This invention relates to a control system, hereinafter referred to as the kind specified, comprising a device to be controlled, a sensor to provide a signal indicative of a predetermined parameter, an electrical circuit to supply said signal to a managing means responsive to said signal to control said device and wherein said signal is zero for a predetermined value of said parameter.

In such a system it is not possible to differentiate between a zero voltage signal being due to said predetermined value of said parameter or being due to failure of the electrical circuit or the sensor.

An object of the invention is to provide a control system of the kind specified in which the above mentioned problem is overcome.

According to one aspect of the invention we provide a control system of the kind specified wherein a continuity testing means is provided to test the electrical continuity of the sensor and said circuit.

Preferably said managing means is adapted to initiate operation of said continuity testing means consequent upon the managing means detecting absence of said signal, i.e. a zero voltage signal.

The continuity testing means may provide a signal when the continuity testing means detects the presence of electrical continuity, to permit normal operation of the device.

The continuity testing means may provide a signal when said continuity testing means detects an absence of electrical continuity, to inhibit normal operation of the device.

Preferably said continuity testing means is arranged to test said electrical continuity after the signal has been determined to have returned to zero after a period of being non-zero. This ensures that the electrical continuity is tested only when it is necessary so to do.

The continuity sensing means may comprise a means to apply a predetermined current to said circuit in parallel with the sensor and with an impedance of a signal conditioning circuit adapted to provide an output to the managing means to indicate said predetermined value of said parameter when

the voltage applied across the input to said conditioning circuit is below a predetermined voltage, said predetermined current being of a value such that when the circuit is continuous the impedance of the testing circuit is a first value, and the voltage across the input to the conditioning circuit is below said predeteπnined voltage whilst when the circuit is not continuous the impedance of the testing circuit is a second, higher, value than the first value _^ and the voltage across the input to the conditioning circuit is not below said predetermined value.

The managing means may be adapted to execute the following routine. a) On detecting that the signal has decreased to zero a continuity testing routine is initiated in which any output from the sensor circuit is regarded as a response to a continuity test, b) said current supply means is activated to apply said current through the parallel combination of the sensor and conditioning circuit input impedance and thereafter said current supply means is de-activated to interrupt said current supply, c) preferably, said cycle of activation followed by de-activation of the current supply is repeated a plurality of times, for example a sequence of three times, within a predetermined time, for example 100 milliseconds and the signal obtained for each cycle is assessed on the basis that if a signal indicative of open circuit or closed circuit is obtained for each cycle of the sequence, the respective signal is accepted by the managing means, whilst if different signals are obtained for different cycles of the sequence, the continuity testing routine is interrupted and normal operation of the device initiated, d) when presence of a signal, or assessed sequence of said signals, indicates absence of continuity the managing means causes said inhibition of the normal operation of the device whilst absence of a signal, or assessed sequence of said signals, indicates presence of continuity and the managing means permits normal operation of the device.

The managing means may initiate a warning signal when absence of continuity is detected.

The control system may be of any desired kind and the device may comprise any suitable device and the sensor means sense any suitable parameter.

For example, the control system may comprise a steering system for vehicles and the device may comprise means to steer a vehicle and the sensor may provide a signal indicative of the steering position of the vehicle.

Alternatively, the control system may comprise a variable suspension system for vehicles and the device may comprise means to adjust the suspension height of a vehicle and the sensor may provide a signal indicative of the suspension height of the vehicle.

Alternatively, the control system may comprise a system to prevent the wheels of a vehicle slipping during acceleration or deceleration of a vehicle and the device may comprise means to control the rate of acceleration or deceleration of the wheel and the sensor may provide a signal indicative of the acceleration or deceleration of the vehicle.

Alternatively, the control system may comprise a fluid pressure brake system for vehicles and the device may comprise a skid control unit to relieve the pressure of an operating fluid supplied to a wheel brake of the vehicle and the sensor may comprise a speed sensor to sense the speed of rotation of a wheel of the vehicle and to provide a wheel speed signal indicative of the speed of rotation of the wheel of the vehicle.

