Calibration process for shape measurement

Details Calibration process for shape measurement Calibration process for shape measurement

1999-09-07

2001-03-06

Assouad, Patrick (Department: 2857)

Data processing: measuring, calibrating, or testing

Calibration or correction system

Position measurement

C033S561000

Reexamination Certificate

active

06199024

ABSTRACT:

FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to measurement of the shape of a workpiece and, more particularly, to a method for calibrating a shape measuring system with a variable distance between the scanning probe and the measuring point of the workpiece.
A layout of a typical shape measuring system, such as a coordinate measuring machine (CMM), is illustrated in
FIG. 1. A
workpiece
10
with a measuring point
11
on the surface of workpiece
10
, is set on a working table
21
of a shape measuring system
20
. A scanning probe
41
senses measuring point
11
, by either touching measuring point
11
in a contact shape measurement, or by pointing toward measuring point
11
and measuring a distance D between measuring point
11
and a scanning probe datum point
43
in a non-contact shape measurement. A scanning probe holder
42
(usually a probe head), as part of a scanning probe assembly
40
, holds scanning probe
41
onto a motion system platform
30
. Motion system platform
30
is movable in three axes (X, Y, Z) of a fixed coordinate frame
22
of shape measuring system
20
. The motion and position of motion system platform
30
are monitored according to the coordinates of a reference point
31
on motion system platform
30
. A vector {right arrow over (r)}
CMM
, with Cartesian components (X
CMM
, Y
CMM
, Z
CMM
) in coordinate frame
22
, represents the position of reference point
31
relative to a fixed origin point
23
of coordinate frame
22
, whereas a vector {right arrow over (r)}, with Cartesian components (X, Y, Z) in coordinate frame
22
, represents the coordinate of measuring point
11
relative to origin point
23
. The coordinate {right arrow over (r)}
CMM
of reference point
31
, is read whenever the scanning probe senses a measuring point on the surface of the workpiece.
However, because of a spatial offset between reference point
31
and measuring point
11
, the coordinate {right arrow over (r)} of measuring point
11
is displaced relative to the measured position {right arrow over (r)}
CMM
of reference point
31
, according to the formula: {right arrow over (r)}={right arrow over (r)}
CMM
+{right arrow over (&dgr;)}, where a displacement vector {right arrow over (&dgr;)}={right arrow over (&Dgr;)}+{right arrow over (D)} is composed of two components: a vector {right arrow over (&Dgr;)}, with Cartesian components (&Dgr;
X
, &Dgr;
Y
, &Dgr;
Z
) in coordinate frame
22
, which is a constant offset between scanning probe datum point
43
and reference point
31
, and {right arrow over (D)}=D·{right arrow over (i)}, which is a distance vector between measuring point
11
and scanning probe datum point
43
, where {right arrow over (i)} is a unit vector pointing from the scanning probe datum point
43
toward the measuring point.
It is worthwhile to mention, that in the case of present art contact scanning probes, the distance D between the scanning probe datum point and the measuring points of the workpiece is constant. It is then common to define the scanning probe datum point as the touching point of the scanning probe with the surface of the workpiece. In such a case, D=0, and hence {right arrow over (&dgr;)}={right arrow over (&Dgr;)}.
Thus, in the case of shape measurement using either a present art contact scanning probe, or a non-contact scanning probe with a constant distance D between the scanning probe datum point and the measuring points of the workpiece, if all points of the workpiece are measured with the same scanning probe configuration, then the displacement {right arrow over (&dgr;)} between reference point
31
and measuring point
11
is the same for all measuring points of the workpiece. Hence, measuring the coordinate {right arrow over (r)}
CMM
of reference point
31
is sufficient in this case for determining the shape of the workpiece.
The situation is more complicated when several different scanning probe configurations are used for measuring the same workpiece, using either a present art contact scanning probe, or a non-contact scanning probe with a constant distance D between the scanning probe datum point and the measuring points of the workpiece. The various scanning probe configurations can differ by the offset {right arrow over (&Dgr;)} between the scanning probe datum point
43
and the reference point
31
, and/or the distance D between the scanning probe datum point
43
and the measuring point
11
, and/or the inclination of the scanning probe. An example of changing the scanning probe configuration, is by attaching an extension, such as a metal shaft, to a contact scanning probe to make it longer, when necessary for measuring hard-to-reach parts of the workpiece.
Thus, in such cases where several different scanning probe configurations are used for measuring the same workpiece, the displacement {right arrow over (&dgr;)} between reference point
31
and measuring point
11
, is not the same for all measuring points of the workpiece. Hence, measuring the coordinate {right arrow over (r)}
CMM
of reference point
31
is not sufficient in this case for determining the shape of the workpiece. Thus, appropriate prior art calibration methods exist, which provide the data that is necessary for deriving the coordinate {right arrow over (r)} measuring point
11
from the measured position coordinate {right arrow over (r)}
CMM
of reference point
31
.
The basic idea of these prior art calibration methods which are suitable for shape measurement using either a present art contact scanning probe, or a non-contact scanning probe with a constant distance D between the scanning probe datum point and the measuring points of the workpiece, is to utilize a calibration object with a known geometry and a particular calibration point.
A vector diagram of a setup of a shape measuring system for a prior art calibration process is shown in
FIG. 2. A
calibration object
50
with a known geometry, and including a calibration point
55
, is fixed by a fixture
59
to working table
21
of shape measuring system
20
. Calibration object
50
is usually a calibration sphere with a known radius R, and the center point of the calibration sphere serving as calibration point
55
.
For each scanning probe configuration individually, several measuring points on the circumference of calibration sphere
50
are scanned, and the corresponding position {right arrow over (r)}
CMM
of the reference point on the motion system platform for each measuring point on the calibration sphere, is recorded.
For the sake of describing the prior art calibration methods, a measuring point
51
″ indicates a measuring point “n” out of a plurality of N+1 measuring points numbered “0”, “1”, “2”, . . . , “N” on the circumference of calibration sphere
50
, for the calibration process of a particular scanning probe configuration. Point
31
″ in
FIG. 2
indicates the corresponding position of reference point
31
of
FIG. 1
when scanning probe
41
senses measuring point
51
″.
Referring further to
FIG. 2
, vector equations between the various positions and distances can be written for each of the plurality of N+1 measuring points. However, for the sake of simplicity, these equations will be explicitly presented for measuring point “n” (
51
″), as a representative for all N+1 measuring points.
The basic relation is: {right arrow over (r)}(n)={right arrow over (r)}
CMM
(n)+{right arrow over (&dgr;)}, where a position vector {right arrow over (r)}
CMM
(n), with Cartesian components (X
CMM
(n), Y
CMM
(n), Z
CMM
(n)) in coordinate frame
22
, represents the measured position of reference point
31
″ relative to an origin point
23
of coordinate frame
22
. {right arrow over (&dgr;)} is the constant but unknown displacement vector between measuring point
51
″ and reference point
31
″, with Cartesian components (&dgr;
X
, &dgr;
Y
, &dgr;
Z
) in coordinate frame
22
. Thus, the coordinate vector {right arrow over (r)}(n), with Cart

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