Method for measuring 3D coordinates of a spherically mounted retroreflector from multiple stations

• US 9,074,869 B2
• Filed: 12/11/2013
• Issued: 07/07/2015
• Est. Priority Date: 12/11/2013
• Status: Active Grant

• Current Assignee: Faro Technologies Incorporated
• Original Assignee: Faro Technologies Incorporated
• Inventors: Bridges, Robert E.
• Primary Examiner(s): Chowdhury, Tarifur
• Assistant Examiner(s): Gray, Sunghee Y
• Application Number:
  US14/102,954
• Publication Number:
  US 20140098383A1
• Time in Patent Office: 573 Days
• Field of Search: 356614-624
• US Class Current: 1/1
• CPC Class Codes: G01B 11/002 for measuring two or more coordinates
• 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 17/87 Combinations of systems using electromagnetic waves other than radio waves
• CPC Class Codes: G01S 7/497 Means for monitoring or calibrating

Abstract

A method of measuring spherically mounted retroreflector (SMR) with a 3D coordinate measurement device such as a laser tracker. The SMR includes an open-air cube corner retroreflector having a vertex point located near a sphere center of the SMR. Measurements of the SMR to the vertex point are corrected to indicate 3D coordinates of the SMR sphere center by accounting for SMR depth error and SMR runout error.

Claims

1. A method of measuring a spherically mounted retroreflector (SMR) with a three-dimensional (3D) coordinate measurement device from two stations, the method comprising the steps of:

providing the 3D coordinate measurement device including a base, a first motor, a second motor, a first angle measuring device, a second angle measuring device, a distance meter, the device configured to send an emitted beam of light to the SMR, the retroreflector returning a portion of the emitted beam as a reflected beam, the first motor and the second motor together directing the emitted beam in an emitted direction, the emitted direction determined by a first angle of rotation about a first axis and a second angle of rotation about a second axis, the first angle of rotation produced by the first motor and the second angle of rotation produced by the second motor, the rotation of the first axis and the second axis made with respect to the base, the first angle measuring device measuring the first angle of rotation and the second angle measuring device measuring the second angle of rotation, the distance meter measuring a target distance from the device to the retroreflector based at least in part on a reflected portion of the reflected beam received by a device;

providing a processor and a memory;

providing computer readable media having computer readable instructions which when executed by the processor determines 3D coordinates of a retroreflector, converts 3D coordinates of a vertex point into 3D coordinates of a sphere center and provides a mathematical transformation between frames of reference;

providing a first target retroreflector;

providing a second target retroreflector;

providing a third target retroreflector;

providing the SMR, the SMR including a body and an embedded retroreflector, the SMR including a reference point, the body having a spherical exterior portion that has a sphere center, the body containing a cavity, the cavity sized to hold the embedded retroreflector, the cavity open to a region outside the body, the embedded retroreflector at least partially disposed in the cavity, the embedded retroreflector being an open-air cube-corner retroreflector, the embedded retroreflector having a set of three mutually perpendicular planar reflectors that intersect in a set of three lines and in the vertex point, the cavity including an air-filled region interior to reflecting surfaces of the set of three planar reflectors, the embedded retroreflector having an axis of symmetry relative to the set of three lines, the SMR having an SMR error vector extending from the vertex point to the sphere center;

providing a nest configured to repeatably receive the spherical exterior portion;

determining the SMR error vector;

storing a numerical representation of the SMR error vector;

reading by the processor the numerical representation of the SMR error vector;

placing the device at a first station of the two stations, the device having at the first station a first device frame of reference, the first device frame of reference being given with respect to the base of the device;

measuring with the device a first set of 3D coordinates of the first target retroreflector;

measuring with the device a second set of 3D coordinates of the second target retroreflector;

measuring with the device a third set of 3D coordinates of the third target retroreflector;

placing the SMR in the nest;

sending a first emitted beam from the device to the SMR;

aligning the axis of symmetry of the SMR to the first emitted beam;

orienting the SMR in the nest in a first orientation based at least in part on a first rule, the first rule being a rule for orienting the SMR in the nest, the first orientation being indicative of a position of the reference point relative to the axis of symmetry in the first device frame of reference;

measuring the SMR with the device in the first station to obtain a first target distance and a first set of two target angles;

executing by the processor the computer readable instructions, the computer readable instructions performing calculations to determine first 3D coordinates of the vertex point in the first frame of reference and to convert the first 3D coordinates of the vertex point into first 3D coordinates of the sphere center, the calculations based at least in part on the first target distance, the first set of two target angles, and the numerical representation of the SMR error vector;

placing the device at a second station of the two stations, the second station having a second device frame of reference;

measuring with the device a fourth set of 3D coordinates of the first target retroreflector;

measuring with the device a fifth set of 3D coordinates of the second target retroreflector;

measuring with the device a sixth set of 3D coordinates of the third target retroreflector;

executing by the processor the computer readable instructions, the computer readable instructions performing calculations to determine a transformation matrix, the transformation matrix transforming a vector in the first frame of reference into a vector in the second frame of reference, the transformation matrix based at least in part on the first set of 3D coordinates, the second set of 3D coordinates, the third set of 3D coordinates, the fourth set of 3D coordinates, the fifth set of 3D coordinates, and the sixth set of 3D coordinates;

executing by the processor the computer readable instructions, the computer readable instructions transforming the numerical representation of the error vector in the first frame of reference into a numerical representation of the error vector in the second frame of reference, the error vector in the second frame of reference based at least in part on the transformation matrix, the numerical representation of the error vector in the first frame of reference, and the preferred orientation;

measuring the SMR with the device in the second station to obtain a second target distance and a second set of two target angles;

executing by the processor the computer readable instructions, the computer readable instructions performing calculations to determine second 3D coordinates of the vertex point in the second frame of reference and to convert the second 3D coordinates of the vertex point into second 3D coordinates of the sphere center, the calculations based at least in part on the second target distance, the second set of two target angles, and the numerical representation of the SMR error vector in the second frame of reference;

storing the first 3D coordinates of the sphere center; and

storing the second 3D coordinates of the sphere center.