GB2280762A - Testing and speed control of ABS pump motors - Google Patents

Abstract

A vehicle anti-lock braking (ABS) system has a motor-driven pump (10) providing a hydraulic supply to the ABS system. The pump motor speed is controlled by an electronic switch (16), such as a MOSFET, Pulse Width Modulated in accordance with a measurement of the e.m.f. generated by the motor between pulses and after a delay to allow the back e.m.f., caused by switching off the motor, to decay (preferably to zero) before the measurement operation begins. The generated voltage is integrated during a motor switch-off period, the motor speed being determined from a knowledge of the voltage/speed characteristic of the particular motor, when acting as a generator. The ignition switch-controlled supply within the vehicle electrical system only operates a logic gate (40) to control the supply of direct battery power to the vehicle voltage regulator (36), the pump motor circuit and its electronic controller (16) being connected directly to the battery supply (B+) whereby to act as an energy sink to protect all silicon devices in the electrical control system except the ignition circuit logic gate means. The motor circuit can be tested by monitoring the effect of a short pulse from the logic, which is not sufficiently long to move the motor. <IMAGE>

Description

DESCRIPTION TESTING AND SPEED CONTROL OF ABS PUMP MOTORS The present invention relates to the testing and speed control of pump motors used in vehicles having ABs (anti-lock brakes).

The pumps driven by such motors provide the hydraulic supply for the ABS system and it is therefore important that the facility be available for testing that the pump motor is operating , or operable, correctly. It is also desirable that the speed of the motor be known and be controllable.

Conventionally, actuation and control of the pump motor is achieved using an electromagnetic relay. To test that the motor is in-circuit" and operational, the relay is turned on temporarily. However, because the speed of operation of such a relay is relatively slow, the motor begins to run before the relay can be turned off again. This has a number of disadvantages, including higher noise and cost, inferior testability and pedal feel. Furthermore, the motor must deliver rated output at 8 volts supply, yet be capable of operating for long periods at 16 volts at the extremes of specified temperature.

Conventionally, ABS pumps motors are not speed controlled because of the significant cost penalty associated with measuring the motor speed using a separate transducer. This cost limitation leads to high noise levels, inferior pedal feel and potential motor unreliability due to the wide voltage range over which the motor is likely to be operating.

It is therefore an objective of the present invention to provide an alternative means of testing and speed control for ABS pump motors which enable the aforementioned problems associated with the prior art to be reduced.

It is known from DE-3830164 that the rotational speed of a motor can be established electrically by making use of the fact that the residual magnetisation in the motor permits it to be operated as a generator for a short period of time which is sufficient for measurement. The current supply to the motor is interrupted at the instant of a current zero crossing and the voltage produced by the motor as a result of the residual magnetisation is monitored. The time period from the instant of the last zero crossing of the motor supply voltage to the first voltage zero crossing of the generated voltage is measured as a first time T1 and the time period from the instant of the first voltage zero crossing of the generated voltage to the second voltage zero crossing of the generated voltage is measured as a second time T2.

The rotation speed of the motor is calculated using T1 and T2.

In accordance with a first aspect of the present invention, the motor supply for the pump motor in a vehicle ABS system is controlled by an electronic switch, such as a MOSFET.

This allows frequent testing of the motor circuit by short pulses which produce no significant noise.

In accordance with a second aspect of the present invention, feedback to control motor speed is obtained by measuring the e.m.f. generated by the motor after it has been switched off and after a delay to allow the back e.m.f. caused by switching off the motor, to decay (preferably to zero) before the measurement operation begins.

Preferably, pulse width modulated control of motor speed is adopted, using as the feedback signal the measured e.m.f. generated by the motor, after the back e.m.f. has decayed substantially to zero, and suitably filtered.

The generated voltage is integrated during a motor switch-off period, the motor speed being estimated in this period using the magnitude of the measured integrated voltage from a knowledge of the voltage/speed characteristic of the particular motor, when acting as a generator.

In accordance with a further aspect of the present invention, the ignition switch-controlled supply within the vehicle electrical system only operates a logic gate means to control the supply of direct battery power to the vehicle voltage regulator, the pump motor circuit and its electronic controller (MOSFET) being connected directly to the battery supply whereby to act as an energy sink to protect all silicon devices in the electrical control system except the ignition circuit logic gate means.

