Opto-electrical position-measuring device

The invention relates to a photoelectric position measuring device in which light from a light source is modulated in a position-dependent manner by a plurality of grids which can be displaced relative to one another, by interfering with light beams which are diffracted on the grids and by providing a plurality of photodetectors for the formation of phase-shifted electrical signals.

Such a position measuring device is described in EP-0 163 362-B1. A reflective scale grating is slidable relative to a scanning grating. The scanning grating is a phase grating with a certain ratio of the ridge widths to the furrow widths in order to generate three electrical signals which are 120 ° out of phase with one another. A group of diffraction beams of the same direction is focused on each of the three detectors. These groups of diffraction beams of the same direction are also referred to as so-called resulting diffraction orders. The diffraction rays n-th resulting diffraction order is the group of rays that emerge directionally from the total system of the two gratings as if, apart from the reflection on the scale, they were deflected by only one of the gratings in the n-th diffraction order.

Another such position measuring device is known from DE-34 16 864-C2. The area of ​​a reference mark of a scale consists of a transverse division, which is scanned with an aperture structure (amplitude division) of the scanning plate. This transverse division consists of several strip-shaped diffraction elements which are arranged next to one another in the measuring direction. The diffraction elements are transverse gratings, the grating bars of which run parallel to the measuring direction. The individual diffraction elements differ with regard to their transverse division periods and therefore deflect an incident light beam in different directions. If this transverse division is illuminated through the column of the scanning plate, then deflected light beams are generated, the deflection angle of which depends on the transverse division period and thus on the illuminated transverse grating region, from which the scale position is derived. Different deflected light beams are focused by a lens on different photo detectors in the focal plane of the lens.

In the position measuring device according to EP-0 220 757-B1, the scale also has a transverse division. The scale consists of reflecting areas arranged one behind the other in the measuring direction and areas with the transverse division.

This transverse division is a phase grating, the grating parameters of which are selected such that the resulting 0th diffraction order is extinguished and the further diffraction orders do not strike the photodetector. The areas with the transverse division are thus seen by the photodetector as non-reflecting areas.

Furthermore, cross grids and checkerboard grids are known as a yardstick for two measuring directions. The grating lines of the position measuring device according to EP-0 482 224-B1 run, for example, diagonally to the two measuring directions, so that incident light is diffracted in two directions. These gratings are not designed to generate mutually phase-shifted scanning signals, since the adjacent transverse grating regions have a transverse phase shift of 0 ° or 180 °.

The invention has for its object to provide a photoelectric position measuring device that is simple and inexpensive to manufacture.

This object is achieved with a position measuring device which has the features of claim 1.

Advantageous refinements are specified in the dependent claims.

The advantages of the position measuring device according to the invention are that position-dependent scanning signals with high levels and limit frequencies can be generated by (quasi) single-field scanning, which are insensitive to contamination and / or scale errors. Furthermore, relatively large manufacturing tolerances of the phase division are permissible, which enables cost-effective manufacturing. Another advantage is that scanning signals which are phase-shifted relative to one another can be generated in a simple manner.

The invention is explained in more detail by means of some exemplary embodiments with reference to the drawings.

It shows:<dl id="dl0001"><dt>Figure 1</dt><dd>a scanning grating according to the invention,</dd><dt>Figure 2</dt><dd>a beam path generated at the scanning grating,</dd><dt>Figure 3</dt><dd>a position measuring device with the scanning grating according to Figures 1 and 2,</dd><dt>Figure 4</dt><dd>another scanning grating,</dd><dt>Figure 5</dt><dd>another example of a scanning grating,</dd><dt>Figure 6</dt><dd>a scanning grating with a continuous course,</dd><dt>Figure 7</dt><dd>a scanning grating for filtering harmonics,</dd><dt>Figure 8</dt><dd>a scanning grid for generating four scanning signals,</dd><dt>Figure 9</dt><dd>8 shows a position measuring device with the scanning grating from FIG.</dd><dt>Figure 10</dt><dd>3 shows a spatial measuring device with the scanning grating from FIG. 8,</dd><dt>Figure 11</dt><dd>another scanning grating,</dd><dt>Figure 12</dt><dd>another example of a scanning grating,</dd><dt>Figure 13</dt><dd>a scanning grating for generating three signals that are 120 ° out of phase with one another,</dd><dt>Figure 14</dt><dd>a possible arrangement of the photodetectors in a position measuring device with the scanning grating according to Figure 13,</dd><dt>Figure 15</dt><dd>13 shows a position measuring device with the scanning grating according to FIG. 13,</dd><dt>Figure 16</dt><dd>another position measuring device according to the invention,</dd><dt>Figure 17</dt><dd>the scanning grating of the position measuring device according to Figure 16,</dd><dt>Figure 18</dt><dd>17 shows a section II of the scanning grating according to FIG. 17,</dd><dt>Figure 19</dt><dd>a section II-II of the scanning grating according to Figure 17 and</dd><dt>Figure 20</dt><dd>a position measuring device according to the prior art.</dd></dl>

