GB2158228A

GB2158228A - Astigmatic non-contact optical probe

Info

Publication number: GB2158228A
Application number: GB08411592A
Authority: GB
Inventors: James L Doty
Original assignee: Spectron Development Laboratories Inc
Current assignee: Spectron Development Laboratories Inc
Priority date: 1984-05-05
Filing date: 1984-05-05
Publication date: 1985-11-06
Also published as: SE8402589L; FR2564602A1; GB8411592D0; SE8402589D0; DE3418767A1; CH658312A5

Abstract

An optical probe, having particular application in contour mapping and small distance gauging, includes a light generation and projection means 14, 17 which projects a light beam 16 through crossed cylindrical lenses 18, 20 to induce astigmatism in the beam. The astigmatic beam is then directed onto a target surface 30 to be mapped. The back-scattered radiation 32 reflected from the target surface is collected by a collection means 34 and focused onto a photodetector 36. The shape of the image projected onto the photodetector is dependent upon the distance between the probe and surface. By observing the output of the detector the distance between the probe end surface may be measured, or maintained constant, and a contour map generated using known mapping techniques. In an alternative embodiment, astigmatism is induced in the reflected beam, instead of in the incident beam. <IMAGE>

Description

SPECIFICATION Astigmatic non-contact optical probe 1. Field of the Invention: The present invention relates to the field of dimensional metrology, and more particularly, to non-contact surface mapping utilizing an optical gauging probe.

2. Art Background: In many scientific and engineering applications, it is desirable to obtain an accurate surface profile of an object. For example, the development of ground, air and space vehicles requires the generation of a contour map of the vehicle's various surfaces for proper analysis of mechanical and aerodynamic characteristics. Design modifications may then be made and an updated surface contour map generated for further design optimization. In addition, in many instances a description of the contour of an article is critical in manufacturing and quality control. High stress components such as gears, turbine and fan blades etc., must be machined to very high tolerances. Frequently, a detailed contour map of the article is obtained in order to act as a template and quality standard for subsequent mass produced articles.

A variety of probes have been developed in order to obtain a contour map of a target surface. For example, contact probes are available which are versatile and exhibit high accuracy at relatively low cost. However, contact probes, by their very nature, mar the surface which is being inspected to some degree and manifest wear at the probe tip. If an error exists in positioning the probe, both the probe and the target surface may be seriously damaged. In addition, in some instances the target surface, such as a compressor blade, is so thin that a contact probe will deform the surface and introduce measurement error.

In response to the difficulties encountered in using contact contour mapping probes, a number of non-contact surface profiling techniques have been attempted. One system which utilizes point optical interferometry yields high accuracy but is frequently too sensitive for many applications. Methods which utilize point optical interferometry require expensive optical components, critical alignment of the optical apparatus, significant vibration isolation and sophisticated electronic processing for proper operation.

Similarly, other techniques have been developed which utilize surface optical interferometry which like point optical interferometry, requires expensive components, vibration isolation and sophisticated signal processing. In addition, surface optical interferometry has been found to be very sensitive to misalignment and must use complex and time consuming image processing techniques to reduce the raw fringe data obtained. The technique has been found to be limited to the inspection of targets with highly polished and specularly reflective surfaces which are subject to the same critical alignment as that required by the measuring system. Other noncontact techniques are disclosed in a U.S.

Patent Nos. 3,481,672; 3,679,307; and 4,299,491.

Accordingly, there exists a need to provide a simple, yet accurate, non-contact optical probe for use in surface contour mapping and small distance gauging systems. The present invention provides a non-contact optical gauging probe which enables detailed contour mapping of a target surface, and which is not subject to the critical alignment and vibration problems typically found in prior art devices.

As will be apparent from the discussion which follows, the present invention has numerous other dimensional metrology applications in addition to surface profiling.

