Time interval phase detection in distance measuring apparatus

• US 3,900,259 A
• Filed: 05/09/1973
• Issued: 08/19/1975
• Est. Priority Date: 05/09/1973
• Status: Expired due to Term

• Current Assignee: Claude M. Mott, Richard J. Clark
• Original Assignee: Claude M. Mott, Richard J. Clark
• Inventors: Mott, Claude M., Clark, Richard J.
• Primary Examiner(s): Wilbur, Maynard R.
• Assistant Examiner(s): Buczinski, S. C.
• Application Number:
  US05/358,771
• Time in Patent Office: 832 Days
• Field of Search: 356/5 343/12R
• US Class Current: 356/5.12
• CPC Class Codes: G01S 17/32 using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated

Abstract

The phase difference between a modulated light signal transmitted to and reflected back from a distant target and that of a signal transmitted through an internal reference path is determined by digital time interval means to automatically provide a direct indication of distance to the target.

Claims

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.

2. Electro-optical distance measuring apparatus as in claim 1 further comprising phase coincidence detection means responsive to an indication of small phase difference between said time multiplexed signal and said reference frequency signal at selected times within a measurement cycle for introducing 180* of phase shift to said reference frequency signal to avoid phase ambiguity due to the effects of noise without thereby eliminating that portion of the measurement cycle already completed at the time of introduction of the phase shift.

3. Electro-optical distance measuring apparatus as in claim 1 wherein said logic means is operative for repeatedly sampling said time multiplexed signal until the computed statistical variance of the resulting plurality of intermediate indications of phase difference satisfies said test limit and for apprising the operator of an excessive measurement noise condition if said test limit is not satisfied after a predetermined number of samples.

4. Electro-optical distance measuring apparatus as in claim 3 wherein said test limit is computed as a function of the number of repeated samples of said time multiplexed signal, a desired confidence level, and a required degree of accuracy of the measurement result.