Optical translation measurement

Details Optical translation measurement Optical translation measurement

2000-09-07

2002-07-23

Buczinski, Stephen C. (Department: 3662)

Optics: measuring and testing

Velocity or velocity/height measuring

With light detector

C345S166000, C356S400000, C356S493000, C356S505000

Reexamination Certificate

active

06424407

ABSTRACT:

FIELD OF THE INVENTION
The present invention is related to the field of velocity and translation measurement and more particularly to methods and apparatus for the non-contact optical measurement of velocity and translation.
BACKGROUND OF THE INVENTION
Various optical methods for the measurement of the relative velocity and/or motion of an object with respect to a measurement system exist. The kinds of objects and the kinds of motions on which it operates characterize each method and apparatus.
The kind of measurable objects may be broadly divided into several groups, including:
A specially patterned object, for example, a scale.
A reflecting surface, for example, a mirror.
A small particle (or few particles), for example precursor particles or bubbles suspended in fluid.
An optically contrasting surface, for example, a line or dot pattern.
An optically diffuse object, for example, blank paper.
The kind of measurable motions may be broadly divided into several groups, including:
Axial movement toward or away from the measuring device.
Transverse (or tangential) motion, where the spacing between the measuring device and the object is essentially constant.
Rotational motion, where the object orientation with respect to the measurement device is changing.
It is also useful to classify the measurement devices according to the number of simultaneously obtainable measurement directions (one, two or three dimensional) and the number of critical components (light sources, light detectors, lenses, etc.).
It should be noted that a specific method may be related to more than one group in the above classification schemes.
A number of systems capable of non-contact measurement of the transverse velocity and/or motion of objects using optical means have been reported. These methods can include Speckle Velocimetry methods and Laser Doppler Velocimetry methods. Other methods of interest for understanding the present invention are Image Velocimetry methods, homodyne/heterodyne Doppler Velocimetry or Interferometry methods and Optical Coherence Tomography (OCT).
Speckle Velocimetry methods are generally based on the following operational principles:
A coherent light source illuminates the object the motion of which needs to be measured.
The illuminated object (generally an opaque surface) consists of multiple scattering elements, each with its own reflection coefficient and phase shift relative to the other scattering elements.
The individual reflection coefficients and phase shifts are substantially random. At a particular point in space, the electric field amplitude of the reflection from the object is the vector sum of the reflections from the illuminated scattering elements, with an additional phase component that depends on the distance between the point and each element.
The light intensity at a point will be high when contributions generally add in phase and low when they generally add out of phase (i.e., subtract).
On a planar surface (as opposed to a point), an image of random bright and dark areas is formed since the relative phase retardation of the source points depends on the location in the, plane. This image is called a “speckle image,” composed of bright and dark spots (distinct “speckles”).
The typical “speckle” size (the typical average or mean distance for a significant change in intensity) depends primarily on the light wavelength, on the distance between the object and the speckle image plane and on the size of the illuminated area.
If the object moves relative to the plane in which the speckle image is observed, the speckle image will move as well, at essentially the same transverse velocity. (The speckle image will also change since some scatterers leave the illuminated area and some enter it).
The speckle image is passed through a structure comprising a series of alternating clear and opaque or reflecting lines such that the speckle image is modulated. This structure is generally a pure transmission grating, and, ideally is placed close to the detector for maximum contrast.
The detector translates the intensity of the light that passes through the structure to an electrical signal, which is a function of the intensity (commonly a linear function).
When the object moves with respect to the measuring device, the speckle image is modulated by the structure such that the intensity of light that reach the detector is periodic. The period is proportional to the line spacing of the structure and inversely proportional to the relative velocity.
By proper signal analysis, the oscillation frequency can be found, indicating the relative velocity between the object and the measurement device.
For these methods high accuracy frequency determination requires a large detector while high contrast in the signal requires a small detector. A paper by Popov &, Veselov, entitled “Tangential Velocity Measurements of Diffuse Objects by Using Modulated Dynamic Speckle” (SPIE 0-8194-2264-9/96), gives a mathematical analysis of the accuracy of speckle velocimetry.
U.S. Pat. No. 3,432,237 to Flower, el. al. describes a speckle velocimetry measuring system in which either a transmission pattern or a pinhole is used to modulate the speckle image. When the pinhole is used, the signal represents the passage of individual speckles across the pinhole.
U.S. Pat. No. 3,737,233 to Blau et. al. utilizes two detectors in an attempt to solve the problem of directional ambiguity, which exists in many speckle velocimetric measurements. It describes a system having two detectors each with an associated transmission grating. One of the gratings is stationary with respect to its detector and the other moves with respect to its detector. Based on a comparison of the signals generated by the two detectors, the sign and magnitude of the velocity may be determined.
U.S. Pat. No. 3,856,403 to Maughmer, et al. also attempts to avoid the directional ambiguity by providing a moving grating. It provides a bias for the velocity measurement by moving the grating at a velocity higher then the maximum expected relative velocity between the surface and the velocimeter. The frequency shift reduces the effect of changes in the total light intensity (DC and low-frequency component), thus increasing the measurement dynamic range and accuracy.
PCT publication WO 86/06845 to Gardner, et al. describes a system designed to reduce the amplitude of DC and low frequency signal components of the detector signal by subtracting a reference sample of the light from the source from the speckle detector signal. The reference signal is proportional to the total light intensity on the detector, reducing or eliminating the influence of the total intensity variations on the measurement.
This reference signal is described as being generated by using a beam-splitter between the measured surface and the primary detector by using the grating that is used for the speckle detection also as a beam-splitter (using the transmitted light for the primary detector and the reflected light for the reference detector) or by using a second set of detectors to provide the reference signal. In one embodiment described in the publication the two signals have the same DC component and opposite AC components such that the difference signal not only substantially removes the DC (and near DC) components but also substantially increases the AC component.
In U.S. Pat. No. 4,794,384, Jackson describes a system in which a speckle pattern reflected from the measured surface is formed on a 2D CCD array. The surface translation in 2 dimensions is found using electronic correlation between successive images. He also describes an application of his device for use as a “padless optical mouse.”
Image velocimetry methods measure the velocity of an image across the image plane. The image must include contrasting elements. A line pattern (much like a grating) space-modulates the image, and a light-sensitive detector is measuring the intensity of light that passes through the pattern. Thus, a velocity-to-frequency relation is formed between the image velocity and the detector AC compo

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