GB2493214A - Metrological apparatus - Google Patents

Abstract

A stylus support carries a stylus 14 having a stylus tip 14a for following a workpiece surface which rotates relative to the stylus support 300. A transducer senses a displacement of the stylus tip as follows the surface. A support structure (24, fig.1) includes a mounting arm 200 and an attitude arm 100 carrying the stylus support and mounted to the mounting arm 200 by a kinematic coupling (figs 3 and 4) so as to be moveable between the first attitude in which a stylus extends in a first direction and a second attitude in which the stylus extends in a direction transverse to the first, the movement being substantially centred at the stylus tip. Different aspects of the invention have been specified in four independent claims. A first aspect defines a biasing mechanism for biasing the attitude arm into the first attitude when the attitude arm is moved towards the first attitude and for biasing the attitude arm into the second attitude when the attitude arm is moved towards the second attitude. Another aspect specifies the coupling comprising protrusions (104) and recesses (204) provided on opposing faces of the attitude and mounting arms, the protrusions and recesses being configured to cooperate to constrain relative motion of the opposing faces when the attitude arm is moved into one of the first and second attitudes.

Description

METROLOGICAL APPARATUS

This invention relates to metrological apparatus and is especially concerned with apparatus for measuring form, such as roundness and straightness, or errors therein.

For many manufactured products and components it is important that surface characteristics such as form and shape are within defined tolerances. Metrological instruments are known which measure such surface characteristics for quality control purposes. These rnetrological instruments must be manufactured to a high precision in order to minimise the effect of systematic errors introduced by the metrological instrument on measurement readings.

EP-A-0240151 discusses a metrological instrument which measures the form of a workpiece mounted on a turntable. In particular, a stylus gauge is supported relative to the turntable and generates a signal representative of the deflection of the stylus along a measurement direction as the stylus follows a surface of the workpiece on rotation of the turntable about a spindle axis. The stylus gauge is mounted on an attitude arm which is rotatable about an axis so that the stylus can be switched between a first configuration, or attitude, in which the stylus is generally parallel with the spindle axis and a second configuration, or attitude, in which the stylus is generally transverse to the spindle axis while generally maintaining the stylus tip in the same position. The stylus gauge also includes a stylus orientation mechanism for rotating the measurement direction.

In instruments such as those discussed in EP-A-0240151, mechanisms are provided for stopping the rotation of the attitude arm at a desired point corresponding to a required attitude. Such a mechanism may comprise a pin intersecting the rotational path of the attitude arm and fixing means such as screws or other adjusters for securing the arm in position so as to eliminate the degree of freedom provided on the side of the arm remote from the pin. The inventors in the present case have recognised that instruments such as those discussed in EP-A-0240151 can be used with greater speed and accuracy when a mechanism is provided for facilitating placement and maintenance of the attitude arm in one the first and second attitudes) mitigating the need for manual fixing and adjustment. Accordingly, there is provided an attitude arm mounting mechanism which accurately defines first and second attitudes corresponding to respective required measurement directions of a stylus carried on the attitude arm and which holds the attitude arm securely in place once the arm is moved to one of those attitudes.

According to a first aspect of the invention, there is provided metrological apparatus for measuring a surface characteristic of a workpiece, the metrological apparatus comprising a stylus support for carrying a stylus having a stylus tip, the stylus tip for following a surface of the workpiece as relative rotation about an axis 5 is effected between the workpiece and the stylus support, a transducer for sensing a displacement of the stylus tip as it follows the surface, and a support structure including an attitude arm and a mounting arm, the attitude arm carrying the stylus support, the attitude arm being moveably mounted to the mounting arm so as to be moveable between a first attitude in which the stylus extends in a first direction and a second attitude in which the stylus extends in a direction transverse to the first direction, the movement being substantially centred at the stylus tip so that the tip is in substantially the same position for the first and the second attitudes of the attitude arm, and a biasing mechanism for biasing the attitude arm into the first attitude when the attitude arm is moved towards the first attitude and for biasing the attitude arm into the second attitude when the attitude arm is moved towards the second attitude.

Thus, an attitude arm according to the invention can be moved from a first attitude corresponding to a first stylus direction to a second attitude corresponding to a second stylus simply by manual application of a force of sufficient magnitude to overcome a force biasing the attitude arm into a given attitude. The arm is accurately biased into the desired attitude by the biasing mechanism, avoiding the need to determine the attitude by eye or to define it using stops, for example. Once in the desired one of the two attitudes, the attitude arm is held securely in place by the biasing mechanism, mitigating the need to fix the attitude arm in place with fixing means such as screws or adjusters.

