Geometrical instruments – Gauge – Coordinate movable probe or machine
Patent
1995-05-15
1997-05-06
Bennett, G. Bradley
Geometrical instruments
Gauge
Coordinate movable probe or machine
33559, G01B 7004
Patent
active
056259573
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
In recent times, probes for coordinate measuring apparatus have become known which apply only a very slight measuring force of less than 1 mN to the workpiece to be measured. These probes operate with a so-called solid-state oscillator having an amplitude of vibration which changes when approaching the object to be measured. Such a probe is disclosed in German utility model 9,213,059.
This known probe has a thin glass rod as a probe element which is attached with adhesive to a prong of a tuning fork crystal in such a manner that the longitudinal axis of the glass rod is aligned parallel to the direction of vibration of the tuning fork crystal.
When a probe of this kind is used in a coordinate measuring apparatus and when the longitudinal axis of the rod or probe element is aligned parallel to one of the measuring directions of the coordinate measuring apparatus as shown in FIG. 1, it can then be determined that the attainable contact uncertainties in the coordinate direction z (that is, in the longitudinal direction of the rod 3a and parallel to the direction S of vibration of the tuning fork crystal) are significantly less than the contact uncertainties which are measured in the coordinate directions x and y perpendicular to the longitudinal axis of the rod 3a. The measuring uncertainties lie apart from each other by a factor of 5. While, for example, a contact uncertainty of only 1.0 .mu.m was determined for the measuring axis z, this uncertainty amounts to about 5 .mu.m for the contact directions x and y.
An asymmetry of this kind in the contact uncertainty is, however, unwanted because many sides must be contacted when making measurements of geometric elements such as bores, et cetera. The measurement uncertainty should correspond for all contact points to the same value specified for the measuring apparatus.
A further problem when working with the above-mentioned probe is obtaining signals which announce clearly and reliably the contact of time probe element with the workpiece to be measured to the control of the coordinate measuring apparatus. If this time point is not clearly and reproducibly determined, then this likewise increases the contact uncertainty of the probe. The circuit described in German utility model 9,213,059 does supply usable results. However, the circuit is rather sensitive to external disturbances. For example, the transfer of measured values is influenced by turbulences in the air. When such disturbances occur shortly ahead of contact on the workpiece surface, then these disturbances are not clearly recognized as disturbances and a false coordinate measuring value is supplied, that is, these disturbances contribute to an increased contact uncertainty.
SUMMARY OF THE INVENTION
In view of the above, it is an object of the invention, in a coordinate measuring apparatus having a probe of the kind referred to above, to so mount the probe or provide a circuit therefor that equal contact uncertainties reliably result, which are as low as possible, in the three measuring directions x, y and z.
In the arrangement of FIG. 2, the solid-state oscillator (1/2) is mounted with respect to the direction S of vibration at an angle of preferably 45.degree. to all three measurement directions x, y and z of the coordinate measuring apparatus. With the arrangement shown in FIG. 2, contact uncertainties of 2.5 .mu.m result in all three contact directions with these contact uncertainties being low and comparable. However, the sketched arrangement is disadvantageous insofar as the probe element, that is the rod 3a, is inclined to the three contact directions which are possible in the coordinate measuring apparatus. The objects O to be measured are, as a rule, clamped to the table of the coordinate measuring apparatus so as to be aligned to the measuring axes x, y and z. For this reason, measurement is improved when the probe element, that is, the rod placed on the solid-state oscillator, is aligned parallel to one of the three measuring directions of the coordinate measu
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Breyer Karl H.
Heier Helmut
Meyer Gerhard
Bennett G. Bradley
Carl-Zeiss-Stiftung
Ottesen Walter
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