Method for automatic compensation of probe offset in a coordinate measuring machine

Claim 2

In a machine for measuring coordinates of a generally planar surface on a workplace by contacting points with a probe having a prescribed diameter and continuously sampling and recording the coordinates of the probe, a method for automatically compensating the planar surface being measured for probe offset comprising the steps of: (a) moving the probe toward the planar surface and measuring the coordinates of a direction point D just before contact between the probe and the planar surface; (b) moving the probe along the planar surface and measuring the coordinates of at least three contact points C 1 -C N with the probe and the planar surface in contact; (c) fitting by electronic data processing means a plane S to the contact points C 1 -C N , the plane S being defined by a normal vector V n passing through the first contact point C 1 and a coordinate of the intersection between plane S and vector V n ; (d) calculating by electronic data processing means a directional vector V d between the first contact point C 1 and direction point D; (e) calculating by electronic data processing means an angle A between the normal vector V n and the directional vector V d , and when the angle A is greater than 90°, reversing the direction of the normal vector V n ; (f) when the angle A is in the range between the predetermined range of acceptance and 90°, rejecting the direction point D as invalid and repeating the compensation procedure; and (g) when the angle A is within a predetermined range of acceptance, moving the plane S by electronic data processing means by the dimension of the probe offset in a direction opposite the direction of the normal vector V n so that the plane S coincides with the planar surface, thereby compensating for probe offset.

Claim 3

A method for probe offset compensation as defined in claim 1 wherein the step of measuring the coordinates of direction point D includes the steps of measuring the coordinates of two sample points just before contact between the probe and the planar surface and averaging the coordinates of the two sample points.

Claim 4

A method for probe offset compensation as defined in claim 1 wherein the step of fitting a plane S to the contact points C 1 -C N includes the step of utilizing a least squares fit algorithm to calculate a definition of the plane S.

Claim 5

A method for probe offset compensation as defined in claim 1 wherein the predetermined range of acceptance is about ±88 degrees relative to the normal vector V n .

Claim 6

A method for probe offset compensation as defined in claim 1 further including the steps of defining a plane S max parallel to plane S and passing through the contact point determined by electronic data processing means to have maximum deviation from plane S in the direction of the normal vector V n , defining a plane S min parallel to plane S and passing through the contact point determined by electronic data processing means to have maximum deviation from plane S in the direction opposite to the normal vector V n , and rejecting the direction point D as invalid it is located between the planes S max and S min and repeating the compensation procedure.

Claim 7

A method for probe offset compensation as defined in claim 1 wherein the step of repeating the compensation procedure further includes the steps of measuring the coordinates of a new direction point D' approximately perpendicular to the last contact point C N by moving the probe off the surface in a direction approximately perpendicular to the contact point C N and taking a coordinate measurement, defining a new normal vector V n ' passing through the last contact point C N , calculating a new directional vector V d ' between the last contact point C N and the new direction point D', and repeating steps (e)-(g) with vectors V n and V d replaced by vectors V n ' and V d ', respectively.

Claim 8

A method for probe offset compensation as defined in claim 5 wherein the step of repeating the compensation procedure further includes the steps of measuring the coordinates of a new direction point D' approximately perpendicular to the last contact point C N by moving the probe off the surface in a direction approximately perpendicular to the contact point C N and taking a coordinate measurement, defining a new normal vector V n ' passing through last contact point C N , calculating a new directional vector V d ' between the last contact point C N and the new direction point D', and repeating steps (e)-(g) with the vectors V n and V d replaced by vectors V n ' and V d ', respectively.

Claim 9

In a machine for measuring coordinates of a curved surface on a workpiece by contacting points on the curved surface with a probe having a prescribed diameter and continuously sampling and recording the coordinates of the probe, a method for automatically compensating the curved surface being measured for probe offset comprising the steps of: (a) moving the probe toward the curved surface measuring the coordinates of a direction point D just before contact between the probe and the curved surface; (b) moving the probe along the curved surface and measuring the coordinates of at least three contact points C 1 -C N with the probe and the curved surface in contact; (c) fitting by electronic data processing means a curve S to the contact points C 1 -C N , the curve S being defined by a center O and a radius R; (d) calculating by electronic data processing means a tangential vector V t tangent to the curve S at the first contact point C 1 ; (e) calculating by electronic data processing means a directional vector V d between the first contact point C 1 and direction point D; (f) calculating by electronic data processing means an angle A between the tangential vector V t and the directional vector V d ; (g) when the angle A is outside the predetermined range of acceptance, rejecting the direction point D as invalid and repeating the compensation procedure; (h) when the angle A is within a predetermined range of acceptance and the distance between direction point D and the center O of curve S is greater than the radius R of curve S, subtracting the probe offset from the radius of the curve S by electronic data processing means so that the curve S coincides with the curved surface, thereby compensating for probe offset; and (i) when the angle A is within the predetermined range of acceptance and the distance between the direction point D and the center O of curve S is less than the radius R of curve S. adding the probe offset to the radius R of the curve S by electronic data processing means so that the curve S coincides with the curved surface, thereby compensating for probe offset.