In one embodiment of the invention, the moveable mounting is provided by cooperating recesses and protrusions of opposing faces of the attitude arm and mounting arm.

In another embodiment of the invention, the protrusions and recesses are configured to cooperate in first and second relative orientations of the opposing faces to constrain relative motion therebetween, the first and second orientations corresponding to the first and second attitudes of the attitude arm.

According to a second aspect, there is provided metrological apparatus for measuring a surface characteristic of a workpiece, the metrological apparatus comprising a stylus support for carrying a stylus having a stylus tip, the stylus tip for following a surface of the workpiece as relative rotation about an axis is effected between the workpiece and the stylus support, and a transducer for sensing a displacement of the stylus tip as follows the surface a support structure including an attitude arm and a mounting arm, the attitude arm carrying the stylus support, the attitude arm being moveably mounted to the mounting arm by way of a bi-stable coupling having a first stable state in which a stylus extends in a first direction and a second stable state in which the stylus extends in a direction transverse to the first direction, wherein the stylus tip is in substantially the san-ic position for the first and the second stable states.

According to a third aspect of the invention, there is provided metrological apparatus for measuring a surface characteristic of a workpiece, the metrological apparatus comprising a stylus support for carrying a stylus having a stylus tip, the stylus tip for following a surface of the workpiece as relative rotation about an axis is effected between the workpiece and the stylus support and a transducer for sensing a displacement of the stylus tip as follows the surface, a support structure including an attitude arm and a mounting arm, the attitude arm carrying the stylus support, the attitude arm being mounted to the mounting arm by way of a kinematic coupling so as to be moveable between the first attitude in which a stylus extends in a first direction and a second attitude in which the stylus extends in a direction transverse to the first direction, the movement being substantially centred at the stylus tip so that the tip is in substantially the same position for the first and the second attitudes of the attitude arm, wherein the kinematic coupling comprises protrusions and recesses provided on opposing faces of the attitude and mounting arms, wherein the protrusions and recesses are configured to cooperate to constrain relative motion of the opposing faces when the attitude arm is moved into one of the first and second attitudes.

According to a fourth aspect of the invention, there is provided a support structure for carrying a stylus having a stylus tip and a stylus support of a metrological apparatus, the stylus support structure including an attitude arm and a mounting arm, the attitude arm carrying the stylus support, the attitude arm being moveably mounted to the mounting arm so as to be moveable between a first attitude in which the stylus extends in a first direction and a second attitude in which the stylus extends in a direction transverse to the first direction, the movement being substantially centred at the stylus tip so that the tip is in substantially the same position for the first and the second attitudes of the attitude arm, and a biasing mechanism for biasing the attitude arm into the first attitude when the attitude arm is moved towards the first attitude and for biasing the attitude arm into the second attitude when the attitude arm is moved towards the second attitude.

In an embodiment, apparatus includes a stylus support carries a stylus having a stylus tip for following a workpiece surface which rotates relative to the stylus support. A transducer senses a displacement of the stylus tip as follows the surface. A support structure includes a mounting arm and an attitude arm carrying the stylus support and mounted to the mounting arm by a kinematic coupling so as to be moveable between the first attitude in which a stylus extends in a first direction and a second attitude in which the stylus extends in a direction transverse to the first, the movement being substantially centred at the stylus tip. The coupling comprises protrusions and recesses provided on opposing faces of the attitude and mounting arms, the protrusions and recesses being configured to cooperate to constrain relative motion of the opposing faces when the attitude arm is moved into one of the first and second attitudes.

FIG.1 is a perspective view of a metrological system comprising metrological apparatus according to an embodiment of the invention and a data acquisition and control system; FlG.2 is a more detailed view of an attitude arm and a mounting arm forming part of the apparatus of FlG.1; FIG.3 is a perspective view of a portion of the mounting arm of FIG.2; FIGS.4A and 4B respectively show first and second perspective views of the attitude arm of FIG.2; FIG.5 is a first exploded view of a kinematic coupling between the mounting arm and the attitude arm of FIG.2; FIG.6 is a second exploded view of a kinematic coupling between the mounting arm and the attitude arm of FIG,2; and Figures 7 is a perspective view of a stylus support according to an embodiment of the invention.

