Method for determining position within the measuring volume of a coordinate measuring machine and the like and system therefor

• US 4,945,501 A
• Filed: 04/12/1989
• Issued: 07/31/1990
• Est. Priority Date: 01/20/1987
• Status: Expired due to Term

• Current Assignee: Giddings & Lewis Machine Tools LLC (MAG IAS Holdings Incorporated)
• Original Assignee: Warner & Swasey Company
• Inventors: Gale, Michael T., Brown, Stephen N., Bell, Frederick K.
• Primary Examiner(s): Lall, Parshotam S.
• Assistant Examiner(s): Dixon, Joseph L.
• Application Number:
  US07/338,378
• Time in Patent Office: 475 Days
• Field of Search: 364/571.05, 364/571.04, 364/167.01, 364/164, 364/474.35, 364/550, 364/551.02, 364/559-563, 33/503, 33/505, 33/545, 33/546, 33/553-558, 318/632
• US Class Current: 702/95
• CPC Class Codes: G01B 21/045 Correction of measurements G01B9/02055 takes precedence
• CPC Class Codes: G01B 7/008 using coordinate measuring machines

Abstract

Axis correction or machine charateristic data is stored (108) in memory (110) within a control or measurement processor (36) of a CMM (10) (FIG. 8). The axis correction data includes error data representing 21 different geometry errors (FIG. 4A through 4C) of the CMM. As position information is required, the measurement processor captures the actual position signal from each machine axis sensor and uses this position signal to locate (112) a set of the axis correction data which describes the machine characteristics at that position. The measurement processor then mathematically combines (114) (FIG. 9) the position signal with the subset of axis correction data to obtain a corrected position signal having a corrected set of cooridinate values which describe the probe position in a true Cartesian coordinate system.

Claims

1. A method for determining the absolute position of a point in the measuring volume (102) of a CMM (10) having a probe shaft (22) and a probe (14) supported on a distal end of the probe shaft, the probe shaft having three axes of motion (X, Y, Z), the CMM including devices for providing a position signal in determining the position of the probe for each of the axes of motion in a coordinate frame of the CMM, and control means (36) for receiving the position signals and for providing a coordinate measurement signal which identifies the position of the probe, the method including the steps of storing (108) sets of axis correction data to compensate for CMM geometry errors for a range of travel in each of the axes;

and determining (112) a subset of the axis correction data for a given position of the probe shaft relative to each of the axes of motion as a function of the position signals;

wherein the improvement comprises;

the step of storing (108) includes storing the axis correction data in storage means of a measurement processor included in the control means;

the step of determining includes determining compensated rotation data (48 through

     64) representing the amount of angular rotation about each of the axes for each of the axes;

squareness data (84, 94,

     96) representing the orthogonality of three different pairs of the axes;

normalized straightness data (72 through

     82) for each of said axes with respect to each of the other axes; and

scale error data (66, 68,

     70) representing error in each of the devices; and

wherein the improvement further comprises the steps of;

measuring the position of an artifact with the CMM by moving the probe of the CMM so that each of the devices provides an artifact position signal and the measurement processor provides a corresponding measurement signal as a function of the artifact position signals;

calculating probe offset data (98) as a function of the measured position of the artifact, the probe offset data representing the offset of the probe from the probe shaft along each of the axes;

storing the probe offset data in the storage means of the measurement processor;

retrieving the probe offset data from the storage means;

determining compensated probe offset data as a function of the probe offset data and respective compensated rotation data; and

modifying (114) the position signals from the devices as a function of the compensated probe offset data and the subset of axis correction data including the compensated rotation data, the squareness data, the normalized straightness data and the scale error data, the step of modifying being performed immediately after the step of determining (112) the subset of axis correction data to obtain a corrected position signal in a standard coordinate frame.

2. The method as claimed in claim 1 wherein said corrected position signal has a component representing a corrected position along each of the axes and wherein each of said components is a function (FIG. 9) of the product of its respective compensated rotation data and the position signal of one of the other axes.

3. The method as claimed in claim 2 wherein at least one of said components is a function (FIG. 9) of the product of its respective squareness data and the position signal of one of the other axes.

4. The method as claimed in claim 3 wherein each of said components is a function of the normalized straightness data in its respective axis.

5. The method as claimed in claim 4 wherein each of said components is a function of the scale error data for its respective axis.

6. The method as claimed in claim 1 wherein the artifact comprises a calibration sphere (100) and wherein the step of measuring includes the step of coupling he probe to a plurality of positions on the sphere.

7. A system for determining the absolute position of a point in a measuring volume (102) of a CMM (10) having a probe shaft (22) and a probe (14) supported on a distal end of the probe shaft, the probe shaft having three axes of motion (X, Y, Z), the CMM including devices for providing a position signal in determining the position of the probe for each of the axes of motion in a coordinate frame of the CMM, and control means (36) for receiving each of the position signals and for providing a coordinate measurement signal which identifies the position of the probe, the system including storage means (110) for storing sets of axis correction data to compensate for CMM geometry errors for a range of travel in each of the axes;

means for determining (36 and

     112) a subset of the axis correction data for a given position of the probe shaft relative to each of the axes of motion as a function of the position signals; and

an artifact, wherein the improvement comprises;

said means for determining determines compensated rotation data (48 through

     64) representing the amount of angular rotation about each of the axes for each of the axes;

squareness data (84, 94,

     96) representing the orthogonality of three different pairs of the axes;

normalizes straightness data (72 through

     82) for each of said axes with respect to each of the other axes; and

scale error data (66, 68,

     70) representing error in each of the devices;

each of the devices provides an artifact position signal upon moving the probe of the CMM to the artifact;

the control means of the CMM includes a measurement processor which provides a corresponding measurement signal as a function of the artifact position signals; and

calculating means for calculating probe offset data as a function of the measured position of the artifact, the probe offset data representing the offset of the probe from the probe shaft along each of the axes;

wherein the storage means stores the probe offset data; and

wherein the measurement processor determines compensated probe offset data as a function of the probe offset data and its respective compensated rotation data, and the measurement processor modifies the position signals from the devices as a function of the compensated probe offset data and the subset of axis correction data including the compensated rotation data, the squareness data, the normalized straightness data and the scale error data to obtain a corrected position signal in a standard coordinate frame.

8. The system as claimed in claim 7 wherein the artifact comprises a calibration sphere (100).