DISTANCE MEASURING APPARATUS AND METHOD UTILIZING PHASE COMPARISON OF MODULATED LIGHT BEAMS

• US 3,778,159 A
• Filed: 03/30/1972
• Issued: 12/11/1973
• Est. Priority Date: 03/10/1970
• Status: Expired due to Term

• Current Assignee: Robin H. Hines, Thomas O. Bolden, William L. Hollinshead
• Original Assignee: Robin H. Hines, Thomas O. Bolden, William L. Hollinshead
• Inventors: Hines, Robin H., Hollinshead, William L., Bolden, Thomas O.
• Primary Examiner(s): Borchelt, Benjamin A.
• Assistant Examiner(s): Buczinski, S. C.
• Application Number:
  US05/239,456
• Time in Patent Office: 621 Days
• Field of Search: 356/4,5 343/5DP,12
• US Class Current: 356/5.11
• CPC Class Codes: G01S 17/32 using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated

Abstract

Apparatus and method for accurately measuring and digitally displaying the distance between remotely separated points involves modulating a light beam at a known radio frequency, transmitting the light beam from one of the points to reflect from the remote point and return to the transmitting point, digitally measuring the phase difference between the transmitted and returned beams, and determining from such phase difference the distance between the points. The light beam is modulated in sequence with a plurality of RF signals of different frequency in order to resolve ambiguities associated with phase comparison distance measuring techniques, the primary one of such modulating signals being obtained by mixing a reference radio frequency (RF) signal with a reference intermediate frequency (IF) signal derived by division of that reference RF signal, thus retaining a fixed frequency ratio with the reference RF signal. The return RF signal is also mixed with the reference RF signal to derive a second IF signal for comparison with the reference IF signal in a digital phase comparator which utilizes as the clock signal the reference RF signal. This method of deriving from a single oscillator source the various comparative signals greatly increases the tolerances of the system and reduces the need for expensive oscillators of extreme stability. The remaining modulating signals are likewise derived by mixing various RF signals with the reference IF signal, yet all modulating signals are maintained within a narrow frequency band, thus greatly reducing compensation requirements while improving accuracy, by utilizing as the additional RF signals those having frequency differences with the reference RF signal corresponding to halfwavelengths which are related to the half-wavelength of the primary beam-modulating RF signal by different powers of 10.

Claims

1. In a distance measuring apparatus coMprising a light source, modulating means for modulating the intensity of a beam of light from said source at a predetermined radio frequency with an RF electrical signal, means for transmitting said modulated light beam to a remote target, photo detection means for detecting a reflection of said beam from said remote target and for providing a return RF electrical signal in response thereto, means for providing an intermediate frequency reference IF electrical signal indicative of the phase of said modulating signal, means for providing a second IF electrical signal at said intermediate frequency indicative of the phase of said return signal, digital phase comparison means for determining the phase difference between said reference IF signal and said second IF signal, and digital computation means for computing a distance proportional to said phase difference;

the improvement in signal-providing means which comprises;

a. a primary oscillator for generating a reference RF electrical signal;

b. frequency divider means operatively connected to said primary oscillator for providing said reference IF signal having a fixed frequency ratio with said reference RF signal;

c. first mixing means operatively connectable to said primary oscillator and operatively connected to said divider means for mixing said reference RF signal and said reference IF signal to thereby provide a primary beam-modulating RF signal having said predetermined frequency; and

d. second mixing means operatively connectable to said primary oscillator and operatively connected to said detection means for mixing said reference RF signal and said return RF signal to thereby provide said second IF signal;

e. said digital phase comparison means being operatively connected to said frequency divider means and to said second mixing means to receive therefrom, respectively, said reference IF signal and said second IF signal.

2. control means for manipulating the stored group of phase difference determinations in accordance with appropriate mathematical formulae to thereby compute the total distance between said apparatus and said remote target.

3. In an improved distance measuring apparatus according to claim 2, the further improvement comprising:

a. logic control for said selection means to effect said operative connection of each of said primary and additional oscillators in a predetermined sequence and for a predetermined duration;

b. said digital computation means including;

4. The improvement according to claim 3 including digital display means operatively connectable to said computation means for visual display of said total distance.

5. In an improved distance measuring apparatus according to claim 3, the further improvement wherein said digital computation means includes:

a. means for inserting and storing data indicative of the influence of ambient atmospheRic conditions on the propagation of a modulated beam of light therein; and

b. compensating means for digitally altering said computed total distance in accordance with the values of said inserted data.

6. In a method of measuring distance comprising modulating the intensity of a beam of light at a predetermined radio frequency with an RF electrical signal, transmitting said modulated light beam to a remote target, detecting a reflection of said beam from said remote target with photo detection means providing a return RF electrical signal in response thereto, providing an intermediate frequency reference IF electrical signal indicative of the phase of said modulating signal, providing a second IF electrical signal at said intermediate frequency indicative of the phase of said return signal, comparing said reference IF signal and said second IF signal and determining the phase difference therebetween, and computing a distance proportional to said phase difference;

the improvement which comprises;

a. generating a reference RF electrical signal;

b. dividing the frequency of said reference RF signal to thereby provide said reference IF signal having a fixed frequency ratio with said reference RF signal;

c. mixing said reference RF signal and said reference IF signal to thereby provide a primary beam-modulating RF signal having said predetermined frequency; and

d. mixing said reference RF signal and said return RF signal to thereby provide said second IF signal.

7. The improvement according to claim 6 which further comprises:

a. generating a plurality of additional RF signals differing in frequency and respectively having a frequency difference with said reference RF signal corresponding to a half-wavelength which is related to the half-wavelength of said primary beam-modulating RF signal by a different power of 10; and

b. mixing one or another of said reference and additional RF signals respectively with said reference IF signal to provide one or another of a plurality of beam-modulating RF signals of difference predetermined frequency and with the relevant resulting return RF signal to provide one or another of a plurality of second IF signals.

8. In an improved method according to claim 7, the further improvement comprising:

a. mixing each respective one of said reference and additional RF signals in a predetermined sequence and for a predetermined duration with said reference IF signal to thereby provide for each said duration a different one of said plurality of beam-modulating RF signals;

b. modulating said light beam for each said duration with each different one of said beam-modulating RF signals;

c. mixing said reference RF signal with each relevant resulting return RF signal to provide said plurality of second IF signals;

d. comparing said reference IF signal with each of said second IF signals and determining the respective phase differences therebetween;

e. storing the phase difference determinations relevant to each of said plurality of second IF signals; and

f. manipulating the stored group of phase difference determinations in accordance with appropriate mathematical formulae to thereby compute the total distance between said apparatus and said remote target.

9. In an improved method according to claim 8, the further improvement comprising:

a. deriving data indicative of the influence of ambient atmospheric conditions on the propagation of a modulated beam of light therein; and

b. utilizing said data in said mathematical formulae to thereby alter said computed total distance in accordance therewith.