GB1573447A

GB1573447A - Apparatus for measuring displacement in at least two orthogonal dimensions

Info

Publication number: GB1573447A
Application number: GB1188176A
Authority: GB
Original assignee: Rolls Royce PLC; Renishaw Electrical Ltd
Current assignee: Rolls Royce PLC; Renishaw Electrical Ltd
Priority date: 1976-03-24
Filing date: 1976-03-24
Publication date: 1980-08-20

Description

(54) APPARATUS FOR MEASURING DISPLACEMENT IN AT LEAST TWO ORTHOGONAL DIMENSIONS (71) We, ROLLS-ROYCE LIMITED, a British company of 65 Buckingham Gate, London, SWIPE, 6AT, formerly known as Rolls-Royce (1971) Limited, a British company of Norfolk House, St. James's Square, London, SWlY 4JR, and RENISHAW ELECTRICAL LIMITED, a British company of Gloucester Street, Wotton-under-Edge, Gloucestershire GL12 7DN, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to apparatus for measuring displacement in at least two orthogonal dimensions.

It is known for such apparatus to comprise an elongate stylus supported on a support member for pivotal movement about a centre and for linear movement along an axis containing said centre, means biasing the stylus into a rest position in which the stylus extends along said axis and from which the stylus is displaceable by said movements, and a sensor arranged between the stylus and the support in a position offset from said axis and so as to sense a said pivotal as well as a said linear movement.

In the known apparatus it is impossible to discriminate between sensor output due to a said pivotal and due to a said linear movement in cases where the stylus is moved simultaneously in the pivotal and the linear mode. It is an object of this invention to overcome this difficulty.

According to this invention there is provided apparatus for measuring displacement in at least two orthogonal dimensions comprising an elongate stylus supported on a support member for pivotal movement about a centre and for linear movement along an axis containing said centre, means biasing the stylus into a rest position in which the stylus extends along said axis, and from which the stylus is displaceable by said movements, a first sensor arranged between the stylus and the support in a position offset from said axis and so as to sense a said pivotal as well as a said linear movement, a second sensor arranged between the stylus and the support member in a position to sense at least a said linear movement, and means for forming the algebraic difference between the outputs of the two sensors thereby to produce in respect of the first sensor a signal indicative of a said pivotal movement only.

Examples of apparatus according to this invention, including probes and associated electrical systems, will now be described with reference to the accompanying drawings wherein: Fig. 1 is a sectional elevation of first example of a probe, Fig. 2 is a section on the line II--II in Fig.

1, Fig. 3 is a diagram of an electrical system connected to the probe shown in Figs. 1 and 2, Fig. 4 is a sectional elevation of a second example of a probe, Fig. 5 is a section on the line V-V in Fig.

4, Fig. 6 is a diagram of an electrical system connected to the probe shown in Figs. 4 and 5, Fig. 7 is a third example of a probe Fig. 8 is a section on the line VIlI-VIlI in Fig. 7, Fig. 9 is a diagram of an electrical system connected to the probe shown in Figs. 7 and 8, Fig. 10 is a sectional elevation of a machine supporting a probe for movement, and Fig. 11 is a modification of the system shown in Fig. 3.

Referring to Figs. 1 and 2, a probe 9 comprises a stylus 10 secured to a spring 11 which has four limbs 12 extending between the stylus and a support member of housing 14. The spring is made of sheet material and extends in a plane A-A perpendicular to the axis of symmetry, denoted B, of the probe. The stylus comprises parts 16, 17 extending respectively below and above a central part 15 of the spring 11, the parts 16, 17 being screwed one into the other to grip the part 15 between them. The limbs 12 have between them two mounting flanges 18 whereby the spring is secured to the housing 14.

The basic shape of each limb 12 is shown shaded at 12A in Fig. 2 from which it is clear that the limb extends curvedly, generally in shape of the letter "U", between the central part 15 and a said flange 18. The spring shape includes, at each limb, a part 19 providing a connection between the limb and the flange 18.

The housing 14 has an arbour 21 for attachment to the operating head of a coordinate measuring machine (known per se and not illustrated) wherein the head is supported by respective linear carriages for movement in three dimensions indicated X, Y, Z in Figs. 1 and 2. The carriages are connected to measuring devices for the continuous monitoring of the head position relative to a given datum. In operation the head is moved relative to a workpiece until a spherical end 22 of the stylus touches a face of the workpiece. Any movement of the head beyond the point of touch results in displacement of the stylus relative to the head, and such displacement is used for determining the position of said face of the workpiece relative to a datum in the measuring machine.

