FR2564602A1 - NON-CONTACT ASTIGMATE OPTICAL SENSOR - Google Patents

Abstract

AN IMPROVED OPTICAL SENSOR OF WHICH A SPECIAL APPLICATION IS THE DRAWING OF THE CONTOURS AND THE MEASUREMENT OF LOW DISTANCES, IS DESCRIBED. THE SENSOR FEATURES A MEANS OF LIGHT GENERATION AND PROJECTION WHICH PROJECTS A BEAM OF LIGHT THROUGH CYLINDRICAL LENSES CROSSED IN A WAY TO ARTIFICIALLY CAUSE ASTTIGMATISM. THE ASTIGMATE BEAM IS THEN DIRECTED ON THE SURFACE OF A TARGET TO BE DRAWED. BACKWARD SCATTERED RADIATION REFLECTED BY THE SURFACE OF THE TARGET, IS COLLECTED BY A COLLECTOR MEDIA AND FOCUSED ON A PHOTODETECTOR. THE SHAPE OF THE IMAGE PROJECTED ON THE PHOTODETECTOR DEPENDS ON THE DISTANCE BETWEEN THE SENSOR AND THE SURFACE. BY OBSERVING THE OUTPUT OF THE DETECTOR, THE DISTANCE BETWEEN THE SENSOR AND THE SURFACE CAN BE MEASURED OR MAINTAINED CONSTANT, WHICH ALLOWS AN OUTLINE DRAWING USING KNOWN REPRODUCTION TECHNIQUES.

Description

-1-

BASIS OF THE INVENTION

  1. Field of the invention: The present invention belongs to the field of dimensional metrology and more particularly to the production of drawings of contactless surfaces in

using an optical sensor.

  2. Basics of the technique: In many scientific and engineering applications, it is interesting to obtain a profile

  precise of the surface of an object. For example, the develop-

  Pement of land, air and space vehicles requires the production of a plan of the outline of the various surfaces

  of a vehicle allowing a correct analysis of the characteristics

  mechanical and aerodynamic teristics. We can then

  make changes at the study stage and produce

  re for an updated optimization of the design an updated surface contour plan. In addition, in

  many cases a description of the outline of an item

  is critical for the manufacture and control of

  lity. Parts subject to severe stress,

  such as gears, turbine and fan fins

  tilators, etc. must be machined to very tight tolerances. A detailed outline of the part is frequently obtained to serve as a template and standard for

  quality for the following parts produced in series.

  A number of sensors have been developed to obtain a contour map of the surface of a target. For example, there are multi-purpose contact sensors with high precision at a relatively low price. However, by their very nature, contact sensors damage the inspected surface to a certain extent and exhibit wear of their test probes. If there is an error in the positioning of the sensor, both the sensor and the target surface can be seriously damaged. In addition, in some cases, the target surface, for example a compressor fin, is so thin that a contact sensor will deform the surface and introduce

a measurement error.

  Following the difficulties encountered in the

  Using contact sensors for contour drawing, attempts have been made to develop techniques for drawing non-contact surface profiles. A system that uses point optical interferometry has high efficiency

  but is often too sensitive for many applications.

  cations. Methods that use interferometry

  point optics require optical components

  teux, a critical alignment of the optical apparatus, an important isolation of vibrations, and a treatment

  sophisticated electronics if you want them to work

nent properly.

  Similarly, other techniques have been developed that use optical interferometry of

  surface which, like the optical interferometry ponc-

  tual, proves to be very sensitive to misalignments and must use complex and time consuming image processing techniques to reduce the raw fringe data obtained. It was discovered that the technique in question was limited to the control of targets with extremely well polished and reflective surfaces like mirrors, subject to the same alignment criteria as those required by the measurement system. Other contactless techniques are described in

  U.S. Patent Numbers 3,481,672, 3,679,307 and 4,299,491.

  -3- Consequently, there is a need for a simple yet precise, non-contact optical sensor for use in drawing contour plans of

  surface and short distance measurement systems.

  The present invention presents an ope-

  non-contact tick which allows the drawing to be made

  cut from the profile of a target surface, and which is not

  not subject to critical alignment and visualization issues

  brations typically encountered in

  the prior art. As the dis-

  Next, the present invention has many other applications in dimensional metrology, in addition to

drawing of surface profiles.

