Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2001-04-09
2002-10-29
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S231140, C356S616000
Reexamination Certificate
active
06472658
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a photoelectric position measuring system for measuring the relative position of two elements and, more particularly, to a three-grating sensor that optimizes the degree of modulation of the scanning device and the intensity of a reference marker signal. The invention can be used for the measurement of lengths and angles.
BACKGROUND OF THE INVENTION
Photoelectric measuring systems are used to a great extent with machine tools. To this end, one grating, in the form of a scale, is fastened on a machine element; and a scanning unit, with at least a second grating, is fastened to a further machine element which can be displaced in relation to the first machine element. A mostly collimated light beam from a light source is modulated as a function of the relative displacement of both gratings with respect to each other, and a periodically changing light signal is generated by light-sensitive detector elements when there is relative displacement. Beam portions which are transmitted in the case of a transmitted light system or reflected in the case of a reflective light system by the scale can be evaluated.
In connection with incremental position measuring systems, a reference marker, which fixes an absolute position and therefore permits an absolute association of the incremental counting signals to a zero point on the machine, is often placed on the scale besides the incremental counting track. Such a position measuring system utilizing collimated illumination and an amplitude grating is described, for example, in the magazine entitled “Feinwerktechnik & Me&bgr;technik” [Precision Mechanics & Measuring Technology] 97 (1989) 1-2 at pages 43 to 46. This reference describes the dependency of the degree of modulation of the scanning signals on the scanning distance, i.e., on the distance between the two grating graduations. A maximum degree of modulation occurs at distances of
Z
=
n
·
P
2
λ
where Z equals the spacing between the amplitude grating and a second grating along the optical axis, n=0, 1, 2 . . . ; P=the graduation period of the amplitude grating; and &lgr;=the mean radiation wavelength of the light source.
With small graduation periods, the amplitude maximum at n=0 can not be practically used because the graduations can become too easily scratched at such close distances. For this reason a scanning distance Z at n=1 is used. However, it has been shown that at this distance and with the use of amplitude scales, the degree of modulation drops relatively strongly because of the finite divergence of the light source, and a smearing of the intensity modulation occurs at the second grating. This is disadvantageous in view of the highest possible degree of modulation required in connection with present-day demands made on resolution and measuring accuracy. Furthermore, the scanning distance was selected without taking into consideration the reference marker structure.
In an article by Pettygrew “Analysis of Grating Imaging and Its Application to Displacement Metrology”, SPIE vol. 136 (1977), pages 325 to 333, the advantages of a three-grating position measuring system are explained where the grating arrangement is illuminated by a divergent light beam, or respectively diffusely, instead by a collimated light beam. The three-grating position measuring system includes a first amplitude grating with the graduation period P
1
, a second grating in the form of an amplitude grating with a graduation period of P
2
, and of a third grating in the form of an amplitude grating with the graduation period P
1
. The position of the planes of maximum modulation on the optical axis is determined by the equation:
Z
=
n
·
P1
·
P2
λ
where n=0, 1, 2, . . . ; P
1
=the grating period of the second granting; P
2
=the grating periods of the first and third grating; &lgr;=the mean radiation wave length of the light source; and P
1
=2•P
2
. With this scanning method, the divergence of the light source hardly affects the degree of modulation. However, it has the disadvantage that the intensity of the light beam captured by the detector greatly diminishes with the scanning distance.
Another three-grating sensor is described in the Pettygrew article where the second grating is a phase grating having a bar/gap ratio of 1:1 and an effective phase deviation of &lgr;/2. The zero diffraction order is surpressed by such a grating. It can be seen from
FIG. 9
of the Pettygrew article that the degree of modulating of such “diffraction imaging” systems is less than with “geometric imaging” systems in accordance with FIG.
8
. It should be mentioned as a further disadvantage that this system only has a sufficient degree of modulation starting at a distance of Z of and P
1
•P
2
/&lgr; with P
1
=P
2
. Thus, with relatively large graduation periods P
1
, a relatively large distance Z is required, so that, particularly with divergent illumination the light intensity is low.
A further incremental, interferentially operating position measuring system is described in German Patent Publication DE 27 14 324 C2, wherein a first grating is divergently illuminated and a second grating in the form of a reflecting amplitude scale is arranged at a distance therefrom, and the reflected light again passes through the first grating. To increase the degree of modulation and of the intensity (signal amplitude), the first grating was designed as a phase grating with a &lgr;/4 phase deviation. Here, use was made of the knowledge that, aside from the inevitable reflection losses at air/glass surfaces, with a phase grating almost the entire light passes through the grating. A phase grating with a phase deviation of approximately &lgr;/4 generates light intensities in the +1st, the −1st and in the zero diffraction order and, in regard to its diffraction characteristics, behaves similar to an amplitude grating of the same grating constant while having the advantage of a greater light intensity.
An incremental position measuring system using two gratings is described in European Patent Publication EP 0 729 013 A2. Here, the first grating is a phase grating with a phase deviation of &lgr;/2, and the second grating is an amplitude grating. The first grating is the scale on which, in addition to the periodic phase grating, a reference marker in the form of an amplitude structure has been applied. The illumination of the scale is performed by collimated light which has the disadvantage that a compact structure is not possible. This grating system can be employed and is functional only when collimated light is used.
In order to obtain a compact design of a position measuring system, it is furthermore known to integrate a reference marker, or generally speaking an additional marking, into a periodic incremental graduation track. German Patent Publication DE 25 01 373 A1 shows a possibility for this, in that the periodic pattern is partially changed. To do this, fields of the periodic pattern are omitted in accordance with the teachings of U.S. Pat. No. 3,985,448.
The integration of a reference marker into a periodic phase grating is explained in German Patent Publication DE 23 62 731 A1. A microstructure is overlaid on a phase grating with a phase deviation of &lgr;/2 and a 1:1 bar/gap ratio, so that defined diffraction orders are amplified at a reference location. Scanning is performed by collimated illumination which, as already mentioned, has the disadvantage that a compact structure cannot be realized.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a position measuring system for the generation of periodic signals, as well as further signals, in particular reference marker signals, which have a high degree of modulation for all signals and which, at the same time, can be realized simply and in a compact structure.
The invention will be explained in greater detail with reference to the preferred embodiments represented in the
Franz Andreas
Holzapfel Wolfgang
Mayer Elmar
Brinks Hofer Gilson & Lione
Dr. Johannes Heidenhain GmbH
Le Que T.
Luu Thanh X.
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