FIG. 1 shows a metrological system 1 which is supported by a bench 4. The system 1 comprises metrological apparatus 2 and an acquisitions and control system 3. The metrological system 1 has a turntable 6 for supporting a workpiece being measured (not shown in FIG. 1) relative to a support structure 24 of the apparatus 2. The turntable 6 has a spindle axis which, in use, provides an axis of rotation 5 of the workpiece relative to the support structure 24 (and which, therefore, is hereinafter referred to as the axis of relative rotation). The apparatus includes a stylus support 300 carrying a stylus 14, the stylus having a stylus tip 14a for abutting a surface of the workpiece being measured. A transducer (not shown in FIG. 1) is coupled to the stylus for monitoring movement of the stylus tip 14a. The transducer may be inductive or capacitive or optical, for example. The stylus support 300 is carried on an attitude arm 100 of the support structure 24. The attitude arm is moveably mounted to a free end of a mounting arm 200 of the support structure for rotational movement about an axis 10 (hereinafter referred to as the attitude axis), the axis being angled at 45 degrees to a plane which is normal with respect to the axis of relative rotation 5. In the example shown in Figure 1, the turntable 6 is mounted horizontally on the workbench 4 and the attitude axis is angled at 45 degrees relative to the horizontal. The moveable mounting of the attitude arm 100 allows the arm to be moved between a first attitude and a second attitude. A biasing mechanism (not shown in FIG. 1) coupled through or about the moveable mounting biases the attitude arm 100 into the first attitude when the attitude arm 100 is moved towards the first attitude and biases the attitude arm 100 into the second attitude when the attitude arm 100 is moved towards the second attitude. In the example shown, a vertical support body 8 of the support structure 24 carries a carriage 9. The mounting arm 300 is moveably mounted to the carriage 9 for movement relative to the carriage 9 in a horizontal plane.

The carriage 9 is moveable between different vertical positions on the support body 8. In this way, the mounting arm 300 and therefore stylus tip 14a is moveable in directions parallel with and transverse to the axis of relative rotation 5, as described in EP-A-0240151. In another example, the mounting arm 300 is statically mounted to the support body 8.

In use, as the turntable 6 rotates, the stylus tip 14a follows the surface of a workpiece. Any deflection of the stylus tip 14a is converted by the transducer into an electrical signal which is processed by a remote processing unit 20 to determine at least one of a number of surface characteristics of the workpiece. The results may be displayed on a screen 16, or printed by a printer 22. The apparatus 2 may be used to measure the surface characteristics of components. For example, for a rotationally symmetrical component the measurements which the metrological apparatus 2 can perform include the roundness of exterior circumferential surfaces and interior circumferential surfaces and the straightness and surface roughness of surfaces parallel to the plane of the turntable 6.

The turntable 6 may include a centring motor (not shown) which allows transverse movement of a platform of the turntable 6 on which the workpiece rests relative to the axis of relative rotation 5 along a traverse direction to allow the workpiece to be centred about the axis of relative rotation 5.

In the embodiment shown in FIG.1, the metrological apparatus 2 is able to perform a conventional automated centring operation using the centring motor, and also a user control is provided for manually nudging the platform in a stepwise manner along the traverse direction.

In this embodiment, the stylus 14 is constrained to move to and from a rest position in only one nominally linear direction (hereinafter called the measurement direction) on one side of the rest position, and the transducer is arranged to measure deflections of the stylus 14 in the measurement direction. The stylus 14 is maintained in a given measurement direction by a biasing mechanism 400 providing a biasing force in a direction opposed to the measurement direction and by any appropriate additional support mechanism. In the present embodiment, the stylus 14 is pivotable about a pivot axis. In another example, the stylus is axially moveable such that displacement of the tip 14a in response to the measurement surface causes the stylus 14 to move in a direction defined by its axis.

As shown in more detail in FIG. 2, the attitude arm 100 is moveably coupled to the mounting arm 200 so as to be rotatable through 180 degrees about the attitude axis 10 between a first attitude (represented by the solid lines in FIG. 2) in which the stylus 14 extends in a first direction, being in this example a direction substantially parallel with the axis of relative rotation 5, and a second attitude (represented by the phantom lines in FIG. 2) in which the stylus 19 extends in a direction transverse to the first direction, being in this example a direction substantially transverse with the axis of relative rotation S. Rotation of the attitude arm 100 between the first and second attitudes is substantially centred at the stylus tip 14a so that the tip is in substantially the same position for the both the first and the second attitudes. In this way, the stylus tip 14a can access different measurement surfaces and perform different types of measurement operation. For example, an edge of the workpiece may be measured when the stylus 14 extends in the first direction (with the attitude arm in first attitude) and an upper surface of the workpiece when the stylus 14 is in the second direction. In a possibility, described with reference to FIG.7, the orientation of the stylus support 300 may be adjusted to provide access to further measurement directions and measurement types.

