WO1992021933A1

WO1992021933A1 - Method of and apparatus for optically measuring rotation

Info

Publication number: WO1992021933A1
Application number: PCT/GB1992/000931
Authority: WO
Inventors: Raymond John Chaney
Original assignee: Renishaw Transducer Systems Limited
Priority date: 1991-05-25
Filing date: 1992-05-22
Publication date: 1992-12-10
Also published as: GB9111383D0

Abstract

Rotation of one machine part relative to another is measured by determining the amount by which the plane of polarisation of a linearly polarised light beam is changed by the rotation. Two methods are disclosed along with apparatus for carrying out the methods. In a first method a polariser (3) attached to a linearly moving machine part (11) produces from an input laser beam (2) a linearly polarised output beam (4) of known polarisation angle. The output beam is passed to a polarising beam splitter (5) to produce orthogonally polarised light beams which are directed onto detectors (8, 9). Any change in the intensity of the light beams at the two detectors can be used as a measure of the roll of the moving machine part during its linear movement. Changes of intensity of the input laser beam are determined from the changes in the total intensity of the two beams reaching the detectors. In an alternative method a linearly polarised light beam is passed through a half-wave plate carried by the moving member.

Description

METHOD OF AND APPARATUS FOR OPTICALLY MEASURING ROTATION

The present invention relates to the measurement of rotation using optical techniques.

In co-ordinate measuring machines and machine tools errors can arise during measuring operations because of pitching, rolling and yawing movements of the moving parts of the machine. In order to be able to determine those errors it is desirable to be able to measure small relative rotational movements of machine parts very accurately.

The present invention is concerned with the accurate measurement of roll errors.

Also it is often desirable to be able to measure rotation of a machine spindle or a rotary table accurately throughout its whole 360° movement.

According to the present invention there is provided a method of measuring the rotation of one machine part relative to another the method comprising the steps of: a) generating a linearly polarised light beam which emanates from one of the machine parts, b) directing said linearly polarised light beam towards an optical component mounted on the other one of the machine parts, and which has the property of producing from an incident beam thereon an output beam the plane of polarisation of which depends upon the angular orientation of the optical component, c) directing the output beam from the optical component towards a photo-sensitive detector system. d) determining from the photo-sensitive detector system the effect of the relative rotation of the machine parts on the intensity of the light beam falling on the said detector thereby to determine the amount of said relative rotation. Two preferred methods may be used for determining the effect of the relative rotation of the machine parts on the intensity of the light beam falling on the photo-sensitive detector.

In the first method, referred to as the d.c. method, the linearly polarised light beam has a fixed plane of polarisation relative to the machine part from which it emanates, the relative rotation of the machine part being determined from the variations caused thereby in the intensity of the light beam falling on the photo-sensitive detector system in a pre-selected polarisation plane.

In the second method, referred to as the a.c. method, the linearly polarised light beam has a continuously rotating plane of polarisation, the relative rotation of the machine part being determined from the variation caused thereby in timing of a reference pulse train, derived from the source of rotation of the plane of polarisation of the beam, and a measurement pulse train derived from the detector system.

The optical component which has the property of producing an output beam the polarisation plane of which depends on its own angular orientation is preferably a polariser or a wave-plate, but the invention is not limited to these two examples.

Various arrangements of the overall system of measurement are envisaged in which a laser, the detector system and said optical components are variously mounted on the fixed machine part or the movable machine part.

The detector system may comprise a polarising beam splitter for producing a second output beam polarised in a plane orthogonal to the first-mentioned output beam, and a second photo-sensitive detector for measuring the intensity of the second output beam to provide a comparison with the intensity measured by the first photo-sensitive detector. Alternatively the detector system may comprise a non- polarising beam splitter for producing two light beams with polarisers disposed in the paths of the beams generated thereby to produce the two mutually orthogonally polarised secondary light beams.

One or more lenses may be used in the detector system to focus the light beams onto small areas of the detector to improve accuracy.

The invention also includes apparatus for carrying out the method described above.

Examples of the invention will now be more particularly described with reference to the accompanying drawings in which:

Fig. 1 is a schematic diagram of one embodiment of the invention showing the d.c. method,

Fig. 2 is a circuit diagram of the signal conditioning electronics,

Fig. 3 is a schematic illustration of the invention applied to a machine also using the d.c. method,

Fig. 4 is an illustration of an alternative detector system of the invention, Fig. 5 is a schematic illustration of an embodiment of the invention including lenses in the detector means,

Fig. 6 is a schematic illustration of an alternative embodiment using the d.c. method,

Fig. 7 is a schematic illustration of an alternative embodiment of the invention using the a.c. method, and

Fig. 8 is a schematic illustration of an alternative embodiment of the invention using the a.c. method.

