Patent ID: 7388655

Claim:
A rangefinder for finding a distance to a measurement object based on a measuring beam and a reflected beam reflected from the measurement object, comprising: a light source for outputting the measuring beam; a burst drive unit for pulse-driving the light source to operate in burst emission; an AM modulation unit for AM modulating the measuring beam that is emitted by the burst drive unit with a modulation signal consisting of a sinusoidal wave; a light receiving unit for detecting the reflected beam that is part of the measuring beam modulated by the AM modulation unit and reflected by the measurement object; an AD conversion unit that converts an analog reflection signal corresponding to the reflected beam detected by the light receiving unit into a digital reflection signal with a sampling signal whose frequency is shifted from that of the modulation signal and synchronizes with a specific phase of the modulation signal once every N times; a phase difference detection unit for Fourier transforming the digital reflection signal as converted by the AD conversion unit to detect the phase difference between the measuring beam and the reflected beam; a wave position detection unit for calculating average values for the digital reflection signal as converted by the AD conversion unit within one modulation period while a time is shifted by one sampling timing each time to get one average value, performing differential calculus on an average waveform obtained by arranging the average values in time sequence, and calculating the wave position of the sinusoidal wave based on a delay time obtained based on a timing showing a position of the center of gravity of an obtained differential waveform and an output timing of the measuring beam; and a distance arithmetic unit for calculating the distance to the measurement object based on the wave position detected by the wave position detection unit and the phase difference detected by the phase difference detection unit.