The electronic switch, although preferably a MOSFET or NOSFETS, could be any proprietary "electronic relay", "smart switch", bipolar device or similar fast acting electronic device.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a circuit diagram illustrating one embodiment of an ABS pump motor control system in accordance with the present invention; Fig. 2 comprises a series of curves illustrating the operation of part of the apparatus of Fig. 1; Fig. 3 is a circuit diagram illustrating a second embodiment of an ABS pump motor control system in accordance with the present invention; Fig. 4 is a flow diagram illustrating one means of controlling the rate of increase of motor current; Fig. 5 shows an example of the variable mark/space ration signal across the motor using the arrangement of Fig. 4; Fig. 6 shows the use of multiple MOSFETS in parallel to control the pump motor; and Fig. 7 illustrates an embodiment wherein the control switch for the motor is on the positive supply side.

Referring first to Fig. 1, an ABS pump motor 10 is connected at one side to the vehicle battery supply B+ via a line 12 containing a fuse 14. The other side of the pump motor is coupled to ground B- by way of a MOSFET 16. The drive signal for the MOSFET 16 is supplied by a line 18 from a "motor drive" output pin MD of a microcontroller unit 20. The pump motor e.m.f. is monitored via a line 22 containing a sampling switch 24, controlled by a "sampler control" output Sc of the microcontroller 20, and coupled to a filter formed by a capacitor 26 whose one side is earthed and whose other side is connected to the line 22. The line 22 leads between one side of the motor 10 and an input ADC2 of the microcontroller 20. The other side of the motor is connected to an input ADC1 of the microcontroller 20 by a line 28.

For simplicity, a direct connection is shown between the motor and ADC1; in practice, this line (28) would also be filtered and controlled by duplicates of elements 24, 26 and Sc.

The vehicle ignition switch 30 is connected in a line 34 to the vehicle supply B+, downstream of the connection of the line 12. Arrows 32 indicate the usual connections to other vehicle loads from the line 34 controlled by the ignition switch. The conventional voltage regulator 36 for generating a regulated supply Rs is energised by the vehicle supply B+ by way of a line 38 coupled to the line 12 and containing a series switch 40 controlled by an OR gate 42. One output of the OR gate 42 is connected firstly to the ignition switch-controlled line 34 via a diode D1 and resistor R1 and secondly to ground via a further resistor R2. The other input of the OR gate 42 is connected by way of a line 44 and resistor R3 to a "supply control" terminal Su of the microcontroller 20.

The ABS system components are those which are enclosed by the chain line 46 and also include output solenoid drive transistors T1, T2 ... for solenoids S1, S2 .... and a failsafe relay RF, again coupled to the line 12 which is independent of the ignition switch 30.

Thus, in the present system, the conventional "high side" pump motor relay is replaced by the power MOSFET 16 controlling the motor ground supply. This allows frequent testing of the motor circuit using short pulses which produce no significant noise. The low resistance path of the motor and drive MOSFET allows size and cost reduction of all the output drive transistors T1, T2 etc.

Normally, the power to the active silicon devices in an electronic control unit (ECU) is derived from the Ignition Switch - controlled supply. However, in the illustrated embodiment, the ignition supply only operates the logic switch formed by the OR gate 42 to control the supply of direct battery power to the voltage regulator 36. Hence the motor and MOSFET act as an energy sink to protect all the silicon devices except the ignition input logic gate 42. Because of the low input current resulting in the ignition switch-controlled line 34, the components D1, R1 and R2 can be much smaller and cheaper than is usually the case.

A feedback function to control motor speed is obtained by measuring the e.m.f. generated by the motor after it has been switched off. The drive signal to the motor control FET 16 is arranged to be pulse-width-modulated (PWM) so that the supply voltage is proportional to the ratio of the width of the ON pulses relative to the off pulses. The sampling of the generated e.m.f. is arranged to take place only after the back e.m.f. produced by the Off pulse has diminished sufficiently for the generated e.m.f. of the motor 10 to be measured reliably. This requires filtration of the sampled signal and is performed by the filter capacitor 26. This filtration is arranged to be achieved in such a way that the effect of the conducted emissions normally produced by a PWM motor controller are reduced by a significant amount.The immunity of the controller to external interference is also improved by this filter 26.

