Geometrical instruments – Gauge – Pivoted probes
Patent
1986-07-28
1987-10-27
Martin, Jr., William D.
Geometrical instruments
Gauge
Pivoted probes
33503, G01B 728
Patent
active
047020130
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a contact-sensing probe for use in apparatus for measuring the dimensions of workpieces.
Contact sensing probes generally include a first, fixed part connected to a machine which supports the fixed part for movement relative to a workpiece to be measured. The probe includes a movable part, having a stylus which is intended to engage the workpiece. Support means are provided to support the movable part in a rest position on the fixed part and the movable part is displaced from the rest position when the stylus engages the workpiece. Sensing means are provided for sensing such engagement and producing a signal to be sent to a system for measuring the position of the fixed part.
Contact sensing probes are known in which the contact of a stylus with a workpiece is sensed by a plurality of piezo electric devices. Examples are shown in the UK Patent Specifications Nos. 1,586,052 and 2,049,198.
The known probes offer increased sensitivity over probes which sense physical displacement of the stylus since they can produce a signal triggered by initial contact of the stylus with the workpiece.
The known probes are relatively complex in that the stylus itself or the stylus holder is made up of an assembly of parts which incorporate a plurality of sensing devices. Also, in both known probes the sensing devices are piezo-electric elements clamped rigidly between the two parts of the assembly.
Although no relative movement is intended between the parts of the assembly, there must inevitably be some relative movement to produce the force necessary to deform the piezo-electric elements in response to the pressure exerted on the stylus. This produces some uncertainty as tot he exact position of the stylus after each contact has been made. For example, there may be permanent deformation of one or more of the sensing devices, or vibration of the machine may cause the clamping pressure holding the assembly together to ease.
It is an object of the present invention to provide a probe which avoids this uncertainty and offers a simplified construction compared to the above-mentioned probes.
The invention as claimed achieves these objects by providing a sensing means for sensing engagement of a probe stylus with a workpiece, said sensing means being in the form of an accelerometer having oppositely disposed parts, one of which is supported on a surface of the movable part to participate in any movement thereof, the other part of the accelerometer being free, with the inertia of the accelerometer providing a yielding resistance, or being connected to means which provides a yielding resistance to such movement.
Thus, the accelerometer can be made from a strain sensitive element which may be deformed in compression or bending with high sensitivity to sense sudden movements of the movable member such as the vibration due to the shock wave generated on contact of the stylus with the workpiece, or a sudden displacement of the stylus. The accelerometer may be supported on a surface of the movable part on the opposite side thereof to the stylus so that there is no interference of the accelerometer with the stylus position.
The term accelerometer is used throughout this specification to include any force or strain sensitive elements, on the basis that, since force equates to the mathematical produce of mass and acceleration, such devices are also inherently responsive to acceleration.
Examples of probes according to this invention will now be described with reference to the accompanying drawings wherein:
FIG. 1 is a part-sectional elevation of the probe,
FIG. 2 is a section on the line II--II in FIG. 1,
FIG. 3 is an enlarged view of the sensing element of FIG. 1.
FIG. 4 is a view of an alternative form of sensing element for the probe of FIG. 1,
FIG. 5 is a part-sectional elevation of a modified form of the probe.
FIG. 6 is an enlarged view of an alternative sensing element for the probe of FIG. 5, and
FIG. 7 is a further alternative form of a probe according to the invention.
Referring to FIGS. 1 and 2, the prob
REFERENCES:
patent: 3945124 (1976-03-01), Jacoby et al.
patent: 4177568 (1979-12-01), Werner et al.
patent: 4294121 (1981-10-01), Inoue
patent: 4333238 (1982-06-01), McMurtry
patent: 4364180 (1982-12-01), Willhelm et al.
Patents Abstracts of Japan, vol. 7, No. 192, (P-218) (1337), 8/23/83.
Patents Abstracts of Japan, vol. 7, No. 156, (P-209) (1301), 7/8/83.
Martin, Jr. William D.
Renishaw plc
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