Laser fresnel distance measuring system and method

A method and system for determining range to a target are provided. A beam of electromagnetic energy is transmitted through an aperture in an opaque screen such that a portion of the beam passes through the aperture to generate a region of diffraction that varies as a function of distance from the aperture. An imaging system is focused on a target plane in the region of diffraction with the generated image being compared to known diffraction patterns. Each known diffraction pattern has a unique value associated therewith that is indicative of a distance from the aperture. A match between the generated image and at least one of the known diffraction patterns is indicative of a distance between the aperture and target plane.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to high-precision distance measuring systems and techniques. More specifically, the invention is a high-precision, laser Fresnel distance measuring system and method.

2. Description of the Related Art

Non-intrusive and highly accurate determination of distance to a “target” with a precision on the order of a micron or less would be useful in a variety of industrial, commercial, and government-related applications. For example, large adaptive mirrors used in space or ground applications require knowledge of the location of mirror segments to a very high precision in order to make corrections to an optical wavefront.

Conventional high-precision rangefinders can use lasers and complex processing systems/algorithms. For instance, U.S. Pat. No. 6,456,383 discloses a method and apparatus for making absolute distance measurements using Fresnel diffraction. Briefly, after a laser beam is reflected by a target, the reflected beam is passed through an aperture. A detector spaced from the aperture detects the central intensity of the beam (passed through the aperture) as well as intensities displaced from the beam's center. The intensities are then used to calculate the distance to the target using complex mathematical relationships. However, this system is limited to determination of distances on the order of a few centimeters.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a method and system for determining distance to a target.

Another object of the present invention is to provide a method and system for determining distance to a target with a precision on the order of a micron or better.

In accordance with the present invention, a method and system for determining distance to a target are provided. A beam of electromagnetic energy is transmitted through an aperture in an opaque screen such that a portion of the beam passes through the aperture to generate a region of diffraction (e.g., a Fresnel region) that varies as a function of distance from the aperture. An imaging system is focused on a target plane in the region of diffraction to generate an image of the target plane. The generated image is compared to a plurality of known diffraction patterns with each known diffraction pattern having a unique value associated therewith that is indicative of a distance from the aperture. A match between the generated image and at least one of the known diffraction patterns is indicative of a distance between the aperture and target plane.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring now to the drawings and more particularly toFIG. 1, a laser Fresnel distance measuring system in accordance with the present invention is shown and is referenced generally by numeral10, the optical elements of which can be affixed to a platform12. System10can be used in a wide variety of applications. Accordingly, it is to be understood that any references made herein with respect to specific applications are not limitations of the present invention.

System10includes an electromagnetic beam generator14(e.g., laser) mounted on platform12and capable of producing a beam14A of electromagnetic energy. Depending on the approach used, beam generator14can be realized by (i) a single laser producing a single-wavelength laser beam, (ii) multiple lasers arranged to output their respective beams onto a shared optical path to produce beam14A, or (iii) a tunable laser capable of outputting a variable-wavelength beam14A.

In the illustrated embodiment, beam14A is passed through a beam expander16(mounted on platform12) to produce a parallel wavefront beam represented by parallel arrows16A. Parallel wavefront beam16A is incident on an opaque screen18mounted on platform12. In general, a portion22of beam16passes through an aperture20formed through screen18. After passing through aperture20, portion22undergoes diffraction within a region that lies between lines24. In terms of the present invention, diffraction region24is any region of diffracted radiation behind aperture20that is predictable and demonstrates a unique intensity pattern as a function of distance from aperture20. Thus, the optical pattern of the diffraction region at a target plane (e.g., the target plane represented by dashed vertical line26) located in diffraction region24varies with the distance between that target plane and aperture20. By way of example, if the electromagnetic energy incident on screen18is parallel wavefront beam16A, diffraction region24is a Fresnel diffraction region as is well known in the art.

Another of the optical elements fixedly mounted on platform12is a telescope30having a shallow or narrow depth-of-field focused on some plane in diffraction region24. More specifically, the focal path length of telescope30includes the path to a reflective surface100and the path from reflective surface100to some target plane in diffraction region24. Thus, telescope30and reflective surface100are part of an imaging system used to view a target plane in diffraction region24. If telescope30has a shallow or narrow depth-of-field, the focal plane imaged by telescope30is similarly narrowly defined. As will become more apparent later in the description, the “shallowness” of the telescope's depth-of-field should be commensurate with the precision required of the distance that is to be measured.

