PURPOSE: To measure easily and high accurately a speed by multiplying a positional signal based upon relative movement to a scale by a carrier signal for processing, etc., and detecting a relative speed as the frequency detuning of the carrier signal through a PLL circuit.
CONSTITUTION: The positional signals S1, S2 outputted from photodetecting elements 4a, 4b on the basis of the relative movement to the scale P engraved as a periodical pattern with a pitch P and expressed by the formulas I and II are multiplied by carrier signals cosωst, sinωst of multipliers 5a, 5b respectively. The multiplied results are added by an adder 6 and an output K based upon the formula III is generated. In the formulas, ωs=2πfs. The relative speed V corresponding to the detuning of the carrier signals in the output K is detected as an analog value by the PLL circuit formed by a voltage control oscillator 9, 1/N frequency divider 10, phase comparator 7, and an LPF8, etc. Consequently, the detecting method is prevented from the influence of ripple which may be generated when detecting a speed as alternative signals and the speed can be measured easily and highly accurately together with the detection of the position. In the formulas, X is a relative moving distance.
ONODERA HIROMI
SUGIZAKI TAIJI
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