WO1992000211A1

WO1992000211A1 - Electrically powered brake system and method

Info

Publication number: WO1992000211A1
Application number: PCT/US1991/004518
Authority: WO
Inventors: William C. Eddy, Jr.; Tamas Pattantyus
Original assignee: AlliedSignal Inc
Current assignee: Honeywell International Inc
Priority date: 1990-06-29
Filing date: 1991-06-24
Publication date: 1992-01-09
Anticipated expiration: 1992-12-29
Also published as: AU8201691A

Abstract

A system for achieving a method of operating brakes such as the rear brakes (34, 36) of a vehicle. The method including the steps of: generating a brake pressure error signal by comparing commanded brake pressure and actual brake pressure; operating a pump (16) during instances wherein the error signal is of one magnitude (+) to effect an increase in actual brake pressure; decreasing brake pressure during instances wherein the error signal is of an opposite magnitude (-).

Description

ELECTRICALLY POWERED BRAKE SYSTEM AND METHOD

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to an electrically powered, pressure or pedal effort referenced single axle braking system.

It is an object of the present invention to provide an electrically referenced braking system. A further object of the invention is to provide an electrically powered rear axle braking system. A further object of the present invention is to provide an electrically braked system at least comparable to the prior master cylinder controlled systems.

Accordingly the invention comprises a system and method for achieving electrically operated brakes. While the invention discloses a rear brake control system the invention is applicable to the front brakes as well. If configured as a rear brake system, such system is hydraulically isolated from the front brake. The system follows the following method steps of operating and includes apparatus to accomplish the method: generating a brake pressure error signal by comparing commanded brake pressure and actual brake pressure; operating a pump during instances wherein the error signal is of one magnitude (+) to effect an increase in actual brake pressure; decreasing brake pressure during instances wherein the error signal is of an opposite magnitude (-). Many other objects and purposes of the invention will be clear from the following detailed description of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIGURE 1 illustrates an electrically powered brake system.

FIGURE 2 is a schematic diagram showing many of the components of the present invention.

FIGURES 3 and 4 illustrate alternate embodiments of the invention.

FIGURE 5 illustrates an electric control unit usable in the present invention. FIGURE 6 illustrates various brake pressure time traces generated by the present invention.

FIGURE 7 illustrates a further embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIGURE 1 illustrates the major components of an electrically powered braking system 10. As shown, the braking system 10 has been included within the rear axle 12 of a truck. The braking system 10 includes a motor 14 powering a pump 16 positioned in a housing 18. Situated above the housing 18 is a fluid reservoir 20. The output of the pump is communicated to the respective brake cylinders (34, 36) in the left hand 22 and right hand 24 vehicle brakes/wheels through an outlet plate 30 of the housing 18 which supports an optional solenoid valve 32. An electronic control unit 26 may be mounted atop the motor 14 to provide for a compact package.

FIGURE 2 schematically illustrates the above described components showing their placement within the housing 18. During normal operation of the pump 16, it is communicated through the optional solenoid 32 directly to left hand and right hand brake cylinders 34 and 36. Upon activation of the solenoid 32, brake fluid within these cylinders is permitted to flow back to the reservoir 20 to relieve brake pressure. A rear pressure transducer such as sensor 38 is provided to sense the rear brake pressure.

The alternate embodiment shown in FIGURE 3 includes a pump 16 feeding the brake cylinders directly. Brake pressure application is accomplished by activating the pump 16. Upon halting the pump, as described below, the pressurized brake fluid is permitted to drain to the reservoir 20 through leak passages in the pump which are illustrated by the orifice 100. The embodiment of FIGURE 4 is similar to that shown in FIGURE 3 with the exception that an discrete orifice 102 is connected, in line 104, between the brake cylinders 34, 36 and the reservoir. It should be appreciated that the flow rate of the pump 16 used in the embodiments of FIGURES 3 or 4 will be larger than that used in FIGURE 2 (assuming that a non-leaky or "tight" pump is used) since the pump must be able to pressurized -4-

the brake cylinders in view of the flow returned to the reservoir through internal leakage or the orifice 102.