SUMMARY OF THE INVENTION An improved optical probe having particular application for use in surface contour mapping and small distance gauging systems is disclosed. The probe includes a laser, or other light source, which projects a beam of light through crossed cylindrical lenses to artificially induce astigmatism into the beam. The astigmatic beam is then projected, using appropriate light directing means, onto a target surface to be mapped. The back-scattered radiation reflected from the target surface is collected by a collection means and focused onto a photo-detector or other appropriate radiation detection means. By nature, the shape of the astigmatic projected light beam varies in accordance with the distance from the optical probe.A point downstream from the optical probe where the astigmatic beam forms a generally circular spot is defined as a central focus "null" point (Z = O). Upstream from this point toward the optical probe the astigmatic beam forms a generally elliptical spot (at Z = - Zo) and similarly, downstream from the null point the beam forms a generally elliptical spot (at Z = + ZO) rotated substan tially 90 with respect to the upstream - Z, spot. Thus, the shape of the image projected onto the photo-detector by the collection means is dependent upon the distance between the optical probe and the target surface.Accordingly, by observing the output from each element of the photo-detector, the distance between the probe and the target surface may be maintained constant and a contour map of the surface may be generated using known mapping techniques.

An alternate embodiment of the invention is provided wherein a non-astigmatic light beam is projected onto the surface to be mapped.

The back-scattered radiation reflected from the target surface is collected and passed through crossed cylindrical lenses to induce astigmatism into the received beam. The astigmatic beam is then focused onto a photo-detector.

As in the case of the first embodiment, the shape of the received beam is a function of the distance between the probe and the target surface, thereby permitting a contour map to be generated.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates the projection of an astigmatic light beam onto the quadrant detector of the present invention.

Figure 2 is a diagrammatical representation of one embodiment of the present invention.

Figure 3 is a graphic illustration of the inducement of astigmatism into a light beam using crossed cylindrical lenses.

Figure 4 is a graph of the electrical output of the quadrant detector of Fig. 1 as a function of distance between the probe of the present invention and a target surface.

Figure 5 is another embodiment of the present invention disclosed in Fig. 2.

Figure 6 is a diagrammatical illustration of the use of the present invention to obtain a surface contour map of a target object.

DETAILED DESCRIPTION OF THE INVEN TION A non-contact optical probe having particular application for use in surface contour mapping and small distance gauging is disclosed.

In the following description for purposes of explanation, numerous details are set forth such as specific numbers, lenses, materials and configurations in order to provide a thorough understanding of the present invention.

However, it will be apparent to one skilled in the art that the invention may be practiced without the specific details. In other instances, well known components such as detectors, electrical processing means, etc., have not been described in detail in order not to obscure the present invention unnecessarily.

Referring briefly to Fig. 1, the present invention comprises a probe, indicated generally by the numeral 10, which projects an astigmatic light beam 1 2 onto a target surface to be mapped. The cross section of light beam 1 2 varies in shape as a function of the distance from the probe 10. As illustrated, the shape of the astigmatic light beam 1 2 is substantially circular at a point defined as the central focus (CF) of the beam (Z = O). Upstream (Z = - Z,) toward the probe 10, the light beam 1 2 generally takes the shape of an ellipse oriented substantially vertically.The downstream (Z = ZO) shape of the astigmatic beam 12 generally forms an ellipse which is oriented approximately 90 with respect to the upstream shape. As will be discussed, the use of an astigmatic beam permits the distance between the probe 10 and the target surface to accurately be maintained in order to generate a contour map of a target surface.

Referring now to Fig. 2, the structure of the present invention will be disclosed. Non-contact optical probe 10 includes a light source 14 which may comprise a laser, light emitting diode, or the like, for generating light of a desired wavelength. The generated light 1 6 is passed through a collimating lens 1 7 in order to obtain substantially parallel rays. Artificially induced astigmatism is introduced into light beam 16 using a pair of crossed cylindrical lenses 1 8 and 20. As best shown in Fig. 3, the present invention's use of crossed cylindrical lenses induces the required astigmatism into beam 1 6 such that at the central focus (CF) of the astigmatic beam a substantially symmetric and circular spot is generated.This central focus is defined as Z = 0 along a Z axis passing substantially perpendicular and through lenses 1 8 and 20, as illustrated in Fig. 3. The shape of beam 1 6 upstream of the central focus (Z = - Z,) is generally an ellipse with its major axis oriented vertically and parallel to the major longitudinal axis of cylindrical lens 20. Downstream from the central focus (Z = + ZO) the shape of beam 1 6 once again takes the general form of an ellipse but with a major axis rotated approximately 90 with respect to that of the upstream ellipse.Thus, the shape of beam 1 6 at Z = + Z,, is that of an ellipse with its major axis generally parallel to the longitudinal axis of cylindrical lens 1 8. The cross sectional shape of beam 1 6 is therefore dependent upon the distance from the crossed lenses. It will be appreciated, that although the present invention utilizes cross cylindrical lenses in order to induce artificial astigmatism, other similar methods utilizing combinations of lenses or the like may also be used to accomplish the same result.

