Optical device for measuring position

Details Optical device for measuring position Optical device for measuring position

2002-01-25

2004-09-21

Fulton, Christopher W. (Department: 2859)

Radiant energy

Photocells; circuits and apparatus

Optical or pre-photocell system

C250S231160, C341S013000

Reexamination Certificate

active

06794637

ABSTRACT:

Applicants claim, under 35 U.S.C. §120, the benefit of priority of the filing date of Apr. 15, 2000 of a Patent Cooperation Treaty patent application, copy attached, Serial Number PCT/EP00/03441, filed on the aforementioned date, the entire contents of which are incorporated herein by reference.
Applicants claim, under 35 U.S.C. §119, the benefit of priority of the filing date of Apr. 22, 1999 of a German patent application, copy attached, Serial Number 199 18 101.2, filed on the aforementioned date, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical position measuring device, in particular a so-called four-grating measuring system.
2. Description of the Related Art
As a rule, such position measuring devices include a light source, a transmitting graduation arranged in front thereof, a measuring graduation, a scanning graduation, as well as a detector arrangement with a periodic structure of individual detector elements, which constitutes a fourth graduation. Here, the light source, the transmitting graduation, the scanning graduation, as well as the detector arrangement are as a rule arranged in a scanning unit, which is movable in a defined measuring direction with respect to the measuring graduation. It is possible here to lay out such systems in the form of transmitted light systems, as well as incident light systems; it is also possible to create rotatory arrangements as well as linear arrangements. A corresponding measuring device is disclosed in WO 97/16704, for example. WO 97/16704 corresponds to U.S. Pat. No. 5,886,352, the entire contents of each of which are incorporated herein by reference. This document does not contain any further information regarding the dimensioning of the detector device.
Different variations of multi-grating transducers are also known from a publication by R. M. Pettigrew under the title “Analysis of Grating Imaging and its Application to Displacement Metrology” in SPIE, vol. 136, 1st European Congress on Optics Applied to Metrology (1977), pp. 325 to 332. The imaging conditions in connection with multi-grating transducers, wherein a divergent illumination is provided, are also discussed on page 328 of this publication. In this connection, divergent illumination should be understood to be one where no optical collimator device is arranged downstream of the light source used, i.e. the light beams emitted by the light source do not impinge exactly parallel on the first graduation in the beam path. Here, the discussion of the imaging conditions leads to the result that the enlargement factor in the beam path is determined by an arrangement, wherein the enlargement factor is determined from a centered elongation, starting in the measurement graduation plane. The enlargement factor is an important parameter in a system with divergent illumination, in that the optimal geometric dimensioning of the detector arrangement, or of the detector elements, is a deciding function of knowing the enlargement factor in the beam path.
However, in actual applications it has been shown that a beam path model as proposed in the mentioned publication leads to a four-grating system which has a poor degree of modulation of the generated scanning signals.
SUMMARY AND OBJECTS OF THE INVENTION
It is therefore an object of the present invention to disclose an optical position measuring device on the basis of a four-grating measuring system, wherein the problems discussed above are reduced as much as possible. In particular, a design of the detector arrangement in such a position measuring device, which is optimized in regard to the respective system prerequisites, is to be disclosed, which assures a sufficient degree of modulation of the position-dependent scanning signals.
This object is attained by an optical position measuring device that includes a measuring graduation and a scanning unit, which moves relative to the measuring graduation along a measuring direction. The scanning unit includes a light source, a transmitting graduation arranged distant from the light source at a distance D
LQ
and a scanning graduation with a graduation period P
AT
, on which a periodic strip pattern with a strip pattern period P
SM
results in case of a relative movement between the scanning unit and the measuring graduation. A detector arrangement arranged at a distance D
DET
from the scanning graduation with a plurality of blocks of individual detector elements, wherein the plurality of blocks are arranged periodically with a detector period P
DET
in the measuring direction. The transmitting graduation is at the distance u from the measuring graduation, the scanning graduation is at a distance v from the measuring graduation, and the detector period P
DET
has been selected in accordance with the equation P
DET
=m*1*P
v
, wherein m=(1+D
DET
/(v+v+D
LQ
)) and 1=1, 2, 3, . . . and 1/P
v
=|1/P
SM
=1/P
AT
|.
The measures in accordance with the present invention now assure that it is possible in connection with any arbitrary system configurations of an optical position measuring device based on the four-grating principle with divergent illumination to disclose an optimized design of the detector arrangement. A good degree of modulation of the position-dependent scanning signals in particular is assured by this.
Furthermore, the exact knowledge of the above mentioned enlargement factor also permits the dimensioning of the position measurement device in accordance with the present invention, which assures a defined tolerance regarding the scanning distance. Accordingly, a deviation of the actual scanning distance from an ideal scanning distance is also possible without the signal quality being decisively negatively affected.
As a further advantage of the four-grating system in accordance with the present invention is should be mentioned, that the detector arrangement here is not arranged directly adjoining the measuring graduation, which is movable with respect to the former. Instead, the scanning graduation, which protects the detector arrangement from possible damage by the measuring graduation, is arranged within the scanning unit in front of the detector arrangement.
The position measuring device in accordance with the present invention can of course be realized as an incident light, as well as a transmitted light system. In the same way it is possible to design linear, as well as rotatary arrangements in accordance with the invention.
Further advantages, as well as details, of the optical position measuring device in accordance with the present invention ensue from the following description of several exemplary embodiments by means of the attached drawings.


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Pettigrew, R.M., “Anaylsis of Grating Imaging and Its Application to Displacement Metrology,” SPIE vol. 136, 1stEuropean Congress on Optics Applied to Metrology, 1977, pp. 325-333.
Pending Patent Application Assigned to Dr. Johannes Heidenhain GmbH: Ser. No.: 09/496,984, Filing Date: Feb. 02, 2000, Inventor: Hermann et al.

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