CH658312A5 - NON-CONTACT ASTIGMATIC OPTICAL PROBE. - Google Patents

Description

The present invention relates to an optical probe used in dimensional metrology and more particularly to the contactless establishment of surface maps, using an optical measurement probe according to the introductory part of claims 1 and 12.

In different scientific and technical applications, it may be desired to obtain the profile of the surface of an object. For example, the development of land, air and space vehicles requires the establishment of contour maps of different vehicle surfaces in order to allow a correct analysis of the mechanical and aerodynamic characteristics. Project modifications can then be undertaken and a map of an improved outline can be produced to allow further optimization of the project. In addition, in some cases, describing the outline of an object is important for manufacturing and quality control. Components subjected to high forces, such as gears, turbine and fan fins, etc., must be machined with very high tolerances. Often, a detailed outline map of an object is drawn up for use as a template and quality standard in subsequent mass production.

Different probes have been developed to allow a contour map of a target surface to be established. For example, there are inexpensive, high-precision, multipurpose contact probes. However, the contact probes damage, to some extent, the controlled surface and their feeler tip wears out. If an error is made in the positioning of the probe, the probe and the surface can be seriously damaged. In addition, in certain cases, the target surface, for example a compressor fin,

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658,312

is so thin that a contact probe can deform it and introduce measurement errors.

To avoid the difficulties encountered when using contact probes for establishing contour maps, various techniques of contactless profile tracing have been tried. A system using point optical interferometry leads to high precision, but is often too sensitive in certain applications. Processes using point optical interferometry require expensive optical components, critical alignment of the optical apparatus, good resolution against vibration and sophisticated electronic processing.

Other techniques have been developed using optical surface interferometry which, like point interferometry, require expensive components, vibration isolation and sophisticated signal processing. In addition, optical surface interferometry has been shown to be very sensitive to misalignments and must use long and complex image processing methods to reduce the fringes obtained. This technique is limited to the inspection of targets with highly polished surfaces, spectacularly reflected and subject to the same critical alignment as that required for the measurement system.

Other contactless techniques are described in the U.S. patent submissions Nos. 3,481,672, 3,679,307 and 4,299,491.

It is thus seen that the need exists for a contactless optical probe, simple and precise and intended to be used in the establishment of contour maps of surfaces and the measurement of small distances. The present invention is characterized by the characterizing parts of claims 1 and 12. It relates to a non-contact optical measurement probe which allows the establishment of a detailed map of the contour of a target surface and which is not subject critical alignment and vibration problems such as those encountered in known devices. As will follow from the description which follows, the invention finds numerous other applications in dimensional metrology, in addition to the profiling of surfaces.

We will describe an improved optical probe intended in particular to be used in the establishment of contour maps of surfaces and gauge systems at a short distance. The probe includes a laser or other light source, which projects a beam of light through crossed cylindrical lenses, to artificially induce astigmatism in the beam. The astigmatic beam is then projected onto the surface to be traced, using appropriate light-indicating means. The backscattered radiation, reflected from the target surface, is captured by sensor means and focused on a photodetector or other suitable means of detecting radiation. The shape of the projected astigmatic light beam varies depending on the distance from the optical probe. A point located downstream of the optical probe where the optical beam forms a spot of generally circular shape is defined as “zero” central focal point (Z = 0). Upstream from this point towards the optical probe, the astigmatic beam forms an elliptical spot (at Z = Zo) and similarly downstream from point Zero the beam forms an elliptical spot (at Z = + Zo) which is turned substantially by 90 ° relative to the downstream spot Zo. Thus, the shape of the image projected onto the photodetector by the sensor means depends on the distance between the optical probe and the target surface.

Consequently, by observing the output of each element of the photodetector, the distance between the probe and the target surface can be kept constant and a map of the outline of the surface can be established using known mapping techniques.