Referring once again to Fig. 2, beam 16 after passing through cross cylindrical lenses 1 8 and 20 is directed, by for example beam splitter 28, onto a target surface 30 to be mapped. Surface 30 scatters beam 16 in accordance with the nature of its surface. For example, if target surface 30 is relatively smooth, a significant portion of the beam 1 6 will be reflected. If target surface 30 is rough or otherwise pitted and uneven beam 1 6 will be scattered to a greater or lesser extent. As will be discussed, although the surface characteristics of target surface 30 will vary the amount of light reflected, scattered or absorbed, the summed output of the present invention is dependent on the distance between the probe 10 and surface 30.

Back-scattered light 32, which has been reflected by target surface 30, is collected by an imaging lens 34 and focused onto a quadrant detector 36 coupled to suitable electronics 37. Although the present invention utilizes an imaging lens 34 in order to collect the scattered radiation from beam 1 6 which is projected onto the target surface 30, it will be apparent that a variety of other collection means may be used to accomplish the same result.

As illustrated best in Fig. 1, quadrant detector 36 comprises four photo-sensitive ele ments 40, 42, 44 and 46. When the distance between the crossed cylindrical lenses 1 8 and 20 and the target surface 30 equals the distance to the central focus, the back-scattered radiation 32 focused onto quadrant detector 36 forms a substantially symmetric and circular spot 35. Thus, the output from each quadrant detector will be substantially equal.

In the case where the target surface 30 falls within the upstream portion of the astigmatic beam 1 6 which has a general elliptical shape with a major axis parallel to the Y axis (see Fig. 3), the combined output of quadrants 40 and 44 will exceed that of quadrants 42 and 46. Conversely, if the target surface falls in the downstream portion of the projected astigmatic beam having an elliptical cross section disposed generally 90 with respect to the upstream cross section, the output of photosensitive elements 42 and 46 will exceed the output of elements 40 and 44 of detector 36.

The outputs of the four detector elements are summed and subtracted to produce voltages of the form: Vcu = KP~ V0 = KP+ where, K = a proportinality constant P = P42 + P46 - P40 - P44 p+ = P42 + P46 + P40 + P44 and Pm = the total optical power incident upon each detector element m, where m = 40, 42, 44 or 46.

Referring now to Fig. 4, the output of the probe is taken as the ratio Vout/Vo, which is graphically represented as a function of the distance between the target surface 30 and the central focus 1 6 of the probe 1 0. Distance from the central focus point of the astigmatic beam 16 is measured along the abscissa in Fig. 4, and the quadrant detector output normalized as VOU,/VO is plotted along the ordinate.

As expected, the detector output reaches a maximum when the detector 36 is located near either foci (ZO and - Z0). Between the two foci it is generally linear and single valued. Outside the two foci the signal asymptotically approaches zero.

Although the presently preferred embodiment of the invention utilizes a four quadrant photo-detector 36, it will be apparent that other detector element combinations are possible. For example, the present invention may use a two element detector satisfactorily. In such case, the output from each element is compared in order to identify the position of the probe 10 relative to the target surface 30, based on the nature of the back-scattered radiation.

Referring now to Fig. 5, another embodiment of the present invention is disclosed.

Inasmuch as many of the elements comprising the embodiment of Fig. 5 are substantially the same as those described with reference to Fig.

2, similar reference numerals are used for simplicity. Light source 14, generates light beam 42 which is passed though collimating lense 44 in order to project the beam onto a target surface 30. The back-scattered radiation 50 reflected from the surface 30 is directed by a beam splitter 52 or the like to crossed cylindrical lenses 54 and 56. As in the embodiment of Fig. 2, crossed cylindrical lenses 54 and 56 induce an artificial astigmatism into the collected beam 50 and focus the beam onto a quadrant detector 36 coupled to appropriate electronics 37. As previously described, the output of quadrant detector 36 provides signals indicative of the position of probe 10 with respect to the target surface 30. It has been found, that the use of the "receive astigmatism" system illustrated in Fig. 5 provides a detector output substantially the same as that illustrated in Fig. 4.Both embodiments disclosed in Fig. 2 and 5 utilize artificially induced astigmatism, in order to vary the geometrical shape of a light beam directed onto a quadrant detector as a function of the distance between probe 10 and the target surface.

