GB2104686A - Signal detector for a wire guidance system for vehicles - Google Patents

Abstract

The signal detector comprises a pair of spaced apart pick-ups which are intended to be positioned in use such that a line drawn through the pick-up is transverse to the wire track, and a signal processing circuit for producing output signals representative of the intensity of signals detected by the pick-ups and a steering control signal representative of the difference between the amplitudes of the output signals, whereby a vehicle can be steered so as to minimise the difference between the amplitudes of the output signals. The gain of the signal processing circuit is variable so that the effective aperture of the signal detector can be adjusted. The gain may be adjusted by signals transmitted over the wire track, or automatically in response to the intensity of the detected signals or by the rate of change of the steering control signal.

Description

SPECIFICATION Signal detector for a wire guidance system for vehicles The present invention relates to a signal detector for a wire guidance system for vehicles.

It is known to guide a vehicle along a wire track by transmitting a guidance signal of predetermined amplitude over the unscreened wire, detecting signals radiated by the wire track, and steering the vehicle in dependence upon the amplitude of the detected signals. It is also known to control the vehicle to for example start or stop by transmitting a control signal over the same or a separate wire, detecting the control signal radiated by the wire, and controlling the vehicle accordingly.

The vehicle mounted guidance signal detector is generally in the form of two spaced apart identical pick-ups such as coils mounted transversely with respect to the intended direction of travel of the vehicle. A transverse position signal is obtained by subtracting the output of one coil from that of the other so that the magnitude of the resulting difference signal represents the distance between the wire and a reference point half way between the two coils.

If a high positioned accuracy is required, for example less than 1 cm deviation from the wire, the effective aperture of the detector must be small. If the aperture is small, a relatively small deviation from the wire causes a large change in the difference signal which makes it easier to rapidly steer the vehicle back to a position in which the deviation is reduced to zero. If however the deviation is large enough to cause the system to become non-linear, the system is no longer able to determine its distance from the wire, and therefore cannot smoothly steer the vehicle back to the wire. The vehicle can be caused to oscillate back and forth across the wire, a condition known as "hunting". In extreme cases the system can produce entirely erroneous data due to the wire only being effectively detected by one coil. This is obviously very dangerous.

It is an object of the present invention to provide an improved signal detector for a wire guidance system for vehicles.

According to the present invention, there is provided a signal detector for mounting on a vehicle adapted to be guided along a wire track over which a guidance signal is transmitted, the signal detector comprising a pair of spaced apart pick-ups which are intended to be positioned in use such that a line drawn through the pick-up is transverse to the wire track, and a signal processing circuit for producing output signals representative of the intensity of signals detected by the pick-ups and a steering control signal representative of the difference between the amplitudes of the output signals, whereby a vehicle can be guided along the wire track by steering the vehicle so as to minimise the difference between the amplitudes of the output signals, characterised in that the gain of the signal processing circuit is variable so that the effective aperture of the signal detector can be adjusted.

Preferably means are provided for automatically adjusting the gain in dependence upon the intensity of the signals detected by the pick-ups.

The means for adjusting the gain may comprise means for detecting saturation of the steering control signal and means for reducing the gain in response tithe detection of saturation.

Alternatively, the means for adjusting the gain may comprise means for monitoring the rate of change of the steering control signal and means for reducing the gain in response to detection of a rate of change above a predetermined limit.

Means may be provided for receiving control signals transmitted over the wire track, and for responding to the received control signals to adjust the gain.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. lisa graphical representation of the variation of the difference between coil outputs with displacement; Fig. 2 is a schematic diagram of a vehicle guidance system embodying the invention; Fig. 3 is a circuit diagram of a coil amplifier board of the embodiment of Fig. 2; Fig. 4 shows in detail the analogue to digital converter of Fig. 2; Fig. 5 is a detail diagram of the steering logic board of Fig. 2; and Fig. 6 is a detailed diagram of the steering circuit of Fig. 2.

Referring to Fig. 1 , the full line represents the difference signal developed by the coils at minimum aperture (maximum gain). The aperture is represented by the spacing between lines 101 and 1 02. Providing the deviation of the vehicle supporting the coils from the wire track is less than half this aperture, the difference signal is linear and the system can respond rapidly and smoothly to reduce the deviation. If the deviation is so large that the difference signal reaches saturation, i.e. between lines 101 and 103 or between lines 102 and 104, the system merely knows the direction in which the vehicle has strayed, but not the distance. The vehicle then steers back towards the wire with maximum lock.