The functional principle of the position measuring device according to the invention can be understood if one first considers the functional principle of a known position measuring device, which is shown in FIG. 20 and is to be briefly described. The light from a light source 100 is collimated by a collimator lens 101 and strikes a phase grating 102 with a division period TA. The phase grating 102 has approximately the same web and gap widths and a phase shift of 90 °. The phase shift of 90 ° means that the ridge height, including the refractive index, is dimensioned such that a collimated beam immediately after passing through the phase grating 102, i.e. in the near field, has a wavefront that has a local phase shift 90 ° (λ / 4) of the ridge areas compared to the gap areas. The effect of this phase grating 102 results in the intervals<maths id="math0001" num=""><math display="inline"><mrow><msup><mrow><mtext>Z1 = (n + 1/2) TA</mtext></mrow><mrow><mtext>2</mtext></mrow></msup><mtext>/ λ</mtext></mrow></math><img file="EP0735346A2_D0001.tif" /></maths> Interference fringes 104 with the period TA. With n = 0, 1, 2 ...., λ = light wavelength,<maths id="math0002" num=""><math display="inline"><mrow><msup><mrow><mtext>TA</mtext></mrow><mrow><mtext>2</mtext></mrow></msup><mtext>/ λ = Talbot distance</mtext></mrow></math><img file="EP0735346A2_D0002.tif" /></maths>. At one of these distances Z1 there is a further grating 103 with an amplitude division of the division period<maths id="math0003" num=""><math display="inline"><mrow><mtext>TM = TA</mtext></mrow></math><img file="EP0735346A2_D0003.tif" /></maths>. Depending on the position of the two gratings 102, 103 relative to one another in the measuring direction X and thus of the grating 103 relative to the interference fringes 104, the transmitted light power differs and is detected by a photodetector 105. When the two gratings 102, 103 are mutually displaced in the measuring direction X, the photodetector 105 supplies a periodic scanning signal.

In the invention, the first grating 102 is now designed as a scanning grating 1 with a special transverse division in order to generate a plurality of position-dependent scanning signals that are phase-shifted relative to one another. FIG. 1 shows such a scanning grating 1, which consists of a series of transversely structured first and second grating regions 2, 3 which are periodically arranged in the measuring direction X. When viewed in the measuring direction X, these grating regions 2, 3 are at least approximately the same width. The width of a first and a second grating region 2, 3 forms the division period TA and is identical to the division period TM of the scale grating 4. The first and second grating regions 2, 3 consist of transverse grids of the same transverse division period TT, viewed in the Y direction. The markings (grating strips) of the transverse grating regions 2, 3 are shifted from one another in the Y direction by a quarter of the transverse division period TT. Both grating regions 2, 3 are also designed as phase gratings, the webs and gaps of which are dimensioned in such a way that the even-numbered transverse diffraction orders (0, ± 2, ± 4, ...) are preferably suppressed. For this purpose, the transverse web and gap elements have approximately the same width (in the Y direction) and the webs have a phase height of 180 ° (λ / 2). If a collimated beam of light strikes this scanning grating 1, each transversely structured grating region 2, 3 essentially splits the incident light beam into a +1. and a -1. transverse diffraction order. Because of the uniform transverse division period TT of both grating regions 2, 3, the same transverse diffraction orders of both grating regions 2, 3 have the same deflection angle (in the Y direction). The offset of the grating strips of both grating regions 2, 3 in the Y direction results in two partial beams in each diffraction order, which are phase-shifted with respect to one another in the near field, that is to say directly on the scanning grating 1. This phase shift of the partial beams is shown in Figure 2.

