Radiant energy – Photocells; circuits and apparatus – With circuit for evaluating a web – strand – strip – or sheet
Reexamination Certificate
2000-07-20
2001-06-12
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
With circuit for evaluating a web, strand, strip, or sheet
C356S400000
Reexamination Certificate
active
06246067
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to measurement systems, and more particularly to a system for taking displacement measurements of an object having a regular pattern preprinted thereon.
2. Discussion of the Related Art
There are a wide variety of known systems in which detecting and/or measuring the position or displacement of an object is important. For example, the system disclosed in U.S. Pat. No. 5,578,813, assigned to the assignee of the present invention and also co-invented by the inventor of the present invention, is a system and method for determining relative movement between a handheld scanner and a web of material (i.e., piece of paper). Specifically, that system utilized an illumination/imaging sensor to detect relative movement between the scanner and the web of material by identifying structure-related properties of the web of material. The inherent structure-related properties of the web (such as paper fibers, or other constituents) were used for navigational purposes, namely to identify the navigational path of the scanner, so that the image scanned could be reconstructed electronically.
Another system is disclosed in U.S. Pat. No. 5,291,131, which discloses an apparatus for measuring the elongation of a circulating chain (elongation resulting from component wear, stretching, or otherwise). The system disclosed therein uses two sensors (e.g., magnetic or optical) disposed a predetermined distance apart along the path of the circulating chain. Iic distance between the two indices is calculated on the basis of the calculated speed of the moving body and the time which elapses from when a first index passed a first sensor until a second index passed a second sensor. By continuing this observation over time, and comparing the calculated distances, chain elongation can be measured. Further, similar systems predating the '131 patent include those disclosed in U.S. Pat. Nos. 4,198,758 and 4,274,783, both entitled “Chain Measuring and Conveyor Control System”, co-invented by the inventor of the present invention.
As is also known in the prior art, the print head of some ink jet printers is configured to move across the print width of a sheet of paper. The deposition of ink from the print head to the sheet of paper is closely controlled based in part on positional information of the print head. In some such printers, this positional information is obtained by stretching a transparent sheet of material across the span covered by the width of the sheet of paper. This sheet of material is passed through a slot in the print head, and contains periodically-spaced demarcation lines. An optical emitter/detector pair is disposed across the slot, and is configured to count the demarcation lines. By maintaining an accurate count of the demarcation lines, the system can maintain information related to the position of the print head along the sheet of paper. In such a system, the sheet of material is held stationary, while the emitter/detector pair (which is fixed to the print head) moves in relation to the sheet of material.
In systems like those mentioned above, as well as many other systems, there is a desire to obtain accurate and precise displacement measurements of an object. Accordingly. it is desired to provide an apparatus and method that effectively measures object displacements.
SUMMARY OF THE INVENTION
Certain objects, advantages and novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the devices and combinations particularly pointed out in the appended claims.
To achieve the advantages and novel features, the present invention is generally directed to a system for taking displacement measurements of an object. In accordance with one aspect of the invention, the system includes an object having a visible pattern disposed thereon (the term “visible pattern” will be used throughout to represent any pattern sensed with a photosensor array, whether or not the illumination involved is in the visible spectrum or not). In this regard, the pattern is defined by areas of systematically alternating contrast (e.g., black and white or other colors). A plurality of photosensors is uniformly spaced apart from the visible pattern, and are further disposed in a configuration similar to the visible pattern. In this regard, if the visible pattern is a repeating circular pattern, then the photosensors are disposed in a repeating circular pattern. If the visible pattern is a plurality of parallel, linear demarcations, then the plurality of photosensors may be disposed in a linear array. A lens is disposed to image the visible pattern onto the plurality of photosensors. Preferably, the pitch, or spacing between adjacent photosensor elements, is slightly different from the spacing in the image of the contrasting areas defining the visible pattern. As will be discussed in further detail below, this allows the invention to utilize the Moiré effect to take precise displacement measurements of the object.
In accordance with this aspect of the invention, a circuit is electrically connected to the plurality of photosensors, wherein the circuit is configured to evaluate an electrical signal generated by the plurality of photosensors to determine the displacement of the object. In this regard, the electrical signal will embody a repeating envelope pattern resulting from the difference in the pitch of the photosensors and the pitch of the image of the visible pattern. This envelope has a spatial frequency that is significantly lower than the frequency of either the visible pattern in the image or the photosensor array, where the frequencies of the visible pattern in the image and the photosensor array are equal to the reciprocals of the distances separating adjacent pattern demarcations in the image or adjacent photosensor elements respectively. In this regard, lateral motions of the object bearing the visible pattern, made parallel to the direction of the repetition of the repeating patterns, produces a shift in the position of the lower spatial frequency signal envelope. Even slight displacements of the object bearing the visible pattern can be readily detected since they cause relatively large displacements of the envelope pattern which itself has a lower spatial frequency. Object motions that include rotation within the object plane of the imaging optics produce more complicated motions of the envelope signal but also allow increased precision in detection of these generalized motions.
In accordance with this preferred embodiment, the lens is a telecentric lens. A telecentric lens may include an aperture at a focal distance behind a first lens or lens group to prevent changes in an object's field position from causing magnification changes. A second lens may be placed behind the aperture at a distance equal to its focal length, for the purpose of similarly preventing changes in the focal position of a detector from also causing magnification changes. The individual lenses of the telecentric lens are aligned along a central axis.
Depending upon system application needs, another but alternative implementation doesn't require an imaging lens. In this form of implementation. an object surface may be placed in close proximity to the photosensor array with illumination provided through the object or from light sources interstitial to the array.
In accordance with another aspect of the invention, a system is provided for measuring the tilt of an object and doesn't require use of patterns that are necessarily systematic. In this aspect, a first photosensor array is disposed alongside a central axis and angled with respect to the central axis. Similarly, a second photosensor array is disposed alongside the central axis and, like the first photosensor array,
Agilent Technologie,s Inc.
Le Que T.
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