Patent GB-821892-A (GB)
Title: Improvements in control device comprising a null-balance bridge system
Inventor: MALTBY FREDERICK LATHROP
IPC: G05D9/12
Filed: 1957-05-03
Abstract: 821,892. Bridge measurements. ROBERTSHAW-FULTON CONTROLS CO. May 3, 1957 [May 21, 1956], No. 14239/57. Class 37. A control device comprises a bridge circuit energized with high-frequency voltage modulated with low-frequency voltage and means for unblancing the bridge in response to a change in a condition to be controlled, the output of the bridge being mixed with the highfrequency voltage and demodulating the mixed voltage to obtain a low-frequency voltage for energizing means for rebalancing the bridge. Modulator 16 is connected to an R.F. oscillator 10 and to a 60 cycle per second reference voltage source L1, L2, the R.F. voltage signal from 10 being amplified and modulated at 60 cycles per second. This output is connected to energize an R.F. bridge 28 whose unbalance signal is connected through rectifier 32 to amplifier 34; a resistance 39 and capacitor 38 being connected across the input terminals of the amplifier. Oscillator 10 is also connected to mix its unmodulated signals with the modulated unbalance output from the bridge 28 to supply rectifier 32, whereby such signals are demodulated to a 60 cycle per second voltage, the phase sense of which depends on the direction of the bridge unbalance and which energizes amplifier 34 connected to a field winding 45 of a reversible biphase motor 46 having a winding 48 connected to the 60 cycle reference voltage source, the motor 46 being mechanically coupled to indicator or controller 50 by linkage 52. The bridge 28 comprises branches 54, 56, 58 containing variable capacitors 62, 64, 66 respectively, a branch 60 including a condition responsive capacitative probe 68 which in the embodiment shown, comprises plates 84 and 86 suspended in a container 82 containing material 80, variations in the depth or quantity of material 80 changing the capacity between the plates 84 and 86. If the bridge 28 is balanced no modulated voltage is supplied to rectifier 32 and motor 46 is not driven, but if the bridge is unbalanced a R.F. signal modulated in phase or out of phase with the unmodulated reference voltage reaches rectifier 32 and after de-modulation the resultant signal in winding 45 drives motor 46 in one direction or other until capacitor 64, mechanically connected to linkage 52, has varied sufficiently to re-balance the bridge and stop the motor, while linkage 52 may be coupled to operate means indicating the level of material 80 or controlling this level. In order to vary the span of measurement of the bridge, that is, to change the ratio of variation of the arms 60 and 56, a pair of identical capacitors 88 and 90 are connected in parallel with capacitors 62 and 64 respectively, with a variable capacitor 94 connected between their junction and the common terminal of capacitors 88 and 90. Capacitors 88, 90 and 94 form a Y-network reduceable to a # network consisting of three capacitors; two being equal of value half that of capacitor 94 respectively in shunt with capacitors 62 and 64, and the third across the modulator output having half the value of 88 or 90. Variable capacitor 94 enables the span of the measurement of the bridge to be varied without affecting the zero setting since adjustment will add capacity symmetrically to 62 and 64.