Reflective surface100is representative of the surface of an object of interest that will experience movement referenced by two-headed arrow102. The distance moved as a result of movement102is to be measured with a high degree of precision. The reflective nature of surface100can be inherent in the object experiencing movement102(e.g., a mirror). However, the reflective nature of surface100could also be provided by attaching a mirror to a non-reflective object of interest (not shown) to define reflective surface100, or by reflectively coating the object of interest.

The image “viewed” by telescope30is provided to an optical receiver32(e.g., an image detector) that prepares the image for processing by a processor34. Coupled to or integrated with processor34is a database36that stores known Fresnel diffraction patterns along with an associated unique distance at which each such pattern would be generated with the given arrangement of optical elements used to generate diffraction region24. Receiver32, processor34and/or database36can be located/mounted on or off platform12without departing from the scope of the present invention.

Each diffraction pattern defining region24is a function of the size of aperture20, the wavelength of beam14A, and the distance from aperture20. Thus, for a given arrangement of beam generator14, its operating wavelength, and aperture20, a set of diffraction patterns from diffraction region24can be generated, mapped, and then stored (in database36) as a function of distance from aperture20.

In terms of measuring an absolute distance that reflective surface100moves, system10must be calibrated. During calibration, the optical elements of system10must remain in a fixed relationship with reflective surface100. Since the optical elements of system10are typically fixed to platform12, reflective surface100and platform12could be rigidly coupled to one another to achieve the fixed relationship. Once fixed in this fashion, the distance between telescope30and reflective surface100is measured using any conventional distance measuring tool. This distance becomes the base distance to which a distance change is added/subtracted to determine the absolute distance to reflective surface100. Note that if only changes in distance are of interest, the calibration step can be omitted.

To calibrate system10while reflective surface100and platform12are rigidly coupled to one another, diffraction region24is generated and telescope30is focused on diffraction region24. Specifically, a baseline target plane26A in diffraction region24is imaged by telescope30with the location of baseline target plane26A being governed by the relative position of reflective surface100and “width” or “thickness” of baseline target plane26A being governed by the depth-of-field of telescope30. The diffraction pattern viewed by telescope30is indicative of a baseline distance from aperture20to baseline target plane26A. This baseline distance is determined by processor34via a comparison with the above-described mapped set of diffraction patterns stored in database36for the particular arrangement of generator14/aperture20and the operating wavelength of generator14.

When it is time for system10to measure the distance that reflective surface100moves via movement102, the fixed relationship between the optical elements of system10and reflective surface100is abolished so that reflective surface100is free to undergo movement102while the optical elements of system10remain fixed. System10is then operated so that diffraction region24is generated and telescope30is focused on diffraction region24via reflection off reflective surface100. The diffraction pattern (within diffraction region24) at the focal plane of telescope30is indicative of the amount of movement of reflective surface100. For example, if reflective surface100moves a distance D away from telescope30as shown, the target plane focused on by telescope30(i.e., target plane26B) will also be a distance D from baseline target plane26A. In the present invention, distance D can be determined (by processor34) by

(i) matching the diffraction pattern at target plane26B with one of the stored patterns in database36in order to determine its associated distance from aperture20, and

(ii) determining the difference between the distance from aperture20associated with baseline target plane26A and the distance from aperture20associated with target plane26B.

The present invention is not limited to the particular system described above. For example, another distance measuring system50in accordance with the present invention is illustrated inFIG. 2where the approach and optical elements used to generate diffraction region24are the same as described above with respect to system10. However, rather than using a telescope to directly image a target plane in diffraction region24, a lens arrangement40mounted to platform12and positioned in diffraction region24optically displaces the target plane to optical receiver32. For example, lens arrangement40can be designed to have a fore and aft focal plane with the fore focal plane being calibrated through an initial calibration procedure (i.e., similar to that described above) to determine its distance to aperture20when the optical elements of system50are fixed in relation to reflective surface100.

