Photo-electrical position measuring device.

(42) Start of patent term: 15. 5.1998 (45) Date of issue: 25. 1.1999 (51) Int.Cl.⁶ : G01B 11/02 (56) Documentation:

EP 0223009A2 EP 0509979A2 (73)

Patentee:

RSF ELECTRONICS SOCIETY MBH

A-5121 TARSDORF, UPPER AUSTRIA (AT).

QQ

AT 404 637 (54) A photoelectric position measuring device, comprising a scale phase grating (1), at least one scanning phase grating (2) adjustable relative to the scale phase grating (1), at least one relative to the the Abtast-Phasengittem fixed light source (3) with collimator and photoreceptor (5-8), the groups of certain diffraction order from the by the two-time diffraction of the light on the or the sampling phase gratings (2) and the at least one time diffraction at the scale phase grating (1) resulting interference images and according to their intensity changes in the Relatiwerstellung of Abtast- and scale Phasengittem periodic, generate mutually phase-shifted measuring signals, is to achieve easily evaluable signals and to allow greater cultivation tolerances in the light beam path (10, 11) at least one conductive phase grating (9) with opposite the scale and Abtastphasengitter transverse webs and furrows turned on, which diffractively subjects the light beam to the diffraction direction on scale and scanning phase gratings (1, 2) into two sub-beam areas (12, 13) fanning out. For the groups of the same diffraction direction of the partial beams are separate photoreceptor (5,6 or 7,8) available.

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AT 404 637 Β

The invention relates to a photoelectric position measuring device, with a scale phase grating, at least one relative to the scale phase grating adjustable scanning phase grating, at least one relative to or the scanning phase fixed light source with collimator and photoreceivers, the groups of certain diffraction order from the by the two-time diffraction of the light on the or Receive the Abtast-phase gratings and the at least one time diffraction at the scale phase grid resulting Interterenzbildem and generate according to their intensity changes in the relative adjustment of sampling and scale phase gratings periodic, mutually phase-shifted measurement signals.

Such a position measuring device is known inter alia from EP 0 509 979 A. In such position measuring is carried out according to the principle of the so-called tri-pacemaker, the bases of which in 1978 published dissertation by J. Wilhelm Dreigitterschrittgeberphotoelektrische Aufnehmer for measuring position changes (TU Hannover) are explained in detail in theory. With reflective phase gratings as a benchmark, the light in the sampling phase grating is diffracted twice. At a transparent scale, a further step-off grating may be provided on the second scale side and finally it is also possible to diffract the light diffracted by the first scanning phase grating and the transparent scale over a diverting prism on the scale back over a portion of the yardstick length and then through the scale and to pass another scanning grid to the receivers, so that there is a two-fold diffraction of the light on the scale.

According to EP 0 509 979 A, ​​signal pairs each containing two signals which are phase-shifted by 180 * are generated, which are 90 ° out of phase with each other, provided on the scale two mutually adjacent graduation tracks offset by 1 / g of the lattice constant, the sampling phase grating being common to both graduation tracks and providing, for each graduation track, two photoreceivers for receiving the interference of the beams diffracted in that direction, so that by appropriate combination of the signals from the signal pairs in anti-parallel circuits each other depending on the direction of adjustment and lagging analog measurement signals are obtained, which can be evaluated to generate digital count signals, whereby similar evaluation circuits can be used, as used in conventional Inkrementalmeßsystemen in use.

In principle, phase errors can occur in all tri-level encoders of the previous design, in particular by division errors of the individual phase gratings and / or mounting tolerances of the scanning phase grids relative to the scale phase grids and due to tilting of the sampling phase grating, so that lengthy adjustment adjustments to achieve useful signals in the measurement become necessary.

