Abstract:
An apparatus and method for measuring the surface deformation or displacement in objects. In one embodiment the apparatus includes an interference pattern generator which projects an interference pattern onto a detector mounted to the surface which is to be measured. As the surface deforms, the detector moves and sweeps across the interference pattern. By noting the changes in the light intensity, the deformation or displacement in the surface at the detector may be determined. In another embodiment, both the detector and the interference pattern generator are located on the surface and as the surface deforms, the relative change in the surface at the detector and the relative deformation or displacement of the surface at the interference pattern generator may be determined. In another embodiment a plurality of detectors and interference pattern generators are positioned to map the deformation or displacement of the surface at many locations on the surface.

Description:
GOVERNMENT SUPPORT 
     This invention was made with government support under Contract Number F19628-95-C-0002 awarded by the U.S. Air Force. The government has certain rights in the invention. 
    
    
     This is a continuation of application Ser. No. 08/617,446 filed on Mar. 18, 1996, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to the field of metrology and more specifically to the measurement of surface displacement or deformation using non-contact methods. 
     BACKGROUND OF THE INVENTION 
     The non-contact measurement of the surface deformation or displacement in large structures is important when active or passive measures are employed to correct or compensate for the deformation or displacement. For example, a large parabolic surface which is utilized as the reflector of microwave or optical radiation in a microwave or optical telescope is subject to wind and temperature deformations and variations in sag due to dish orientation. To correct for the distortions caused by such deformations either mechanical actuators are used to selectively correct the surface so as to counter the unwanted deformation or mathematical techniques are applied to correct the received signal as reflected by the deformed surface. 
     Further, such measurements are useful in determining the reaction of many kinds of physical structures to the effects of load even if deformation compensation is not to be applied. For example, the deformation of a building due to wind shear, the deformation of a bridge under traffic load, and the deformation of an airplane wing under flight conditions are all important measurements for the purpose of maintenance and safety. 
     In each case the amount of surface displacement or deformation which may be very small in magnitude, must be measured precisely, both if the compensation to be applied is to be effective and if the behavior of the structure under load is to be compared with the predicted behavior. Laser range finders may be used in such situations because such range finders permit small displacements to be measured accurately and precisely. However, the cost of such range finders typically requires that one range finder be used to measure the displacement of a surface at multiple points on the surface. In such a case, the displacement or deformation at the various points on the surface is measured sequentially, thereby precluding an accurate measurement of the displacement or deformation of the surface at any point in time. 
     The present invention provides an inexpensive system for making accurate surface displacement or deformation measurements. 
     SUMMARY OF THE INVENTION 
     The invention relates to an apparatus and method for measuring the surface displacement or deformation in objects. In one embodiment the apparatus includes a fringe pattern generator which projects a fringe pattern onto a detector. The detector is mounted to the surface which is to be measured. As the surface deforms, the detector moves and sweeps across the fringe pattern. By noting the changes in the light intensity, the deformation or displacement of the surface may be determined. In another embodiment, both the detector and the fringe pattern generator are located on the surface and as the surface deforms, the relative change in the surface between the detector and the sources may be determined. In another embodiment a plurality of detectors and fringe generators are positioned to map the deformation or displacement of the surface at many locations on the surface. 
     In one embodiment the interference generator is a transmissive plate fringe generator with a laser light source. In another embodiment a plurality of transmissive plate fringe generators are located in close proximity to provide multiple fringe generators for a plurality of detectors located at locations on the surface to be measured. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     This invention is pointed out with particularity in the appended claims. The above and further advantages of this invention may be better understood by referring to the following description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a highly schematic diagram of an embodiment of the invention; 
     FIG. 2 is a highly schematic diagram of the embodiment of the invention of FIG. 1, used to measure the deformation of a parabolic reflector; 
     FIG. 3 is an embodiment of a transmissive plate fringe generator of the invention; 
     FIG. 3a is an embodiment of an array of transmissive plate fringe generators; 
     FIG. 3b is a top view of the array of FIG. 3a; 
     FIG. 4 is a diagram of the interference pattern generated by the embodiment of the interferometer of FIG. 3 as seen at the detector; and 
     FIG. 4A is an embodiment of a strip element array used with masks. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In brief overview and referring to FIG. 1, one embodiment of the invention includes two coherent light sources 10, 10&#39; separated by a distance D and located at a distance R from a detector 14 which is attached to the surface 18 whose deformation or displacement is to be measured. The two sources 10, 10&#39; are herein collectively referred to as a fringe source or fringe generator and are contemplated to be held at a fixed distance apart. The output of the detector 14 is connected to a processor 16. The two sources 10, 10&#39; produce an interference pattern 22 at the location of the detector 14. The interference pattern 22 consists of regions of varying light intensity (only three periods shown for clarity). The distance d between adjacent regions of equivalent intensity (The distance of one period.) is described by the equation: 
     
