Patent GB-871562-A (GB)
Title: Improvements in or relating to automatic electronic controllers
IPC: G05B11/42
Filed: 1956-03-16
Abstract: 871,562. Automatic temperature-control systems. STEGHART, F. L. F. Feb. 22, 1957 [March 16, 1956; Nov. 17, 1956], Nos. 8355/56 and 35200/56. Class 38 (4). In an electronic control system for automatically maintaining a physical property of a medium at a desired value, an electrical signal dependent in sign and magnitude on any error is applied to a D.C. amplifier stage operating a regulating unit feedback from which is applied across the entire amplifier stage through a resistance-capacity circuit to introduce an integral component of control. Fig. 1 shows an automatic temperature control system in which the supply of steam to a heat exchanger in an air duct leading to a space is controlled in dependence on the temperature of the space. A resistance 1 responsive to the space temperature is arranged in a bridge circuit which becomes unbalanced when the temperature deviates from a desired value to apply an A.C. output through a two-stage thermionic valve amplifier 9 to a phase-sensitive rectifier 23 which produces across resistances 26, 27 a direct voltage corresponding in magnitude and polarity to the magnitude and sense of temperature deviation. This voltage is applied through a D.C. amplifier 44 to operate a twoway differential relay 33 so that a reversible motor 38 runs in the appropriate direction to correct the temperature by adjustment of the steam valve (not shown) through gearing 39. A feedback is introduced by causing motor 38 to vary a direct voltage supply potentiometer 40 associated with a condenser 52 which introduces an integral component of control adjustable by ganged rheostats 53, 56. The following modifications are described: (a) use of a spring-loaded relay, two separate relays or a vibrating relay instead of the differential relay to control the servomotor. In the case of the vibrating relay the period of operation is determined by a resistance-condenser network; (b) use of a magnetic instead of a thermionic valve amplifier for the output of the bridge circuit; (c) an additional condenser in the input circuit of valve 44 to introduce a differential component of control; (d) introduction into the input circuit of valve 44 of an over-riding signal to prevent the temperature in the air duct from falling below or rising above a predetermined value. This is effected by a thermallyresponsive switch which initiates the charging of a condenser or the heating of a temperature variable resistor to introduce a gradually increasing signal; (e) temperature overshoot soon after starting up is prevented by introducing into the input circuit of valve 44 a signal derived from the heating of a temperature-variable resistor which takes place when a switch is closed by the steam valve when the latter is in its initial fully-open position; (f) an arrangement for regulating the roof temperature of an open hearth furnace by controlling the gas supply in which discharge of the integrating condenser following a mains failure is arranged to cause the gas valve to move towards a closed position when the supply is restored; (g) switches and indicators to facilitate initial setting up of the apparatus. Specifications 871,563, 871,564, 871,565, 871,566 and 871,567 are referred to.