FIGURE 5 illustrates the electronic control unit 26 used to control the operation of the motor 14 and pump 16. It also contains circuitry for controlling the solenoid 32, if used. The electric control unit 26 includes a power supply circuit 40 and a control circuit 42. The purpose of the power supply circuit is to apply battery or alternator supply voltage to the motor 14 and to various portions of the control circuit 42. Upon closure of the ignition switch 44 or alternatively application of the pedal 50, i.e., closure of a brake switch 52, a switching transistor 46 is enabled which activates a voltage supply switch 48 to communicate battery or alternator voltage to the motor 14 and to the control circuit. As will be seen from the description below, the brake system 10 is electrically powered and as such, the supply circuit further includes a timer delay circuit 60 which maintains supply voltage to the motor 14 and control circuit 42 for a predetermined time (.5 to 5 seconds) after the brake pedal is released. This time delay avoids premature turning off of the motor due to bounce of the brake switch 52 and also prohibits the supply voltage from being abruptly turned on and off, thereby energizing the control circuit and motor 14, such as when the operator taps on the brake pedal when the vehicle is parked. As shown, the front brakes of the vehicle are hydraulic, but can also be electrically powered. Upon application of the brake pedal, the master cylinder 54 applies brake pressure to the front right 56 and front left 58 brake cylinders. It should be appreciated that the system 10 is hydraulically isolated from the front brakes. A front brake pressure command signal P_ (also see numeral 60) is obtained by measuring master cylinder pressure or the applied front brake pressure with a transducer 62. Instead of sensing pressure a pedal effort, force transducer can be used. The output of this transducer 62 is communicated to a proportioning circuit 66, the output of which forms the commanded rear brake pressure signal. As it is known in the art, conventional proportioning valves have a nonlinear characteristic. This nonlinear characteristic can be approximated electronically by a look-up table, a piece-wise linear curve fit or other technique as is known in the art. A summing circuit 70 compares the commanded brake pressure signal with the pressure in the rear brake line P. as sensed by the rear brake pressure transducer 38 forming an error signal P„ (also see numeral 72). The error signal is communicated to a sign sensitive dead-band circuit 74 of known construction. The dead-band region in the circuit 74 (74a) is used to minimize noise propagation throughout the control circuit 42. As shown schematically, if the commanded brake pressure signal P- is greater than the rear brake pressure P., a positive error signal is generated, shown by block 72, which is communicated to a proportional plus integral motor controller 76, the output of which is received by a constant frequency variable pulse width modulator 78. The output of the modulator 78 is fed to a conventional arrangement of motor power transistors QA and QB. These power transistors QA and QB are of the sense-fet variety which include a current monitoring lead which is connected to a motor current limiting circuit 80 of known variety. The output of the pulse width modulator 78 represents the command input to the motor 14. This output signal comprises a plurality of pulses having a constant frequency such as 25KHz and a varying pulse width proportional to the error signal P„ . If for some reason, such as during motor start up, the binding of the pump 16, hydraulic load, etc., the current in the motor, as sensed by the motor current limiting circuit 80, exceeds a preset value, the output of the motor current limiting circuit 80 clamps the output of the pulse width modulator 78 to ground, thereby reducing the effective motor commanded signal, i.e., pulse width to the motor. This technique is one known in the art and not described in detail herein. It should be appreciated that during the first phase of braking, the pump 16 operates in its normal pumping mode moving fluid to the brake cylinders 34, 36. After the brake pressure achieves its commanded valve, the pump operates somewhat as a rotary solenoid, that is, with the brake line fully pressurized only a modest rotation of the pump 16 contributes to additional braking force. During this phase of operation it is expected that pump rotation will be proportional to commanded motor current. As can be appreciated, in some regard the motor/pump combination operates as an electric master cylinder. As an example, a conventional hydraulic master cylinder pumps a relatively large amount of fluid to initiate braking. After the brake line has been pressurized, relatively small displacements in the master cylinder contribute directly to increased brake forces.