Another embodiment is provided in which a non-astigmatic beam of light is projected onto the surface to be reproduced. Backscattered radiation reflected from the target surface is captured and passed through crossed cylindrical lenses to induce astigmatism in the received beam. The astigmatic beam is then focused on a photodetector. 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, which makes it possible to establish a map of the contour. The accompanying drawing shows, by way of example,

two embodiments of the probe:

FIG. 1 illustrates the projection of an astigmatic light beam on a quadrant detector according to the present invention; FIG. 2 is a schematic representation of a first embodiment;

Figure 3 is a graphical representation of the induction of astigmatism in a light beam using crossed cylindrical lenses;

FIG. 4 is a graph representing the electrical output of the quadrant detector of FIG. 1 as a function of the distance between the probe according to the invention and the target surface;

FIG. 5 is a variant of the embodiment shown in FIG. 2, and

Figure 6 is a graphical representation of the use of the present invention to plot the outline of the surface of a target object.

We will describe a contactless optical probe intended to be used in the establishment of surface contour maps and the calibration of short distances. In the description which follows, numerous details are mentioned such as specific numbers, lenses, materials and configurations in order to allow a perfect understanding of the present invention. However, it is clear to those skilled in the art that the invention can be implemented without these specific details. In other cases, well-known components such as detectors, electrical processing means, etc., have not been described in detail so as not to unnecessarily complicate the description.

FIG. 1 represents a probe 10, which projects a beam of astigmatic light 12 onto the target surface whose map is to be established. The shape of the cross section of the light beam 12 varies as a function of the distance from the probe 10. As shown, the shape of the astigmatic light beam 12 is substantially circular at a point defined as being the central focus (CF) of the beam. -35 skin (Z = o). Upstream (Z = Zo) towards the probe 10, the light beam 12 takes the form of an ellipse oriented substantially vertically. The downstream shape (Z = Zo) of the beam 12 is an ellipse oriented substantially 90 ° relative to the upstream shape. As will be discussed later, the use of an astigma-40 tick beam makes it possible to precisely maintain the distance between the probe 10 and the target surface in order to establish the contour map of a target surface.

The embodiment illustrated in FIG. 2 will now be described. The non-contact optical probe 10 comprises a light source 14 constituted by a laser, a light-emitting diode or the like, and intended to emit light of length d wave wanted. The light emitted 16 is passed through a collimating lens 17 in order to form substantially parallel rays.

Astigmatism is artificially introduced into the beam of light 50 using two crossed cylindrical lenses 18 and 20. As can best be seen in FIG. 3, the use, in the invention, of crossed cylindrical lenses induces the astigmatism desired in the beam 16, so as to form a substantially circular spot at the central focus (CF) of the astigmatic beam. This central focal point 55 is defined by Z = 0 along an axis Z passing substantially perpendicularly through the lenses 18 and 20, as illustrated in FIG. 3. The shape of the beam 16 upstream from the central focal point (Z = Zo) is an ellipse whose main axis is oriented vertically and parallel to the main longitudinal axis of the cylindrical lens 60 20. Downstream of the central focus (Z = + Zo), the shape of the beam 16 is again an ellipse, but whose main axis is rotated about 90 ° relative to the upstream ellipse. Thus, the shape of the beam 16 in Z = + Zo is that of an ellipse whose main axis is parallel to the longitudinal axis of the cylindrical lens 18. The dirty cross-65 section of the beam 16 thus depends on the distance from the crossed lenses. It should be noted that, although the invention uses crossed cylindrical lenses to induce artificial astigmatism, other similar methods, making use of combinations of lenses or

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analogues, can also be used to achieve the same result.

Referring again to FIG. 2, it can be seen that the beam 16, after having passed through the crossed lenses 18 and 20, is sent, for example by a beam splitter 28, to the target surface 30 whose card must be established.

The surface 30 disperses the beam 16 as a function of the nature of this surface. For example, if the target surface 30 is relatively smooth, a large part of the beam 16 will be reflected. If the target surface 30 is rough, or pitted and uneven, the beam 16 will be more or less scattered. As will be seen below, although the surface characteristics of the target surface 30 vary the amount of light reflected, dispersed or absorbed, the overall output obtained according to the invention depends on the distance between the probe and the surface 30.

The backscattered light 32, which has been reflected from the target surface 30, is picked up by the objective lens 34 and focused on a quadrant detector 36 coupled to appropriate electronics 37. Although the present invention uses an objective lens 34 to capture the scattered radiation of the beam 16 which has been projected onto the target surface 30, it is clear that other collecting means can be used to obtain the same result.

As best seen in FIG. 1, the quadrant detector 36 comprises four photosensitive elements 40, 42, 44 and 46.