In the latter case, from a more specific aspect the invention relates to a fluid pressure operated braking system for vehicles, hereinafter referred to as the kind specified, comprising a wheel brake, a speed sensor to provide a wheel speed signal, an electrical circuit to supply said signal to a managing means responsive to said signal to cause a skid control unit to relieve the pressure of an operating fluid supplied to the wheel brake.

For example, the speed sensor may generate a voltage to provide said wheel signal and hence provides a zero voltage when the wheel skids. Alternatively the speed sensor may modify an externally applied voltage so as to produce a zero voltage when the wheel skids.

In either case, when the wheel is skidding the voltage and hence the wheel speed signal produced by the sensor is zero. Hence the managing means

cannot differentiate between such a wheel speed signal due to lack of wheel rotation and a failure in the electrical circuit or the sensor.

A further object of the invention is to provide a fluid pressure operated braking system of the kind specified in which the above mentioned problem is overcome.

According to a second aspect of the present invention we provide a fluid pressure operated braking system of the kind specified wherein a continuity testing means is provided to test the electrical continuity of the sensor and said circuit.

Preferably said managing means is adapted to initiate operation of said continuity testing means consequent upon the managing means detecting absence of said wheel speed signal.

The continuity testing means may provide a signal when the continuity testing means detects the presence of electrical continuity, to permit normal anti¬ skid operation of the skid control unit to release the brake.

The continuity testing means may provide a signal when said continuity testing means detects an absence of electrical continuity, to inhibit normal anti¬ skid operation of the skid control unit to release the brake.

Preferably said continuity testing means is arranged to test said electrical continuity after the wheel speed has been determined to have returned to zero after a period of being non-zero. This ensures that the electrical continuity is tested only when it is necessary so to do.

The continuity sensing means may comprise a means to apply a predetermined current to said circuit in parallel with the speed sensor and with an impedance of a wheel speed signal conditioning circuit adapted to provide an output to the managing means to indicate absence of wheel rotation when the voltage applied across the input to said conditioning circuit is below a predetermined voltage, said predetermined current being of a value such that when the circuit is continuous the impedance of the testing circuit is a first value, and the voltage across the input to the conditioning circuit is below said predetermined voltage whilst when the circuit is not continuous the impedance of the testing circuit is a second, higher, value than the first value, and the

voltage across the input to the conditioning circuit is not below said predetermined value.

The managing means may be adapted to execute the following routine. a) On detecting that the wheel speed has decreased to zero a continuity testing routine is initiated in which any output from the wheel speed sensor circuit is regarded as a response to a continuity test, b) said current supply means is activated to apply said current through the parallel combination of the wheel speed sensor and conditioning circuit input impedance and thereafter said current supply means is de-activated to interrupt said current supply, c) preferably, said cycle of activation followed by de-activation of the current supply is repeated a plurality of times, for example a sequence of three times, within a predetermined time, for example 100 milliseconds and the signal obtained for each cycle is assessed on the basis that if a signal indicative of open circuit or closed circuit is obtained for each cycle of the sequence, the respective signal is accepted by the managing means, whilst if different signals are obtained for different cycles of the sequence, the continuity testing routine is interrupted and normal anti-skid operation initiated, d) when presence of a wheel speed signal, or assessed sequence of said signals, indicates absence of continuity the managing means causes said inhibition of the normal anti-skid operation of the skid control unit whilst absence of a wheel speed signal, or assessed sequence of said signals, indicates presence of continuity and the managing means permits normal anti-skid operation of the skid control unit.

The managing means may initiate a warning signal when absence of continuity is detected.

An embodiment of the invention is illustrated in the accompanying drawings, wherein

FIGURE 1 is a diagrammatic illustration of a two axle vehicle provided with a braking system embodying the invention, and

FIGURE 2 is a diagrammatic illustration of parts of the electric circuit of the braking system of Figure 1.

