Abstract:
A contour reading method for an item such as an eyewire for spectacles, the method using an appliance including a support for holding the item, and a rotary platform which is rotatably mounted in relation to the support about a rotational axis and carries a reading sub-set provided with a sensor consisting of a finger and a rod, the finger having a distal end. The distal end is displaced along the contour and successive positions thereof are detected. The method is characterized in that the displacement step includes a measuring step wherein the effort exerted between the platform and the rod is measured parallel to the rod, and a control step wherein the distance between the finger and the platform is controlled according to the effort in such a way that the intensity of the effort remains lower than a pre-determined threshold.

Description:
FIELD OF THE INVENTION 
   The invention relates to the general field of dimensional measurement devices used in the fabrication of eyeglasses adapted to a particular wearer. 
   The invention relates more particularly to a contour reading method and a contour reading device adapted, with the aid of a feeler, to determine the shape of an article disposed on a support. The palpated article is generally an eyeglass frame rim or an ophthalmic lens. 
   BACKGROUND OF THE INVENTION 
   A device of this kind is used, for example, to determine the shape of the bezel of a frame rim, i.e. the groove that runs around the interior of the frame rim and retains an ophthalmic lens in the frame rim. To this end, the feeler is inserted into the bezel and follows the contour of the bezel while the device measures the coordinates of the position of the feeler along its path, thereby storing a digital image of the contour of the bezel. An ophthalmic lens blank can then be trimmed to the dimensions of the bezel so that it can be inserted perfectly into the frame rim. 
   A contour reading device of the above kind is known from the document U.S. Pat. No. 5,121,550. The device includes a support for holding an article to be palpated. A circular platform is mounted to turn relative to the support and carries a reading subassembly that includes a feeler. The feeler includes a rod and a finger extending from said rod. The distal end of the finger is adapted to move along the contour. 
   The reading subassembly includes a slideway on which a mobile carriage attached to the feeler is mounted. 
   This device measures polar coordinates along the path of the feeler where the angular dimension (θ) corresponds to the rotation of the rotary platform and the radial dimension (ρ) corresponds to the movement in translation of the carriage on the slideway. 
   The finger moves in the bottom of the bezel of the rim and remains in contact with the latter because of the effect of a radial force applied to hold the finger against the bezel. 
   The article to be palpated being in three dimensions, the feeler must be able to move along an axis (z) corresponding to a movement transverse to the plane comprising the dimensions (ρ, θ). 
   These devices include a motor initially driving the feeler along the axis (z) until it faces the bezel. Once the finger has been brought into contact with the bezel, by movement in translation along the slideway, said motor is disengaged or passive so that, during the reading of the contour, movement along the axis (z) is unimpeded. 
   If the article to be read is a frame rim having moderate curvature, the movement of the feeler along the axis (z) is of small amplitude and the finger easily remains in contact with the bottom of the bezel. 
   On the other hand, in the case of frame rims with highly curved portions, where the amplitude along the axis (z) is therefore high, the free movement of the finger along the axis (z) makes it more difficult for the finger to remain in the bottom of the bezel. 
   To limit the risk of movement away from the bottom of the bezel, it is necessary to reduce the rotation speed when the finger includes a highly curved portion, for example. 
   There is also known from the document U.S. Pat. No. 6,325,700 a contour reading device including calculation means for predicting the amplitude of the contour to be read in three directions, and in particular along the axis (z), with the aim of preventing the finger moving away from the bottom of the bezel. These calculation means estimate the evolution along the axis (z) of the portion to be traveled as a function of the measured coordinates of the portion already traveled. A motor for positioning the feeler along the axis (z) is controlled as a function of the calculated estimate. The feeler is guided as it follows the contour but there is nothing to guarantee that the finger will remain in the bottom of the bezel. 
   SUMMARY OF THE INVENTION 
   The object of the invention is to provide a contour reading method for alleviating the drawbacks mentioned above at the same time as being particularly simple and convenient to use. Another object of the invention is to propose a reading device adapted to any type of contour suitable for the implementation of a method of this kind. 
