GB2377029A

GB2377029A - Sensors and signal encoding

Info

Publication number: GB2377029A
Application number: GB0223136A
Authority: GB
Inventors: Kenneth Vincent; David Charles Hurst
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1998-08-27
Filing date: 1999-08-27
Publication date: 2002-12-31
Anticipated expiration: 2019-08-27
Also published as: GB2377029B; GB0223136D0

Abstract

A method of encoding two or more input signals (OP1,OP2) on a single line comprising processing the two or more input signals to produce a pulse width modulated (PWM) output signal comprising at least two periodically repeated PWM portions encoding the first and second signal respectively, and in which the mark-space ratio of the first portion of the pulse width modulated output signal is dependent upon the value of the sum of the first and second input signals and the mark-space ratio of the second portion of the pulse width modulated signal is dependent upon the value of the first input signal, and in which during normal operation the mark-space ratio of the first portion lies outside an allowable range of mark-space ratios for the second portion. The two input signals may comprise demand signals from a dual potentiometer brake pedal sensor. In another aspect, a current limiting circuit is provided between the sensors and the voltage supply so that in the event that the two sensors short together, the voltage supply is pulled down and the sum signal drops. This drop can than be detected.

Description

IMPROVEMENTS RELATING TO SENSORS This invention relates to improvements in sensors, and in particular to sensors for use in electro-hydraulic braking systems of the brake-by-wire kind.

In the known kind of brake-by-wire braking systems, a brake demand signal is generated from a sensing means operatively connected to a brake pedal. The sensor produces an output signal indicative of brake demand which varies with pedal position. Typically, this function is performed by a potentiometer of the rotary or slide type. A demand signal is taken from a connection to a wiper arm on the potentiometer which slides along a resistive or capacitive track connecting a positive and negative supply.

Of course, other suitable sensors can be used. This brake demand signal is then processed by a control unit which controls one or more electrohydraulic valves. Operation of the valves modulates hydraulic pressure applied to a brake for a wheel.

Braking systems are quite safety critical. Failure of a single sensor producing an erroneous demand signal must be detected and/or compensated. To do this, it is known to provide two sensors, each producing an output demand signal which varies in an opposite sense, the sum of the two demand signals remaining constant.

Integrity checks can be performed by monitoring the sum signal. If it varies, one of the sensors must be at fault. The system could then flag an error signal.

In the past, two separate signals have been produced, one for the sum and one for the pedal position. However, this requires two separate output limits.

A problem with the simple sum approach occurs when the wipers of two potentiometers short together. In this case, the sum signal may provide a plausible reading even though the sensors are not working. This is a dangerous failure mode.

In accordance with a first aspect, the invention provides a method of encoding two or more input signals on a single line comprising processing the two or more input signals to produce a pulse width modulated output signal comprising at least two periodically repeated pulse width modulated portions encoding the first and second signal respectively, and in which the mark-space ratio of the first portion of the pulse width modulated output signal is dependent upon the value of the sum of the first and second input signals and the mark-space ratio of the second portion of the pulse width modulated signal is dependent upon the value of the first input signal and in which during normal operation the markspace ratio of the first portion lies outside an allowable range of markspace ratios for the second portion.

The sum of the first and second input signals is preferably constant.

In this manner, two signals can be encoded as pulse width modulated signals on a single line and can easily be discriminated without the need for a synchronising signal by monitoring the mark-space ratios. The sum signal is used to generate a synchronising pulse.

In a refinement, a third, or fourth or even fifth additional input signal can be encoded in the output signal. The additional signals may be independent of the first and second signals. They may be encoded as a periodic pulse width modulated portion of the output signal in a similar manner to the second portion of the output signal.

The input signals may be demand signals from a sensor and may be analogue or digital signals.

The sensors may comprise part of a brake-by wire electro-hydraulics braking system.

In accordance with a second aspect, the present invention provides apparatus for use in combination with a sensing means comprising at least a first and a second sensor in which the first sensor is adapted to produce a first demand signal indicative of a measurand and the second sensor is adapted to produce a second demand signal indicative of the said measurand and which varies in the opposite sense to the first signal, in normal operation the sum of the first and second demand signals being substantially constant, the apparatus comprising sensor signal processing means adapted to produce a first intermediate signal indicative of the sum of the first and second demand signals and a second intermediate signal indicative of only one of the demand signals, and an intermediate signal processing means adapted to generate a pulse width modulated output signal having at least first and second periodically repeating portions within which the value of the first intermediate signal and the second intermediate signal are respectively encoded and in which during normal operation of the sensors the mark-space ratio of the portion of the pulse width modulated signal corresponding to the first intermediate signal lies outside an allowable range of mark-space ratios for the portion of the

pulse width modulated signal corresponding to the second intermediate signal.

