Touch probe

Abstract: A touch trigger probe for a coordinate measuring machine or machine tool has a stylus (14), which is biased into a rest position. The stylus has two independent supports, provided within a housing (10). In the embodiment of Fig 1, the first support comprises an axially projecting skirt (18) of a stylus holder (12), which is axially constrained on a surface (20) of the housing (10). The second support comprises a support element biased into contact with an inclined surface; more particularly a kinematic arrangement of cylinders (34) and balls (36), which provide lateral constraint. The second support is distinct from the skirt (18) and the surface (20) and is closer to the axis than the skirt. This improves the lateral constraint during the commencement of tilting of the stylus, for example by reducing wobble transverse to the plane of tilting when the support element disengages from the inclined surface.

Description: Field of the Invention This invention relates to touch probes for position determining apparatus such as machine tools and coordinate measuring machines. Description of Prior Art Such machines have a spindle, quill or arm which is movable in two or three dimensions and which can carry a touch probe or other tool. Scales or other measuring devices measure the position of the probe or tool in the two or three dimensions. A touch probe for use with such a machine has a deflectable stylus extending from its body, for contacting a workpiece surface. Soviet Union invention disclosure no. SU 460426 describes a touch probe for position determining apparatus, comprising: a fixed housing member having an axis; a movable member having a central portion for carrying a workpiece-contacting stylus, and a portion rigidly connected thereto which extends radially outwards from the central portion; bias means for biasing the movable member into a rest position relative to the fixed member, the movable member being movable out of the rest position against the action of the bias means when a force is applied to the stylus; a first annular surface carried on the radially extending portion of the movable member, and a second annular surface on the fixed member, the annular surfaces axially confronting each other and extending around the axis, said movable member tilting about a point of contact between the annular surfaces when the stylus contacts a workpiece; a seating, including at least one pair of mutually engageable elements, one of said elements being provided on the movable member, and the other of said elements being provided on the fixed member, said elements engaging at inclined surfaces thereof and being axially biased by the bias means into contact with each other so as to thereby provide lateral constraint of the movable member when in contact; and means for detecting deflection of the movable member out of the rest position. In this probe, the seating element on the movable member is provided together with the first annular surface on an axially projecting skirt. Touch probes with confronting annular surfaces are also known from German Patent Applications DE 37 15 698 and DE 32 46 252. In these probes, however, lateral constraint is provided by a planar spring, rather than by mutually engageable elements which engage at inclined surfaces. International Patent Application WO91/10887 (published 25th July, 1991) shows a probe having various features similar to SU 460426. However, in this probe, the first annular surface, which confronts the annular surface on the fixed housing member, is not carried on a radially extending portion which is rigidly connected to the central portion. Summary of the Present Invention In a touch probe according to the present invention, said at least one element provided on the movable member is carried by said central portion of the movable member, said elements being independent from said annular surfaces and located closer to said axis than said annular surfaces. Brief Description of the Drawings The preferred embodiments of the present invention will now be described with reference to the accompanying drawings, wherein:- Fig 1 is a vertical cross section through a first probe, Fig 2 is a horizontal cross section on the line II-II in Fig 1, Fig 3 shows a detector arrangement for the probe of Fig 1, Fig 4 is a view corresponding to Fig 2, but showing a modification, Fig 5 is a view on the line VIII in Fig 1, showing another modification, and Fig 6 is a view corresponding to Fig 5, showing a still further modification. Description of Preferred Embodiments Referring firstly to Figs 1 and 2, the probe shown comprises a housing 10, within which is located a stylus holder 12. The stylus holder 12 carries a stylus 14, which projects through an aperture 16 in the housing 10. The stylus has a tip 15 for contacting a workpiece, and such contact causes deflection of the stylus. The stylus holder 12 is cup-shaped, having a depending skirt 18. The annular bottom edge 22 of the skirt 18 rests on a flat internal surface 20 of the housing 10. The surfaces 22 and 20 are manufactured by lapping them together, so that they are extremely flat and conform with each other. This ensures that the stylus holder 12 is supported in a precise