Abstract:
A probe head for a coordinate measuring machine has a stylus resiliently suspended in a housing. A sensor arrangement serves for detecting deflections of the stylus relative to the housing. The sensor arrangement comprises at least one Hall sensor having a magnet and a Hall element arranged in a vicinity to each other. When the stylus is deflected, the magnet is laterally passed by the Hall element in a direction defined from the North Pole to the South Pole of the magnet or vice versa. A Hall voltage of changing polarity can be tapped at the Hall element then. A preferably linear range around the polarity change is processed in order to determine the deflection of the stylus.

Description:
RELATED APPLICATIONS 
   This application is a continuation of copending international patent application PCT/EP2003/011473, filed on Oct. 16, 2003 and published in German language, which claims priority from German patent application DE 102 50 812.7 filed on Oct. 23, 2002. 

   BACKGROUND OF THE INVENTION 
   The invention generally relates to a probe head for a coordinate measuring machine. More specifically, the invention relates to a probe head comprising a stylus resiliently suspended in a housing, and a sensor arrangement for detecting deflections of the stylus relative to the housing. 
   A prior art probe head designed for determining dimensions of a shaft is disclosed by DE 37 21 682 A1. This probe head has an elongated arm with a spherical probe tip that bears against the circumference of a rotating shaft. The arm is part of a parallelogram-type structure whose articulations are formed by segments of diminished cross section that are designed in the manner of film hinges. The entire arrangement is designed in a single-piece fashion. In order to convert the movement of the arm with the probe tip into an electrically detectable signal, use is made of an inductive sensor arrangement in which a core moved indirectly by the probe tip dips into a measuring coil. As an alternative embodiment, a Hall element is proposed, but without specifying any details of the arrangement or the evaluation of the Hall voltage. 
   DE 198 23 059 C2 discloses a method and a device for detecting the spatial position of a body. Use is made of an array of 4×4 sensors, such as Hall sensors. 
   DE 37 08 105 A1 discloses a measuring probe for a coordinate measuring machine that proposes magnetoresistive sensors for three-dimensional position measurements. A stylus is situated with one of its poles opposite a resistor dependent on a magnetic field, the deflection of the stylus effecting a variation in the spacing between the magnetic pole and resistor. 
   DE 26 20 099 C2 discloses a probe head with a stylus that can be deflected in all directions. In order to measure and detect a movement of the stylus, use is made of an inductive sensor arrangement in which there are provided two inductive sensors on each of all three axes. The sensors are arranged on opposite sides of the stylus. The inductive sensors are connected up to form a bridge circuit. 
   DE 37 20 524 A1 discloses a caliper rule having a Hall sensor. The Hall sensor is located next to a magnet such that, in the absence of a ferromagnetic disturbance, the Hall element is penetrated symmetrically by the field lines of the magnet, the Hall voltage thereby being precisely zero. If a runner with the Hall sensor is displaced on the bar, ferromagnetic rungs of a ladder-type arrangement run past the Hall sensor and distort the magnetic field, with the consequence that the Hall element is permeated asymmetrically by field lines, a finite Hall voltage thereby being produced. The Hall voltage is fed to a threshold stage that forms pulses, which are subsequently counted in, order to determine the position of the runner on the bar. 
   DE 197 12 829 A1 discloses a device for detecting the position of a piston in a pneumatic cylinder. A Hall voltage with a polarity change is utilized, specifically by laterally passing a Hall by a magnet. The Hall signal is further processed by means of threshold stages. 
   DE 196 39 801 A1 discloses a sensor arrangement for detecting the position of movable parts by means of a Hall sensor in which the Hall sensor is laterally passed by a magnet in order to control a windscreen wiper system for a motor vehicle. Again, use is made of a threshold stage in order to process the Hall signal. 
   SUMMARY OF THE INVENTION 
   Against this background, it is an object of the invention to provide a probe head for a coordinate measuring machine that allows a high-resolution measurement, preferably in three coordinate directions, with a low outlay on apparatus and at low costs. 
   According to one aspect of the invention, this object is achieved with a probe head, wherein a magnet is laterally passed by a Hall element such that a Hall voltage of changing polarity is generated, and wherein a range of the Hall voltage that lies around the polarity change is processed as a continuous measure of the deflection of the stylus. 
   It is thus proposed to measure the deflection of the stylus in any coordinate direction by means of a Hall sensor. Generally, only a single Hall sensor per coordinate direction is required here, even though for other reasons two or four Hall sensors can be advantageous for certain applications, as will be explained further on. 
