Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1998-12-02
2001-02-13
Huber, Paul W. (Department: 2753)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S053130
Reexamination Certificate
active
06188652
ABSTRACT:
The present invention relates to a method and to a device for automatically correcting positioning errors of optical elements of an optical system.
In the scope of the present invention, an optical system consists of a plurality of optical elements, for example lenses, mirrors, diaphragms and optical detectors, which are centered with respect to the optical axis of said system and are intended to be subjected to a light beam.
Although not exclusively, the present invention can be applied particularly well to a system of the laser type.
In order for such an optical system, in particular a laser system, to be capable of operating normally and effectively, it is necessary for said optical elements to be positioned properly with respect to the optical axis of said system.
It is consequently necessary to correct any positioning error existing at the start of the operations or appearing during said operations.
In known fashion, in order to make such a correction a plurality of sequential processes are performed, each of which consists in measuring the position of an image and in intervening on one of the optical elements to recenter this image. The same process is carried out for a plurality of images, one image after another, each time by choosing a suitable optical element.
It is generally found that intervention on any one optical element of the system leads to effects on a plurality of images lying downstream of this optical element. This is due, in particular, to the fact that a light beam is defined with respect to two plans, namely a focal plane and a pupil plane. These two plans are imaged by the various optical elements all along the system. Intervention on one optical element thus simultaneously displaces these two images plans.
It is consequently necessary, in order to achieve full and satisfactory adjustment of the optical system, to repeat the same sequences of checking and correcting position a plurality of times. This known correction method is therefore iterative, and the time which it takes increases rapidly as a function of the desired precision.
The object of the present invention is to overcome these drawbacks. It relates to a method for automatically correcting, in a single operation, positioning errors of optical elements of an optical system, with respect to the optical axis of said system, said method being one which does not need iterative interventions and the desired precision being obtained after the first application.
To this end, according to the invention, said method is noteworthy in that:
m pairs of measurements are taken in measurement planes distributed over said system, m being an integer greater than or equal to 2, each of said m measurement pairs being representative of an image shift with respect to said optical axis in the corresponding measurement plane;
a matrix relationship:
|M|=|A|.|C|
is defined, in which
. |M| is a column vector comprising said m pairs of measurements;
. |C| is a column vector comprising 2n values which are respectively representative of the positioning errors of n optical elements, n being an integer less than or equal to m; and . |A| is a transformation matrix;
the transformation matrix |A| is determined, which matrix may be assumed to be constant;
the 2n values of the column vector |C| are calculated on the basis of the matrix expression:
|C|=|A|
−1
.|M|,
|A|
−1
being the inverse or “pseudo-inverse” matrix of |A|; and
the positioning of said n optical elements is modified respectively in accordance with the 2n values previously calculated, so as to correct the positioning errors.
Thus, by virtue of the invention, after the first application of said method, said n optical elements (n not necessarily being the total number of optical elements in the optical system, as will be seen below) are positioned so that the correction is made. No iterative intervention is consequently necessary.
Further, since it is possible to use a working light beam of the optical system during the measurements, it is possible to make the correction at the same time as the work done by the optical system is being carried out (measurements, calculations, corrections, etc.). In this case, the correction does not cause any time to be lost.
Of course, in the scope of the present invention, said images may also be formed with the aid of an auxiliary light beam which is specifically intended for implementing the method according to the invention.
Moreover, in the preferred embodiment, the transformation matrix |A| is determined through calculation, by simulating the path of a light beam in said optical system with the aid of geometrical optics. As a variant, this matrix may also be determined experimentally on the system itself.
Further, the matrix |A|
−1
is preferably calculated on the basis of the transformation matrix |A|, with the aid of an inversion method employing decomposition into singular values. This method also has the advantage of determining the minimum number of optical elements needed for making the corrections. This number is generally less than the total number of optical elements in the system, which makes it possible to reduce and minimize the number of corrections to be made.
According to the invention, a measurement plane may be:
a pupil plane of an optical element; or
a focal plane of an optical element; or
an abitrary plan lying in the system.
Moreover, a relationship between the measurement errors and the correction errors is advantageously determined on the basis of the expression:
|&egr;
2
|=|A|
−1
.|&sgr;
2
|.(|
A|
−1
)
5
in which:
|&egr;
2
| is a matrix made up of the variance of the positioning errors of at least one optical element;
|&sgr;
2
| is a matrix made up of the variance of the corresponding measurement errors; and
(|A|
−1
)
5
is the transpose of the matrix |A|
−1
.
This relationship makes it possible for the characteristics and the properties of the measurement means and of the correction (or positioning) means to be optimally matched.
The present invention also relates to a device of the aforementioned type for automatically correcting positioning errors.
According to the invention, said device is noteworthy in that it has:
first means for taking 2m measurements in measurement planes of said system, m being an integer greater than or equal to 2, each of said 2m measurements being representative of an image shift with respect to said optical axis in the corresponding measurement plane, and namely in a first direction of said measurement plane for one half of said measurements and in a second direction for the other half;
a calculation unit for determining, on the basis of said 2m measurements, 2n values which are respectively representative of positioning errors, both in said first and in said second directions, of n optical elements, n being an integer less than or equal to m, said calculation unit using for this purpose the matrix expression |M|=|A|.|C| with |M| and |C| being column vectors respectively comprising the 2m measurements and the 2n values and |A| being a transformation matrix; and
second means for automatically modifying the positioning of said n optical elements, respectively in accordance with the 2n values calculated by the calculation unit, so as to correct the positioning errors.
Further, advantageously:
at least some of said first means comprise image detectors and associated automatic image-processing means; and/or
at least some of said second means comprise electromechanical positioning components.
REFERENCES:
patent: 4627725 (1986-12-01), Nishio et al.
patent: 5282016 (1994-01-01), Shen et al.
pate
Compagnie Industrielle des Lasers Cilas
Huber Paul W.
Marshall O'Toole Gerstein Murray & Borun
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