In order to control the phase error and still obtain evaluable signal pairs with a corresponding phase offset, a measuring system according to Patent Abstracts of Japan, Vol. 7, no. 10 (P168) (1155), 14. January 1983 &amp; JP-A-57 169 613, with a phase grating as scale, but two coupled, consisting of light source, collimator, Abtast-phase gratings and photodetectors measuring systems in which at least the light source of a measuring system, but possibly also the entire measuring system in the scanning direction relative to the other measuring system is adjustable. This embodiment is extremely complicated and has the fundamental disadvantage that signals from the two measuring systems are forcibly located at a great longitudinal distance from each other, possibly processed by graduation errors different scale points.

A similar construction is known from EP 0 232 919 A. In this case, optical guide means are used which project the coming of a light source through the scanning phase grating on two again longitudinally spaced scanning fields of the scale phase grating, wherein for the Abtast-phase grating of 1: 1 deviating ratio of land width to furrow width is provided.

The same disadvantages also have a suggestion in the dissertation of J. Wilhelm, the back of a correspondingly large scanning phase grating in Meßverstellrichtung successively and gegengleich transverse to the measuring direction inclined optical prism wedges to order to get out of the deflected sub-beams again phase-shifted signals. Again, the signals are obtained from successive in Maßstablängsrichtung and not of the same scale ranges.

The object of the invention is to provide a measuring device of the type mentioned in the large cultivation tolerances in simple production are allowed, a high accuracy is achieved and a simple adjustment to adjust the phase error is possible.

The object is achieved in principle by the fact that in the light beam path at least one LeitPhasengitter is turned on with respect to the scale and Abtastphasengitter transverse webs and grooves, which fanned the diffracted light beam transverse to the diffraction direction of scale and Abtastphasengittern in two sub-beam areas and that for the groups are the same

AT 404 637 Β

Diffraction direction of the partial beams separate photoreceptor are present. Basically, the position measuring device according to the invention allows the use of simple scale and Abtastphasengitter, yet the phase-shifted signal pairs are generated at the same longitudinal region of the scale phase grating, so there is the possibility, even the phase-shifted signal pairs from each other in the transverse direction adjacent or to generate mutually passing sampling points on the scale phase grating. The fanning of the light beam on the Leitphasengitter is preferably driven so far by appropriate design and arrangement of this Leitphasengitters that pass in a three-grid encoder conventional design, the incidence areas of the partial beams into each other, but still a sufficiently large local separation of the interference images at the site of the photoreceptor is achieved ,

According to a preferred embodiment, it is provided that the guide phase grating has a ridge / groove ratio of 1: 1 at a groove depth corresponding to the wavelength of the light used, so that the light predominantly in the diffraction directions +1. and -1. order bends.

In the embodiment described so far results in this photoelectric position measuring device over the construction according to EP 509 979 A simplification of the overall structure because of the elimination of the second graduation track on the scale phase grating and an increase in accuracy, since tilt errors of the scanning unit can only have a smaller effect, because closely spaced scale ranges are scanned in a scan zone substantially neutral to tilting. Thus, one-field push-pull sampling is widely realized.

In order to be able to compensate still occurring phase errors, in particular by distance tolerances caused by phase errors, the Leit-phase grating is rotatable about a normal to its main plane extending axis according to a development. In principle, the guiding phase grating can be arranged between the scanning phase grating and the scale phase grating, so that it diffracts the light twice in the case of a reflecting scale phase grating. Another variant allows the arrangement of the guiding phase grid between the light source and the collimator or between the collimator and the scanning phase grating.

In order to simplify the overall structure, according to a further development, the conductive phase grating is combined with the or a scanning phase grating to form a cross grating, wherein preferably the grating lines intersect each other at an angle deviating from 90 ', e.g. At 45 °, if, as mentioned above, there is a two-fold diffraction of the light at this grating.

As already indicated, an embodiment is recommended according to which the optical guide phase grating only spreads the partial beams so far that their incidence ranges largely overlap on the scale phase grating.

Further details and advantages of the subject invention is taken from the following description of the drawing.