FIG. 2 shows a section II-II of FIG. 1. A light beam impinging on the first grating region 2 becomes +1. and -1. Diffraction order split, the resulting partial beams are shown in full lines. A light beam impinging on the second grating region 3 is also +1. and -1. Diffraction order split, the resulting partial beams are shown in broken lines. In +1. transverse diffraction order is the phase shift of the partial beams of both areas 2, 3 + 90 ° (λ / 4) and in -1. transverse order of diffraction -90 ° (- λ / 4). These local phase shifts can be compared with the local phase shift of the above-described position measuring device according to the prior art (FIG. 16). The local effect of the scanning grating 1 according to the invention is therefore similar to that of a conventional phase scanning division, if one considers only one of the two transverse diffraction orders (+1. Or -1.) Alone and disregards the transverse deflection of the partial beams. With a phase shift of + 90 ° of area 2 with respect to area 3, area 2 also corresponds to the web of a conventional phase scanning division. With a phase shift of -90 °, however, area 2 corresponds to a gap. The effects of the division according to the invention in +1. and -1. The transverse diffraction order is thus that of a conventional scanning graduation, but shifted by half a division period (180 °).

If one now illuminates such a scanning grating 1 with collimated light, then approximately at the known intervals <maths id="math0004" num=""><math display="inline"><mrow><msup><mrow><mtext>Z1 = (n + 1/2) TA</mtext></mrow><mrow><mtext>2</mtext></mrow></msup><mtext>/ λ</mtext></mrow></math><img file="EP0735346A2_D0004.tif" /></maths> essentially two interference fringe systems, one starting from the +1. transverse diffraction order and one starting from the -1. transverse diffraction order. For the reasons mentioned above, however, both interference fringe systems are out of phase with one another by 180 °, so that the interference fringe maxima of one interference fringe system coincide with the minimas of the other. Is now in one of the spaces<maths id="math0005" num=""><math display="inline"><mrow><msup><mrow><mtext>Z1 = (n + 1/2) TA</mtext></mrow><mrow><mtext>2</mtext></mrow></msup><mtext>/ λ</mtext></mrow></math><img file="EP0735346A2_D0005.tif" /></maths> with n = 0, 1, 2 ... the scale grating 4 in the form of an amplitude division, the transmitted light becomes +1. and the -1. transverse diffraction order with a relative shift of the two gratings 1, 4 modulated in phase opposition.

The +1. and -1. transverse diffraction orders passed to separate photodetectors 6, 7, so that they accordingly supply phase-shifted signals.

A position measuring device with the scanning grating 1 on a scanning plate 19 is shown in FIG. The light from a light source 8, preferably an LED or a semiconductor laser diode, is collimated by a first lens 9 and reaches the scanning grating 1, which is described in detail in FIGS. 1 and 2. The transmitted light beams then fall onto the scale grating 4, which is at a distance of approximately<maths id="math0006" num=""><math display="inline"><mrow><msup><mrow><mtext>Z1 = (n + 1/2) TA</mtext></mrow><mrow><mtext>2</mtext></mrow></msup><mtext>/ λ</mtext></mrow></math><img file="EP0735346A2_D0006.tif" /></maths> with preferably n = 0. In the case of a spatially extended light source 8 (LED), somewhat smaller distances are preferably chosen because the divergence of the lighting unit, consisting of light source 8 and collimator lens 9, causes the contrast of the interference fringing systems and thus the degree of modulation of the scanning signals to decrease with larger scanning distances. Even at significant transverse deflection angles, due to small transverse division periods TT, the optimal scanning distance is shortened.

The scale grating 4 is an amplitude grating with transparent and non-transparent strips arranged one behind the other in the measuring direction X, the edges of the strips run in the Y direction (a section is shown in FIG. 10). The beams of rays transmitted by the scale grating 4 are focused by a further lens 5 onto the photodetectors 6, 7, which are arranged at a distance from one another in the Y direction, that is to say perpendicular to the measuring direction X. The +1 hit the photo detector 6. transverse diffraction orders of the two grating regions 2, 3 of the scanning grating 1 and on the photodetector 7 meet the -1. transverse diffraction orders of the two grating regions 2, 3 of the scanning grating 1. The photodetectors 6, 7 thus supply two signals which are 180 ° out of phase with one another. The effect of the lenses 5, 9 creates an image of the light source 8 on the photodetectors 6, 7, which image is generally very small. Despite large scanning areas on the scale grating 4, very small and thus fast photo detectors 6, 7 can therefore be used. The two scanning signals generated in this way are derived from a common scanning area of ​​the scanning grating 1 and the scale grating 4, so that the advantages of the single-field scanning are achieved with the invention.

