GB873742A - Improvements in or relating to control devices for automatically controlling physicalvariables - Google Patents

Abstract

873,742. Automatic temperature-control systems. KELVIN & HUGHES Ltd. Oct. 9, 1958 [July 9, 1957], No. 21707/57. Class 38 (4). A control device for the automatic control of a variable-for example the temperature of an electric furnace-comprises a galvanometer 14 having a pointer 16 attached to a moving-coil element 15; the pointer 16 carrying vanes 20, 25 which cooperate with photo-cells 23, 28 respectively which are mounted on two frictionally engaged arms 17, 18, the two arms being movable relatively to the galvanometer 14 and relatively to each other whereby control of the variable may be effected at two pre-selected values thereof. In an electric furnace having a threephase heater the furnace may have an initial rate of heating with the supply connected to the furnace in delta. When the first pre-selected temperature is reached one of the vanes masks one of the photo-cells whereupon switch means are operated to change the connections to start to provide a reduced heating rate. When the second pre-selected temperature is reached the other of the vanes masks the other photo-cell whereupon the heater of the furnace is deenergized. The galvanometer 14 operates in conjunction with a scale 12 indicating the furnace temperature the arm 17 carrying a pointer 31 and a sub-scale 30 whilst the arm 18 carries a pointer 32 arranged to move over the sub-scale 30. Each photo-cell is arranged to receive light from a lamp reflected by a mirror. Alternatively, reactances may be used in lieu of the photo-cells, the moving vanes being arranged to vary the reactance values thereof. Specifications 813,277 and 821,552 are referred to.