GB2049198A - Probe for use in measuring apparatus - Google Patents

Abstract

A stylus (17) is mounted to one part (22) of a stylus holder the other part (21) of which is mounted to a base (10) for resilient displacement against a spring (16). Three radial arms (14) on the holder part (21) seat in respective pairs of balls (15) to define a return position for the holder after displacement. Three stress transducers (20), e.g. piezoelectric devices, are sandwiched between the two-holder parts (21,22) which are secured together by a screw (23). The transducers detect omnidirectionally contact of the stylus with a workpiece, and the resilient mounting of the holder to the base allows over-run of the workpiece measuring machine without involving the transducer elements. The transducer elements could alternatively be incorporated in a two- part base (10), e.g. in the plane X-X. <IMAGE>

Description

SPECIFICATION Probe for use in measuring apparatus This invention relates to a probe for measuring workpieces. A known such probe (United Kingdom Patent No. 1,445,977) comprises a stylus projecting from and supported by a base. In use, the base is secured to a machine for moving the probe in three dimensions relative to the workpiece and the machine automatically measures and records the coordinate position of any point on the workpiece with which the stylus may be brought into engagement. The measuring operation of the machine is initiated by a pulse signal output by the probe substantially at the instant of engagement with the workpiece.

The machine is adapted to move the probe rapidly from one measuring point to the next, causing the probe to alternately engage with and be withdrawn from the workpiece. Since the movement of the probe cannot be stopped at the instant of engagement between the stylus and the workpiece, the stylus is connected to the base by a kinematic positioning device which allows limited displacement of the stylus relative to the base so that the probe can overtravel the engagement point by a limited amount. The overtravel takes place in opposition to a spring which returns the stylus to its initial or rest position when the probe is withdrawn from the workpiece.

It is desirable for the probe to have the following properties. The displacement of the stylus allowed by the positioning device should be substantial, the rest position should be positively and accurately defined, the signal response should be rapid, i.e. the signal should occur as nearly as possible at the instant of engagement with the workpiece, the force acting on the stylus during its engagement with the workpiece should be reasonably low, and the signal should not be produced spuriously as a result of momentum forces acting on the stylus or other components of the probe during acceleration and deceleration of the probe by the machine.

Insofar as the above probe produces its signal substantially at the instant of engagement, it is referred to as a "touch probe".

Such probe have attained wide use because of the high operating speeds made possible by them. This arises from their simple and robust construction and their relatively good conformity with the above properties especially as regards freedom from producing the signal on account of inertia forces. It is however an object of this invention to improve the response of the probe to engagement with the workpiece with a view to meeting higher accuracy requirements.

In another known probe (United States Patent No. 3945124) the positioning device comprises a parallelogram made of two rigid members, being respectively a base and a stylus holder, connected by two parallel leaf springs. A piezo-electric rod connected between the rigid members is stressed in bending when an appropriately directed force acts on the stylus, the bending stress producing the probe signal. The connection between the rod and the base is through the intermediary of two coil springs which give way during the displacement of the stylus relative to the base during a said overtravel and which return the stylus to the rest position when the probe is withdrawn from the workpiece. The spring arrangement comprises two heads urged towards one another by the respective springs but held in spaced relationship by fixed ball stops situated between confronting parallel faces of the heads.The bending rod has a spherical end also situated between the faces of the heads. To ensure an accurately defined rest position for the stylus, there must be no play between the spherical end of the rod and the faces of the head. When a given force is exceeded any further displacement is accommodated by the springs. To provide for the necessary stylus displacement in three dimensions, three said parallelograms, each including a said bending rod and coiled springs, are connected in series between the stylus and a final base.

The piezo-electric rods in the last-mentioned known probe have, by themselves, good response characteristics but difficulties arise due to inter-action between the spherical end of the rod and the flat surfaces of the heads between which that end is situated. It is difficult to avoid play between the spherical end and the heads and inasmuch as there is a possibility of the spherical end being gripped between the heads there arises undesirable friction when the end turns as the rod bends.

