Optical arrangement for detecting the intensity modulation of partial ray beams

An optical arrangement or length- or angle-measuring device comprising an illumination device for generating partial ray beams and an image lens to receive and transmit the partial ray beams, wherein each of the received partial ray beams defines an angle of inclination with respect to the image lens. The optical arrangement further comprises a plurality of detectors to receive the transmitted partial ray beams and the detectors are positioned so that the distance between the image lens and the individual photodetectors is a function of the angle of inclination of the partial ray beams received by the image lens.

Applicants claim, under 35 U.S.C. .sctn. 119, the benefit of priority of 
the filing date of Jun. 27, 1992, of a German application, copy attached, 
Serial Number P 42 21 165.4, filed on the aforementioned date, the entire 
contents of which are incorporated herein by reference. Applicants further 
claim, under 35 U.S.C. .sctn. 119, the benefit of priority of the filing 
date of Jul. 21, 1992, of a European application, copy attached, Serial 
Number 92112487.1, filed on the aforementioned date, the entire contents 
of which are incorporated herein by reference. 
FIELD OF THE INVENTION 
The invention relates to an optical arrangement for detecting the intensity 
modulation of partial ray beams inclined in various directions, 
particularly an interferentially-operating position-measuring device 
having an illumination device and a plurality of grids that are 
displaceable relative to each other. 
DESCRIPTION OF PRIOR ART 
European Patent Document EP 0 163 362 B1 discloses a measuring device 
wherein the photodetectors are disposed in the focal plane of the lens. 
The arrangement of the photodetectors in one plane has the disadvantage, 
however, that photodetectors with a relatively large surface area are 
required in order to concentrate the total intensity of the individual ray 
beams onto the photodetectors. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the invention to create an optical 
arrangement, particularly a length- or angle-measuring device, with which 
the ray beams detected by an image lens can be detected completely with 
photodetectors that have a small surface area and are subsequently faster. 
This object is attained by means of an optical arrangement or length- or 
angle-measuring device comprising an illumination device for generating 
partial ray beams and an image lens to receive and transmit the partial 
ray beams, wherein each of the received partial ray beams defines an angle 
of inclination with respect to the image lens. The optical arrangement 
further comprises a plurality of detectors to receive the transmitted 
partial ray beams and the detectors are positioned so that the distance 
between the image lens and the individual photodetectors is a function of 
the angle of inclination of the partial ray beams received by the image 
lens. 
The particular advantages of the invention are that photodetectors can be 
used that have a small surface area, and are therefore faster. 
Moreover, the entire ray beams are detected by the photodetectors, 
guaranteeing a high intensity of the signals derived from the 
photodetectors. It is assured by means of the invention that the 
peripheral rays of the ray beams also impact completely on the 
photodetectors, and that the scanning area of the graduated grid is also 
completely and uniformly imaged onto the photodetectors. 
Further advantageous features of the invention will become apparent from 
the detailed description of exemplary embodiments of the invention, taken 
in conjunction with the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The photoelectric length-measuring device shown in FIG. 1 includes a light 
source 1, a lens 2, a scanning plate 3 with a grid 301, a reflecting 
graduated grid 401, and three photodetectors 5, 6 and 7. As explained in 
detail in EP 0 163 362 B1, the light generated by the light source 1 and 
collimated by the lens 2 is diffracted in three different directions as it 
passes through the grid 301. These first diffracted, partial ray beams 
comprise 0th, +1st and -1st diffraction order and are identified as 
reference numerals 8, 9 and 10, respectively, in FIG. 1. 
The first partial ray beams 8, 9 and 10 are reflected on the graduated grid 
401, and deflected one more time into +1st and -1st components so as to 
produce second diffracted partial ray beams 11, 12, 13 and 14. These 
partial ray beams 11, 12, 13 and 14 pass again through the grid 301, and 
are again diffracted and come to interference. The third diffracted 
partial ray beams 15 through 20 are received by and transmitted through 
lens 2. Thus, source 1, lens 2, grid 301, and graduated grid 401 act as an 
illumination device for generating diffracted partial ray beams to impact 
on lens 2. Those third diffracted partial ray beams having the same angle 
of inclination with respect to lens 2, are collected by the lens 2 and 
directed at the three photodetectors 5, 6 and 7, which convert the 
intensity modulations into three electrical signals phase-shifted by 
120.degree. from each other. These electrical signals form a value for the 
direction and size of the displacement of the grid 301, in relation to the 
graduated grid 401. 
The angle of inclination of the partial ray beams 15, 18 and 17, 20 
impacting on the lens 2 depends on the separation period of grids 301 and 
401. The smaller the graduation period at a given wavelength .lambda., the 
larger the angle of inclination of the third diffracted partial ray beams 
15, 18 and 17, 20 (.lambda.=constant in this case). For example, ray beam 
20 enters lens 2 at an angle of inclination .alpha., relative to a line 
parallel to the optical axis of lens 2. The ray beam is bent by lens 2 so 
that it exits lens 2 at an angle of inclination .alpha., as shown in FIG. 
1. 