   To this end, the invention proposes a contour reading method, for articles such as an eyeglass frame rim, wherein a device is used including a support for holding said article, a rotary platform mounted to rotate relative to the support about a rotation axis, this rotary platform carrying a reading subassembly which includes a feeler featuring a finger and a rod extending transversely to said finger, said finger having a distal end, said method including the step of displacing said distal end along said contour and of measuring successive positions of said distal end, characterized in that said displacement step includes the step of measuring the force exerted between the platform and the rod, parallel to the latter, and the step of controlling the distance between said finger and said platform as a function of said force so that the intensity of said force remains below a predetermined threshold. 
   Accordingly, the finger is at all times at a distance from the rotary platform corresponding to a particular force exerted by the article to be read on the feeler, which enables the finger to follow a predetermined path and in particular prevents the finger from dropping off the contour. Moreover, the force is measured in only one direction, which simplifies measurement. 
   The invention also proposes a contour reading device, for articles such as an eyeglass frame rim, including a support for holding said article, a rotary platform mounted to rotate relative to the support about a rotation axis, this rotary platform carrying a reading subassembly which includes a feeler featuring a finger and a rod extending transversely to said finger, said finger having a distal end adapted to be displaced along said contour, characterized in that said subassembly further includes a member, to which said rod is fixed, extending globally transversely to said rod, a drive motor of said member for controlling the distance between said finger and said platform, a force sensor disposed on said member to determine the force to which it is subjected in a direction parallel to said rod, and control means connected to said sensor and to said motor adapted to control said motor so that the intensity of said force remains below a predetermined threshold. 
   The position of the member relative to the rod enables use of the relatively large receiving space available under the platform. Moreover, the force sensor being away from the finger in this receiving space, it is rendered inaccessible from the outside so that it is protected from unintentional misadjustment by the user or from dust. 
   In one embodiment, said control means are adapted to control said motor so that the intensity of said force remains below a first threshold if the force is applied in a first sense and remains below a second threshold if the force is applied in a second sense opposite the first sense. If the feeler is reading a frame rim, the finger is then at all times very close to the bottom of the bezel where there is no force and reading can be effected at high speed. 
   It will be noted that there is already known from the document U.S. Pat. No. 5,341,079 a copying machine including a feeler head and a finger, in which machine the feeler head includes a force sensor for determining the force applied to the finger by the groove. This sensor measures microdisplacements of the finger in three directions, taking the feeler head as reference, these microdisplacements corresponding to the forces applied to the finger by the groove. 
   The document U.S. Pat. No. 5,477,119 describes a groove tracing device including a feeler head of the above kind in which, as a function of the sensed force, the means for driving the feeler head are commanded to zero the force in a direction transverse to the finger and to make the resultant of the force along the main axis of the finger equal to a predetermined value. The finger then remains centered in the groove and in contact with the groove bottom. The device is furthermore provided with a position sensor for sensing the position of the feeler along the contour in three directions. The position is measured with the fixed support of the device as a reference. 
   Apart from the positioning of the force sensor at the distal end, where it is particularly exposed, whereas it should have a small volume, this sensor measures the forces in three directions and adjust the position of the feeler head in these three directions, which is relatively complex (the device according to the invention is capable of achieving the same result with only one direction). 
