GB2057676A

GB2057676A - Incremental Angle Measurement

Info

Publication number: GB2057676A
Application number: GB8024989A
Authority: GB
Original assignee: Carl Zeiss Jena GmbH
Current assignee: Jenoptik AG
Priority date: 1979-07-31
Filing date: 1980-07-30
Publication date: 1981-04-01
Also published as: GB2057676B; DD143953B1; DD143953A1; CH645716A5

Abstract

An optical-electronic device for the incremental measurement of angles comprises a disc 11 with a radial ruled grating 1 and rotatably mounted in a basic body 8 by shaft 10 in bearings 9. Two sectors of a circular ring with a sensing ruled grating 2 are diametrally opposite each other beneath disc 11 and radially adjustable. Gratings 1;2 have identical constants, grating 2 being arranged eccentrically to grating 1. Illuminating means 15 are in vertical alignment with the two sectors. Each is rigidly attached to body 8 and comprises a light source 13 and a condenser 14 preferably giving parallel light. Four spaced photodetectors 7 are attached to each sector and to an electrically conducting plate 12 for sensing moire structures when gratings 1 and 2 are relatively rotated. The detectors of each sector lie in a plane tangential to disc 11. The spacing between the outer photo-detectors 7 of each detector group corresponds to three- quarters of the moire period. <IMAGE>

Description

SPECIFICATION Optical-electronic Measuring Device for the Incremental Measurement of Angles This invention relates to an optical-electronic measuring device for the incremental measurement of angles, down to the resolution of fractions of seconds or arc, primarily for precision measuring instruments and surveying instruments.

The literature on moire techniques describes many possible ways in which moire structures occur, which, in general terms, always form when a luminous flux traverses at least two splitting devices or gratings in succession.

An optical-electric angle measuring device is described in German Patent Specification 1,798,368, which comprises a rotatable disc carrying a ruled grating. A second disc, carrying an opposing grating with identical grating constants, is located eccentric to the first disc. In this angle measuring device, the eccentrically coarranged discs, which possess radial gratings of the same type, generate moire structures in the form of apollonian circles, which are sensed photoelectrically. The sensing positions, with the photo-detectors, are diametrally displaced relative to the discs, in order to eliminate eccentricity errors, which could falsify the measurements, as the discs are moved relative to each other. In such device, sensing of the moire lines is effected by photo-detectors using the four-phase scanning technique.

however, the known angle measuring device contains photo-detectors with large detecting surfaces, which are unsuitable for many particular applications, with regard to the dynamics of producing the measurements and to miniaturisation. Moreover, it is disadvantageous, above all when the diameters of the gratings on the discs are small, that a relatively large number of degrees of freedom, resulting from the lack of a rigid connection between the photo-detectors and the disc carrying the opposing grating, akes it difficult to adjust the parts relative to each other in a precise manner. Furthermore, residual errors, in the form of fluctuations of the moire spacings, cannot be corrected. These errors have an adverse effect, especially when the disc diameters are small, on the sensing accuracy, and hence on the measurement accuracy of the entire device.

The invention aims at obviating the deficiencies of the prior art, at reducing the physical size and the cost of the equipment and at increasing the measurement accuracy of angle measuring devices.

The invention aims at providing an incremental angle measuring device in which a high resolving power and high measurement accuracy result from an advantageously adjustable location of the sensing positions and a particular configuration and arrangement of the photo-detectors.

Accordingly, the present invention consists in an optical-electronic measuring device for the incremental measurement of angles, comprising illuminating means, a disc provided with a first ruled grating, this disc being rotatable about an axis, a second sensing ruled grating arranged eccentrically to the grating of said disc, and photo-detectors for sensing moire structures which occur when the two gratings are rotated relative to each other, characterised by the combination that two sensing gratings are provided, diametrally opposite to each other and capable of separate adjustment relative to the ruled grating disc, their centres lying off the axis of rotation of the ruled grating disc but on a line including the axis of rotation, that the sensing gratings are rigidly attached to the photodetectors, four spaced apart photo-detectors being provided for each sensing grating, these detectors being located in a plane running tangentially to the ruled grating disc and that the spacing between the two outer photo-detectors of each detector group corresponds to threequarters of the periodic spacing of the moire structures (moire period).

Advantageously, the two ruled gratings have identical grating constants.

The sensing gratings are conveniently rigidly connected to the illuminating means and are adjustable relative to the ruled grating disc, both in the radial direction and in the tangential direction, in order to generate an optimum moire structure.

It is advantageous to be able to correct the residual sensing errors, resulting from fluctuations in the period of the moire structures, by means of the separate adjustable arrangement of the opposing gratings. Furthermore, the opticalmechanical expense is reduced and the adjustment is simplified by reducing the number of adjustment conditions. By choosing the separation of the centres of the gratings and by positioning them, together with the axis of rotation, on a straight line, it has become possible to set up moire signals of defined magnitude and phase, relative to the arrangement of the photodetectors, and thus to achieve a high interpolation and accuracy.

In order that the invention may be more readily understood, reference is made to the accompanying drawings which illustrate diagrammatically and by way of example an embodiment thereof, and in which: Fig. 1 shows the generation of moire structures by means of two radially-aligned ruled gratings, arranged eccentrically to each other.

Fig. 2 shows the conversion of the moire structure, thus generated, for the purpose of a measuring device, and Fig. 3 shows a cross-section through a measuring device.

