Position encoding optical device and method

Details Position encoding optical device and method Position encoding optical device and method

2000-09-18

2003-04-22

Lester, Evelyn A (Department: 2873)

Optics: measuring and testing

Shape or surface configuration

C356S602000, C356S614000, C356S623000

Reexamination Certificate

active

06552809

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the field of optical position detecting devices, and more particularly to such devices capable of encoding the position of a light spot generated by a light source, which find applications in 3D vision and object measurement (profilometry), object detection, pattern recognition and target tracking.
DESCRIPTION OF THE PRIOR ART
Position detectors including components whose relative position or movement is measured are well known. Such detectors have been abundantly described in the literature, as by N. A. Agarkova et al. in “The design of a digital electro-optical displacement sensor” Optical Technology, vol. 38, no. 9, 1971, pp. 532-534; by W. Scholz in “Determining tape position with optical markers” Magnettontechnik, Funkschau, Heft 1, 1976, pp. 42-44; by R. Ogden in “A high resolution optical shaft encoder” Journal of IERE, vol. 55, no. 4, 1985, pp. 133138; by U. Griebel et al. in “A new method to determine accuracy and repeatability of robots” Proceedings of the IASTED, 21-26 June 1985, Lugano, Switzerland; by T. Bohme in “A digital potentiometer for position indication using a microcomputer, Elektroniker, Nr. 8, 1987, pp. 86-88; by D. Varshneya et al. in “Applications of time and wavelength division multiplexing to digital optical code plates” SPIE, vol. 838, 1987, pp. 210-213; by P. Auvert et al. in “Monolithic optical position encoder with on-chip photodiodes” IEEE Journal of Solid-State Circuits, vol. 23, no. 2, 1988, pp. 465-473; and by A. Kwa et al. in “Optical angular displacement sensor with high resolution integrated in silicon” Sensors and Actuators A, vol. 32, 1992, pp. 591-597. Examples of such moving part-based position detectors are also disclosed in patent documents, namely in U.S. Pat. No. 3,500,055 issued on Mar. 10, 1970 to Russell et al.; in U.S. Pat. No. 3,702,471 issued on Nov. 7, 1972 to Kennedy et al.; in U.S. Pat. No. 30 4,180,704 issued on Dec. 25, 1979 to Pettit; in U.S. Pat. No. 4,388,613 issued on Jun. 14, 1983 to Rush et al.; in U.S. Pat. No. 4,405,238 issued on Sep. 20, 1983 to Grobman et al.; in U.S. Pat. No. 4,971,442 issued on Nov. 20, 1999 to Okutani et al.; in U.S. Pat. No. 4,948,968 issued on Aug. 14, 1990 to Matsui; in U.S. Pat. No. 5,497,226 issued on Mar. 5, 1996 to Sullivan; in U.S. Pat. No. 6,080,990 issued to Watanabe et al. on Jun. 27, 2000; in Deutche Democratic Republic Patent Specification no. 283001, 1985, naming Rossler et al. as co-inventors; and in European Patent Specification published under no. 490206 on Jun. 17, 1992, naming Durana et al. as co-inventors.
In many fields there is a need for finding the position of a light spot or peak of a relatively small size, wherein known position detectors involving lo relative movement between detector components cannot be used. Some applications can be found in artificial vision where a light beam is scanned over a surface or a volume and the position of the spot is indicative of either the position or the thickness of an object. In pattern recognition, applications can be found in optical processing (e.g. optical correlator) where the optical device transposes the presence of an object into a sharp light peak. In other applications such as in the fields of object detecting and target tracking, a light source or the illuminated part of an object is imaged as a moving small spot whose position must be rapidly detected.
Existing technologies for light spot position detection generally use three different approaches.
According to a first one, a scene containing the luminous spot or peak is acquired with a video camera. The image is then processed by a computer to detect the maximum intensity value to find the corresponding position. However technologies using this approach are generally characterized by limitations related to processing speed, system complexity and cost. Speed limitations are due to the acquisition process with the video camera and to the data processing performed by the computer. For conventional existing cameras, the acquisition process typically takes {fraction (1/30)} sec. Although high-speed cameras with image acquisition frequency around a few kHz are available, they may not be suitable for high rate scanning or fast moving spot applications, such as object tracking. Furthermore, even using a high-performance, high-speed camera, the processing time necessary to detect the maximum intensity value from raw image signals to find the corresponding position of the light spot may still significantly limit detection performance of the system. Such system requiring a high performance camera with a computer running particular analysis software or equivalent high level processing instrumentation, it may be complex to program, calibrate and/or operate, as well as expensive. Such a video position sensor is proposed by E. Lanz in “Electro-optic sensor for position detection and object identification” Automation Technology with Microprocessors, Interkawa congress 1977, pp. 95-106, which sensor is based on electronic sequential processing of a two-dimensional video signal.
Another way to proceed is to use position-sensitive electronic devices. A photodiode-based position measuring system is taught by H. Janocha in “Universally usable position measuring system with analog displaying position sensitive photodiodes” Technisches Messeen tm, Heft 11, 1979, pp. 415-420. Such system combines photodiodes that are sensitive to the two-dimensional position of a light source, with an electronic processing circuit generating a position indicative analog signal. Such system is disadvantageous because additional encoding is required to further process the position signal with a digital computer, therefore increasing processing time. A one-dimensional position detector requiring signal pre-processing to generate a digital output is also described by Smith et al. in “An integrated linear position sensitive detector with digital output” Transducers 1991, Digest of Technical Papers, 24-27 June 1991, San Francisco, pp. 719-722. A coded aperture light detector for use with a three-dimensional camera is disclosed in U.S. Pat. No. 4,830,485 issued on May 16, 1989 to Penney et al., which detector provides a direct digital representation of a range or height position of a reflecting surface of an object. A light spot reflected from the surface is optically spread into a line segment so it can be shared among a number of light detection channels coupled through a segmented fiber optic bundle to a corresponding number of photo-multipliers or solid state detectors. Although not requiring pre-processing, the proposed detector is significantly limited in its resolution due to the mechanical coupling required between each fiber optic of the bundle and each corresponding channel of the coded aperture. Furthermore, several rows of channels being required on the coded aperture to generate a multi digit signal, such detector would be hardly practicable for bi-dimensional spot positioning. Another position-sensitive electronic device is disclosed by Yamamoto et al. in “New Structure of Two-dimensional Position Sensitive Semiconductor Detector and Application” IEEE Trans. Nucl. Sci., NS-32, 1985, pp 438-442. The voltage output of such semiconductor device depends on the position of the centroid of the illumination pattern projected on it. This device has the potential to be very fast (around 100 kHz) and is less complex than the camera/processing computer system. However, in computing the mass center of the peak, this device is more sensitive to noise coming either from background of from other sources showing lower intensity. Moreover, resolution and speed are affected by the intensity of the light peak.
The third detection scheme is based on the use of diffractive devices such as diffraction gratings. One-dimensional and two-dimensional light spot position detecting devices are disclosed in U.S. Pat. No. 4,025,197 to Thompson. The one dimensional detecting device disclosed uses a first linear grating disposed before the focal point of an incident laser

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