POSITION SENSING PROBE

Abstract: A position sensing probe has a fixed member (10) and a moveable member (16) which is free to translate in three mutually orthogonal directions, but is constrained from rotational movement. A transducer arrangement (50,60,70) is provided to measure the degree of translation along each of the said directions. The arrangement comprises three pairs of elements (51,53; 61,63; 71,73) attached to the fixed member (10). Three transducer plates (52,62,72), attached to the moveable member (16) form differential capacitors with corresponding pairs of transducer elements (51,53; 61,63; 71,73). The plates (52,62,72) have a substantially larger effective surface area than the corresponding transducer elements (51,53; 61,63; 71,73) which ensures that the differential capacitance does not alter as a result of a change in the overlap area of the plates (52,62,72) and the elements (51,53; 61,63; 71,73).

Description: This invention relates to a probe for sensing the position of an object. Such probes are used in co-ordinate measuring machines wherein the probe and the object are supported for relative three-dimensional movement. The probe is adapted to output a signal when, by such movement, the probe is brought into engagement with a surface of the object, all with a view to determining the position of said surface relative to a datum. A known such probe is disclosed in US 4,078,314. The probe comprises a stylus-supporting member, itself supported for movement relative to a fixed structure with 3 linear degrees of freedom. A transducer arrangement provided for determining displacement of the stylus supporting member relative to the fixed structure comprises three perpendicular flat conducting surfaces on the stylus-supporting member, each confronted by an e.g. inductive proximity sensor. The present invention provides a position-sensing probe comprising: a fixed member a movable member which is translatable relative to the fixed member on each of three orthogonal axes, and a transducer arrangement for measuring the translation of the moveable member on each of the orthogonal axes, the transducer arrangement comprising three transducer plates on one said member, the plates being arranged as three orthogonal sides of a cube; characterised by three pairs of transducer elements on the other said member, the elements of each pair confronting a respective said plate on opposite sides thereof and acting differentially therewith. Two 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 a first probe, Fig. 2 is a section on the line II-II in Fig. 1, Fig. 3 is a section on the line III-III in Fig. 1, Fig. 4 is a part-sectional elevation of the second probe. The probe shown in Figs. 1 to 3 has a fixed member or housing 10 having a spigot 11 whereby the housing is secured to the head 12 of a co-ordinate measuring machine (not shown). The machine supports the housing for primary three-dimensional displacement relative to a workpiece 13 with a view to determining the position of a surface 14 thereof relative to a datum. The probe further has a movable sensing member or stylus 15 supported by the housing for secondary three-dimensional displacement relative thereto. When, during a said primary displacement of the housing relative to the workpiece, the stylus is engaged with the surface 14, there occurs a said secondary displacement of the stylus 15 relative to the housing 10. The machine is of course halted within the range of the secondary displacement. The machine has a primary measuring system for said primary displacement, and the sensor has a secondary measuring system for said secondary displacement. The position of the surface 14 is determinable in terms of the difference between the respective measures of the primary and secondary displacements. Such a measuring process is well understood per se. The support of the stylus 15 on the housing 10 comprises a movable member or cube 16 to which the stylus 15 is secured, and three support units 20,30,40 each connected, one independently of the other, between the cube 16 and the housing 10. The units 20,30,40 permit three-dimensional translation of the cube 16 relative to the housing but prevent rotation of the cube 16 about respective mutually perpendicular axes X,Y,Z. The units are more fully described in EP 239 337, the parent application to the present application and will not be described further. The secondary measuring system, i.e. the system for measuring translational displacement of the stylus 15 relative to the housing 10, comprises three differential capacitance transducers 50,60,70 for measuring translation in the directions of the axes X,Y,Z respectively. The transducer 50 comprises a differential capacitor defined by two outer capacitor plates 51,53 situated at opposite sides of an inner capacitor plate 52. The plates 51,53 are mounted on a bracket 27 secured to the housing 10. The plate 52 is mounted on a bracket 28 secured to an elongate connecting member 29 one end of which is secured to the cube 16 and the other end of which has the stylus 15 secured thereto. The plate 52 may be surrounded by a guard ring. The plates 51,52,53 lie in planes parallel to the YZ plane and are spaced apart in the direction of the axis X so that any translation of the stylus in the latter direction changes the spacing between the plates 51,52 relative to that between the plates 52,53. This change is detected by a transducer circuit having an output proportional to the magnitude of the translation. The translation of the stylus 15 in the direction of the axis Y or Z does not affect the output of the transducer partly because the spacing between the plates 51,52,53 is not affected by such translation and partly because, as shown, the plates 51,53 are made to have substantially larger surface areas than the plate 52 so that the latter plate does not pass out of the range of the former plates by any Y or Z translation within the translation range of the support units 30,40. For similar reasons, any guard ring should be made larger than would be conventional. The transducers 60,70 have the same plate construction as the transducer 50 and are mounted on the same brackets 27,28 except in that the plates denoted 61,62,63 and 71,72,73, of the transducers 60 and 70 are spaced apart in the directions of the axes Y and Z respectively. In other words, the spacing of the plates of any of the transducers is perpendicular to the spacing of the plates of the other two transducers. The arrangement is such that the plates 52,62,72 lie at three adjacent surfaces of a notional cube, and the plates 51,61,71 and 53,63,73 are of course parallel to the plates 52,62,72 respectively. In this way there is achieved a compact construction of the transducers with good rigidity of the relative position of the plates. The probe shown in Fig. 4 will now be described. Referring to Fig. 4, this probe has a fixed member 110 comprising a housing 100 and two vertical pillars 101, 102. A stylus 15 is fast with a movable member 116 which consists of a top plate 105, a bottom plate 106, and three vertical struts 107, 108, 109 which extend between the top and bottom plates. As previously, the housing 110 is supported by a spigot 11 in the head 12 of a co-ordinate measuring machine. Also as previously, the movable member 116 is supported within the housing 110 by three connecting units, which permit translation of the movable member on each of three orthogonal axes, each connecting unit preventing rotation about a respective one of these axes. As previously, these units will not be described further were. The transducer arrangement can be seen in Fig. 4. It comprises a fixed block 127, which is attached to and suspended between the fixed pillars 101, 102. A movable transducer block 128 is attached to and suspended between the three vertical struts 107, 108, 109. As previously, the movable part of the transducer comprises three capacitor plates which are arranged on three adjacent, orthogonal sides of a notional cube. Since the transducer arrangement is the same as shown in Figs. 1 and 3, it will not be described further. Of course, if desired it would be possible for the transducer block 128 to be mounted on the fixed structure, with the transducer block 127 being on the movable member. If desired, non-capacitative transducer arrangements can be provided in either embodiment. For example, the fixed capacitor plates 51, 53 can be replaced by transducer elements in the form of inductive coils, and the movable plate 52 replaced by a somewhat large plate of magnetic or conductive material. Movement of the movable plate then has a differential effect on the mutual inductances of the coils, which can be measured. Similar inductive transducer arrangements then also replace the plates 61,62,63 and 71,72,73.

