GB1506397A - Shaft encoders - Google Patents

Abstract

1506397 Shaft angle measurement FERRANTI Ltd 17 Oct 1975 [24 Oct 1974] 46154/74 Heading G1A and G1N A shaft angle measuring apparatus comprises: means for continuously and repetitively moving a marker past a fixed detector and past a movable detector responsive to shaft displacement; means responsive to the detectors and to a fixed-frequency source to provide two counts of pulses whose numbers represent the shaft angle and one drum rotation respectively; and means for providing further pulses whose number represents the ratio between the two counts. In a photo-electric transducer Fig. 4 a drum 11, driven at high speed (e.g. 6000 rpm) about an axis aligned with a shaft 10, carries extensions 11a, 11b which interrupt respective light beams between a fixed lamp 15 and detector 17 and between a lamp 14 and detector 16 mounted to rotate with the shaft 10. Signals from the movable detector 16 are extracted via slip rings 121 and brushes 122. In an electromagnetic transducer Fig. 5 the drum 11 carries embedded magnetic elements 111 arranged to induce pulses of e.m.f. in a fixed coil 117 and a movable coil 116, signals being transmitted from the latter via a rotary transformer coupling 130, 131. In the circuit of Fig. 2, pulses 21 from the fixed detector, and pulses 20 from the movable detector, cause counters 68, 71 to count, during alternate drum rotations, numbers of first clockpulses 67 respectively representing the times taken for the drum to rotate through one circle and through the angle subtended between the fixed and movable markers. A rate multiplier 72, multiplying second clockpulses 75 by a factor proportional to the counter 68 contents, uses the product to empty the counter 71, whereby the number of second clockpulses 80 gated out during the emptying is an accurate measure of the ratio of counter contents 71 to 78, and errors due to long-term drum-speed variation are thus avoided.