In the example described so far, each division period TA of the scanning grating 1 in the measuring direction X has two grating regions 2 and 3, which have a phase offset transversely which deviates from 0 ° and 180 °. According to the invention, however, each division period TA can also be subdivided into a plurality of grid regions, each with any phase offset. It is essential that several grating regions are involved in the generation of the mutually phase-shifted scanning signals, which have a phase shift that deviates transversely from 0 ° and 180 °. FIG. 4 shows such a scanning grating 10 with four regions 11, 12, 13, 14 of approximately the same width per division period TA. Each area 11, 12, 13, 14 has a transverse division with markings (grid strips) of the same division period TT running in the measuring direction X. The grid strips of the second area 12 are arranged with a phase shift of 90 ° with respect to the grid strips of the first area 11, the grid strips of the third area 13 again with respect to the grid strips of the second area 12 by 90 ° and the grid strips of the fourth area 14 are with the grid strips of the second Range 12 in phase. The phase positions are thus 0 °, 90 °, 180 °, 90 °.

If such a scanning grating 10 is used in a position measuring device according to FIG. 3, the optimum scanning distance is approximately <maths id="math0007" num=""><math display="inline"><mrow><msup><mrow><mtext>Z1 = (n + 1/4) TA</mtext></mrow><mrow><mtext>2</mtext></mrow></msup><mtext>/ λ</mtext></mrow></math><img file="EP0735346A2_D0007.tif" /></maths> , in which the photodetectors 6, 7 from the +1. and -1. transverse diffraction order two signals 180 ° out of phase with each other are generated. This reduced scanning distance Z1 is particularly advantageous if the degree of modulation in a larger scanning distance Z1 would be reduced too much by the divergence of the lighting units 8, 9.

For certain applications, it may also be advantageous to maintain a larger scanning distance Z1, for example in order to reduce the harmonic content of the scanning signals due to the increased effect of the divergence of the lighting units 8, 9. A scanning grating 20, as shown in FIG. 5, is suitable for this. The scanning grating 20 again consists of a grating with a division period TA in the measuring direction X, which has four transverse grating regions 21 to 24. The markings (grating strips) of the transverse grating regions 21 to 24 are arranged so as to be out of phase with one another in such a way that the phase positions 0 °, 90 °, 0 °, -90 ° result. The optimal scanning distance Z1 is approximately<maths id="math0008" num=""><math display="inline"><mrow><msup><mrow><mtext>(n + 3/4) TA</mtext></mrow><mrow><mtext>2</mtext></mrow></msup><mtext>/ λ</mtext></mrow></math><img file="EP0735346A2_D0008.tif" /></maths> .

In a further exemplary embodiment, a scanning grating 30 is proposed which has a continuous course of the phase shift over a longitudinal division period TA (that is to say in the measuring direction X), as shown in FIG. Certain properties of the position measuring device can be optimized by this continuous course of the markings of both transverse grating regions 31 and 32. In this way, the harmonic content of the scanning signals can be reduced or the optimal scanning distance to a predetermined value <maths id="math0009" num=""><math display="inline"><mrow><msup><mrow><mtext>Z1 = (n + C) TA</mtext></mrow><mrow><mtext>2</mtext></mrow></msup><mtext>/ λ</mtext></mrow></math><img file="EP0735346A2_D0009.tif" /></maths> set with any C With the sinusoidal curve chosen in FIG. 6, the third harmonic in particular can be reduced very strongly. The amplitude a of the transverse web displacement is given by the following condition:<maths id="math0010" num=""><math display="block"><mrow><msup><mrow><mtext>a / TT = X3 / (4πsin (3π * Z1 * λ / TA</mtext></mrow><mrow><mtext>2</mtext></mrow></msup><mtext>))</mtext></mrow></math><img file="EP0735346A2_D0010.tif" /></maths> With<ul id="ul0001" list-style="none" compact="compact"><li>ΔY = <maths id="math0011" num=""><math display="inline"><mrow><mtext>a * sin (2πX / TA)</mtext></mrow></math><img file="EP0735346A2_D0011.tif" /></maths> local, transverse web displacement</li><li>X3 = zero of the Bessel function <maths id="math0012" num=""><math display="inline"><mrow><mtext>J3 (X3) = 0</mtext></mrow></math><img file="EP0735346A2_D0012.tif" /></maths></li></ul>

A scanning distance <maths id="math0013" num=""><math display="inline"><mrow><msup><mrow><mtext>Z1 = (n + 1/2) TA</mtext></mrow><mrow><mtext>2</mtext></mrow></msup><mtext>/ λ</mtext></mrow></math><img file="EP0735346A2_D0013.tif" /></maths> is particularly advantageous since the fundamental wave takes on a large value. On the other hand, a maximum degree of modulation of the scanning signals in a given scanning distance Z1 is obtained<maths id="math0014" num=""><math display="inline"><mrow><msup><mrow><mtext>a / TT = X1max / (4πsin (π * Z1 * λ / TA</mtext></mrow><mrow><mtext>2</mtext></mrow></msup><mtext>))</mtext></mrow></math><img file="EP0735346A2_D0014.tif" /></maths> with the maximum X1max of the Bessel function J1. The extension of the light source must still be taken into account in this case and leads to somewhat smaller scanning distances Z1.

