Optics: measuring and testing – By alignment in lateral direction
Reexamination Certificate
1998-08-07
2001-03-06
Kim, Robert (Department: 2877)
Optics: measuring and testing
By alignment in lateral direction
C356S400000, C250S23700G
Reexamination Certificate
active
06198534
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a scanning unit for a position measuring system which, besides periodic incremental signals, also generates at least one reference pulse signal at a defined reference position of a scale and of a scanning unit which can be moved in relation to the latter, wherein the scanning unit contains at least one reference mark scanning field and/or at least one reference pulse detector element for scanning a reference mark. The present invention also relates to an optical measuring system which, besides incremental signals, also provides at least one reference pulse signal at a defined reference position of a scale and a scanning unit which is movable relative to it, wherein a reference mark is arranged in at least one incremental graduation of the scale.
BACKGROUND OF THE INVENTION
As a rule, known optical position measuring systems provide, in addition to the incremental signals with respect to the relative displacement of two elements, so-called reference pulse signals. At a defined relative position it is possible by these signals to establish an exact absolute relation of the elements which can be moved with respect to each other. To generate the reference pulse signals, fields with reference marks are arranged at one or several places on the scale. Among other elements, the scanning unit of the optical position measuring system includes at least one reference mark scanning field, by which an appropriate reference pulse signal can be generated at the relative position of the scale and the scanning unit to be detected.
There are various options regarding the arrangement of the reference marks on the scale, or respectively the associated scanning fields on the scanning unit.
For example, it is known from U.S. Pat. No. 4,263,506 to arrange the reference marks laterally on the scale next to the graduation track with the incremental graduation. However, a problem here is that, if the scale and the scanning unit are twisted on an axis vertically with respect to the plane of the scale or scanning, the exact association of the reference pulse signal to a defined period of the incremental signal is possibly no longer assured.
Besides this it is also possible to integrate the reference marks directly into the graduation track with the incremental graduation, such as is suggested in U.S. Pat. No. 3,985,448. To this end it is possible, for example, to omit one or several strips or lines of the incremental graduation at the desired position of the scale in the incremental graduation. Further variations regarding the integration of reference marks into the track with the incremental graduation are known from German Patent Publication DE 35 36 466 A1, or from U.S. Pat. No. 4,866,269. It is proposed in these references to design the reference marks in the incremental graduation as a periodic sequences of lines or strips, or to utilize areas with optical properties, which differ from the remainder of the incremental graduation, as reference marks.
However, it has been shown to be problematic in connection with the integration of reference marks into the incremental graduation that next to the actual reference position it is customary to superimpose a periodic signal resulting from the incremental graduation of the scale on the detected reference pulse signal. An exemplary representation of the signal progression in this area is shown in FIG.
8
. As a result, the modulation of the reference pulse signal is greatly reduced, i.e., assured detection of the reference pulse signal at the reference position is correspondingly difficult.
Of course it should also be pointed out as a further requirement in connection with the integration of a reference mark into the incremental graduation, that the smallest possible interference with the incremental signals by the reference pulse signal should result.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to produce a scanning unit for an optical position measuring system, wherein the above mentioned problems in connection with the detection of a reference pulse signal are minimized.
Because of the optical filtering effect provided in accordance with the invention, it is now assured that the reference pulse signal can be definitely detected at the respective reference position. The periodically modulated incremental signal portion now no longer exists at the reference pulse signal next to the reference position, instead a signal portion with a very constant, or respectively steady amplitude exists. In this case at least the fundamental wave of the incremental signal portion of the reference pulse signal is now filtered. A more dependable, or respectively simpler detection of the reference pulse signal is possible at the reference position.
A number of options exist regarding the required optical filter effect. For example, it is possible to employ a suitably designed screen, or respectively amplitude structures in connection with the reference mark scanning field. However, it is also possible to achieve this filtering effect with the aid of phase structures in the transparent areas of the reference mark scanning field. Furthermore there are of course combined possibilities between these different filtering arrangements.
Further advantages of the scanning unit in accordance with the invention result if the phase structures mentioned are used for filtering in connection with an advantageous embodiment of the scanning unit in accordance with the invention, and if these areas furthermore have a focusing optical effect. In particular, the result is a reduced sensitivity to tilting of the scanning unit around the line direction of the scanned graduation structures. Thus, in case of slight tilting of the scale in the line or strip direction, a very much smaller portion of the light beams passing through the scanning plate during the second passage is cut off because of the embodiment selected than with conventional embodiments of the scanning unit. The association of the reference pulse signals with the corresponding incremental signal period remains assured because of the closeness of the neutral rotating point to the scale, more so than is the case with conventional scanning arrangements.
It is furthermore possible to achieve a higher signal intensity on the detector side with this embodiment of the scanning unit, or respectively of the reference mark scanning field, if focusing optical elements are employed.
A number of embodiments result in connection with the design of the optical filter structure, or respectively with the selection of suitable focusing optical elements in the reference mark scanning field, which can be selected depending on the intended use.
As an alternative to filtering a suitably designed reference mark scanning fields, filtering in accordance with the invention can also take place by suitable embodiment, or respectively structuring, of one or several detector elements.
Further advantages as well as details of the scanning unit designed in accordance with the invention ensue from the following description of the preferred embodiments with the aid of the attached drawings.
REFERENCES:
patent: 3985448 (1976-10-01), Wiklund et al.
patent: 4263506 (1981-04-01), Epstein
patent: 4866268 (1989-09-01), Tang et al.
patent: 5739911 (1998-04-01), Holzapfel et al.
patent: 5777322 (1998-07-01), Holzapfel et al.
patent: 5994692 (1999-11-01), Holzapfel
patent: 35 36 466 A1 (1987-04-01), None
patent: 195 30 560 A1 (1997-02-01), None
patent: 0 714 015 A2 (1996-05-01), None
patent: 0 735 346 A2 (1996-10-01), None
patent: 0 754 933 A2 (1997-01-01), None
Hofer Volker
Holzapfel Wolfgang
Huber Walter
Mayer Elmar
Brinks Hofer Gilson & Lione
Johannes Heidenhain GmbH
Kim Robert
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