Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Patent
1992-05-19
1993-12-14
Strecker, Gerard R.
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
324158P, 324662, 324687, G01R 2726, G01B 734
Patent
active
052706648
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a capacitance sensing probe for making measurements of the distance between the probe and a surface. In particular, the probe is used to make measurements of the roughness of the surface being measured.
It is known for example from U.S. Pat. No 4,814,691 to measure the profile of a surface using a capacitance probe, in which the distance between a single electrode of the probe and the surface is determined by measuring the capacitance between the electrode and the surface at any given point as the probe is moved over the surface. The probe described includes a thin flat-plate electrode which is mounted in the probe with its edge flush with the end of the probe and a determination is made of the variation of the fringe-field capacitance between the probe and the surface.
While such probes give satisfactory results they are limited in amount of information about the surface which they can provide in a single pass across the surface.
This deficiency of the known probes is overcome with a probe as claimed in the appended claims by providing two electrodes in the probe which are spaced apart in the direction of the intended movement of the probe with insulation therebetween, and by measuring the fringe field capacitance between the electrodes and the effect the surface has on this capacitance
By using the principle of measuring the change of fringe-field capacitance between a pair of plates the probe designer has greater flexibility in the probe design. The probe is basically an integrating device which determines a capacitance which is related to the average of the distances of the electrodes from the surface over an area.
By choosing different spacings between the electrodes the size of the area over which the probe will average can be changed. Thus a range of probes of different sensitivities can be made for a variety of jobs.
In addition, by measuring the fringe field capacitance between each electrode and the surface at the same time, information can be derived about the local slope of the surface at the point of measurement.
In a preferred form of the invention the two electrodes each take the form of a very thin layer applied to internal surfaces of parts of the stylus by sputtering or vapour deposition, and the two parts are firmly connected together with the electrodes confronting each other and a very thin layer of insulation between them.
By this means the area of surface over which the capacitance is averaged becomes small enough that the probe will detect individual peaks and troughs of the surface roughness and effectively acts as a device for measuring a surface profile having a high spatial frequency variation.
Increasing the spacing of the electrodes increases the fringe-field and gives the probe greater sensitivity thus enabling the electrode to be mounted at a greater distance from the surface. However the spatial frequency which can be detected will decrease as the spacing of the electrodes increases. This means that greater variations in the distance between the electrode and the surface can be measured i.e greater roughness in the surface, but only at lower frequencies.
There is therefore an optimum spacing for the electrodes for any given measurement task.
In another embodiment of the invention the stylus terminates in a rounded skid within which the electrodes are mounted with their edges adjacent to, but spaced a small fixed distance from the surface of the skid.
By this means the skid can be passed over a workpiece surface in contact therewith and the skid will keep the electrode a constant distance from the surface. The skid will thus follow the low frequency variations in the surface profile and eliminate them allowing the probe to measure the higher frequencies only.
The probe may be mounted on a co-ordinate measuring machine (CMM) for providing movement thereof over the surface being measured. The machine may be provided with a servo control system to raise or lower the probe relative to the surface responsive to the capacitance measurement increasi
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McMurtry David R.
Thomas David K.
Do Diep
Renishaw plc
Strecker Gerard R.
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