Patent GB-776163-A (GB)
Title: Improvements in and relating to vacuum tube balancing networks for metal detection systems
IPC: G01V3/10
Filed: 1954-04-26
Abstract: 776,163. Bridge measurements; detecting metallic bodies. GENERAL ELECTRIC CO. April 26, 1954 [April 24, 1953], No. 12018/54. Class 37. In a metal detection system a first impedance bridge circuit comprises in two arms reactive components incorporated in the sensing means responsive to metal particles, a resistor connected across one diagonal of the bridge and an electron discharge tube shunting the whole or part of one of the non- responsive impedance arms; and a second bridge circuit comprises two adjacent impedance arms of the first bridge in conjunction with two additional impedance arms, one of which is shunted wholly or in part by an electron discharge tube, with a reactance connected across one diagonal of the circuit; there being provided means responsive to non- transitory unbalance signals arising from longterm changes in the circuit and not from the presence of metal particles which apply control signals over delay circuits to the grids of the electron discharge tubes in such manner as to vary their impedances to restore balance of the bridges. Fig. 2 shows a double bridge vacuum tube balancing network for use in metal detector systems which is automatically held balanced over an appreciable time interval; in which a first bridge comprises inductances 11, 12 forming part of a metal sensing coil assembly, together with a resistor 13, and an adjustable resistor 15 in series with a fixed resistor 14; and a second bridge comprises inductances 11, 12 together with a resistor 22 and an adjustable resistor 24 in series with a fixed resistor 23. From the junction of inductances 11, 12 a fixed resistor 16 and a fixed capacitance 25 respectively shunt the diagonals of the first and second bridges, while the inductances are coupled to excitation windings 45, 46 energized by a high-frequency source 47. The bridges are initially balanced by adjustable resistors 15, 24, and the signal across the unbalance diagonal (comprising components in phase and in quadrature with the bridge excitation according to the magnetic or non-magnetic nature of metallic particles contained in material traversed by the sensing coil assembly), is amplified and applied to the anodes of electron discharge devices contained in phase-sensitive detector 49; their control grids being respectively excited in phase and in quadrature with the bridge excitation through transformer 51 and phase-shift capacitance 52 so that the passage of metal particles is shown on unbalance indicator 53 comprising instruments sensitive to the D.C. output signals from the detector corresponding respectively to the in-phase and in quadrature imbalance signals. Non-transitory unbalance signal components of the phase detector outputs having a long persistence are coupled through long time constant circuits 54 (which exclude transitory signals due to the occurrence of metal particles) to bias the grids of triodes 17, 26 whose anode to cathode circuits shunt resistors 15, 24 and whose anodes are energized through resistors 21, 29 and meters 19, 28 from a D.C. source, so that long term changes of characteristics of the bridge circuit or of the electric or magnetic environment of the sensing coil assembly are compensated by bridge rebalance to an extent indicated by meters 19, 28. Initially the bridges are adjusted for balance with triodes 17, 26 passing currents midway along their working ranges, and to indicate unbalance conditions necessitating saturation or cut-off, there are provided gas discharge tubes 31, 32 connected through resistors 33, 36 between the anodes of triodes 17, 26 and a reference voltage source in series with resistor 39. On saturation of either triode, the appropriate gas tube ignites to transmit a negative pulse to the grid of triode 35 and release normally energized relay 43 in its anode to operate an alarm, while on cut-off of either triode its gas tube ignites to positively pulse the triode grid. The voltage drop across resistor 39 then biases the gas tubes to extinction, at which the voltage drop disappears and the tubes re-ignite and are extinguished successively, at a period established by capacitor 38, so that negative impulses are developed at the grid of triode 35 to release relay 43 and operate the alarm. In a modification (Fig. 3, not shown) the inductances 11, 12 are replaced by capacitors sensitive to the dielectric presence of metallic bodies, in series with inductances and capacitances, while the two bridges are diagonally interconnected by an inductance or capacitance.