Supply voltage is also communicated from the supply circuit 40 to the coil 90 of the solenoid valve 32. A Zener diode 92 and diode 94 are connected across the coil 90 in a conventional manner to speed up the current decay in the coil, on turn-off. Absent a signal supplied to the solenoid drive transistor 96, the valve 32 will remain in a condition as shown in FIGURE 2 communicating the pump 16 to the rear brake lines and rear brake cylinders 34 and 36. If during the operation of the system 10, the rear brake pressure achieves a value greater than the commanded brake pressure, the error signal P„ is negative. This negative error signal P_E is communicated using the negative going portion of the dead-band circuit 74 (74b) to a second proportional plus integral controller 100 which is communicated to another constant frequency variable pulse width modulator 102 which varies the on time of the solenoid drive transistor 76 so that this on time is proportional to the magnitude of the error signal. The frequency of the pulse width modulator 102 should be compatible with the valve 32. A constant frequency of 100 cycles has been chosen for the second pulse width modulator 102. It should be appreciated that if the embodiments of FIGURES 3 and 4 are employed, the solenoid control circuitry is not used.

The operation of the system 10 is as follows. Upon application of the brake pedal 50 by the operator, the front brake pressure 200, as shown in the time traces of FIGURE 4, will increase. These time traces are representative of actual test data. As mentioned, pedal effort can be measured as an alternative to measuring master cylinder or front brake pressure as by using transducer 62. The output of the electric proportioning control circuit 68, defines the commanded rear axle brake pressure signal. In the system which generated the curves shown in FIGURE 6, such proportioning circuit or control 68 had a front to rear proportioning of 60:40. Upon the initial application of the brake pedal, a large magnitude positive error signal P_, is generated causing the controller 76 and modulator 78 to cause transistors QA and QB to turn on during the duration of each positive pulse generated by the modulator 78. With the power transistors QA and QB turned on, the motor 14 causes pump 16 to supply pressurized brake fluid from the reservoir 20 to the rear brakes 34 and 36. The operation of the pump increases rear brake pressure 202 (see FIGURE 6) such that at or about a time Tl the rear brake pressure has achieved a substantial steady state value as established by the electric proportioning circuit 68. The control circuit 42 will attempt to match the actual rear brake pressure P. with the commanded rear brake pressure P_. During the regulation of the rear brake pressure, the actual rear brake pressure may exceed the commanded brake pressure. In this situation the now negative going error signal

P_ ___ is communicated to the controller 100 and the second pulse width modulator 102 to activate the solenoid drive transistor 96, thereby causing the valve 32 to change state and communicate the rear brake cylinders 34 and 36 to the reservoir, thereby reducing rear brake pressure. In the embodiments of FIGURES 3 and 4 which do not employ a solenoid to relieve pressure, any momentary overpressurization will be reduced by virtue of leakage flow or flow through the discrete orifice 102.

Returning to the discussion of the system of FIGURE 6, the various oscillations in the rear brake pressure time trace of FIGURE 6, such as at time T2, are indicative of the fact that the actual rear brake pressure had exceeded the magnitude of the commanded rear brake pressure and as such the valve 32 was commanded under the influence of the proportional plus integral controller 100 and pulse width modulator 102, to periodically return rear brake fluid to the reservoir 120. During the time that the error signal P_E is communicated to the controller 100 and pulse width modulator 102, the error signal is removed from the motor controller 76 and modulator 78. As such, the motor 14 and pump 16 will tend to slow down. Due to the inertia of the motor 14 and pump 16, this slowing does not occur instantaneously and as such, the motor/pump combination continues to generate additional though diminishing pressure. During the operation of the control circuit 42 when the error signal P„ is once again positive, such signal is communicated to the motor 14 for continued brake pressure build. Reference is again made to the time traces in FIGURE 6. At time T3 the operator of the vehicle slightly reduced the applied brake force, which is accompanied by a reduction in both front and rear brake pressure. As the vehicle approached a full stop condition, the operator continued to relieve the applied brake force at time such as T4. Subsequent to time T4, that is, as the operator further reduces brake pedal effort as the vehicle is approaching a stop, the rear brake pressure closely tracks the front brake pressure. The oscillations in the rear brake pressure show the periodic activation of the valve 32 wherein rear brake pressure is further relieved.