A further feature with regard to the synchronously switched filter described above is that the sampling of the generated e.m.f. from the motor 10 via the analogue switch 24 activates the (low-pass) filter 26, which allows a longer time constant to be used for the filter than if it was connected directly to the drain of the FET. The filter capacitor 26 retains a charge voltage corresponding to motor speed during the time that the analogue switch 24 is open circuited, and hence performs a "sample and hold" function, which improves measurement accuracy.Since the filter is connected to the motor only when the motor is switched off, this means that the time constant of the filter is longest when the motor speed is highest, which is advantageous for optimum control of the motor; the filter components can be chosen so that at low speeds, the loading of the filter does not significantly increase the motor response time, whilst at higher speeds optimal filtering of the motor e.m.f. is achieved.

In the present system described above, the generated voltage is integrated over a period of length varying from a fraction of a millisecond to a few milliseconds during the motor switch-off period: this has the effect of improving the noise immunity, hence accuracy, of the measurement process. The speed of the motor is estimated in this brief period from a knowledge of the voltage/speed characteristic of the particular motor, when acting as a generator. Thus, in the present invention, the actual magnitude of the measured e.m.f. is used to infer a particular motor speed.

As explained initially hereinbefore, with the conventional motor actuation by a relay, to test that the motor is in-circuit, the relay is turned on. As the speed of operation of a relay is relatively slow, the motor will run before the relay can be turned off.

By using the MOSFET 16 i.e. the motor in-circuit test can be performed by test firing the motor with a short duration pulse, such that the microprocessor driving it can "see" a change in state of the monitor line without enough time for the motor to start running.

Using a HOSFET to drive the motor also allows absorbtion of the load dump energy. This can be achieved in two ways: 1. By allowing the MOSFET and the motor to dissipate the energy by either causing the MOSFET to voltage limit by connecting a zener diode between drain and gate or by letting the MOSFET self avalanche, if a low voltage MOSFET is used.

This could however cause excessive heating of the MOSFET.

2. By turning the MOSFET fully on by the micro controller, so that the motor absorbs all of the energy.

With the second method there is small time delay, between the load dump occurring and the FET turning on. Therefore both methods can be used, such that when the load dump occurs both the MOSFET and the motor absorb the energy until the micro-controller turns the MOSFET fully on so that the motor then absorbs all of the energy.

As the motor, or motor and MOSFET, are absorbing the load dump energy on the B+ input to the controller, then anything else connected to the B+ line will also be protected. Therefore if the ignition input is used solely as a logic input to switch power from the B+ line, to the controller, then the controller can be turned on and off by the ignition input without high current transient protection.

Referring now to Fig. 2, the speed of the motor is determined by filtering and measuring the generated voltage from the motor between points E and F on trace 4. The rate at which the filtered voltage can change is determined by the RC time constant; this only changes between the points E and F as shown on trace 2. Between samples, the previous voltage is held on by the capacitor, which act as a low pass filter. As RFI is usually high frequency and AC coupled, virtually all of the RFI is filtered out.

This filtered voltage is measured by ADC2 as shown on trace 3. This measured voltage is then subtracted from B+ as measured by ADC1. This resultant voltage is proportionate to the speed of the motor. The speed of the motor can be kept constant by negative feedback closed loop control by adjusting the mark to space ratio of the Pulse Width Modulated (PWM) on the motor MOSFET drive.

Thus the advantages gained by adopting the features of the present invention include the fact that the motor 10 and its fuse 14 can be checked without affecting system reliability or generating pump noise; the motor and its MOSFET 16 absorb "load dump" energy, thus saving costs in other components; and low-cost closed-loop control of the motor speed which permits system noise reduction, and pedal feel and ABS performance improvements. Furthermore, a smaller motor may be specified than previously because the present motor needs to be specified at, for example, 12 volts, in order to operate without overload at 16 volts. Hence a motor specified at 12 volts is more powerful than required in order to provide the required output at 8 volts. The use of the invention allows the motor to be specified to give rated power output at 8 volts, such that the motor speed controller then acts as a voltage regulator when the supply is at 16 volts, thus preventing damage to the motor.

In general ABS operation can be performed with the motor operating at a relatively slow speed, hence generating less noise. Only when full hydraulic pump output is required (recognised by ABS control software) will the motor be driven at full power. A further advantage is that a smaller motor can be overdriven for short periods without damage.

