FR2541002A1 - OPTICAL INSPECTION SYSTEM - Google Patents

Abstract

SYSTEM USING TRIANGULATION PRINCIPLES AND A SINGLE LENS FOR PROJECTION AND RECEPTION. IT INCLUDES A FIRST LENS 18 HAVING A FIRST OPTICAL AXIS 21 TO FOCUS A LIGHT BEAM TOWARDS AN OBJECT 33 LOCATED AT A FIRST FOCAL PLANE 30 AND TO COLLECT AND COLLIMATE THE LIGHT REFLECTED FROM THE DIRECTION OF THE FIRST FOCAL PLANE 57, AND B TO DETECT THE APPEARANCE OF A PREDETERMINED TYPE OF COLLIMATION OF A. APPLICATION TO INSPECTION OF GAS TURBINE ENGINE BLADES.

Description

2 to 1 OO 2

  The present invention relates to optical triangulation systems and, more particularly, systems of this type which uses a single lens to project an indexing light beam towards an object as well as to collect light reflected from the object.

  In virtually all manufacturing operations

  The measurement can be very long, especially when human beings are measuring so that, in general, it is desirable to use automated machines to perform the measurement. The engineers who make a measurement commonly use probes or micrometers that physically come into contact with a part to be measured. In this case, a small but finite force is applied to the part as a result of this contact and this force can disturb the position of the piece or deform the piece, thereby altering

  a characteristic of the piece itself during the

  Moreover, the mechanical feelers themselves are subject to wear and they themselves generate their own inaccuracies as and when they use them.

  Consequently, there has been a growing use of

  optical measuring systems without contact and, in particular,

  of optical triangulation systems These latter systems

  commonly project a beam of light (an indexing beam) to an object using a

  first lens system and collects reflected light.

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  using a second lens system.

  The object of the present invention is to provide a

  new advanced optical inspection system.

  Another object of the present invention is to provide

  a new and improved optical inspection system

  who inspects objects without physical contact with them.

  Finally, the object of the present invention is to provide a new and improved optical inspection system which

  uses principles of triangulation, and only one

  as both a projection lens and a lens

reception.

  A form of the invention projects a beam of light through a lens to an object located in a

  a focal plane of the lens, collects the light

  inflected by the object using the same lens, and detects a predetermined type of collimation of light

  collected by the lens The appearance of the prede-

  completed collimation indicates that reflection is taking place

  on the object at a predetermined distance from the lens.

  The following description refers to the figures

  attached which represent, respectively Figure 1, a form of the present invention;

  Figure 2, optical paths of light beams

  neux differently reflected in the invention of the figure

1; -

  FIG. 3, images on photodetectors FIG. 3A, a distribution of the intensity of the cross section of a light beam which forms an image 63A in FIG. 3; Figures 4A-C, different optical paths of the light reflected by a knife blade mirror; Figure 5, a scanning feature of the present invention; Figure 6, a gas turbine engine blade; and FIG. 6A, a light beam focused towards a

dawn of gas turbine engine.

  FIG. 1 shows a shape of the -

  In the present invention, a light source 3, which is preferably a neon helium laser (He Ne laser), projects an indexing light beam 6 to a beam splitter or mirror 9 which reflects the indexing light beam 6 to a scanning mirror 12 The scanning mirror 12 preferably pivots around a

  point 15 in order to be able to rotate and thus selectively occupy

  the positions indicated by the dashed lines

  12 A and 12 B, as well as intermediate positions

  These rotational features of this invention are used in a scanning function and are discussed further.

details below -

  The indexing light beam 6 is reflected by the scanning mirror 12 towards a peripheral region

  17 (that is, a region near an edge) of a long

  projection screen 18 having an optical axis 21 designated

  hereby a first axis 21 The indexing light beam

  6 arrives at the projection lens 18 parallel-

  The projection lens 18 focuses the indexing light beam 6 along, but not parallel to, the first axis 21 so that the indexing light beam 6 propagates from the peripheral region 17 to an first focal point 27

  on an object 33 located near a first focal plane 30.

