Patent GB-629363-A (GB)
Title: Improvements in electric control apparatus for regulating a variable
IPC: G05D23/24
Filed: 1947-06-02
Abstract: 629,363. Automatic temperature control systems. BAILEY METERS & CONTROLS, Ltd. June 2, 1947, No. 14523. Convention date, June 6, 1946. [Class 38 (iv)] [Also in Group XXXVIII] Apparatus for regulating the temperature of a furnace of the on-off type is supplied with an alternating voltage, which in alternate periods is of one phase and in other periods is of different phase, together with a control voltage, the two voltages affecting control by variation of the " on " relatively to the " off periods. An A.C. bridge 1 (Fig. 1) has a temperaturesensitive resistance 5 located in the furnace 6. The output of the bridge is applied to an amplifier 16 and motor control 17 controlling a reversible motor 9 operating a rheostat 7, 8 in a sense to restore balance of the bridge, and at the same time a pointer 10 on a scale 11 so as to give an indication of the instantaneous temperature in the furnace. The motor 9 also operates the arm 27 of a potentiometer 28 which, together with a potentiometer 30 set at the required standard temperature of operation as indicated on a scale 50, form a further A.C. bridge. The output ec from the bridge varies in amplitude and phase according to the extent and sense of the temperature deviation from the normal as determined by the setting of the potentiometer 30. This voltage is applied together with an alternating voltage em to an electronic 'relay 26 for effecting on-off operation of a power controller 25 in series with the furnace heater 13, according as to whether the magnitude and phase of the combined voltage ec and em, i.e. eo, are sufficient to cause the negative bias on a thyratron 45 to be overcome thus causing it to fire and operate a relay 49. The proportion of " on time " to " off time " is controlled by the voltage em superposed on the voltage ec. The voltage em is the unbalance voltage of a modulating bridge including an adjustable resistance Sp and the secondary windings 54, 55 of a movable core transformer 56 having an A.C. energized primary 57. The movable core is oscillated by means of an eccentric 61 operated at a constant predetermined rate by a motor 62. The control voltage eo is as shown in Fig. 3. In a modification (Fig. 4) a resistance bridge modulator comprising a 360 degree slide wire 67 tapped at two points 180 degrees apart with a slide wire driven by the motor 62 is employed instead of the movable core transformer. In a further embodiment (Fig. 5) the voltage em is used to control' a further electronic relay 26a controlling the sense of rotation of a further motor M2. through contacts 49A and 49B. The time of rotation of the motor M2 in one direction or the other is controlled by the setting of the potentiometer Sp. The motor M2 is geared to the standard setting potentiometer Ss. Thus by means of the setting of potentiometer Sp a rate of increase or decrease of temperature may be obtained between limits fixed by two movable switches 76 and 77 on the potentiometer Ss. Reversal of the motor may be manually effected by a switch 70, so that after a standard rate of increase of temperature MN (Fig. 6) operation of the switch 70 will effect a similar uniform rate of decrease of temperature NO. In a modification (Fig. 7, not shown) automatic reversal of the motor sense may be obtained by a relay connected in circuit with the limit switches and in further modifications (Fig. 8 and 9, not shown) automatic tracing of the curve MNPQ may be obtained by including in circuit a timer driven by a further motor controlled by the limit switches, and automatic tracing of the curve MNPR may be obtained by replacing the potentiometer Sp by two potentiometers, one controlling the rate of increase and one the rate of decrease of temperature. In this case limit switches are not provided on the potentiometer Ss but a further potentiometer for setting the maximum value of temperature is included in parallel with it.