While the above discussion has described one cycle of normal braking action, it should be apparent that the present invention as shown is FIGURE 2 is readily usable in an adaptive braking/antiskid mode of operation. That is, if during the above described braking cycle the wheels 22, 24 begin to skid, as sensed by adaptive control electronics and wheel sensors 28 of known variety, the pump 16 command signal is reduced or set to zero, the solenoid 32 is opened thereby diminishing brake pressure, on a per-axle basis until the wheel has stopped skidding. Thereafter brake pressure, i.e. pressure command signal can be increased (build) and/or held constant under control of the antiskid/adaptive braking control electronics. While the solenoid 32 yields a means to precisely and controllably reduce brake pressure, such reduction during antiskid operation, can be achieved by use of the embodiments of FIGURES 3 and 4. As previously described, reductions in brake pressure are accomplished by permitting fluid to return to the reservoir 20 through pump leakage 100 or a discrete orifice 102. As such, if a wheel skid or impeding skid condition is sensed the commanded brake pressure is reduced. Thereafter the pressurized brake fluid in the cylinders will rapidly dump to the reservoir 20, permitting the wheel to come out of its skid condition. Thereafter the commanded pressure is again increased, held constant, etc., under control of the antiskid electronics.

The advantages of the above described system can be enhanced by adding a second control valve 32b (see

FIGURE 7) . The control valves 32a and b can be operated simultaneously to relieve brake pressure during normal braking in the manner that the single control valve 32 is operated. However, by virtue of •12-

the use of two control valves each wheel 22, 24 can be independently controlled during antiskid and/or adaptive traction modes of operation. It should be apparent that is this configuration individual control valve circuits 100, 102 would be employed to operate the respective control valves 32a or b.

Many changes and modifications in the above described embodiment of the invention can, of course, be carried out without departing from the scope thereof. Accordingly, that scope is intended to be limited only by the scope of the appended claims.

Claims

IN THE CLAIMS

1. A method of operating a brake system of a vehicle comprising a pump (16), at least one brake or brakes (34, 36), receiving fluid from the pump and including a reservoir (20), the method comprising the steps of: 1) generating a brake pressure error signal by comparing commanded brake pressure and actual brake pressure;

2) operating a pump (16) during instances wherein the error signal is of one magnitude (+) to effect an increase in actual brake pressure;

3) decreasing brake pressure during instances wherein the error signal is of an opposite magnitude (-).

2. The method as defined in Claim 1 wherein step 1.1 of generating includes the steps of:

1) generating a first signal indicative or brake pedal effort.

3. The method as defined in Claim 2 wherein step

2.1 of generating includes: generating a first signal indicative of the pressure in the front brakes of a vehicle and scaling such first signal to generate a rear brake commanded brake signal related to the pressure in the front brakes by a desired proportioning ratio between front and rear brakes of the vehicle.

4. The method as defined in Claim 1 wherein the step 1.2 of operating includes: controlling the speed of operation of a motor (14), connected to the pump (16) in response to the error signal.

5. The method as defined in Claim 4 wherein the step of controlling includes: utilizing the error signal to generate a constant frequency, variable pulse-width motor current control signal.

6. The method as defined in Claim 1 wherein step 1.3 of decreasing brake pressure includes terminating the operation of the pump (16) and thereafter permitting brake pressure to decrease.

7. The method as defined in Claim 6 wherein the step of permitting brake pressure to decrease includes permitting brake fluid to flow from the brake(s) (34, 36) through leakage paths in the pump (16) to a reservoir (20).

8. The method as defined in Claim 6 wherein the step of permitting brake pressure to decrease includes permitting brake fluid to flow from the brake(s) (34, 36) through a discrete orifice (102), substantially in parallel with the pump (16) to a reservoir (20) .

