GB988964A - Magnetic field detector - Google Patents

Abstract

988,964. Magnetic tests. THOMPSON RAMO WOOLDRIDGE Inc. March 15, 1963, No. 10430/63. Heading G1N. A magnetometer of the flux gate type includes a detecting magnetometer 10 and a biased reference magnetometer 11 which provides an output of constant phase. The output signals from the transducers are amplified in band-pass filters 30 and 31 and ring demodulator 38 causes a current to flow through meters 49 and 50 of magnitude and direction corresponding to the amplitude and direction of the external field. The band-pass filters 30 and 31 are tuned to twice the driving frequency to and the impedance of the demodulator, which may employ vacuum or semiconductor diodes, is sufficiently high to prevent loss of "Q" in the filters. Fig. 3 illustrates a gradiometer employing two detectors 56, 57, screened to give directional qualities, in which the outputs are compared with a reference phase signal and passed through low-pass filters, to remove the effects of high frequency changes, to cathode followers and a subtractor. To make the gradiometer sensitive to the rates of change rather than the magnitude of the external field filters 74 and 76 and filters 73 and 75, which act as band-stop filters supply negative feedback to their associated transducer but band-pass filters 77 and 78 supply positive feed back to the opposite transducer. The circuit of Fig. 5 has a power supply of variable amplitude so that different even harmonies can be excited in the transducers. The outputs are passed through turnable band-pass filters to a phase detector. Part of the output is returned as negative feedback to the output of the sensing transducer through a cathode follower and tunable band stop filter thus producing a circuit sensitive only to rates-of-change in a particular frequency band. By varying the power amplitude and tuning the filters a full spectrum may be obtained. By adding further detectors the apparatus may be adapted to indicate the strength of the field in a plane or in space. The bias field may be produced by a permanent magnet instead of the direct current arrangement of Fig. 1. The maximum field to be detected should produce two thirds saturation in the element and the flux may be adjusted by means of ferrite or "Mumetal" (Registered Trade Mark) ears. The saturable element itself may be Mumetal of Permalloy. The detected. signal passed to the demodulator of Fig. 1 should always be less than half the reference signal. The apparatus may be used in an aircraft for submarine searching.