Photo-electronic position-measuring device.

0001The invention relates to a photoelectric position measuring device in which the relative adjustment of a scanning grating designed as a phase grating compared to a scale designed as a reflecting phase grating is detected, light emitted by at least one light source on the scanning grating, upon reflection on the scale and finally again on the scanning grating, and only groups with a specific diffraction order are detected from the resulting interference image via photo-receivers and periodically changing, phase-shifted measuring signals are converted during the relative adjustment of the scanning grating and scale.

0002The basic principle of such position measuring devices is known from GB-PS 1 474 049. In these known measuring devices, the Oth, i.e. undiffracted group and the same positive and negative phase offset groups of the diffraction image are always detected for evaluation by means of the photodetector, the signals moving the direction of movement of the scanning grating relative to the scale and reflecting phase grating the extent of the adjustment of the scanning grating relative to the scale can also be determined. According to DE-OS 23 16 248 three photo receivers are provided in a corresponding arrangement, which detect the Oth group of the diffraction pattern and the positive and negative first order diffraction patterns. A special version of the latter arrangement is known from EP-BO 163 362. There is a special design of the scanning grating by a web: gap ratio that differs from 1: 1, for example in the order of 1: 3rd and corresponding adjustment of the step height and by appropriate adjustment of the arrangement of the photoreceivers an increase in insensitivity to a change in the phase offset by changes in the spacing of the gratings and a reception of the diffraction groups Oth and positive and negative first order, three phase-shifted as precisely as possible by 120 ° To receive measurement signals. The phase shift can essentially be achieved, but both this latter position measuring device and all other known devices have decisive disadvantages. First, the absolute height or change the absolutely considered change of the signals depending on the illuminance, the distances between the grids and the optical losses during the measurement process, so that the measurement signals must be normalized before processing and furthermore, three signals that are phase-shifted by 120 ° cannot be immediately used for derive usable measurement signals from the length or angle measurement. Rather, it is necessary to first convert and convert these three signals into two 90 ° phase-shifted analog, sine-like signals, from which digital count signals are then obtained according to the usual methods used in normal incremental measuring systems, which are used to control up-down counters and thus can subsequently be used to indicate the linear or rotary path covered.

0003To avoid interference from internal reflections, it is known in the case of other position measuring devices of the generic type to use at least one transparent phase grating and to guide the diffracted partial beam bundles back to receivers via an optical deflection device through the grating (s) at a distance from the area of ​​incidence of the light. Here, a laser is preferably used as the light source. In the following, diffraction patterns of the Oth and positive and negative diffraction patterns of an Nth order are acquired again via photodetectors, used to obtain three signals which are phase-shifted by 120 °, and measurement signals are generated therefrom by the conversion and transformation already mentioned. From EP-A-0 387 481 it is known to use only a phase grating as a yardstick, to work in the transmitted light method and to introduce two coherent partial light beams onto the grating with angles of incidence relative to the normal, so that these partial light beams are diffracted on the grating, interfere with each other and measurement signals can be obtained again from the interference image by photo receivers of a measuring device. In two-grating arrangements, too, it is known to introduce the light into the first grating at a given angle of incidence in order to be able to detect the adjustment direction more easily during the measurement and to obtain sufficiently differentiated diffraction images for detection by the photodetectors.

0004From EP-A-0 223 009 it is known to provide two phase grids staggered by a quarter of the grating constant on the scanning grating, as viewed in the adjustment direction, which must moreover have a web / groove ratio that deviates from 1: 1. Here the measuring range extends over a relatively long length of the scale and the arrangement becomes complex. However, there is the advantage that signal normalization is possible by combining the received signals from the photo receivers. for each half of the grating, three photoreceivers are again provided for the beams which are diffracted in the respective direction.

0005The object of the invention is to provide a photoelectric position measuring device of the type mentioned, in which a simple structure is made possible, there is largely insensitivity to fluctuations in the absolute level of the received signals, and in which the received signals are easily converted into measuring signals suitable for further processing , in particular digital count signals can be converted.

0006The problem is solved in principle by the fact that the scale has two adjacent graduation tracks offset by 1/8 of the grating constant, the scanning grating is common to both graduation tracks and two photodetectors for each graduation track to receive the interference of the beams of rays diffracted in this direction to generate in particular signals are provided that are phase-shifted by 180 °, which have a phase shift of 90 ° in pairs with the signals obtained from the other graduation track because of the offset of the graduation tracks on the scale.

0007The basic idea of ​​the invention is that by using two graduation tracks on the scale, it is possible to obtain two signals that are exactly 90 ° out of phase with photodetectors arranged side by side in a row running transversely to the measuring direction. that there is therefore no need to record the interference for the diffraction order running in the vertical direction by means of a separate photoreceiver, and that, finally, by dimensioning the grating accordingly (web / furrow ratio, furrow depth) there is the possibility of phase-shifting two on each scale track by 180 ° To get signals.