Inasmuch as the springs have to be relatively stiff (they have to be sufficiently stiff not to give way until a certain stylus force is exceeded) there can result forces at the contact point between the stylus and the workpiece sufficient to cause undesirable deflection of either. If, on the other hand, the springs are made sufficiently weak to avoid such deflection, the probe becomes correspondingly more subject to generating signals due to inertia forces. This arises essentially from the serial arrangement of the three positioning devices whereby the device nearest the final base is subject to the inertia of the other two devices.

The known probe is therefore not suitable as a touch probe in machines having high operating speeds. Lastly, the need to provide three said positioning devices with their individual piezo-electric rods constitutes a complex and expensive arrangement.

The invention as claimed in claim 1 hereto is based on the finding that a satisfactory signal is obtainable from a single stress sensor means embodied directly in the base or in the stylus holder. That is, the stress sensor element, which could for example be a piezoelectric or piezo-resistive device, forms a part of the base or a part of the holder, but not both. It is therefore separate from the elements of the positioning device responsible for locating the holder in the rest position.

The single stress sensor means may comprise a single sensing element, or more than one such element arranged in a compact unit.

Preferably the positioning device used in the context of the invention is a kinemetic device which may be known per se, comprising only two rigid members being respectively the base and the stylus holder, the stress sensor means being preferably embddied in the stylus holder.

Further, the stress sensor means may be connected to an electric circuit for sensing a pulse signal when that signal has a peak exceeding a given magnitude. This requires that the machine is operated sufficiently fast to produce an impact between the stylus and the workpiece which is sufficiently strong to produce said peak. Although a certain strength of impact is required for such operations, this is in practice sufficiently small not to cause damage to the stylus or the workpiece.

One embodiment is described with reference to the accompanying drawings, wherein: Figure 1 is a sectional elevation of the probe, and Figure 2 is a section on the line ll-ll of Fig.

1.

Referring to the drawings; a housing 10 is secured at 11 to a first carriage (not shown) of a co-ordinate measuring machine. Such a machine conventionally comprises three carriages: a first carriage mounted to a second carriage and movable relative thereto along a first axis; the second carriage being mounted to a third carriage and movable relative thereto along a second axis; and a third carriage being mounted to the machine frame and movable relative thereto along a third axis; the three axes being generally at right angles to each other. Thus, a probe carried by the first carriage can be contacted with the surface of a workpiece, and the co-ordinates of the position of contact can be determined in terms of the relative positions of the three carriages.

In this embodiment, the probe is mounted to the housing 10 on the first carriage. The probe comprises a stylus 1 7 having a ball 9 at one end for contacting the workpiece 1 8. The stylus 1 7 is screwed into the bottom end of a lower part 22 of the support member 1 2. An upper part 21 of the support member 12 is located over the lower part 22 with a clearance 1 9 between them. The top region of the support 1 2 is generally triangular in plan (see Fig. 2) providing three equiangularly disposed arms 13. The parts 21,22 are separated by piezo-electric crystals sandwiched between respective arms of the upper and lower parts 21,22.A screw 23 secures the parts 21,22 together. - Cylindrical section extensions 14 from the arms 1 3 overlap an inturned flange 8 at the mouth of the housing 10 and pairs of balls 1 5 on the flange accurately locate the extensions 14 relative to the flange. A compression spring 1 6 acts between the housing and the support member 1 2 to keep the extensions 14 normally located by the balls 15, with the axis 1 2A of the stylus parallel to the axis of travel of the first carriage to which the housing 10 is mounted.

In use, the force F, acting on the stylus by virtue of engagement with the workpiece 18, displaces the support member 1 2 from its rest position relative to the housing against the reactive force of the spring 1 6. When, on disengagement from the workpiece, the force F ceases, the spring and the convergent surfaces of the pairs of balls 1 5 co-operate to return the member 1 2 to the rest position.

The application of the force F causes stress in the support member 1 2 which is sensed by the three stress-responsive elements 20 being, in this example, piezo-electric crystals embodied in the arms 1 3. No relative movement between the parts 21,22 is intended, and the screw 23 is provided to secure the parts and elements firmly together and avoid displacement of the elements under the operating forces acting on the support member 1 2. The elements themselves are attached between the parts 21,22 by adhesive.

