DE29624216U1 - Photoelectric length and angle measuring device - Google Patents

Description

DR. JOHANNES HEIDENHAIN GmbH 9 September 1996

Photoelectric length or angle measuring device

Known photoelectric length and angle measuring systems usually comprise a scanning unit with several scanning fields, which are designed in the form of scanning grids. A scale is arranged so that it can be moved relative to it, so that position-dependent measuring signals can be generated with a resulting relative movement. The individual scanning fields are arranged so that they are shifted relative to one another by a certain fraction of the grid constant of the scanned scale, so that when the scale is scanned

rod at the detector elements that are assigned to the scanning windows, defined phase-shifted signals are present. Using evaluation electronics arranged downstream of the detector elements, it is possible to determine both the amount and the direction of the relative movement of the scale and scanning unit to one another.

In photoelectric measuring systems constructed in this way, light from different areas of the scanned scale reaches the detector elements, i.e. the phase-shifted signal components do not originate from the same scale areas. If the different scale areas now exhibit local contamination, locally uneven scale divisions or other locally different properties,

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properties, this has a detrimental effect on the signal quality. In particular, undesirable changes in the duty cycle and/or amplitude ratios occur in the phase-shifted signals that originate from the different scale ranges; this ultimately results in measurement errors.

A number of possibilities are now known for solving this problem. For example, EP 0 163 362 describes a so-called interferential measuring system in which three output signals, each 120° out of phase, are generated from a common field of the scanning unit, so that any local changes on the scale side affect all signals equally. However, due to the selected scanning principle, the three resulting output signals have a different degree of modulation. In the case of locally differing conditions on the scale side, fluctuations in the duty cycle and amplitude ratios in the three output signals also arise in such a measuring system.

Another approach to solving the problem mentioned is known from DE 43 03 162. There it is proposed to assign a pair of scanning gratings to each of the detector elements provided in the scanning unit, with the scanning gratings forming an angle relative to the scale grating. This angle is chosen so that the scanning gratings deflect or diffract the incident light beams in the direction of the same scale range. All phase-shifted signals that are detected at the detectors thus originate from the same scale range. The disadvantage of this approach is that very small graduation periods of the scanning gratings are necessary due to the large diffraction angles required, which in turn requires complex production. In addition, the proposed arrangement proves to be relatively sensitive to fluctuations in the scanning distance.

The present invention is therefore based on the object of eliminating the above-mentioned disadvantages of photoelectric length or angle measuring devices

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to be eliminated, i.e. the insensitivity of such a measuring system to local changes on the scanned scale division is required. In addition, a relative insensitivity to variations in the scanning distance should be guaranteed, as should a simple manufacture of the scanning unit.

This object is achieved by a length or angle measuring device with the features of claim 1.

Possible embodiments of the length or angle measuring device according to the invention result from the measures in the dependent claims.

With the aid of the length or angle measuring device according to the invention, at least two phase-shifted output signals are now generated, each of which results from scanning the same scale range. For this purpose, spatially separate, direction-selective deflection means are arranged on the scanning unit side next to the partial scanning fields, which deflect the partial beams of different partial scanning fields into the same scale range. All phase-shifted signals thus originate from the same scanned scale range, so that, for example, local contamination on the scale no longer has a detrimental effect on the various phase-shifted signals. This results in a so-called single-field scan, in which the phase-shifted signals to be evaluated are all equally influenced by any local irregularities in the scale structure that may be present.

By appropriately designing the scanning unit, the amplitudes, phase relationships and modulation degrees of the output signals in the length or angle measuring device according to the invention can be adjusted in the desired manner.

Within photoelectric length or angle measuring devices, direction-selective means assigned to the scanning units, designed as

Prism wedges are already known, for example from EP 0 223 009 or from the dissertation by J. Willhelm (page 65). However, these do not cause the various partial beams of different scanning fields to be diverted to the same scale ranges, i.e. there is no single-field scanning provided, but only the spatial separation of the phase-shifted signal components necessary for detection.

