Photoelectric position measuring device.

The invention relates to a photoelectric position measuring device according to the preamble of claim 1.

Such an arrangement is described in DE-C2-24 31 551. A first embodiment of so-called three-grating measuring arrangements consists of three transparent grids, which are successively penetrated by the incident light. In a second embodiment, an arrangement is described which consists of only two gratings, one of which is transparent and the second is designed as a reflection grating. In the arrangements described there, the grating constant of the grating used cannot be less than the wavelength of the light used. This limits the resolution of the measuring arrangement.

Another three-grating measuring arrangement is described in EP-A1-163 362. Due to the special design of the phase grating shown there with respect to phase shift and duty cycle (ridge / furrow), three light beams are created which, when the grids move relative to one another, experience intensity modulations that are staggered in phase (preferably 120 °). With this arrangement, too, the grating constant cannot be smaller than the wavelength of the incident light.

A so-called two-grating measuring arrangement is known from DE-A1-21 46 310, in which the grating constants of the grids used are smaller than the wavelength of the incident light. A fundamental disadvantage of such interferometric two-grating measuring arrangements is that they are sensitive to fluctuations in the spacing of the grids from one another and are dependent on the spatial coherence of the light used.

The object of the invention, in contrast, is to provide a position measuring device with three grids in terms of effectiveness, which are scanned interferometrically and also allow grating constants which are smaller than the wavelength of the light used. In addition, the scanning signals should be harmonic free.

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

With the help of exemplary embodiments, the invention will be explained in more detail with reference to the highly schematic drawings.

It shows<ul id="ul0001" list-style="none"><li>FIG. 1 shows a transmitted light arrangement,</li><li>Figure 2 shows an enlarged detail of a phase grating and</li><li>FIG. 3 shows an enlarged detail of another phase grating.</li></ul>

In the transmitted light arrangement shown in FIG. 1, a phase grating 2 is illuminated at an angle α to the grating normal 0 by a light source 1 which emits narrowband light. The angle α is determined according to the formula sin α = λ / 2 g, where λ is the wavelength of the light used and g is the grating constant. Behind the phase grating 2, two partial beams 3 and 4 of the diffraction orders "0" and "1" are created by diffraction. These partial beams 3 and 4 hit a further grating 5, which is preferably also designed as a phase grating. This grating 5 is designed so that no zeroth diffraction order is created. This design can be done in a known manner by the surface design of the grating 5, for example in the case of a grating with a rectangular profile by varying the step height, which corresponds to the variation of the phase shift (see FIG. 2).

Since the grating constant g in the grating 5 is also smaller than the wavelength λ, there are also no diffraction orders with an ordinal number greater than "1". Of the four partial beams 6, 6 'formed on the grating 5 by diffraction; 7, 7 'only the partial beams 6 and 7 bent towards the grating normal 0 are used. The partial beams 6 and 7 meet another grating 2 '- which is identical in physical properties to the grating 2 - and are in turn diffracted at this grating 2'. Behind the grating 2 'you get four diffracted partial beams 8, 9, 10, 11. Each two of the partial beams, namely 8 and 9 and 10 and 11 have the same direction and can thus interfere with each other.

When the grating 5 is shifted by the amount x compared to the grids 2 and 2 ', the partial beams 6 and 7 diffracted on the grating 5 receive a phase shift Φ = 2πx / g or Φ = -2πx / g.

The gratings 2 and 2 'are designed so that when a beam is diffracted, the resulting partial beams of the zero and first order receive a constant phase shift by the amount Θ. The partial beams 3 and 4 are therefore delayed in their phase by the amount Θ to each other. Likewise, the partial beams 8 and 10 formed from the partial beam 6 on the grating 2 'and the partial beams 9 and 11 formed from the partial beam 7.

The aforementioned phase shift by the amount Θ is achieved by a special design of the grids 2 and 2 '. A possible embodiment is disclosed in EP-A1-163 362. This particular design consists of a phase grating with a rectangular profile in that the land / groove ratio (duty cycle) a: b differs markedly from 1: 1 and the step height h is designed according to Figure 2 so that the grating 2, 2 'a clearly phase deviation deviating from π radian.

The interfering partial beams 8 and 9 or 10 and 11 fall on photo elements 12 and 13 and are converted into electrical signals which are evaluated in a manner known in the case of incremental position measuring devices.

The signals can be described by the following formulas: For the photo element 12 I 12 = K (1-cos (2.2πx / g)) and for the photo element 13 I 13 = K (1-cos (2 (2π<maths id="math0001" num=""><math display="inline"><mrow><mfrac><mrow><mtext>x</mtext></mrow><mrow><mtext>G</mtext></mrow></mfrac></mrow></math><img file="EP0333929A2_D0001.tif" /></maths> - Θ))), where K is a constant, x denotes the shift of the grids 2, 2 'and 5 to each other and Θ the phase shift already described.

According to the example described above, a position measuring device based on the incident light principle can also be implemented in an analogous manner. The grating 5 is designed as a reflection grating and the grating 2 and 2 'are physically identical. In this case, FIG. 1 can be regarded as an "unfolded" representation.

The advantages of the position measuring device according to the invention over the arrangements mentioned in the introduction to the description are that a high resolution is achieved without subsequent interpolation, and that the spacing of the gratings from one another can vary within wide limits without the signals being adversely affected. Furthermore, there is only a small requirement for the spatial coherence of the light source. Furthermore, the signals generated at the photo elements are harmonic-free, since they are caused by the interference of only two partial beams, so that an optimal evaluation of these signals is guaranteed.