GB780036A - Apparatus for measuring rotational speed - Google Patents

Abstract

780,036. Measuring rotational speed electrically. NAPIER & SON, Ltd., D. Aug. 18, 1955 [Sept. 20, 1954], No. 27221/54. Class 40 (1). Apparatus for measuring rotational speed of a shaft comprises an electromagnetic means associated therewith producing signals in phase quadrature which are fed through an amplifier to the X and Y plates of a cathode-ray tube causing the spot to move about an axis at the angular speed of the shaft. A control apparatus is arranged periodically to initiate a cycle com. prising switching in at a predetermined point in the travel of the spot a timing circuit to illuminate the spot for a fixed time interval and to inject a signal into the amplifier causing the spot to move radially outwards during the interval, whereby the length of spiral trace thus produced is a measure of speed. In addition the control apparatus may be arranged to make the interval short, e.g. equivalent to 1 turn of spiral and to illuminate the spot again momentarily after a total time interval of say 10 turns of spiral. The length of spiral gives a rough indication of speed while the position of the momentarily illuminated spot allows an accurate measure of the position of the end of the spiral. For instance, in Fig. 2 the two intervals are in the ratio 10: 1 so that the angular displace of spot J3 from 0 corresponds to 10 times the angular displacement of J1 from line 1 which is not readily measured directly. At a predetermined position of the shaft simultaneous pulses are applied to illuminate the tube and start a timer consisting e.g. of a crystal-controlled oscillator feeding pulse counters adapted to apply a switching-off pulse to the tube when a certain count is reached and to apply a flashing pulse to the tube after a longer interval. This pulse also resets the counter and switching circuits. The screen persistence and the periodicity of the cycle are such that a persistent trace is obtained.