9. The method as defined in Claim 1 wherein step 1.3 of decreasing brake pressure includes: diverting brake fluid flow between the pump and brakes and connecting the brakes (34, 36) to the low pressure reservoir.

10. The method as defined in Claim 9 wherein the step of diverting includes activating at least one solenoid valve (32) connected between the pump (14), brakes (34, 36) and reservoir (20) to communicate the brakes and reservoir.

11. The method as defined in Claim 10 wherein the step of activating includes the step of utilizing the error signal to generate a constant frequency, variable pulse-width solenoid valve control signal.

12. The method as defined in Claims 1 wherein the step of decreasing is initiated during instances indicative of a wheel skid or impending wheel skid condition.

13. A braking system (10) comprising: front brakes (54, 56, 58) rear brakes isolated from the front brakes; a pump for pressurizing rear brakes; a motor for powering the pump (16); a reservoir (20) for storing brake fluid communicated to the pump (16); control unit means for controlling rear brake pressure comprising: a first pressure sensor for generating a signal indicative of front brake pressure; electric proportioning means (66) for generating a commanded rear brake pressure signal (P_r) ; a second pressure sensor for generating a signal of actual rear brake pressure (P_A) ,* comparison means (70) for comparing the command brake pressure signal to the actual brake pressure signal and for generating an error signal (P„ ]__) ; motor control means (74, 76, 78, QA, QB, 80) responsive to a positive error signal for activating the motor.

14. The system as defined in Claim 13 wherein control valve means (32) interpose the pump (16) and the rear brake(s) (34, 36) and have a first state to communicate the pump (16) to the rear brakes (34, 36) and a second state to communicate the rear brakes to the reservoir (20); and wherein the control unit means includes valve control means (74, 100, 102) responsive to a negative error signal, for causing the control valve to operate in its second state.

15. The system as defined in Claim 13 wherein the motor control means comprises: a proportional plus integral first controller (76); a first pulse width modulator (78) responsive to the output of the first controller (76), for generating a signal to initiate motor operation.

16. The system as defined in Claim 15 wherein the valve control means comprises: a proportional plus integral second controller (100) a second pulse width modulator (102) responsive to the output of the first controller (100) for generating a signal to activate the control valve means.

17. A braking system comprising: a brake associated with a vehicle wheel, a pump for supplying pressure fluid to the brake; a motor for powering the pump; a reservoir for storing brake fluid communicated to the pump; control unit means for controlling brake pressure comprising^*: first means for generating a first signal indicative of operator applied braking effort; command means responsive to the first signal for generating a commanded brake pressure signal (P_c); second means for generating a signal indicative of actual brake pressure (P_*)/* comparison means for comparing the command brake pressure signal to the actual brake pressure signal and for generating an error signal (P_,v . motor control means responsive to a positive error signal for activating the motor.

18. The system as defined in Claim 17 wherein control valve means interpose the pump and the brake and have a first state to communicate the pump to the brake and a second state to communicate the brake to the reservoir; and wherein the control unit means includes valve control means responsive to a negative error signal, for causing the control valve to operate in its second state.

19. The system as defined in Claim 17 wherein the motor control means comprises: a proportional plus integral first controller; a first pulse width modulator responsive to the output of the first controller, for generating a signal to initiate motor operation.

20. The system as defined in Claim 19 wherein the valve control means comprises: a proportional plus integral second controller, a second pulse width modulator responsive to the output of the first controller for generating a signal to activate the control valve means.

21. The system as defined in Claim 17 wherein the command means comprises proportioning means such that commanded brake pressure supplied to rear brakes of the vehicle is smaller than the brake pressure applied to the front brakes of the vehicle. -19-

22. The system as defined in Claim 17 wherein the first means comprises one of: a first pressure sensor for generating a signal indicative of front brake pressure and a brake pedal sensor indicative of operator applied brake pedal effort.

23. The system as defined in Claim 22 wherein the second means comprises a second pressure sensor.