Movement between the first and second attitudes is provided by a kinematic coupling between the mounting arm 200 and the attitude arm 100. An example of a kinematic coupling is shown in FIGS. 3 and 4, in which respective faces 102, 202 of an attitude arm 100 and mounting arm 200 provide a kinematic coupling therebetween. In a general example, the kinematic coupling comprises six elongate recesses on one face configured to mate with three hemispherical protrusions on the other face. When mated, each hemispherical protrusion has two points of contact with a respective groove, that is, a point one point of contact with each elongate edge of the groove, so that in total the three protrusions have six points of contact so that the face having the protrusions is kinematically constrained in each of its six degrees of freedom.

FIG.3 shows a portion of the mounting arm 200 having a face 202 with a plurality of recesses 204 configured to cooperate with a plurality of protrusions 104 of a face 102 of the attitude arm 100 as shown in FIGS. 4A and 4B. In the present embodiment, the recesses comprise six grooves 204a, b, c, d, e, f extending radially from a notional common centre point on the face 202. The grooves 204a, b, c, d, e, fare spaced apart by an aperture (the face aperture) centred on the notional centre point. A through-hole 220 extends between the face aperture and a rear aperture in a rear surface of the mounting arm 200 for receiving a biasing element. The grooves 204a, b, c, d, e, f have substantially the same size and geometry as one another and the combination of grooves has rotational symmetry of order 2.

FIG.4B shows an attitude arm 100 having a face 102 with a configuration of protrusions 104. At a centre point of the configuration is a projection 420 providing part of a biasing mechanism 400. In the present embodiment, the protrusions 104 are embedded ball bearings 104a, b, c but alternatively they could be hemispheres. The protrusions 104a, b, c have substantially the same size as one another and each is configured to be received into a corresponding one of the grooves 204a, b, c, d, e, f. The configuration is triangular and has rotational symmetry of order 1 such that in a first relative orientation of face 102 to face 202 the protrusions 104a, band c cooperate with a first set of three recesses 204a, b, and c. The cooperation between this combination of protrusions and recesses kinematically constrains face 102 relative to face 202. In this example, this corresponds to the attitude arm 100 being in its first attitude. In a second relative orientation of the faces 102, 202 which is angularly spaced from the first orientation by 180 degrees, the protrusions 104a, b and c cooperate with a second set of recesses 204d, e, and f, the second set comprising recesses distinct from those of the first set. In this example, this corresponds to the attitude arm 100 being in its second attitude.

The symmetry of the faces 102, 202 thus provides a bi-stable coupling with a first stable state providing a first attitude and therefore a first stylus measurement direction, and the second stable state providing a second attitude angularly spaced by 180 degrees from the first attitude and a second measurement direction transverse to the first measurement direction.

Couplings comprising other configurations of protrusions and recesses 104, 204 are possible. In one possibility, the attitude face 102 has the recesses and the mounting face 202 has the protrusions. In another possibility, a combination of protrusions and recesses 104, 204 of one of the faces 102, 202 is configured to cooperate with a corresponding combination of protrusions or recesses 104, 204 of the other face 102, 202.

For every configuration of the attitude arm 100, the coupling is biased into or towards one of the first and second stable states by a biasing mechanism 400. Figures Sand 6 are exploded views showing S the components of a biasing mechanism 400 according to an embodiment of the present invention.

In this example, the biasing mechanism comprises a pin 410 configured to pass through a through-hole 200 of the mounting arm 200 to cooperate with a receiving member 420 of the attitude arm 100, and a spring 402 for providing a biasing force. The through-hole 220 of the mounting arm 200 is configured to receive the receiving member 420 through a face aperture and the pin 410 through a rear aperture. In this example, spring 402 is carried on the pin 410. The pin 410 is secured relative to the rear aperture by a flange 404 coupled to the pin 410.

In use the pin 410 and retaining member 420 cooperate within the through-hole 220. The pin 410 is received through an aperture in a surface 424 of the receiving element 420. In the present example, the surface 424 is supported on the spring 402 which is mounted in an extended configuration on the flange 404 and about the pin 410 so as to bias the pin 410 and receiving member 420 into closer cooperation. In this way, a biasing force is applied to the coupling faces 102, 202 to urge the faces together.