axial rest position. It is biased into this axial rest position by a spring 24, which causes a conical member 26 to bear in a recess 28 in the stylus holder. As well as being located in a precise axial rest position, the stylus holder 12 and its depending stylus 14 are also located in a very precise lateral rest position. This is ensured as follows. A diaphragm-like planar spring 30 is secured in its central region to the stylus holder 12, at the point from which the stylus 14 depends from the stylus holder 12. At its outer perimeter, the planar spring 30 is secured to a ring 32. Three cylinders 34 project radially from the ring 32, and seat in the convergent surfaces provided by respective pairs of balls 36, which are fixed on the housing 10. Thus, the ring 32 is located in a kinematically-defined rest position, both axially and laterally, in the same manner as described in US patent US 4,153,998. With respect to the ring 32, the planar spring 30 holds the stylus holder 12 in an extremely well-defined lateral rest position. However, the planar spring 30 can flex to allow vertical movement and tilting of the stylus holder 12 with respect to the ring 32; such movements are only constrained by the axial rest position provided by the surfaces 20 and 22. Thus, the stylus 14 is held in a precisely defined rest position by a combination of an axial constraint and a lateral constraint. Although the balls 36 and cylinders 34 provide axial constraint as well as lateral constraint, the axial flexibility of the planar spring 30 ensures that the stylus holder 12 is not over-constrained. A dowel 38 is fixed to the housing 10 and extends through a hole 40 in the stylus holder 12, with sufficient clearance so as not to interfere with normal operation of the probe. The purpose of this dowel is to prevent rotation of the stylus holder 12 when an exchangeable stylus 14 is being screwed into the stylus holder. The dowel also provides a measure of guidance to ensure that the cylinders 34 correctly reseat in the pairs of balls 36 after a deflection of the stylus. The planar spring 30 is pre-stressed, and in its relaxed state it is bowed upward, in the manner of a Belleville washer. However, it assumes the flat, stressed condition shown in Fig 1 under the biasing provided by the spring 24. Considering the forces acting upon the stylus holder 12 in the rest position, the downward force provided by the spring 24 is only slightly greater than, and is largely counterbalanced by, the upward bowing force of the planar spring 30. Consequently, the pressure between the surface 20 of the housing and the surface 22 of the skirt 18 is very small. Since the friction between the surfaces 20 and 22 is proportional to this pressure, the friction is also very small. When the stylus 14 contacts a workpiece, from any direction, the stylus is deflected. For example, if the contact is in a horizontal direction, the stylus 14 tilts, about a point of contact between the surfaces 20 and 22. At this time, the cylinders 34 and balls 36 remain engaged with each other, and the tilting is accommodated by flexing of the planar spring 30. As deflection of the stylus 14 continues, and the pre-stressing of the planar spring 30 is relieved, the further deflection is accommodated by lifting of at least one of the cylinders 34 out of its seating with the associated balls 36. Because the initial deflection is accommodated only by the tilting of the skirt 18 on the surface 20, and not by movement of the kinematically-mounted ring 32, the force required to produce the deflection is the same from all horizontal directions. This is because of the annular nature of the skirt 18. As a result, this initial deflection is entirely non-lobing. It is also extremely sensitive, because of the low contact force between the surfaces 20 and 22. When the deflecting force on the stylus 14 ceases (i.e. when the probe is moved so that the stylus 14 no longer contacts the workpiece) the stylus member 12 is returned to its axial and lateral rest position by the action of the spring 24. In doing so, it is likely that there will be some rubbing of the surfaces 20 and 22 against each other, and also of the cylinders 34 against the balls 36. The friction inherent in such rubbing is a source of hysteresis in conventional probes, when the spring force is unable completely to overcome the frictional forces to return the stylus holder to the precise rest position. In the present probe this is reduced because of the very light pressure between the surfaces 20 and 22, which means that the frictional forces are extremely low. Thus, there is little frictional force preventing the skirt 18 moving laterally to the lateral rest position; and since the surfaces 20 and 22 are flat, there is no resistance preventing the skirt 18 from moving into the axial rest position. During the reseat operation, the lateral rest position is in fact assured positively by the cylinders 34 and balls 36, and by the planar spring 30. It is of course still possible for frictional forces between the cylinders 34 and balls 36 to have an effect. Thus, friction between the cylinders 34 and balls 36 may prevent one or more of the cylinders 34 from completely reseating in its rest position