Referring now to Fig. 6, one possible application of the present invention will be described. Probe 10 is shown coupled by an articulated arm assembly 60 to a coordinate measuring machine 62. Articulated arm 60 includes transducer couplings 64, 66 and 68 in order to permit arm 60 to move in three dimensions (X, Y, Z). In order to obtain a surface contour map comprising X,Y,Z coordinates for points on a target (for example aircraft 70), articulated arm 60 is maintained such that it has a constant stand-off distance "d" from target 70. This distance d equals the distance to the central focus of the astigmatic beam from probe 10. The present invention's output signal at this stand-off distance is defined as the "null" point (Z = 0).

Probe 10's output signal may then be used to control the probe position such that the central focus is maintained on the target surface.

Thus, articulated arm 60 may be used to move probe 10 over the surface 70 while maintaining a constant stand-off. Coordinates for (X,Y,Z) each point of target 70 over which probe 10 passes may then be obtained by appropriate processing of the signals provided by transducers 64, 66 and 68, such that a surface contour map of the target is generated relative to an initial starting point arbitrarily defined as the origin.

It will be appreciated that although Fig. 6 illustrate one use of the present invention wherein probe 10 is utilized in order to maintain articulated arm 60 at a constant distance from the target surface, that probe 10 may also be utilized to directly measure variations of the target surface. The measurement range of probe 10 would generally be defined as the distance between the two foci - Z, and + Z0.

Inasmuch as the output of quadrant detector 36 is substantially linear between each foci, small distances and contour changes may be detected. Thus, the present invention has utility in maintaining quality control for high tolerance articles such as for example, gears, optical and magnetic storage mediums, etc.

For improved accuracy, non-linear processing may be employed to linearize the signal over a larger measurement range.

Thus, an improved non-contact optical probe has been disclosed having particular application for use in contour mapping and small distance measurement. The probe is simple, reliable, and may be easily interfaced with existing mechanical and electrical contour mapping devices. Although the present invention has been described with reference to Figs. 1-6, it will be understood that the figures are for illustration only and should not be taken as limitations upon the invention.

Claims

1. An optical probe for use in determining characteristics of a target surface, comprising: projection means for projecting an astigmatic beam of light onto said target surface; collection means for collecting a portion of said light beam back-scattered from said surface; detection means coupled to said collection means for determining the position of said probe relative to said surface based on the scattering of said astigmatic beam on said surface; whereby characteristics of said target surface may be determined by passing said probe over said surface.

2. The optical probe as defined by Claim 1 wherein said detection means includes a light sensitive photo-detector.

3. The optical probe as defined by Claim 2 wherein said projection means includes two cylindrical lenses disposed 90 degrees with respect to one another for providing said astigmatic beam.

4. The optical probe as defined by Claim 3 wherein said detector is comprised of at least two detector elements, each element providing an output signal in response to light incident on said element.

5. The optical probe as defined by Claim 4 wherein the cross-section of said astigmatic beam varies as a function of distance from said probe to said surface.

6. The optical probe as defined by Claim 5 wherein said collection means includes an imaging lens for collecting said back-scattered astigmatic beam reflected off of said surface.

7. The optical probe as defined by Claim 6 wherein said projection means includes light generation means.

8. The optical probe as defined by Claim 6 wherein said projection means includes light generation means.

8. The optical probe as defined by Claim 7 wherein said projection means further includes a collimating lens disposed between said light generation means and said cylindrical lens.

9. The optical probe as defined by Claim 8 further including a beamsplitter for directing said astigmatic beam onto said surface.

10. The optical probe as defined by Claim 5 wherein said astigmatic beam forms a generally circular cross-section image at its central focus.

11. The optical probe as defined by Claim 10 wherein said detector includes four light sensitive elements (40, 42, 44, 46), the output of said elements of said detector being described by the following relationship: Vout = KP VO= KP+ where, Vout = total voltage output of the detector, K = a proportionality constant P = P42 + P46 - P40 - P44 p+ = P42 + P46 + P40 + P44 P40 = optical power incident on element 40, P42 = optical power incident on element 42, P44 = optical power incident on element 44, P46 = optical power incident on element 46.