         d=λR/D 
    
     where λ is the wavelength of the light emitted by the sources 10, 10&#39;. 
     As the surface 18 moves in the direction shown by arrow M, the detector 14 sweeps across the interference pattern. The detector 14 detects periodic light 26, 26&#39;, 26&#34; and dark 28, 28&#39; regions. If the amount of deformation or displacement is less than the spacing of the periods of the interference pattern, the detector 14 can determine the amount of deformation or displacement by determining the change in light intensity, as discussed in detail below. If the deformation or displacement exceeds one period, the number of light and dark periods or fringes can be detected and counted and the amount of deformation or displacement determined. 
     As an example, for a wavelength λ of 0.5 μm, a source separation D of 1.4 mm and a distance R from sources 10, 10&#39; to detector 14 of 20 m, the resulting fringe spacing is 10 mm. Thus the deformation or displacement of the surface 18 would have to exceed 10 mm before fringe counting is necessary. 
     Although the apparatus is described in terms of the generation of an interference pattern, any device which is capable of projecting a pattern of periodic light and dark regions of accurately known spacing (hereinafter referred to generally as a fringe pattern) may be used as the source. For example a Moire pattern generator may be used as the source. 
     Further, although the detector 14 is described above as being located on the surface 18 to be measured and the sources 10, 10&#39; are located some distance from the detector 14 and the surface 18, it is also possible to locate the sources 10, 10&#39; on the surface 18 to be measured, and the detector 14 some distance from the surface 18. Yet further, it is also possible to locate both the source 10, 10&#39; and detector 14 on different parts of the surface 18. 
     Referring to FIG. 2, an example of where this system may be used is in the determination of the deformation in a parabolic reflector 30. In the embodiment shown, a multiplicity of detectors, generally 14 (only two shown for clarity), and fringe sources, generally 10 (only four shown for clarity), are used to measure the deformation of various portions of the surface 18 of the reflector 30 simultaneously. By receiving signal data from all the detectors, generally 14, in response to the fringe pattern 22, simultaneously, the deformation of the surface 18 of the reflector 30 may be mapped. 
     Referring to FIG. 3, although the embodiment of the invention is described using two coherent light sources 10, 10&#39;, a suitable source of coherent optical radiation may be obtained by using a transmissive plate interferometer 38. One embodiment of such an interferometer includes a glass plate 40 oriented at an angle θ to a laser beam 44 emitted by a laser 48. A portion 50 of the laser beam 44 is reflected by the surface 52 of the plate 40, while a portion 58 penetrates the plate 40 and is reflected by the other surface 62 of the plate 40. The result is that two coherent beams 50, 52 are formed which are transmitted to the detector 14. The two beams 50, 52 are separated by a distance D which is given by the equation: ##EQU1## where t is the thickness of the plate 40 and η is the index of refraction. Additionally two light shields 64, 64&#39; may be provided to prevent the formation of additional beams as the light is internally reflected within the glass plate. 
     Referring to FIG. 3a, an embodiment of a plurality of transmissive plate interferometers of FIG. 3 located adjacent one another to produce a plurality of fringe patterns. In addition, the transmissive plates 38 may be oriented so as to project the light beams in different directions (see FIG. 3b). 
     In another embodiment a laser beam is passed through a cylindrical lens to narrow the beam prior to the beam entering the transmissive plate. In this way beam energy is concentrated in one dimension across one or more detectors 14. 
     Referring to FIG. 4, although the embodiments described above have been discussed in terms of a single detector 14, increased resolution and the avoidance of the intensity ambiguity may be achieved by using a plurality of detectors. The intensity ambiguity arises because generally a given intensity occurs twice during a cycle and so it is not easy to determine from where in the periodic cycle of intensity (for example point I 2  or I 3 ) the signal is arising. By using a detector 70 having three parallel detecting elements 72, 74, 76 in the form of strips each separated from an adjacent strip by 1/4 d, and orienting the strips parallel to the fringes, the ambiguity may be removed. Each strip 72, 74, 76 produces a signal (I 1 , I 2 , and I 3  respectively) proportional the intensity of optical radiation detected. The phase φ of the intensity cycle can be determined by the following relationship: 
     