SUMMARY OF THE INVENTION

  An improved optical sensor which can be used particularly in the drawing of contour maps of

  surface and in small distance measurement systems

  these. The sensor comprises a laser or any other light source, which projects a light beam through crossed cylindrical lenses so as to artificially produce an astigmatism in the beam. The astigmatic beam is then projected using a

  suitable means of directing the light, on an over-

  target face whose plan is to be drawn. The rays scattered backwards, reflected by the surface of the

  target are collected by means of reception and foca-

  read on a photo-detector or any other means of

  tection of appropriate radiation. By nature, the shape of the projected astigmatic light beam varies as a function of the distance from the optical sensor. A point

  located downstream of the optical sensor, and o the beam as-

  tigma forms a generally circular spot is defined -4-

  as being a "zero" point of central focus (Z.O). In -

  upstream from this point towards the optical sensor, the astigmatic beam forms a generally elliptical spot (at Z = -ZO) and, identically, downstream from zero point the beam forms a generally elliptical spot (at Z = + ZO) oriented essentially at 90 from the upstream spot - ZO. So the form of

  the image projected on the photo-detector by the collec-

  tor depends on the distance between the optical sensor and the target surface. Consequently, by observing the output of each element of the photodetector, the distance between the sensor and the target surface can be kept constant and one can obtain a plane of the contours of the surface using

known drawing techniques.

  Another version of the invention is planned, a version in which a non-astigmatic light beam is projected onto the surface to be mapped. Spurious rear radiation reflected by the target surface is collected and passes through crossed cylindrical lenses to cause astigmatism in the received beam. The astigmatic beam is then focused on a photo-detector. As in the case of the first version, the shape of the received beam is a function of the distance separating the sensor from the target surface, which

  thus allows the production of a contour plan.

BRIEF DESCRIPTION OF THE DRAWINGS

  FIG. 1 represents the projection of an astigmatic light beam on the quadratic detector of the present invention. Figure 2 is a schematic representation of a

version of the present invention.

  -5- Figure 3 is a graphic illustration of the production of astigmatism in a light beam

  by the use of crossed cylindrical lenses.

  Figure 4 is a graph of the electrical output

  of the quadratic detector of Figure 1 in function

  tion of the distance between the sensor of this in-

vention and a target area.

  Figure 5 is another version of this

  invention shown in Figure 2.

  Figure 6 is a schematic illustration of the use of the present invention to obtain a

  drawing the surface contour of a target object.

  DETAILED DESCRIPTION OF THE INVENTION

  A contactless optical sensor with an application

  particular cation in the production of con-

  surface tour and measuring short distances is

  described. In the following description, given ex-

  many details are presented in the form

  specific quantities, lenses, materials and confi-

  gurations in order to allow an excellent understanding

  of the invention. However, it is evident to the spe-

  specialists that the invention can be implemented without specific details. In other cases, well known components such as detectors, electrical processing means, etc. have not been described in detail in order to

  not to unnecessarily interfere with the understanding of the invention

  tion. If we refer briefly to Figure 1, the --6--

  present invention comprises a sensor, indicated generally

  symbol 10, which projects a light beam

  astigmatic neux 12 on a target surface to reproduce

  in drawing. The section of the light beam 12 has a form

  me which varies as a function of the distance separating it from the sensor 10. As shown in FIG., the shape of the

  astigmatic light beam 12 is essentially circu-

  area at a point which is defined as the center hearth

  beam tral (CF) (Z = 0). Upstream (Z = -Zo) towards the cape-

  tor 10, the light beam 12 generally takes the

  shape of an ellipse oriented essentially vertical-

  is lying. The downstream form (Z = Zo) of the astigmatic beam 12 forms

  generally an ellipse which is oriented approximate-

  ment at 90 compared to the upstream form. As will be discussed, the use of an astigmatic beam makes it possible to precisely maintain the distance between the sensor 10 and the target surface constant, in order to produce the

  outline drawing of a target surface.

  If we now refer to Figure 2, the

  structure of the present invention will now be described

  you. The contactless optical sensor 10 comprises a source

  this light 14 which may include a laser, a light emitting diode or the like, to produce light of the desired wavelength. The light produced 16 passes through a collimating lens 17 to obtain

  essentially parallel rays. A provocative astigmatism

  that artificially is introduced into the light beam

  neux 16 using a pair of crossed cylindrical lenses 18 and 20. As best shown in Figure 3,

  the use in the present invention of cy-

  crossed lindrics cause the desired astigmatism in beam 16, so that there is production at home

  central (CF) of the astigmatic beam of an essential spot-

  symmetrical and circular. This central focus is defined - f -

  by ZwO along an essentially perpendicular Z axis

  Cular to the 18 eL 20 lenses it crosses, -vmm * the

  represents Figure 3. The shape of the beam 16 in u'ont.

  of the central focal point (Z - Zo) is generally an ellipse whose largest sat axis oriented vertically and is parallel to the main longitudinal axis die with a cylindrical lens 20 .. Downstream of the central focal point (3- +} Z la shape of the beam '16 once again takes the general form of an ellipce but with a larger axis * é "alés

  about 90 'compared to that of the upstream ellipse.