The lens 2 does not image the partial ray beams 15 through 20, which have 
varying inclinations, onto a common plane. The larger the angle of 
inclination of the partial ray beams 15 through 20 impacting on the lens 
2, the closer the point of optimum imaging--that is, the image point of a 
light source point--is to the lens 2. This is taken into account by the 
present invention in that the photodetectors 6, 7 of the more steeply 
inclined partial ray beams 15, 18 and 17, 20 are disposed closer to the 
lens 2 than the photodetector 5 of the partial ray beams 16, 19, which are 
not inclined with respect to lens 2. The distance between the lens 2 and 
the individual photodetectors 5, 6, 7 is therefore selected as a function 
of the angle of inclination of the partial ray beams 15 through 20 with 
respect to the lens 2. 
FIG. 1 includes a schematic drawing of the image shell 21, which is 
generated by the convexity of the image and on which the points of optimum 
imaging of the variously inclined partial ray beams 15 through 20 
collected by the lens 2 lie. When the photodetectors 5, 6, 7 are disposed 
on this image shell 21, photodetectors 5, 6, 7 having small surface areas 
can be used, and the entire partial ray beams 15 through 20 can be 
detected despite this. Particularly with high-resolution measuring devices 
with grids 301, 401 with short graduation periods and, thus, large angles 
of inclination of the partial ray beams with respect to the optical axis 
of the lens, this has the great advantage that very fast photodetectors 5, 
6, 7 can be used. It is known that the rapidity, i.e., the frequency 
response of photodetectors 5, 6, 7, is a function of the size of the 
receiving surface. The smaller the receiving surface, the faster and 
better the photodetectors 5, 6, 7 can detect the intensity modulation of 
the partial ray beams 15, 18 and 17, 20. 
The photodetectors 5, 6, 7 can be secured in a known way on a plate. To 
attain the varying distances between the lens 2 and the photodetectors 5, 
6, 7, the photodetectors 5, 6, 7 can be secured to raised parts 22, 23. 
A multilayer plate technique can also be implemented in a way that is not 
shown, in that the photodetector 5 is disposed on a first plate, and the 
photodetectors 6, 7 are disposed on a second plate, placed above the 
first. Flexible plates that are arched in a suitable way can likewise be 
used. 
A further exemplary embodiment of a length-measuring device is shown in 
FIG. 2 in which reference marks 25, 26 are disposed next to the graduated 
grid 401. The principal design and mode of operation of this 
length-measuring device with reference marks are known per se from German 
Patent Publication DE 38 34 676 A1. The same reference numerals are used 
for the same components in FIGS. 1 and 2. Only the differences with 
respect to the prior art and the measuring device described in FIG. 1 are 
described below. 
The light source 1 and the photodetectors 5, 6, 7 are disposed on a plate 
24. The photodetectors 5, 6, 7 are closer to the lens 2 than the 
photodetector 5. Aside from the graduated grid 401, reference marks 25, 26 
are located on the scale 4. Besides the grid 301, the scanning plate 3 has 
reference marks-scanning fields 27, 28 for scanning the scale graduation 
401. A prism 29 deflecting crosswise with respect to the measuring 
direction is disposed above the grid 301, by means of which an optical 
graduation of the scanning ray paths is achieved during scanning of the 
graduated grid 401 and the two reference marks 25, 26. To generate a 
reference signal, at least one further photodetector 30, 31 is associated 
with the reference marks 25, 26. 
A number of prisms that direct the ray beams at a plurality of 
photodetectors can also be used for optical graduation of the scanning ray 
paths. In this case it is also advantageous when the prisms are disposed 
between the photodetectors and the scanning plate. For better handling and 
adjustment of the prisms, it is advantageous to mount them on a plate or, 
when a plurality of prisms are used, to integrate them into a common prism 
plate. 
Up to this point a length-measuring device with a reflective scale 4 has 
been described. The invention is not limited to this, however; it can also 
be used in angle-measuring devices with a reflective index disk as a scale 
and, in length- or angle-measuring devices with a transparent scale, i.e., 
in transillumination systems. The number of grids passed through is also 
not limited to three. An arrangement of two grids is particularly 
advantageous in transillumination devices. 
FIG. 3 shows a schematic of a three-grid transmitter in accordance with the 
transillumination method. Light is transmitted by an illumination device 
13 towards a lens 23. Lens 23 generates light which is uncollimated and 
directed convergently at a first diffraction grid 33. At grid 33 the light 
is diffracted and transmitted to penetrates a graduated grid 43. Grid 43 
diffracts and transmits the light to a third grid 33'. Grid 33' then 
diffracts the light and transmits the diffracted light to photodetectors. 
These processes correspond in analogous fashion to those described for 
FIG. 1. 
The distances of the photodetectors 53, 63, 73 from the image lens 23 
likewise correspond to those of FIG. 1 and are a function of the 
inclination of the interfering ray beams. 
This arrangement further has the advantage of a very simple design and high 
efficiency in conjunction with good signal behavior when distances are 
altered. 
The result of the principles underlying the invention is that the spacing 
dependency of the photodetector on the angle of inclination of the partial 
ray beams does not necessarily require a position-measuring device that 
operates interferentially. This is only one particularly advantageous 
application for which the invention is well-suited. 
The invention may be embodied in other forms than those specifically 
disclosed herein without departing from its spirit or essential 
characteristics. For example, the grids used can be both phase grids as 
well as amplitude grids. Furthermore, it is also possible, however, to 
generate partial ray beams inclined toward each other with other optical 
elements besides grids, such as prisms. The described embodiments are to 
be considered in all respects only as illustrative and not restrictive, 
and the scope of the invention is commensurate with the appended claims 
rather than the foregoing description.