WO1988002845A1

WO1988002845A1 - Optical probe

Info

Publication number: WO1988002845A1
Application number: PCT/GB1987/000748
Authority: WO
Inventors: David Roberts Mcmurtry
Original assignee: Renishaw PLC
Current assignee: Renishaw PLC
Priority date: 1986-10-20
Filing date: 1987-10-20
Publication date: 1988-04-21
Anticipated expiration: 1989-04-20
Also published as: GB8625053D0

Abstract

The probe comprises a light source (12) and a lens system (14, 16) for focussing a spot of light (18) on a workpiece surface (20). A detector system (26, 27, 28A, 28B) detects whether the spot is in focus, and a servo motor (29) drives the lens (16) axially in order to keep it in focus. The axial movement necessary for this is detected by a sensor (82) to provide a measurement of the axial position of the surface (20). In order to measure the slope of the surface (20), a refractive plate (30) can be tilted to deflect the light beam to focus on adjacent points (18', 18'').

Description

OPTICAL PROBE

This invention relates to optical probes.

Optical probes are known for use on position determining apparatus such as co-ordinate measuring machines (CMMs) and machine tools. Such probes are used to sense the position of a surface of a workpiece along a measurement axis of the probe, in order to measure or inspect it for dimensional accuracy. Examples are shown in patent specifications GB 2,183,418, WO 87/01886 (both published after the priority date of the present application) and WO 83/00216.

There is a need for such a probe in various applications, such as where the contour of the workpiece surface is to be followed, e.g. to check the shape of a complicated surface. In such circumstances it is desirable to be able to measure the slope of the surface relative to the probe measurement axis, e.g. s that the probe can be manoeuvred into an orientation normal to the surface. This slope may sometimes need to be measured in more than one direction, e.g. two orthogonal directions (in X and Y directions if the probe axis is taken to be the Z axis). It would be desirable to be able to do this without moving the probe's position on the machine, since moving the probe's position increases the time taken to perform the measurement.

The present invention provides an optical probe comprising: a light source; means for focussing light from the light source in the vicinity of a first point on a workpiece surface; ■eans for deflecting the path of the light whereby to focus it in the vicinity of a second point on the workpiece surface, adjacent the first; and means for detecting the light reflected from the surface and for determining therefrom the positions of the first and second points.

From the positions thus determined of the adjacent first and second points on the surface, the slope between them (or a vector describing this slope) can easily be calculated.

Preferred embodiments of the invention will now be described by way of example, with reference to the accompanying drawings, wherein:-

Fig. 1 is a diagrammatic vertical elevation of an optical probe,

Fig. 2 is a plan view of a mounting arrangement for such a probe, Figs. 3 and 4 are sections on the lines III-III and IV-IV in Fig. 2,

Fig. 5 is a plan view of another mounting arrangement,

Figs.6,7 and 8 are diagrammatic elevations of parts of three further probes, and

Fig. 9 is a plan view of a modification of Fig. 8.

Referring firstly to Fig. 1, there is shown an optical probe 10 comprising a light source 12 such as a laser diode, a lens 14 for producing a parallel beam of light, and a lens 16 for focussing the light as a spot 18 on a workpiece surface 20. The light source 12 and lenses 14,16 are arranged within a housing 42 along an optical axis 22 of the probe, which will be referred to as its Z axis. The image of the spot of light 18 passes back through the lens 16 and is reflected by a beam splitter 24 to a detection system comprising a lens 26 and a beam dividing 'V prism 27 which focuses the image onto a pair of position-sensitive detectors 28A.28B. Each detector 28A.28B gives an indication of the degree to which the spot 18 is in or out of focus, in a manner well understood from compact disc technology (e.g. see European Patent Application EP-0, 177, 108-A) . The detector outputs are combined differentially in a differential amplifier 80.

