GB2297813A - A road vehicle anti-lock (ABS) braking system - Google Patents

Abstract

In an anti-lock braking system for a road vehicle, in which the brake pressure at any particular wheel is reduced/increased when in ABS mode, hydraulic fluid is delivered to, and exhausted from, wheel brake actuators by a hydraulic pump driven by an electric motor 111. In order to minimise pump noise and subsequent driver/passenger discomfort, rotation of the electric motor is stopped quickly after pumping has finished, by shorting the motor to ground when the motor is switched off, at switch 10a, so that on over-run the motor acts as a generator to create an extra braking torque on the motor rotor. A warning signal is generated if the motor stops too quickly or not quickly enough (i.e. stopping time lies outside a predetermined time range).

Description

DESCRIPTION ROAD VEHICLE ANTI-LOCK (ABS) BRAKING SYSTEM The present invention relates to anti-lock (ABS) braking systems for road vehicles and is concerned in particular with the problem of reducing the noise produced by ABS pump motors after the ABS has been switched off.

ABS systems operate by responding to an impending wheel-lock/skid condition at any particular wheel to reduce (dump) the actuating pressure applied to the brake associated with that wheel and to later re-apply the pressure when the tendency of that wheel to lock has reduced.

Such ABS systems are controlled by a dedicated electronic control unit (ECU) which is powered by the vehicle-battery/alternator supply. The system includes control of solenoid valves disposed in the hydraulic supply to brake actuators at the individual axles or wheels, the hydraulic fluid being delivered to the solenoid valves from a hydraulic pump driven by an electric motor.

Anti-lock braking systems using the abovedescribed "dump and pump" principle, require the electric pump motor to be run-on for a short time after the last solenoid firing. The time that the motor runs-on must be long enough to empty the low pressure/expander chambers of the dump solenoids.

Once the pump motor has finished this operation, it is switched off and allowed to coast to rest.

The result is that the time taken for the motor to come to rest after it has been switched off is longer than the time that the motor is performing real work, thereby extending considerably (possibly more than doubling) the time and therefore the noise that the driver/passengers hears at the end of an ABS cycle. The pump noise and subsequent driver/passenger discomfort is mainly apparent at the end of a stop when the vehicle comes to rest and other "road noises" have stopped.

It is not possible to shorten the time before the motor is switched off because one must guarantee that the low-pressure expander chambers have been emptied.

In accordance with the present invention, the aforegoing problem of overrun noise is mitigated by arranging for the motor to be stopped as quickly as possible after pumping has finished.

In a preferred embodiment, this is achieved by arranging for the pump motor to experience an electrical load during its over-run phase which produces an extra braking torque to the motor rotor to assist in bringing this rapidly to rest.

In one embodiment one side of the motor is permanently grounded and the other side is disposed in a series circuit with a switch controlled by an electrical control unit of the ABS system, closure of the switch connecting said other side of the motor to a current supply, and wherein said other side of the motor is arranged to be also grounded when said switch is opened to disconnect the motor from the current supply.

This can be achieved for example, by arranging for the switch to be of the double pole type wherein a movable switch contact couples said one side of the motor to the current supply in one switching position and to ground in another switching position.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a schematic diagram of the basic hydraulic and electrical arrangement of a typical ABS braking system to which the present invention can be applied; Fig. 2 is a diagrammatic partially sectional view of a typical conventional pressure modulator and a single control channel; Fig. 3 is a simplified circuit diagram of a conventional pump motor control system; Fig. 4 is a simplified circuit diagram of one embodiment of a pump motor control system in accordance with the present invention; Fig. 5 is a circuit diagram of a pump motor control system in accordance with the present invention having a monitoring facility for providing a warning if the motor control is faulty; Fig 6 is a detail view showing an alternative to part of the system of Fig. 5; and Figs 7 and 8 are curves of motor speed versus time for the conventional motor control system of Fig.

3 and the new system of Fig. 4, respectively.

