Patent GB-787253-A (GB)
Title: Improvements in or relating to recording voltmeters
IPC: G01R13/04
Filed: 1954-11-03
Abstract: 787,253. Voltage measurement. COLE, Ltd., E. K., COX, V. J., and REEVES, R. J. D. Jan. 11, 1956 [Nov. 3, 1954], No. 31740/54. Class 37. In a recording voltmeter for repetitive waveforms, the instantaneous input voltage is compared with a reference voltage at isolated short time intervals determined by a train of gating pulses at the same frequency or a sub-multiple frequency of the input waveform, and each value of difference voltage operates a servomotor to regulate a feed-back voltage to balance the appropriate difference and to correspondingly operate a recording stylus ; a slight time asynchronism being introduced between the gating pulse repetition frequency and the input waveform repetition frequency. The input waveform is (Fig. 1) applied to a gated difference measuring circuit 2 for instantaneous comparison with a voltage derived from a potentiometer 7; the circuit 2 developing a difference voltage which operates a servomotor 6 to readjust the potentiometer for zero error and deflect the stylus of a recorder to plot a point representing the value of the input waveform at the instant of comparison. A waveform at terminal 8 synchronous with that of the waveform at terminal 1 is connected to a device 9 introducing a delay time variable by potentiometer 11 coupled to the recording chart carrier for adjustment according to the position of the fatter; the delayed output waveform operating a strobe pulse generator 12 arranged to open the gate of circuit 2 at each pulse. Thus the input waveform at terminal 1 is plotted in successive points against an expanded time scale ; the gated comparison circuit 2 opening at successive instants timed relatively to the waveform by the chart movement. In Fig. 2 the synchronous waveform applied to terminal 8 operates a pulse repetition frequency generator 13 which energizes a delay circuit, controlled from the output of potentiometer 11 adjusted from the axial movement of the chart, to trigger a gating pulse generator 12 coupled to the first control grids of a pair of hexodes 2, one of which has its second control grid returned to a reference voltage from potentiometer 7 coupled mechanically to the recording pen, while the other second control grid is connected to the input waveform at terminal 1. The amplitude difference between the anode pulses of the hexodes is determined by comparison circuit 14, amplified and stretched by circuit 15, and applied to servo amplifier 5 controlling motor 6 driving the recording pen and potentiometer 7 in such sense as to reduce the pulse difference to zero. The chart is movable by hand wheel 17. In a modification (Fig. 3, not shown) for producing a comparison curve of two waveforms of similar frequency, two servo controlled potentiometric comparison circuits as shown in Fig. 1 above are provided, one for each signal, and are gated from a common delay circuit; the delay being manually controlled independently of the chart movement, which is mechanically coupled to the rebalancing servo system of the second comparison circuit; the first such circuit driving the recording pen, so that the curve produced is a function of the two input waveforms.