GB797721A - Measuring apparatus - Google Patents

Abstract

797,721. Fluid-pressure servomotor-control systems. FOXBORO CO. Sept. 2, 1955 [Sept. 14, 1954], No. 25325/55. Divided out of 797,722.- Class 135. [Also in Groups XXXV, XXXVI, XXXVIII and XL (c)] The D.C. output of a thermocouple is converted to A.C. by a second harmonic converter, amplified and rectified by a phase-sensitive detector, the output current, a proportion of which is fed back through a bridge to balance the thermocouple output being also fed to a. transducer which converts it to a proportional change in pneumatic pressure for control or measurement. As shown, the output from a thermocouple 8 (Fig. 1) is fed to the signal winding 14 of a second harmonic converter 16 via a bridge 24 and a filter 28. The converter has two input windings 46, 48 fed from an oscillator 58, and are connected in opposition to give no signal in winding 14 except when a current from the thermocouple 8 unbalances the flux in the converter core, when a proportional A.C. signal having twice the oscillator frequency is induced in the signal winding 14 and, after amplification at valves 76, 112, is fed to a valve 138 which has a resonant circuit 154 from grid to earth slightly mistuned from the double frequency signal, which together with the oscillator frequency fed to the anode, causes phasesensitive detection at valve 138. The rectified output is amplified at 186 and fed through coils 190, 192 of a transducer 194 (described below) in the anode circuit, and through resistor 202 in one arm of the bridge 24, thus unbalancing the bridge, the D.C. output current of the bridge being fed through the signal winding 14 to neutralize the current from the thermocouple. Temperature compensation is effected by temperature-sensitive resistors 222 in the bridge 24, and 206 adjusting the bias on valve 186 in accordance with the temperature of the transducer 194. The transducer 194 comprises a pivoted plate 334 (Fig. 3) carrying the two coils 190, 192 in the air gaps between two magnets 316, 318 and the cases 312, 314, the intensity of the current in the coils determining the twist on the plate 334, and the amount of movement of a flap 358 from a nozzle 360 which is the outlet of pressurized air fed into a pipe 366. The resultant change in pressure in the pipe 366 due to the movement of the flap 358 is proportional to the current in the cores 190, 192.