Patent GB-575573-A (GB)
Title: Electrical indicating system
IPC: G01V3/10
Filed: 1943-12-24
Abstract: 575,573. Correspondence control. HAZELTINE CORPORATION. Dec. 24, 1943, No. 21684. Convention date, Dec. 31, 1942. [Class 40 (i)] [Also in Groups XXXVI and XL] An indicating system particularly for locating buried metal objects, comprises an amplifier having a positive phase slope over a predetermined frequency range, a single feed-back path which is non-selective over this frequency range and has quadrature-related components of coupling of which one is effective to cause sustained oscillation of the amplifier at a frequency within the predetermined range and the other is variable in response to conditions to be indicated and causes variations of the frequency of oscillation in accordance with variations of these conditions, and means responsive to the frequency for indicating variations of the component of coupling caused by variations of the conditions to be indicated. A stable amplifier 10, Fig. 1, has input terminals 11 and output terminals 12 and a positive phase slope over a predetermined frequency range, which may be an audible-frequency range. A feed-back path includes a coupling inductance 13 coupling the output terminals 12 to the input terminals 11 and substantially non-selective over the operating frequency range. The feed-back path has, by virtue of the coupling impedance 13, quadrature related components of coupling. The major component is provided by an auxiliary reference impedance 14 and the minor component by an exploring unit 15 which normally has substantially no coupling between its separate sections 16 and 17a, 17b but furnishes coupling in response to a condition to be indicated. The reference impedance 14 comprises a primary coil 18, connected in series with the coil 16 to the output terminals 12, and a secondary coil 19 connected in series with the coils 17a, 17b to the input terminals 11. An oscillator 20, having a constant frequency at the centre of the range of operation of the amplifier 10, is connected through a beatnote indicator 21 to the output terminals 12. The indicator 21 includes a non-linear detector for deriving a beat note from the oscillations of the amplifier 10 and those of the oscillator 20 and includes head-phones or a loud-speaker for audibly reproducing the beat note. Variations of the major component of coupling may be indicated by an indicator 22 connected to the output of the amplifier. A phase adjuster 23 enables the phase shift through the amplifier and therefore its frequency of oscillation to be adjusted. The phase adjuster 23 may be provided with a scale to indicate the change of phase angle to compensate for an object in the neighbourhood of the search coil 15. An A.V.C. rectifier 24 automatically adjusts the magnitude of the major component of coupling provided by the feed-back path inversely with amplitude variation to maintain constant the oscillation amplitude of the amplifier 10. For testing the operation of the system a single turn of wire 25 is arranged coaxially with the coils 16, 17a. 17b, and is in series with a resistor 26 and key 27. If the exploring unit 15 is brought near a buried metal object M the balance of the coupling between the coils 16, 17a, 17b is disturbed and a change of phase is produced in the oscillations transmitted through the feed-back path. The amplifier 10 consequently changes its oscillation frequency to such a value that the phase shift occurring in the amplifier itself just compensates for the phase shift produced by the resistance component of coupling occurring in the exploring unit 15. The frequency of oscillation of the amplifier then differs from that of the oscillator 20 and a beat note is produced. The phase adjuster 23 is adjusted to return the frequency of the amplifier to its original value, the amount of adjustment required being a measure of the distance of the conductive rod M from the exploring unit 15. Undesired resistance component of coupling in the unit 15 due to metal screws and structural members may be compensated for by an adjustable conductive member such as a thin-walled brass cylinder as described in Specification 575,574, [Group XXXV]. For measuring the magnetic properties of dry ground, detecting the presence of magnetic ore, or locating a magnetic object in salt water, the inductive coupling 14 is replaced by a variable resistance coupling. In a modification, Fig. 6, the major component of coupling is automatically adjusted inversely with the amplitude of oscillation of the amplifier. This is effected by a bias-controlled repeater 29 included in the feed-back path and means for controlling its repeating ratio inversely as the oscillation amplitude. The repeater 29 comprises a pair of valves 30, 31 having their input electrodes coupled in push-pull with a pair of resistors 32, 33 which are included in the output circuit of the amplifier 10. The output electrodes of the valves 30, 31 are coupled in push-pull through a transformer 34 to the input terminals 11 of the amplifier 10 to modify the major or reactance component of coupling provided by reference impedance 14 and which is effective to cause sustained oscillation of the amplifier. A.D.C. meter 35 is included in the anode and screen circuits of the valves 30, 31. A balanced rectifier 36 is coupled to the output terminals 12 of the amplifier 10 to provide a bias voltage proportional to the output of the amplifier for reducing the reactive component of coupling furnished by the repeater 29 as the amplifier output increases. This reduced feedback causes the amplifier 10 to oscillate less strongly thus reducing the amplitude of its oscillations. Specifications 574,941 and 574,955, [both in Group XXXVI], also are referred to.