In order to reduce the harmonic content of the scanning signals, it is also possible to make the transverse markings (grating strips) along the measuring direction X with different widths b, as shown in FIG. The diffraction efficiency in the individual diffraction orders ± 1, ± 2, ... transverse order of diffraction thus depends on the location X along the measurement direction X. Viewed in the near field, these transverse diffraction orders correspond to the effect of a combination of conventional phase and amplitude divisions, since both the phase and the amplitude of the partial beams are modulated in the near field. The width b (in the Y direction) of the individual markings varies and if they correspond, for example, to the solution of the equation<maths id="math0015" num=""><math display="block"><mrow><mtext>sin (πb (x) / TT) = </mtext><msqrt><mtext>sin (2πX / TA)</mtext></msqrt></mrow></math><img file="EP0735346A2_D0015.tif" /></maths> is selected, the scanning signals are harmonic-free at approximately the following distance Z1:<maths id="math0016" num=""><math display="block"><mrow><msup><mrow><mtext>Z1 = (n + 1/2) TA</mtext></mrow><mrow><mtext>2</mtext></mrow></msup><mtext>/ λ</mtext></mrow></math><img file="EP0735346A2_D0016.tif" /></maths>

FIG. 7 shows this scanning grating 40, consisting of two grating regions 41, 42 which are phase-shifted by TT / 4 relative to one another, in which the width b (x) of the markings (web or gap) varies in accordance with the above equation.

FIG. 8 shows a scanning plate 59 which has two groups of scanning grids 51, 52 which are arranged nested in one another in the measuring direction X. Each group consists of three identical scanning grids 51 and 52. The scanning grids 51 and 52 of a group are arranged offset by a multiple of the longitudinal division period TA. Each scanning grating 51, 52 in turn consists of three longitudinal division periods TA. The scanning grids 51, 52 of the two groups differ in the transverse division periods TT1 and TT2, which have values ​​of 5 μm and 7 μm, for example. In addition, the scanning grids 52 of the second group are around<maths id="math0017" num=""><math display="inline"><mrow><mtext>(m + 1/4) TA</mtext></mrow></math><img file="EP0735346A2_D0017.tif" /></maths> arranged at a distance from the scanning grids 51 of the first group in the measuring direction X, with m = 1, 2 .... As a result of this geometric offset, the intensity modulations of the ± 1st transverse diffraction order of the scanning gratings 52 of the second group are each phase-shifted by 90 ° to those of the first group. By separate detection of the +1. and -1. Diffraction orders of both groups are thus obtained with four scanning signals, each phase-shifted by 90 ° relative to one another. Of course, each group can consist of more than three scanning grids 51, 52 and each scanning grating 51, 52 of more than three division periods TA.

FIG. 9 shows a position measuring device with the scanning plate 59 according to FIG. 8. The light from a light source 8 is collimated with the lens 9 and reaches the scanning plate 59. The partial beams ± 1. Diffraction order of the scanning gratings 51, 52 of both groups hit the scale grating 4 at the known scanning distance Z1 and the transmitted partial beams are focused by the further lens 5 onto four photodetectors 6, 7 and 61, 71. The four photodetectors 6, 7, 61, 71 are arranged one above the other in the Y direction. Due to the different transverse division periods TT1 and TT2, the light beams are deflected differently from the scanning grating 51 than from the scanning grating 52 and thus through the second lens 5 onto different photodetectors 6, 7; 61, 71 focused. The light beams which are from the scanning grids 51 in +1. Diffraction orders are deflected, hit the photodetector 61 by the scanning grating 51 in -1. Diffraction order are deflected onto the photodetector 71. The light beams, which from the scanning gratings 52 in +1. Diffraction order are deflected, hit the photodetector 6 by the scanning gratings 52 in -1. Diffraction order are deflected onto the photodetector 7. Each of the scanning gratings 51, 52 can be designed in accordance with the previously described embodiments (FIGS. 1 to 7) or combinations thereof.

The two position measuring devices described all work in the transmitted light method. However, the invention can also be implemented in the so-called incident light method, as shown in FIG. 10. The result is a particularly simple construction with only one lens 9, which also takes over the function of the second lens 5. The scanning plate 59 is designed according to FIG. 8. Since the scale grating 4 is designed to be reflective, the scanning plate 59 is run through twice, so that the first pass in +1. and -1. transverse diffraction order deflected partial beams again transversely in +1 in the second pass. and -1. Diffraction order can be distracted and in +2. and -2. resulting transverse diffraction order (related to the direction) emerge. The four photo detectors 6, 7, 61, 71 are in or arranged near the focal plane of the lens 9 so that it ± 2. the resulting transverse diffraction orders of the two groups of scanning gratings 51, 52.

