Patent GB-812777-A (GB)
Title: Temperature control
Inventor: EVANS ARTHUR DUNN
IPC: G05D23/22
Filed: 1956-11-19
Abstract: 812,777. Automatic temperature control systems. TEXAS INSTRUMENTS Inc. Nov. 19, 1956, No. 35320/56. Class 38 (4). In a system for controlling the temperature of an electric furnace supplied through at least one grid controlled gaseous discharge device, a D.C. error voltage is derived from a D.C. voltage indicative of furnace temperature and converted to an A.C. voltage wherein the phase and magnitude are indicative of the error, the A.C. being added to a further A.C. voltage to produce a phase modulated sine wave signal which is converted to time spaced voltage spikes for controlling the firing of the tube. The difference in voltage between a thermocouple 15, Fig. 2, in an induction furnace 13 from which it is desired to draw a germanium crystal and a thermocouple 17 in melting ice, is opposed by a battery 30 in a potentiometer 19-26 to produce a zero error signal input to chopper 31 at the predetermined temperature. The chopper output, after amplification and shaping in a unit 35 is applied with an alternating voltage from phase A to the two grids of a double-triode 36 having a common anode load 40, a phase difference of 90 degrees being achieved by a phase-shifting network 3 7 in  the phase A supply. A sine wave signal is thus produced in line 42 the phase of which changes with crucible temperature and when applied to phase-shifting network 43 produces at the grids of " thyratrons " 51-53 which are normally negatively biased. Three spiked pulses at identical time positions with respect to the cycles of the three phases of the mains supply, the time position being determined by the signal phase so that the input power to the furnace and thus the furnace temperature is continuously controlled. A meter 125 is provided for the initial adjustment of the system and thermocouples of platinum and 90 per cent platinum, 10 per cent rhodium are stated to be most satisfactory. The system may be modified for any number of phases and for full-wave working. In a modification, Fig. 3, a self-balancing potentiometer 60 is introduced and the chopper and phase modulating valve 36 are replaced, between terminals 27, 28 and 58, 59, by a continuously variable phaseshifting circuit including a saturable reactor 72 the control winding of which is in the anode circuit of an amplifier valve 66. Improved sensitivity to sudden changes is provided by a resistor 67 and a condenser 68, and internal feed-back is produced by R.F. induction in a single turn pick-up 79 placed near the generator output coils. A stabilizing rate-of-change signal which is applied also to the grid of amplifier 66 is derived from a quartz rod 92 in heat transfer relation with the furnace, and a photo-electric cell 93 which detects the infrared emission therefrom. A cam programme arrangement, which utilizes a reversible motor to adjust the datum set by the potentiometer whereby the temperature may be controlled in conjunction with the withdrawal of the crystal is described with reference to Fig. 5 (not shown). Performance figures for the system are given and the operation of the continuously variable phase-shifting network is described.