Referring now to Figure 1, a vehicle has two front wheels 10, 11 on a first axle of the vehicle and two rear wheels 12, 13 on a second axle of the vehicle. Each wheel is provided with a wheel brake 14 - 17 respectively and each wheel is provided with a respective wheel speed sensor 18 - 21 respectively. The wheel speed sensors 18 - 21 comprise an alternating voltage generator and the magnitude and frequency of the generated voltage varies in proportion to the speed of wheel rotation. When a wheel is stationary no voltage is generated. If desired the wheel speed sensors may be of other type and may, for example, be arranged to modify an externally applied voltage so that zero voltage is provided when a wheel is stationary.

The variable voltage and frequency current thus generated is fed by an electric circuit 22 - 25 respectively to a managing means 26 which comprises a micro-processor and, for each of the sensors 18 - 21, a sensor signal conditioning circuit.

The managing means 26 provides a control signal for operating front and rear skid control units 28, 29, via lines 30.

Each skid control unit 28, 29 controls the supply of an operating pneumatic pressure to a relay valve 31 - 34 for each respective wheel and each relay valve 31 - 34, in turn, controls the supply of air under pressure from a reservoir 35, 36 to the brakes 14 - 17. If desired the operating fluid may be hydraulic fluid and whether hydraulic or pneumatic the operating fluid may be supplied to the brakes independently of the control units, the control units being arranged to reduce the fluid pressure thus applied to the brakes.

Referring now to Figure 2, the sensor 18 is shown with its associated circuit 22. It should be appreciated that the three other sensors 19 - 21 and their associated circuits 23 - 25 are similar and thus do not require further discussion.

The sensor 18 comprises a voltage generator as described in EP-B- 0190879 although, if desired, any other suitable voltage generator type sensor may be used. The sensor 18 is connected in circuit 22 in parallel with an input impedance 35 of a sensor signal conditioning circuit 36 of conventional type. The sensor signal conditioning circuit 36 is arranged to provide a signal to the

micro-processor 37 of the managing means 26 when the voltage across the input of the circuit 36 is above a predetermined value, whilst when the voltage across the input to the circuit 36 is below said .predetermined value, no signal is provided from the conditioning circuit 36 to the micro-processor 37.

A continuity testing circuit 38 is connected in parallel with the sensor 18 and impedance 35 and incorporates suitable switch means 39 actuated by a switch signal provided on a line 40 from the micro-processor 37.

Lines 41 extend from the micro-processor 37 for control signals to the skid control units 28, 29 whilst a line 42 extends to a warning means drive circuit.

The micro-processor is programmed with an algorithm such that it performs the following routines.

When the wheel 10 is rotating the sensor 18 generates a voltage which is conditioned by the circuit 36 and an appropriate signal indicative of wheel rotation is fed to the micro-processor 37.

When the wheel stops rotating as a result of a skid, or if the electrical continuity of the wheel sensor 18 or circuit 22 is interrupted, no signal is provided from the conditioning circuit 36 to the micro-processor 37 which thus a)initiates normal anti-lock operation by providing a signal on the line 41 to the respective control device 28, 29 to cause appropriate relief of pressure to the associated wheel brake 14, then b) a wheel speed sensor continuity testing routine is initiated and this occurs until such time as the wheel speed sensor circuit continuity is confirmed or otherwise.

If wheel speed sensor continuity is confirmed, normal wheel speed measurement continues providing anti-lock activity as demanded. If wheel speed sensor continuity is not confirmed anti-lock activity is disabled, at least on the wheel where continuity is not confirmed and an anti-lock warning lamp is illuminated. If desired the warning means device circuit may activate any other desired warning means.