This can cause the vehicle to hunt the wire, oscillating backwards and forwards as the vehicle steering fails to respond quickly enough once the wire comes back into the aperture. In extreme cases the deviation is so large that the difference signal decreases as the coils get progressively farther away, as represented by the area beyond 103, 104. The system then assumes that the wire is within the aperture and steers accordingly. The vehicle is thus effectively lost.

The present invention proposes the adjustment of the aperture to increase the linear region of the 'difference versus deviation' curve by varying the gain of the system. Thus for example if the difference signal reached saturation (between lines 101 and 103) the gain could automatically be reduced to open up the aperture to the extent represented by dashed line 105, the aperture then being increased to the distance between lines 106 and 107. The system would then be operating in a linear region again and could steer smoothly back to the wire.

As an alternative the aperture could be controlled in response to the rate of change of the difference signal. Thus if a vehicle was travelling rapidly across the wire the aperture could be opened up to give the system a better opportunity to bring the vehicle smoothly back to the wire.

Referring now to Fig. 2, the basic layout of a system according to the invention will be described. Four coils are provided, coils 108, 109 being spaced apart transversely at the front of the vehicle, coils 110, 111 being spaced apart transversely at the rear of the vehicle. The four coils may be mounted on a common support providing the appropriate transverse and longitudinal spacing is provided.

The coils 108, 109 will be used when going forward, the coils 110, 111 when reversing, although all four coils may be used to give an indication of the attitude relative to the wire of the entire vehicle.

The analogue outputs of the coils are applied via a pre-amp and filter stage 112 and a multichannel analogue-to-digital converter 11 3 to a microprocessor 114. The guidance signal picked up from the wire track by the coils is a sine wave which is sampled and then a peak recovery operation is carried out which gives the maximum and minimum values. The ADC 113 accepts a O to 5 volt range so that the detected sine wave is centred on 2.5 volts. Thus if the sine wave is not at full strength the minimum will not be at zero.

The sampled maximum and minimum values give the amplitude of the signal. The ADC thus delivers signals representing the amplitude of the coil outputs and their difference is calculated in software.

The guidance signal on the wire track is FM modulated using FSK techniques. The outputs of one pair of coils are added to maintain a strong signal regardless of wire position (assuming the vehicle does not get lost) and applied to an FM detector 1 5 the output of which is fed to the ADC 11 3. This makes it possible for a central controller to communicate with the vehicle. Each vehicle on the wire may be separately addressed by appropriately coded signals. Thus full control of a number of vehicles on the wire track may be achieved.

The difference signal derived from the coil outputs is compared with a reference, the reference normally being zero. If however one wished to cause a vehicle to move parallel to but to one side of the guidance wire, (e.g. to pass an obstacle) this could be achieved by opening up the aperture and changing the reference. In this respect the reference and aperture could be controlled over the FM link from the central controller.

The microprocessor 114 provides a two bit output to a 2 to 4 converter 116. This supplies gain control signals to the pre-amps 112 as described below. The microprocessor controls the gain either in dependence upon signals from the central control over the FM link, or in dependence upon the outputs of the coils reaching saturation, or in dependence upon the rate of change of the coil outputs, or in dependence upon any combination of these factors. The generator of the software necessary to enable the microprocessor to perform the functions described will be within the capabilities of persons skilled in the art.

A VHF transmitter unit 11 7 transmits back to the central control data indicating current receipt of instructions from the central control and the status and position of the vehicle. Thus complete two-way communication is achieved to enable the central control both to control and monitor the operations of the vehicle.

To achieve correct steering of the vehicle, the microprocessor 114 produces an error word representative of the deviation of the vehicle from the desired path. This is converted to a word suitable for application to a logic board 118 to give an appropriate duty cycle to a steering circuit 11 9. The steering circuit controls the steering motor 120 which drives the steering wheels 121.

A potentiometer 1 22 provides a signal indicative of steering wheel angle to the ADC 113, enabling the microprocessor 114 to monitor wheel angle directly to ensure that the steering circuit operates correctly.

The microprocessor 114 comprises a number of dedicated devices performing respective functions. For example one device will control steering, one device will control the vehicle traction motor, one the transverse position relative to the wire etc.

An emergency return sequence can be provided to cope with loss of the word. The vehicle may be disabled if the wire is lost for a predetermined period. Tactile sensors will also be provided to prevent the vehicle pushing any obstacle over.

Referring now to Fig. 3, the coils 108 and 109 are shown. Coils 110 and 111 (Fig. 2 only) are connected to circuits identical to those shown in Fig. 3 connected to coils 108 and 109.

