Patent GB-1305228-A (GB)
IPC: G05D23/24
Filed: 1970-02-11
Abstract: 1305228 Automatic temperature control SOC D'INSTRUMENTATON SCHLUMBERGER 11 Feb 1970 [3 March 1969] 6648/70 Heading G3R The temperatures of a plurality of electric ovens particularly tubular ovens for the continuous thermal treatment of spun synthetic fibres, are regulated individually so that each equals a common reference temperature. Full positive half waves of an A.C. supply are applied to the oven heaters via respective thyristors. A visual alarm is actuated and the heater current is terminated if the temperature of an oven exceeds a certain threshold. To initiate operation of the ovens a push button switch 28 in a central unit CU is actuated switching the state of a bi-stable unit 30 whereby relay coils are energized in respective regulator units RU 1  to RU n  for example the unit RU 1  wherein coil 58 is energized via an OR gate 54 and relay control 56 and a thyristor 42 and oven heater 44 are connected across A.C. source A.B. Pulses from a source 10 in the central unit CU are applied to a potentiometer 16, 17 whereon the common reference temperature is pre-set, and also to temperature probes 32 1  to 32 n in regulator units RU 1  to RU n  respectively for the respective ovens. The probe in RU 1  for example, includes an oven temperature sensitive resistor 36 1  and a temperature stable resistor 34 1 . The amplitudes of voltage pulses at point 35 of the probe 32 1  and slider 17 of the potentiometer are compared in a respective amplifier 38 1 . If the temperature of the oven is low, as when commencing operation, positive pulses from the amplifier output and synchronized with the zero going positive crossings of the A.C, supply are applied to the trigger electrode of thyristor 42 which conducts and the heater 44 is energized. Each firing pulse is maintained for a full half cycle. If the oven temperature is higher than the reference temperature the amplifier output pulses will be negative and the thyristor will not fire. As soon as the oven temperature reaches the regulating range which extends on both sides of the reference temperature by a value fixed by the threshold of a logic detector 46, the detector goes into its "O" state and after inversion of its output at inverter 48 a lamp 52 lights up. The temperature of the oven is henceforth controlled to the reference temperature, the thyristor 42 being triggered ON only for positive half cycles when temperature is below the reference but not when it is above the reference temperature. An AND gate 64 detects when all the ovens are so controlled and switches the state of bi-stable 30. Thereafter the energization of the relay coil 58 is dependent upon a respective AND gate 50. Excessive temperature protection. If the threshold imposed on the error signal output of amplifier 38 1  by detector 46 is exceeded the detector assumes its "1" state and AND gate 50 assumes its "O" state causing relay 60 to cutoff thus switching off the oven heater 44. At the same time an AND (or NAND) gate 66 illuminates an alarm lamp 24 and lamps 52, 22 go out, also relay contact K 2  connects the second input of AND gate 50 to earth which prohibits reenergization of the relay coil. Thus reenergization of the oven heater is prevented even when the oven temperature has returned to normal.