Method for calibration of coordinate measuring machine

• US 4,939,678 A
• Filed: 05/12/1988
• Issued: 07/03/1990
• Est. Priority Date: 11/19/1987
• Status: Expired due to Fees

• Current Assignee: Hexagon Metrology AB (Hexagon AB)
• Original Assignee: Brown & Sharpe Manufacturing Company (BNS Holding Incorporated)
• Inventors: Beckwith, Walter L. Jr.
• Primary Examiner(s): Teska, Kevin J.
• Application Number:
  US07/193,069
• Time in Patent Office: 782 Days
• Field of Search: 356/375, 356/363, 356/152, 356/358, 356/356, 356/353, 356/239, 364/513, 364/167.01, 364/571.01, 364/571.02, 364/571.04, 364/571.05, 364/474.35, 364/474.37, 33/1 M, 33/503, 33/505, 33/556, 33/504, 318/480
• US Class Current: 702/95
• CPC Class Codes: G01B 11/005 coordinate measuring machines
• CPC Class Codes: G01B 11/306 for measuring evenness

Abstract

Methods for determining nonrigid error parameters that result from deflection or deformation of machine elements and correcting scale readings in a coordinate measuring machine. The nonrigid error parameters are a function of probe position along two directions. In a bridge type coordinate measuring machine, the y-direction error parameters are a function of probe position in both x and y directions and thus exhibit nonrigid behavior. Nonrigid error parameters are measured by making error measurements at a plurality of corresponding points along two parallel, spaced-apart measurement lines. The error measurements are substituted into simultaneous equations for the error parameters, and the equations are solved for the coefficients. The coefficients are used to determine the axial errors at any point in the measuring volume. The axial errors are subtracted from the scale readings.

Claims

1. In a coordinate measuring machine including a first element and a second element, said first element being movable relative to said second element, said coordinate measuring machine further including scales for monitoring the position of the first element relative to the second element and providing scale readings, and computer means for storing said scale readings, said coordinate measuring machine being subject to errors in said scale readings, said errors being characterized by a set of error parameters including rigid error parameters which are a function only of relative position along an associated measurement direction and nonrigid error parameters which are a function of relative position along an associated measurement direction and of relative position along a second direction, a method for providing corrected scale readings of a workpiece, comprising the steps of:

measuring each of said rigid error parameters at point along a line that is parallel to the measurement direction, the step of measuring each of said rigid error parameters being performed by a calibration apparatus;

measuring each of said nonrigid error parameters at corresponding sets of n points along n spaced-apart lines that are parallel to the measurement direction, each set of n points lying on a line parallel to said second direction to provide sets of nonrigid errors parameter measurements, the step of measuring each of said nonrigid error parameters being performed by said calibration apparatus;

substituting each of said sets of nonrigid error parameter measurements and corresponding first element positions into n simultaneous equations having n coefficients, said equations describing the relationship between said nonrigid error parameters and the position of the first element along said second direction, the step of substituting each of the sets of nonrigid error parameter measurements and corresponding first element positions being performed by said computer means;

for each of said sets of nonrigid error parameter measurements, solving the associated n simultaneous equations for said n coefficients, the step of solving the associated n simultaneous equations being performed by said computer means;

measuring a workpiece by bringing said first element into contact with the workpiece and storing scale readings in said computer means; and

using said rigid error parameters and said sets of n coefficients to correct said scale readings and provide corrected scale readings of the workpiece, the step of correcting said scale readings being performed by said computer means.

2. A method as defined in claim 1 wherein the step of using said rigid error parameters and said sets of n coefficients to correct said scale readings and provide corrected scale readings of the workpiece includes the steps of calculating said errors from said rigid error parameters and said sets of n coefficients, and subtracting said errors from said scale readings, said steps of calculating and subtracting being performed by said computer means.

3. A method as defined in claim 1 wherein the step of measuring each of said nonrigid error parameters includes the step of measuring each of said nonrigid error parameters at corresponding pairs of points along two spaced-apart lines parallel to the measurement direction.

4. A method as defined in claim 3 wherein the step of measuring each of said nonrigid error parameters at corresponding pairs of points along two spaced-apart lines parallel to to measurement direction includes the steps of

positioning the first element at a plurality of points along a first line parallel to the measurement direction and measuring the nonrigid error parameter at each such point,

shifting the first element in the second direction to a second line parallel to the measurement direction, and

positioning the first element at a like plurality of points along the second line and measuring the nonrigid error parameter at each such point.

5. A method as defined in claim 1 wherein the step of measuring each of said nonrigid error parameters includes the steps of attaching a first measuring device of said calibration apparatus to the first element, attaching a second measuring device of said calibration apparatus to the second element and directing at least one laser beam between the first and second measuring devices for determining the position of the first element relative to the second element.

6. A method as defined in claim 5 wherein the step of measuring each of said nonrigid error parameters further includes the step of offsetting the first measuring device from the first element during measurement of the nonrigid error parameters along one of said lines.

7. In a coordinate measuring machine including a first element and a second element, said first element being movable relative to said second element, said coordinate measuring machine further including scales for monitoring the position of the first element relative to the second element and providing scale readings, and computer means for storing said scale readings, said coordinate measuring machine being subject to errors in said scale readings, said errors being characterized by a set of error parameters including rigid error parameters which are a function only of relative position along an associated measurement direction and nonrigid error parameters which are a function of relative position along an associated measurement direction and of relative position along a second direction, a method for providing corrected scale readings of a workpiece, comprising the steps of:

measuring each of said rigid error parameters at points along a line that is parallel to the measurement direction, the step of measuring each of said rigid error parameters being performed by a calibration apparatus;

measuring each of said nonrigid error parameters at corresponding sets of n points along n spaced-apart lines that are parallel to the measurement direction, each set of n points lying on a line parallel to said second direction to provide sets of nonrigid error parameter measurements, the step of measuring each of said nonrigid error parameters being performed by said calibration apparatus;

measuring a workpiece by bringing said first element into contact with the workpiece and storing scale readings in said computer means; and

using said rigid error parameters and said nonrigid error parameter measurements to correct said scale readings and provide corrected scale readings of the workpiece, the step of correcting said scale readings being performed by said computer means.

8. A method as defined in claim 7 wherein the step of measuring each of said nonrigid error parameters includes the step of measuring each of said nonrigid error parameters at corresponding pairs of points along two spaced-apart lines parallel to the measurement direction.

9. A method as defined in claim 8 wherein the step of measuring each of said nonrigid error parameters at corresponding pairs of points along two spaced-apart lines parallel to the measurement direction includes the steps of

positioning the first element at a plurality of points along a first line parallel to the measurement direction and measuring the nonrigid error parameter at each such point,

shifting the first element in the second direction to a second line parallel to the measurement direction, and

positioning the first element at a like plurality of points along the second line and measuring the nonrigid error parameter at each such point.

10. A method as defined in claim 7 wherein the step of measuring each of said nonrigid error parameters includes the steps of attaching a first measuring device of said calibration apparatus to the first element, attaching a second measuring device of said calibration apparatus to the second element and directing at least one laser beam between the first and second measuring devices for determining the position of the first element relative to the second element.

11. A method as defined in claim 10 wherein the step of measuring each of said nonrigid error parameters further includes the step of offsetting the first measuring device from the first element during measurement of the nonrigid error parameters along one of said lines.