Method for correcting a spherically mounted retroreflector when resetting a distance meter

• US 9,121,689 B2
• Filed: 12/11/2013
• Issued: 09/01/2015
• 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): Punnoose, Roy M
• Application Number:
  US14/102,935
• Publication Number:
  US 20140313521A1
• Time in Patent Office: 629 Days
• Field of Search: 356/614
• US Class Current: 1/1
• CPC Class Codes: G01B 11/002 for measuring two or more coordinates
• CPC Class Codes: G01C 15/002 Active optical surveying means optical plumbing G01C15/105
• 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 method of correcting centering errors of a spherically mounted retroreflector (SMR) when the distance meter of a 3D coordinate measurement device is reset to a home reference distance.

Claims

1. A method for resetting a distance meter in a three-dimensional (3D) coordinate measurement device when measuring a spherically mounted retroreflector (SMR) in a home position nest, the method comprising:

providing the 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, 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 a 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 a 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 the 3D coordinate measurement device including a base, a first motor, a second motor, a first angle measuring device, a second angle measuring device, the distance meter, the home position nest, the device having a device frame of reference, 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 first axis and the second axis configured to rotate about a gimbal point, the rotation of the first axis and the second axis made with respect to the base, the gimbal 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 device to the retroreflector based at least in part on a reflected portion of the reflected beam received by a device, the home position nest being affixed to the base and being configured to receive the spherical exterior portion;

providing a processor and a memory;

providing computer readable media having computer readable instructions which when executed by the processor determines a reset distance;

determining a home reference distance, the home reference distance being a distance from the gimbal point to the sphere center of the SMR in the home position nest;

determining the SMR depth error;

recording the home reference distance and 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 device to the SMR in the home position nest a first emitted beam;

executing by the processor the computer readable instructions to determine a reset distance based at least in part on the home reference distance and the SMR depth error; and

resetting the distance meter to read a distance equal to the reset distance.

2. The method of claim 1 wherein:

the method further includes providing for the device a preferred home angle, the preferred home angle being an angle between the axis of symmetry and the first emitted beam; and

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.

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 the 3D coordinate measurement device, the 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 placed in the home position nest, the first image being an image on the photosensitive array;

the method further includes determining a home orientation of the SMR based at least in part on the first image, 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.

6. The method of claim 5 wherein:

in the step of providing the SMR, the SMR further includes a barcode, the barcode configured to hold SMR information, the SMR information being at least one of an SMR compensation parameter or an SMR serial number; and

the method further includes reading the barcode with the camera and extracting the SMR information.

7. A system for measuring three-dimensional (3D) coordinates of a spherically mounted retroreflector (SMR) with a coordinate measurement device, the system comprising:

the SMR including a body and a retroreflector, the body having a spherical exterior portion that has a sphere center, 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 SMR including a barcode, the barcode configured to hold SMR information, the SMR information being at least one of an SMR compensation parameter or an SMR serial number;

the 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 position detector, a control system, a home position nest, a camera, and a processor, the coordinate measurement device configured to send a first beam of light to the SMR, the retroreflector target returning a portion of the first beam as a second beam;

the first motor and the second motor together directing the first beam of light to a first direction, the first 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 first axis and the second axis configured to rotate 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 first distance from the coordinate measurement device to the retroreflector target based at least in part on a first portion of the second beam received by a first optical detector;

the position detector receiving a second portion of the second beam, the position detector configured to produce a first signal in response to a position of the second portion on the position detector;

the control system sending a second signal to the first motor and a third signal to the second motor, the second signal and the third signal based at least in part on the first signal, the control system configured to adjust the first direction of the first beam to track the SMR;

the home position nest being affixed to the base and being configured to hold the SMR;

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 camera being configured to receive a first image and in response to provide a camera electrical signal to the processor, the first distance being selected, based at least in part on the focal length, to bring the barcode on the SMR into focus on the first image when the SMR is placed in the home position nest, the camera having sufficient resolution to unambiguously resolve the barcode; and

the processor configured to process the image of the barcode to extract the SMR information based at least in part on the camera electrical signal and to provide three-dimensional coordinates of the retroreflector target, the three-dimensional coordinates based at least in part on the first distance, the first angle of rotation, the second angle of rotation, and the SMR information.