   By contrast to known probe heads for coordinate measuring machines, a substantial reduction of the number of active measuring elements can be achieved. It is an advantage that the Hall voltage profile is evaluated not only in the sense of a threshold value detection. Rather, the present invention turns to its advantage the fact that the Hall voltage profile as a function of travel is linear over a relatively wide range in the region of the zero crossing such that continuous detection and processing of measured values is possible here. 
   In preferred embodiments of the invention, the range is substantially linear. 
   This measure has the advantage that it is possible to process measured values more simply when, for example, an inherently nonlinear characteristic curve of a Hall element is linearized, for example by means of previously determined correction values. 
   In a preferred refinement of the inventive probe head, there is defined a plane in the housing, which plane forms a radial plane of the stylus in the rest position of the stylus. Preferably, the stylus is adapted to swivel about a fulcrum lying in the plane, and the stylus is provided with at least one first magnet and the housing is provided with at least one first Hall element, or vice versa, in order to detect the swiveling. 
   It is particularly preferred in this context when at least one first magnet is arranged with its axis parallel to the axis of the stylus and, in the rest position of the stylus, it is located in the plane with its symmetry plane defined between its poles. 
   This measure has the advantage that accurate measurements are possible in the so-called X-Y plane. 
   In a specific exemplary embodiment, the stylus is cardanially suspended in the plane by means of a diaphragm, and the first magnet and the first Hall element project into a first cutout in the diaphragm. 
   This measure has the advantage that the measurement of the X-Y deflection in the cardan plane is possible without the elements required for this purpose causing an obstruction. 
   It is particularly preferred in this context when the first magnet is arranged on a holder extending radially away from the stylus, and the first Hall element is arranged on an axially extending inner wall of the housing. 
   This measure has the advantage of intensifying the swiveling of the stylus over the length of the holder, the arrangement of the Hall element on the inner wall of the housing corresponding to a maximum length of the holder. 
   In further embodiments, at least one first magnet is arranged with its axis orthogonal to the axis of the stylus, and the first magnet is located on a first holder extending radially away from the stylus and at an axial distance from the fulcrum, and the first Hall element is located on a radially extending inner wall of the housing. 
   This embodiment has the advantage that the elements required for detecting the swiveling in the X-Y plane can be spatially separated from one another and can therefore be implemented more simply. 
   In further embodiments of the invention, there is defined a longitudinal axis in the housing which axis coincides in the rest position of the stylus with a longitudinal axis of the stylus, wherein the stylus is elastically displaceable along its longitudinal axis, and wherein the stylus is provided with at least one second magnet, and the housing is provided with at least one second Hall element, or vice versa, for detecting the displacement 
   This embodiment relates to measurements along the so-called Z-axis, which can likewise be carried out in an advantageous manner within the scope of the present invention. 
   This holds, in particular, whenever the stylus can be swiveled about a fulcrum lying in the plane, wherein the second magnet is arranged with its axis parallel to the axis of the stylus and, in the rest position of the stylus, it is located in the plane with its symmetry plane defined between its poles. 
   This measure also has the advantage that very precise measurements can be carried out even if the sensor arrangement of the Z-axis is located in the cardan plane of the stylus. 
   In another embodiment, the stylus is cardanically suspended in the plane by means of a diaphragm, and the second magnet and the second Hall element project into a second cutout in the diaphragm. 
   This method has the advantages already mentioned above, namely that the required elements can all be arranged in the plane of the diaphragm without risk of collision. 
   Furthermore, with the abovementioned exemplary embodiments of arrangements for measuring the displacement on the Z-axis, it is particularly preferred when the second magnet is arranged on the stylus, and the second Hall element is arranged on a holder extending radially away from an inner wall of the housing. 
   This arrangement, which is thereby designed in a fashion directly opposed to the arrangement of the sensor elements for measurements in the X-Y plane, thus likewise has the advantage of being able to carry out particularly precise measurements that are independent on the Z-axis of a simultaneous deflection in the X-Y plane. 
   It is preferred in another exemplary embodiment when the second magnet is arranged with its axis parallel to the axis of the stylus and, in the rest position of the stylus, is located with its symmetry plane defined between its poles at a spacing from the plane. 
   This measure has the advantage that a simpler construction is possible owing to the spatial separation of the measuring elements, on the one hand, and of the cardan plane, on the other hand. 
   In the case of the last named exemplary embodiment, a particularly good action is further obtained by virtue of the fact that two magnets are provided at an axial spacing from one another. 
   This measure has the advantage that the direction of the displacement along the Z-axis can be reliably detected. 
   Finally, it is preferred in this context when the second magnet is arranged on the stylus, and the second Hall element is arranged on a holder extending radially away from an inner wall of the housing. 