When the distance between the crossed cylindrical lenses 18 and 20 and the target surface 30 is equal to the distance to the central focus, the backscattered radiation 32 focused on the detector 36 forms a substantially symmetrical and circular spot 35. Thus, the outputs of the quadrants are substantially equal. In the case where the target surface 30 falls inside the upstream part of the astigmatic beam 16 which has a generally elliptical shape with the main axis parallel to the y axis (see FIG. 3), the combined output of the quadrants 40 and 44 exceeds that of quadrants 42 and 44. Conversely, if the target surface falls in the downstream part of the astigmatic beam which has an elliptical cross section arranged at 90 ° from the upstream cross section, the exit of the photosensitive elements 42 and 46 exceeds the output of elements 40 and 44 of detector 36.

The outputs of the four elements are summed and subtracted to produce tensions of the form:

You. = KP

Vo = KP +

or

K = Constant of proportionality

P = P42 + P45 P40 P44

P + = P42 P «+ P40 + P44

pm = Total optical power striking each of the detector elements m where m = 40, 42, 44 or 46.

FIG. 4 represents the output Vout / V „plotted as a function of the distance between the target surface 30 and the central focus 16 of the probe 10. The distance from the central focus of the astigmatic beam 16 is measured along the abscissa of FIG. 4, and the output of the detector normalized by V0Ut / V0 is reported on the ordinate.

As expected, the detector output reaches its maximum when the detector 36 is located in the vicinity of one of its homes (Zo and + Zo). Between the two foci, it is generally linear and has only one value. Outside the two foci, the signal tends asymptomatically towards zero.

Although the presently preferred embodiment uses a four-quadrant photodetector 36, it is clear that other combinations of detector elements are possible. For example, the invention can satisfactorily use a two-element detector. In this case, the output of each element is compared in order to identify the position of the probe 10 relative to the target surface 30, this being based on the nature of the backscattered radiation.

FIG. 5 illustrates another embodiment of the invention.

The elements of the embodiment according to Figure 5, which are the same as those described with reference to Figure 2, are referenced by the same numbers, for simplicity. The light source 14 emits a light beam 42 which passes through a collimating lens 44 in order to project the beam onto a target surface 30. The backscattered radiation 50 reflected from the surface 30 is directed by a beam splitter 52 or the like towards the lenses crossed cylindrical 54 and 56. As in the embodiment according to FIG. 2, the crossed lenses 54 and 56 artificially induce astigmatism in the captured beam 50 and focus the beam on a quadrant detector 36 coupled to appropriate electronics 37. As described above, the output of the quadrant detector 36 produces signals indicating the position of the probe 10 relative to the target surface 30. It has been found that the use of the astigmatic reception system illustrated in FIG. 5 produces an output detector substantially the same as that shown in Figure 4. The two embodiments of Figures 2 and 5 use astigmatism formed artificially cially in order to vary the geometry of the beam directed on a quadrant detector as a function of the distance between the probe 10 and the target surface.

Figure 6 illustrates a possible application of the present invention. The probe 10 is coupled, by an articulation 60, to a machine 62 for measuring the coordinates. The articulation 60 includes transducer connections 64, 66 and 68 to allow the arm 60 to move in three dimensions (X, Y, Z). To obtain a map of the contour of the surface, consisting of the X, Y, Z coordinates of the points of a target (for example the plane Zo), the articulated arm 60 is maintained at a constant measurement distance "d" of the target 70. This distance d is equal to the distance between the central focus of the astigmatic beam and the probe 10. The output signal according to the invention, for this measurement distance, is defined as the point "Zero" (Z = 0). The output signal from probe 10 can then be used to control the position of the probe so that the central focus is maintained on the target surface. Thus, the articulated arm 60 can be used to move the probe on the surface 70 while maintaining a constant measurement distance. The coordinates (X, Y, Z) of each point of the target 70 over which the probe passes can be obtained by appropriate processing of the signals supplied by the transducers 64, 66 and 68, so that a map of the contour of the target surface is established relative to a starting point arbitrarily chosen as the origin.

