Radiant energy – Photocells; circuits and apparatus – With circuit for evaluating a web – strand – strip – or sheet
Reexamination Certificate
2001-12-10
2003-11-18
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
With circuit for evaluating a web, strand, strip, or sheet
C250S559290
Reexamination Certificate
active
06649925
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods and apparatus for precise position determination and in particular to a combination of electro-optical methods and apparatus and advanced data processing methods and means for the very precise determination of absolute position and incremental displacement of objects.
BACKGROUND OF THE INVENTION
Prior art electro-optical apparatus for the determination of the position of a rotational objects is shown in
FIG. 1. A
light-opaque mask
12
having light-transparent apertures
20
is rigidly attached to and moves with an object
22
, whose angular position is to be determined. Light rays emitted from a light source
10
, impinge on the mask
12
and a light flux passing through the apertures in the plate is detected by a light detecting device
14
, which is aligned with the mask and the source to measure the light flux.
As different apertures
20
, in the mask
12
pass in front of detector
14
, an output signal is obtained from detector
14
which determines the position of object
22
with the aid of a position reporting device
18
.
The problems associated with this type of apparatus and this method for position determination relate principally to limitations in position determination accuracy. Best positional accuracy obtained with this method and similar prior art methods is of the order of 1 micrometer.
The reasons for this accuracy limitation can be traced to the fact that light passing through an aperture and detected by a light sensor is registered either as a zero or as a one, i.e., there are no fractional measurements; thus, the positional accuracy is determined only by detection and identification of a particular slit through which light passes. Thus, in order to determine very small distance displacements the aperture width and the aperture intervals must be made impossibly small. Physical size, therefore limits the fine range measurements.
Other systems are capable of finding a position of an edge to greater accuracy than the size of the detector by utilizing digitized values at a number of positions as a single detector moves past the edge. The values are used to find the position of the edge. However, such methods are capable of less than an order of magnitude of improvement of resolution and furthermore, they do not measure the position at a point without making measurements at adjacent points. Thus the position of the edge is known only after the detector has moved away from the edge and is therefor not known in real-time, i.e., while the measurement is being made. Such determination of edges is used, for example, in the inspection of reticules for microcircuit production.
The apparatus of
FIG. 1
is also sensitive to vibrations which can lead to large errors in position determination. Such prior art apparatus and similar apparatus (as described for example in Japanese Patent Publication No. He-1-22884) is, thus, not suitable for very precise, high resolution position determination.
For more precise positional accuracy determination, of better than 1 micrometer, most prior art devices rely on optical interferometry, which suffers from high sensitivity to vibration. Furthermore, such interferometers are high cost, very delicate apparatus. Examples of such apparatus are described in the General Catalog of Dr. Johannes Heidenhain, GmbH of Traunreut Germany dated June 1996, in particular on pages 6 and 7.
SUMMARY OF THE INVENTION
The object of this invention is to provide techniques, measurement and calibration methods to determine an object's position and displacement with high resolution and accuracy, better than 0.01 micrometers. This accuracy is at least one order of magnitude better than that quoted for the most accurate linear encoder in the above mentioned catalog and two orders of magnitude better than almost all of the listed encoders. Various preferred embodiments of the invention perform such determination for various object positional displacement conditions and geometries. It is noted that the accuracy of the method and apparatus of the present invention is greater (better) than the diffraction limit of the light to which the detectors are sensitive.
In the realization of the above mentioned object of the invention a linear array of light detectors is used to detect the light received from a striped mask which is attached to an object whose position is to be precisely determined. The mask provides a pattern of bright and dark light strips over the linear detector array surface, when it is suitably illuminated.
The method employed in this invention for positional determination is based on determination of the positions of the boundary lines between a plurality, generally a large number of, bright and dark parallel strips of light obtained from the mask. This positional line information is obtained from the detected light level of each of the light detectors in an array (such as a linear or matrix type CCD array). The output of each individual detector is converted into one of many possible digital values, i.e., a gray scale of values is determined. The line position, and thus the object position, are determined from the values provided by the plurality of detectors.
By averaging or otherwise processing the positional data of the light detectors, high accuracy in positional determination is obtained. Preferably, hundreds or thousands of detectors are used, resulting in improved accuracy.
In a preferred embodiment of the invention, apparatus is provided in which a pattern of alternating bright and dark strips illuminate an array of light detectors. In one preferred embodiment the source of the mask strips is a plurality of alternating light transparent and light opaque strips. In another, embodiment a mask utilizes a plurality of alternating light reflecting (white) and light absorbing strips (black). In some preferred embodiments of the invention the strips are oriented at an angle to the direction of position measurement and provide a pattern of alternating bright and dark angled strips across an array of detectors.
The strips are preferably at an angle to the direction of motion (and to the repeat direction of the cell array) which angle is preferably chosen to optimize the number of linear detecting cells. Linear detecting cells are cells in which the boundary lines between bright and dark strips cross both sides of the detector cell which are generally perpendicular to the direction of motion. In this case the output signal from the cell is linearly related to the fraction of the cell area covered by the bright strip. In most embodiments of the invention, only “linear” cells are used in the determination of the position. Generally, the cells are rectangular and the cells are in the linear region and greater accuracy is achieved when the long sides of the detector are perpendicular to the direction of motion.
The detected output signal of each linear cell, is preferably digitized into one of 256 levels which are proportional to the fraction of the cell area illuminated by the bright strip and which enables determination of the bright-dark interface lines position.
Each cell's fractional illumination information is fed to a signal processor which determines the position of the object. The positional information provided by each “linear cell” and by all the linear cells together is used to provide an average position calculation which is more accurate than the positional determination provided by a single cell (typically by at least a factor of 10 and more typically by a factor of 100).
Another aspect of the invention includes the calibration of the sensitivity of each detector cell by moving the subject on which the mask is mounted, so as to allow the area of each cell to be completely illuminated by a bright strip and alternately, to be completely in the shadow of a dark strip, Normalization factors for each cell can be accurately determined.
This normalization is part of a broader aspect of the invention which includes the provision of methods and apparatus for the Auto-Calib
Fenster & Company
Le Que T.
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