Referring now to the drawings the simplest embodiment of the invention is shown in which an unpolarised light source 1, which may be a laser, produces a collimated light beam 2 which is directed through a polariser 3 to produce a linearly polarised light beam 4 having a known polarisation plane for a given angular orientation of the polariser. In this example the polariser 3 is mounted on the moving machine part, for example, the spindle 11 of a measuring machine or machine tool.

Clearly a polarising light source could be used in place of the light source 1 and the polariser 3.

For the purposes of this specification we define a polarised light beam generating means as either: i) a polarising light source, or ii) the light source 1 together with the polariser 3.

The linearly polarised light beam 4 emanates from the polariser 3 in the direction of movement of the spindle 11 and is directed towards a polarising beam splitter 5 on the fixed machine part which produces two mutually orthogonally polarised output light beams 6 and 7 respectively. The output light beams are directed towards two photo-sensitive detectors 8 and 9 respectively which form part of the detector system whereby the intensities of each of the two output light beams can be determined.

In the present example the polariser 3 is set to produce a primary light beam polarised in a plane at 45°, to a datum plane, and the polarising beam splitter is set such that its planes of polarisation are at 0° and 90° respectively to the same datum plane.

Both the polariser 3 and the polarising beam splitter 5 are optical components which have the property of producing from an incident beam thereon an output beam, the plane of polarisation of which is dependent upon the angular orientation of the component.

It can be seen that in the example described above, if the machine spindle 11 is moving in the direction of the beam 4, and rolls about an axis parallel to the axis of movement, the plane of polarisation of the beam 4 will be rotated about its own axis, and this will cause a change in the intensity of the plane polarised beam 6 as measured at the detector 8.

Clearly the same would be the case if the laser 1 and polariser 3 were mounted on the fixed part of the machine and the polarising beam splitter 5 and the photo-sensitive detectors were carried on the movable spindle.

All that is needed for the operation of the method is that there is relative rotation between the two optical components when relative rolling movement occurs between the two machine parts. The rolling movement will cause rotation of the plane of polarisation of the optical component carried by the moving .machine part because of the property of the polarising optical component in producing an output beam dependent upon is angular orientation.

With such systems the relative rotation between the polariser 3 and the polarising beam splitter 5 can be determined as follows:

Let the orientation of the polariser relative to the polarising beam splitter alter by Θ, then the changes in signal intensities Dl and D2 on the two detectors 8 and 9 is given by:

Dl = E² sin² (45 + Θ) and D2 = E² cos² (45 + θ)

Where E is a constant.

The difference in signal level Dl - D2 is given by Dl - D2 = 2 sin θ cos Θ = Sin 2 Θ

Thus for small angular rotations Dl - D2 is proportional to θ. Signal strength may be monitored in order to allow for appropriate gain control to be applied to the signal conditioning electronics in order to eliminate errors in the measurements taken by the detectors which are caused by variations in the intensity of the light source. This enables the resolution of the system to be maintained.

Signal strength may be determined by summing the two detector signals since their sum is proportional to unity. Alternatively the signal strength may be monitored by inserting a non-polarising beam splitter 10 in the path .of the primary beam 4, and making a direct measurement from the deflected portion 12 of the beam 4.

For small angles of rotation it is important that the two secondary light beams are as near as possible wholly polarised in their respective planes. Since the extinction ratio of current polarisers is in the region of 1 in 10⁵ a second polariser may be necessary in each of the secondary beam paths to ensure that no significant quantity of light polarised in the wrong plane arrives at the detectors. Alternatively from a calibration of the polariser it is possible to electronically correct for light polarised in the wrong plane arriving at the detectors.

A diagram of the detector system conditioning electronics in a very simple arrangement is shown in Fig. 2.

In Fig. 2 the two detector signals Dl and D2 are passed to operational amplifiers 20 and 21.

The outputs from the amplifiers are passed to a summing junction 22 and a subtracting junction 23 to produce outputs proportional to Dl +D2 and Dl - D2 respectively.

The output Dl + D2 is fed back to each of the amplifiers 20 and 21 and is used to control the gain of the amplifiers to maintain the value of Dl + D2 constant to maintain the full resolution of the system.

