Patent GB-908107-A (GB)
Title: Electrical measuring and control system
IPC: G05B11/01
Filed: 1959-11-17
Abstract: 908,107. Balanced - inductance systems. LEEDS & NORTHRUP CO. Nov. 17, 1959 [Nov. 28, 1958], No. 39011/59. Class 40 (1) [Also in Groups XXXV and XXXVI] In a control or measuring system, a variable alters the flux in a magnetic core and hence the inductance of a winding thereon, an oscillator whose tuned circuit includes this inductance generates a signal in accordance with the value of the inductance, and a circuit responsive to this signal passes D.C. through a further winding on the core to balance out the change in flux caused by the original variable. In Fig. 1, the variable is represented as the displacement of a plunger 14, which may be connected e.g. to a Bourdon tube, bellows, diaphragm or bimetallic element. The plunger comprises a permanent magnet 14a flanked by two soft iron polepieces 14b (Figs. 5 and 6, not shown), and moves in a gap in a core 15 to vary the flux therein. The plunger has a permeability near to that of air, to minimize changes in reluctance of the flux path as it moves, and stops 17 limit the movement to the range where the flux changes linearly with position. A compensating disc at one end prevents the flux changing with temperature. The magnetic circuit is completed by a ferrite ring 16 around which is a winding 18 whose inductance thus changes with movement of the plunger. This inductance, together with a capacitor 21, forms a timed collector circuit for a crystal-controlled transistor oscillator 23, and is tuned to a frequency slightly higher than the crystal frequency. Movement of the plunger thus varies the magnitude of the collector current, and this flows through a further transistor 28 and part only of the winding 18 so that its variation has a negligible back effect on the inductance. The transistor 28 is in a bridge network with resistors 41, 42 and 43, the bridge output being applied to a winding 22 on the core 15 in a sense to vary the flux in opposition to changes caused by movement of the plunger. Temperature variations are rendered ineffective by a thermistor 28t across the transistor 28. The output signal is taken from terminals T1 and T2. In a modification (Fig. 4, not shown), which is intended mainly for measuring voltage (see Group XXXVI), the transistor 20 is replaced by a pentode, which provides sufficient output for the amplifier 28 to be omitted. The input signal energizes a recording motor which also moves the plunger 14, and the output signal is applied in opposition to the input to bring the motor to rest. The plunger may be rotary (Fig. 7) comprising a cam 14<SP>1</SP> on a shaft 80 driven by the variable to be measured, and providing a linear relationship between change of flux in the core 15 and angular rotation of the shaft. A further adjusting magnet 82 is provided for zero setting (also in Fig. 8, not shown), and this may also be incorporated in the device of Fig. 1. Alternatively, a biasing current may be injected into the winding 22 or an auxiliary winding. The rotor 14<SP>1</SP> may be shaped to produce a non-linear flux variation if desired. The output current can be used for control purposes, and a rate signal can be added.