The length and angle measuring device according to the invention can be designed both as an interferential measuring system according to EP 0 163 362, as well as as a photoelectric measuring system which is operated, for example, in shadows.

Further advantages and details of the present invention will become apparent from the following description of embodiments with reference to the accompanying drawings.

This shows

Figure 1 shows the schematic representation of the beam path

course of a first embodiment of the inventive

appropriate length or angle measuring device;

2) Figure 2 shows the front and back of a scanning plate, which is

the first embodiment of the length or angle measuring device according to the invention from Figure 1

is used;

Figure 3 shows the schematic representation of the beam path

run of a second embodiment of the length or angle measuring device according to the invention;

Figure 4 is a view of the scale used in the

second embodiment according to Figure 3;

Figure 5 shows the arrangement of the detector elements in the second

Embodiment according to Figure 3;

Figure 6 shows the schematic representation of the beam path

course of a third embodiment of the inventive

appropriate length or angle measuring device operated in transmitted light.

The beam path in a first embodiment of the length or angle measuring device according to the invention is shown in a highly simplified manner in Figure 1. Here, an embodiment as an interferential measuring system

which operates according to the principle of a three-grid encoder, as is known, for example, from EP 0163 362. The measuring direction of this embodiment, designated by the coordinate X, is oriented perpendicular to the plane of the drawing, i.e. the scanning plate 35 and the scale 45 are displaced relative to one another in the X direction. In the first embodiment shown, the length or angle measuring device according to the invention is designed as an incident light measuring system, which is not specific to the invention. Of the scanning unit, only the scanning plate can be seen in the illustration in Figure 1.

The light emitted by a light source and collimated by a suitable upstream optics first passes through the scanning plate 35 and hits

the reflective scale 45, from which it is reflected and then passes the scanning plate 35 once again. After passing through the scanning plate 35 for the second time, the light beams reach detector elements (also not shown) which record the position-dependent modulated signals in the case of a relative movement.

In order to ensure the desired single-field scanning, certain measures are now provided on the scanning plate 35, which will be explained below. For example, on the scanning plate 35, on the side 36 facing the scale 45, adjacent parallel tracks are arranged in the Y direction, which represent the individual partial scanning fields 3a5, 3b5,

3c5. The partial scanning fields 3a5, 3b5, 3c5 are designed in the form of scanning gratings, with the respective grating lines being oriented in the Y direction. The various partial scanning fields 3a5, 3b5, 3c5 are in turn assigned detector elements (not shown) designed as photo elements. The grating lines of the scale division 4a5 on the scale 45 are also arranged in the Y direction. The various partial scanning fields 3a5, 3b5, 3c5 of the scanning plate 35 are also arranged relative to one another in such a way that a defined phase offset results in the signals assigned to the partial scanning fields 3a5, 3b5, 3c5. For this purpose, a relative offset of the grating lines of adjacent partial scanning fields 3a5, 3b5, 3c5 is provided by a defined fraction of the graduation period TP of the partial scanning fields 3a5, 3b5, 3c5, which in the embodiment according to Figure 1 is 1/6 of the graduation period TP of the scale graduation 4b5. The graduation periods of the various partial scanning fields 3a5, 3b5, 3c5 are otherwise selected to be identical to one another and to be identical to the graduation period TP of the scale graduation 4b5. The two tracks with the partial scanning fields 3a5 and 3c5 are arranged symmetrically and parallel to the central partial scanning field 3b5, so that with the aid of a scanning plate 35 designed in this way, a total of three scanning signals can be obtained which are each 120° out of phase with one another within an interferential measuring system according to EP 0 163 362.

A view of the front side 36 of the scanning plate 35 is shown in the right part of Figure 2 in order to illustrate the explained design of this side of the scanning plate 35. The offset arrangement of the individual partial scanning fields 3a5, 3b5, 3c5 and the symmetry with respect to the middle partial scanning field 3b5 are clearly visible.