   According to other implementation features that are particularly simple and convenient both in terms of fabrication and in terms of use:
         said member is a first support member, the subassembly further including a second support member projecting transversely relative to said platform and on which the first support member is mounted in translation in a direction parallel to the rotation axis; and, where applicable   the rotary platform includes an opening through which said rod extends, said finger being situated on a first side of said platform whereas said second support member, said first support member and a proximal end of said rod opposite said finger are situated on a second side of said platform; and/or   the second support member includes a support shaft; and/or   the first support member is a support arm including a plate having two ends from which two parallel blades extend transversely, each blade being deformable parallel to said rotation axis, and a sleeve extending between two ends of the blades opposite the plate, said sleeve being adapted to grip said proximal end of said rod; and, where applicable   each blade has at one of its ends, opposite the plate, a circular opening adapted to receive said rod gripped in said sleeve; and/or   said second support member includes a support shaft and said support arm includes a ring centered on said support shaft via a ball bush enabling said translation of said ring relative to said support shaft; and, where applicable   said drive motor includes a pinion meshing with a rack fastened to said ring; and/or   said drive motor includes a coder for measuring the position of the first support member relative to the second support member; and/or   said first member is further mounted to turn about said second member; and/or   said force sensor includes a detection cell and a flag mounted face to face; and, where applicable   said detection cell is an optoelectronic cell; and/or   said first support member is a support arm including a plate having two ends from which two parallel blades extend transversely, said detection cell and said flag being situated between the two blades; and, where applicable   the detection cell and the flag are fixedly mounted on the plate and on a sleeve extending between two ends of the blades opposite the plate, respectively, the subassembly including a signal processing unit connected to said drive motor, the cell including means for detecting the position of the flag relative to itself and means for sending a signal to said processing unit as a function of said position;   said feeler is mobile both parallel to said rotation axis and transversely to said rotation axis.       

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The characteristics and advantages of the invention will emerge from the following description, given by way of preferred example, with reference to the appended drawings, in which: 
       FIG. 1  is a perspective view of a contour reading device according to the invention; 
       FIG. 2  is similar to  FIG. 1 , the contour reading device receiving an eyeglass frame whereof the shape of the rims is to be read by the feeler; 
       FIG. 3  is a perspective view from below of the rotary platform showing the reading subassembly carried by the rotary platform; 
       FIG. 4  is a perspective view of a portion of the reading subassembly including the feeler, the support arm and the force sensor; 
       FIG. 5  is a diagrammatic view of the rod and of the blades of the support arm, in a situation in which a force is applied to the feeler; 
       FIGS. 6   a  to  6   c  are diagrammatic representations of three configurations of the orientation of the forces operative on the feeler, the finger and the groove as a function of the position of the finger in the groove; and 
       FIG. 7  is a theoretical diagram of the control of the position of the feeler. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is a general view of a contour reading device  1  as seen by its user. This device  1  includes an upper cover  2  covering the whole of the device except for an upper central portion. 
   A rotary platform  6  is mounted to rotate relative to the chassis  5  of the device  1 . This rotary platform  6  includes an oblong opening  7  of circular arc shape through which projects a feeler  8  including a rod  12  provided at its distal end with a finger  9 . 
   The device  1  from  FIG. 1  is intended to receive a frame  10 . 
   As seen in  FIG. 2 , the device  1  includes a device for gripping the frame  10  featuring two parallel jaws  3  and four clamps. Each clamp includes two studs  4  and is situated on one jaw  3  facing a clamp of the other jaw  3 . 
   At least one of the jaws  3  is adapted to move towards or away from the other one to grip the frame  10 . In the position with the frame  10  gripped, the clamps are situated face-to-face on each rim, in a direction that corresponds to a vertical direction when the frame is being warm and a horizontal direction when the frame is placed in the device  1 . The studs  4  of the same clamp may be moved towards each other to clamp the rims and hold the frame  10  for reading. 
   In  FIG. 2 , the jaws  3  have been moved towards each other to hold the frame  10  horizontal. The studs  4  have been closed onto the frame rims. Each of the rims of the frame  10  is therefore ready to be palpated along a path starting with the insertion of the feeler at a predetermined location situated between two studs  4  and then along the bezel of the frame  10  to cover the whole of the circumference of the frame rim. 
   Note that in this example the machine  1  is equipped with jaws adapted to hold a frame but that any other gripping device may be employed, for example a clamp for holding an ophthalmic lens whose external contour must be read. 
   The chassis  5  takes the form of a support table in which there is a circular cutout  11  of slightly larger diameter than the rotary platform  6 . The circular cutout  11  receives the rotary platform  6 , which is guided by three guide rollers (not visible) regularly disposed along its periphery. 