The sensors may form part of a brake-by-wire braking system. Of course, in a modification, the apparatus could easily be adapted for use in combination with any two or more sensor producing two demand signals.

These sensors need not be for use in brake-by wire systems, but could be sensors for use in engine diagnostics or steering systems. It is envisaged ., that it could be modified to encode signals which do not have a substantially constant sum value and which could be independent.

The apparatus thus produces a single modulated output signal which encodes information about the sum and the pedal position which can easily be discriminated from the single output signal.

The apparatus may further include a switching means adapted to switch between at least a first state and a second state and in which the switching means is adapted to connect the first intermediate signal to an intermediate signal processing means when the switching means is in the first state, and adapted to connect the second intermediate signal to the intermediate signal processing means when the switching means is in the second state.

The apparatus may further comprise ramp signal generation means adapted to produce a sawtooth or triangular ramp signal.

Pulse generation means may be provided which is adapted to produce a periodic control signal with a period substantially equal to the period of the ramp signal. The switching means may be adapted to switch states as a function of the periodic control signal.

The intermediate signal processing means may comprise a comparator. The ramp signal may be connected to one input of the comparator whilst the switching means alternatively connect the first intermediate signal and the second intermediate signal to the other input of the comparator. This provides one way in which each pulse width modulated portion of the modulated output signal can be produced.

The ramp signal and control signal may comprise different waveforms.

The control signal may be a square wave.

Means may be provided for monitoring the modulated output signal to detect failure of one or more of the sensors. In this case, the monitoring means may be adapted to identify the portion of the modulated signal which corresponds to the sum signal. The monitoring means may be adapted to identify a fault by monitoring variation in the mark/space ratio of the sum portion of the modulated output signal.

The first and second sensors may comprise potentiometers. They may be provided in a common sensor body or housing. They may be connected between a supply rail and ground, and have an output demand signal taken from a wiper. The demand signals for each sensor may vary over substantially the same range of values.

The sensor signal processing means may be adapted to produce the first intermediate (sum) signal using the equation: first intermediate signal = A (first demand signal + second demand signal)

where A is a scaling factor which may lie between 0 and 1. A value of A of 0.9 to 0.95 is preferred. The value of A is chosen so that the maximum value of the first intermediate signal is less than the peak ramp signal value.

The second intermediate signal may be produced without modifying the single demand signal. Of course, this demand signal could also be multiplied by a factor, denoted B, as long as B < A. Again, the value of B is chosen so that the second intermediate value is less than the peak ramp signal value.

This achieves the desired effect of ensuring that the normal mark/space ratio from the sum portion of the modulated output signal lies outside of any possible range of the mark/space ratio for the portion of the modulated output signal corresponding to the single demand signal.

The ramp signal may vary over the same range of values as the first and second demand signals, or over a higher or lower range of values. In each case, the scaling factors A and B must be carefully chosen. The ramp signal may vary substantially linearly over its rising slope.

Encoding the sum signal and single demand signal as the mark-space ratio of a portion of a PWM signal, i. e. the ratio of on to off time, the frequency of the ramp signal could vary without significantly effecting the accuracy of the signal. Such changes could be due to component value tolerances or thermal drifts.

The ramp generator may comprise a Miller Integrator.

The pulse generation means which supplies the switching means may comprise a pulse generator adapted to produce an intermediate control signal comprising a periodic sequence of short duration pulses of duration corresponding to the time during which the ramp signal exceeds a predetermined threshold value. It may include a comparator, one input of which is connected to the ramp signal output of the ramp generator, the other being connected to a reference signal. The output of the comparator may be adapted to remain low until the ramp signal exceeds the reference value, and return low when it drops back below the reference value.

The reference signal may be generated by a current mirror, such as a long tail pair. One end of the mirror may provide the supply voltage for the sensors. The other end at which the voltage is mirrored to the same value may define the reference value and be connected to the comparator.

An advantage of this technique is that the reference fed to the comparator will not drop as the sensor attempts to pull down the rail by drawing excess current in a short circuit failure.

The output of the comparator of the pulse generation means may be fed to the input of a bistable. This may trigger from the rising or falling edge of the short duration pulse signal. The effect of the bistable is to divide the period of the short duration pulse signal by two to produce the periodic control signal for the switching means.