On the rear side 37 of the scanning plate 35, deflection means 3d5, 3e5, 3f5 are arranged according to the invention, which act in a spatially direction-selective manner and are each clearly assigned to the various partial scanning fields 3a5, 3b5, 3c5 on the front side of the scanning plate 35. In this case, the deflection means 3d5, 3e5, 3f5 and the partial scanning fields 3a5, 3b5, 3c5 are spatially separated. As can be seen from the left part of Figure 2, the

the rear side 37 of the scanning plate 35 shows, the directionally selective deflection means 3d5, 3e5, 3f5 are designed in the form of grating graduations, the graduation lines of which are oriented parallel to the measuring direction X and consequently cause a transverse deflection of the light beams passing through. Each of the partial scanning fields 3a5, 3b5, 3c5 on the front side 36 of the scanning plate 35 is assigned a spatially separated deflection means 3d5, 3e5, 3f5 with a specific deflection effect on the rear side 37 of the scanning plate, which is achieved in the embodiment shown by the different selection of the grating constants GK1, GK2, GK3 of the deflection means 3d5, 3e5, 3f5. Corresponding to the symmetrical arrangement of the partial scanning fields 3a5, 3b5, 3c5 on the front side 36, the symmetry of the

Deflection means 3d5, 3e5, 3f5 on the back 37 of the scanning plate 35 are selected, i.e. the deflection means 3e5 on the back 37 of the scanning plate assigned to the two partial scanning fields 3a5 on the front 36 each have the same deflection effect, etc.

The directionally selective deflection means 3d5, 3e5, 3f5 on the rear side 37 of the scanning plate 35 are dimensioned according to the invention such that the incident light beams of all partial scanning fields 3a5, 3b5, 3c5, which provide the phase-shifted signal components, are deflected to a common partial area 4b5 of the scanned scale division 4a5 and thus ensure the desired single-field scanning.
After the second passage of the light beams through the scanning plate 35 and the subsequent re-diffraction, the phase-shifted

Signal components are spatially separated and can be detected in the focal plane of the condenser optics.

There are basically different possibilities for the design of the direction-selective deflection means 3d5, 3e5, 3f5 on the back 37 of the scanning plate 35. For example, the deflection means 3d5, 3e5, 3f5 shown in Figures 1 and 2 are designed in the form of binary phase gratings.

It is also possible to use blazed phase gratings, refractive optical elements, diffractive optical elements, holographic optical elements, etc.

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The scanning structure in the form of partial scanning fields 3a5, 3b5, 3c5 on the front side 36 of the scanning plate 35 and the directionally selective deflection means 3d5, 3e5, 3f5 on the back side 37 of the scanning plate 35 are arranged spatially separated from one another. As an alternative to the representation in Figures 1 and 2, it is also possible in principle to arrange the required scanning and deflection structures spatially separated on the same side of a carrier substrate by using a multi-stage photolithography process.

The beam path of a second embodiment of the length or angle measuring device according to the invention is shown in Figure 3. Again, a scanning plate 3 is designed in the manner according to the invention in order to enable a single-field scanning of a scale 4 which is provided with a scale division 4a. The generation of the phase-shifted signal components is based on the principle of an interferential measuring system, as is known from EP 0 163 362.
For this purpose, partial scanning fields 3a, 3b arranged in parallel are provided on the front side 3.6 of the scanning plate 3 in the form of scanning graduations arranged parallel to one another. In contrast to the first embodiment in Figure 1, only two different scanning graduations are arranged, offset from one another in the measuring direction x. The orientation of the graduation lines of the partial scanning fields 3a, 3b is identical to the embodiment explained above. The two identical partial scanning fields 3a are arranged symmetrically to the partial scanning field 3b. To generate the desired output signals that are phase-shifted by 90°, at least one scanning field unit is required from the middle partial scanning field 3a and the two symmetrically arranged partial scanning fields 3b. The arrangement of these scanning field units 3a, 3b, 3a is now repeated several times in the Y direction, which overall improves the energy balance on the detector side.

The offset of the partial scanning fields 3a, 3b to each other is a quarter of the graduation period of the selected scanning graduations in the measuring direction X. In conjunction with the interferential measuring principle of a three-grating encoder according to EP 0 163 362, phase-shifted by 180° to each other result.

