and the linear differential pressure control valve 414.
 As mentioned above, this brake system is constituted by a hybrid brake by
 wire and mechanical brake system.
 (Other Embodiments)
 In each embodiments mentioned above, it may be easily realized to employ an
 anti-skid control. In this case, a slip of each wheel will be detected by
 the wheel velocity based on each signal of the wheel speed sensors 7, 8, 9
 and 10 and each of the differential pressure control valves may be
 energized to control each of the wheel cylinder pressures so that an
 adequate wheel slip condition may be secured.
 In each of the second to fifth embodiments, it is possible to incorporate a
 control by learning flow regarding the fluctuation adjustment of the
 detected wheel cylinder pressures from the sensors 11, 12, 13 and 14 in
 the brake by wire control flow.
 The brake fluid conduit line for the front right and left wheels in the
 third embodiment may be controlled as the brake by wire system as
 described in the fourth embodiment.
 Though the pressure sensors 11, 12, 13 and 14 are disposed respectively for
 each wheel cylinders in the embodiments mentioned above, it is possible to
 employ only one pressure sensor in each of the brake fluid conduit lines
 50 and 60 in the second to fifth embodiments. For example, if a pressure
 is disposed in the brake fluid conduit within the area surrounded by three
 of the linear differential pressure control valves 20, 21 and 70 in the
 FIG. 4A, the pressure in each of the wheel cylinders can be presumed by
 the duty rate current applied to each of the linear differential pressure
 control valves 20 and 21.
 While the present invention has been shown and described with reference to
 the foregoing preferred embodiments, it will be apparent to those skilled
 in the art that changes in form and detail may be made therein without
 departing from the scope of the invention as defined in the appended
 claims.