GB2367962A - A single current sensor for several solenoids in a hydraulic braking system - Google Patents

Abstract

A number of separately controllable solenoids 10a, 10b, 10c are coupled to a single current sensing resistor 24 for output to an analogue-to-digital converter via an amplifier 26. The switches 12a,12b,12c may be FETs.

Description

DESCRIPTION MULTIPLE CHANNEL SOLENOID CURRENT MONITOR The present invention is concerned with the monitoring of multiple channel solenoid currents particularly, but not exclusively, in automotive electrical and electronic control systems.

There are many situations in electrical and electronic control systems where there is a requirement for the current flowing in a solenoid coil to be monitored and measured. Conventionally, each channel containing an individual solenoid coil has its own current sensing element associated with it. Usually, the sensing element comprises a resistive element, eg. a simple resistor disposed in series with the solenoid coil, whereby the voltage drop across the resistor is proportional to the current flowing through it, and hence proportional to the current flowing through the solenoid coil. The voltage across the resistor is conditioned and read by an analogue to digital converter (ADC).

The known arrangement thus has the associated cost disadvantage that individual sensing elements and conditioning are required for each channel of current to be measured, ie. for each solenoid coil to be monitored.

It would be advantageous to provide, particularly for automotive applications, an arrangement whereby it is no longer necessary for there to be individual sensing elements for each solenoid channel.

In accordance with the present invention, a plurality of separately controllable solenoid coils are coupled commonly by a single current measurement element to one side of a current supply.

The measurement element can, for example, be coupled to an analogue to digital converter via a signal conditioning amplifier for measurement purposes.

Preferably, the solenoid coils are coupled commonly by said single current measurement element to the low side of the current supply.

In a preferred embodiment, in order to enable the current through any one particular solenoid coil to be measured, means are included for, firstly, enabling a current measurement reading to be made only while a respective drive element for that particular solenoid coil is switched on, and, secondly, switching on the drive element for only that particular coil when the current measurement reading is made, with all other drive elements being switched off.

The first and second means would normally be realized by logic circuit arrangements which are implemented by hardware or software. Software implementation is preferred since the microcomputer which is present in the system for control of the braking system is available for this purpose, so that the additional hardware costs do not arise.

Usually the measurement of the current through the solenoid coils is required for"closed-loop"operation whereby, preferably, the duty cycle of a PWM-signal is varied to control the current through the solenoid coil. Since the present circuit arrangement has a common current measurement element,"closed-loop"operation is not possible. Therefore a so-called"calibration cycle"can be arranged to be passed through for each solenoid coil when the two before-mentioned first and second conditions can be met. During the calibration cycle, the optimum setting for the PWM duty cycle can be learned. Because the calibration cycles are repeated periodically, a reliable operation can be ensured over the whole running period, even though only"normal"control is possible. This means e. g. that in applications where valves are driven, a correct switching behaviour ("OPEN","CLOSED"respectively) can be guaranteed.

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 a typical example of a known arrangement for monitoring multiple solenoid coils; Fig. 2 is a circuit diagram illustrating one embodiment of an arrangement in accordance with the present invention for monitoring multiple solenoid coils; and Fig. 3 is an example of an electro hydraulic braking system (EHB) having a plurality of solenoid operated valves to which the present invention can be applied.

Referring first to Fig. 1, there is shown a circuit arrangement having three

solenoids whose solenoid coils 10a, 10b, 10c are to be monitored. The solenoid coils 10a, 10b, 1 Oc are disposed between a supply line 16 and ground 18 and are controlled by respective series low side switches 12a, 12b, 12c, for example drive FETs. In parallel with each coil 10a, lOb, 1Oc is a respective recirculation diode 14a, 14b, 14c. In order to measure the current passing through the solenoids 10a, lOb, lOc, there is disposed between each drive FET 12a, 12b, 12c and ground 18 a respective resistor 20a, 20b, 20c, the voltage drop across each resistor 20a, 20b, 20c being measured by a respective amplifier 22a, 22b, 22c. The outputs of the amplifiers 22a, 22b, 22c lead to respective ADC inputs (not shown) for measurement purposes.