   Alternatively, the rollers are driven by a motor-coder (not shown) enabling controlled rotation of the rotary platform  6  and reading of its angular position at any time. 
   The circular-arc-shaped opening  7  has a length approximately corresponding to the radius of the rotary platform  6  and extends between the center of the rotary platform  6  and its periphery. 
   Also, the opening  7  is centered with respect to an axis A ( FIG. 3 ). 
   As seen in  FIG. 3 , a groove  14  is provided on the edge of the rotary platform  6 , over the whole of its circumference. This groove  14  is used to retain and to drive the platform  6  relative to the chassis  5 , thanks to the motorized rollers. 
   The rotary platform  6  carries a reading subassembly  15  including a bearing  16  on which is mounted a support shaft  17  fixed to the rotary platform  6  by a rivet (not visible). This shaft  17  is centered on the axis A. 
   A support arm  18  is mounted on the support shaft  17  by means of a ball bush  19 . The support arm  18  has at one of its ends a ring  20  surrounding the ball bush  19  and the support shaft  17 , the ball bush  19  enabling the support arm  18  to move in rotation about the axis A and to move in translation along that axis. 
   At its end opposite the ring  20 , the support arm  18  includes a cylindrical sleeve  21  in which the rod  12  is gripped so that it is parallel to the support shaft  17 . 
   The support arm  18  is described next with reference to  FIG. 4 . 
   The support arm  18  is a globally square component. At the two ends of a substantially rectangular plate  40  there are two identical transversely extending blades  41 ,  42 . 
   Here each blade  41 ,  42  is triangular and has a rounded end opposite the plate  40 . At this end, each blade  41 ,  42  has a circular opening (not shown) intended to receive the sleeve  21  gripping the rod  12 . 
   The plate  40  carries a reinforcing plate  49  fixed to the plate  40  by two pairs  43 ,  44  of screws. The pairs  43 ,  44  of screws also fix the plate  40  to the ring  20 . A first pair  43  comprises two screws flush with the surface of the plate  40 . 
   The second pair  44  comprises two longer screws each of which extends transversely to the plate  40  through a sheath  45  to fix a cell support  46  to its surface opposite the plate  40  and parallel to the plate  40 . The support  46  carries an opto-electronic cell  47  having two walls  48  parallel to each other and transverse to the blades  41 ,  42  and to the plate  40 . This cell  47  is adapted to sense the position of a flag  51  between its two walls  48  by measuring a luminous flux. One of the walls  48  includes an optical emitter and the other wall includes an optical receiver, the receiver producing an output signal having a value that varies as a function of the quantity of light received from the emitter. The blades  41 ,  42 , the cell  47  and the flag  51  form a force sensor  64 . The electrical signal is transmitted by signal sending means (not visible) from the cell  47  to a motor-coder  31  described hereinafter. The sending means may be a radio sender, a connecting wire, etc. 
   The feeler  8  includes the feeler rod  12  carrying the finger  9  at its distal end opposite the arm  18 . The rod  12  is centered and fixed relative to the two openings by the sleeve  21  that surrounds it between the two openings and extends a short distance towards the finger  9 . The rod  12  is fixed at its proximal end, opposite the finger  9 , to the opening of the corresponding blade  41  by a nut and bolt  50 . 
   The sleeve  21  carries between the two blades  41 ,  42  the flag  51 , which extends so that one of its corners occupies a portion of the space between the two walls  48 . 
   By virtue of its shape, each blade  41 ,  42  has an overall stiffness. Nevertheless, each blade  41 ,  42  has slight flexibility so as to be able to curve in a direction parallel to the rod  12 . 
   The presence of the two blades  41 ,  42  retaining the feeler  8  provides transverse flexibility of the support arm  18 . Moreover, retaining the feeler  8  at the level of the two openings avoids the pivoting of the feeler  8  that could occur if the latter were on a single blade. 
