GB944018A - Improvements in or relating to electronic apparatus for detecting a change in position of a conductive or magnetic body - Google Patents

Abstract

944,018. Locating conductive or magnetic bodies. R. G. F. ZINNASER. July 25,1960 [July 28,1959], No. 25824/60. Heading G1N. In apparatus for detecting the proximity of a conductive or magnetic body, the sensing head comprises an oscillatory circuit, fed from a pulse generator the frequency of which is automatically adjusted so that it remains in fixed relationship to the natural frequency of the oscillatory circuit. Compensation is thus arranged for slow changes in reactance of the sensing head due to deposition of moisture or metallic powder. Fig. 1 shows a block diagram suitable for use with the railway axle counting system described in Specification 845, 460 [Group XXX]. The sensing head A, located adjacent a railway track, is fed with pulses from a multivibrator A2 the frequency of which is maintained at 1. 1 times the natural frequency of the sensing head A. This results in the waveform shown in Fig. 3 being present across the sensing head A. The positive pulses are passed by a circuit A4 and integrated at A5 and A6 to provide a D.C. control voltage dependent on the pulse repetition frequency which is used to control the frequency of multivibrator A2. A further sensing head B, located adjacent the track near head A, is connected to a similar circuit. The outputs of integrators A4, B4 are compared by a circuit 14 which gives two outputs of small pulses unless the outputs from heads A, B become different due to the proximity of a train wheel to one head. The difference in the output pulses from circuit 14 is now amplified at 29 and operates a pulse routing gate 15 to direct pulses from a generator 20 to operate bi-stable circuits A1 or B1. Circuits 29, 15 are corrected against long term drift by feedback via integrators 21, 22. Figs. 7, 8 (not shown) relate to suitable transistor circuits not described in detail in the Specification.