Optical waveguides – Optical waveguide sensor
Reexamination Certificate
2001-11-14
2003-02-25
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
Optical waveguide sensor
C385S010000, C385S033000, C385S036000, C356S499000, C356S618000, C250S23700G, C250S231160
Reexamination Certificate
active
06526190
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a position measuring system, having a graduation support with a measuring graduation which can be scanned by light, having a planar waveguide, by which light generated by a light source is conducted to the measuring graduation, wherein the light is reflected at the surfaces of the planar waveguide, and having a scanning unit for scanning the measuring graduation, which has at least one light- sensitive scanning area to which the light modulated by the measuring graduation is conducted.
2. Discussion of Related Art
An angle measuring device is known from EP 0 589 477 A2, which includes a light source, a disk, which is provided with a measuring graduation and can be connected, fixed against relative rotation, to a shaft to be measured, as well as a photodetector. For realizing the angle measurement, the light coupled out by the light source is initially reflected at a first location on the measuring graduation. Subsequently the reflected light is conducted by a waveguide to another location on the measuring graduation, where it is again reflected and from where it reaches the photodetector.
A photoelectric position measuring device for measuring the relative position of two objects is known from EP 0 426 882 B1, wherein a graduation support with a measuring graduation made of transparent strips and light-reflecting strips is scanned by a scanning unit, which has an illumination unit, a scanning plate with at least one scanning graduation, as well as photodetectors. In this case, the bundle of light rays emanating from the illuminating unit is split into a partial bundle of rays passing through the transparent strips for acting on a first photodetector, and into a partial bundle of rays reflected by the light-reflecting strips for acting on a second photodetector, and the position measuring device is designed in such a way that the partial bundle of rays passing through the measuring graduation is modulated by the scanning graduation, and the partial bundle of rays reflected by the measuring graduation is not modulated by the scanning graduation. A plan-parallel waveguide, which includes the graduation support of the measuring graduation, is provided for conducting the bundle of light beams emanating from the illumination unit to the measuring graduation.
OBJECTS AND SUMMARY OF THE INVENTION
An object of the present invention is based on providing a position measuring system of the type mentioned at the outset, which is distinguished by an improved delivery of the light to the measuring graduation.
In accordance with the present invention, this object is attained by providing a position measuring system that includes a graduation support having a measuring graduation, a planar waveguide and a light source that generates light that is conducted to the measuring graduation, wherein the light is reflected at surfaces of the planar waveguide. A scanning unit for scanning the measuring graduation that includes a light-sensitive scanning area to which light modulated by the measuring graduation is conducted. The modulated light conducted to the light-sensitive scanning area has at least two different light portions, which differ in a number of reflections at the surfaces of the planar waveguide, and wherein the reflecting of the at least two different light portions occurs prior to reaching the measuring graduation.
In accordance therewith the light conducted to the scanning area has at least two different light components, wherein the light of the different components emanates from the waveguide at different locations and differs in the number of reflections which the light beams of the respective light component undergo prior to reaching the measuring graduation in the planar waveguide.
The design in accordance with the present invention of the position measuring system is particularly advantageous in connection with measuring graduations which can be scanned by an incident light method, i.e. which reflect the light conducted to them. If in such a measuring graduation the scanning unit is arranged at a short distance above the graduation support, the problem arises that the scanning unit makes the illumination of the measuring graduation more difficult. In this case it is made possible by the use of a planar waveguide to conduct the light specifically to the measuring graduation. A planar waveguide is understood to be a waveguide having two planar surfaces located opposite each other, at which the light is reflected while it is conducted along the waveguide.
The solution in accordance with the present invention has the advantage that, based on the different number of reflections which different light rays undergo prior to impinging on the measuring graduation, and thereafter on the same scanning area of the scanning unit, the surface section of the plane waveguide through which the light emanates from the waveguide (to finally reach the scanning area) can be considerably larger than that surface section, through which the light is coupled into the waveguide. Because of this, an area of a large surface of the measuring graduation, as well as a corresponding scanning area of the scanning unit can be completely illuminated by the light, even if this light is coupled into the planar waveguide through a comparatively smaller surface section.
The area of the measuring graduation, which is scanned, or illuminated, over a large surface with a different number of previous reflections in the waveguide, is here understood to be a uniform area which provides the same position information. Thus, this does not relate to the illumination of completely separate sections of a graduation support, which are arranged next to each other, such as for example an incremental track on the one side and a reference bar track on the other, but to the scanning of a uniform (incremental or absolute) measuring graduation.
Several analogous, sinusoidal scanning signals which are offset with respect to each other are generated in incremental position measuring systems, namely as a rule four scanning signals, which are each phase-shifted by 90° with respect to each other. These scanning signals are linked together for evaluation. In this regard it is important that they have equal offset portions. This is only assured if all scanning signals are obtained from a common uniform area of the measuring graduation. To compensate for partial contamination of the measuring graduation it is important here that not only one graduation bar of the measuring graduation is a partial component of the generation of each one of the scanning signals, but a plurality of graduation bars, so that they can be averaged. For this reason it is required to illuminate the largest possible scanning area of the measuring graduation which provides a uniform position information, and to derive all scanning signals from this coherent area (“single field scanning”).
The measuring graduation of absolute position measuring systems includes a single-track or multi-track coding with code elements, from which a multi-digit code word is formed. During scanning, the code elements required for forming the code word should be illuminated by a common light source, if possible. The illumination of the largest possible uniform area of the measuring graduation, or of the coding which provides the code word characteristic for a defined position in the measuring direction, is therefore also important here.
The coupling element for coupling the light into the waveguide, as well as the coupling element for coupling the light out from the waveguide, can be integrated in a simple manner into the surface of the waveguide, for example in that these coupling elements are stamped as a single-piece component of the waveguide into its surface. A Fresnel grating, or a prism, for example, are suitable for coupling in the light into the planar waveguide.
In accordance with a variation of the present invention, the light rays of the bundle of light rays coupled into the planar waveguide exte
Holzapfel Wolfgang
Reichhuber Siegfried
Brinks Hofer Gilson & Lione
Dr. Johannes Heidenhain GmbH
Palmer Phan T. H.
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