between the corresponding pairs of balls 36. However, any resulting mis-positioning will largely be in the vertical direction rather than the horizontal direction. This is accommodated by a flexure of the planar spring 30, and therefore does not disturb the axial rest position of the stylus holder 12 (since this is dependent upon the skirt 18 returning to its axial rest position). Such mis-positioning between the cylinders 34 and balls 36 has a much less significant effect upon the lateral rest position. Furthermore, any lateral mis-positioning which might occur causes only a lateral translation of the rest position of the stylus 14; it does not cause it to tilt as in the case of the prior art. Thus, the magnitude of the mis-positioning of the stylus tip 15 is only the same as the magnitude of the lateral mis-positioning of the ring 32, whereas in the prior art probes the leverage effect caused by the length of the stylus 14 would greatly magnify the mis-positioning of the stylus tip 15 when the stylus 14 tilts. Fig 5 shows a modification which can reduce any lateral mis-positioning of the ring 32 even further. In place of the balls 36, each roller 34 is located between two cylindrical rollers 36a, forming a V groove. The sides of the V groove are deliberately made very steep, so that any mismatch is more evident in the vertical direction than in the lateral direction. If desired, a similar effect could be achieved with the balls 36 of Fig 1 by setting them slightly further apart than would be conventional. Fig 6 shows a further modification of the type shown in Fig 5. It comprises two cylinders 106, in place of the pair of balls 36. The cylinders 106 are seen end-on in Fig 6, and are parallel to each other, lying radially with respect to the axis of the probe. Between them, there is a recess 108 in the housing 10. In place of the cylinder 34 in Fig 1, a conical element 110 depends from the ring 32. In the kinematic rest position, this conical member 110 is urged against the pair of cylinders 106 as shown in the figure. The apex of the conical element 110 has a cylindrical extension 112, which in the rest position is accommodated in the recess 108. The cone angle of the conical element 110 is made very steep. In consequence, any mis-positioning due to friction effects when the ring 32 is brought back to the rest position, occurs primarily in the vertical direction rather than in the lateral direction. The steepness of the cone angle thus performs the same function as the steep sided V formed by the cylinders 36a in Fig 5. The extension 112 remains between the cylinders 106 when the ring 32 lifts and the conical element 110 disengages. This extension 112 thus acts as a guide to ensure correct return of the conical element 110 into contact with the cylinders 106, and it also obviates the need for the dowel 38. The instant at which the stylus tip 15 first contacts a workpiece can be detected in various possible ways. For example, it is possible to mount piezo-electric detectors or strain gauges on the stylus holder 12 to detect the initial contact (for example as described in International Patent Specification No. WO86/03829). Alternatively, the contact can be detected by arranging that an electric circuit between the stylus tip 15 and the workpiece is closed by the contact. As shown in Fig 1, however, our presently preferred method is photo-electric. A light emitting diode 42 produces a beam of light which passes through an opening 44 in the stylus holder 12. This produces a spot of light 48 upon a pair of light detectors 46 arranged side by side as seen more clearly in Fig 3. The outputs of the detectors 46 are taken to an auto zero circuit 50 having differential inputs. The function of the circuit 50 is to track any long-term drift of the outputs of the detectors 46, so that the output of the circuit 50 is a constant value (e.g. zero) despite any such long-term drift. However, if there is any more sudden mismatch between the differential inputs received from the detectors 46, the auto zero circuit 50 reacts, its output increases, and a trigger circuit 52 produces a trigger signal as a result. Thus, the auto zero circuit 50 is responsive to rates of change of the differential inputs which exceed a given value, such as would be caused when the light spot 48 moves towards one or other of the detectors 46 when the stylus 14 begins to deflect upon contact with a workpiece. Further description of such an auto zero circuit can be found in EP 0242747. Fig 4 shows a modification of the probe of Figs 1 and 2. The planar spring 30, ring 32 and cylinders 34 are replaced by a generally triangular planar spring 54. At each of the apices of the triangle, a radial slot 56 is cut into the planar spring. This defines a fork at each apex. In place of the three pairs of balls 36 fixed to the housing 10, there are three single balls 58, and each slot 56 locates over one of the balls 58. This arrangement provides a precise lateral rest position in the same manner as Fig 1, but is less expensive to manufacture. The skirt 18 in Fig 1 may be made from a ceramic material, and may bear against a ceramic annulus in the housing 10 forming the surface 20. The ceramic materials are preferred because