When the spring 11 is free, i.e. when there is no force, other than gravity, acting on the end 22 of the stylus, the stylus is said to be in the rest position. A said displacement of the stylus, i.e. of the end 22, is accommodated by deflection of the spring involving bending or twisting of the limbs 12. During displacement in the X and Y direction the stylus pivots about the centre, denoted C, of the spring and during Z displacement the centre C travels along the axis B. Parallel motion of the stylus in the X or Y direction is avoided by the limbs 12 having substantially greater extent in the plane A-A than the thickness of the spring sheet material so that while the limbs can bend or twist they can resist forces lying in the plane A-A itself.

Displacement of the stylus is measured by four sensing transducers 25A, 25B, 25C, 25D arranged on a common pitch circle about the axis B and on two diameters lying respectively in the X and Y directions (Fig.

2). Each transducer comprises induction windings 26 and a movable core 27 such that the inductions of the windings vary in accordance with a change in core position.

Such transducers are known per se. Each core 27 is connected by a flexible wire to a respective arm 28 secured to the stylus.

Each transducer 25 is connected to an induction bridge 30 (Fig. 3) for producing an output which is positive, negative or zero according to the balance of the bridge, The outputs of the four bridges are denoted 29A, 29B, 29C, 29D respectively. The transducers are so positioned and the bridges so adjusted that all transducer outputs are zero when the stylus is at rest.

The bridge outputs 29A, 29B are connected, one through an inverter 31, to a summing amplifier 32 for forming the difference 29A-29B. It will be clear that, since during movement of the stylus in the X direction the outputs 29A, 29B become respectively positive and negative, the amplifier output is a signal 33 proportional to X displacement of the stylus and has a sign indicative of the sense of direction away from the rest position.

The bridge outputs 29C, 29D are connected through, the output 29C going to an inverter 35, to a summing amplifier 36 to produce a signal 37 proportional to Y displacement of the stylus.

Further, all the bridge outputs 29A, 29B, 29C, 29D are connected to a summing amplifier 39 whose output 40 is therefore proportional to Z displacement of the stylus and has a sign corresponding to the sense of the Z displacement. Thus Z displacement is sensed by the same transducers as X and Y displacement without loss of discrimination between the three dimensions.

If the stylus is moved such that there are component movements in two or three dimensions the respective amplifier outputs show these components separately.

The outputs 33, 37, 40 are each connected to a respective calibrating potentiometer 42, analogue to digital converter 43, and digital display units 44. The separate potentiometers are necessary because the outputs 33, 37 are not to the same scale as the output 40, the relationship being as 2:4, due to amplifiers 32, 36 each having two unit inputs while the amplifier 40 has four unit inputs.

In a modification, not illustrated, only one output of each pair of transducers is used for determining the Z dimension. For example, the outputs 29A and 29B only may be connected to the summing amplifier 39.

This will generate a true Z reading though possibly not to as high an accuracy as in the case of using both outputs of each pair of transducers.

The probe shown in Figs. 4, 5 has a stylus 110 secured to a spring 111 supported in a housing 114. The arrangement of the stylus, spring and housing is generally the same as in Figs. 1 and 2, the spring being arranged in a plane A-A and permitting displacement of the stylus from a rest position in the X, Y and Z directions, the Z direction being that of the axis of symmetry B of the probe.

Displacement of the stylus is measured by three sensing transducers 125A, 125B, 125C each having induction windings 126 and a movable core 127. Two of the transducers, 125A and 125C are situated in positions offset from the axis B respectively in the X and Y directions the windings and cores being secured to the housing 114 and stylus 110 respectively. The third transducer 125B is situated on the axis B in a hollow part 150 of the stylus and its winding is secured to the latter part while the core is secured to the housing 114 by a wire 151.

The transducers 125 are connected to respective induction bridges 130A, 130B, 130C (Fig. 6) having outputs 129A, 129B, 129C respectively. The outputs 129A, 129B are connected, one through an inverter 131, to a summing amplifier 132 which, accordingly, has an output 133 proportional to displacement of the stylus in the X direction. The outputs 129B, 129C are connected, one through an inverter 135 to a summing amplifier 136 which, accordingly, has an output 137 proportional to displacement of the stylus in the Y direction. It will be clear that as fas as the measurement of X and Y dimensions is concerned the output 129B plays no part unless there is also a displacement in the Z direction. In the latter case the connections through the inverters 131, 135 ensure that the Z displacement does not falsify the measurement in the X or Y dimension.

The output 129B itself is proportional to displacement in the Z direction and is simply connected to an amplifier 139 having an output 140.

The three outputs 133, 137, 140 are taken through respective potentiometers for being calibrated into 1:1 correspondence with the respective stylus displacements.