  That is to say, the indexing light beam 6, after having been refracted by the lens, propagates out of the axis with respect to the first axis 21 (in FIG.

  the presentdescription a focal point, which is a point

  in the space at which a lens focuses light, and a point of reflection, which is a region

  of an object from which light is reflected).

  An object 33, present at the first focal point 24, will reflect the indexing light beam 6 (thus making the first focal point 24 a point of reflection) in many directions indicated by the arrows 36 A-D Accordingly, the reflection appearing in FIG.

  first focal point, in fact, produces emanate light

  from a source point, to have, the first focal point

  24 This is a diffuse reflection of this reflected light

  On the other hand, the light reaching the projection lens 18 within its aperture, namely between the lines 39 and 42, will be collected and focused back to the scanning mirror 12. This light collected, such as that indicated by the radii 44 A and 44 B is reflected by the scanning mirror 12 along a second axis 45 to an image forming lens 48 in the form of

  radii 51 and 54 The second axis 45 is preferably

  pendicular to the first axis 21 and is the optical axis of the image forming lens 48 The image forming lens 48 focuses the rays 51 and 54 to a photodetector 57 located in a second focal plane 60, which is a The focal areas of the imaging lens 48 The spokes 51 and 54 are focused in the form of an image 63A in FIG. 3 at a second focal point 63 in FIG. 1. The photodetector 57 may consist of two distinct photodetectors. 57 AB placed beside one

  on the other, and separated by a space 57 C

  57 A-B detectors respond mainly to light

  striking surfaces 57 AA and 57 BB.

  The outputs 66 A-B of the photodetectors 57 A-B are

  preferably connected to the inputs 65 A-B of an amplifier

  two-input differential generator 68 which compares the

  signals from the outputs 66 A and 66 B with each other and ampli-

  the difference between these signals When the difference is zero, equal amounts of light strike each photodetector 57 A-B Object 33 is then assumed to be

  located at the first focal point 24 of the lens of

  section 18 and, like the position of the lens of

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  projection 18 is known in advance, it is known that the object 33 is located at a focal distance along the first axis 21, that is to say, the fact that the object 33 is located at the first focal point 24 is known by the zero difference signal produced by the photodetectors 57 The zero difference signal is itself a focusing signal which indicates that the light received by the projection lens 18 has

  has been reflected by the object 33 at the first focal point 24.

  Thus, the light reflected at the first focal point 24 will be collimated by the projection lens 18, reflected by the scanning mirror 12, and focused by the imaging lens 48 at the second focal point.

  63 In these circumstances, only the light that is -

  accurately reflected in the first focal point 24 will be

  focused on the second focal point 63 The light reflected by an object placed far from the first focal point,

  object 68 in FIG. 2 will be focused differently.

  on the second focal plane 60.

  For example, as shown in more detail in Figure 2, when an object moves to positions 72,

  33 and 68, the indexing light beam 6 is reflected

  from the respective reflection points 72A, 33A (which coincides with the first focal point 24 in FIG. 1), and 68A. As a result, the light beams 75, 77 and 78 are shifted in the vertical direction (FIG.

  direction of the arrow 81) For reasons of simplicity

  the light beams 75, 77 and 78 are truncated

  in the region 82 and illustrated as single rays 75A, 77A and 78A. Reflected by the scanning mirror 12, the rays 75 B, 77 B, and 78 B occupy the

  represented positions and these rays are then

  as reflected in light beams 75 C, 77 C and 78 C Thus, the reflection of light by objects 72, 33 and 68 results in a translation of light beams 75 C, 77 C and 78 C in the direction of the light. As a result, these light beams are translated through the photodetector 57, as and when

  the object 33 moves relative to the lens of

  jection 18 along the first axis 21.