The output of the bistable may be directly fed to the switching means.

A regulated voltage power supply circuit may be provided which is adapted to reduce a high voltage supply line a to a voltage suitable for a

supply rail for the ramp circuit, pulse generator and sensors etc.

This may convert a 12 volt negative supply into a lower voltage such as 7.5 volts or 5 volts. The lower voltage may be selected to be compatible with standard electronic devices (such as FET, bipolar, mosfet).

The optional monitoring means may be adapted to detect when the markspace ratio of the sum signal and/or the pedal position signal lies outside of an allowable range by an amount exceeding an allowable tolerance error range.

The sensors may be connected to a current limited supply. Any attempt to draw current in excess of a predetermined allowable level would result in a reduced voltage being applied across the sensors.

The failure of the sensors in a mode in which the two wipers are fused together can be detected in this invention as such a failure would result in the voltage rail supplying the devices being pulled down.

The apparatus may be implemented in hardware using analogue electronics circuit components. Of course, it may be implemented at least partially using digital techniques. The circuitry may be provided within a sensor housing for the sensing means.

In accordance with a third aspect, the invention provides a braking system incorporating an apparatus according to the second aspect of the invention.

In accordance with a fourth aspect, the invention provides a sensor circuit including a sensor assembly comprising at least a pair of sensors, in which one sensor is adapted to produce a first output signal and the other

sensor is accepted to produce a second signal, the sum of the signals being constant during normal operation, and a current limiting means provided between the sensor assembly and its supply voltage adapted to limit the current which can be drawn by the sensor assembly so that, in the event of a fault in which the sensors are at least partially shorted together, the value of the sum signal is reduced.

Each sensor may comprise a potentiometer comprising a track and a wiper. The track may be connected across the supply. The output signal from a respective sensor may be taken from the wiper. The wiper may comprise a conducting element which contacts the track. The track may be resistive so the sensor acts as a potential divider. The output signals may comprise voltage signals.

The sensors may be housed in a common body, with the two wipers connected together physically. A single wiper carrying two conducting elements, one for each track, may therefore be provided.

The sensors may be arranged so that the tracks lie parallel to one another. A common positive side of the supply may be connected to one end of a first sensor track and the other end of the second track. The two remaining ends of track may be connected to earth. This ensures that as one signal goes up, the other goes down and vice versa.

Because the two wipers are connected together, there is a possibility of a short circuit forming between the two wipers. This type of fault can be difficult to detect. Because the invention limits the supply of current, a short circuit fault will pull down the supply positive voltage, producing a reduced sum value which can be detected. An error flag can be lowered if the sum falls outside of an allowable range.

The current limiting means may comprise one arm of a current mirror.

The sensor circuit may form part of an electro-hydraulic braking system.

The wiper may be connected to a brake pedal.

There will now be described, by way of example only, one embodiment of the present invention with reference to the accompanying drawings in which Figure 1 is a block diagram illustrating the various component parts of an apparatus for use in monitoring a demand signal; Figure 2 is a timing diagram illustrating the output of the various blocks of the system; Figure 3 is a circuit diagram illustrating one circuit which implements an embodiment of the present invention; Figure 4 shows the output from two demand signal sensors; and Figure 5 is a circuit diagram for a modified circuit which enables four sensor signals to be encoded.

Apparatus for monitoring a brake pedal demand signal and producing an encoded demand signal is shown in figure 3. A schematic block diagram equivalent is shown in figure 1. Of course, the skilled man will appreciate that the circuit is not intended to be limiting, and that many possible ways of implementing the scheme shown in figure 1 exists which will fall within the scope of the invention.

As shown in figure 1, the circuit can be broken down into several constituent stages.

In a first stage 1, a ramp is generated which varies linearly between a first voltage level and a second voltage level periodically to produce a sawtooth or triangular waveform. The first voltage level may correspond to zero volts. The second voltage level may substantially correspond to the line voltage for the circuit supply system. This may be five to seven volts.

In one embodiment, illustrated in figure 3, the ramp generator comprises a Miller Integrator connected to a 7 volt supply rail and an earth. The ramp waveform produced by the Miller Integrator is in the form of a sawtooth waveform which varies from 0 volts to approximately 5 volts as shown in figure 2a. This rise is set by R16 and the fall time by R15.

