Optical coordinate measuring system for large objects

• US 5,305,091 A
• Filed: 12/07/1992
• Issued: 04/19/1994
• Est. Priority Date: 12/07/1992
• Status: Expired due to Term

• Current Assignee: Creo Products Incorporated
• Original Assignee: OREO Products Incorporated
• Inventors: Gelbart, Daniel, Laberge, Michel G.
• Primary Examiner(s): Rosenberger, Richard A.
• Assistant Examiner(s): Pham, Hoa Q.
• Application Number:
  US07/987,976
• Time in Patent Office: 498 Days
• Field of Search: 356/1, 356/4, 356/4.5, 356/373, 356/375, 356/376, 356/141, 356/152, 356/345, 356/356, 356/363, 356/358, 250/561, 250/231.11, 33/502, 33/503, 33/504, 33/556
• US Class Current: 356/620
• CPC Class Codes: G01B 11/002 for measuring two or more coordinates
• CPC Class Codes: G01S 17/66 Tracking systems using electromagnetic waves other than radio waves
• CPC Class Codes: G01S 7/497 Means for monitoring or calibrating

Abstract

A coordinate measuring system consists of multiple optical transceivers (transmitter-receivers) mounted onto a stable reference frame such as the walls of a room. The object to be measured is touched with a hand-held measuring probe. To measure, the probe triggers the transceivers to read the distance to two retroreflectors mounted on the probe. The location of the probe tip relative to the reference frame is computed from at least six transceiver readings (three for each retroreflector). The accurate determination of the distance between the transceivers and retroreflectors is done by measuring the phase shift of a high frequency modulation superimposed on light beams. To avoid ambiguity two high frequencies are used, one for coarse and one for fine resolution. The initial location of the transceivers can be found out by measuring an accurately known object and computing the transceiver locations. The transceivers include a beam steering device to track the position of the probe.

Claims

1. A three dimensional coordinate measuring system comprising:

a plurality of optical transmitter-receivers (transceivers) mounted at accurately known locations on a fixed and stable reference frame;

a freely moveable probe containing a tip and at least two reference points, the distance from the tip to said reference points accurately known and each of said reference points being within line-of-sight to at least three of said transceivers regardless of the position of said tip;

an omnidirectional retroreflector located at each of said reference points;

a laser modulated by a plurality of accurately known frequencies and coupled to said transceivers using optical fibers, said transceivers including optical means of forming output of said fibers into a collimated light beam steerable in two dimensions, said collimated beam can be independently steered in each one of said transceivers;

means of tracking said retroreflectors with said collimated light beams and means of collecting the retroreflected light of each one of said beams and focusing it onto a separate photo-detector;

means of measuring the phase shift of the output of each one of said photo-detectors relative to the phase of said accurately known frequencies and means of converting this phase shift into distance information, the plurality of said accurately known frequencies used to resolve the ambiguity in the valve of said distance present when only a single frequency is used;

means of computing the location of each retroreflector from its distance to at least three of said transceivers;

and means of computing the location of the said probe tip from the computed location of said retroreflectors.

2. A system as claimed in claim 1 wherein said omnidirectional retroreflectors consist of two concentric spheres made of transparent material and having the refractive index of the inner sphere higher than the refractive index of the outer sphere, the outside sphere coated with a partially reflective coating.

3. A system as claimed in claim 1 wherein said tracking means are a light source located at the center of each of said retroreflectors and being modulated with a different frequency for each one of said retroreflectors in order to allow said transceivers to distinguish between said retroreflectors;

said transceivers inducing servomechanism means of pointing to said light sources.

4. A system as claimed in claim 1 wherein said means of collecting the retroreflected light consists of multi-mode optical fibers.

5. A system as in claim 1 wherein said probe is hand-held.

6. A system as in claim 1 wherein said probe is moved by a computer controlled machine.

7. A system as in claim 1 wherein the said probe tip is a sphere with an accurately known radius.