they can be lapped against each other to produce a seating having a very high degree of flatness, and thus precision. It is desirable to ensure that the respective weights of the stylus holder 12 and stylus 14 are chosen such that the centre of gravity CG of the stylus and stylus holder combination is roughly in the same horizontal plane as the axial and lateral constraints which define the rest position. This helps to ensure that, despite the relatively low axial loading which keeps the skirt 18 in place on the surface 20, nevertheless there is little risk of accidental deflection of the stylus 14 owing to vibration or inertial effects as the probe is moved. All of the embodiments of Figs 4-6 may have any of the arrangements for detecting the instant of contact between the stylus tip and a workpiece, as discussed above in relation to the Fig 1 embodiment. The centre of gravity of the stylus and stylus holder may also be as discussed in Fig 1. It is a feature of all the embodiments described that the planar spring 30 does not have to flex for any great distance. In the case of the probe of Fig 1, the overtravel of the stylus can be accommodated by disengagement of the cylinders and balls 34,36 without flexing of the planar spring 30. This is in contrast to prior art probes in which lateral constraint is provided by a planar spring, (such as US 4,451,987) which must also permit overtravel. Such prior art probes require a relatively floppy planar spring to allow for such stylus overtravel. With the probes described above, this is not necessary, and so the planar spring can in each case be made substantially stiffer. This considerably improves the force with which the planar spring resists any residual lateral displacement of the skirt 18 in Fig 1. The greater lateral resistance results in further reductions in the hysteresis of the frictional contact between the surfaces 20,22 (Fig 1). A further feature of all the probes described is that while the force required for the initial deflection of the stylus is deliberately made very low, to reduce or eliminate the lobing and hysteresis problems, nevertheless the force causing the cylinders 34 to seat in the ball pairs 36 is still very high, so that good reseating can be assured. In fact, the force with which the cylinders are urged into engagement with the respective balls can be higher than would normally be the case with prior art probes having such kinematic seating arrangements, since increasing this force does not reduce the lobing and hysteresis performance of the probe.

Claims: A touch probe for position determining apparatus, comprising: a fixed housing member (10) having an axis; a movable member (12), having a central portion for carrying a workpiece-contacting stylus (14), and a portion rigidly connected thereto which extends radially outwards from the central portion; bias means (24) for biasing the movable member into a rest position relative to the fixed member, the movable member being movable out of the rest position against the action of the bias means when a force is applied to the stylus; a first annular surface (22) carried on the radially extending portion of the movable member, and a second annular surface (20) on the fixed member, the annular surfaces axially confronting each other and extending around the axis, said movable member tilting about a point of contact between the annular surfaces when the stylus contacts a workpiece; a seating, including at least one pair of mutually engageable elements, one of said elements (34,56,110) being provided on the movable member, and the other of said elements (36,58,106) being provided on the fixed member, said elements engaging at inclined surfaces thereof and being axially biased by the bias means into contact with each other so as to thereby provide lateral constraint of the movable member when in contact; and means for detecting deflection of the movable member out of the rest position; characterised in that said at least one element (34,56,110) provided on the movable member (12) is carried by said central portion of the movable member, said elements (34,56,110,36,58,106) being independent from said annular surfaces (22;20) and located closer to said axis than said annular surfaces. A touch probe according to claim 1, wherein said at least one element (34,56,110) on the movable member remains in contact with the other element (36,58,106) of the pair during the commencement of said tilting, thereby providing lateral constraint during the commencement of tilting, and wherein said at least one element disengages from the other element as further tilting occurs. A touch probe according to claim 1 or claim 2, wherein said first annular surface (22) is provided at a lower edge of a skirt (18) which depends from the radially extending portion of the movable member (12), and said second annular surface (20) faces upwardly. A touch probe according to any one of the preceding claims, further including a planar spring (30,54) for constraining movement of the movable member relative to the fixed member.

Inventors: MCMURTRY DAVID ROBERTS, MCMURTRY, DAVID ROBERTS
Applicants: RENISHAW PLC
Application year: 1991
Main classification: G01B5
IPC codes: G01B7, G01B5, G01B11