The probe shown in Figs. 7, 8 has stylus 210 secured to a spring 211 supported in a housing 214. The arrangement of the stylus, spring and housing is generally the same as in Figs. 1, 2, the spring being arranged in a plane A-A and permitting displacement of the stylus from a rest position in the X, Y and Z directions, the Z direction being that of the axis of symmetry B of the probe.

Displacement of the stylus is measured by three sensing transducers 225A, 225B, 225C each having induction windings 226 and a movable core 227. The transducers are situated in equispaced relationship on a common pitch circle centred on the axis B.

The transducer 225A is situated on a radius extending in the X direction. The transducers 225B, 225C are situated on respective radii each forming an angle of 60 with the X direction and lie on a line parallel to the Y direction. The windings 226 of the transducers are secured to the housing 214. The cores 227 of the transducers are secured to the stylus by respective rods 228.

The transducers 225A, 225B, 225C are connected to respective induction bridges 230A, 230B, 230C (Fig. 9) having outputs 229A, 229B, 229C respectively.

During displacement of the stylus in the X direction the output 229A becomes positive while the two outputs 229B and 229C become both negative, or vice versa.

However, in view of the 60 angle of the arms 228B, 228C the output of the transducers 225B and 225C are each one half of the output of the transducer 225A.

To measure movement in the X direction the outputs 229B, 229C are connected to a summing amplifier 241 whose output is passed through an inverter 231 whose output is in turn passed together with the output 129A to a summing amplifier 232 having an output 233 given by the difference: 233=229A-(-229B +(-229C)) =229A+229B +229C which is proportional to the displacement of the stylus in the X direction. Assuming the magnitude of the signal 229A during an X displacement to be unity, them having regard to the signals 229B, 229C each being one half unit, the signal 233 has a magnitude of two said units.

During movement of the stylus in the Y direction the transducer 225A is unaffected while the transducers 225A, 225C produce outputs which are equal and opposite. Due to the 60 angle of the arms 228B, 228C the latter outputs are each proportional to 0.866 of unity. To measure the movement in the Y direction one of the bridge outputs 229B, 229C is taken through an inverter 235 where output is connected together with the other bridge output to a summing amplifier 236 having an output 237 given by the difference: 237=229B-(-229C) =229B +229C which is proportional to the stylus displacement in the Y direction. This signal has a magnitude of 2x0.866=1.732 units.

During movement of the stylus in the Z direction all three transducers 225A, 225B.

225C having the same magnitude and sign of output. To measure the movement, the outputs 229A, 229B, 229C are connected to a summing amplifier 239 whose output 240 is proportional to the stylus movement in the Z direction and has a magnitude of three said units.

The three outputs 233, 240, 237 are taken through respective potentiometers 242A, 242B, 242C for being calibrated into 1:1 correspondence with the respective stylus displacements, i.e. for having the same output for unit displacement. Thus potentiometer 242A is set to divide the signal 233 by 2, the potentiometer 242C is set to divide the signal 237 by 1.732, and the potentiometer 242B is set to divide the signal 240 by 3.

The output from the potentiometer 242B has a branch 242BX led as in input into the summing amplifier 232 in order to remove the effect of any Z displacement from the signal 232. The signal 237 does not require any similar treatment because the effect of any Z displacement is cancelled by the inverter 235.

Fig. 10 shows a probe e.g. the probe 9 of Fig. 1 connected to a slide 50 supported in a guide 51 for movement in the direction of the axis B, the guide 51 itself comprising a carriage 52 supported for movement along a guide 53 in a direction transverse to the axis B. The arrangement constitutes a machine supporting the probe for movement relative to a workpiece 53 to be measured; In any one of the examples shown the outputs of the induction bridges may be taken through a network which provides single output of uniform polarity so that a single, say, positive signal is produced so long as at least one of the bridges has an output and regardless of whether that output is positive or negative. For example the signal 29A of Fig. 3 may be taken through a polarising network 59A (Fig. 11) wherein the signal 29A is divided into branches 60, 61 each having respective opposite diodes 62, 63.The branch 60 is taken through an inverter 64 to produce a signal 65. The signals 61, 65 are taken through an OR gate 66 to produce a single signal 67A which is positive regardless of the sign of the signal 29A. Similar network 59B 59C, 59D are provided for the signals 29B, 29C, 29D to produce output 67B, 67C, 67D which are taken through an OR gate 68 to produce a single output signal 69.

WHAT WE CLAIM IS: 1. Apparatus for measuring displacement in at least two orthogonal dimensions comprising an elongate stylus supported on a support member for pivotal movement about a centre and of linear movement along an axis containing said centre, means biasing the stylus into a rest position in which the stylus extends along said axis, and from which the stylus is displaceable by said movements, a first sensor arranged between the stylus and the support in a position offset from said axis and so as to sense a said pivotal as well as a said linear movement, a second sensor arranged between the stylus and the support member in a position to sense at least a said linear movement, and means for forming the algebraic difference between the outputs of the two sensors thereby to produce in respect of the first sensor a signal indicative of a said pivotal movement only.