It will be appreciated that the stress in the member 1 2 due to the force F is a stress tending to bend the arms 1 3 which may be regarded as cantilevers. This is so because during a displacement from the rest position the member 1 2 is tilted and the uniform loading which the three arms have in the rest position becomes concentrated in the one or two of the arms on which the member 1 2 remains supported. Regardless of the direction of the force F transversely to the axis 12A, always one or two of the elements 20 are loaded owing to an increase in the bending stress, while the or each remaining elements is not loaded.

The elements 20 are each connected to two conductors 24 by which the voltage changes at the poles of the crystals are transmitted to amplifiers 25, each crystal having its own amplifier. The output of each amplifier is taken through a respective trigger circuit 26 which produces a pulse signal when the trans-t ient voltage exceeds a given threshold value.

The outputs of the trigger circuits are combined in an OR gate 27 whose output 28 is therefore enabled if any one of the elements 20 is subject to a stress level exceeding said threshold. The output 28 can be to electronic counters which, conveniently in respective digital displays, show quantitively the relative positions of the carriages. The output 28 interrupts the counters so that they stop at the moment of contact of the stylus with the workpiece and are not affected by the inevitable over-run of the carriages, which is permitted by the displacement of the support member 1 2 in the housing 10.

Although, as stated, there is no movement between the parts 21,22, there is nevertheless a change of strain between them. The screw 23 subjects the parts 21,22 to an initial stress producing an initial elastic deformation exerting like loads on the crystals.

When the force F is applied the stress pattern changes and the loads on the element 20 change accordingly. This change gives rise to the electrical output of the crystals. If the force on the stylus is axial the elements 20 undergo a uniform change of load and correspondingly produce uniform signals. This applies whether the stylus is pulled or pushed.

Thus the probe is omnidirectional; i.e. it responds to a force on the stylus in any direction.

The members 21,22 could be secured together by means other than a screw. They could for example be bonded together by adhesive, such as an epoxy resin adhesive, so long as contact of the stylus with the workpiece produces a sufficient change of stress in one or more of the piezo-electric crystals 20.

The piezo elements 20 are, in the foregoing embodiment, clamped between the two parts 21,22 of the stylus holder. Alternatively, the housing 10 could be in two parts, e.g. separated at the plane X-X, with the piezo elements secured between them.

Claims (6)

1. Probe for measuring workpieces, comprising two members being respectively a base and a stylus holder, a positioning device connecting the holder to the base for limited three-dimensional displacement from and return to a rest position thereon, sensor means for detecting engagement of the stylus with a workpiece in any of said three dimensions, the base being supportable in a machine for moving the probe in three dimensions relative to a workpiece to bring a stylus connected to the holder into engagement with the workpiece at a selected point thereon, the sensor means cooperating to output a signal responsive to such engagement for use by the machine to determine the coordinate position of the said point, characterised in that the sensor means being a stress sensor means embodied in one of said members.

2. Probe according to claim 1 wherein said stress sensor means is embodied in said holder and comprises at least one stressresponsive element.

3. Probe according to claim 1 wherein said stress sensor means comprise three stress-responsive elements each having an electrical response which goes positive or negative according to the direction of the stress, and the outputs of the elements are connected to a circuit responsive to one sign only of any one of the outputs so that the circuit is activated so long as at least one of the elements outputs a response of said one sign.

4. Prove according to claim 3, one of said members comprising two parts, said stressresponsive elements being arranged in parallel between said parts in firm engagement therewith.

5. Probe according to claim 1 or claim 2 wherein the positioning device comprises a first abutment provided on the holder and constraining the members against displacement in two dimensions, bias means for urging the holder into engagement with the base at said abutments thereby to establish a rest position for the holder, the holder being movable in opposition to the bias means and in the sense of tilting on the abutment of the base or being bodily displaced from the latter abutment.

6. Probe according to claim 5 wherein one of said abutments comprises three portions for engagement with the other abutment at three locations only, and three said stressresponsive elements are arranged adjacent said portions respectively.