Time interval phase detection in distance measuring apparatus

Claim 1

Electro-optical distance measuring apparatus of the type including a modulated target path light signal transmitted to and reflected back from a distant target, and an internal reference path light signal continuously and cyclically time multiplexed therewith to provide a time multiplexed signal of fixed frequency having alternating target path and reference path portions, wherein the improvement comprises: detecting means for separately determining the beginning of a target path portion of said time multiplexed signal and an immediately succeeding reference path portion thereof, and for providing a plurality of pulses, during each of those target path and reference path portions, the number of such pulses being proportional to the phase difference between the reference frequency signal and the respective portions of said time mulTiplexed signal; accumulator means connected to said detecting means for separately receiving and counting the plurality of pulses provided by said detecting means during those target path and reference path portions of said time multiplexed signal; and logic means for combining the counted results of the accumulator means obtained during those target path and reference path portions of said time multiplexed signal to provide an intermediate indication of the phase difference between said target path light signal and said internal reference path light signal, said logic means being operative for enabling said detecting means to repeatedly sample said time multiplexed signal to provide a plurality of pulses, during subsequent target path and immediately succeeding reference path portions thereof, the number of pulses provided during each sample again being proportional to the phase difference between the reference frequency signal and the respective portions of said time multiplexed signal, said logic means being operative to enable said accumulator means to separately receive and count the plurality of pulses provided by said detection means during each of the target and reference path portions of said time multiplexed signal occurring in connection with each such repeated sample, said logic means being operative for combining the counted results of the accumulator means obtained during each of the target and reference path portions of said time multiplexed signal occurring in connection with each such repeated sample to provide a plurality of intermediate indications of the phase difference between said target path light signal and said internal reference path light signal, said logic means being operative for computing the statistical variance of the plurality of intermediate indications of phase difference and for comparing that computed statistical variance with a predetermined test limit to insure a specified degree of measurement accuracy.