JPS6469983A - Distance measuring device - Google Patents

Abstract

PURPOSE:To make it possible to measure the distance with high accuracy in a wide measuring range, by so arranging that approximate distance is measured by a low second frequency and correct distance is measured by a high first frequency. CONSTITUTION:Oscillating circuits 11 and 12 oscillate respectively frequencies f1 and f2. The frequency f1 which is for measuring a correct distance is larger than the frequency f2 for measuring an approximate distance. A reference signal of each of the frequencies f1 and f2 is applied to an AM modulator 13, thereby driving an LD (laser diode) 14. Thus, a laser beam which is modulated in strength by the frequencies f1 and f2 is generated at the LD 14. An output signal from a PD (photodiode) 15 consequent to the receipt of a light reflected from an objective material is, through passing band-pass filters 21 and 22 for respective frequency zones f1 and f2, turned to a signal of a single frequency f1 or f2. In a phase difference detecting circuit 31, a phase difference phi1 between a reference signal of the frequency f1 of the circuit 11 and a signal of the frequency f1 of the filter 21 is detected. Similarly, in a phase difference detecting circuit 32, a phase difference phi2 between a reference signal of the frequency f2 of the circuit 12 and a signal of the frequency f2 of the band-pass filter 22 is detected. A processing circuit 16 finally obtains the distance by the phase difference #1 and #2, and outputs the same.