   This measure has the advantage already mentioned that precise measurements are possible on the Z-axis even if the stylus is simultaneously being swiveled in the X-Y plane. 
   It is particularly preferred in the case of all the above-mentioned embodiments when a plurality of Hall sensors, in particular four first Hall sensors or two second Hall sensors, are arranged distributed over a circumference of the probe head. 
   This measure has the advantage that differential measurements are possible such that, for example, during measurement of the swiveling in the X-Y plane only an inverse signal change is detected at oppositely situated sensors, while a change in the same direction is disregarded because it results from a movement of the stylus along the Z-axis, which is not to be detected by the first Hall sensors, but by the second ones. 
   Finally, it is particularly preferred within the scope of the present invention when at least magnets and/or Hall elements serving for detecting swiveling are provided with a spherical surface, the radius of the surface corresponding to the radius of the swiveling movement. 
   This measure has the advantage that the air gap between the elements of the Hall sensor remains constant when they are swiveled relative to one another. 
   Further advantages follow from the description and the attached drawing. 
   It is self-evident that the features mentioned above and those still to be explained below can be used not only in the respective specified combination, but also in other combinations or on their own without departing from the scope of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention are illustrated in the drawing and will be explained in more detail in the following description. In the drawing: 
       FIG. 1  shows an extremely schematic sectional side view of a first exemplary embodiment of a probe head according to the invention, along the line I-I in  FIG. 2 ; 
       FIG. 2  shows, on an enlarged scale, a plan view of the interior of the probe head in accordance with  FIG. 1 , in the view II-II of  FIG. 1 ; 
       FIGS. 3A and 3B  show a truncated sectional view along the line III-III of  FIG. 2 , in two different operational positions; 
       FIGS. 4A and 4B  show illustrations similar to  FIGS. 3A and 3B , but in a view along the line IV-IV of  FIG. 2 ; 
       FIG. 5  shows a schematic side view of a Hall sensor as used within the context of the present invention; 
       FIG. 6  shows a diagram representing the Hall voltage against the travel, for the sensor in accordance with  FIG. 5 ; and 
       FIG. 7  shows an illustration, similar to  FIG. 1 , but for a second exemplary embodiment of a probe head according to the invention. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   In  FIGS. 1 to 4 , reference numeral  10  denotes a probe head for three-dimensional measurements by means of a coordinate measuring machine. The probe head  10  has a housing  12  with an axially extending inner wall  14 . A diaphragm  16  is clamped on the circumference of the axial inner wall  14 . At its center, the diaphragm  16  supports a stylus  18  whose axis  19  coincides with the longitudinal axis of the housing  12  in the rest position of the stylus  18 , shown in  FIG. 1 . Moreover, axis  19  lies on the so-called Z-axis of the probe head  10 , whose transverse axes are denoted in the usual way by X and Y. 
   The probe head  18  is thereby cardanically mounted at a fulcrum  20  at the center of the diaphragm  16 , the diaphragm  16  simultaneously forming the so-called cardan plane  21  in the rest position. The stylus  18  can thus be swiveled in the X-Y plane about the fulcrum  20 , as indicated by an arrow  22 . 
   The stylus  18  terminates at its lower end in a contact sphere  24 . 
   For sake of clarity, only the X-Y measuring system is shown in  FIG. 1 , whereas the Z-measuring system is not shown here. 
   Further details of the probe head  10  are now to be explained with reference to  FIGS. 2 to 4 : 
   Located on the stylus  18  somewhat above the cardan plane  21  is a star-shaped holder  30  that protrudes radially from the stylus  18  with four arms  32   a  to  32   d , and is rigidly connected to said stylus. 
   Located at the free ends of the arms  32   a  to  32   d  are first Hall sensors  33   a  to  33   d  which serve for detecting the swiveling movement of the stylus  18  in the X-Y plane, i.e. about the fulcrum  20 . 
   The first Hall sensors  33   a  to  33   d  comprise first permanent magnets  34   a  to  34   d  that are arranged at the free ends of the arms  32   a  to  32   d . The first permanent magnets  34   a  to  34   d  project through first cutouts  35   a  to  35   d  at the circumference of the diaphragm  16 . Located directly opposite them are first Hall elements  36   a  to  36   d  that are arranged at the axial inner wall  14  of the housing  12  in such a way that only very small air gaps remain between the first permanent magnets  34   a  to  34   d  and the first Hall elements  36   a  to  36   d.    
