Patent GB-1289474-A (GB)
IPC: G05D1/02
Filed: 1968-10-16
Abstract: 1289474 Leader gear systems INTERNATIONAL HARVESTER CO OF GREAT BRITAIN 6 Oct 1969 [16 Oct 1968 4 Dec 1968] 49140/68 and 57498/68 Heading H4U In a vehicle guidance system, pick-up coils 1, 2, 3, Fig. 1, tuned by capacitors 4, 5; 6 respectively, co-operate with an energized leader cable and means are provided whereby the vehicle may be offset from the cable by differing amounts, e.g. for agricultural purposes. Coils 4 and 5 are connected in phase opposition to an amplifier 7 which provides a signal representing the difference in field strength at opposite sides of the vehicle, this signal being supplied to a differential amplifier 9. The output of coil 3, representing the field strength at the centre of the vehicle, is amplified at 8 and supplied via an adjustable attenuator 10 as the second input to the amplifier 9 and is also supplied directly from the amplifier 8 as a reference frequency to a phase sensitive detector 11. By adjustment of the attenuator 10 the signal from coil 3 may be arranged to balance the difference signal from coils 1 and 2 for any selected offset path of the vehicle relative to the leader cable. When the vehicle is on a selected offset path any departure from this path will cause an input signal to be applied to the phase detector 11, the resulting output signal, representing the direction and extent of the deviation, being used to correct the steering. The coils 1 and 2 may have a common tuning capacitor. An attenuator may be included in the output of amplifier 7 instead of or in addition to that of amplifier 8. In a modification Fig. 3 (not shown), the differential amplifier 9 is replaced by a transformer, or alternatively, by an auto-transformer with an adjustable tap replacing also the attentuator 10, Fig. 4 (not shown). In a further arrangement, Fig. 6 (not shown), the coil 3 and its associated capacitor are dispensed with and the input to amplifier 8 is taken from the junction of coils 1, 2 or alternatively may be taken from a pair of series connected coils coupling with coils 1, 2 respectively, Fig. 7 (not shown). The leader cable is preferably laid in a series of rectangular loops, spaced by a similar area to that enclosed by a loop, Fig. 8 (not shown). The selected paths in these spaces will be a series of parallel lines which extend for the full length of the loop but within a loop the paths tend to curve off towards a corner of the loop where they approach each end of a loop. This may be avoided by using two separate cables 30, 40, Fig. 9, with the loops of the cable 40 arranged between those of cable 30, the two cables being energized at different frequencies. In this arrangement the vehicle traverses the area having a width denoted by line 50 under the control of cable 30 and then traverses the adjacent area of similar width shown as 51 under the control of cable 40 and so on, alternative tuning capacitors being provided for the coils 1, 2, 3 which are switched in accordance with a programme. The vehicle may be turned round at the end of each traverse by a signal from a trip wire. When the vehicle is traversing an area such as that having width 50, the phase of the difference signal from coils 1, 2 reverses at line 53 and a switch is operated automatically to correspondingly invert the signal from coil 3, Fig. 10 (not shown).