         φ=tan.sup.-1  (I.sub.1 -I.sub.3)/(2I.sub.2 -I.sub.1 -I.sub.3)! 
    
     Thus the phase φ can be determined unambiguously within a range of 360°. Although this equation is strictly true only for strip widths of 0, it is a good approximation if the strip width is much less than the fringe period. The amount of deformation or displacement z is then given by the equation: 
     
         z=φd/(2π) 
    
     By using a multiple element detector, variations in the amplitude (A) of the intensity of the interference pattern caused by changes in the sources 10, 10&#39; can be made to cancel out as can changes in the ambient light level or bias (B), as is seen from the equation defining φ. 
     Referring to FIG. 4A, it is also possible to create a detector whose strip elements are oriented perpendicular to the fringes. In such a case, each of the strip elements is masked so as to segment each strip into individual active areas. Each of the masks are offset by 90° with regard to the period of the fringe and with respect to each other. Such an arrangement thereby produces the same effect obtained by the three element detector described above, but with increased efficiency due to the larger overall active area. Alternatively, a two dimensional charge couple device array may be used. 
     Because both of the beams which form the interference pattern travel over nearly the same optical path, temperature gradients and atmospheric effects which do not cause a bending of the optical beams have a minimal effect on the measurement. Transverse temperature gradient ∂T/∂z, which causes a bending of the beam, affects the measurement of the deformation or displacement according to the equation: 
     
         Δz=1/2|∂η/∂T∂T/.differential.z|R.sup.2 
    
     where Δz is the error in the measurement, ∂η/∂T is the change in the index of refraction with temperature, and ∂T/∂z is the transverse temperature gradient. To understand the magnitude of the error in measurement, consider for example with a propagation path (R) of 20 m, a change in the index of refraction with temperature (∂η/∂T) of 5.4×10 -7  /F.° and a transverse gradient (∂T/∂z) of 0.5° F/m. In such a case the error (Δz) will be about 50 μm. 
     In another embodiment, the detector is located adjacent to the source and the fringe pattern is projected onto a comer cube retroreflector located where the detector had been in the previous discussions. When the retroreflector receives the fringe pattern, the pattern is reflected back toward the detector which is adjacent the source. Such an arrangement has the attribute of not requiring electrical connections at both the source and retroreflector locations. 
     In yet another embodiment one or more detectors are illuminated by two widely separated independent fringe sources. In such a case, each fringe source may have a different wavelength each of which is selectable at the detector using an appropriate filter. This arrangement provides for a consistency check which is useful in the monitoring of fringe stability. 
     It is envisioned that the sources and detectors can be attached to a flexible tape or line to aid in placing them on the surface to be measured. 
     Having described the preferred embodiments of the invention, it will now become apparent to one of skill in the art that other embodiments incorporating the concepts may be used. It is felt, therefore, that these embodiments should not be limited to the disclosed embodiments but rather should be limited only by the spirit and scope of the following claims.