A particular form of the moire effect is used in the optical-electronic measuring device for the incremental measurement of angles. Figure 1 shows that the device is based on the generation of moire structures, know per se, by means of two eccentric radially-aligned gratings with identical grating constants, sections of which are shown diagrammatically in the Figure. The grating lines of each grating 1 and 2 mathematically define a single-parameter family of curves, having the family parameters n, and n2 respectively. For the condition shown in Figure 1, the O-grating lines lie on the median line which runs through the centres 1 and 2' of the two gratings. As a result of the eccentric displacement of the centres 1' and 2' at the separation 4, the grating lines of the gratings 1 and 2 intersect with each other.A series of intersection points then contributes to the generation of a moire line 5, provided that their mutual separation is smaller than the distance to the other adjacent intersection points.

Such a condition exists, for example, for two intersection points, which are formed by the line pairs n1=i+p and n2=i and n1=i+j+p and n2=i+j (i, j=O, 1, 2, 3...) and which accordingly differ from each other by the same numerical value p. In this way, a system of moire lines is generated, having the family parameter, or order number, p=n1-n2 (p=O, 1, 2, 3...).

If, for example, the centres are mutually displaced on the median line, the distance 6 between two adjacent moire lines 5 alters, for example between those of the order p=O and the order p=+1,which corresponds to the extent of a moire period. In the measuring device, photodetectors 7 are accordingly located on a line at right-angles to the median line and at a particular distance 3 from the centre 1 r, and, by choosing the separation 4 of the grating centres 1 ' and 2', moire signals are set up, which have a defined magnitude and phase relative to the arrangement of the detectors. The light-modulation of the signals is substantially sinusoidal.

Although the moire structures (moire lines) are, with regard to their shape, apollonian circles passing through the grating centres 1' and 2t, their behaviour in the vicinity of the photodetectors 7 (Figure 2) can, with sufficient accuracy, be regarded as linear, provided that only lines of low order, at most of the 2nd order, contribute to the signal-generation. This can be achieved during the adjustment procedure, by means of a particular mutual arrangement of the centres 1 and 2', relative to the plane of the photo-detectors 7.

The conversion of the generated moire structures, for the purposes of a measuring device, comprises, as shown in Figure 2, a grating 1, which is designed in the form of a light transmitting circular ring and which represents the means of measurement, and a grating 2, which is a sensing grating and which has the form of a light transmitting sector of a circular ring. In this Figure 2, the axis of rotation of a ruled grating disc, which carries the grating 1, and the grating centre 1' are coincident. At a distance 3 from the grating centre 1', which corresponds approximately to the median radius of the grating 1, four photo-detectors 7 are arranged on a straight line tangential to the diameter of the partcircle, these photo-detectors being spaced at regular distances from each other and consisting preferably of photo-detector chips.The grating 2, which forms the sensing grating, is orientated, relative to the grating 1, in such a way that its centre 2' lies on the perpendicular 7' to the plane of the photo-detectors 7, on which the centre 1 ' of the grating 1 or the axis of rotation also lie. In the measuring device, the sensing grating 2 and the photo-elements 7 are advantageously attached to each other in a rigid manner, the sensing grating 2 being orientated in such a way that its centre 2' (Fig. 1) is also located on the perpendicular 7' to the plane of the photodetectors 7, as is the centre 1'.

By radially displacing the sensing grating 2, the moire structures or moire lines 5 are set up in such a manner that the separation between the outer of the four photo-detectors 7 corresponds to three-quarters of the periodic spacing of the moire lines 5. A phase-difference of 90 , between the individual photo-detectors 7 and their signals, thereby occurs, and the four-phase evaluation of the signals, known per se, can be effected.

The measuring device shown in Figure 3 comprises a basic body 8, within which a ruled grating disc 11 is located, on a shaft 10, in bearings 9, this ruled grating disc 11 carrying the grating 1 and serving as the means of measurement. Beneath the ruled grating disc 11, two sensing gratings 2 are adjustably located on the basic body 8, diametrally opposite to each other, each of these gratings being rigidly attached to photo-detectors 7 and to an electrical conductor plate 12. As shown in Figure 3, an illuminating means 1 5 is provided on each side, above the ruled grating disc 11, which means consists of a light source 13 and a condenser 14 and which is also rigidly attached to the basic body 8 and which illuminates the gratings 1 and 2 with light which is preferably parallel.

In order to guarantee the ability to correct the diametral sensing of the ruled grating disc 11, with reference to faults in the mounting, care must be taken that both the gratings 2 are uniformly displaced in the radial direction, either outwards or both inwards. The analogue signals, obtained on each side by sensing with the photodetectors 7, are advantageously digitalised and can be combined, by deriving an average value, in order to eliminate eccentricity errors.

Claims

1. Optical-electronic measuring device for the incremental measurement of angles, comprising illuminating means, a disc provided with a first ruled grating, this disc being rotatable about an axis, a second sensing ruled grating arranged eccentrically to the grating of said disc, and photo-detectors for sensing moire structures which occur when the two gratings are rotated relative to each other, characterised by the combination that two sensing gratings are provided, diametrally opposite to each-other and capable of separate adjustment relative to the ruled grating disc, their centres lying off the axis of rotation of the ruled grating disc but on a line including the axis of rotation, that the sensing gratings are rigidly attached to the photodetectors, four spaced apart photo-detectors being provided for each sensing grating, these detectors being located in a plane running tangentially to the ruled grating disc, and that the spacing between the two outer photo-detectors of each detector group corresponds to threequarters of the periodic spacing of the moire structures (moire period).

2. Measuring device according to claim 1, wherein the two ruled gratings have identical grating constants.

3. Measuring device according to claim 1 or 2, wherein the sensing gratings are each rigidly connected to a respective illuminating means and are adjustable relative to the ruled grating disc, both in the radial direction and in the tangential direction, in order to generate an optimum moire structure.

4. Optical-electronic measuring device for the incremental measurement of angles, substantially as herein described with reference to and as shown in the accompanying drawings.