  In addition, the light beams 75 C, 77 C and 78 C are focused by the image forming lens 48 at the respective points 94 A, B and C. As is well known, under these conditions, the light of each beam C , 77 C, and 78 C strikes the image plane 60 will contain images having different degrees of focus depending on the position of the object 33 This is further illustrated in Figure 3 o onarepresented a picture 94 AA as being projected on the photodetector 57 by the light beam 75 C in FIG. 2 (For the sake of simplicity, the light rays which actually propagate from the point 94 A to the photodetector 57 have not been represented). Similarly, the image 63 A of FIG. Fig. 3 corresponds to point 94 B in Fig. 2, and Fig. 63A of Fig. 3 corresponds to point 94B in Fig. 2, and image 94 CC is projected onto photodetector 57 through light beam 78 C. It should be noted that the light beams 75 A, 77 A and 78 A in Figure 2 can all appear to be

  parallel to the first axis 21 and, similarly, that the

  However, it is known in the art that, in the circumstances described, only the light beams 75B, 77B, and '78B may appear parallel to the axis 45. However,

  beams 77 A and 77 B have these parallel properties

  The other light beams 75 A, 78 A, 75 B and 78 B appear only so for the convenience of the representation. In fact, the beams 75 AB will converge towards the axes 21 and 45 as they propagate towards the imaging lens. 48 and the light beams 78 AB

will diverge from these axes.

  As a result, from a point of view, the 63 A picture,

  seen by the photodetector 57 in terms of both

  The signals produced by the photodetectors 57 AB indicate when the image 63 A of FIG. 3 seen by the photodetectors 57 AB is focused because the object 33 reflects the indexing beam 6. when the image 63 A exclusively illuminates the space 57 C, the differential amplifier 68 produces a zero output signal (the focusing signal). In addition, the question of whether the reflection takes place at the first focal point 24 is noted. , in fact, by the detection of the parallelism of the light beam 77 with the first axis

  21 (which is the optical axis of the projection lens 18).

  The focusing of the light beam 77 at point 94 B between the photodetectors 57 A-B indicates this parallelism, which is, of course, a particular type of collimation of the light.

  light beam 77 by the projection lens 18.

  As shown in FIGS. 4A-C, photodetection

  57 AB (shown in dashed lines only in Figure 4A) are replaced by a knife-blade mirror 95 (whose tip only is shown) and by two other photodetectors 57 DE The edge 95A of the mirror blade The knife mirror 95 reflects the light projected toward it by the imaging lens 48 (not shown in FIG. 4A) to either one or the other the two photodetectors 57 D and 57 E located on each side of the knife-blade mirror 95 The quantity of light received by each photodetector 57 'DE depends on the position of the object 33 in FIG. 1, i.e. as the light beams 75 B, 77 B and 78 B translate in the direction of the arrow 83 in FIG. 2, they strike the knife-blade mirror 95,

  in FIGS. 4A-C at different positions.

  In particular, in FIG. 4A, the light beam

  C 75 is reflected back to the photodetector 57 D In FIG. 4 B, the light beam 77 C is divided in two

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  parts 77 F and 77 G reflected towards the two photodetectors

  In FIG. 4C, the light beam 78C is reflected towards the photodetector 57E. One reason for using a knife blade mirror 95 rather than the two adjacent photodetectors 57 AB of FIG. 1 is that this knife-blade mirror 95 allows the space 57 to be, in fact, defined by the edge 95 C of this knife-blade mirror. Thus, in fact, a space 57 C is defined

  very narrow and we obtain a better resolution and

  detection of deviation conditions from the focus. Consequently, in theory and in practice, to the degree possible with the equipment used, when the reflection

  the object 33 generates the image 63 A in FIG.

  at the focal point 24 of the projection lens 18 in Figure 1, the light carrying the image 63 A is

  propagates exactly along axes 21 and 45, is specified

  half divided by the knife blade mirror in FIG. 4B, and each half is projected towards one of the photodetectors 57D and 57E. In the case of a laser such as the He Ne laser 3, the image will be a light circle 0.02 to 0.05 mm in diameter (representing the Gauss distribution) focused on the mirror edge 95 C in two half-circles projected towards the photodetectors 57 D and 57 E. We will now discuss the scanning of the surface of the object 33 The mirror 12 of FIG.

  to occupy positions 12A and 12B as indicated below.