Both of the outputs Dl + D2 and Dl - D2 are fed to a divider device 24 which forms the quotient Dl - D2.

Dl + D2 This quotient is equal to 2 Sin Θ Cos Θ which in turn is equal to Sin 2Θ which for small angles approximates to 2θ.

If the value of Dl + D2 can be maintained constant to the required level of accuracy this division step can be eliminated.

Hence by directly measuring the quotient of Dl - D2 and Dl + D2 a direct reading of the change in the angle of the plane of polarisation from its 45° nominal angle can be made.

As an alternative to the summing and subtracting of Fig. 2 the signals Dl and D2 can be fed via an A/D converter to a microprocessor which produces outputs equal to Dl + D2 and Dl - D2. The output Dl + D2 is used for the feedback signal to amplifiers 20 and 21 and both outputs are fed to a look-up table which has stored in its memory the relationship between the quotient Dl - D2 and the angle 20

Dl + D2

Hence the angle 2Θ can be read directly from the look-up table. Such an alternative system would be preferable where large changes in the angle Θ are being measured and the small angle approximations cannot be used.

In Fig. 3 there is shown diagrammatically part of a machine including a flat bed 40 and a vertical machine spindle 42. The laser 1 is mounted on the bed 40, and generates a light beam 2. Also mounted on the bed 40 is a beam deflector 44 which deflects the light beam 2 vertically towards a retro- reflector 46 on the machine spindle 42. The retro- reflector 46 returns the beam vertically through the polariser 3 whereby it becomes linearly polarised with a known angle of polarisation. The polarised beam 4 is passed to a further beam deflector 48 which deflects it towards the polarising beam splitter 5 which generates two output beams 6 and 7 which are directed towards the detectors 8 and 9.

In this example rotation of the machine spindle causes relative rotation of the polariser 3 (which forms part of the polarising beam generating means 1,3) and the polarising beam splitter 5 which is mounted on the fixed bed 40. This system can be used to measure rolling movement of the spindle during movement along its axis or simply to measure rotation of the spindle about its axis.

A more sophisticated detector system is shown in Fig. 4. With this system the detector signals can be processed to correct for the effects of stray light and mis-alignments of the apparatus. Due to the complexity of the calculations required it is envisaged that this system will be used with a micro-processor to determine the rotation θ from the four detector signals.

A polarised light beam 4 from the polarised light beam generating means 1,3 is directed towards a combination of non-polarising beam splitters 53,56 and 57 which produce two beams of light 50 and 51 which remain polarised in the known plane produced by the polariser 3. A plurality of non-polarising beam splitters are used because, in practice all non-polarising beam splitters are polarising to some degree. Thus they are arranged as described below so that the polarisation state of the primary beam from the polariser 3 is preserved in the two beams 50 and 51. Non polarising beam splitter 53 produces the two beams 54 and 55 but each is likely to be partially polarised by the beam splitter. However, a second non-polarising beam splitter 56,57 is inserted in the path of each beam and each of the second non-polarising beam splitters is oriented so that p-polarised light from the first beam splitter is in the s-polarisation plane of the second beam splitter.

Since the reflection and transmission co-efficient for the s- and p- polarisation states of the three beam splitters can be made the same, the proportions of the beam 4 coming off the beam splitter 57 in the s- and p- polarisation states will be proportional to Rp x Rs and Rs x Rp respectively and will be equal. Similarly the portions of the beam 4 coming off the beam splitter 56 will be proportional to Tp x Ts and Ts x Tp respectively and will be equal.

The two beams 51 and 52 are further split to form the two mutually orthogonally polarised secondary beams 6,7, 6A and 7A, at two polarising beam splitters 60 and 61 and the intensities of all of the secondary beams is measured at detectors 62,63,64 and 65.

The corrections available from a four detector arrangement can be effected in alternative manner as follows:

To stop the effects of stray light, a monochromatic light source may be required with appropriate filters F ahead of each of the detectors.

To overcome problems of the DC level of the detector signals obscuring the small changes which require to be detected, the intensity of the light source may be modulated and the detector system arranged to operate at the same frequency. A problem which may arise if the apparatus is used in circumstances under which significant air turbulence arises, is that the turbulence causes the beams to move across the diode detectors. Since photodiode detectors do not have uniform surfaces, the errors in signal level produced, which are caused by such beam movement can cause significant measurement errors at the levels of accuracy required for detecting roll of a machine spindle.