In the current arrangement of Fig. 2 in accordance with the present invention, identical components are given the same reference numerals. In this arrangement, the terminals of three FETs 12 remote from the coils 10 are connected together and coupled to ground 18 via a single common resistor 24, the voltage across which is monitored by means of a single amplifier 26. The output of the amplifier is again passed to the input of an analogue to digital converter (ADC) for measurement purposes.

It is emphasised that although the illustrated circuit shows three solenoid coils lOa, lOb, 1Oc, this is purely by way of example and in practice there could be any number of such coils, commonly coupled to ground 18 by the single sensing resistor 24.

Although the circuit illustrated uses FETs as low side drivers, in principle other drive elements, such as relays, transistors and the like, could be used.

The sensing element formed by resistor 24 in the circuit of Fig. 2 detects the sum of all of the currents flowing through the solenoid coils 10a, lOb, 1Oc. In order to read the current through any particular single one of the coils 10, two conditions must be met. Firstly, the ADC reading must only be made while the respective drive element 12 for that particular solenoid channel is switched on. Secondly, only the FET 12 for the particular coil 10 being measured should be switched on when the ADC reading is made. All other drive FETs 12 must be switched off. Although not shown in Fig. 2, suitable control circuitry is provided to achieve these two conditions.

By having only a single sensing resistor and conditioning amplifier, a substantial cost saving can be made.

Fig. 3 shows an example of a typical practical situation where the use of the present invention can be of cost saving advantage. Fig. 3 illustrates an electro hydraulic braking system where braking demand signals are generated electronically at a travel sensor 28 in response to operation of a foot pedal 30, the signals being processed in an electronic control unit (ECU) 32 for controlling the operation of brake actuators 34a, 34b at the front and back wheels respectively of a vehicle via pairs of valves 36a, 36b 36c, 36d. The latter valves are operated in opposition to provide proportional control of actuating fluid to the brake actuators 34 from a pressurised fluid supply accumulator 38, maintained from a reservoir 40 via a motordriven pump 42. For use, for example, in emergency conditions when the electronic control of the brake actuator is not operational for some reason, the system includes a master cylinder 44 coupled mechanically to the foot pedal 30 and by which fluid can be supplied directly to the actuators 34 in a"push-through"condition. In the push-through condition, a fluid connection between the front brake actuators 34a and the cylinder 44 is established by means of digitally operating, solenoid operated valves 46a, 46b. Also included in the system are further digitally operating solenoid valves 48,50 and 52 which respectively connect the two pairs of valves 36a, 36b, the pump 42 and accumulator 38 and the two pairs of valves 36c, 36d.

The present system of monitoring solenoid coil currents can be applied to monitor the currents in the digitally operated valves 46a, 46b, 48,50 and 52 by means of a common sensing element 24 and conditioning amplifier 26. It can also be applied to the control valve pairs 36a, 36b, 36c and 36d in the event that they are not provided for a proportional control mode.

Claims (5)

1. An arrangement for monitoring multiple channel solenoid currents wherein a plurality of separately controllable solenoid coils are coupled commonly by a single current measurement element to one side of a current supply.

2. A monitoring arrangement as claimed in claim I wherein the measurement element is coupled to an analogue to digital converter via a signal conditioning amplifier for measurement purposes.

3. A monitoring arrangement as claimed in claim 1 or 2, wherein said plurality of separately controllable solenoid coils are coupled commonly by said single current measurement element to the low side of the current supply.

4. A monitoring arrangement as claimed in claim 1,2 or 3, wherein in order to enable the current through any one particular solenoid coil to be measured, means are included for, firstly, enabling a current measurement reading to be made only while a respective drive element for that particular solenoid coil is switched on, and, secondly, switching on the drive element for only that particular coil when the current measurement reading is made, with all other drive elements being switched off.

5. A monitoring arrangement for multiple channel solenoid currents, substantially as hereinbefore described, with reference to and as illustrated in Figs 2 and 3 of the accompanying drawings.