Cathode-ray-tube actuator in a noncontact analogue tachometer

The paper describes an electro-optical method for noncontact measurement of instantaneous angular velocity of a shaft. The tachometer employs a standard cathode-ray tube as a voltage to light-spot position actuator, a black-white contrast edge on a diameter of the shaft end, and a photomultiplier light detector. The light spot is projected onto the shaft end and switched rapidly along the contrast edge about the shaft centre. Any detector output (at the switching frequency) is amplified, and by using a sampling function circle generator, the system is able to position the light-spot so as to minimise the detector output. Thus, this closed-loop system operates to maintain the light-spot on the contrast edge and the input to the circle generator is the tachometer output. The instrument must be focused on the shaft end; no special calibration is required and the light spot readily locks onto a rotating contrast edge. An experimental instrument has been used on shafts down to 3 mm in diameter, which has a range of at least 20000 rev/min in either direction, a bandwidth of about 1 kHz with a resolution of about 400rev/min limited by noise, owing to the effect of background light on the photomultiplier. Greater resolution may be obtained at the expense of bandwidth.