Signal components from these two partial scanning fields, which are spatially separated due to the transverse deflection acting in the Y direction by the deflection means 3e, 3f. Since the deflection means 3e, 3f are designed in such a way that a deflection in the +/- 1st diffraction order results, this results in four signal components to be detected that are separated in the Y direction.
Furthermore, by dimensioning the scanning gratings 3a, 3b to only +/- 1st diffraction orders, a splitting of the signal components originating from a partial scanning field 3a, 3b in the measuring direction X can be achieved, resulting in a phase shift of the separated signal components of 90°. The four signal components separated in the Y direction are thus split in pairs in the X direction, thus ultimately creating eight different signal

components with certain phase shifts result, which can be detected and processed further using appropriately arranged detector elements. With regard to the suitable connection and arrangement of the detector elements, reference is made in this context to Figure 5, which will be described below. On the output side, with such a scanning plate design, after the appropriate further processing of the eight phase-shifted signal components, two output signals with a phase shift of 90° result, which can be evaluated in a known manner for position determination.

On the side 3.7 of the scanning plate 3 ) facing the incident light, the direction-selective deflection means 3e, 3f are arranged.

net, which cause a deflection of the incoming light beams of neighboring scanning fields 3a, 3b into the same, applied scale area 4b. In this case, each of the two partial scanning fields 3a, 3b is assigned a spatially separated, direction-selective deflection means 3e, 3f. The design of the deflection means 3e, 3f is provided in the form of binary phase gratings with different graduation periods, the graduation structure of which is oriented parallel to the measuring direction X. The deflection means 3e, 3f in turn have a deflecting effect such that the signal components of partial scanning fields 3a, 3b, which provide the phase-shifted signal components, originate from the same scale area 4b.

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A view of the scale 4 used in the embodiment according to Figure 3 is shown in Figure 4. The partial areas 4b of the scale 4 from which the phase-shifted signals of adjacent partial scanning fields are generated are shown in dash-dotted lines. Any contamination or covering of a partial area of the scale does not result in an uneven influence on signal components of only a certain phase position. If, for example, a part of the scale is covered due to local contamination in the lowest dash-dotted scale area, this only affects the resulting overall intensity, since all phase-shifted signal components involved in signal generation in this partial area are influenced equally.

However, all phase-shifted signal components from the dot-dash areas above are still available for signal acquisition.

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A detector arrangement which can be used in conjunction with the embodiment of the scanning plate according to Figure 3 for detecting the phase-shifted partial beams or eight existing signal components is shown in Figure 5. Reference numeral 20 designates a carrier board on which eight detector elements 5-12, designed as photoelements, are arranged. Reference numeral 1 in Figure 5 designates the passage opening for the incident light beams.
The two detector elements 9, 10 connected together provide the signal component designated as 0° signal, the detector elements 11, 12 provide the signal component designated as 90° signal, the detector elements 5, 6 provide the signal component designated as 180° signal and the detector elements 7, 8 provide the signal component designated as 270° signal.

The signal components reaching the detector elements 6, 8 and 5, 7 originate from the partial scanning field 3a, the signal components reaching the detector elements 10, 12 and 9, 20 originate from the partial scanning field 3b.

To generate the desired 0° and 90° output signals, the detector element pairs 5,6 and 9,10 are connected in push-pull in a known manner; the

•♦ · *

Detector element pairs 7,8 and 11,12. While the first connection results in the 90° output signal, the second connection results in the 0° output signal.