12. An optical probe for use in deriving contour information of a target surface, comprising: projection means for projecting an astigmatic beam of light onto said target surface; collection means for collecting a portion of said light beam back-scattered from said surface; detection means coupled to said collection means for determining the position of said probe relative to said surface based on the scattering of said astigmatic beam on said surface; whereby contour information may be derived by passing said probe over said target surface.

13. The optical probe as defined by Claim 1 2 wherein said detection means includes a photo-detector having four light sensitive elements, each element providing an output signal in response to light incident on said element.

14. The optical probe as defined by Claim 1 3 wherein said projection means includes two cylindrical lenses disposed 90 degrees with respect to one another for providing said astigmatic beam.

1 5. The optical probe as defined by Claim 1 4 wherein the total power contained within each of said four elements of said detector is generally described by the following relationship: VOUt = KP~ V0 = KP + where, Vout = total voltage output of the detector, K = a proportionality constant P = P42 + P46 - P40 - P44 p+ = P42 + P46 + P40 + P44 P40 = optical power incident on element 40, P42 = optical power incident on element 42, P44 = optical power incident on element 44, P46 = optical power incident on element 46.

1 6. An optical probe for use in determining characteristics of a target surface, comprising: projection means for projecting a light beam onto said target surface; collection means for collecting a portion of said beam back-scattered from said surface and inducing astigmatism into said collected beam; detection means coupled to said collection means for determining the position of said probe relative to said surface based on the detected astigmatism induced into said collected beam; whereby the characteristics of said target surface may be determined by passing said probe over said surface.

1 7. The optical probe as defined by Claim 10 wherein the cross-section of said collected astigmatic beam varies as a function of the distance between said probe and said surface.

18. The optical probe as defined by Claim 1 7 wherein said collection means includes two cylindrical lenses disposed 90 degrees with respect to one another for inducing said astigmatism in said collected beam.

9. The optical probe as defined by Claim 1 8 wherein said detection means includes a photo-detector having at least two light sensitive quadrants for providing signals indicative of the amount of light incident on each quadrant.

20. The optical probe as defined by Claim 1 9 wherein said photo-detector has four quadrants (40, 42, 44, 46) having generally equal light sensitive areas.

21. The optical probe as defined by Claim 20 wherein the output voltage from said quadrant detector is described by the following relationship: Vout = KP V0 = KP + where, Vout = total voltage output of the detector, K = a proportionality constant P - = P42 + P46 - P40 - P44 P+ = P42 + P46 + P40 + P44 P40 = optical power incident on element 40, P42 = optical power incident on element 42, P44 = optical power incident on element 44, P46 = optical power incident on element 46.

22. The optical probe as defined by Claim 20 wherein said projection means includes a light generation source and a collimating lens.

23. An optical probe for use in deriving contour information of a target surface, comprising: projection means for projecting a beam of light onto said target surface; collection means for collecting a portion of said beam reflected from said surface and inducing astigmatism into said collected beam; detection means coupled to said collection means for determining the position of said probe relative to said surface based on the detected astigmatism induced into said collected beam, whereby contour information may be derived by passing said probe over said target surface.

24. The optical probe as defined by Claim 23 wherein the cross-section of the astigmatic beam varies as a function of the distance between said probe and said surface.

25. The optical probe as defined by Claim 24 wherein said detection means includes a photo-detector having four quadrants (40, 42, 44, 46) having generally equal light sensitive areas.

26. The optical probe as defined by Claim 25 wherein total power contained within each of said four quadrants is generally defined by the following relationship: VOU, = KP V0 = KP + where, Vou = total voltage output of the detector, K = a proportionality constant P = P42 + P46 - P40 - P44 D n 1 + P46 + P40 + P44 r + = ≈42 T 1 46 T r40 T r44 P40 = optical power incident on element 40, P42 = optical power incident on element 42, P44 = optical power incident on element 44, P46 = optical power incident on element 46.

27. The optical probe as defined by Claim 24 wherein said collection means includes two cylindrical lenses disposed 90 degrees with respect to one another for inducing said astigmatism into said collected beam.

28. The optical probe as defined by Claim 27 wherein said projection means includes light generation means.

29. An astigmatic non-contact optical probe substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.