2. Apparatus according to claim 1 wherein both said sensors are offset from said axis and are situated at opposite sides of said axis in a common plane therewith.

3. Apparatus according to claim 1 wherein the second sensor is situated on said axis.

4. Apparatus according to claim 1 comprising a third said sensor, wherein the three sensors are situated symmetrically about said axis.

5. Apparatus according to claim I wherein the support of the stylus on the support member is by means of a spring extending primarily in a plane containing the centre of said pivotal movement and being perpendicular to said axis, the spring having at least one portion connected between the stylus and the support member and extending curvedly in said plane thereby to accommodate the axial and pivotal displacement of the stylus.

6. Apparatus according to claim 1 wherein the probe is connected to a slide supported in a guide for movement in the direction of said axis, the guide itself comprising a carriage supported for movement along a guide in a direction transverse to said axis.

7. Apparatus according to claims 1, 2 or 3 wherein the two sensors each have outputs of opposite sign in respect of displacement of the stylus to opposite sides of said rest position, and the outputs of the two sensors are connected to a means for forming a signal defining the algebraic difference between the two outputs.

8. Apparatus according to claim 4 wherein the three sensors each have outputs of opposite sign in respect of displacement of the stylus to opposite sides of a rest position, and the outputs of the three sensors are connected to a means for forming a signal defining the algebraic difference between one of the outputs, the sum of the second and third outputs, and one third of the sum of the three outputs.

9. Apparatus according to claim 1 comprising a first pair of said sensors arranged to sense pivoting of the stylus in a first plane including said axis and a second pair of said sensors arranged to sense pivoting of the stylus in a second plane including said axis and perpendicular to the first plane, each sensor having outputs of opposite sign in respect of displacement of the stylus to opposite sides of a rest position, wherein the outputs of each said pair of sensors are connected to a means for

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. 242B, 242C for being calibrated into 1:1 correspondence with the respective stylus displacements, i.e. for having the same output for unit displacement. Thus potentiometer 242A is set to divide the signal 233 by 2, the potentiometer 242C is set to divide the signal 237 by 1.732, and the potentiometer 242B is set to divide the signal 240 by 3. The output from the potentiometer 242B has a branch 242BX led as in input into the summing amplifier 232 in order to remove the effect of any Z displacement from the signal 232. The signal 237 does not require any similar treatment because the effect of any Z displacement is cancelled by the inverter 235. Fig. 10 shows a probe e.g. the probe 9 of Fig. 1 connected to a slide 50 supported in a guide 51 for movement in the direction of the axis B, the guide 51 itself comprising a carriage 52 supported for movement along a guide 53 in a direction transverse to the axis B. The arrangement constitutes a machine supporting the probe for movement relative to a workpiece 53 to be measured; In any one of the examples shown the outputs of the induction bridges may be taken through a network which provides single output of uniform polarity so that a single, say, positive signal is produced so long as at least one of the bridges has an output and regardless of whether that output is positive or negative. For example the signal 29A of Fig. 3 may be taken through a polarising network 59A (Fig. 11) wherein the signal 29A is divided into branches 60, 61 each having respective opposite diodes 62, 63.The branch 60 is taken through an inverter 64 to produce a signal 65. The signals 61, 65 are taken through an OR gate 66 to produce a single signal 67A which is positive regardless of the sign of the signal 29A. Similar network 59B 59C, 59D are provided for the signals 29B, 29C, 29D to produce output 67B, 67C, 67D which are taken through an OR gate 68 to produce a single output signal 69. WHAT WE CLAIM IS:

1. Apparatus for measuring displacement in at least two orthogonal dimensions comprising an elongate stylus supported on a support member for pivotal movement about a centre and of linear movement along an axis containing said centre, means biasing the stylus into a rest position in which the stylus extends along said axis, and from which the stylus is displaceable by said movements, a first sensor arranged between the stylus and the support in a position offset from said axis and so as to sense a said pivotal as well as a said linear movement, a second sensor arranged between the stylus and the support member in a position to sense at least a said linear movement, and means for forming the algebraic difference between the outputs of the two sensors thereby to produce in respect of the first sensor a signal indicative of a said pivotal movement only.

inverting one of the outputs of the pair, and all the inverted and non-inverted outputs are gated to a single output, thereby to make available a single output of the same polarity from any one of the outputs of the respective pairs.

10. Apparatus for measuring displacement substantially as described herein with reference to the accompanying drawings.