1289476 Leader gear systems INTERNATIONAL HARVESTER CO OF GREAT BRITAIN 6 Oct 1969 [16 Oct 1968 4 Dec 1968] 49142/68 and 57499/68 Heading H4U In a vehicle guidance system employing a plurality of energized guide wires, the vehicle carries two pick-up coils responsive to lines of constant magnetic field strength, spaced apart by a distance having a component in the direction of advance of the vehicle and connected in phase opposition, the resultant output signal providing an indication of the relationship of the vehicle to the desired path or controlling the steering to guide the vehicle along the desired path. The coils may be arranged in a line parallel to the direction of advance of the vehicle or if the desired path is oblique to the lines of constant magnetic field strength the coils may be spaced apart in a line oblique to the direction of advance or may be parallel to the direction of advance but with the output of one of the coils passed through an attentuator prior to combination with the output of the other coil. The arrangement may be combined with the system disclosed in Specification 1,289,474, Fig. 3 (not shown), in which separate sets of coils are used for the two systems, or as in Figs. 5 and 6 in which coil 15 is common to both systems and co-operates with coil 16 for the present system and with coil 18 for the system of the above mentioned Specification, coil 19 providing a reference signal. Coils 15, 16 are tuned by capacitor 30 and trimming capacitor 31 via contact 101 and any difference signal is amplified at 32 and supplied via contact 131 and amplifier 33 to a phase sensitive detector 9 receiving the reference signal from coil 19 via amplifier 34 and contact 111. To operate in accordance with the above mentioned Specification, contacts 101, 102 are changed over, contact 131 is opened, contact 130 closed and a selected one of contacts 133 closed in accordance with the particular line the vehicle is to take. Coils 15, 18 are now tuned by capacitor 30 and trimmer 35 and the difference signal from these coils is supplied via amplifier 32 to one end of the potentiometer 36, the signal from coil 19 being supplied via amplifier 34 and contact 130 to the other end. The output of the potentiometer is supplied to the phase detector 9 which receives a reference signal from coil 19 as before. A contact 111 provides an inversion of the signal from coil 19 in accordance with the above mentioned Specification. The selection of the system to control the vehicle, e.g. a tractor used for agricultural purposes, may be effected under the control of a computer program carried on the vehicle or may be effected under the control of the magnetic field produced by a guide wire arranged as in Fig. 7 in which the vehicle traverses the area to be worked in a series of paths parallel to the arms 20, 21, 22 &c. When using the system of Specification 1,289,474, the vehicle tends to diverge as shown by lines K, Fig. 14, at the top and bottom of areas such as that bounded by wires 20, 21, Fig. 7, but under the control of the present system would diverge in the opposite direction as shown by lines H. An average of these two signals may be used to control the vehicle at these points so that it proceeds on a substantially straight course. As shown in Fig. 15, the output of the phase detector 9, Fig. 6, is supplied to track store amplifiers 201, 202 operated alternately by gating signals 203, 204 respectively, the amplifiers giving a continuous output corresponding to the last input signal applied. The vehicle program controls contacts 101, 102, 130, 131, so that the arrangement of Fig. 6 is switched continuously between the two operating modes at the frequency of the gating signals so that amplifiers 201, 202 receive signals resulting from the coil combinations 15, 18 and 15, 16 respectively. The outputs of amplifiers 201, 202 are supplied via contacts 141, 142 respectively and resistors 43, 44 to an operational amplifier 211 providing a signal representing the mean of the signals from the phase detector 9 which is used to control the steering of the vehicle. The error signals from amplifiers 201, 202 are also supplied to an operational amplifier 212, that from amplifier 201 being inverted at 221 so that the combined output from the amplifier is larger than either of the input signals. A level detector 52 is inoperative until the output of the amplifier reaches a predetermined level indicating that the vehicle is approaching the boundary where the deviations of the lines H, Fig. 14, become substantially greater than those of lines K. Detector 52 operates a relay (not shown) which opens contact 141 so that only the error signal from amplifier 202 is applied to amplifier 211 and the vehicle is guided along a line H to turn the corner. When the turn is completed the vehicle starts to return along a reverse track along a line H until it reaches a point where the lines H and K are convergent and the error signal at the output of amplifier 201 reduces until the relay is released and contact 141 closed. The vehicle is now controlled by the combined error signals as before. The output of amplifier 212 may be supplied to another level detector which operates a relay to open contact 142 when the amplifier output falls below a predetermined level so that the vehicle is now controlled by the signal from coil combination 15, 18 only. Alternatively, this switching may be effected by the program device.