Patent GB-1435489-A (GB)
Title: Automatic vehicle guidance systems
IPC: B62D1/28
Filed: 1972-08-02
Abstract: 1435489 Automatic control of vehicles E M I Ltd 23 July 1973 [2 Aug 1972] 36070/72 Heading G3N [Also in Division H4] An automatic vehicle guidance system includes at least two leader gear cables LC1, LC2 energized at different frequencies F1, F2 to enable the vehicles to follow different paths as determined by programme means either carried by the vehicles or located at a remote point. Each vehicle carries two pairs of pick-up coils 1, 2 and 3, 4. When the vehicle is centred on cables LC1 and LC2, the signals picked up by coils 1 and 2 are equal as are those picked up by coils 3 and 4. Coils 1 and 2 are connected in series and produce a sum signal of substantially constant amplitude and phase despite deviations of the vehicle providing these are not too great. Coils 3 and 4 are connected in opposition and produce a difference signal whose amplitude is proportional to the deviation and which is in phase with the sum signal for deviation to one side of the cables and is in antiphase for deviation to the other side. Alternatively, the sum and difference signals may be derived from two  coils. The sum signal is amplified at 6 and supplied to a phase detector 7 where it is compared with the output of an oscillator 9, any error signal being applied via filter 8 to control the oscillator frequency. When the vehicle is approaching a diversion point of the cables LC1, LC2, as indicated by a marker, e.g. a permanent magnet co-operating with a pick-up coil on the vehicle, the oscillator 9 is set by switch 17, controlled by the programme means, and circuit 11 or 12 to produce the appropriate frequency F1 or F2 according to the route to be followed. The phase-locked output of the oscillator is phase-shifted by 90 degrees at 16 to produce a signal in phase with the required component F1 or F2 of the sum signal and supplied together with the difference signal to a phase detector 13. The output of phase detector 13 via filter 14 is a signal having an amplitude and polarity which is used in known manner, e.g. as disclosed in Specification 1097141, to control the steering of the vehicle. When the vehicle is controlled from a central station a communication link may be established by short duration interruptions of the leader cable current, e.g. the current may be interrupted for 1 ms. and during each interruption or time slot one of four tones f 1 , f 2 , f 3 , f 4  may be transmitted, each vehicle being provided with its own time slot in a sequence. The link may be used to transmit positional &c. information and acknowledgment of received signals to the central station and routing or other information from the central station to the vehicles. A detector 19 detects the presence of a gap in the sum signal and blocks a normally open gate 18 to prevent spurious signals from affecting the oscillator 9 during the gaps. The output of detector 19 may also be used to open a gate 20 to apply the coded frequencies via filters f 1 , f 2 , f 3 , f 4  to a control unit 21 where they are decoded and stored. An output from unit 21 controls the switch 17 in accordance with the received route instructions.