  This is further shown in FIG.

  that the scanning mirror 12 turns in the direction of the

  of a watch, the indexing light beam is reflected in the form of light beams 106 A, 106 B and

  106 C to the projection lens 18 The focal beams

  by the projection lens 18 in the form of

  The light reflected from the object 33 at these points will be collimated by the projection lens 18 in the form of collimated light such as the beam 112, which is reflected by the balayaqe mirror 12 along

  the second axis 45 in the form of the beam 114.

  If we consider the parallelism to the second axis 45, a geometrical analysis will show which, if the axes 21 and are perpendicular, if the surface of the scanning mirror 12 pivots at the point of intersection 15 of the axes, and if the part 6 of the laser beam is parallel to the second axis 45, then all the rays such as the rays 114 which come from the reflection at the first focal plane 30 will be parallel to the second axis 45 The rays emanating from reflections out of the first focal plane 30 will not be parallel to the second axis Moreover, when AC reflection points move along the first focal plane 30, the resulting rays like 114 move towards and away from this second axis 45, but remain parallel to this axis. As is well known, this displacement Lateral of the spoke 114 relative to the second axis 45 does not change the point at which the imaging lens 48 in FIGS. 1 and 2 focuses the spoke 114. However, if the object 33 is moved away from the FIG. 5 of the first focal plane 30 (as are the objects 68 and 72 in FIG. 2), the light

  reflected by the displaced object in Figure 5 (not shown

  felt as displaced in Figure 5) from the

  light beams 108 A-C (not shown displaced)

  will carry because it is reflected either to the left or

  to the right of the first focal plane 30, as the

  light beams 75, 77 and 78 in FIG.

  the positions to the left and to the right of the reflection points 110 A-C relative to the first focal plane 30

  are recorded in the same way as the posi-

  left and right of the reflection points 72A, 33A and 68A in FIG. 2 relative to the first focal point 24 in FIG. 1. Furthermore, the observation of FIG. 2 when applied to FIG. object such as

  scanned in Figure 5 is not significantly affected

  by the fact that the points of reflection 11 OB and

  11 OC in Figure 5 are off the first axis 21.

  Therefore, one can scan the object 33 and trace the contour of its surface.

  In a recommended form of representation of con-

  In accordance with the present invention, the object 33 is first placed on a movable carriage 153 in FIG. 1 and brought into the region of the first focal plane 30 under the control of the digital control equipment 156. As

  discussed in connection with Figures 2 and 4 A-C, the photographs

  detectors 57 A-B produce indi-

  as to whether object 33 is closer or further away from the

  projection lens 18 as the first focal plane 30.

  Alternatively, a carriage position sensor 157 may be used to indicate the arrival of the carriage 153 near the first focal plane. In response to these position signals, the digital control equipment 156 outputs a signal to a motor 159 for move the projection lens 18 to or away from the object 33, according to

  the signals from the photodetectors as processed by the amplifier

  differential indicator 68 The displacement of the projection lens 18 continues until the signals

  photodetectors indicate the achievement of

  the indexing beam 6 on the object 33.

  The displacement of the projection lens 18

  along the axis 21 is preferably carried out by

  A threaded assembly 163 for moving the projection lens 18 relative to the object 33 This forces the light beams 75 C, 77 C and 78 C in FIG. 2 to scan the knife blade mirror 95 in FIG. and causes the differential amplifier 68 to output a focus signal when the image received by the photodetectors 57A and 57B are of equal intensities (i.e., when the light received by the photos

2 2541002

  The sensors 57 A-B is reflected at the first focal point 24).

  The digital control equipment 156 receives signals relating to the position of the projection lens 18 of a position sensor 170 and is then able to calculate the position of the focal point 24 on the object 33 when the comparator delivers a signal The focal point 24 is located at a further focal distance

  projection lens 18 along the first axis 21.

