Abstract:
A sensor device is for performing rapid optical measurement of distances according to the confocal optical imaging principle in order to determine distance values and/or height values of a surface. The sensor device includes a point-shaped light-emitting element that is arranged on an optical axis, a reflector, which is arranged perpendicular to the optical axis and which can be displaced along the optical axis, imaging optics centered with regard to the optical axis, and a point-shaped light-receiving element that is situated at the same location as the light-emitting element. The components of the sensor device are arranged in such a manner that: light emitted by the light-emitting element is reflected by the reflector; while the reflector is displaced, the imaging optics focus the light reflected by the reflector at least once onto the surface to be measured; the focussed light is at least partially backscattered by the surface to the measured, and; while the reflector is displaced, the imaging optics focus, via the reflector, the backscattered light at least once onto the light-receiving element.

Description:
This application is the national phase under 35 U.S.C. § 371 of PCT International Application No. PCT/DE02/01748 which has an International filing date of May 15, 2002, which designated the United States of America and which claims priority on German Patent Application number DE 101 25 885.2 filed May 28, 2001, the entire contents of which are hereby incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The invention generally relates to a sensor device. Preferably, it relates to one for performing rapid optical measurement of distances and/or for determining distance values and/or height values of a surface. In particular, it relates to the measurement of three-dimensional surfaces. 
   BACKGROUND OF THE INVENTION 
   For assembly systems in the field of electronics manufacture, such as—for example—for automatic assembly systems for printed circuit boards, ever greater demands are being placed on the positional tolerances of the components due to the increasing miniaturization of components. The requirements in terms of assembly speed have likewise increased sharply in the last few years and will continue to increase still further in the future. These high expectations indicate that, in the future, assembly systems will require high-precision and rapid distance or height sensors as well as high-precision and rapid positional sensors for components. Such sensors will be used, for example, for checking the coplanarity of the terminal posts of the components. Distance or height sensors can also be used to control the distance between a component and the circuit substrate during the assembly process. 
   Until now, known devices for detecting height raster images including a multitude of three-dimensionally located points of object surfaces have essentially been based on the so-called triangulation process, in which a laser beam touches the surface of the object to be inspected. While the two planar location coordinates of a specific point on the surface area are known due to the current positioning of the laser beam, the height coordinate of the surface point currently to be measured is detected by at least one laterally positioned objective combined with a location-sensitive detector. Sequential illumination with the laser beam of the three-dimensional surface to be investigated thus enables the surface to be measured. 
   The resolution of optical distance sensors which use the triangulation procedure depends on the so-called triangulation angle. Since future miniaturization of components will also call for an improvement in the resolution of distance sensors, the triangulation angle in such optical distance sensors must be increased. However, this leads to a considerable increase in existing shadowing problems. 
   A further disadvantage of optical distance sensors based on the triangulation process is that the surfaces to be studied can have different optical diffusion factors. Precise distance measurements may only be possible for objects with surfaces that have an isotropic diffusion factor, i.e. a diffusion factor that is equally strong in all directions, for the laser beam as it falls on the surface. Such an isotropic diffusion factor is not usually guaranteed, particularly in the case of metallic reflective or even transparent surfaces. 
   A known method for the three-dimensional measurement of surface structures is based on the so-called confocal principle, in which a point-shaped light source, which is usually defined by an aperture plate, is imaged onto the surface of the object to be measured. The light backscattered by the surface is in turn imaged onto a virtually point-shaped detector, which is likewise usually defined by an aperture plate. The light falling on the detector is at maximum intensity when both the object level and the detector level are actually in the focal point of the respective lens. If the surface of the object to be investigated is outside the focal point, the measured beam is widened in front of the point-shaped detector and the measurable intensity decreases greatly. 
   A known sensor device for the optical measurement of distances according to the confocal principle is explained below with the help of  FIG. 1 . The sensor device  100  has a light source  101 , a first aperture plate  102 , a beam splitter  103 , a second aperture plate  104 , a light detector  105  and an objective  106 . The first aperture plate  102  is arranged directly in front of the light source  101 , so that the system comprising the light source  101  and the first aperture plate  102  functions as a point source of light, which has an effective light-emitting surface that corresponds to the cross-section of the opening in the first aperture plate  102 . Correspondingly the second aperture plate  104  is arranged directly in front of the light detector  105 . The light detector  105 , the second aperture plate  104 , the beam splitter  103 , the objective  106  and the point that is currently being captured on the surface to be measured (not shown) lie on an axis which coincides with the optical axis of the objective  106 . 