  Therefore, the shape of the beam 16 in Z- + Zo is that of ** ell1ipse whose largest axis is generally by * lt1 to the longitudinal axis of the cylindrical lens 18. The shape of the section of the beam 16 therefore depends from 4iis-,

  tance which separates it from crossed lenses. We pelt themselves.

  realize that, although the present invention uses crossed cylindrical lenses to cause artificial astigmatism, other methods can be

  using combinations of lenses or elements.

  can also be used to arrive at & -! i

me result.

  If we refer again to Figurt 2, after having crossed the crossed cylindrical lenses 18 and 20, the beam 16 is directed for example, by the beam splitter 2S, on a surface of cable 39 f

  draw. The surface 30 disperses the beam 16 selen 11l

  nature of the surface. For example, 'If the surface of ib is relatively smooth, a large portion of

  beam 16 will be reflected. If the target area 30.

  is rough or pitted and not planar, the bundle 16, ara 'dispersed in a certain proportion. As it PeWa di * çué, quvique lez radt * ristiq4es surface of the target surface 30 vary the amount of liet4 t -8- reflected, dispersed or absorbed, the added output value of the present invention will depend on the distance

  separating the sensor 10 from the surface 30.

  The back scattered light 32, which has been reflected from the target surface 30, is collected by a tracing lens 34 and focused on a detector

  quadratic 36 coupled to electronic circuits con-

  venables 37. Although the present invention uses a tracing lens 34 in order to receive the radiation dispersed by the beam 16 and which is projected onto the

  target surface 30, it is obvious that one can use

  a wide variety of collecting means for arriving

worm with the same result.

  As best shown in Figure 1, the

  quadratic protector 36 includes four photo-

  sensitive 40, 42, 44 and 46. When the distance between

  the crossed cylindrical lenses 18 and 20 and the over-

  target face 30 is equal to the distance to the central focus, the light scattered towards the rear 32 focused on the quadratic detector 36 forms a spot 35 essentially

  symmetrical and circular. So the output of each die

  quadratic tector will be essentially equal. In the case where the target surface 30 falls in the upstream portion

  of the astigmatic beam 16 which has a general ellip-

  tick with a main axis parallel to the Y axis (see

  Figure 3), the combined output of quadrants 40 and 44

  will pass that of quadrants 42 and 46. Conversely, if the target surface falls in the downstream portion of the projected astigm beam having an elliptical section generally disposed at 90 relative to the upstream section, the output value of the photosensitive elements 42 and 46

  will exceed that of elements 40 and 44 of detector 36.

  The output values of the four elements

  tectors are added and subtracted to give tensions of the form:

Vout = KP-

  Vo = KP + where K = proportionality factor

P- = P42 + P46 - P40 - P44

P + = P42 + P46 + P40 + P44

  and Pm = the total optical power incident on each detector element m, in which

m = 40, 42, 44 or 46.

  If we now refer to Figure 4, the sensor output is taken as the ratio

  Vout / Vo which is represented graphically as a function-

  tion of the distance between the target surface 30 and the central focus 16 of the sensor 10. The distance from the

  central focal point of the astigmatic beam 16 is me-

  on the abscissa axis in Figure 4, and the

  general output of the quadrant detector normalized as

  being Vout / Vo is carried on the coordinate axis.

  As expected, the detector output value reaches a maximum when the detector 36 is near one of the foci (Zo and Zo). Between the two foci, it is generally linear and of unique value. Outside the two homes, the signal is approaching

asymptotically from zero.

  Although the currently preferred version of the

  vention uses a four quadrant photodetector 36,

  it is evident that other combinations of elements

  tectors are possible. For example, the present invention

Z564602

  -10- tion can use a detector satisfactorily

  with two elements. In this case, the output of each é-

  the element is compared in order to identify the position of the cap-

  10 relative to the target surface 30, while basing

  health on the nature of the back scattered radiation. If we now refer to Figure 5,

  another version of the present invention is shown

  tee. Since many elements comprising the version of Figure 5 are essentially the same

  than those described with reference to Figure 2, we use

  for simplicity's sake, the same re-

  ference. The light source 14 produces the beam

  miners 42 which passes through the collimating lenses 44

  to project this beam onto a target surface 30.