More particularly, when the micro-processor has initiated the continuity testing routine the micro-processor is set so that any output from the conditioning circuit 36 is regarded as a response to a continuity test which is to

be performed. In addition, the switch 39 associated with the circuit 22 is closed by the micro-processor 37 by means of a signal on the line 40 and direct current of a predetermined constant or substantially constant value flows through the circuit 38 and hence through the parallel combination of the wheel speed sensor 18 and the sensor signal conditioning input impedance 35. The constant or substantially constant current may be provided by a series connected fixed resistor of significantly greater impedance than that of the impedance 35, or by a suitable transistor circuit adapted to provide a constant or substantially constant current or by other suitable means.

The value of the predetermined current is chosen such that if the wheel speed sensor 18 or the circuit 22 is continuous, the impedance seen by the testing circuit is at a first value such that there is a relatively small voltage drop across the input to the conditioning circuit 36 of a magnitude below said predetermined value so that there is no output from the circuit 36 to the micro¬ processor 37. Alternatively, if the wheel speed sensor 18 or the circuit 22 is not continuous the impedance seen by the testing circuit is higher and therefore the voltage dropped across the input of the conditioning circuit 36 is greater and not less than said predetermined value so as to cause an output from the circuit 36 to the micro-processor 37.

After a predetermined period of time the switch S is opened by the micro-processor 37 and then the above described cycle of closure of switch 39 and subsequent opening thereof is repeated for a sequence of a predetermined number of times in a predetermined time period, for example, a sequence of at least three times within 100 milliseconds.

The micro-processor 37 then assesses the results obtained in such a sequence. If all the results are identical, i.e. if a signal is either produced or not produced by the conditioning circuit 36 for each cycle of an assessed sequence, the result is accepted by the micro-processor and acted upon as hereinafter to be described.

If the results are not all identical the micro-processor aborts the continuity test routine and normal anti-lock wheel speed assessment continues.

If the results are accepted and indicate that the circuit has continuity, i.e. by virtue of no signal being received from the conditioning circuit 36, normal anti-lock operation continues of the skid control units, whilst if discontinuity is indicated by virtue of a signal being received by the micro-processor 37 from the conditioning circuit 36, then the micro-processor inhibits normal anti-lock activity operation of the skid control unit, at least associated with the wheel 10 associated with the sensor 18 and a signal is provided on the line 42 to an anti-lock warning means drive circuit to warn the driver of' a reduction in performance of the anti- lock system.

After performance of the above mentioned sequence of cycles, the sensor continuity test routine is terminated and normal anti-lock control routine is re-entered.

The micro-processor 37 is programmed so that the above mentioned continuity testing routine is entered each time a sensor 18 -21 senses cessation of wheel rotation after a period of the wheel rotation. Accordingly, when the vehicle is stationary there is no initiation of the wheel speed sensor continuity testing routine.

Although in the above described example a separate wheel speed sensor and skid control circuit is provided for each wheel and wheel brake, if desired a specific wheel speed sensor may provide a wheel speed signal which is used to control one or more skid control units and the or each skid control unit may control one, or more than one, wheel brake.

Although in the embodiment described above a control system comprising a fluid pressure braking system for a vehicle has been described the invention can be applied to any other desired suitable control system to control any desired suitable device by sensing any desired suitable parameter. For example, the control system may comprise a steering system for vehicles and the device may comprise means to steer a vehicle and the sensor may provide a signal indicative of the steering position of the vehicle.

Alternatively, the control system may comprise a variable suspension system for vehicles and the device may comprise means to adjust the suspension

height of a vehicle and the sensor may provide a signal mdicative of the suspension height of the vehicle.

Alternatively, the control system may comprise a system to prevent the wheels of a vehicle slipping during acceleration or deceleration of a vehicle and the device may comprise means to control the rate of acceleration or deceleration of the wheel and the sensor may provide a signal indicative of the acceleration or deceleration of the vehicle.

The features disclosed in the foregoing description, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, or a class or group of substances or compositions, as appropriate, may, separately or in any combination of such features, be utilised for realising the invention in diverse forms thereof.