FIGS. Sand 6 illustrate biasing of the attitude arm 100 in the first attitude. The attitude arm 100 can be moved to the second attitude by the manual application of a force sufficient to overcome the biasing force. The bi-stable kinematic coupling is displaced from the first stable state as the attitude arm 100 is moved from the first attitude. As the arm 100 is moved further towards the second attitude, the biasing mechanism serves to bias the coupling into the second stable state, and thus the attitude arm into the second attitude.

To perform some types of measurement operation it may be desirable to alter the orientation of the stylus tip 14a for a given attitude of the attitude arm 100.

Accordingly, in an embodiment of the invention, the stylus support 300 is moveably coupled to the attitude arm 100 to allow movement of the stylus 14 between different orientations. In one possibility, the moveable mounting is provided by cooperating recesses and protrusions 204, 104 of coupling surfaces 302, 112 of the stylus support 300 and attitude arm 100. In a further possibility, one of the coupling surfaces 302, 112 has the recesses and the other of the coupling surfaces 302, 112 has the protrusions. The cooperating recesses and protrusions 204, 104 are configured to cooperate for a plurality of relative orientations of the coupling surfaces 302, 112, each orientation defining a stylus orientation.

FIG.7 shows an example of a stylus support 300 having a body 306 for carrying the stylus 14. The S stylus is received into aperture 310 on a lower face 312 of body 310. The stylus has a spigot for slotting into recess 308a in the lower face 312 to anchor the stylus 14 relative to the body 310, and optionally has further spigots for slotting into recesses 308b and 308c for further anchoring the stylus 14. A coupling surface 302 of the stylus support 300 is configured to cooperate with a coupling surface 112 contained within the recess 110 of the attitude arm 100. In the present example, the recesses are v-shaped grooves 304 provided in the stylus surface 302 and the protrusions are ball bearings (not shown) embedded in the attitude arm coupling surface 112. As illustrated in Figure 4B, coupling surface 112 has raised portions upon which the body of the stylus gauge 300 is supported, while recessed portions provide cavities (not shown) into which the ball bearings are embedded. In this embodiment, the stylus support coupling surface 302 and attitude arm coupling surface 112 have four equally angularly spaced recesses 304a, b, c, d and protrusions (not shown) respectively so that the stylus support 300 can be rotated between four coupling orientations in which its coupling surface 302 cooperates with the attitude arm coupling surface 112. The four coupling orientations therefore providing four stylus orientations spaced apart by 90 degrees.

MODIFICATIONS AND FURTHER EMBODIMENTS

As described above, a kinematic coupling between an attitude arm 100 and a mounting arm 200 allows movement of the attitude arm 100 between two attitudes which correspond to respective measurement directions of a stylus 14. It will be understood that a kinematic coupling could be provided which allows movement of the attitude arm 100 between more than two attitudes, thereby enabling easy and accurate access to a greater number of measurement directions.

Similarly, the optional kinematic coupling of the stylus support 300 to the attitude arm 100 of the described example provides four equally spaced stylus orientations. In an alternative example, the coupling could provide more or fewer stylus orientations which need not be equally spaced.

In the illustrated embodiment, the protrusions 104 and recesses 204 providing the kinematic coupling are respectively embedded ball bearings and grooves. In another embodiment each of the protrusions 104 could be provided by, for example, hemispheres provided on the face 102, 202 -10 -providing the protrusions 104. Alternatively, the protrusions could comprise any appropriate curved surface. The recesses 204 could comprise, for example, a v-shaped groove, u-shaped groove or any other recess configured to provide two points of contact to a protrusion 104, once received into the recess 204, to kinematically restrict the protrusion 104 in a single direction whilst allowing the individual protrusion 104 a degree of freedom in a direction transverse to the restricted direction so as to increase the ease of receipt of the protrusion 104 into the recess 204. The same applies to the optional kinematic coupling of the stylus support 300 to the attitude arm 100.