In the drawing, the subject invention is illustrated, for example. 1 shows in a highly schematic representation the basic arrangement of a photoelectric position measuring device according to the invention in the diagram, wherein the entire beam path is thus unfolded to illustrate the operation of the assumed as a reflective scale phase grating scanning grating, Leitphase grating and collimator were again shown on the back .

2 is a side view of FIG. 1 and FIG. 3 is a plan view of FIG. 2.

In the drawing, like parts are designated by like reference numerals. The imaginary second arrangement of the components in the unfolded representation was characterized in each case by the addition of a dash.

Basically, one over the length or Scale grating 1 passing through the circumference of a measuring range, a scanning phase grating 2, 2 ', at least one light source 3 and a collimator 4, 4' as well as photoreceivers 5, 6 and 7, 8 are provided. With the exception of the special arrangement of the photoreceptor 5 to 8, the measuring system in the embodiment described so far corresponds to the known three-pancake stepper. The light source 3 may be an LED, LD, incandescent lamp or a laser. The scale phase grating 1 is a reflective phase grating with a certain ridge-ridge ratio. The scanning phase grating 2, 2 'has a significantly different from 1: 1 web-groove ratio. Between the scanning phase grating 2, 2 'and the scale phase grating 1, a guide phase grating 9, 9' is mounted, which has, for example, a ridge to ridge ratio of 1: 1 at a depth corresponding to the wavelength of the light used, so that it the light predominantly in the diffraction directions +1. and -1. Order bends.

During the measurement, the light collimated at the collimator 4 (light beam 10) is first diffracted at the scanning phase grating 2 and the resulting simple light beam 11 is now designated during the first passage through the guide phase grating 9 into two partial beam regions 12, 13 (only in FIG ) fanned out, the

AT 404 637 Β fall with a small center distance from each other on the scale phase grating 1 and are reflected by this under diffraction to the guide phase grating 9 ', here again diffracted and after further diffraction at the Abtast-phase grating 2' via the collimator 4 ' the photoreceptor 5, 6 or 7, 8 illuminate. By a ridge-furrow ratio of 1: 1 at the wavelength of the light corresponding groove depth of the guide phase grating 9 and by appropriate dimensioning of the web-to-valley ratio in the scanning phase grating 2 and the scale phase grating 1 is achieved in the embodiment that the photoreceptor 5 and 6 7 and 8 receive the interferences of the partial beams 12, 13 diffracted in their direction, which in each of the two pairs 5, 6 and 7, 8 against each other have a nominal offset of 180 *. The setpoint offset in the pairs 5, 6 or 7, 8 signals generated compared to the signals of the other pair is 90 *. When set correctly, the signals transmitted by the photoreceivers 5, 7 and 15, respectively offset by 180 *, can be selected under antiparallel switching. 6, 8 two mutually phase-shifted by 90 *, generate in the basic form of sinusoidal analog measurement signals in which the one signal the other adjustment direction dependent in Reiatiwerstellung the parts containing the parts 2 to 9 scanning relative to the scale phase grid 1 or lags so that counting or control devices can be controlled depending on the adjustment direction via these signals.

As indicated by arrows 14, 14 'in Fig. 1, the guide phase grating 9 is rotatably adjustable about an axis normal to its main plane.

As a result, the division of the partial beams 12, 13 and the center distance of their impact on the

Scale phase grating 1 changed and subsequently influenced the phase separation of the signals generated at the receivers 5 to 8. The rotational adjustment of the guide phase grating 9 can be carried out during assembly of the measuring system. But it is also possible to provide a monitoring circuit, the phase difference between the signals occurring at the receivers 5 to 8 and, where appropriate, the signal level or monitors the relative signal level and actuates a rotary drive for the Leit-phase grating 9 in case of deviations from the set value. In addition to the rotational adjustment in the direction of arrow 14, a relative adjustment between the gratings 2, 1 in the direction of the main axis, ie, normal to the grating plane, would also be conceivable.