0008In the position measuring device according to the invention, the four signals customary in incremental length measuring technology are thus obtained<maths id="math0001" num=""><img file="EP0509979A2_D0001.tif" /></maths> in which<maths id="math0002" num=""><math display="block"><mrow><mtext>f =</mtext><mfrac><mrow><mtext>2nd π</mtext></mrow><mrow><mtext>G</mtext></mrow></mfrac><mtext> x</mtext></mrow></math><img file="EP0509979A2_D0002.tif" /></maths> is. X means the displacement, g the lattice constant and A and B constant.

0009By anti-parallel connection of the signals I and II or III and IV obtained from the photo-receivers arranged in a longitudinal row, the measurement signal is freed from the DC voltage component and therefore largely independent of the absolute height of the signal, so that the optical transmission of the grating and changes in the illuminance hardly affect the measurement influence more. The sine-like signals obtained from the anti-parallel circuits can, after appropriate amplification, be used in a known manner as analog measurement signals which advance or lag behind one another in the direction of adjustment for the generation of digital counting signals, with the principle of the same evaluation circuits as in conventional incremental measuring systems with photoelectric scanning of the scale by means of one another phase-shifted gratings can be used. Of course, in the position measuring device according to the invention, in addition to the graduation tracks of the scale, reference marks can be provided in separate tracks or between the graduation tracks, which advantageously also form phase gratings and are detected by cooperating scanning grating regions and photo receivers, which here preferably evaluate the Oth derivative of the interference image can be.

0010Various variants are possible in the technical implementation of the position measuring device according to the invention. First, separate light sources can be provided for the two graduation tracks, which consist of light sources that are as monochromatic as possible, which, if necessary, are followed by a collimator, but which can also be formed by a laser. It is sufficient to use only light in the form of a narrow strip.

0011According to another embodiment, a common light source and optical guide devices are provided for distributing the light emitted by this light source to the scanning grating regions and photo receivers assigned to the two graduation tracks on the scale. An advantageous further development here is when the optical guide device has a diffraction grating that fills the light beam from a light source transversely to the longitudinal direction of the scale, the arrangement and division of which can be selected such that essentially the largest proportion of the light introduced falls on the photoreceivers. By appropriate selection of the scanning grating, its ridge / groove width and its groove depth for both gratings, taking into account the wavelength of the light used, it can be achieved that a high proportion of the light in the selected group order of the diffraction pattern is diffracted and thus used for the measurement.

0012According to another embodiment, the light of the light source can be provided on the scanning grating on the side facing the light source on the guiding prisms which divide the graduation tracks.

0013A particularly advantageous arrangement when using conventional divergent light sources consists in that a condenser lens arrangement is provided which extends over the beam path from the light source to the phase gratings and also the beams for the selected order of the interference image and which also concentrates the corresponding portions of the interference image on the photoreceivers. According to a variant, separate condenser lenses upstream of the scanning grids can also be provided for the two scale graduation tracks.

0014According to a possible variant of the subject matter of the invention, the reflective scale is replaced by a transparent scale and an optical redirection prism reflecting the light diffracted by the scanning grating and passing through the scale tracks, with further diffraction by the scale and scanning grating to the receivers. Such a diversion prism, as already mentioned in the prior art, reduces the effect of internal reflections on the measurement reception. However, a diverting prism of the type mentioned can also be used for the generation of reference pulses at reference marks if, for example, a reference mark is marked on the scale by two gratings, the longitudinal spacing of which is selected as a function of the diverting path of the light in the diverting prism and which can interact with further grating elements on the sensing grating , so that a reference signal is only generated on an assigned receiver, when appropriately diffracted light reaches the reference signal receiver via the first reference mark, the diverter prism and the second reference mark as well as the corresponding grating of the scanning grating.

0015Further details and advantages of the subject matter of the invention can be found in the following description of the drawings.

0016The subject matter of the invention is illustrated in the drawing, for example. Show it<ul id="ul0001" list-style="none"><li>1 is a highly schematic representation of a possible basic arrangement in a position measuring device according to the invention seen from the side,</li><li>Fig. 2 a. 1 to illustrate the relative arrangement of lighting devices and photoelectric receivers,</li><li>3 shows a schematic representation modified with regard to the light source, but otherwise corresponding to FIG. 1, in which the scanning grating for the beam passages has been shown twice, namely in front of and behind the transparent scale, to clarify the mode of operation,</li><li>4 shows a diagram corresponding to FIG. 3,</li><li>5 in a representation corresponding to FIG. 4, an embodiment variant with only one light source,</li><li>6 shows a further embodiment variant in a diagram corresponding to a transverse view of FIG. 1,</li><li>7 shows another embodiment again in a schematic representation according to FIG. 6,</li><li>Fig. 8 shows another embodiment in a schematic representation according to Fig. 6 and</li><li>Fig. 9 is a schematic diagram of a measuring system equipped with a diversion prism.</li></ul>

00171 to 8 of the drawing, the same parts are designated by the same reference numerals. Light sources, which can be LCDs, light bulbs, laser diodes or lasers, were equipped with 1, 2, optical scanning grids with 5, their imaginary second arrangement with 5 ', the two graduation tracks of scale 6 with 7 and 8 and the photo receivers assigned to the two graduation tracks with 9, 10, 11, 12. In the Fig. 1 to 5 and 9, the collimator lenses indicated in the other figures by rectangular representation of the light sources 1, 2 have not been drawn for the sake of simplicity.