While in the illustrated embodiment, the biasing mechanism 400 is described as comprising a receiving member 420 provided on the attitude arm 100 and a pin 410 provided through the mounting arm 300, in another embodiment the receiving member 420 could be provided on the mounting arm 200 and the pin 410 provided through the attitude arm 100. While in the embodiment, the spring 402 is provided about the pin 410 to bias the surface of the receiving member 420 towards the flange 404, in another possibility the spring 402 could be provided in a loaded configuration beneath the pin 410 so as to bias the pin further inside the aperture of the receiving member 420. As a further possibility, the biasing mechanism 400 could be arranged about the attitude arm 100 rather than through the faces 102, 202 that provide the kinematic coupling and could comprise any appropriate components. Any other appropriate mechanical or non-mechanical biasing mechanism, for example a magnetic mechanism, for urging together faces 102 and 202 may be used.

Claims (1)

1. <claim-text>-11 -CLAIMS1. Metrological apparatus for measuring a surface characteristic of a workpiece, the metrological apparatus comprising: a stylus support for carrying a stylus having a stylus tip, the stylus tip for following a surface of the workpiece as relative rotation about an axis is effected between the workpiece and the stylus support; a transducer for sensing a displacement of the stylus tip as it follows the surface; and a support structure including an attitude arm and a mounting arm, the attitude arm carrying the stylus support, the attitude arm being moveably mounted to the mounting arm so as to be moveable between a first attitude in which the stylus extends in a first direction and a second attitude in which the stylus extends in a direction transverse to the first direction, the movement being substantially centred at the stylus tip so that the tip is in substantially the same position for the first and the second attitudes of the attitude arm, and a biasing mechanism for biasing the attitude arm into the first attitude when the attitude arm is moved towards the first attitude and for biasing the attitude arm into the second attitude when the attitude arm is moved towards the second attitude.</claim-text> <claim-text>2. Apparatus according to claim 1, wherein the moveable mounting is provided by cooperating recesses and protrusions of opposing faces of the attitude arm and mounting arm.</claim-text> <claim-text>3. Apparatus according to claim 2, wherein the protrusions and recesses are configured to cooperate in first and second relative orientations of the opposing faces to constrain relative motion therebetween, the first and second orientations corresponding to the first and second attitudes of the attitude arm.</claim-text> <claim-text>4. Apparatus according to claim 3, wherein the second orientation is rotationally spaced from the first orientation by 180 degrees.</claim-text> <claim-text>-12 - 5. Apparatus according to any of claims 2 to 4, wherein one of the opposing faces has the recesses and the other of the opposing faces has the protrusions.</claim-text> <claim-text>6. Apparatus according to any of claims 2 to 5, wherein the recesses are grooves extending radially from a common centre point on the face having the grooves and the protrusions are embedded ball bearings, each hemisphere being configured to slot into a corresponding groove.</claim-text> <claim-text>7. Apparatus according to any of claims 2 to 6, wherein the recesses number six and the protrusions number three, the protrusions having a triangular configuration such that each of the protrusion cooperates with one of a first set of three recesses in the first orientation and one of a second set of three recesses in the second orientation, the recesses of the first set being distinct from those of the second set.</claim-text> <claim-text>8. Apparatus according to any of claims 1 to 7, wherein one of the mounting arm and the attitude arm has a projection of the biasing mechanism and the other of the mounting arm and the attitude arm has a through-hole with a first open end for receiving the projection and a second open end for receiving a biasing member of the biasing mechanism, the biasing member and the projection being configured to cooperate within the through-hole.</claim-text> <claim-text>9. Apparatus according to any of claims ito 8, wherein the biasing mechanism is operable to urge together the opposing faces of the attitude arm and mounting arm.</claim-text> <claim-text>10. Metrological apparatus for measuring a surface characteristic of a workpiece, the metrological apparatus comprising: -13 -a stylus support for carrying a stylus having a stylus tip, the stylus tip for following a surface of the workpiece as relative rotation about an axis is effected between the workpiece and the stylus support and a transducer for sensing a displacement of the stylus tip as follows the surface; a support structure including an attitude arm and a mounting arm, the attitude arm carrying S the stylus support, the attitude arm being moveably mounted to the mounting arm by way of a bi-stable coupling having a first stable state in which a stylus extends in a first direction and a second stable state in which the stylus extends in a direction transverse to the first direction, wherein the stylus tip is in substantially the same position for the first and the second stable states.</claim-text> <claim-text>11. Apparatus according to claim 10, wherein the bi-stable coupling is a kinematic coupling.