Automatic measurement of dimensional data with a laser tracker

• US 8,467,071 B2
• Filed: 01/14/2013
• Issued: 06/18/2013
• Est. Priority Date: 04/21/2010
• Status: Active Grant

• Current Assignee: Faro Technologies Incorporated
• Original Assignee: Faro Technologies Incorporated
• Inventors: Steffey, Kenneth, Steffensen, Nils P., Bridges, Robert E.
• Primary Examiner(s): NGUYEN, TU T
• Application Number:
  US13/740,709
• Publication Number:
  US 20130128284A1
• Time in Patent Office: 155 Days
• Field of Search: 356/614
• US Class Current: 356/614
• CPC Class Codes: G01B 11/002 for measuring two or more coordinates
• CPC Class Codes: G01B 11/14 for measuring distance or clearance between spaced objects or spaced apertures G01B11/26 takes precedence; rangefinders G01C3/00
• CPC Class Codes: G01B 21/045 Correction of measurements G01B9/02055 takes precedence
• CPC Class Codes: G01C 15/002 Active optical surveying means optical plumbing G01C15/105
• 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 17/89 for mapping or imaging
• CPC Class Codes: G01S 7/4808 Evaluating distance, position or velocity data
• CPC Class Codes: G01S 7/481 Constructional features, e.g. arrangements of optical elements
• CPC Class Codes: G01S 7/4811 common to transmitter and receiver
• CPC Class Codes: G01S 7/4813 Housing arrangements
• CPC Class Codes: G01S 7/4815 using multiple transmitters
• CPC Class Codes: G01S 7/4972 Alignment of sensor

Abstract

Measuring with a system having retroreflector targets and a laser tracker includes storing a list of nominal coordinates for three targets and at least one added point; capturing on a photosensitive array of the tracker a portion of the light emitted by a light beam and reflected off the three targets; obtaining spot positions on a photosensitive array of a tracker camera from light reflected off the three targets; determining a correspondence between three spot positions on the tracker photosensitive array and the nominal coordinates of the three targets; directing a beam of light from the tracker to the three targets based at least in part on the nominal coordinates of the first target and the first spot position; measuring 3-D coordinates of the three targets with the tracker; determining 3-D coordinates of the at least one added point based at least in part on the measured 3-D coordinates of the three targets and the nominal coordinates of the at least one added point.

Claims

1. A method for measuring with a system, the method comprising steps of:

providing the system including a collection of retroreflector targets and a laser tracker, the collection of retroreflector targets including at least three non-collinear retroreflector targets, the at least three non-collinear retroreflector targets including a first target, a second target, and a third target, the laser tracker in a first frame of reference fixed with respect to tracker surroundings, the laser tracker having a structure, a first light source, an absolute distance meter, a first angular transducer, a second angular transducer, a tracking system, a first camera, a second light source, and a processor, the structure rotatable about a first axis and a second axis, the first light source producing a first light beam that cooperates with the absolute distance meter, the first angular transducer measuring a first angle of rotation about the first axis, the second angular transducer measuring a second angle of rotation about the second axis, the tracking system configured to move the first light beam to a center of any retroreflector target from among the collection of retroreflector targets, the first camera including a first lens system and a first photosensitive array, the second light source providing a second light beam, and the processor configured to operate the laser tracker;

storing a list of nominal coordinates for the first target, the second target, the third target, and at least one additional point, the nominal coordinates being three-dimensional coordinates in a second frame of reference;

capturing on the first photosensitive array a portion of the light emitted by the second light beam and reflected off the first target, the second target, and the third target;

obtaining spot positions on the photosensitive array from the portion of light reflected off each of the first target, second target, and the third target;

determining a correspondence between a first spot position, a second spot position, and a third spot position on the first photosensitive array and the nominal coordinates of the first target, the second target, and the third target, respectively;

directing the first beam of light to the first target based at least in part on the nominal coordinates of the first target and the first spot position;

measuring three-dimensional coordinates of the first target using the absolute distance meter, the first angular transducer, and the second angular transducer;

directing the first beam of light to the second target based at least in part on the nominal coordinates of the second target and the second spot position;

measuring three-dimensional coordinates of the second target using the absolute distance meter, the first angular transducer, and the second angular transducer;

directing the first beam of light to the third target based at least in part on the nominal coordinates of the third target and the third spot position;

measuring three-dimensional coordinates of the third target using the absolute distance meter, the first angular transducer, and the second angular transducer;

determining three-dimensional coordinates of the at least one additional point in the first frame of reference based at least in part on the measured three-dimensional coordinates of the first target, the second target, the third target, and the nominal coordinates of the at least one additional point;

storing the determined three-dimensional coordinates of the at least one additional point;

providing a maximum allowable discrepancy;

providing a coefficient of thermal expansion for an object under test;

providing a reference temperature;

placing a first reference retroreflector and a second reference retroreflector on the object under test, there being a first distance between the first reference retroreflector and the second reference retroreflector at the reference temperature;

measuring a temperature of the object under test;

calculating a first temperature difference by subtracting the reference temperature from the measured temperature of the object under test;

calculating a scale factor by multiplying the first temperature difference by the coefficient of thermal expansion;

measuring the three-dimensional coordinates of the first reference retroreflector using the absolute distance meter, the first angular transducer, and the second angular transducer;

measuring the three-dimensional coordinates of the second reference retroreflector using the absolute distance meter, the first angular transducer, and the second angular transducer;

calculating a second distance extending from the measured three-dimensional coordinates of the first reference retroreflector to the measured three-dimensional coordinates of the second reference retroreflector;

calculating a third distance by subtracting the first distance from the second distance;

calculating a fourth distance by multiplying the scale factor by the first distance;

calculating a discrepancy value by subtracting the third distance from the fourth distance; and

taking an action when the discrepancy value exceeds the maximum allowable discrepancy, the action being either issuing an alarm or measuring the three-dimensional coordinates at least some retroreflector targets from among the collection of retroreflector targets and, from this data, re-establishing a frame of reference for an object under test.