   The path of the beam in the sensor device  100  is explained below. The light emitted from the point source of light falls first on the beam splitter  103 . This is positioned diagonally relative to the optical axis so that the light emitted from the point source, after being reflected off the beam splitter  103 , is directed onto the objective  106 , and is focused by this objective onto a focal point  107  which is located at least close to the surface to be measured. The light reflected back at least partially by the surface is in turn imaged onto the second aperture plate  104  by means of the objective  106 . Only the light which is transmitted without further deflection by the beam splitter  103  is involved in this imaging process. 
   The two plates  102  and  104  are arranged confocally relative to the focal point of the objective  106 , i.e. the distance between the second aperture plate  104  and the objective  106  is equal to the sum of the distances between the first aperture plate  102  and the beam splitter  103  and between the beam splitter  103  and the objective  106 . For the optical imaging processes inside the sensor device  100 , the aperture plate  102  is imaged onto the focal point  107  via reflection off the beam splitter  103  from the objective  106 , and the focal point  107  is imaged onto the second aperture plate  104  by way of the objective  106 . 
   The actual distance measurement takes place in that the entire sensor device  100  is displaced in direction z  108  relative to the surface to be measured (not shown). While the device is displaced, the light intensity measured by the light detector  105  is detected. The course  109  of the measured light intensity as a function of the distance between the sensor device  100  and the surface to be measured is drawn in the insertion  110 . 
   A maximum level  111  appears precisely when the focal point  107  is lying directly on the surface to be measured. In other words, the maximum level  111  is achieved when the opening in the first aperture plate  102  is imaged onto the smallest possible area on the surface to be measured. In the confocal arrangement of the two aperture plates  102  and  104 , the illuminated area on the surface to be measured (not shown) is imaged by way of the objective  106  onto the smallest possible area, which coincides with the opening in the second aperture plate  104 . The distance from the corresponding point on the surface to be measured to the sensor device  100  can be determined from the course  109  of the light intensity, in particular from the precise position of the maximum level  111 . An entire three-dimensional surface profile of an unknown structure is then determined by sequential distance measurements between the sensor device  100  and individual points on the surface to be measured. 
   According to the confocal principle the illumination and detection path are identical, i.e. the light falling on the surface to be measured and the light reflected by the surface to be measured run coaxially and shadowing phenomena can generally be disregarded. 
   Furthermore measurement of the distance by way of the sensor device  100  does not require the absolute value of the light intensity reflected back to be measured; the relative light intensity which is measured by the point-shaped light detector according to the displacement of the sensor device  100  in direction z  108  is sufficient. Thus any measurement of distance by the sensor device  100  is done almost without regard to the dispersal or reflection characteristics of the object surface to be measured. The use of a point-shaped light detector also has the effect that multiple reflections onto three-dimensional object surfaces do not lead to false measurements. A further important advantage of the confocal method is that it is highly accurate to sub-micrometers, which means that the accuracy requirements associated with the increasing miniaturization of components can easily be met. 
   One disadvantage of the sensor device  100  shown in  FIG. 1  is that the entire sensor device  100  must be displaced relative to the surface to be measured in order to measure the distance. A further disadvantage is that several optical components must be used, and it has therefore not yet been possible, cost-effectively, to produce compact confocal sensors with small dimensions. 
   An optical distance sensor based on the confocal principle is known from WO 93/11403, and includes the following features: 1) the measuring beam is substantially larger than the lighting beam with regard to diameter, 2) the diameters of measuring beam and lighting beam are approximately equal at the point of measurement, whereby the lighting beam has a greater depth of focus than the measuring beam, and 3) a beam splitting unit with a number of beam splitters for dividing the measuring beam into a number of partial beams, with an approximately point-shaped photodetector arranged in each partial beam. 
   A further optical distance sensor based on the confocal principle is known from DE 19608468, with 1) a transmitting unit having a number of point-shaped light sources, which are imaged onto the surface of an object to be measured, 2) a receiving unit having a number of corresponding point-shaped light detectors in the same quantity, which are arranged confocally in the measurement region on the image side, whereby the point-shaped light sources and the corresponding point-shaped light detectors are arranged linearly in a plane which is orthogonal to the optical axis and generate a row of scanning points in a straight line on the surface of the object, 3) coaxial guidance of the illumination and measurement beams, and 4) a periodically variable optical path length between the receiving unit and the imaging optics. 