</claim-text> <claim-text>12. Apparatus according to claim 10 or 11, wherein the second stable state is rotationally separated from the first stable state by 180 degrees.</claim-text> <claim-text>13. Apparatus according to any of claims 10 to 12, wherein the bi-stable coupling is provided by cooperating recesses and protrusions of opposing faces of the attitude arm and mounting arm.</claim-text> <claim-text>14. Apparatus according to claim 13, wherein the protrusions and recesses are configured to cooperate in first and second relative orientations of the opposing faces to constrain relative motion therebetween, the first and second orientations corresponding to the first and second stable states, the second orientation being rotationally separated from the first orientation by 180 degrees.</claim-text> <claim-text>15. Apparatus according to any of claims 10 to 14, wherein one of the opposing faces has the recesses and the other of the opposing faces has the protrusions.</claim-text> <claim-text>-14 - 16. Apparatus according to any of claims 10 to 15, wherein the recesses are grooves extending radially from a common centre point on the face having the grooves and the protrusions are embedded ball bearings, each hemisphere being configured to slot into a corresponding groove.</claim-text> <claim-text>17. Apparatus according to any of claims 13 to 16, wherein the recesses number six and the protrusions number three, the protrusions having a triangular configuration such that each of the protrusion cooperates with one of a first set of three recesses in the first orientation and one of a second set of three recesses in the second orientation, the recesses of the first set being distinct from those of the second set.</claim-text> <claim-text>18. Apparatus according to any of claims 10 to 17, wherein one of the mounting arm and the attitude arm has a projection of the biasing mechanism and the other of the mounting arm and the attitude arm has a through-hole with a first open end for receiving the projection and a second open end for receiving a biasing member of the biasing mechanism, the biasing member and the projection being configured to cooperate within the through-hole.</claim-text> <claim-text>19. Apparatus according to any of claims 10 to 18, wherein the biasing mechanism is operable to urge together the opposing faces of the attitude arm and mounting arm.</claim-text> <claim-text>20. Metrological apparatus for measuring a surface characteristic of a workpiece, the metrological apparatus comprising: a stylus support for carrying a stylus having a stylus tip, the stylus tip for following a surface of the workpiece as relative rotation about an axis is effected between the workpiece and the stylus support and a transducer for sensing a displacement of the stylus tip as follows the surface; a support structure including an attitude arm and a mounting arm, the attitude arm carrying the stylus support, the attitude arm being mounted to the mounting arm by way of a kinematic coupling so as to be moveable between the first attitude in which a stylus extends in a first direction and a second attitude in which the stylus extends in a direction transverse to the first direction, the -15 -movement being substantially centred at the stylus tip so that the tip is in substantially the same position for the first and the second attitudes of the attitude arm, wherein: the kinematic coupling comprises protrusions and recesses provided on opposing faces of the attitude and mounting arms, wherein the protrusions and recesses are configured to S cooperate to constrain relative motion of the opposing faces when the attitude arm is moved into one of the first and second attitudes.</claim-text> <claim-text>21. Apparatus according to claim 20, wherein the protrusions and recesses are configured to cooperate in first and second relative orientations of the opposing faces to constrain relative motion therebetween, the first and second orientations corresponding to the first and second attitudes of the attitude arm.</claim-text> <claim-text>22. Apparatus according to claim 21, wherein the second orientation is rotationally spaced from the first orientation by 180 degrees.</claim-text> <claim-text>23. Apparatus according to any of claims 20 to 22, wherein one of the opposing faces has the recesses and the other of the opposing faces has the protrusions.</claim-text> <claim-text>24. Apparatus according to any of claims 20 to 23, wherein the recesses are grooves extending radially from a common centre point on the face having the grooves and the protrusions are embedded ball bearings, each hemisphere being configured to slot into a corresponding groove.</claim-text> <claim-text>25. Apparatus according to claim 24, wherein the recesses number six and the protrusions number three, the protrusions having a triangular configuration such that each of the protrusion cooperates with one of a first set of three recesses in the first orientation and one of a second set of three recesses in the second orientation, the recesses of the first set being distinct from those of the second set.</claim-text> <claim-text>-16 - 26. Apparatus according to any of claims 20 to 25, wherein one of the mounting arm and the attitude arm has a projection of the biasing mechanism and the other of the mounting arm and the attitude arm has a through-hole with a first open end for receiving the projection and a second open S end for receiving a biasing member of the biasing mechanism, the biasing member and the projection being configured to cooperate within the through-hole.