To permit focussing on the surface 20, either the whol assembly 12, 14, 16, 24, 26, 27, 28A, 28B, or just the lens 16, may be translatable in the Z direction, e.g. by a probe mounted servo motor 29. Preferably just the len 16 is thus translatable, since this reduces the inerti of the system and therefore increases its frequency response. For the same reason, the lens 16 may be replaced by a holographic film reproduction of such a lens. The detectors 28A.28B act on the servo motor 29 in a closed loop via the amplifier 80 for Z-axis movement, tending to keep the spot 18 focussed on the surface 20. Such a system is described in more detail in patent specification GB 2,183,418. The amount of Z-axis movement required to keep the spot in focus is sensed by a sensor 82, from which a position determination circuit 84 calculates the Z-axis positio of the surface 20 from the probe. Readings of this quantity are taken when required by a computer 86 whic controls the operation of a CMM or machine tool in which the probe is mounted, in a conventional manner. Instead of the sensor 82, the circuit 84 may determine the Z-axis position from the servo signal supplied to the servo motor 29 in order to keep the spot 18 in ocus.

Attached to the probe on the workpiece side of the len 16, in the path of the light beam, is a parallel-sided plate 30 of a refractive medium, e.g. glass. When the plate 30 is in the position shown in full lines in Fig. 1, it has no effect on the light, which passes straight through it undeflected. However, it can be tilted about a Y axis 36 to a position as shown (greatly exaggerated) in dashed lines, in which the light beam, both on the outward and return parts of its path, is deflected to follow a path 32, parallel to th Z-axis 24, to focus at an adjacent spot 18' . Tilting the plate 30 in the opposite direction (as shown in chain dotted lines) caused the beam to be deflected along a path 34 to focus at a spot 18". The tilting i performed by a motor system 88 under the control of th computer 86. The system 88 includes a feedback to the computer representing the degree of tilting, and thus the amount of X-axis deflection of the light beam. As this tilting occurs, the detectors 28A,28B and motor 2 keep the spot in focus, and readings are taken by the computer 86 of the Z-axis motion needed for this.

Calculation of the local slope of the surface is then straightforward. The calculation is performed by the computer 86, and can include compensation for the change in path length caused by the tilting of the plate 30. It will be appreciated that very little tilting of the plate 30 is needed to produce a measurable effect from which the slope can be determined. Although not shown in Fig. 1, the plate 30, as well as being tiltable about the Y-axis 36, may if desired also be tiltable in a similar manner about an X-axis 38, giving full information about the slope in both X and Y directions at point 18.

Figs. 2,3 and 4 show a gi bal mounting arrangement for the plate 30 to enable the tilting motion. A gimbal ring 40 is pivotably mounted to the housing 42 of the probe by pivot pins 44, and can be tilted about those pins by motors 46 (as shown by the arrows through the motors 46 in Fig. 4). The plate 30 is provided insid the gimbal ring, pivotably mounted to it (at a locati 90° removed from the pins 44) by pivot pins 48, and c be tilted about those pins by motors 50 (as indicated by the arrows in Fig. 3). The motors 46,50 may be bi orph piezo motors, or they may operate magneticall in the manner of a solenoid.

Fig. 5 shows an alternative mounting arrangement for the plate 30. A leaf spring 52 is mounted to the housing 42, and has three limbs 54 extending in a partly circumferential direction. Three corners of a triangular version of the plate 30 are attached to th free ends of the limbs 54. These three corners can each independently be driven in the direction of the Z-axis 22 by a respective motor 56, secured between t housing and the plate 30. The motors 56 may be solenoid motors. Alternatively, the limbs 54 may be the form of biomorph piezo motors, with no separate motors 56.

Fig. 6 shows an alternative to the glass plate 30. T oblique parallel mirrors 58 are mounted in a ring 60. Tilting of the ring in the sense shown by the arrows causes the axis of the beam of the light to shift as shown by dotted lines 62. Alternatively, one of the mirrors 58 may be moved independently (e.g. in a horizontal direction away from or towards the other) t achieve the same effect. However, this arrangement requires further mirrors or a more complicated mountin arrangement to achieve tilting in both X and Y directions, and if only one mirror is moved, the probe reading must be adjusted to compensate for the increased or decreased path length of the light thereb caused .

