Intensity modulation at two frequencies for interferometric measuring of distance

• US 9,631,921 B2
• Filed: 04/13/2015
• Issued: 04/25/2017
• Est. Priority Date: 04/05/1999
• Status: Expired due to Term

• Current Assignee: Faro Technologies Incorporated
• Original Assignee: Faro Technologies Incorporated
• Inventors: Bridges, Robert E., Brown, Lawrence B., West, James K., Ackerson, D. Scott
• Primary Examiner(s): LEE, HWA S
• Application Number:
  US14/685,076
• Publication Number:
  US 20150211851A1
• Time in Patent Office: 743 Days
• CPC Class Codes: G01B 11/002 for measuring two or more coordinates
• CPC Class Codes: G01B 11/02 for measuring length, width or thickness G01B11/08 takes precedence
• CPC Class Codes: G01B 11/026 by measuring distance between sensor and object G01B11/0608 takes precedence
• CPC Class Codes: G01B 11/03 by measuring coordinates of points
• CPC Class Codes: G01B 9/02012 using temporal intensity variation
• CPC Class Codes: G01C 3/08 Use of electric radiation detectors
• CPC Class Codes: G01S 17/42 Simultaneous measurement of distance and other co-ordinates indirect measurement G01S17/46
• CPC Class Codes: G01S 17/66 Tracking systems using electromagnetic waves other than radio waves
• CPC Class Codes: G01S 5/163 Determination of attitude using inertial means G01C9/00; control of attitude G05D1/49

Abstract

A method for measuring a distance includes modulating the light beam at a first frequency, receiving a second beam by the optical detector to produce a first electrical signal having the first frequency and a first phase; modulating the light beam at a second frequency different than the first frequency; receiving the second beam by the optical detector to produce a second electrical signal having the second frequency and a second. After these steps, the retroreflector is moved while modulating the light beam continuously at the second frequency; and a first distance to the retroreflector is determined based at least in part on a the first and second frequencies and phases.

Claims

1. A method for measuring a distance, the method comprising:

providing a light source, an electronics circuit, an optical detector, and a retroreflector, wherein the light source produces a light beam, a portion of the light beam being reflected by the retroreflector as a second beam received by the optical detector;

in a first instance;

modulating the light beam at a first frequency;

receiving the second beam by the optical detector to produce a first electrical signal having the first frequency and a first phase;

measuring the first phase by the electronics circuit in response to the first electrical signal;

modulating the light beam at a second frequency, the second frequency being different than the first frequency;

receiving the second beam by the optical detector to produce a second electrical signal having the second frequency and a second phase;

measuring the second phase by the electronics circuit in response to the second electrical signal;

in a second instance occurring later than the first instance;

moving the retroreflector while modulating the light beam continuously at the second frequency;

receiving the light beam modulated continuously at the second frequency by the optical detector to produce a third electrical signal having the second frequency and a third phase;

measuring the third phase by the electronics circuit in response to the third electrical signal;

determining a first distance to the retroreflector based at least in part on a speed of light, the first frequency, the second frequency, the measured first phase, the measured second phase, and the measured third phase; and

storing the first distance.

2. The method of claim 1 wherein, in the step of modulating the light beam at a second frequency, the modulating is at a single second frequency.

3. The method of claim 1 wherein, in the step of providing the light source, the light source is a laser.

4. The method of claim 3 wherein, in the step of modulating the light beam at the second frequency, the modulation is direct modulation of the laser optical power by a radio-frequency electrical signal.

5. The method of claim 1 wherein the step of determining the first distance further includes determining an unambiguous range equal to c/(2*f*n), where c is the speed of light, f is the second frequency, and n is an index of refraction of air.

6. The method of claim 5 wherein, in the steps of measuring the second phase by the electronics circuit and measuring the third phase by the electronics circuit, the retroreflector has not moved over a complete unambiguous range.