</claim-text> <claim-text>27. Apparatus according to any of claims 20 to 26, wherein the biasing mechanism is operable to urge together the opposing faces of the attitude arm and mounting arm.</claim-text> <claim-text>28. A support structure for carrying a stylus having a stylus tip and a stylus support of a metrological apparatus, the stylus support structure including: an attitude arm and a mounting arm, the attitude arm carrying the stylus support, the attitude arm being moveably mounted to the mounting arm so as to be moveable between a first attitude in which the stylus extends in a first direction and a second attitude in which the stylus extends in a direction transverse to the first direction, the movement being substantially centred at the stylus tip so that the tip is in substantially the same position for the first and the second attitudes of the attitude arm, and a biasing mechanism for biasing the attitude arm into the first attitude when the attitude arm is moved towards the first attitude and for biasing the attitude arm into the second attitude when the attitude arm is moved towards the second attitude.</claim-text> <claim-text>29. Apparatus according to claim 28, wherein the moveable mounting is provided by cooperating recesses and protrusions of opposing faces of the attitude arm and mounting arm.</claim-text> <claim-text>30. Apparatus according to claim 29, wherein the protrusions and recesses are configured to cooperate in first and second relative orientations of the opposing faces to constrain relative motion therebetween, the first and second orientations corresponding to the first and second attitudes of the attitude arm.</claim-text> <claim-text>-17 - 31. Apparatus according to claim 30, wherein the second orientation is rotationally spaced from the first orientation by 180 degrees.</claim-text> <claim-text>32. Apparatus according to any of claims 29 to 31, wherein one of the opposing faces has the recesses and the other of the opposing faces has the protrusions.</claim-text> <claim-text>33. Apparatus according to any of claims 29 to 32, wherein the recesses are grooves extending radially from a common centre point on the face having the grooves and the protrusions are embedded ball bearings, each hemisphere being configured to slot into a corresponding groove.</claim-text> <claim-text>34. Apparatus according to any of claims 29 to 33, wherein the recesses number six and the protrusions number three) the protrusions having a triangular configuration such that each of the protrusion cooperates with one of a first set of three recesses in the first orientation and one of a second set of three recesses in the second orientation, the recesses of the first set being distinct from those of the second set.</claim-text> <claim-text>35. Apparatus according to any of claims 28 to 34, wherein one of the mounting arm and the attitude arm has a projection of the biasing mechanism and the other of the mounting arm and the attitude arm has a through-hole with a first open end for receiving the projection and a second open end for receiving a biasing member of the biasing mechanism, the biasing member and the projection being configured to cooperate within the through-hole.</claim-text> <claim-text>36. Apparatus according to any of claims 28 to 35, wherein the biasing mechanism is operable to urge together the opposing faces of the attitude arm and mounting arm.</claim-text> <claim-text>-18 - 37. Apparatus according to any preceding claim, wherein the stylus support comprises a biasing mechanism for biasing the stylus in a measurement direction.</claim-text> <claim-text>38. Apparatus according to claim 37, wherein the stylus support is moveably mounted to the attitude arm to allow movement of the stylus between different orientations.</claim-text> <claim-text>39. Apparatus according to claim 38, wherein the moveable mounting is provided by cooperating recesses and protrusions of coupling surfaces of the stylus support and attitude arm.</claim-text> <claim-text>40. Apparatus according to claim 39, wherein one of the coupling surfaces has the recesses and the other of the coupling surfaces has the protrusions.</claim-text> <claim-text>41. Apparatus according to claim 39 or 40, wherein the cooperating recesses and protrusions are configured to cooperate for a plurality of relative orientations of the coupling surfaces, each orientation defining a stylus orientation.</claim-text> <claim-text>42. Apparatus according to claim 39,40 or 41, wherein both coupling surfaces have rotational symmetry of order 4 so that the stylus support can be rotated between four orientations in which its coupling surface cooperates with the attitude arm coupling surface, the four orientations being angularly spaced by 90.</claim-text> <claim-text>43. Apparatus according to any preceding claim, wherein the attitude arm is mounted to the mounting arm for rotational movement about an attitude axis at substantially 45 degrees to a plane which is normal with respect to the axis of relative rotation.</claim-text> <claim-text>-19 - 44. Apparatus according to any of claims ito 42, wherein the attitude arm is mounted to the mounting arm for rotational movement about an attitude axis that substantially bisects an angle subtended at the stylus tip by the first and second stylus directions.</claim-text> <claim-text>45. Apparatus according to any preceding claim, comprising a turntable for effecting the relative rotation of the workpiece and the stylus support about the axis of relative rotation.</claim-text> <claim-text>46. Metrological apparatus as substantially described herein, and/or with reference to the accompanying drawings.</claim-text>