Patent GB-702307-A (GB)
Title: Improvements in or relating to electric control systems
IPC: G08C19/16
Filed: 1952-01-29
Abstract: 702,307. Potentiometric measurements ; electrical instruments ; variable contact wire resistances. WESTERN ELECTRIC CO., Inc. Jan. 29, 1952 [Jan. 31, 1951], No. 2389/52.' Class 37 [ Also in Group XXXVIII] Apparatus responsive to a remote signal voltage comprising a moving coil movement freely pivoted in the airgap of a magnetic field and energized from the signal source over oppositely-wound coiled wire conductors exerting negligible opposing torque thereon, rotates a shaft operating the moving element of a potentiometer across a voltage source, whose output is opposedly fed back to the input of a summation amplifier energizing the moving coil to balance out the input signal voltage, so that the coil and potentiometer slider are deflected in correspondence with the input magnitude.  The movement 1 comprises, Figs. 2, 3, 5, two horseshoe magnets having polepieces 10, 11 provided with concentric pole faces 12, 13 defining an arcuate airgap 14. Pole face 13, machine screwed to polepiece 10, is centrally bored to accommodate a pivoted meter shaft 17 which carries a rectangular moving coil 18 rotatable through 180 degrees in the airgap, receiving input signals over soft thin wire' connections 21, 22 wound oppositely in loose coils adjustable by elements 19 to exert substantially zero torque on the shaft. The free end of arm 23 is engaged by the bifurcated end of balanced arm 24 on shaft 25, rotating the brush 28 over the winding 27 of potentiometer 5 which is energized by a voltage source centre-tapped to earth. The moving coil is energized, Fig. 4 (not shown), by the output of a summation amplifier receiving a signal voltage, and the voltage is derived from the potentiometer;slider over separate series resistances so that at an angular deflection dependent on the input signal the voltages cancel. Shaft 25 may operate additional potentiometers. In a modification, Figs. 8, 9, 10, the movement comprises a horseshoe magnet 15, has polepieces 46, 47. defining an arcuate gap wherein a counterbalanced rectangular moving coil is rotatable about one leg thereof attached to shaft 52 which passes through the central bore of a pole face screwed to the polepiece 47, and is supported in adjustable pivot bearings 55, 56. The coil is electrically connected to terminals through oppositely wound 'spiral filaments 61, 62 exerting substantially zero torque to the shaft, which operates plural potentiometer units 42, 43, Each of these comprise two separate potentiometers 67, 68, wound toroidally on a cylindrical member 66 which is mounted concentrically with the shaft in an  annular insulator 70 clamped by screws 73 to 76 into the accurately fitting recess 71 of unit 42 (or 43). Electrical connections to the ends of each potentiometer winding 67, 68 are made over terminals, e.g. 80, 81 clamping adjustable brushes 82, 83 which are movable through a few degrees to accurately align the electrical limits of the potentiometers with the mechanical limits of brushes 85, 88, mounted on 180 degrees spaced arms 86, 89 fixed to shaft 52 and adjustable for pressure on the windings. The brushes are connected to terminals through oppositelywound spiral filaments 92, 93 exerting zero torque. Further potentiometer units may be fitted to the shaft. The remote signal is applied through resistor 104 to the grid of triode 100 of a summation amplifier, Fig. 11, cathode coupled to a degenerative feedback triode 101, whose anode is energized through resistor 116 from variable potentiometer 117. The amplified anode signal is directly coupled to the grid of triode 102 of a balanced pair 102, 103, the grid of the first valve being adjustably biased from potentiometer 117 and that of the second from potentiometer 126. The anodes are energized through resistances 121, 123 and interconnected by the moving coil of a movement, such as described above, the first potentiometer resistance 130 of which derives a variable voltage from the centretapped earthed supply 124 proportional to the deflection, which is fed back in opposition through resistance 139 to balance out the signal voltage on the grid of triode 100. The deflection of the movement will vary until balance is obtained, when the position of the several potentiometer brushes will correspond to the signal voltage magnitude. The remaining potentiometers may be connected in other circuits, e.g. computers.