   Second Hall sensors  37   a ,  37   b  are provided for detecting a displacement of the stylus  18  in the Z-direction. Said sensors are located directly at the stylus  18  in two diametrically opposite positions. Second permanent magnets  38   a,    38   b  are supported by the holder  30  and project through second cutouts  39   a ,  39   b  in the diaphragm,  16 . The associated second Hall elements  40   a ,  40   b  are located at the free end of arms  42   a,    42   b  that protrude radially from the axial inner wall  14  of the housing  12 . 
     FIG. 3A  indicates that the first permanent magnets  34   a  to  34   d  are aligned parallel to the axis  19  of the stylus  18  with their axis  44 , which is defined by the poles N and S. The same applies for the second permanent magnets  38   a,    38   b.    
   Furthermore, the first permanent magnets  34   a  to  34   d  are positioned in the Z-direction such that their symmetry plane  45  is located precisely in the cardan plane  21  between the poles N and S in the rest position of the stylus  18  ( FIG. 1  and  FIG. 3A ). This also applies in the same way for the second permanent magnets  38   a ,  38   b.    
   The first permanent magnets  34   a  to  34   d  are all identically aligned in the direction of the axis  19 . In the exemplary embodiment illustrated, all the North Poles N are at the top. By contrast therewith, the second permanent magnets  38   a ,  38   b  are oppositely aligned, as is clearly shown in  FIGS. 4A and 4B . 
   Finally, it is further indicated in  FIG. 3A  that the surface  46 , facing the first Hall element  36   a , of the first permanent magnet  34   a  is designed, just like the opposite surface  48  of the first Hall element  36   a , to be spherical, with the spherical radius corresponding precisely to the spacing of the respective surface from the fulcrum  20 . 
   The mode of operation of the probe head  10  in accordance with  FIGS. 1 to 4  is as follows: 
   When the stylus  18  is located in the rest position in accordance with  FIGS. 1 ,  3 A and  4 A, a zero signal is present both at the first Hall sensors  33   a  to  33   d  for detecting swiveling in the X-Y plane, and at the second Hall sensors  37   a,    37   b  detecting swiveling in the Z-direction. This is the case because the field lines of the permanent magnets  34   a  to  34   d  and  38   a ,  38   b  respectively, symmetrically penetrate the Hall elements  36   a  to  36   d , and  40   a ,  40   b  respectively, assigned to them. 
     FIG. 3B  shows the situation when the stylus  18  is swiveled out of its rest position  18  into a swiveled position  18 ′, the swivel angle in  FIG. 3B  being denoted by α. In  FIG. 3B , the reference symbols of all the swiveled elements are marked by the addition of a prime. 
   As may easily be seen, the signals, that is to say the Hall voltages, change in opposite directions at the Hall elements  36   a ,  36   c , for which purpose reference may briefly be made to the illustrations of  FIGS. 5 and 6 : 
   In  FIG. 5 ,  50  denotes a base fixed to the housing and on which a Hall sensor  51  is located. The Hall element  52  of the latter is permanently connected to the base  50 . A Hall voltage U H  can be tapped at a connection  54  of the Hall element  52 . 
   The permanent magnet  56  of the Hall sensor  51  is moved across the Hall element  52  at a slight spacing. This takes place in such a way that the axis  61 , defined by the North Pole  58  and the South Pole  60 , of the permanent magnet  56  coincides with the direction x of movement. 
   In the basic position illustrated in  FIG. 5 , the plane  62  of symmetry of the permanent magnet  56  lies exactly in the middle over the Hall element  52 . This is therefore penetrated symmetrically by the field lines of the permanent magnet  56  such that Hall effects occur in the Hall element  52  on both sides of the plane  62  of symmetry which are exactly symmetrical but oppositely directed, and so the charge carriers are deflected in an identical way but in an opposite direction. 
   If the permanent magnet  56  is now moved in a direction of the axis  61  from left to right beyond the Hall element  52 , a Hall voltage U H  over the travel x can be tapped at the terminal  54  as illustrated in the diagram  64  of  FIG. 6 . 
   It is to be seen that the Hall voltage U H  traverses a zero crossing from plus to minus, a preferably linear range  66  being set up in the region of the zero crossing such that a corresponding dependence of the Hall voltage U H  on the travel x can be assumed within a voltage range  68  or a travel range  70 . If the Hall element  52  inherently has a non-linear characteristic curve, this can be linearized by means of correction values or the like determined in advance. 
   If the situation according to  FIGS. 3A and 3B  is now considered again, it is easy to see that exactly opposed Hall voltages are present at the first Hall elements  36   a  and  36   c  in the case of a pure swiveling movement by an angle α in accordance with  FIG. 3B . This is an indication that a pure swiveling movement is present. 
