Reseat system of touch signal probe

Details Reseat system of touch signal probe Reseat system of touch signal probe

2000-06-16

2004-01-20

Fulton, Christopher W. (Department: 2859)

Geometrical instruments

Gauge

Having a movable contact probe

C033S561000

Reexamination Certificate

active

06678966

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a reseat system of a touch signal probe installed in a coordinates measuring machine. More specifically, it relates to a reseat system of a touch signal probe having a fixed component and a movable component, the reseat system allowing displacement of the movable component relative to the fixed component when a force is applied to the movable component from the outside and accurately returning the movable component to a rest position when the force applied to the movable component ceases to exist.
2. Description of Related Art
A touch signal probe is used in a coordinates measuring machine for detecting contact. In the coordinates measuring machine having the touch signal probe, a probe movable in three-dimensional directions touches a workpiece on a fixed table and a coordinate value of respective axes (respective axes in the three-dimensional directions) when the probe touches the workpiece is read as an electric trigger, so that dimensions and configuration of the workpiece are measured based on the coordinate values. Accordingly, a position of the probe can be detected by an electric touch signal based on contact between the probe and the workpiece.
FIG. 5
shows a conventional touch signal probe. In the figure, a stylus
1
is fixed to a movable component
2
. A contact ball
4
is provided at a distal end of the stylus
1
. Three cylindrical bodies
3
radially projecs with 120 degree intervals around an axis of the stylus
1
from a periphery of the movable component
2
on a plane perpendicular to the axis of the stylus
1
. On the other hand, the fixed component
5
has three pairs of cylindrical bodies
6
at positions corresponding to cylindrical bodies
3
of the movable component
2
. The cylindrical bodies
3
and the cylindrical body
6
constitute a reseat component for defining the relative position of the fixed component
5
and the movable component
2
at one place.
According to the above arrangement, the movable component
2
is pressed to the fixed component
5
by virtue of a biasing force F of a biasing component (not shown) and the movable component
2
is forcibly brought into contact with the fixed component
5
through the reseat component. When the pressing force from the workpiece is not applied to the distal end of the stylus
1
, the movable component
2
rests on the fixed component at six contact points. In other words, respective cylindrical bodies
3
of the movable component
2
rest on the respective cylindrical bodies
6
at two points for a total of six points. Accordingly, the reseat system is called as a six-point contact reseat system.
According to the six-point contact reseat system, the reseat position of the movable component after an escape movement can be located at only one place. In other words, assuming that the stylus
1
displaces parallel to axial direction at the rest position of the stylus
1
while maintaining contact between the reseat component on the movable component side and the reseat component on the fixed component side at the respective contact points, the respective loci drawn by the distal end of the stylus crosses the axis of the stylus at the rest position. According to the arrangement, the stylus
1
returns to a unique rest position only by restoring contact with the respective contact points by the biasing force F during return movement after an escape movement of the movable component
2
by the pressing force from the workpiece, so that the rest position of the stylus
2
can be kept constant.
Since the position of the movable component relative to the fixed component can be set unique by the six-point contact reseat system, the six-point contact reseat system has high anti-vibration rigidity. Further, irrespective of the direction of the outside pressing force, the six-point contact reseat system has high reseat ability in a relatively rough unit of, for instance, 10 &mgr;m.
However, the above-described six-point contact reseat system causes an error (“reseat shift error”) in a further fine unit of, for instance, 1 &mgr;m observed in return movement after contact, the error being caused because the movable component is pushed by the workpiece during escape movement of the movable component to cause displacement relative to the fixed component.
Specifically, as shown in FIG.
6
(A), when the contact ball
4
touches the workpiece W in the conventional reseat system, the stylus
1
moves in the left direction in the figure as shown in FIG.
6
(B). At this time, a small reaction force is caused between the movable component
2
and the fixed component
5
, so that the movable component
2
slightly slides in the left direction in the figure. When the workpiece W and the stylus
1
connected to the movable component
2
are no more in contact with each other as shown in FIG.
6
(C), the movable component
2
conducts the return movement by virtue of the biasing force F, where the axial position of the movable component
2
is shifted on account of the aforesaid slide movement. The shift directly affects on measurement accuracy of the probe.
The Applicants of the present invention have proposed a reseat system capable of correcting reseat position shift after return movement shown in
FIG. 7
(European Patent Publication No. 0764827 A2), where the reseat error is corrected by a piezoelectric element for administrating the direction of the friction force applied to a contact point between the movable component and the fixed component of the reseat system.
The reseat system has a fixed component
11
, a movable component
21
and a biasing force generator (not shown) capable of allowing displacement of the movable component
21
relative to the fixed component
11
when a force is applied to the movable component from the outside and capable of returning the movable component
21
to a rest position when the force is not applied to the movable component
21
.
The movable component
21
has a stylus
22
having a contact ball
24
to be in contact with the workpiece projecting therefrom and three cylindrical bodies
23
extending radially around the axis of the stylus
22
at 120 degree intervals to be in contact with the fixed component
11
.
A central portion of the fixed component
11
is secured to a housing of the probe (not shown), the fixed component
11
having three arms
12
extending radially around the axis of the stylus
22
at 120 degree intervals. A pair of hard balls
13
is disposed on an upper surface of an end of the respective arms
12
.
Further, a piezoelectric element
14
as a displacement generator is provided on an inner portion relative to the hard balls
13
of the respective arms
12
, the piezoelectric elements being stretchable radially approximately along the axis of the stylus
22
.
When a voltage is applied to the respective piezoelectric elements
14
, the respective piezoelectric elements
14
synchronously displace, so that the respective hard balls
13
displace in approximately radial direction around the axis of the stylus
22
. Incidentally, the displacement in the present arrangement is a kind of “static” displacement, which is different from vibration where the movement of the piezoelectric elements is minutely repeated.
The direction of the friction force at respective contact points between the cylindrical body
23
and the hard ball
13
aligns by the displacement, so that the reseat position can be adjusted to return the movable component by the biasing force.
However, in the above-described mechanism, though “axial shift”, i.e., the reseat shift error in axial direction of the cylindrical body
23
can be effectively corrected, “circumferential error”, i.e., the reseat shift error in a circumferential direction around the axis of the stylus
22
cannot be sufficiently corrected.
Specifically, as shown in FIGS.
8
(A) (seen from an upper direction in
FIG. 7
) and
8
(B) (seen from an outside on the axis of the cylindrical body
23
), when the movable component
21
conducts the

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