  Of course, it would be possible to move the object 33 together with the carriage, rather than to move the projection lens 18, but it is generally easier to quickly change the distance between the object 33 and the projection lens 18. move the projection lens 18 rather than the carriage because the mass of the projection lens 18 is generally much

less than that of the trolley 153.

  The representation of the outline of an object can be

  The object, which may be a gas turbine engine blade 172 in FIG. 6 (or a master molding from which a

  series of molds for such blades) and which is

  FIG. 6A is placed near the focal plane 30 of the projection lens 18. Part of a wall 175 of the blade 172 has been torn off to allow the indexing beam 6 to strike the internal members. The blade 172 is swept by the indexing beam 6 in the vertical direction (arrow 174 by rotation of the scanning mirror 12 as described with reference to FIG. 5 and if the surface of the blade 172 deviates of the focal plane 30 (as it will do because of the ribs), the image 63 of the laser beam moves on the knife blade mirror 95 in the manner described in connection with FIGS. 3 and 4 AC, thereby inducing signals In response to this, the digital control equipment 156 in FIG. 1 will produce the photodetectors 37.

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  moves the projection lens 18 along the first axis 21 as described above until the focusing signal is obtained. Thus, it can be seen that the digital control equipment 156 adjusts the projection lens 18 to maintain the focal plane always at the surface of the dawn 172 The leftward and rightward movement of the projection lens 18 along the first axis 21 corresponds to the changes to the left and to the

  right of the outline of the dawn 172 When it is scanned.

  In a sense, the distance between the projection lens 18 and the blade 172 is kept constant as if a mechanical cam attached to the projection lens 18 was

  surface of the dawn 172 Thus, the recording of the

  to the left and to the right of the projection lens 18 and the report of the amplitude of these displacements as a function of the vertical scanning of the laser beams 108A-C in FIG. 5 is equivalent to the recording of the contour

  the surface of a strip (ie, the illuminated region)

  by the indexing beam 6) on the dawn 172 After this

  vertical scan, swirls the dawn 172 in the plane of the

  6A and another strip, vertical, adjacent is swept Thus, one sweeps parallel strips of the surface

  of the dawn 172 and the contour of the surface is recorded.

  An optical inspection system has been described which uses a single lens (the projection lens 18 described above) to project an indexing light beam to, as well as to capture light reflected from a reflection point on a object to be inspected (Of course, the single lens can consist of a series of coaxial lenses and so be

  composite lens). The degree of focus is detected.

  When the focal point is reached, the focal point is known to be located at a focal distance from the reflected light and the lens is moved relative to the object to bring the reflected light to the focus.

1 2541002

  lens and the position of the focal point in space is thus verified from the position of the lens One can undertake the scanning of the object with the projected light and the system maintains a constant distance between the lens and the surface of the lens. object located at the focal point of the lens, thus making it possible to determine the

outline of the object.

  It should be noted that the distance from the first focal point 24 to the projection lens 18 in FIG. 1 can be measured at many different points on the projection lens 18. However, distances such as the focal length of this lens are determined. in a way

  known in the art, by choosing particular points

  on a lens for distance measurement.

  The space 57 C is preferably infinitely narrow. However, in practice, the narrower space that is thought to be possible in a commercial unit such as the UDT spot 2D device, available from United Detector Technology, Cuber City, California, is about 0, 125 mm Also, the 63 A image in Figure 3 is not a discrete circle as shown but is

  actually a circular distribution of light intensity

  In this way, as in such a distribution, the intensity level never reaches theoretically zero at any distance from the central axis 63 C of the horizontal axis. image, some illumination, in theory, will hit the photodetectors 57 AB in Figure 3 even when the image 63 A is precisely focused at the midpoint between them This, however, does not pose in practice a significant problem, but causes a Certain degradation of the resolving power theoretically available in the photodetector assembly. The focused image may accordingly be treated as a circle of light having boundaries 63 D in FIG.