0018The scanning grating 5 is designed as a step grating in which the crossbeams 13 are narrower than the depressions 14. In contrast, the graduation tracks 7, 8 of the scale are designed as a grid with the same width of the bars and gaps 15, 16. In the exemplary embodiments, the scanning grating and the graduation tracks have the same graduation period, but arrangements are also readily possible in which the graduation period of the scale tracks 7, 8 deviates from the graduation period of the scanning grid 15. The two graduation tracks 7, 8 of the scale 6 are mutually offset by 1/8 of the lattice constant. 1 to 4, a separate light source 1, 2 is provided for each scale track 7, 8. The light emitted by these light sources is diffracted as it emerges from the scanning grating 5 and is thus fanned out in the longitudinal direction of the scale 6, a reflection pattern being produced by reflection and diffraction of the light on the gratings of the graduation tracks 7, 8, which again falls on the scanning grating 5 and is bent again by the scanning grating. Photo receivers 9, 10 and 11, 12 are arranged so that they detect the interference of the light beams diffracted in the respective direction, the scanning grating 5 being designed in such a way that the signals from the photo element 9 and 10 and the signals from the photo element 11 and 12 are 180 ° out of phase. When the scanning grating 5 is adjusted relative to the scale 6, the intensity of the light falling on the photo elements 9 to 12 also changes depending on the direction of adjustment, so that the direction of adjustment can be defined from the signals. The receivers 9, 11 and 10, 12, which are arranged next to one another in transverse rows, receive signals that are 90 ° out of phase with each other because of the offset of the division tracks 7, 8. The receivers 9, 10 and 11, 12 are subsequently connected in anti-parallel connection, so that the signals that can be removed from this parallel connection are largely independent of interference by changing the overall illuminance and the optical density in the beam path. These signals arise in their basic form as sinusoidal signals, the signal length of which corresponds to half the division constant of the grating of the scale graduation tracks 7, 8. The signals can be amplified in a known manner, converted and via trigger circuits or the like into digital counting signals for the control of display units, machine controls and the like. Like. Converted, although in the exemplary embodiments only linear scales 6, which in practice extend over the entire measuring length, were illustrated, but of course an analog arrangement with a measuring rod 6 lying on a circular ring is also possible for the angle measurement. Preferably, scale 6 will be attached to the circumference of a cylindrical drum. On the scale, between the graduation tracks or on both sides outside of these graduation tracks 7, 8 reference marks can be attached, for which counter-marks, lighting devices and receivers are provided on the scanning grating and which, in a very specific relative adjustment of the scanning unit to the scale, provide a detectable reference pulse for defining a scale zero or od. Generate.

0019Of course, the grids of the scanning grating 5 and the scale graduation tracks 7, 8 have been greatly enlarged and exaggerated in the exemplary embodiments. In practice, grating constants of the order of μm and with step heights of the gratings kept even smaller than the grating constant are used, the step heights and the gap-bar ratio of the scanning grids 5 also being dependent on the wavelength of the light used.

00205, only a single light source 1 is provided, which emits a light beam which is fanned out across the grating 5 via an optical diffraction grating 17, so that these fanned out partial beams which are diffracted for the first time on the grating 5 fall on the two partial scale tracks , be reflected there with further diffraction and finally get back to the receivers 9, 10, 11, 12 via the grating 5 '.

00216 shows a similar arrangement, in which the light from a light source 1 is additionally collected by a condenser lens arrangement 18 and then fanned out on the grating 17, the partial beams of the selected interference image group also being guided through the condenser lens again before they open the receivers 9 to 12 fall.

00227, the optical grating 17 of FIGS. 5 and 6 is replaced by two triangular prisms 19, 20 deflecting the light beam through the grating 5 to the scale graduation tracks 7, 8.

00238, the two scale sub-tracks 7, 8 and the receivers 9, 10 and 11, 12 assigned to them are each assigned their own condenser lens arrangements 21, 22, which separate the light coming from the light source 1 via the grating 5 to the scale sections and direct selected interference image groups to the receivers 9, 10 or 11, 12.

0024The embodiment according to FIG. 9 uses instead of the reflective scale 6 according to FIGS. 1 to 8 a transparent scale 6a, which is also provided on the side facing the scanning grating 5 with two offset graduation tracks forming optical diffraction gratings. The light coming from the light source 1 is diffracted at the scanning grating, falls under further diffraction and interference imaging through the graduation tracks of the scale 6a and is then redirected via a triangular prism 23 first in the longitudinal direction of the scale and then back to the scale 6a, where it passes under new diffraction and finally is bent again on the grating 5 before it falls on the photodetectors 9 to 12.