Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2000-08-30
2003-04-01
Allen, Stephone B. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S231160, C356S616000, C033S707000
Reexamination Certificate
active
06541761
ABSTRACT:
Applicants claim, under 35 U.S.C. §119, the benefit of priority of the filing date of Aug. 31, 1999 of a German patent application, copy attached, Ser. No. 199 41 318.5, filed on the aforementioned date, the entire contents of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical position measuring system that is suitable for precise determination of the relative spacing of two objects movable relative to one another.
2. Discussion of Related Art
Known position measuring systems include a scanned graduation structure as a measuring graduation as well as a scanning unit, movable relative to it in the measurement direction, that has a light source, one or more graduation structures, and a detector arrangement. In the position measuring systems of interest here, the generation of the position-dependent scanning signals is based on the fundamental principles explained below. Thus, in such position measuring systems, with the aid of one or more first graduation structures, a fine fringe pattern is created, which is scanned with the aid of one of more second graduation structures to generate the scanning signals.
In the simplest case, such a position measuring system is a so-called two-grating transducer, in which a first graduation structure is illuminated by a usually collimated light source. The result is a light pattern on the downstream second graduation structure, and the graduation period of the second graduation structure matches the graduation period of the light pattern. In the case of relative motion of the two graduation structures with respect to one another, the result is a periodic modulation of the light beams that pass through the second graduation structure. The light beams that pass through the second graduation structure are detected by optoelectronic detector elements disposed downstream of the second graduation structure. Because of the functional effects of the first and second graduation structures, as they have been explained in the above example, the first graduation structure will hereinafter be called the projection graduation; conversely, the second graduation structure will be called the detection graduation.
For generating a plurality of phase-shifted scanning signals, it is often provided in such position measuring systems that the graduation periods of the detection graduation and of the projection graduation are selected as slightly different from one another. In this case, a fringe pattern whose fringes are oriented parallel to the graduation lines of the detection graduation emerges from the detection graduation. The graduation period of the emerging fringe pattern furthermore has a markedly greater graduation period than the fringe pattern on the detection graduation. For the emerging fringe patterns generated in this way, the term used below will be a so-called Vernier fringe pattern.
Alternatively, it is also possible to rotate the projection graduation and detection graduation slightly relative to one another. Then the graduation lines are not oriented parallel to one another as in the previous cases but instead have a defined small angle from one another. The result then is again an emerging fringe pattern with a markedly greater graduation period, but whose fringes are oriented perpendicular to the fringes of the detection graduation. The term used in this case is so-called moire fringe patterns.
In both cases, that is, in the creation of both Vernier fringe patterns and moiré fringe patterns, the actual scanning of whichever fringe pattern results is done with the aid of a further graduation structure. This graduation structure will be called a Vernier graduation for the sake of simplicity below, but it must be understood that this term does not preclude the case where a moiré fringe pattern is generated. The Vernier graduation must in principle have the same graduation period and graduation orientation as the Vernier or moiré fringe pattern generated. Only the light transmitted through the Vernier graduation then finally strikes one or more detector elements.
With regard to the Vernier graduation, it should be noted that it is also already known to embody the Vernier graduation together with a plurality of detector elements as an integral component, which is used for scanning the resultant Vernier or moiré fringe pattern and for generating the phase-shifted scanning signals.
While the position measuring systems discussed thus far each have included a optical collimator element, systems have also become known that work without such a optical collimator element. In this respect, see for instance the publication by R. Pettigrew, entitled “Analysis of Grating Imaging and its Application to Displacement Metrology” in SPIE, vol. 36, First European Congress on Optics Applied to Metrology (1977), pages 325-332. In such position measuring systems, a further graduation structure, hereinafter called a transmitting graduation, is additionally disposed between the light source and the projection graduation. From each transparent gap or subregion of the transmitting graduation, a beam emerges that with the aid of the projection graduation generates a periodic fringe pattern on the detection graduation. The graduation period of the transmitting graduation is selected here such that the fringe patterns emerging from the various gaps are superimposed on one another to an increased extent on the detection graduation. In this way, even light sources, such as LEDs, that are spatially far apart from one another can be used in these position measuring systems.
In summary, the above-discussed position measuring systems accordingly include at least one projection graduation and one detection graduation. Optionally, a transmitting graduation and/or a Vernier graduation can also be provided, each of which take on the functions discussed above. As a scale or measuring graduation, either the transmitting graduation, the projection graduation or the detection graduation can serve in principle in these position measuring systems.
Both the transmitting graduation and the Vernier graduation are as a rule embodied as amplitude graduation structures; conversely, the projection graduations or detection graduations can also be embodied as phase graduation structures.
An important variable in position measuring systems constructed in this way is in principle their behavior upon a change in the scanning gap. This spacing is as a rule defined by the spacing between the graduations that are movable relative to one another. Any mechanical inadequacies that may occur can then lead to more or less major fluctuations in the scanning gap during measurement operation. However, the least possible influence of such fluctuations in the scanning gap on the position-dependent output signals is to be desired.
In the aforementioned position measuring systems, there is a priori a relatively great dependency of the signal quality, detected by the detector, on the applicable scanning gap. In
FIG. 9
, to illustrate these problems, the resultant grating pattern contrast in a two-grating transducer is plotted in the detector plane with regard to the scanning gap. A projection graduation is used here that is embodied as a phase graduation structure, with a phase depth &phgr;=&pgr; and a line-to-gap ratio of 1:1. It is quite clear from
FIG. 9
that there are major interruptions in contrast between the maximum contrast values, and in these interruptions only low-contrast scanning signals can be detected. This in turn means that in the event of a possible undesired fluctuating scanning gap, marked sacrifices in the signal modulation result. Because of this strong dependency of the signal quality on the applicable scanning gap, stringent demands are accordingly made of the mechanical components of the applicable overall system, for instance in terms of guidance precision, etc.
From U.S. Pat. No. 5,646,730, the entire contents of which are incorporated herein by reference, a position measuring system is know
Holzapfel Wolfgang
Huber Walter
Allen Stephone B.
Brinks Hofer Gilson & Lione
Dr. Johannas Heidenhain GmbH
Spears Eric J
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