Note that, although Figure 6 illustrates a use of the invention in which the probe 10 is used to maintain the articulated arm at a constant distance from the target surface, this probe can also be used to directly measure variations in the target area. The measurement range of the probe 10 will generally be defined as the distance between the two focal points Zo and 4- Zo. Provided that the output of the detector 36 is substantially linear between each of the focal points, small changes in outline and distances can be detected. Thus, the present invention is useful for maintaining quality control for objects with close tolerances such as, for example, gears, magnetic storage media, etc. For greater accuracy, non-linear processing means can be used to linearize the signal over a wide measurement range.

Thus, an improved contactless optical probe has been presented, intended for use particularly in establishing contour maps and measuring short distances.

The probe is simple, reliable and can easily be interfaced with existing mechanical and electrical devices for reading contour maps.

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4 sheets of drawings

Claims (18)

658,312 1. Optical probe intended to determine the characteristics of a target surface by passing the probe over the surface, characterized in that it comprises projection means for projecting an astigmatic beam of light onto this target surface, sensor means for picking up a part of the beam of light re-scattered from this surface, detection means coupled to said sensor means for determining the position of the probe relative to the surface on the basis of the dispersion of the astigmatic beam on the surface, due to the astigmatism of this one. 2. Optical probe according to claim 1, characterized in that the detection means comprise a photoelectric detector. 2 3. Optical probe according to claim 2, characterized in that the projection means comprise two cylindrical lenses arranged at 90 ° relative to each other, this to produce the astigmatic beam. 4. Optical probe according to claim 3, characterized in that the detector comprises at least two detector elements, each of which produces an output signal in response to the light which it receives. 5. Optical probe according to claim 4, characterized in that the cross section of the astigmatic beam varies as a function of the distance between the probe and the surface. 6. An optical probe according to claim 5, characterized in that the sensor means comprise an image forming lens for picking up the backscattered astigmatic beam which is reflected from the surface. 7. Optical probe according to claim 6, characterized in that the projection means comprise light generating means. 8. Optical probe according to claim 7, characterized in that said projection means further comprise a collimating lens disposed between the light generating means and the cylindrical lenses. 9. Optical probe according to claim 8, characterized in that it comprises a beam splitter for directing the astigmatic beam on the surface. 10. Optical probe according to claim 5, characterized in that the astigmatic beam forms an image of generally circular section in its central focus. 11. Optical probe according to claim 10, characterized in that the detector comprises four elements a, b, c, d sensitive to light (40,42,44,46), the outputs of these elements of the detector being represented by following relationships: Vau. = KP- Go = KP + or Wants = Total detector output voltage K = Proportionality constant P- = P; 2 + P; t. - P :: i - P44 P + = P42 + P. " + P40 + P44 P40 = Optical power striking the element a (40) P42 = Optical power striking element b (42) P44 = Optical power striking element c (44) P46 = Optical power striking element d (46) 12. Optical probe intended to be used to determine characteristics of a target surface by passing the probe over the surface, characterized in that it comprises projection means intended to project a beam of light onto the target surface, means sensors intended to capture part of the beam backscattered from the surface and to introduce astigmatism into said captured beam, detector means coupled to the collecting means and intended to determine the position of the probe relative to the surface on the basis of the detection of the dispersion induced in the captured astigmatic beam due to the astigmatism thereof. 13. Optical probe according to claim 12, characterized in that the cross section of the detected astigmatic beam varies as a function of the distance between the probe and the surface. 14. Optical probe according to claim 13, characterized in that the sensor means comprise two cylindrical lenses arranged at 90 relative to each other to induce astigmatism in the captured beam. 15. Optical probe according to claim 14, characterized in that the detection means comprise a photodetector comprising at least two light-sensitive quadrants to produce signals indicating the amount of light received by each quadrant. 16. Optical probe according to claim 15, characterized in that the photodetector comprises four quadrants a, b, c and d (40, 42, 44, 46) having surfaces sensitive to light of equal quantities. 17. Optical probe according to claim 16, characterized in that the output voltage of the quadrant detector is determined by the following relationships: You, = KP- Vo = KP. or Wants = Total detector output voltage K = Proportionality coefficient P- = P42 + P - ;:> - P40 - P44 P + = P42 + P46 + P40 + P44 Pw = Optical power striking element a (40) P42 = Optical power striking element b (42) P " = Optical power striking element c (44) P4 & = Optical power striking element d (46) 18. Optical probe according to claim 17, characterized in that the projection means comprise a light source and a collimating lens.