Spherically mounted retroreflector and method to minimize measurement error

• US 9,347,767 B2
• Filed: 12/11/2013
• Issued: 05/24/2016
• 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): Hellner, Mark
• Application Number:
  US14/102,877
• Publication Number:
  US 20150285615A1
• Time in Patent Office: 895 Days
• Field of Search: 356/4.01
• 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 17/08 for measuring distance only indirect measurement G01S17/46; active triangulation systems G01S17/48
• 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 7/481 Constructional features, e.g. arrangements of optical elements
• CPC Class Codes: G01S 7/4863 Detector arrays, e.g. charge-transfer gates
• CPC Class Codes: G01S 7/497 Means for monitoring or calibrating
• CPC Class Codes: G02B 5/122 cube corner, trihedral or triple reflector type

Abstract

A spherically mounted retroreflector (SMR) having a reference point placed on a body of the SMR in a fixed and predetermined relationship to a runout error vector as given in a manufacturer'"'"'s data sheet. A method for aligning the reference point to minimize measurement error.

Claims

1. A spherically mounted retroreflector (SMR) comprising a body and a retroreflector, the SMR including a reference point, the reference point placed on the SMR, 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 a runout plane perpendicular to the axis of symmetry and passing through the sphere center, the SMR having an intersection point, the intersection point being a point of intersection of the axis of symmetry with the runout plane, the SMR having a runout error vector component, the runout error vector component being a vector that extends from the intersection point to the sphere center, the SMR having a reference plane that includes the reference point and the axis of symmetry, there being a reference ray coincident with a line of intersection between the reference plane and the runout plane, the reference ray beginning at the intersection point and lying in a half of the reference plane that includes the reference point, the runout error vector component having a runout reference angle, the runout reference angle being an angle between the reference ray and the runout error vector component, wherein the reference point is placed on the SMR at a location that gives the runout reference angle a preferred and predetermined value, the preferred and predetermined value given in a manufacturer data sheet.

2. The SMR of claim 1 wherein the reference point is placed on the SMR to give the runout reference angle the preferred and predetermined value of zero degrees.

3. The SMR of claim 1 wherein the reference point is placed on the SMR to give the runout reference angle the preferred and predetermined value of 180 degrees.

4. The SMR of claim 1 wherein the reference point is placed on the SMR to give the runout reference angle the preferred and predetermined value of +90 degrees or −

90 degrees.

5. A method of measuring a spherically mounted retroreflector (SMR) with a device, the method comprising the steps of:

providing a first SMR, the first SMR including a first body and a first retroreflector, the first SMR including a first reference point, the first reference point placed on the SMR, the first body having a first spherical exterior portion that has a first sphere center and a first sphere radius, the first body containing a first cavity, the first cavity sized to hold the first retroreflector, the first cavity open to a region outside the first body, the first retroreflector at least partially disposed in the first cavity, the first retroreflector being an open-air cube-corner retroreflector, the first retroreflector having a first set of three mutually perpendicular planar reflectors that intersect in a first set of three lines and in a common first vertex point, the first cavity including an air-filled region interior to reflecting surfaces of the first set of three planar reflectors, the first retroreflector having a first axis of symmetry relative to the first set of three lines, the first SMR having a first SMR runout plane perpendicular to the first axis of symmetry and passing through the first sphere center, the first SMR having a first SMR intersection point, the first SMR intersection point being a point of intersection of the first axis of symmetry with the first SMR runout plane, the first SMR having a runout error vector component, the first SMR runout error vector component being a vector that extends from the first SMR intersection point to the first sphere center, the first SMR having a first SMR reference plane that includes the first reference point and the first axis of symmetry, there being a first SMR reference ray coincident with a line of intersection between the first SMR reference plane and the first SMR runout plane, the first SMR reference ray beginning at the first SMR intersection point and lying in a half of the first SMR reference plane that includes the first reference point, the first SMR runout error vector component having a first SMR runout reference angle, the first SMR runout reference angle being an angle between the first SMR reference ray and the first SMR runout error vector component, wherein the first reference point is placed on the first SMR at a location that gives the first SMR runout reference angle a preferred and predetermined value, the preferred and predetermined value given in a manufacturer data sheet;

providing the device, wherein the device has a device frame of reference fixed with respect to a base of the device and a light source that emits a first beam of light, the device being configured to measure a target distance and two target angles from the device to the first vertex point along a first beam direction, the two target angles given with respect to the base;

providing a processor and a memory;

providing computer readable media having computer readable instructions which when executed by the processor calculates first three-dimensional (3D) coordinates of the first sphere center in the device frame of reference;

performing a first measurement, the first measurement including steps A through I;

A) aligning by the operator the first axis of symmetry to the first beam direction;

B) providing a rule for orienting the first SMR to obtain a first preferred orientation of the first SMR, the rule based on a first dimensional quantity of interest, the first dimensional quantity of interest given with respect to a first measurement line;

C) determining, by the operator, the first preferred orientation of the first SMR based at least in part on the rule;

D) rotating by the operator the first SMR about the first axis of symmetry to obtain the first preferred orientation of the first SMR;

E) sending the first beam of light from the light source to the first vertex point in a first beam direction and in response receiving at the device a first reflected light;

F) measuring, from the device to the first vertex point, a first target distance and a first set of the two target angles based at least in part on the first reflected light, the first target distance further based at least in part on a speed of light over a path traveled by the first beam of light;

G) determining with the processor 3D coordinates of the first vertex point in the device frame of reference based at least in part on the first target distance and the first set of the two target angles;

H) executing by the processor the computer readable instructions to determine the 3D coordinates of the first sphere center, the 3D coordinates of the first sphere center based at least in part on the 3D coordinates of the first vertex point; and

I) storing the 3D coordinates of the first sphere center.

6. The method of claim 5 wherein, in the step of providing a first SMR, the reference point is placed on the SMR to give the runout reference angle the preferred and predetermined value of zero degrees.

7. The method of claim 5 wherein, in the step of providing a first SMR, the reference point is placed on the SMR to give the runout reference angle the preferred and predetermined value of 180 degrees.

8. The method of claim 5 wherein, in the step of providing a first SMR, the reference point is placed on the SMR to give the runout reference angle the preferred and predetermined value of +90 degrees or −

90 degrees.

9. The method of claim 5 wherein, in the step of providing a rule for orienting the first SMR to obtain a first preferred orientation of the first SMR, the first dimensional quantity of interest is a first length between two points on the first measurement line, and the rule is to make the first SMR reference ray perpendicular to the first measurement line.

10. The method of claim 5 wherein, in the step of providing a rule for orienting the first SMR to obtain a first preferred orientation of the first SMR, the first dimensional quantity of interest is first 3D coordinates along a direction perpendicular to the measurement line, the first 3D coordinates given in the device frame of reference.