   SUMMARY OF THE INVENTION 
   An object of an embodiment of the invention is to provide a compact, high-resolution distance sensor which includes a high data rate and low sensitivity both with regard to the diffusion factor of the surface to be measured and also with regard to secondary light reflexes. 
   An object may be achieved in that a sensor device for performing rapid optical distance measurement according to the confocal imaging principle in order to determine distance values and/or height values of a surface, in particular for three-dimensional surface measurement, with a point-shaped light-emitting element arranged on an optical axis, a reflector which is arranged perpendicular to the optical axis and which can be displaced along the optical axis, imaging optics centered with regard to the optical axis and a point-shaped light-receiving element that is situated at the same location as the light-emitting element, whereby the light transmitted from the light-emitting element is reflected by the reflector. Further, while the reflector is displaced, the imaging optics focus the light reflected by the reflector at least once onto the surface to be measured, the focussed light is at least partially backscattered by the surface to be measured. Further, while the reflector is displaced, the imaging optics focus, via the reflector, the backscattered light at least once onto the light-receiving element. 
   Compared to conventional sensor devices for performing optical measurement of distances according to the confocal imaging principal, in the sensor device according to an embodiment of the invention, in particular, it is no longer necessary for the entire sensor device to be displaced relative to the surface to be measured; only the reflector arranged on the optical axis needs to be displaced. Thus only a relatively small mass needs to be displaced compared to the entire sensor device, so that the displacement can be done with high frequency, and therefore a high data rate is guaranteed. 
   According to one embodiment of the invention, the reflector is a plane mirror. Implementation of the reflector using a plane mirror has the advantage that the reflector can be produced at low cost. Furthermore, the reflector fashioned as a plane mirror has very small spatial dimensions and therefore also has a very small mass, which means that the reflector can be moved with very high frequency. 
   According to another embodiment of the invention, the reflector is displaced periodically. This enables the distance from a point on the surface to be measured to be measured several times in succession by the sensor device and this distance to be determined with particular accuracy by calculating the mean of several individual measurements. 
   According to a further embodiment of the invention, the reflector is displaced by way of an electromagnetic, piezoelectric and/or micromechanical capacative drive mechanism. These forms of drive mechanism enable the reflector to be moved with high frequency, so that a particularly high data rate can be achieved by the distance sensor according to an embodiment of the invention. 
   According to a preferred embodiment of the invention, the imaging optics have a converging lens. In the simplest case the imaging optics consist of a single converging lens. This enables the imaging optics to be implemented in a simple and—in particular—economical manner. 
   According to another, particularly preferred embodiment of the invention, the light-emitting element and light-receiving element are implemented by way of one end of an optical fiber. This has the advantage that it enables the components of the sensor device to be installed in two different and spatially separate modules. This is particularly advantageous if the first module contains the light source, light detector and the electronic components of the sensor device, and the second module contains only those components of the sensor device that need to be close to the object to be studied according to the confocal imaging principle. Thus, the sensor device on the object side can be of particularly small construction. 
   According to a further, particularly preferred embodiment of the invention, the second end of the optical fiber is split into two partial ends, whereby the first partial end is optically coupled to the light source and the second partial end is optically coupled to the light detector. According to the invention, the splitting of the optical fiber into two partial ends is preferably performed by way of an optical fiber coupling element. This further, particularly preferred embodiment has the advantage that it is no longer necessary to adjust the light source and the light detector relative to the second end of the optical fiber. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description of preferred embodiments given hereinbelow and the accompanying drawings, which are given by way of illustration only and thus are not limitative of the present invention, and wherein: 
       FIG. 1  shows an optical sensor device according to the confocal imaging principle according to the prior art. 
       FIG. 2  shows an optical sensor device having a first and a second module according to the confocal imaging principle, according to an initial exemplary embodiment of the invention. In the second module, one end of the optical fiber is split into two partial ends, whereby the first partial end is optically coupled to the light source and the second partial end is optically coupled to the light detector. 
       FIG. 3  shows the second module of an optical sensor device according to a second exemplary embodiment of the invention. The light source and light detector are coupled to the optical fiber according to the invention by means of a beam splitter. 
       FIG. 4  shows the first module of an optical sensor device according to the confocal imaging principle according to a third exemplary embodiment of the invention. A deflection mirror is additionally arranged on the optical axis. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2  shows an optical sensor device  200  according to the confocal imaging principle according to an initial embodiment of the invention. The sensor device  200  has a first module  201  and a second module  202 . The first module  201  contains an electronic module  203 , a laser  204  which acts as the light source, and a photodiode  205  which acts as the light detector. The laser  204  is coupled to the electronic module  203  by means of an electrical connection cable  206 . The photodiode  205  is coupled to the electronic module by way of the electrical connection cable  207 . 