  The back scattered radiation 50 reflected by the over-

  face 30 is directed by a beam splitter 52 or

  an identical element on the cross cylindrical lenses

  54 and 56. As in the version of Figure 2, the crossed cylindrical lenses 54 and 56 cause a

  artificial astigmatism in the received beam 50 and foca-

  read this beam on a 36-quadrant detector

  plé to appropriate electronic circuits. Like this

  was previously described, the detector output signal

  quadrant 36 provides signals that indicate the position of sensor 10 relative to the target surface

  30. It has been found that the use of the "astigma-

  reception tism "illustrated in Figure 5 delivers a detector output signal which is essentially the

  same as that shown in Figure 4. The two ver-

  sions shown in Figures 2 and 5 use a

  artificially induced astigmatism in order to make

  rier the geometric shape of a light beam directed towards a quadrant detector as a function of the distance

  between the sensor 10 and the target surface.

  If we now refer to Figure 6, a possible application of the present invention will be

  described. The sensor 10 is shown coupled by a

  seems to articulate arm 60 to a machine 62 for measuring coordinates. The articulated arm 60 has couplings 64, 66 and 68 with transducers to allow the arm to move in three dimensions (X, Y, Z). To obtain a contour drawing of the surface including the coordinates X, Y, Z for the points placed on a target (for example the aircraft 70), the articulated arm 60 is

  maintained so as to have a constant distance from

  geometry "d" of the target 70. This distance d is equal to the distance from the central focus of the astigmatic beam of the sensor 10. The output signal of the present invention at this spacing distance is defined as the point "zero" (Z = O). The output from sensor 10 can then be used to control the position of the sensor so that the central focus is maintained on the surface of the target. So the articulated arm 60 can be used to move the sensor 10 on the surface

  while maintaining constant spacing. The co-

  data for (X, Y, Z) each point of target 70 above

  above which the sensor 10 passes can then be obtained by appropriate processing of the signals delivered by the transducers 64, 66 and 68 so that a drawing of the outline of the surface of the target is produced by

  compared to an arbitrarily chosen initial starting point

finished as original.

  It can be seen that, although Figure 6 illustrates a use of the present invention in which the sensor 10 is used to keep the articulated arm 60 at constant distance from the surface of the target, this sensor 10 can also be used for - 12- directly measure the variations in the surface of the

  target. The measurement range of sensor 10 would be general-

  ment defined as the distance between the two foci -Zo and + Zo. Provided that the output of the quadrant detector 36 is essentially linear between each focal point, small distances and small variations in contour can be detected. So this

  invention has its utility for ensuring the control of

  for parts with tight tolerances, such as

  example of gears, optical and magnetic memories, etc. For better accuracy, non-linear processing can be used to linearize the signal

  over a wider range of measurement.

  So an improved non-contact optical sensor

  has been described, a sensor which has particular applications

  res for use in contour drawing and short distance measurements. The sensor is simple,

  reliable, and can be easily interfaced with the devices

  existing electrical and mechanical contour drawing devices. Although the present invention has been described with reference to Figures 1-6, it is obvious that the Figures are given only for illustrative purposes and should not be considered as limiting the invention. -13-

Claims (28)