The outputs of the coils are applied to amplifiers 123 the gains of which are controilable by switches 124 in series with feedback resistors 125. The switches 1 24 are controlled by inputs 1 26 supplied by the converter 11 6 (Fig. 2). The gain control achieved by switchings 124 controls the effective aperture of the detector comprising coils 108 and 109.

The outputs of the amplifiers 123 are passed through high pass filters 127 and low pass filters 1 28. These filters provide a band pass response between 2 and 6 kHz. This prevents noise generated by the pulse control of the steering motor (typically at 700 kHz) from disrupting operation. Filtering is also necessary for antialiasing before conversion to digital signals.

The outputs of the filters are applied via buffer amplifiers 129 to terminals 1 30 which are connected to the ADO 11 3 (Fig. 2).

The ADO 113 is illustrated in more detail in Fig.

4. It comprises an 0816 ADO 131 connected to the microprocessor via a PL4 socket 132. Sixteen inputs are available, four which are used for the four coil output signals. The ADO 131 is fed by a clock 1 33 which is counted down from the master clock input provided by a crystal oscillator (not shown). The A ports control logic operations, ports A0 to A4 being the address input allowing sixteen codes which pick which input is being used. Ports A4 to A6 respectively control start of conversion, end of conversion, and tri-state control lines. Data is read via ports BO to B7. A digital to analogue converter 134 and latch 135 provide an analogue output at terminal 136 if such an output is required.

One logic word is sent to the steering logic board 11 8 (Fig. 2) from the microprocessor. The logic board is shown in detail in Fig. 5. The signals at the A7 and B0 to B7 ports of the ADO 131 (Fig.

4) are fed to the logic board. A7 and B1 to B6 are applied to a latch 137 the output of which is compared by comparators 138, 139 with the output of a divider 140. B0 enables a flip-flop 141 and B7 determines the direction (left or right) in which the steering is to be adjusted. B7 is inverted by inverter 142 and inverted and noninverted B7 control a pair of AND gates 143.

The output from the divider 140 to the flip flop 141 is reset every millisecond. This output resets the divider and the flip-flop. The comparators 138,139 compare the input data word with the divider output and when a true comparison is reached a high signal is applied to line 144, setting the flip-flop 141. The output of the flipflop is thus high until the next clock period begins when it goes low. When and only when the output of the flip-flop is high either output 145 or output 146 of the AND gate pair 143 will be high in dependence upon input B7. Thus at any one time either output 145 is high, instructing steering to the right, or output 146 is high, instructing steering to the left, or both outputs 145, 146 are low. The signals appearing on the outputs 145, 146 are thus in the form of pulse trains.

Referring now to Fig. 6, the outputs 145, 146 are shown. These signals amplified by amplification chain 147, control SCR's 148, 149 and FET's 150, 151 to drive a steering motor having an armature 152 and field winding 1 53.

The motor is switched on in the appropriate direction for as long as a signal is present at one of the inputs 145, 146. Thus the motor sets the wheel angle. The wheel angle is directly sensed to prevent it exceeding a predetermined angle as described with reference to Fig. 1.

Claims (7)

Claims

1. A signal detector for mounting on a vehicle adapted to be guided along a wire track over which a guidance signal is transmitted, the signal detector comprising a pair of spaced apart pickups which are intended to be positioned in use such that a line drawn through the pick-up is transverse to the wire track, and a signal processing circuit for producing output signals representative of the intensity of signals detected by the pick-ups and a steering control signal representative of the difference between the amplitudes of the output signals, whereby a vehicle can be guided along the wire track by steering the vehicle so as to minimise the difference between the amplitudes of the output signals, characterised in that the gain of the signal processing circuit is variable so that the effective aperture of the signal detector can be adjusted.

2. A signal detector according to claim 1, comprising means for automatically adjusting the gain in dependence upon the intensity of the signals detected by the pick-ups.

3. A signal detector according to claim 2, wherein the means for adjusting the gain comprise means for detecting saturation of the steering control signal and means for reducing the gain in response to the detection of saturation.

4. A signal detector according to claim 2, wherein the means for adjusting the gain comprise means for monitoring the rate of change of the steering control signal and means for reducing the gain in response to detection of a rate of change above a predetermined limit.

5. A signal detector according to any preceding claim, comprising means for receiving control signals transmitted over the wire track, and means responsive to the received control signals for adjusting the gain.

6. A signal detector according to any preceding claim, wherein each coil is connected to a preamplifier comprising a switchable feedback path, the feedback path being switched to vary the gain.

7. A signal detector substantially as hereinbefore described with reference to the accompanying drawings.