In both calibration and operation, the focused diffraction pattern passing through lens arrangement40is converted to a parallel wavefront that reflects off reflective surface100and onto optical receiver32where the parallel wavefront is converted back to an image of the diffraction pattern. That is, the diffraction pattern is optically moved to optical receiver32. Thus, as the distance between optical receiver32and reflective surface100changes, the effective imaging location of receiver32in diffraction region24also changes. Thus, a different diffraction pattern indicative of such change is imaged by optical receiver32. The functions of processor34and database36are the same as those described with respect to system10.

While the present invention can utilize the entire image of a generated Fresnel diffraction pattern, processing speeds and efficiency associated with such “whole image” processing may limit the overall value of the system especially in high frequency applications. Accordingly, the present invention can also be practiced by using only one or more portions of the Fresnel diffraction pattern at a given target plane therein. Several non-limiting examples will be discussed herein.

In a first approach, processor34uses just the central portion (e.g., a center pixel) of the Fresnel diffraction pattern at a given target plane. In general and as is known in the art, the intensity of a Fresnel diffraction pattern varies as a function of source distance in accordance with a wavelike or sinusoidal function. Further, this intensity and its variability is greatest at the central portion of a Fresnel diffraction pattern so that even a very small change in source distance results in a detectable change in intensity.

If the range of distances of interest is sized to coincide with a single cycle of the wavelike or sinusoidal function governing the Fresnel diffraction pattern, then the central portion of the pattern will uniquely define the distance. However, as the range of distances of interest increases, the Fresnel diffraction pattern will undergo multiple cycles so that comparisons using just the intensity of the pattern's central portion produces ambiguous distance measurements. For example, if the range of distances of interest was such that the Fresnel diffraction pattern experienced five cycles over the range, a given intensity of the central portion of the pattern might be repeated five times with each such repeat being indicative of a different distance value. To remedy this ambiguity problem, a second approach of the present invention (e.g., programmed into processor34) utilizes a second distinct portion (i.e., another pixel) of the pattern in combination with the central portion of the pattern. That is, processor34would compare the central and second portion of the imaged pattern with the corresponding central and second portion of the known patterns stored in database36. The distance ambiguity problem is solved by requiring a match between both compared portions.

The ambiguity problem could also be addressed by means of a third approach of the present invention. More specifically, generator14could be a variable-wavelength laser capable of generating beam14A of different wavelengths. Since the Fresnel diffraction pattern cycle will be unique for each wavelength of beam14A and a given aperture20, processor34could be programmed to collect pattern images at each of two (or more) different wavelengths. The central (or other) portion from each collected pattern/wavelength can then be used during the comparison with known Fresnel diffraction patterns/wavelengths to unambiguously determine distance.

Still further, a fourth approach of the present invention could utilize a single-wavelength laser for generator14but vary the size of aperture20. For example, as shown inFIG. 3, screen18can have an aperture adjustment mechanism18A incorporated with or coupled thereto to make aperture20smaller or larger. Since changing the size of aperture20changes the Fresnel diffraction pattern cycle, processor34could collect and utilize imaged patterns (or portions thereof) for two aperture sizes to unambiguously determine distance.

The advantages of the present invention are numerous. Distance measurements are accurately made without any complex hardware or software. It is only necessary to provide an optical arrangement (e.g., telescope30) that has a depth-of-field precision on the order of the precision of the distance changes to be measured. The simple comparison approaches described herein can be implemented with minimal processor complexity without sacrificing measurement accuracy.

Although the invention has been described relative to a specific embodiment thereof, there are numerous variations and modifications that will be readily apparent to those skilled in the art in light of the above teachings. For example, the present invention is not limited to use with Fresnel diffraction as it will work with any radiation field region behind an aperture that is predictable and demonstrates a unique intensity pattern as a function of distance from the aperture.

Still further, should there be a concern about losing the absolute distance information momentarily during operation, the present invention can include a series of platform “stops” at known fixed distances from the screen's aperture. Each “stop” would provide the system with a new baseline distance in the event the system had to be re-started. This will reduce the amount of time lost each time the system must be re-started. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described.