   Thus, the force sensor  64  is positioned in an area of the device  1  featuring a relatively large receiving space. When the device  1  is ready for use, the force sensor  64  is not accessible from the outside and is therefore protected from dust and also from intervention, in particular involuntary intervention, by the user of the device  1 , which prevents any misadjustment of the sensor  64 . 
   The fact that the support arm  18  is mounted to turn about the axis A enables the feeler  8  to move along the opening  7  in a circular arc in a plane transverse to the rotation axis (R) of the rotary platform  6 , that rotation axis (R) here being parallel to the axis A. Moreover, the feeler  8  can effect an entry/exit movement of the finger  9  relative to the surface of the rotary platform  6  if the support arm  18  is caused to slide along the axis A. When the device is in operation, only the finger  9  and a portion of the rod  12  project above the platform, the remainder of the subassembly  15  being situated on its other side. 
   The reading subassembly  15  also includes a guide arm  22  attached to the base of the shaft  17  via a bearing (not visible) allowing the guide arm  22  to move only in rotation about the axis A. This guide arm  22  has a length sufficient to reach the opening  7  and includes a fixed rest  24  and a bearing  25  facing the opening  7 . 
   The fixed rest  24  and the bearing  25  are disposed side by side with a mutual separation substantially corresponding to the thickness of the rod  12 . 
   By virtue of the rotational mounting of the guide arm  22  on the support shaft  17 , the fixed rest  24  and the bearing  25  remain facing the opening  7  whatever the angular position of the guide arm  22  about the axis A. The support arm  18  and the guide arm  22  are disposed so that the rod  12  is gripped between the fixed rest  24  and the bearing  25 . Angular movements of the support arm  18  and the guide arm  22  about the axis A are therefore effected conjointly. 
   The guide arm  22  further includes a toothed semicircular portion  26  of arcuate shape centered on the axis A. The teeth of the portion  26  mesh with an intermediate pinion  27  that itself meshes with the pinion (not visible) of a motor-coder  28  mounted on a yoke  29  fixed to the rotary platform  6 . To make the drawings clearer, the teeth of the intermediate pinion  27  are not represented. 
   The guide arm  22  further includes a vertical yoke  30  parallel to the axis A, to which is fixed a motor-coder  31  the pinion  32  whereof meshes with a rack  33  fixed to the ring  20  of the support arm  18 . The rack  33  is parallel to the axis A. For the same reasons of clarity as before, the teeth of the pinion  32  are not represented. 
   The motor-coder  31  includes control means (not visible) adapted to receive an electrical signal and selectively to command movement of the pinion  32  as a function of that signal. 
   The coders of the motor-coders  28 ,  31  measure the position of the feeler  8  in the radial and vertical directions. 
   As seen in  FIGS. 5 and 6   a  to  6   c,  during reading, the feeler  8  is liable to exert a contact pressure on the contour. This pressure results from a radial force applied by means of the motor-coder  28 . 
   Here the contour to be read is a bezel  60  of a frame rim. The bezel  60  takes the form of a V-shaped groove with two branches  61 ,  62 . The finger of the feeler moves in the bezel on the two branches  61 ,  62 . The junction of the branches  61 ,  62  is referred to as the bottom  63  of the bezel  60 . 
   The direction parallel to the axis A and to the feeler rod  12  is referred to as the direction along z and the direction of the radius of curvature of the bezel  60  is referred to as the direction along ρ. 
   The contact pressure is applied along ρ, which generates a force along z applied by the bezel  60  to the tip  65  of the finger  9 . When the tip  65  is in contact with the branch  61 , the force along z is directed in a first sense, towards the branch  62 . When the tip  65  is in contact with the branch  62 , the force along z is directed in a second sense, towards the branch  61 . When the tip is against the bottom  63  of the bezel  60 , the resultant of the force applied to the bezel in the direction along z is zero. 