During the first pass through the scanning grids 51, 52, the partial beams are deflected in the Y direction and thus, after reflection on the scale grating 4, are displaced in the Y direction again onto the scanning plate 59 Partial beam. At a given scanning distance Z1, this shading is different for both scanning gratings 51, 52 due to the different transverse division periods TT1, TT2.

This shadowing effect can be compensated for by correspondingly different dimensions of the scanning gratings 51, 52 in the Y direction.

The second pass through the scanning plate 59 in ± 2. resulting partial diffraction order emerging partial beams are overlays of different partial beams, which are deflected differently transversely in the two passes. This also occurs in the first round in +3. and in the second round in -1. transverse diffraction order deflected partial beam in +2. resulting transverse diffraction order and arrives at the photodetector 6 or 61. Since this partial beam in comparison to that twice in +1. transverse diffraction order deflected partial beams a different path, a scanning signal can be generated with a coherent superposition of both partial beams, which is strongly dependent on the scanning distance Z1. A coherent superposition should therefore be avoided, for example, by using a light source which is incoherent in time and / or space, such as an LED or a longitudinally or transversely multimode laser, in particular a semiconductor laser diode. Another possibility to avoid a coherent superposition is to design the grating regions 81, 82 of a scanning grating 80 as a so-called chirped grating, the transverse division period TT of which changes continuously by a small amount depending on the path in the Y direction (FIG. 11). Due to this constant variation of the local transverse division period TT (Y), the incident light beams are deflected differently in the Y direction. These partial beams which are deflected to different degrees are directed onto a common photodetector 6 or 7. The path length differences between the superimposed partial beams which fall on one of the photodetectors 6, 7 are thus location-dependent in the Y direction and are averaged so that the coherent superimposition is destroyed. This training is particularly advantageous when using a laser light source.

A particular advantage of the position measuring device shown in FIG. 10 is the position of the so-called neutral pivot point. The neutral pivot point is defined as the point about which the scanning arrangement 6 to 9, 61, 71 or the scale 4 can be tilted without the scanning signals being phase-shifted relative to their nominal position, so that the position measurement value determined by an evaluation electronics remains constant. The neutral pivot point of the position measuring device according to FIG. 10 lies in the plane of the scale grid 4. A ripple of the scale surface - in particular in the measuring direction X - which corresponds to a local tilting of the scale grid 4 about an axis of rotation lying in the plane of the scale grid 4, does not influence the determined measured position value, which leads to considerable increases in the accuracy of the measuring device. This is because the light beams are diffracted in the Y direction regardless of the position in the X direction on the second pass through the scanning plate 59. The intensity of the partial beams passing through is independent of the point of impact in the measuring direction X. For this purpose, the photodetectors 6, 7, 61, 71 should detect all longitudinal diffraction orders, at least every 0th and ± 1. and possibly also the ± 2. Diffraction orders.

The scanning plate 59 according to FIG. 8 can be modified in such a way that pure single-field scanning is also possible. Such a scanning plate 57 is shown in FIG. Compared to FIG. 8, the grid regions 53, 54 of the scanning grating 51 with the transverse graduation period TT1, which are offset from one another by 1/4 of the transverse graduation period TT1, are not arranged directly next to one another in the measuring direction X; of the transverse division period TT2 arranged in between. Each graduation period TA of the resulting scanning grating 50 consists of four grating regions 53 to 56 of the same width. The first and third grating regions 53, 54 each have the same transverse graduation period TT1 of, for example, 5 μm, the markings (grating stiffeners) of the grating regions 53, 54 are out of phase with each other by TT1 / 4 corresponding to 90 °. In the assigned first transverse diffraction orders, they generate two first scanning signals that are 180 ° out of phase with one another. The intermediate second and fourth grating regions 55, 56 also have a common, but different TT1 transverse division period TT2 of, for example, 7 μm, the transverse grating strips being mutually phase-shifted in relation to one another in the Y direction by TT2 / 4 corresponding to 90 °. In the assigned first transverse diffraction orders, they also generate two scanning signals that are 180 ° out of phase with each other. However, these two scanning signals are 90 ° out of phase with respect to the first two scanning signals by the geometric offset (in measuring direction X) of the grating regions 53, 54 with respect to 55, 56, so that four scanning signals 0 °, 90 ° with respect to each other are 90 ° out of phase , 180 °, 270 ° can be derived from a common area of ​​the scanning plate 90 and thus also from the scale grid 4.

