Patent GB-764341-A (GB)
Title: Improvements in automatic temperature control systems
IPC: G05D23/24
Filed: 1954-09-24
Abstract: 764,341. Automatic temperature control systems. GENERAL ELECTRIC CO. Sept. 24, 1954 [Sept. 25, 1953], No. 27679/54. Class 38 (4). [Also in Groups XI and XXXVII] In a system for maintaining the temperature of a body constant, wherein temperature is detected by a temperature-sensitive resistor and an electric heater is energized through a magnetic amplifier controlled by deviations of temperature from a desired value, the amplifier is also controlled by the difference of currents in two further temperature-sensitive resistors .heated by an auxiliary element in series with the main heater, so that the heater is switched off before or after the desired temperature is reached depending on the heat capacity of the body. The further resistors have equal negative coefficients but unequal heat capacities, the one of smaller capacity having a series fixed resistor for limiting its maximum current below the maximum current of the one of larger capacity; thus the net control current N, Fig. 5, varies with time and actually changes polarity at a certain instant, after which switching off of the heater follows rather than precedes the attainment of the desired temperature. Control' circuits. The main and auxiliary elements 4, 42, Fig. 4, are energized from supply leads 45, 46 over the contact 49 of a hot-wire relay switch 48, the latter being operated by the output current of the magnetic amplifier 53 flowing in the hot-wire 50 of switch 48. The amplifier load windings 51, 52 are energized over a transformer 56, and D.C. control windings 60, 61, 62, 63 are energized in parallel over a rectifying system 57, 58, 59. The opposed windings 60, 61 carry current representing the desired temperature and the actual body temperature, as set on a rheostat 64 and sensed by the temperature-sensitive resistor 28 respectively ; and the opposed windings 62, 63 carry the currents flowing in temperature-sensitive resistors 30, 31 heated by the auxiliary element 42, resistor 66 limiting the maximum current in resistor 30 below that in resistor 31. In operation, switch contact 49 is closed to energize heaters 4, 42 when rheostat 64 is set to a resistance value below that of temperature-sensing resistor 28, the latter having a high negative coefficient. As the temperature rises the current in winding 61 increases with a tendency to cancel the opposing reference current in winding 60, and meanwhile resistors 30, 31 decrease in value, the former more rapidly since it has the lower thermal capacity. The net control current N, Fig. 5, initially aids that in winding 61, so that contact 49 is opened before the desired temperature is reached provided current' N has not reversed polarity. With a body of high thermal capacity the current N will begin to oppose that in winding 61 before the desired temperature is reached, so that contact 49 is opened after this value is attained. In a modification, Fig. 6 (not shown), resistors 30, 31 are in adjacent arms of a Wheatstone bridge network, the other arms carrying resistance 28 and rheostat 64 respectively. The bridge output energizes a single control winding for the magnetic amplifier. Both the above systems may be operable with resistors of positive rather than negative coefficients. Constructional details. An electric hot-plate is described in which temperature-sensitive resistor 28 is contained within a metal button placed at the centre of a helical element, the button with its associated leads being insulated from the element by a series of concentric cylindrical shields and biased upwards by a leaf spring against the base of a utensil placed on the element. Resistors 30, 31 are mounted on hollow conducting cylinders of unequal mass, which are placed end to end and are thermally insulated from one another. Element 42 passes through the cylinders, and the whole is spring- mounted in an outer cylindrical casing.