   If the stylus  18  were displaced only along its axis  19 , as indicated by ΔZ in  FIG. 3A  and dashed and dotted, as well as by arrows, this would result at the first Hall elements  36   a  and  36   c  in identical signals that could be counted out with the aid of conventional means in order to avoid falsification in the measurement of the swiveling. 
   It is further to be seen from  FIGS. 3A and 3B  that the relatively long arms  32   a  to  32   d  effect a large transmission of the swiveling of the stylus  18  at the measuring point of the first Hall sensors  33   a  to  33   d.    
     FIGS. 4A and 4B  show the situation in connection with the measurement of the displacement of the stylus  18  along the Z-axis, with  FIG. 4A  again illustrating the initial situation, and  FIG. 4B  illustrating the measuring situation, in which the stylus  18  has been displaced from the rest position  18  to  18 ′, as illustrated by ΔA in  FIG. 4B . 
   As a consequence of the opposite alignment of the second permanent magnets  38 A,  38 B, this now yields the same result as was previously stated in relation to  FIGS. 3A and 3B . In the case of the quantity to be measured, specifically in the case of a linear displacement of the stylus  18  in the Z-direction, opposite voltages are present at the two Hall elements  40   a ,  40   b  while identical signals are produced in the case of the quantity not to be measured, specifically in the case of a swiveling of the stylus  18 . Here, as well, it is thus possible to separate signals into desired and undesired ones. 
   Since no transmission of the movement takes place when measuring the displacement in the Z-direction, the measurement is thus performed in the vicinity of the stylus  18  here, which is again in contrast to the situation of  FIGS. 3A and 3B . 
   Finally,  FIG. 7  shows yet another exemplary embodiment of a probe head  80 . 
   The probe head  80  again comprises a housing  82  with an interior  84  in which a diaphragm  86  is clamped. 
   The diaphragm  86  supports a stylus  88  whose longitudinal axis is denoted by  89 . The stylus  88  again is suspended at the center of the diaphragm  86 , such that a fulcrum  90  and a cardan plane  91  are defined there. 
   A circle or a spherical surface  92  indicates here that the measurement of the swiveling of the stylus  88  takes place at a spacing from the cardan plane  91 . 
   A contact sphere  94  is located once again at the lower end of the stylus  88 . 
   A first sensor arrangement  96  serves for measuring the swiveling of the stylus  88  in the X-Y plane. The first sensor arrangement  96  comprises a first holder  98  that is designed as a plate in a radial plane of the stylus  88  and rigidly connected to the latter. 
   Located below the first holder  98  are first Hall sensors, of which only two are to be seen in  FIG. 7 , specifically the Hall sensors  99   a  and  99   b . However, four such Hall sensors are provided, offset by 90° in each case, around the stylus  88 . 
   The Hall sensors  99   a ,  99   b  comprise first permanent magnets  100   a ,  100   b.    
   Located at the lower end of the housing  82  is a second holder  102 , which forms a radial inner wall  103  of the housing  82  facing to the top. Located on this inner wall  103  are first Hall elements  104   a ,  104   b  that form the first Hall sensors  99   a ,  99   b  together with the first permanent magnets  100   a ,  100   b.    
     106  indicates that here, as well, the mutually opposite surfaces of the permanent magnets  100   a ,  100   b  or Hall elements  104   a ,  104   b  have the shape of a spherical surface whose radius is determined by the circle or the spherical surface  92 . Here, as well, the effect of swiveling the stylus  88  about the fulcrum  90  is that the air gap between the first permanent magnets  100   a ,  100   b  and the first Hall elements  104   a ,  104   b  is constant. 
   Located in the upper region of the stylus  88 , between the fulcrum  90  and first holder  98 , is a second sensor arrangement  110  that serves for measuring the displacement of the stylus  88  in the Z-direction. 
   The second sensor arrangement  110  comprises two Hall sensors  111   a ,  111   b  that are arranged at an axial spacing from one another on opposite sides of the stylus  88 . The second Hall sensors  111   a ,  111   b  each comprise second permanent magnets  112   a ,  112   b  and second Hall elements  114   a ,  114   b . The latter are located at the free ends of arms  116   a ,  116   b  that protrude radially from the inner wall  84  of the housing  82 . 
   When the stylus  88  is swiveled about the fulcrum  90 , the first permanent magnets  100   a ,  100   b  move past the first Hall elements  104   a ,  104   b , and a signal profile corresponding to  FIG. 6  is obtained here as well. 
   A corresponding statement holds for a movement of the stylus  88  in the Z-direction when the second permanent magnets  112   a ,  112   b  move past the second Hall elements  114   a,    114   b.