Patent GB-660105-A (GB)
Title: Improvements in apparatus for measuring or/and controlling a variable
IPC: G01D5/12
Filed: 1947-08-26
Abstract: 660,105. Temperature indicating systems. BAILEY METERS & CONTROLS, Ltd. Aug. 26, 1947. [Aug. 27, 1946] No. 23503/47. Class 40 (i). [Also in Groups XXXV and XXXVI] Apparatus for measuring and/or controlling a variable e.g. temperature or rate of flow, comprises an a.c. motor having a winding excited through a full-wave rectifier and fed through an amplifier and phase detector, the amplifier being fed from an a.c. bridge controlled by deviations in the variable. As shown in Fig. 1, a thermocouple 2 is situated in a furnace 1 supplied with fuel through a burner 3 controlled by a valve 4, and is connected in a potentiometer circuit including a balancing potentiometer 8. Assuming a change in the temperature of the furnace from a predetermined value a d.c. voltage is fed to an electrical bridge 19 having reactor converters A, B and fixed resistance legs C.D. The bridge 19 changes the direct current signal of given polarity into an amplified alternating current signal of given phase depending upon the polarity of the d.c. signal, as described in U.S.A. Specification 2,447,338. A switch 12 is provided for connecting the bridge 19 to a standard cell 15 for checking purposes. The alternating voltage e derived from the bridge 19 is fed through an amplifier 27 to a motor control 39 comprising a valve 31 having an anode supplied with unsmoothed d.c. by a rectificer 33. As a result half wave pulses of d.c. EM, flow into the motor circuit including the energising winding 32 and capacitor 36 in parallel. The phase of this pulsating current depends on the phase of the grid signal on valve 31 and therefore upon the deviation of the temperature. The motor comprises a second winding 37 in series with a condenser 3S fed with a voltage EM2 of constant phase. The resulting direction of rotation of the motor then depends on phase difference between the voltages, EM1 and EM2. The motor is geared to adjust the balancing potentiometer 8, move a pointer 7 on a scale 5 to give an indication of the temperature in the furnace 1 and adjust the valve 4 in a sense to restore the temperature to the predetermined value. In a modification Fig. 3 (not shown), a temperature variable resistance is located in the furnace and forms one leg of an alternating current bridge, a balancing resistance being connected in the bridge circuit. The out of balance a.c. from the bridge is applied to the amplifier and motor control, the motor operating the pointer and balancing resistance to give an indication of the temperature. In a further embodiment (Fig. 4) the rate of flow in a conduit 48 is measured, pressure pipes being connected on either side. of a restriction 49, the difference in pressure on either side operating a movable core 54 of a transformer. The secondary of the transformer forms two legs of a bridge circuit the other two legs being formed by a variable resistance 60. The output e, applied to the amplifier and motor control circuits as above, is derived between the centre tap of the secondary of the transformer and the variable contact 61. of the resistance 60. Upon movement of the core 54 an out of balance voltage e is derived causing the motor to move the indicator 62 and restore balance by moving the contact 61.