  1. An optical sensor for determining the characteristics of the surface of a target, comprising: projection means for projecting an astigmatic beam of light onto said surface of the target; receiving means for collecting a portion of said light beam scattered behind said surface; detector means coupled to said receiving means for determining the position of this sensor relative to said surface, and based on the dispersion of said astigmatic beam on said surface; the characteristics of said target surface can thus be determined by passing said sensor on said surface.   2. The optical sensor defined by the claim   tion 1, wherein said detector means comprises a   light sensitive photodetector.   3. The optical sensor defined by the claim   tion 2, wherein said projection means comprises two cylindrical lenses arranged at 90 one of   the other so as to deliver an astigmatic beam.   4. The optical sensor defined by the claim   tion 3, wherein said detector is composed of at least two detector elements each of which provides a signal   output in response to light incident on said é- element.   5. The optical sensor defined by the claim   tion 4 in which the section of said astigm beam -14- varies as a function of the distance separating said sensor of said surface.   6. The optical sensor defined by the claim   tion 5, wherein said receiving means includes a representation lens for collecting said beam   astigma scattered back and reflected by said over- face.   7. The optical sensor defined by the claim   tion 6 and wherein said projection means comprises a light generating means.   8. The optical sensor defined by claim 7 and wherein said projection means further comprises a collimator lens disposed between said   medium light producer and said cylindrical lens than.   9; The optical sensor defined by the claim-   tion 8 and further comprising a beam splitter   to direct said astigmatic beam onto said surface.   10. The optical sensor defined by the claim   tion 5, in which said astigmatic beam forms an image of generally circular section in its central focus.   11. The optical sensor defined by the claim   tion 10 and in which said detector comprises four photosensitive elements (40, 42, 44, 46), the output signal of said elements of said detector being described by means of the following relations: -15- Vout = KP_ Vo = KP + o Vout = voltage total detector output K = proportionality factor 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 sensor for use in   information guard on the outline of a target surface, comprising: projection means for projecting an astigmatic light beam on said target surface; collecting means for collecting a portion of said light beam dispersed behind by said surface   detector means coupled to said collection means   tor to determine the position of said sensor relative to   port to said surface and based on the dispersion of said astigmatic beam on said surface; by which information on the contour can be obtained by passing said sensor over said target surface.   13. The optical sensor defined by the claim   tion 12 and wherein said detection means comprises   a photodetector with four photosensitive elements   bles, each element delivering an output signal in   response to the light incident on said element. -16-   14. The optical sensor defined by the claim   tion 13 and in which said projection means comprises   te two cylindrical lenses arranged at 90 one of   the other so as to provide an astigmatic beam.   15. The optical sensor defined by claim 14 in which the total power found in each of said four elements of said detector is generally described by the following relationships: Vout = KP-   Vo = KP + in which Vout = total detector output voltage K = proportionality factor 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 16. An optical sensor for determination   characteristics of the surface of a target, including   nant: a projection means for projecting a light beam on said target surface; a collecting means for collecting a portion   of said beam dispersed rearward from said over-   face and to cause astigmatism in said beam   collected skin; detector means coupled to said collecting means for determining the position of said sensor relative to said surface based on the detected astigmatism   produced in said collected beam; - -17-   by means of which the characteristics of ladi-   The target area can be determined by doing   pass said sensor over said surface.   17. The optical sensor defined by the claim   tion 10 and in which the section of said beam astig-   mate collected varies according to the distance between re said sensor and said surface.   18. The optical sensor defined by the claim   tion 17, in which said collecting means comprises two cylindrical lenses arranged 90 apart from each other to cause said astigmatism in said beam collected.   19. The optical sensor defined in the claim   tion 18 and in which said detector means comprises a   photodetector having at least two photosensitive quadrants   to produce signals indicating the amount of   incident light on each quadrant.   20. The optical sensor defined in the claim   tion 19 and wherein said photodetector has four qua-   drants (40, 42, 44, 46), whose photosensitive surfaces are generally identical.   21. The optical sensor defined by the claim   tion 20 and wherein the output voltage of said detector   quadrant is described by the following relation: Vout = KP-   Vo = KP + in which: Vout = total detector output voltage K = proportionality factor -18- 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 sensor defined by the claim   tion 20 in which said projection means comprises   a light-generating source and a colli- matrix.   23. An optical sensor to draw information   tion on the outline of a target surface, comprising: projection means for projecting a light beam on said target surface; a collecting means for collecting a portion   of said beam reflected by said surface and for pro-   evoke astigmatism in said collected beam; detector means coupled to said collecting means for determining the position of said sensor with respect to   said surface and based on the astigmatism detected, pro-   voiced in said collected beam by means of which information on the contour   can be obtained by passing said sensor over the said target surface.   24. The optical sensor defined by the claim   tion 23, in which the section of the astigmatic beam varies as a function of the distance which separates said sensor and said surface.   25. The optical sensor defined by the claim   tion 24, wherein said detector means comprises a photodetector having four quadrants (40, 42, 44, 46)   whose photosensitive surfaces are generally identical ticks.   26. The optical sensor defined by the claim   tion 25, in which the total power found in each of said four quadrants is generally defined by the following relation: Vout = KP-   Vo = KP + in which, Vout = total detector output voltage K = proportionality factor 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 F46 = optical power incident on element 46   27. The optical sensor defined by the claim   tion 24 in which said collecting means comprises two cylindrical lenses arranged at 90 from one another to cause said astigmatism in said collected beam.   28. The optical sensor defined by the claim   tion 27, wherein said projection means comprises a light generating means.