     FIG. 5  shows the state of the blades  41 ,  42  when the tip  65  is situated against the branch  62 . The force applied to the tip  65  in the direction along z is transmitted to the remainder of the feeler  8 , which is reflected in deformation of the blades  41 ,  42  fastened to the feeler  8 . 
   The flag  51  fastened to the feeler  8  effects a microdisplacement in front of the stationary cell  47  resulting from the deformation of the blades  41 ,  42 . A change of luminous flux is detected by the cell  47 . 
   As shown in  FIG. 7 , the sensor  64  is connected by a cable to a processing unit  66  adapted to receive an electrical signal emitted by the sensor  64  and corresponding to the luminous flux. The processing unit  66  is itself connected by a cable to the motor  31 . The electrical connection between the processing unit  66  and the motor  31  is a bidirectional connection. 
   The processing unit  66  processes the signal received from the sensor  64  by comparing it to a predetermined set point, corresponding here to a zero value of the force, and then sends a corresponding output signal to the motor  31 . On the other hand, the motor  31  sends the processing unit  66  a signal corresponding to the position of the feeler  8  along the axis (z). 
   In concrete terms, the detection of a microdisplacement of the flag  51  relative to a reference position leads to the cell  47  of the sensor  64  sending the processing unit  66  a signal corresponding to the microdisplacement and in particular to its orientation. The processing unit  66  effects a comparison with a predetermined set point held in the processing unit  66 . Provided that the value is greater than the set point, the processing unit  66  commands the motor  31  to activate the pinion  32 . 
   The pinion  32  then meshes with the rack  33  in the direction opposite to the microdisplacement of the feeler  8 , i.e. in the direction reducing the force on the finger  9  along the axis (z). 
   More generally, the reading of a contour may start with the placing of the feeler finger  9  against the contour, disposing it at the required height by means of the motor  31  and maintaining the contact between the finger  9  and the contour by means of the motor  28 , which is controlled so that the feeler  8  exerts a constant contact pressure against the contour. It will be noted here that retaining the finger  9  in the bottom  63  of the bezel  60  reduces the value of the applied radial force compared to the applied radial force in prior art devices. It will also be noted that it suffices to maintain the pressure in the radial direction constant. 
   The rollers are then driven so that the rotary platform  6  performs a complete turn, corresponding to a complete circuit of the contour to be palpated. 
   Also, the rotation speed of the rotary platform  6  may be relatively fast. 
   During this rotation, the sensor  64  measures the force exerted on the arm  18  corresponding to the force applied by the article to the finger  9 . The motor  31  controls the distance between the finger  9  and the platform  6  as a function of the force by driving the feeler  8 . 
   More precisely, the motor-coder  31  tracks the set point coming from the processing unit  66 . If the force corresponds to the set point, the motor operates only as a coder for measuring the successive positions of the support arm  18  along the axis A (corresponding to the height of the finger  9  relative to the platform  6 ) while the finger  9  is following the shape in which it is engaged. 
   According to a variant, a non-zero threshold value of the force sensed by the cell  47  is defined below which the tip  65  is considered to be sufficiently close to the bottom  63  of the bezel  60  even if the tip  65  is not exactly at the bottom of the bezel. 
   According to another variant, two distinct sensed force threshold values are defined. When the finger is in contact with the branch  62 , i.e. when the force is directed towards the branch  61 , the force threshold value not to be exceeded is a first of the preceding two values. When the finger is in contact with the branch  61 , it is the second threshold value that is taken into account. 
   According to an embodiment not shown, and subject to the necessary adaptations, the invention applies equally to reading a lens: if the lens includes a groove, the finger is then identical to that previously described for a frame rim. If the lens has a convex profile, the back of the feeler then features a recess. The recess also receives forces in the direction z that are sensed by a force sensor as previously described. 
   According to embodiments that are not shown, the force sensor is a strain gauge or a piezo-electric spacer. 
   One embodiment of the device according to the invention has been described, but the invention nevertheless applies equally to devices featuring different embodiments of the kinematics of the feeler, the necessary adaptations of the components enabling the various movements then being effected.