To reduce this error one or more lenses may be introduced to focus the beams onto the detectors so that only a small area of the photodiode is used. The area of the photodiode detector should preferably be one hundred times the focused spot size.

A single lens may be placed ahead of the beam splitter 5 with the two photodiodes 8 and 9 at the focal point of the lens, or two lenses may be used, one placed in each of the secondary beams 6 and 7 to focus them onto the photodiodes.

In a more expensive, but more accurate system however, three lenses 80,81 and 82 can be used as shown in Fig 5, whereby the effect of lack of uniformity in the beam splitter can also be reduced by focusing the polarised beam 4 onto the beam splitter surface, and also focusing the emerging secondary beams onto their respective photodiodes.

Another optical component which has the property of producing from an incident beam thereon an output beam the plane of polarisation of which depends on its angular orientation is a wave-plate.

Fig 6 shows an alternative arrangement of a measurement system using the d.c. method for measuring roll of a machine part about the axis of movement thereof and using a wave-plate to cause rotation of the plane of polarisation of a linearly polarised light beam. A laser 70 produces a non-polarised or circularly polarised light beam 72 which is passed to a fixed polariser 74. The linearly polarised output light beam 76 from the polariser 74 passes through a non-polarising beam splitter 78 towards the moving part 80 of a machine. On the moving part of the machine are a quarter wave-plate 82 and a retro-reflector 84. The beam 76 passes through the quarter wave-plate once and then is returned through it again by reflection from the retro-reflector 84, whereby a linearly polarised light beam emanates from the quarter wave-plate. Thus, the quarter wave-plate behaves like a half wave-plate.

As described in the previous embodiments, the linearly polarised light beam is directed towards a polarising beam splitter and detector•system 86 which may be similar to that shown in any of Figs 1 to 4. If rolling movement occurs about the laser beam axis during movement of the part 80 in the direction of that axis, the wave-plate 82 will cause a change in the plane of polarisation of the returning laser beam and will cause a variation of intensity measured by the photodiode detectors. The amount of roll can be determined as explained with reference to Figs 1 to 4.

For a more accurate measurement, the detector system may incorporate the lens system shown in Fig 5.

Fig 7 shows how the a.c. method can be used for the measurement of rolling movement of a machine part about an axis parallel to the axis of movement.

In this method a laser 100 produces a non-polarised or circularly polarised light beam 102 which is passed through a rotating polariser 104 which produces a linearly polarised light beam 106. The polariser is rotated by a motor 108 at a constant speed at say 100 Hertz so that the plane of polarisation of the beam 106 rotates at 200 Hertz. A reference pulse is produced from the motor 108 every time the polariser passes through a nominal reference position. This may be the zero or horizontal polarisation position.

The linearly polarised light is passed through a half wave- plate 110 mounted on the moving part 112 of the machine, the roll or rotation of which about an axis nominally parallel to the laser beam axis is to be measured. The half wave-plate 110 rotates the plane of polarisation of the linearly polarised light by a fixed angle and the beam 114 emanating from the half wave-plate is passed through a fixed polariser 116 and focused by a lens 118 onto a photodiode detector 120.

The detector 120 produces a sinusoidal signal at twice the frequency of the reference pulse train. If, during its linear movement, the part 112 rolls and causes rotation of the wave-plate relative to the polariser 104 about the laser beam axis, the plane of polarisation of the output beam 114 from the wave-plate will rotate relative to that of the input beam 106 and consequently the phase of the output signal from the detector 120 will be advanced or retarded relative to the reference pulse train, depending on the degree of rotation of the wave-plate. Thus, by comparing timing between the reference pulse train from the motor 108 and a pulse train derived from the detector output, e.g. at the zero crossing points of the sinusoidal output, the degree of rolling movement of the part 112 can be measured.

In place of the single detector, a polarising beam splitter and detector system similar to that shown in Figs 1 to 4, may be used.

The use of a half-wave plate on the moving machine part eliminates the requirement for any trailing leads on the moving part. If trailing leads are not a problem the polariser 116, with or without the detector system 120, may be carried on the moving machine part. Alternatively the rotating polaroid, with or without the laser, may be mounted on the moving part and the polariser 116 remains fixed.

If there is a variation in distance between the rotating polaroid and the detector, allowance may have to be made for the finite speed of light in order to maintain the highest accuracy.