Fig. 7 shows an arrangement which can be used to inspect relatively inaccessible surfaces, e.g. inside bore. The light beam is internally reflected through 90° by a prism 64. The prism may be rotated about the Z-axis 22 to take measurements at points circumferentially at either side of the point 18, to inspect for deviations of concentricity of the bore. It may also be tilted about a horizontal axis, or translated horizontally or vertically, to cause this spot 18 to move vertically (i.e. parallel to the Z-axis) to inspect the slope of the surface 20 in that sense. Again, the probe output should be compensated for such motion where it would alter the path length o the light.

Fig. 8 shows an arrangement similar to Figs. 2 to 4. However, in place of the gimbal ring, the plate 30 is fixedly mounted at a swash angle in a horizontal ring 66. The ring 66 is rotatable on the housing 42 about the Z-axis 22, by means of bearings 70, driven by a conventional servo or stepping motor 68. As with the motor system 88 in Fig. 1, this rotation is carried ou under the control of the computer 86, with feedback of angular position to the computer. As the ring 66 rotates, the swashed plate 30 causes the spot 18 to describe an orbit on the surface of the workpiece, the centre of the orbit being the true axis 22 of the probe. In use, the drive may be continuous, or it may be stepped to particular points at which measurements are required. In either case, measurement can be take at various points around the orbit, and the measuremen position of the true centre of the orbit determined by interpolation in the Z direction. The slope can be determined as a vector normal to an elemental plane which is a best fit to the points measured. Travellin in an orbit in this manner is an efficient way of taking the required readings in a short time, and can also be done by the probes of the previous embodiments For example, the motors 46,50 of Figs. 2 to 4 can each be driven by a sinusoidal current, at 90° to those of its nearest neighbours. The motors 56 of Fig. 5 can b driven by sinusoidal currents at 120° . Either three o four readings around the circle will generally be sufficient. In any of these cases, for continuous rotation, the frequency response of the servo system 28A,28B,80,29 should be faster than the speed of rotation, to ensure that the spot 18 remains in focus.

The bearings 70 may be air bearings. As shown in

Fig. 9, in place of the servo or stepping motor 68, th ring 66 may be driven around by a tangential air blast from a nozzle 94 onto suitable projecting blades 90. In this case, it is desirable to have a marker on the ring, by means of which its angular position may be determined, so that the computer is able to work out the orientation of the calculated slope about the Z-axis. A suitable marker would be a white stripe, detected by a photo detector on the housing; or a magnetic marker may be used; or a detector 92 may detect the passing blades 90 and feed a count back to the computer (one of the blades being thicker than the others to provide a reference).

Claims

CLAIMS :

1. An optical probe comprising: a light source; means for focussing light from the light source in the vicinity of a first point on a workpiece surface; means for deflecting the path of the light whereby to focus it in the vicinity of a second point on the workpiece surface, adjacent the first; and means for detecting the light reflected from the surface and for determining therefrom the positions of the first and second points.

2. An optical probe according to Claim 1 wherein the deflecting means can deflect the path of the light in either of two non-parallel planes, whereby to focus the light in the vicinity of a third point on the workpiece surface adjacent the first, and to determine the position of said third point, the first, second an third points being non-colinear.

3. An optical probe according to Claim 1 or

Claim 2, wherein the deflecting means comprises at least one reflective or refractive member arranged in the path of the light, the member being movable to deflect the light.

4. An optical probe according to Claim 3, wherein said member comprises a tiltable refractive plate.

5. An optical probe according to Claim 3, wherein said member comprises a refractive plate tilted at a swash angle with respect to the path of the light and rotatable to cause the focussed light beam to describ an orbit on the workpiece surface.

6. An optical probe according to Claim 3, wherein said member is a reflective member arranged at an angl to the path of the light and tiltable to vary said angle.

7. An optical probe according to Claim 6, wherein the light passes from a first said tiltable reflective member to a second said tiltable reflective member.

8. An optical probe according to Claim 3, wherein said member is a reflective member arranged at an angl to the path of the light and translatable in a manner to cause said deflection of the path of light.

9. An optical probe according to Claim 6 or

Claim 8, wherein the reflective member is rotatable about an optical axis of the light prior to said deflection.

10. An optical probe according to any one of

Claims 3 to 9, including means for determining the position to which said member has been moved.