Referring now to Figure 3, there is shown a modification of the arrangement of Figure 1. In the arrangement of Figure 1, the voltage generated by the motor 10 during the switch-off period is calculated in software within the microcontroller 20, by subtracting the voltage measured on line 22 via ADC2 from the voltage measured on line 28 via ADC1. In the modified system of Figure 3, a differential amplifier A is used to calculate the difference between the voltages on lines 22 and 28. Resistors R4 and R5 form a precise potentiometer to provide the positive (non-inverting) input to the operational amplifier A, with a signal proportional to the voltage on line 12.Resistors R6 and R7 are also precision resistors to provide the negative (inverting) input to the amplifier A with a signal proportional to the voltage on line 22, and to adjust the close loop feedback of amplifier A in order to present ADC1 with a correctly scaled voltage.

The latter arrangement offers the benefit that only one ADC port is required to perform the relevant function and the software overhead is less.

In the arrangement of Figure 1, the use of a pulse width modulated (PWM) FET 16 to control the motor 10 permits a motor to be used which has a lower voltage rating than is conventionally specified for a 12 volt electrical system (for example 5-8v. compared to 8-16v.). A possible problem in some instances with such an arrangement is that the higher current surge (in rush) associated with a lower operating voltage may lead to a partial demagnetisation of the motor's permanent magnets. This possible problem can be overcome by controlling the initial rate of increase of motor current using software (referred to as softstart). This can be achieved by increasing the pulse rate of the modulation for a short period of time (e.g. 100 ms) to 100 ps intervals. A simple routine for performing this operation and operating open-loop is illustrated in Figure 4.On switch-on of the pump motor, a ramp generator is started which adjusts the mark/space ratio of the motor voltage as illustrated in Figure 5 by way of example via an output buffer and the FET 16. An equivalent function could alternatively be achieved in hardware (not illustrated).

The basic arrangement of Figure 1 uses a single MOSFET 16 to control the motor 10. In the alternative arrangement of Figure 6, the simple MOSFET is replaced by a plurality (four in this case) of similar MOSFETS operating in parallel. An advantage of the latter arrangement is that larger d.c. motors may be controlled than the present "state of the art" allows with a single (relay) device. Furthermore, economies may be made when driving motors of the size associated with the Figure 1 arrangement, by the substitution of several lower-cost FET devices to replace the single device of Figure 1.

The function of the presently proposed system can also be achieved if the switching device for the motor, disposed on the grounded side in the arrangements of Figures 1 and 3, is replace by a proprietory "solid state relay", "smart switch", bipolar device or similar solid state device controlling the positive supply to the motor, rather than the negative supply. Fig. 7 shows such an arrangement using a single MOSFET 16' in the positive supply side of the motor 16.

Claims (9)

1. A vehicle anti-lock braking (ABS) system having a motor-driven pump for providing a hydraulic supply to the ABS system, the pump motor being connected between a source of voltage and ground (usually the vehicle chassis) and being controlled by opening and closing a series switch, wherein said series switch is an electronic switch.

2. A system as claimed in claim 1 in which said series switch is a MOSFET.

3. A system as claimed in claim 1 in which the series switch comprises a plurality of MOSFETS in parallel.

4. A system as claimed in claim 1, 2 or 3, wherein feedback to control motor speed is obtained by measuring the e.m.f. generated by the motor after it has been switched off and after a delay to allow the back e.m.f., caused by switching off the motor, to decay before the measurement operation begins.

5. A system as claimed in claim 4 wherein the e.m.f. is allowed to decay to zero before the measurement operation begins.

6. A system as claimed in claim 4 or 5, wherein pulse width modulated control of motor speed is adopted, using as the feedback signal the measured e.m.f. generated by the motor, after the back e.m.f.

has decayed substantially to zero, and suitably filtered.

7. A system as claimed in claim 4, 5 or 6 wherein the generated voltage is integrated during a motor switch-off period, the motor speed being estimated in this period using the magnitude of the measured integrated voltage from a knowledge of the voltage/speed characteristic of the particular motor, when acting as a generator.

8. A vehicle equipped with an ABS system as claimed in any of claims 1 to 7, wherein the electrical supply controlled by the vehicle ignition switch only operates a logic gate means to control the supply of direct battery power to the vehicle voltage regulator, the ABS pump motor circuit and its electronic control switch being connected directly to the battery supply, independently of the ignition switch, whereby to act as an energy sink to protect all silicon devices in the vehicle electrical control system except said ignition circuit logic gate means.

9. A vehicle ABS system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.