Method for finding a home reference distance using a spherically mounted retroreflector

Patent metadata

Field	Value
Publication number	US-9423492-B2
Application number	US-201314102921-A
Country	US
Kind code	B2
Filing date	Dec 11, 2013
Priority date	Dec 11, 2013
Publication date	Aug 23, 2016
Grant date	Aug 23, 2016

Abstract

A method of finding a home reference distance of a 3D coordinate measurement device in which a mathematical adjustment is made to move the vertex point to the sphere center of a spherically mounted retroreflector (SMR).

First claim

What is claimed is:

1. A method for determining a home reference distance, the method comprising:
providing a spherically mounted retroreflector (SMR), the SMR including a body and a retroreflector, the SMR including a reference point, the body having a spherical exterior portion that has a sphere center and a sphere radius, the body containing a cavity, the cavity sized to hold the retroreflector, the cavity open to a region outside the body, the retroreflector at least partially disposed in the cavity, the retroreflector being an open-air cube-corner retroreflector, the retroreflector having a set of three mutually perpendicular planar reflectors that intersect in a set of three lines and in a common vertex point, the cavity including an air-filled region interior to reflecting surfaces of the set of three planar reflectors, the retroreflector having an axis of symmetry relative to the set of three lines, the SMR having an SMR runout plane perpendicular to the axis of symmetry and passing through the sphere center, the SMR having an SMR intersection point, the SMR intersection point being a point of intersection of the axis of symmetry with the SMR runout plane, the SMR having an error vector extending from the vertex point to the sphere center, the SMR error vector having an SMR depth error vector component and an SMR runout error vector component, the SMR depth error vector component being a vector that extends from the vertex point to the SMR intersection point, the SMR runout error vector component being a vector that extends from the SMR intersection point to the sphere center, the SMR depth error vector component having a magnitude equal to an SMR depth error;
providing a three-dimensional (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, a home position nest, and a processor, the 3D coordinate measurement device having a device frame of reference, the 3D coordinate measurement 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 second axis having a rotation point lying on a line coinciding with the emitted beam, 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 3D coordinate measurement device to the retroreflector based at least in part on the reflected beam received by the 3D coordinate measurement device, the home position nest being affixed to the base and being configured to receive the spherical exterior portion;
providing a second nest and a third nest, the second nest and the third nest configured to repeatably receive the spherical exterior portion, the second nest having a second nest point corresponding to the sphere center when the SMR is in the second nest, the third nest having a third nest point corresponding to the sphere center when the SMR is in the third nest, a distance between the second nest point and the third nest point being a nest-to-nest distance, the second nest and the third nest having a nest center line, the nest center line being a line passing through the second nest point and the third nest point, the nest center line having an interior portion and an exterior portion, the interior portion being that portion of the nest center line between the second nest point and the third nest point, the exterior portion being the nest center line excluding the interior portion;
providing a processor and a memory;
providing a non-transitory computer readable media having computer readable instructions which when executed by the processor determines the home reference distance;
determining the SMR depth error;
recording the SMR depth error as a recorded data set;
reading by the processor the recorded data set;
placing the SMR in the home position nest;
sending from the 3D coordinate measurement device to the SMR in the home position nest a first emitted beam;
measuring with the 3D coordinate measurement device a first target distance to the SMR in the home position nest;
placing the rotation point of the 3D coordinate measurement device on the interior portion;
placing the SMR in the second nest;
sending from the 3D coordinate measurement device to the SMR in the second nest a second emitted beam;
measuring with the 3D coordinate measurement device a second target distance to the SMR in the second nest;
placing the SMR in the third nest;
sending from the 3D coordinate measurement device to the SMR in the third nest a third emitted beam;
measuring with the 3D coordinate measurement device a third target distance to the SMR in the third nest;
executing by the processor the computer readable instructions, the computer readable instructions performing calculations to determine the home reference distance, the home reference distance being a distance from the rotation point to the sphere center of the SMR in the home position nest, the home reference distance based at least in part on the nest-to-nest distance, the first target distance, the second target distance, the third target distance, and the SMR depth error; and
storing the home reference distance.

2. The method of claim 1 wherein:
the method further includes providing for the 3D coordinate measurement device a preferred home angle, the preferred home angle being an angle between the axis of symmetry and the first emitted beam;
the step of placing the SMR in the home position nest further includes placing the SMR into the home position nest at the preferred home angle;
the step of placing the SMR in the second nest further includes aligning the axis of symmetry to the second emitted beam; and
the step of placing the SMR in the third nest further includes aligning the axis of symmetry to the third emitted beam.

3. The method of claim 2 wherein:
the method further includes determining the SMR runout error vector component; and
the step of recording the SMR depth error as a recorded data set further includes recording a numerical representation of the SMR runout error vector component in the recorded data set.

4. The method of claim 3 wherein:
in the step of placing the SMR in the home position nest, the SMR is placed in a home orientation based at least in part on a home rule, the home rule being a rule for orienting the SMR in the home position nest, the home orientation being indicative of a position of the reference point relative to the axis of symmetry in the device frame of reference; and
in the step of executing by the processor the computer readable instructions, the home reference distance is further based on the numerical representation of the SMR runout error vector component, the home orientation, and the preferred home angle.

5. The method of claim 3 wherein:
in the step of providing a three-dimensional (3D) coordinate measurement device, the 3D coordinate measurement device further includes a camera, the camera having a photosensitive array and a lens, the lens having a focal length, the camera having a first distance between the lens and the photosensitive array, the first distance being selected, based at least in part on the focal length, to bring a mark on the SMR into focus on a first image when the SMR is p

Assignees

• Faro Tech Inc

Inventors

• Bridges Robert E

Classifications

• G01B11/002

  for measuring two or more coordinates · CPC title

• G01S7/4972 Primary

  Alignment of sensor · CPC title

• G01S17/08

  for measuring distance only (indirect measurement G01S17/46; active triangulation systems G01S17/48) · CPC title

• G01C15/06

  Surveyors' staffs; Movable markers · CPC title

• G01S17/66

  Tracking systems using electromagnetic waves other than radio waves · CPC title