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  Gaussian distribution decreased to a selected fraction 63 F (such as 10%) of the peak value 63 E. An important aspect of the present invention resides in the fact that measuring the distance from the projection lens 18 to the object can be done by moving the projection lens 18 only The projection lens 18 is moved along the first axis 21, but the rest

  components in Figure 1 may remain fixed.

  Although particular achievements have been described

  In particular, the scanning mirror 12 can be rotated in a second direction, such as around the second axis 45, for the purpose of the present invention.

  get a two-dimensional scan.

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Claims (11)

  An optical inspection system characterized by comprising: (a) a first lens (18) having a first optical axis (21) for focusing a light beam toward an object (33) at a first focal plane (30); ) and for receiving and collimating light reflected from the direction of the first focal plane, and (b) means (57) for detecting the occurrence of a   predetermined type of collimation of (a).   System according to Claim 1, in which the first type of collimation comprises the collimation of the light reflected parallel to the first optical axis (21) 3 Optical inspection system, characterized in that it comprises:   (a) a light source (3) to provide a   bright water; (b) a first lens (18) having a first optical axis (21) for X (i) receiving the light beam parallel to the first optical axis, (ii) projecting the light beam to a region enclosing a first focal plane (30) of the first lens (18), (iii) receiving the light reflected by an object (33) in the region enclosing the first focal plane,   (iv) direct the received light to a second   lens (48) for focusing the light toward a second focal plane (60) of the second lens and (c) detection means (57) located in the region   the second focal plane to receive the light of the se-   lens conde and to generate a signal when the 16 2541002   reflection of the indexing beam (6) by the object takes place at a predetermined position.   System according to Claim 1, characterized   in that the light source is a laser.   An optical inspection system characterized by comprising (a) a projection lens (18) having a first focal plane (30) and a first optical axis (21); (b) an imaging lens (48) having a second focal plane (60) and a second optical axis (45), a second optical axis perpendicularly intersecting the first optical axis; (c) means for projecting a light beam parallel to the second optical axis; (d) a scanning mirror (12) rotating about the intersection point of the first and second optical axes for: (i) receiving the light beam (c) (ii) rotating to reflect the light beam to different locations on the projection lens of (a) for (A) focusing the light beam by means of the projection lens to different locations in the first focal plane (30)   (B) receive, by means of the lens of   jection, the light reflected by an object (33) located near the first focal plane to focus the received light to the scanning mirror, and (C) reflecting the focused light from (ii) B to the imaging lens (48) for focusing to the second focal plane (60); and (e) a detector (57) located near the second focal plane for receiving the reflected light of (d) (ii) (C) 254-002 and for producing a signal when the light of (d) (ii)   (C) is reflected at the first focal plane (30).   System according to one of the claims   1 to 5, characterized in that it further comprises: detection means (57A-C) for detecting an angular deviation of the light beam from a predetermined reference, and for generating an error signal, and   correction means (156, 159, 163, 170)   detection means for suppressing in response to the error signal the influence of the angular deviation in   a light beam derived from the light beam.   7 A method of optical inspection of an object (33) located near a first focal plane (30), characterized in that it consists in: (a) projecting an indexing light beam (6) towards a first lens (18),   (b) use the first lens (18) for focusing   aligning the indexing light beam (6) with the first focal plane (30);   (c) receive and collimate with the first   mirror (18) of the light reflected by the object and (d) producing a focus signal in response to the collimated light of (c) when the object (33) reflects the indexing beam (6) by focal foreground (30).   Process according to Claim 7, characterized   in that step (d) comprises the steps of (i) focusing   collimated light to a second focal plane (60) using a second lens (48) and (ii) generating the focus signal in response to the light focused in the second focal plane.   Process according to claim 7 or 8, characterized in that it further comprises the steps of stabilization 18 2541002   of the indexing light beam by (a) focusing the indexing light beam (6) by a plate (12) using a first lens means to a point on a focal plane and then to a second lens means for the projecting to the projection lens (18), (b) detecting a deviation of the point from a predetermined point, and (c) rotating the plate (12) in response to the   deviation to reduce this deviation.