The output of the ramp generator is connected to one input of a pulse generation means 2. The pulse generation means 2 comprises a comparator, one input of the comparator being fed from the output of the ramp generator and the other being tied to a reference voltage that is nominally set to just lower than the peak ramp voltage. The output of the pulse generator 2 defines an intermediate control signal which comprises a series of short duration pulses of period equal to the period of the sawtooth waveform. This is shown in figure 2 (b). It will also be appreciated that the output of the pulse generation means is fed back into the ramp generation means.

The intermediate control signal produced within the pulse generation means is used to trigger a simple bistable circuit 3. This produces a square wave signal having equal periods at a low voltage and a higher

voltage. The bistable output defines a periodic control signal which is shown in figure 2 (c).

The periodic control signal (as described herein before) operates a switching means 4. When the bistable output is"low"the switch is in a first state. When the bistable is"high"the switch is in a second state.

The circuit is adapted for use in monitoring the operation of a brake demand sensor assembly. The sensor assembly 5 is shown in figure 3 and comprises two potentiometers. Each potentiometer is connected between a positive supply and ground, nominally five volts and zero volts. A demand signal is taken from each potentiometer from a sliding wiper. These demand signals are denoted OP1 and OP2 respectively. A further feature apparent in Figure 4 is that the output of the two sensors are clipped at 10% and 90% displacement. The operating range of the sensors should therefore be restricted to within the unclipped region.

The two demand signals are adapted to vary with movement of the brake pedal by providing an interconnection between the brake pedal and the potentiometer wipers. Importantly, the potentiometers are arranged in such a manner that as the output OP1 from one potentiometer increases, the other decreases as shown in figure 4. Another important feature is that the sum of the two outputs OP1 and OP2 remains constant. In this case, the sum is always 5 volts. This is represented by the dotted line in figure 4. By ensuring that the two outputs vary from zero volts to the supply voltage linearly with pedal travel, the output values sum to a constant value. This provides a means whereby many types of sensor failure can be detected by monitoring the sum signal.

The two demand signals OP1 and OP2 are connected to the input of an adder circuit 6 to produce a first intermediate signal equal to their sum

multiplied by a fractional scaling factor A such as 0.9. Thus, a sum value of 5 volts from the outputs of the two sensors leaves the adder as 5 x 0.9 = 4.5 volts. This summed and scaled signal is connected to one side of the switching means 4.

The other side of the switching means 4 is connected directly to one of the demand signals OP1 or OP2 which is multiplied by a fractional amount B. In this case a value of B of 0.5 is chosen so the single demand signal is multiplied by 0.5x, i. e. to half its value. This defines a second intermediate signal.

As the bistable switches states, the switching means 4 alternately connects the first and second intermediate signals to one input of a comparator 7.

The other input of the comparator is permanently connected to the ramp signal.

As the switching means 4 periodically switches from the first state to the second state, the output of the comparator 7 comprises a pulse width modulated signal having two portions which alternate periodically of the switching rate. This is shown in figure 2 (e) and its origin is shown in figure 2 (d).

A first portion (from To to T1) comprises a signal having a mark-space ratio dependent upon the value of the first intermediate signal. For the example with a 4.5 volt modified sum signal, the output of the comparator remains high until the value of the ramp signal exceeds 4.5 volts. It then remains low until the bistable switches to connect the second intermediate signal to the comparator.

With the second intermediate signal connected to the comparator, the modulated output signal goes high until the ramp value exceeds the second intermediate value, whereafter it goes low until the bistable switches again. This repeats periodically. This defines the second portion from T, to T. The maximum value possible for the second intermediate value is 0.5 x 5 volts = 2.5 volts. Thus, the maximum mark-space ratio for the second portion is always lower than that for the first portion when the sensors are functioning correctly.

As will be appreciated, the modulated output signal shown in figure 2e comprises information about the sum of the demand signals and the pedal position which can be passed down a single line. Provided that the expected mark/space ratio for the first portion (from To to T1) of the modulated output signal remains within a predetermined allowable range, the sensors can be presumed to be working correctly.

The system described is ratiometric in so far as changes in the ramp frequency will not affect the mark-space ratio and so will not produce erroneous or unreliable outputs.

Also, the use of the purely analogue circuitry provides advantages over digitally encoded systems.

The skilled man will appreciate that in a refinement, the bistable could be replaced by a divide-by-three counter and the switching means may be a three way device. This would allow an encoded sum signal and two encoded measurement signals to be encoded on the same line. This could of course be extended to three, four, five or more signals. Not all the measurement signals need to be demand signals. For example, yaw rate signals from yaw rate sensor could be encoded. An example circuit in

which four signals are encoded is shown in Figure 5. As will be appreciated, the presence of the sum signal defining the sync pulse allows easy discrimination of one sensor signal from another within the output signal. Two additional sensors are shown in the figure.