Claims: 1. A position-sensing probe comprising a fixed member (10) a movable member (16) which is translatable relative to the fixed member (10) on each of three orthogonal axes (X, Y, Z), and a transducer arrangement (50, 60, 70) for measuring the translation of the movable member (16) on each of the orthogonal axes (X, Y, Z), the transducer arrangement (50, 60, 70) comprising three transducer plates (52, 62, 72) on one said member the plates (52, 62, 72) being arranged as three orthogonal sides of a cube; characterised by three pairs of transducer elements (51, 53; 61, 63; 71, 73) on the other said member, the elements (51, 53; 61, 63; 71, 73) of each pair confronting a respective said plate (52, 62, 72) on opposite sides thereof and acting differentially therewith. 2. A probe according to claim 2 wherein the transducer plates (52, 62, 72) are on the moveable member (16) and the transducer elements (51, 53; 61, 63; 71, 73) are on the fixed member (10). 3. A probe according to claim 1 or claim 2 wherein each transducer plate (52, 62, 72) forms a differential capacitor with its corresponding pair of transducer elements (51, 53; 61, 63; 71, 73). 4. A probe according to any one of the preceeding claims wherein each transducer plate (52, 62, 72) has a substantially larger effective surface area than its corresponding transducer elements (51, 53; 61, 63; 71, 73).

Inventors: THOMAS, DAVID KENNETH
Applicants: RENISHAW PLC
Application year: 1987
Main classification: G01B5
IPC codes: B23Q1, G01B5, G01B21, G01B7