Referring first to Fig. 1, there is illustrated highly schematically the braking system for a vehicle having four wheels 114, two at the front 114a,114b and two at the rear 114c,114d. The system is of the Xsplit type having independent hydraulic circuits, with the first circuit 107a (black) actuating the front offside and rear nearside wheel brakes 1OSa 105d and the second circuit 107b actuating the front rearside and rear offside wheel brakes 105b,105c. A brake pedal (not shown) is mechanically connected to the actuating plunger 120 of a dual circuit master cylinder 101.The master cylinder 101 is connected to a modulator 102 comprising a plurality of control channels 113 having control valves 103, connected to an electronic control unit (ECU) 108 by control lines 104, the control valves 103 being adapted to control the communication between the master cylinder 101 and the brake actuators 105 controlling the wheels 114.

Rotational speed of each wheel 114 is sensed individually by wheel speed sensors 106a - 106d connected to the ECU 108, which is adapted to control the control valves 103 of the modulator 102 in dependence upon signals received from the wheel speed sensors 106.

The ECU 108 is powered from a battery/alternator circuit 109 of the vehicle via an ignition switch 110.

The battery/alternator circuit 109 also supplies power to modulator pump motor 111 via a relay valve 112 controlled from signals generated by the ECU 108.

Should the ECU 108 determine that an appropriate fault has occurred then a fault warning indicator (warning lamp) 115 is activated.

Referring now to Fig. 2, there is illustrated one possible embodiment of a modulator 102, showing the construction of a single control channel 113. The same reference numerals as in Fig. 1 have been used for equivalent components. Fig. 2 shows the ECU 108 receiving signals from a single wheel speed sensor 106 which senses the rotational speed of a wheel 114 braked by a brake actuator 105.

The master cylinder 101 supplies fluid to the brake actuator 105 via the modulator 102 which has at least one control channel 113, consisting for example of a flow valve 132 and solenoid-controlled dump valve 131. Alternatively, the control channel could be formed by two solenoid controlled valves of the conventional inlet and dump configuration.

An electrically powered pump motor 111 is adapted to drive a pump 135 to scavenge fluid, dumped from the brake by opening of the dump valve 131, from a low pressure reservoir or expander chamber 133, in order to return the fluid dumped from the brake actuator 105 back to the master cylinder 101 for controlled reapplication of the brake by the control channel 113.

Referring now to Fig. 3, in conventional arrangements for the control of the pump motor 111 (usually a permanent magnet motor), one side of this pump motor is simply connected in series with a solenoid controlled relay switch 10 and a power supply 12, the other side being connected to earth (vehicle chassis). The on/off state of the solenoid switch 10 is controlled by a solenoid 14 which is selectively energised by the ABS electronic control unit (ECU) 108.

Fig. 4 shows the circuit of Fig. 3 modified in accordance with one embodiment of the control system of the present invention. In the system of Fig. 4, wherein the same components have been given the same reference numerals as in Fig. 3, a modified, doublepole switch 10a is used by which, when the switch 10a is in its motor - OFF position, the switch contact, and hence the normally non-grounded side of the motor 111, is connected to ground (chassis) via a line 17.

By so shorting the motor to ground when it is switched off, it stops more quickly than if simply opencircuited as a result of the fact that on over-run (free-wheeling) the motor acts as a generator once the ground circuit has been completed so that its "rotor" then experiences an electrical load/braking torque as a result of the opposing forces generated in its rotor and stater.

Fig. 7 shows a typical curve of pump motor speed against time for a conventional control circuit of the type shown in Fig. 3 and Fig. 8 shows a typical curve of pump motor speed against time for a control circuit in accordance with the present invention. In this example, the time taken for the pump motor to stop in the conventional system was 100ms whereas by use of the present invention the time had been reduced to 64ms. This is just an example of the benefits that can be achieved and much higher percentage time reductions are possible in practice.

Referring now to Fig. 5, there is shown a development of the circuit of Fig. 4 which enables a warning to be generated in the event of certain faults arising.

