Patent GB-855991-A (GB)
Title: Improvements in electrical null transmission networks
Inventor: WALTON JOHN HUGH DAVEY
IPC: G01V3/10
Filed: 1957-10-18
Abstract: 855,991. Locating conductors; measuring bridges. MEC - TEST Ltd. Oct. 15, 1958 [Oct. 18, 1957], No. 32686/57. Class 37 Fig. 1 shows a circuit for detecting the presence of a conductive body between search coils 10, 14 in which the coils 10, 12 are series aiding and 14, 16 series opposing, coils 12, 16 being on a ferro-magnetic core. The circuit fed from a transistor oscillator 22 is normally near the balanced condition the small resultant voltage across coil 14, 16 is amplified in unit 24 and applied to phase-sensitive rectifiers 26, 28 being fed to the latter via a phase-shifting unit 30 which produces a 90 degrees phase shift. The output from rectifier 26 is fed to operate polarised relay 36 in one direction. A manually variable signal derived from network 40, 42, 44 which may operate the relay in either direction is set so that in the initial condition the relay is operated in a given direction. The output from phase-sensitive rectifier 28 is fed back to coil 20 associated with coil 10, 14, 12, 16 so that the circuit will be sensitive to a small change in mutual inductance between coils 10, 14 produced by the presence of a metal object to change the direction of operation of relay 36. The rectifier 28 responds to induced voltages 90 degrees out of phase with the primary current by means of resistor 46 which causes the voltage across the oscillator output to be substantially in phase with the primary current so that voltages produced by mechanical movement of the coils will be cancelled by the variation of the mutual inductance element and will not affect the operation of relay 36. In the modification shown in Fig. 3 both of the mutual inductance elements are duplicated. Balancing coils 48 and 50 are connected in series-aiding and series-opposition with coils 10 and 14 the mutual inductance of the balancing coils being manually variable so that the voltage induced in secondary winding 50 is equal and opposite to that induced in winding 14. Coils 52 and 54 series-aiding and secondary coils 56 and 58 in series-opposition constitute the windings 12 and 16 respectively as shown in Fig. 1. A separate polarising winding is provided for each magnetic path these being shown at 62 and 64. A large D.C. current from a source 66 divides equally between winding 62 and 64. If the output from rectifier 28 is zero then the two paths in core 60 are equally polarised and the resultant voltage in secondaries 56 and 58 is zero. An output from rectifier 28 of either polarity causes the currents in winding 62 and 64 to be unequal hence the polarisation of the two magnetic paths are unequal and the resultant signal in windings 56 and 58 is not zero thus allowing feedback action to take place as described in connection with Fig, 1. This balanced system possesses great immunity to temperature and supply voltage variations. The search coils 10 and 14 can consist of a single loop of coaxial cable the outer conductor forming the primary. The coils 14 and 50 of Fig. 3 may be wound upon opposite ends of a ferrite rod the windings 10 and 18 forming one primary wound round the centre of the rod. In another arrangement the windings 52, 54, 56 and 58 together with core 60 are omitted and the windings 62 and 64 are placed on the ferrite rod close to the ends the primary windings 10 and 48 being positioned close to their secondaries. The device may be used to count the number of vehicles on a road the pair of coils without the magnetic core being laid in the form of cable loops under the road surface. A number of loops could be provided at the end of an aerodrome runway in order to determine the position and height of incoming aircraft. The device may be used for monitoring flow of material on a conveyer for metallic impurities or for detecting the presence of a foreign metallic body for example in the human eye. Electric components (resistors, reactors or impedances) may be tested by replacing the two halves of potentiometer 40 (Figs. 1 and 3) by the component to be tested and a standard component respectively. The out-of-balance signal then indicates when the component under test lies within permitted limits as described in Specification 832,609.