Geometrical instruments – Gauge – Movable contact probe – per se
Patent
1991-02-06
1993-06-29
Will, Thomas B.
Geometrical instruments
Gauge
Movable contact probe, per se
33558, G01B 520, G01B 1124
Patent
active
052223043
DESCRIPTION:
BRIEF SUMMARY
This invention relates to probes, such as are used with co-ordinate measuring and similar machines, to indicate a touch to an object.
In co-ordinate measuring and similar machines an object is measured by relative motion of a probe and an object in a controlled manner, usually from a datum, until a "touch" on the object is indicated by the probe. The co-ordinates of the touch position are recorded. By repeated such actions the dimensions of the object can be determined.
The aim of such measurements is to achieve precision to better than one micron with a repeatability of half a micron or less. Accordingly it is clear that the "touch" action must be very responsive and free from error. Probes to achieve such touch action usually use a light, thin stylus with a ball-end of hard material to touch the object. The touch action deflects the stylus in a mounting in the probe and the occurrence of the deflection of the stylus is sensed to cause the co-ordinates to be recorded.
Known probes provide good results in many measurements but in some measurements errors can arise which cannot be compensated by corrections to the recorded co-ordinates. One cause for such errors can be that deflections of the stylus in different directions require different amounts of deflection before sensing occurs. Another cause can be that an electromechanical switch operated by the stylus is used for the sensing action and the making or breaking of the switch is not consistent because of arcing or corrosion at the switch contacts. Probes used hitherto have electrical or mechanical connections to the stylus.
Furthermore the probes require precision mechanical engineering manufacturing techniques which are expensive, resulting in a high cost for a probe. Also such a probe can easily be damaged by excess load on the stylus. Various of the problems with present probes are discussed in articles in Quality Today, February 1988, page 56 and May 1988, page 22.
It is an object of this invention to provide a probe which overcomes such shortcomings.
According to the invention there is provided a probe for position-determining apparatus and the like having a housing and in the housing a sensing space and an aperture to the sensing space, a stylus extending out of the sensing space through said aperture, a low-friction stylus support means around said aperture, means for constraining said stylus on to said support means and generally towards a neutral axis while permitting position-determining displacement on said support means against said constraining means, characterised in that said constraining means is outside said sensing space and that in said sensing space is a sensing means separate from the constraining means to sense directly significant movement of the stylus from said neutral axis without contact with the stylus and indicate said significant movement of the stylus as a position-determining displacement.
Conveniently the low-friction stylus support is a plurality of balls spaced around the aperture and the stylus has a surface at least partly-spherical to move on said support. The stylus thus pivots about a point which is the centre of curvature of the part-spherical surface. The constraining means may be a resilient, flexible element such as a sheet of rubber.
Preferably the sensing means includes in said sensing space a source of light directed towards the stylus and the stylus carries a mirror to reflect light from the source back to the sensing means. The amount of light received by the sensing means from the mirror varies in dependence on the displacement of the stylus. The mirror is preferably concave. The source of light may be one end of a flexible fibre optic extending from a radiator of light at the other end of the fibre optic remote from the probe. The light reflected by the stylus mirror may be collected by the or another fibre optic and carried to means to detect a change in the amount of light collected. The sensing means may include means to compare the amount of light radiated from the radiator and the amount collected
REFERENCES:
patent: 4158768 (1979-06-01), Lavelli
patent: 4177568 (1979-12-01), Werner et al.
patent: 4513507 (1985-04-01), Laskowski
patent: 4523382 (1985-06-01), Werner et al.
patent: 4532713 (1985-08-01), Feichtinger
patent: 4995170 (1991-02-01), Brule et al.
Dr. Derek Smith, "Probing Errors of a CMM", Quality Today, Feb. 1988, p. 56.
Patent Abstracts of Japan, vol. 8, No. 144, (p-284) (1581), Mar. 1984.
Patent Abstracts of Japan, vol. 10, No. 322, (P-511)(2378) Jun., 1986.
British Technology Group Limited
Will Thomas B.
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