Claim 10

In a machine for measuring coordinates of a curved surface on a workpiece by contacting points on the curved surface with a probe having a prescribed diameter and continuously sampling and recording the coordinates of the probe, a method for automatically compensating the curved surface being measured for probe offset comprising the steps of: (a) moving the probe toward the curved surface measuring the coordinates of a direction point D just before contact between the probe and the curved surface; (b) moving the probe along the curved surface and measuring the coordinates of at least three contact points C 1 -C N with the probe and the curved surface in contact; (c) fitting by electronic data processing means a curve S to the contact points C 1 -C N , the curve S being defined by a center O and a radius R; (d) calculating by electronic data processing means a tangential vector V t tangent to the curve S at the first contact point C 1 ; (e) calculating by electronic data processing means a directional vector V d between the first contact point C 1 and direction point D; (f) calculating by electronic data processing means an angle A between the tangential vector V t and the directional vector V d ; (g) when the angle A is outside the predetermined range of acceptance, rejecting the direction point D as invalid and repeating the compensation procedure; (h) when the angle A is within a predetermined range of acceptance and the distance between direction point D and the center O of curve S is greater than the radius R of curve S, subtracting the probe offset from the radius of the curve S by electronic data processing means so that the curve S coincides with the curved surface, thereby compensating for probe offset; and (i) when the angle A is within the predetermined range of acceptance and the distance between the direction point D and the center O of curve S is less than the radius R of curve S. adding the probe offset to the radius R of the curve S by electronic data processing means so that the curve S coincides with the curved surface, thereby compensating for probe offset.

Claim 11

A method for probe offset compensation as defined in claim 8 wherein the step of measuring the coordinates of direction point D includes the steps of measuring the coordinates of two sample points just before contact between the probe and the curved surface and averaging the coordinates of the two sample points.

Claim 12

A method for probe offset compensation as defined in claim 8 wherein the step of fitting a curve S to the contact points C 1 -C N includes the step of utilizing a least squares fit algorithm to calculate a definition of the curve S.

Claim 13

A method for probe offset compensation as defined in claim 8 wherein the predetermined range of acceptance is between about 2 and 178 degrees relative to the tangential vector V t .

Claim 14

A method for probe offset compensation as defined in claim 8 further including the steps of defining a curve S max parallel to curve S and passing through the contact point determined by electronic data processing means to have maximum deviation from curve S in one radial direction, defining a curve S min parallel to curve S and passing through the contact point determined by electronic data processing means to have maximum deviation from curve S in the opposite radial direction; and rejecting the direction point D as invalid if it is located between the curves S max and S min and repeating the compensation procedure.

Claim 15

A method for probe offset compensation as defined in claim 8 wherein the step of repeating the compensation procedure further includes the steps of measuring the coordinates of a new direction point D' approximately perpendicular to the last contact point C N by moving the probe off the surface in a direction approximately perpendicular to the contact point C N and taking a coordinate measurement, calculating a new tangential vector V t ' passing through the last contact point C N and tangent to the curve S, calculating a new directional vector V d ' between the last contact point C N and the new direction point D', and repeating steps (f)-(i) with the vectors V t and V d replaced by vectors V t ' and V d ', respectively.

Claim 16

A method for probe offset compensation as defined in claim 12 wherein the step of repeating the compensation procedure further includes the steps of measuring the coordinates of a new direction point D' approximately perpendicular to the last contact point C N by moving the probe off the surface in a direction approximately perpendicular to the contact point C N and taking a coordinate measurement, calculating a new tangential vector V t ' passing through the last contact point C N and tangent to the curve S, calculating a new directional vector V d ' between the last contact point C N and the new direction point D', and repeating steps (f)-(i) with the vectors V t and V d replaced by vectors V t ' and V d ', respectively.