Referring to Fig 8, an embodiment using the a.c. method for measuring rotation of a machine spindle or rotary table is shown. A laser 150 mounted on a fixed machine part directs a non-polarised or circularly polarised light beam through a rotating polariser 152. The polariser is driven by a motor 154 mounted on the fixed machine part and from which a reference pulse train is generated as described with reference to Fig 7. The linearly polarised light beam 155 so produced is directed towards a quarter wave-plate 156 and a retro-reflector 158 mounted on the moving machine part 160. The returning light beam 159 after passing twice through the quarter wave-plate 158 emanates towards a fixed polaroid 162 and the output beam from this polaroid is directed towards a detector system 164. The detector produces a sinusoidal output signal from which a series of pulses can be generated and compared with the reference pulse train produced from the motor.

The movement of the moving part is in this embodiment, rotation of the part about an axis 170 parallel to the axis of the laser beam. By this means any relative rotation between the fixed and movable parts can be determined throughout the whole 360° range by noting changes in phase between the measuring reference pulse trains.

Other variations may be made, for example, using the various detection system with the different positions of the parts of the system, and mounting different parts of the system on fixed or movable machine parts as appropriate.

Claims

15 CLAIMS

1. A method of measuring the rotation of one machine part relative to another, the method comprising the steps of: a) generating a linearly polarised light beam which emanates from one of the machine parts, b) directing said linearly polarised light beam towards an optical component mounted on the other one of the machine parts and which has the property of producing from an incident beam thereon an output beam, the plane of polarisation of which depends on the angular orientation of the optical component, . c) directing the output beam from the optical component towards a photo-sensitive detector system, d) determining from the photo-sensitive detector system the effect of the relative rotation of the machine parts on the intensity of the light beam falling on said detector, thereby to determine the amount of said relative rotation.

2. A method as claimed in claim 1 and wherein the linearly polarised light beam is generated by a polarised light beam generating means as defined herein.

3. A method as claimed in claim 2 and including the further steps of causing the plane of polarisation of the linearly polarised light beam to rotate, generating from the source of rotation of the linearly polarised light beam a reference pulse every time the plane of polarisation of the beam passes through a reference plane, and comparing the phase of pulses derived from the detector signal with the reference pulse train to determine the amount of relative rotation between the two machine parts.

4. A method as claimed in claim 2 and including the further steps of generating from the optical component two beams polarised in orthogonal planes, and comparing the intensities of the two beams to determine the effect of the relative rotation of the machine parts on the intensity of the light falling on said detector system.

5. Apparatus for measuring the rotation of one machine part relative to another comprising polarised light beam generating means for producing a linearly polarised light beam emanating from one of the machine parts, an optical component mounted on the other one of the machine parts which has the property of producing from an incident beam thereon an output beam the plane of polarisation of which depends upon the angular orientation of the optical components, and to which the linearly polarised light beam is directed, a photo-sensitive detector system towards which the output beam of said optical component is directed, and means for determining the effect of the relative rotation of the machine part on the intensity of the light beam falling on the photo-sensitive detector system.

6. Apparatus as claimed in claim 5 and in which the polarised light beam generating means is a polarised laser.

7. Apparatus as claimed in claim 5 and in which the polarised light beam generating means is a non-polarised laser and a polaroid fixed relative to said one machine part.

8. Apparatus as claimed in claim 5 and in which the polarised light beam generating means is a non-polarised laser, a polariser and a motor for rotating the polariser.

9. Apparatus as claimed in any one of claims 5 to 8 and in which the polarised light beam generating means further includes a wave-plate.

10. Apparatus as claimed in claim 5 and in which the optical component is a polariser. 17

11. Apparatus as claimed in claim 5 and in which the optical component is a polarising beam splitter.

12. Apparatus as claimed in claim 5 and in which the optical component is a wave-plate.

13. Apparatus as claimed in claim 7 and in which the optical component produces two output beams polarised in orthogonal planes and the detector system includes means for comparing the intensities of the two output beams.

14. Apparatus as claimed in claim 8 and in which the motor provides a reference pulse every time the plane of polarisation of the light beam passes through a reference plane, and the detector means includes means for producing a pulse every time the intensity of the light falling on the photo-sensitive detector passes through a reference value and means for comparing the phase of the two pulse trains.

15. Apparatus according to any one of claims 5 to 14 and in which the detector system includes one or more lenses to focus the light beam onto a photo-sensitive detector or a beam splitter.