A further embodiment of the length or angle measuring device according to the invention is shown schematically in Figure 6. In contrast to the two variants explained above, the third embodiment shown works in transmitted light, i.e. the scale 37 with the periodic I scale division 470 is not reflective but transmissive. On the sides of the scanning plate 370, the direction-selective deflection means A ₇ , AV, A ₇ , A' _7l A ₇ -, AV- and the correspondingly assigned partial scanning fields 3a ₇ -, 3b ₇ -, 3c 7 , 3a 7 , 3b ₇ , 3c ₇ , 3a ₇ ·-, 3b ₇ , 3c ₇ - are provided spatially separated from one another on the front and _back sides 371 _, 372. The partial scanning fields 3a ₇ , 3a ₇ , 3a ₇ - are arranged offset from one another by one third of their graduation period relative to the partial scanning fields 3b ₇ , 3b ₇ -, 3b ₇ - and the partial scanning fields 3c _7l 3c ₇ -, 3c ₇ - and thus provide three scanning signals which are each 120° out of phase with one another.
The partial scanning fields 3a ₇ -, 3b ₇ , 3c ₇ -, 3a ₇ , 3b ₇ , 3c ₇ , 3a ₇ -, 3b ₇ -, 3c ₇ - on the front side 372 of the scanning plate 370 are assigned, analogously to the previously explained embodiments, the deflection means A ₇ -, A' ₇ -, A 7 , A' ₇ , A ₇ -, A V- on the side 371 of the scanning plate 370 facing the incident, collimated light, which deflection means allow the superposition of the partial beams assigned to the various partial scanning fields 3a ₇ , 3b ₇ -, 3c ₇ , _{3a 7 , 3b 7 , 3c 7 , 3a 7 -} , 3b ₇ -, 3c ₇ ·· on common areas of the Scale division 4a7. Furthermore, the deflection means A ₇ -, A>, A ₇ , A' ₇ , A ₇ -, A' ₇ - bring about the desired directional separation of the phase-shifted partial beams, which impinge on three correspondingly arranged detector elements in the focal plane of a downstream lens 270, which bring about a conversion of the signals into three phase-shifted electrical signals.

In this embodiment, the deflection means A ₇ , AV, A ₇ , A' ₇ , A ₇ -, AV- are designed as blazed phase gratings whose -1st diffraction order is optimized and whose division period is selected according to the desired deflection.

Claims (14)