   The first module  201  is optically coupled to the second module  201  by way of an optical fiber  210 . On the side turned toward the first module  201 , the optical fiber  210  is split into a first partial end  211  and a second partial end  212 . The first partial end  211  is optically coupled to the laser  204 . The second partial end  212  is optically coupled to the photodiode  205 . One end  213  of the optical fiber  210  arranged inside the second module  202  acts both as a point-shaped light source and as a point-shaped receiving surface. 
   The components of the second module  202  are described below. The second module  202  has a converging lens  220  and a movable reflector  221 . The movable reflector  221  is arranged perpendicular to the optical axis of the converging lens  220 . The reflector  221  is a plane mirror according to the first exemplary embodiment of the invention described here. This reflector is moved in an axial direction along the optical axis of the converging lens  220  by way of a drive mechanism  222 . 
   The drive mechanism  222  is an electromagnetic drive mechanism, a piezoelectric drive mechanism or a micromechanical capacitive drive mechanism. The drive mechanism  222  is further coupled to the electronic module  203  via the control cable  223 . 
   Between the reflector  221  and the converging lens  220 , the end of the optical fiber  213  is arranged on the optical axis of the converging lens  220  such that the light coupled into the optical fiber  210  from the laser  204  in the first partial end  211  of the optical fiber  210  exits from the optical fiber  210  at the end of the optical fiber  213  and falls on the reflector  221 . The end of the optical fiber  213  here represents almost a point source of light. The light emitted from the end of the optical fiber  213  is reflected by the reflector  221  and then focused by the converging lens  220 . 
   The focused light falls on the surface to be measured (not shown) and is at least partially reflected by this surface. The reflected light is in turn imaged by the converging lens  220  and falls on the reflector  221 , which directs the backscattered light to the end of the optical fiber  213 , where the light reflected back into the optical fiber  210  is at least partially coupled in and the coupled in light is in turn at least partially captured, via the second partial end  212  of the optical fiber  210 , by the photodiode  205 . According to an embodiment of the invention the end of the optical fiber  213  acts as a point detector. 
   If the reflector  221  is now displaced in an axial direction by use of the drive mechanism  222 , a shift in focus takes place in the object area, i.e. on the side of the converging lens  220  that is turned away from the reflector. At precisely the point where the focus falls, with a minimum cross-section, on the surface to be investigated, the photodiode  205  detects a maximum light intensity and delivers a maximum output signal. The measurement of the light intensity detected by the photodiode  205  as a function of the position of the reflector  221  enables the distance of the object from the converging lens  220  or from the entire second module  202  to be measured according to the confocal detector principle. 
   Since only a very few optical components are required inside the second module  202  according to the first exemplary embodiment of the invention described here, the second module  202  can be implemented with a highly compact design and spatially separate from the first module  210 . According to the first exemplary embodiment of the invention described here, the module  202  has a height of approximately 6 cm, a width of approximately 5 cm and a depth also of approximately 5 cm. 
   The sensor device  200  described here according to the first exemplary embodiment of the invention has a height measurement range of approximately 2 mm. Thus, three-dimensional objects with a height difference of up to 2 mm can be measured. The height resolution of the sensor device  200  described here is approximately 2 μm. Since the reflector  221  can be implemented in the form of a small, light plane mirror, according to an embodiment of the invention only a very small mass needs to be displaced along the optical axis of the converging lens  220 . 
   The displacement of the reflector  221  may be done advantageously by use of an electromagnetically driven oscillator. Such an oscillator, which has the functionality of a tuning fork, is already described in DE 196 08 468 C2. 
   Alternatively the reflector  221  may also be displaced by use of a plunger coil drive mechanism, which is used in commercial loudspeakers. The drive mechanism  222  may however be implemented to particularly good effect by way of a micromechanical solution, in which the oscillator is produced using the silicon etching technique. 
   The reflector  221  can be fashioned as a fixed component of the oscillator system. This solution, which is used in the first embodiment of the invention described here, permits an oscillation frequency of up to 10 kHz. Since the distance can be measured for each half period of the displacement of the reflector  221 , up to 20,000 distance measurements per second are possible with the sensor device  200  described here according to the first exemplary embodiment of the invention. 