In principle, any phase grating according to the prior art, in which the phase shift is realized by webs and gaps with different heights and different refractive indexes arranged alternately in measuring direction X, can be replaced by a transverse grating with transversely geometrically phase-shifted markings of several grating regions. Thus, the scanning grating (reference grating) used in EP-0 163 362-B1 can also be implemented in a particularly simple manner by the invention. In order to generate three signals that are phase-shifted by 120 ° with respect to one another in this further inventive scanning grating 90, which is shown in FIG are out of phase. The grating regions 91, 92 can in turn be designed as an amplitude or as a phase grating. It is also particularly advantageous here to design the transverse grating regions 91, 92 as a phase grating, the parameters of which are selected such that the 0th, ± 2nd, ± 4th, ... transverse diffraction orders are suppressed.

The width of the two grid areas 91, 92 forms the division period TA. One of the grid areas 91 has a width of 2TA / 3 and the other area has a width of TA / 3 (viewed in the measuring direction X).

FIG. 15 shows a position measuring device with the scanning grating 90 according to FIG. 13. the light source 8 illuminates a scanning plate 99 with the scanning grating 90 via a collimator lens 9. The transmitted light beams hit the reflecting scale grating 4 and are in turn directed onto the scanning grating 90. Partial beams diffracted in the measuring direction X as well as transversely diffracted hit the photodetectors 6, 62, 63. By twice transverse diffraction in +1. Diffraction order hit the partial beams of the +2. resulting diffraction order on the photodetectors 6, 62, 63.

FIG. 14 shows a possible arrangement of photodetectors 6, 62, 63 and 7, 72, 73. However, since the photodetectors 7, 72, 73 generate the same signals (0 °, 120 °, -120 °) as the photodetectors 6, 62, 63, they are not used in the position measuring device in FIG. 15.

The use of a transversely deflecting scanning grating 90 has the advantage that when a reference mark 93, 94 is scanned simultaneously, the partial beams can be separated by the selection of the grating parameters of the transverse grating regions 91, 92 and the reference mark 93. No additional deflection elements in the form of prisms are required in order to prevent crosstalk between the partial beams from the reference mark 93, 94 and the scale and scanning grating 4, 90. It is particularly advantageous to design the reference marks 93, 94 in a known manner in the measuring direction X as a chirped grating, in particular according to FIG. 3 of the as yet unpublished European patent application 95102328.2. The chirped lattice strips of the reference mark 93 have a transverse division, so that by means of the photodetectors 95, 96 two signals which are 180 ° out of phase with one another can be generated.

FIG. 16 shows a further position measuring device according to the invention. The scanning grids 151 and 152 are formed on the scanning plate 159. Two groups of scanning gratings 151, 152 are provided for scanning the scale grid 4. Each group consists of two identical scanning grids 151 and 152, respectively. The two scanning grids 151 and 152 of each group are offset from one another by a multiple of the longitudinal division period TA. Each scanning grating 151, 152 in turn consists of five longitudinal division periods TA. In contrast to the exemplary embodiment according to FIG. 8, the scanning gratings 151, 152 of the two groups do not differ by different transverse division periods, but by the blaze angle φ1, as can be seen from FIGS. 18 and 19. In addition, the scanning grids 152 of the second group are around<maths id="math0018" num=""><math display="inline"><mrow><mtext>(m + 1/4) TA</mtext></mrow></math><img file="EP0735346A2_D0018.tif" /></maths> arranged at a distance from the scanning grids 151 of the first group in the measuring direction X, with m = 0, 1, 2 .... The scanning grids 152 essentially direct the incident light bundle into the +1 by the blaze effect. transverse diffraction order. After reflection on the scale grating 4, it becomes +1 again on the second pass through the scanning grating 152. transverse diffraction order so that it is in +2. resulting transverse diffraction order emerges and is focused by the lens 9 onto the photodetectors 64, 65 which detect the various longitudinal diffraction orders.

In an analogous manner, the associated light beam is directed onto the photodetectors 74, 75, which are in turn longitudinally spaced apart, by the reversed blaze angle φ1 of the scanning grating 151.