1. Photoelectric length or angle measuring device with a scanning unit with several partial scanning fields ( 3 a, 3 b; 3 a5, 3 b5, 3 c5; 3 a _7' , 3 b _7' , 3 c _7' , 3 a ₇ , 3 b 7 , 3 _c 7 _, 3 a _7' , 3 b _7' , 3 c _7' ) arranged offset from one another and detector elements ( 5 , 6 , 7 , 8 ) assigned to the partial scanning fields ( 3 a, 3 b; 3 a5, 3 b5, 3 c5 ; 3 a 7 _' , 3 b _{7 '} , 3 c _{7 '} , 3 a ₇ , 3 b 7 , 3 c ₇ , 3 a 7 _' , 3 b 7' , 3 c _7' ) . , 9 , 10 , 11 , 12 ), which deliver modulated and phase-shifted output signals during the photoelectric scanning of a scale (45, 4, 470), wherein the partial scanning fields ( 3 a, 3 b; 3 a5, 3 b5, 3 c5; 3 a _7' , 3 b _7' , 3 c _7' , 3 a ₇ , 3 b ₇ , 3 c ₇ , 3 a _7' , 3 b _7' , 3 c _7' ) are further assigned spatially separated, directionally selective deflection means (3d5, 3e5, 3f5; 3e, 3f; A _7' , A' ₇ , A ₇ , A' ₇ , A _'7' , A'_7' ) which deflect at least one Part of the partial beams passing through the partial scanning fields ( 3 a, 3 b; 3 a5, 3 b5, 3 c5; 3 a _7' , 3 b _7' , 3 c _7' , 3 a ₇ , 3 b ₇ , 3 c ₇ , 3 a _7' , 3 b _7' , 3 c _7' ) in the direction of a jointly applied partial area on the scale (45, 4, 470). 2. Photoelectric length or angle measuring device according to claim 1, wherein the partial scanning fields ( 3 a, 3 b; 3 a5, 3 b5, 3 c5; 3 a _7' , 3 b _7' , 3 c _7' , 3 a ₇ , 3 b ₇ , 3 c ₇ , 3 a _7' , 3 b _7' , 3 c _7' ) and the directionally selective deflection means (3d5, 3e5, 3f5; 3e, 3f; A _7' , A'_7' , A ₇ , A' ₇ , A'_7' , A'_7' ) are arranged on the front and back of a common carrier substrate. 3. Photoelectric length or angle measuring device according to claim 1, wherein the directionally selective deflection means are designed as diffraction gratings. 4. Photoelectric length or angle measuring device according to claim 1, wherein the diffraction gratings assigned to the different partial scanning fields ( 3 a, 3 b; 3 a5, 3 b5, 3 c5; 3 a _{7' ,} 3 b 7', 3 c _7' , 3 a ₇ , 3 b ₇ , 3 c ₇ , 3 a _{7 '} , 3 b 7', 3 c _7' ) have different grating constants. 5. Photoelectric length or angle measuring device according to claim 1, wherein the directionally selective deflection means are designed as diffractive optical elements. 6. Photoelectric length or angle measuring device according to claim 1, wherein the directionally selective deflection means are designed as refractive optical elements. 7. Photoelectric length or angle measuring device according to claim 1, wherein the direction-selective deflection means ( 3 d5, 3 e5, 3 f5; 3 e, 3 f) are designed as binary step gratings. 8. Photoelectric length or angle measuring device according to claim 1, wherein the directionally selective deflection means are designed as blazed phase structures (A _7' , A'_7' , A ₇ , A' ₇ , A _'7' , A'_7' ). 9. Photoelectric length or angle measuring device according to claim 1, wherein the directionally selective deflection means are designed as holographic optical elements. 10. Photoelectric length or angle measuring device according to claim 1, wherein the partial scanning fields ( 3 a, 3 b; 3 a5, 3 b5, 3 c5; 3 a _7' , 3 b _7' , 3 c _7' , 3 a 7, 3 b ₇ , 3 c ₇ , 3 a _{7 '} , 3 b _7' , 3 c _7' ) are arranged shifted relative to one another in the measuring direction. 11. Photoelectric length or angle measuring device according to claim 10, wherein the partial scanning fields ( 3 a, 3 b; 3 a5, 3 b5, 3 c5; 3 a _7' , 3 b _7' , 3 c _7' , 3 a ₇ , 3 b ₇ , 3 c ₇ , 3 a _7' , 3 b _7' , 3 c _7' ) are arranged parallel to one another in the measuring direction (X). 12. Photoelectric length or angle measuring device according to claim 11, wherein the shifted further partial scanning fields ( 3 b; 3 a5, 3 c5; 3 a 7', 3 b 7 _' , 3 c _7' , 3 a ₇ , 3 b 7, 3 c ₇ , 3 a ₇ ', 3 c 7') are arranged symmetrically to a central partial scanning field (3 b5, 3 a, 3 _{b 7} ' ) _{on the} two adjacent sides. 13. Photoelectric length or angle measuring device according to claim 2 and 10, wherein on the back of the carrier substrate, transversely to the partial scanning fields ( 3 a, 3 b; 3 a5, 3 b5, 3 c5; 3 a ₇ ' , 3 b _7' , 3 c _7' , 3 a ₇ , 3 b ₇ , 3 c ₇ , 3 a _7' , 3 b _7' , 3 c _7' ) areas with deflection means are arranged, which are assigned to the various partial scanning fields ( 3 a, 3 b; 3 a5, 3 b5, 3 c5; 3 a _7' , 3 b _7' , 3 c _7' , 3 a ₇ , 3 b ₇ , 3 c ₇ , 3 a _7' , 3 b _7' , 3 c _7' ) and each have a different distracting effect. 14. Photoelectric length or angle measuring device according to claim 10, wherein the partial scanning fields ( 3 a, 3 b; 3 a _7' , 3 b _7' , 3 c _7' , 3 a ₇ , 3 b ₇ , 3 c ₇ , 3 a _7' , 3 b _7' , 3 c _7' ) and the deflection means (3e, 3f; A _7' , A'_7' , A ₇ , A' ₇ , A'_7' , A'_7' ) are arranged repeatedly perpendicular to the measuring direction (X).