   A first module  301  according to a second exemplary embodiment of the invention is explained below with the help of  FIG. 3 . The first module  301  has an electronic module  303 , a light diode  304  which acts as a light source, and a photodiode  305  which acts as a light detector. The light diode  304  is coupled to the electronic module  303  via an electrical connection cable  306 . The photodiode  305  is coupled to the electronic module  303  via an electrical connection cable  307 . The light diode  304  and the photodiode  305  are both optically coupled to the optical fiber  310  via a fiber launch device. The optical fiber  310  runs into a second module (not shown), which is constructed according to the second module according to the first exemplary embodiment of the invention (see  FIG. 2 ). 
   Analogously to the first exemplary embodiment the electronic module  303  is coupled to the drive mechanism of the second module (not shown) via a control cable  323 . The fiber launch device has a first converging lens  331 , a beam splitter  332  and a second converging lens  333 . The first converging lens  331 , the beam splitter  332  and the second converging lens  333  are arranged such that the light emitted from the light diode  304  is focused onto the end of the optical fiber  309  and is thus coupled into the optical fiber  310 . On the other hand the second converging lens  333  and the beam splitter  332  are arranged such that the light emitted from the end of the optical fiber  309  is captured by the photodiode  305 . 
   In one variant of the first module of the second exemplary embodiment, a laser diode is used as the light source. The light from the laser diode is collimated, runs through a beam splitter and is coupled into a fiber by way of a focusing lens. The light falling on the fiber on the detection side is deflected by the beam splitter and focused onto a light detector by way of a further lens. 
   A second module  402  according to a third exemplary embodiment of the invention is explained below with the help of  FIG. 4 . The second module  402  has a deflection reflector  450 , a reflector  421 , a drive mechanism  422  and a converging lens  420 . The reflector  421 , which is displaced along the optical axis of the converging lens  420  by way of the drive mechanism  422 , is positioned perpendicular to the optical axis of the converging lens  420 . The deflection reflector  450 , which is likewise located on the optical axis of the converging lens  420 , is arranged at an angle to this axis between the reflector  421  and the converging lens  420 . The drive mechanism  422  is electrically coupled to an electronic module (not shown) by way of a control cable  423 . 
   The second module  402  is optically coupled to a first module (not shown) by way of an optical fiber. An appurtenant first module (not shown) is designed according to the third embodiment of the invention as the first module  201  shown in  FIG. 2  or the first module  301  shown in  FIG. 3 . 
   The optical fiber  410  and the deflection reflector  450  are arranged relative to the optical axis of the converging lens  420  such that the light emitted from one end of the optical fiber  413  falls on the deflection reflector  450  and is reflected by this reflector such that the light reflected by the deflection reflector  450  falls on the movable reflector  421  in the shape of a cone that is coaxial to the optical axis of the converging lens  420 . The light reflected by the movable reflector  421  is then imaged by the converging lens  420  onto the surface of the object to be measured. 
   At least part of the light falling on the surface to be measured is coaxially backscattered from that surface, and is then imaged onto the end of the optical fiber  413  by the converging lens  420  by way of a first reflection off the displaced reflector  421  and a second reflection off the deflection reflector  450 . It is thus fed to the optical fiber  410 . 
   As already explained in the description of  FIG. 2 , a shift in focus in the object area takes place along the optical axis of the converging lens  420  when the reflector  421  is displaced by the drive mechanism  422  in an axial direction parallel to he optical axis of the converging lens  420 . The light intensity backscattered by the surface to be measured, and thus also the light intensity which is coupled into the optical fiber  410 , is at its maximum precisely when the focus that is shifted along the optical axis of the converging lens  420  falls on the surface to be measured. 
   The distance to the surface to be measured from the second module  402  is then determined in that the light intensity at least partially backscattered by the surface, fed to the optical fiber  410  and measured by the light detector (not shown) which is optically coupled to the optical fiber  410 , is measured as a function of the deflection of the displaced reflector  421 . 
   At this point is should be mentioned that the deflection reflector  450  results in a shadowing of the light intensity near the optical axis of the converging lens  420 . On the basis of the cylindrically symmetrical beam path inside the second module  402  between the deflection reflector  450  and the surface to be measured, the area shadowed by the diagonally positioned deflection reflector  450  contributes only slightly to the total light intensity. In addition the marginal beams contain the greatest proportion of the distance information. Thus the reduction in light intensity caused by shadowing by the deflection reflector  450  can be more or less disregarded. 
   The deflection reflector  450  is then particularly advantageous if the second module  402  is to be implemented inside a highly compact module and if, consequently, small bending radii in the optical fiber  410  are to be avoided. 
   Exemplary embodiments being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.