The scanning signals obtained from the photodetectors 64, 65 and 74, 75 can be shifted in phase with respect to one another by a suitable design by a suitable design of the grating regions 153, 154 and 155, 156 of the scanning gratings 151, 152. For example, a phase shift of 90 ° between the scanning signals of the photodetectors 64 and 65 can be achieved if the transverse grating regions 155, 156 have the same division period TT and are arranged transversely offset from one another by 2 TT / 3. Viewed in the measuring direction X, a grating area 155 and a grating area 156 each form a division period TA, the width of the grating area 155 being approximately 2 TA / 3 and the width of the grating area 156 being approximately TA / 3. As shown in FIG. 17, the same conditions apply to the grating regions 153 and 154 of the scanning grating 151 in order to achieve a phase shift of 90 ° between the scanning signals of the photodetectors 74, 75. The phase shift of the scanning signals of the photodetectors 64, 65 with respect to the photodetectors 74, 75 can be set by a geometric offset of the grating region 152 with respect to the grating region 151. If this offset is TA / 4, for example, then the scanning signals of the photodetectors 64, 65 are 180 ° out of phase with respect to the photodetectors 74, 75. The four required scanning signals, which are usually 90 ° out of phase with one another, are thus obtained in a simple manner and have the advantages of single-field scanning. The light yield in the (± 1.) Transverse diffraction orders used is particularly high in this exemplary embodiment if the suitable blaze angle φ1 of the grating regions 153 to 156 is selected. The conditions for this are known per se to the person skilled in the art and can be found in the relevant specialist books under "Echelette grid". The use of the echelette grids, which are shown in particular in FIGS. 18 and 19, is particularly advantageous if, as in so-called three-grating transmitters (EP-0 163 362-B1), the + n-th and -n-th transverse diffraction orders are modulated in phase and therefore none can provide additional information.

For better differentiation of the blaze angles φ1, the grating regions 153, 154 with the downward-facing blaze angle φ1 in FIG. 17 are hatched differently than the grating regions 155, 156 with upward-directed blaze angle φ1.

In this exemplary embodiment, the reference mark 193 also consists of a chirped scanning grating which consists of grating regions 195 and 196 which are alternately arranged in the measuring direction X and whose widths decrease continuously in the measuring direction X. The grating regions 195 and 196 have opposite blaze angles φ2, as can be seen in section II and II-II in FIGS. 18 and 19. The grating regions 195 direct the incident light bundle onto the photodetector 95, the grating regions 196 direct the incident light bundle onto the photodetector 96. The photodetectors 95 and 96 thus supply the clock pulse. Push-pull signal of the chirped reference mark 194.

By using transverse grating areas 153 to 156 and 195, 196 for both the incremental and for the reference mark scanning, it is easily possible for the individual diffracted by different choice of the transverse division periods TT, TTR and / or the blaze angle φ1, φ2 Separate light beams in the focal plane of the lens 9. This eliminates the need for complex, light-deflecting means such as prisms or mirrors. The blazed grids shown can be produced particularly advantageously by embossing.

A particular advantage of the scanning gratings 1, 10, 20, 30, 40, 51, 52, 80, 50, 90 is that the local phase shift of the in + nth and - nth (n = 1, 2, 3 ...) transverse diffraction order diffracted beam does not depend on the phase shift and the web width of the transverse grating, but solely by the geometric offset of the transverse grating regions 2, 3, 11 to 14, 21 to 24, 31, 32, 41, 42, 53 to 56, 81, 82, 91, 92 is given. As a result, the tolerances of phase shift and web width of the transverse grating regions 2, 3, 11 to 14, 21 to 24, 31, 32, 41, 42, 53 to 56, 81, 82, 91, 92 are large, so that the production is conventional Phase splitting is inexpensive.

In all embodiments, the scanning grating can be designed as a phase structure in the form of the surface relief shown in FIG. 2 or as a phase structure by means of a location-dependent variable refractive index or else as an amplitude structure in the form of location-dependent variable reflection, absorption or transmission. The phase structure has the particular advantage that - as already explained - the light intensity can be directed into certain diffraction orders. It is particularly advantageous to use the ± 1. To use diffraction orders, but other diffraction orders could also be used according to the invention.

Since both the position of the markings in the Y direction and their width in the Y and X directions can be chosen locally arbitrarily in the grid according to the invention, any combination between a phase division and an amplitude division can thus be produced in a simple manner. As a result, the desired phase shifts of the scanning signals derived directly from this grid can be selected with the grid according to the invention, and the scanning signals can also be optimized.

It is particularly advantageous if the scanning plate, which is relatively small in the measuring direction X, is designed according to the invention. However, it is also within the scope of the invention to design the scale accordingly.

In all examples, the photodetectors can be arranged at different distances, that is, depending on the angle of incidence of the partial beams falling on the lens 5 or 9, as explained in detail in EP-0 576 720-A2.

The invention can be used in length and angle measuring devices.