By ensuring that the mark-space ratio for the sum signal lies outside of the allowable range of mark-space values for the other encoded signals on the line, synchronisation of the signals can be achieved without any need to access a clock signal from within the circuit as the sum signal portion can be instantly identified.

Another important feature of the circuit is the provision of a current limited supply to the sensors. This is achieved by providing a current limiting circuit 8. As shown, supply transistors Q2 and Q4 are connected in a long tail pair to produce the supply voltage for the sensors. R4 acts as a current limited resistor. Since the reference voltage is taken from Q4 and not Q2, it will remain at 5 volts in current limit. This is arranged by providing for the supply to the potentiometer to be fed from a current mirror.

By limiting the supply current, it is possible to detect a fault in which the two wipers of the potentiometers fuse together. In this case, the apparent sum value from a non-current limited source would appear to be within an allowable limit. However, if current is limited, the sum signal will reduce as the sensor attempts to draw more current. This can be readily detected.

To detect partial shorts, the current limit should be set as close as possible to the normal working current drawn by the sensors. Protection for the use of a current limited supply to feed a pair of sensors as

described hereinbefore may be sought. This feature provides many advantages when sensors housed in a common body are provided.

The skilled man will also readily understand that whilst the invention described relates to a pulse width modulated output signal in which the mark-space ratio increases with the sum signal and the single output signal, it could equally be made to decrease. Each portion of the modulated output may comprise a low value followed by a high value. In this case, the alteration can be readily achieved by connecting the inputs to the camparator the other way round to that shown in the figure, with the ramp connected to the negative input terminal. In such a case, the value of B should exceed that of A.

The faults which can be detected by monitoring changes in the sum signal are as follows: A high resistance track ground connection would add to both demand signals so their sum would increase. This would be detected quickly before the demand value was significantly affected.

A short from track ground to pedal frame would create an earth loop causing very little effect. This could possibly cause the signals to enter their error bands.

A high resistance connection to a wiper. This would affect the sum value and be detected very quickly.

A partial short from a wiper to track ground or frame ground. This would distort the demand signal and possibly cause it to enter the error

band. When the pedal is depressed the additional current draw would reduce the sum signal, thereby detecting the fault.

A partial short between the wipers. This would distort both demand signals in opposite directions so the sum signal would remain in range.

The short was serious enough it would cause the potentiometer supply to current limit, reducing the sum signal and, therefore, generate an error. To detect a partial short would require the current limit range to be set close to the working current of the potentiometers.

Partial shorts from wiper to potentiometer track supply are similar to ground shorts.

A high resistance between any potentiometer track end and its power supply or ground connection would cause that demand signal to move into the demand range. This would also effect the sum signal, and the error to be detected.

A high resistance between the supply and both potentiometers would affect both demand signals again resulting in a sum error.

A partial short from the potentiometer track supply to ground or frame would cause a current limit condition thereby reducing the sum signal.

Claims

CLAIMS 1. A method of encoding two or more input signals on a single line comprising processing the two or more input signals to produce a pulse width modulated output signal comprising at least two periodically repeated pulse width modulated portions encoding the first and second signal respectively, and in which the mark-space ratio of the first portion of the pulse width modulated output signal is dependent upon the value of the sum of the first and second input signals and the mark-space ratio of the second portion of the pulse width modulated signal is dependent upon the value of the first input signal and in which during normal operation the mark-space ratio of the first portion lies outside an allowable range of mark- space ratios for the second portion.
2. The method of claim 1 in which the sum of the first and second input signals is constant.
3. The method of claim 1 or claim 2 in which a third, or fourth or even fifth additional input signals are encoded in the output signal.
4. The method of claim 3 in which the additional signals are independent of the first and second signals.
5. The method of claim 3 or claim 4 in which the additional signals are encoded as a periodic pulse width modulated portion of the output signal in a similar manner to the second portion of the output signal.
6. The method of any preceding claim in which the input signals are demand signals from a sensor.

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7. The method of any preceding claim in which the input signals are analogue signals.
8. The method of any one of claims 1 to 6 in which the input signals are digital signals.
9. The method of any preceding claim in which the sensors comprise part of a brake-by-wire electro-hydraulics braking system.
10. A method of encoding two or more input signals on a single line as substantially described herein with reference to the accompanying drawings.