A low value resistor 20 is connected in series with the permanently grounded side of the motor 111 and the voltage across this resistor 20 is monitored by an amplifier 22 to establish a signal representative of the prevailing motor current. The output of the amplifier 22 is compared in a comparator 24 with a low/zero voltage to provide a high output from the comparator 24 in the event that the motor voltage, and hence the current is greater than said low/zero voltage. The output of the comparator 24 controls a first switch 26 connected to a "stop counting" input of a timer 28.The timer 28 is also connected via a second switch 30 and inverter 32 to the solenoid 14 such that the converter provides a high signal to the switch 30, which is used to start the timer 28 counting when the solenoid 14 is switched OFF, corresponding to the motor being disconnected from the supply 12 by the switch 10a. The count output of the timer 28 is compared in a second comparator 34 with a signal X representative of a predetermined or prescribed time, whereby the comparator 34 provides an input signal to the ECU 108 via a line 36 if the aforegoing system determines that the time taken for the motor to come to rest (zero output from the amplifier 22) exceeds said predetermined/prescribed time represented by the signal X.

The aforegoing system of Fig. 5 issues a warning signal if the motor overrun time exceeds the predetermined value X applied to the comparator 34.

This would provide an indication that, for example, the relay contacts 10a or switching mechanism which apply the electrical load across the motor is/are not switching in correctly and the motor overrun time is too long. However, it can be useful also, or alternatively, to detect that the motor overrun time is below a predetermined value to provide an indication, for example, that the motor itself is beginning to seize or that the motor driven pump is experiencing or seeing a hydraulic load. Thus, the system can include a means for generating a warning signal if the time taken for the motor to come to a stop is outside a predetermined range or is less than a predetermined value. One possible manner in which such means might be configured is shown in Fig. 6 which is intended to replace the comparator 34 of Fig.

5 by two comparators 34a, 34b such that comparator 34a responds to input B < input A and comparator 34b responds to input B > input A, where input A of comparator 34a is connected to a lower predetermined limiting value Y and input A of comparator 34b is connected to an upper predetermined limiting value X.

Claims (8)

1. An anti-lock braking system for a road vehicle of the type which operates by responding to an impending wheel-lock/skid condition at any particular wheel to reduce the actuating pressure applied to a brake actuator at the brake associated with that wheel and to later re-apply the pressure when the tendency of that wheel to lock has reduced, the system being controlled by an electronic control means and including a plurality of solenoid valves disposed in the hydraulic supply to brake actuators at the individual wheels or axles, hydraulic fluid being delivered to the solenoid valves from a hydraulic pump driven by an electric motor, wherein the system includes means by which the rotation of the electric motor is arranged to be stopped quickly after pumping has finished.

2. An anti-lock braking system as claimed in claim 1, wherein said motor-stopping means is arranged to cause the pump to experience an electrical load during its over-run phase whereby to produce an extra braking torque to the motor rotor to assist in bringing this rapidly to rest.

3. An anti-lock braking system as claimed in claim 2, wherein one side of the motor is permanently grounded and the other side is disposed in a series circuit with a switch controlled by said electrical control means, closure of the latter switch connecting said other side of the motor to a current supply, and wherein said other side of the motor is arranged to be also grounded when said switch is opened to disconnect the motor from the current supply.

4. An anti-lock braking system as claimed in claim 3, wherein said switch is of the double pole type wherein a movable switch contact couples said one side of the motor to the current supply in one switching position and to ground in another switching position.

5. An anti-lock braking system as claimed in any of claims 1 to 4, including means for generating a warning signal if the time taken for the motor to come to a stop is outside a predetermined range.

6. An anti-lock braking system as claimed in any of claims 1 to 4, including means for generating a warning signal if the time taken for the motor to come to a stop is less than a predetermined value.

7. An anti-lock braking system as claimed in any of claims 1 to 4, including means for generating a warning signal if the time taken for the motor to come to a stop exceeds a predetermined value.

8. An anti-lock braking system as claimed in claim 7, when appendant to claim 3 or 4, wherein said warning signal generating means comprises means generating a signal representative of the motor current, a timer which is arranged to be started when said switch is opened to disconnect the motor from the current supply, means responsive to said motor current signal to generate a timer stopping signal when said motor current signal is zero and means for generating said warning signal if the timer count exceeds a predetermined level.