Abstract:
A probe head for a coordinate measuring machine has a replaceable, miniaturized probe system with a stylus. The stylus is rigid and forms a structural unit together with a membrane-type sensor system that is deformed in a predetermined way when a force acts on the stylus. The structural unit is permanently fixed to a probe holder and can be handled together with the probe holder when the stylus is being replaced.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   The present application is a continuation of international patent application PCT/EP2003/014584, filed on Dec. 19, 2003 designating the U.S. and published as WO 2004/068068 A1 in German language, which claims priority under the Paris Convention from German patent applications DE 103 04 827.8 filed on Jan. 31, 2003 and DE 103 14 304.1 filed on Mar. 25, 2003. 

   BACKGROUND OF THE INVENTION 
   The invention relates to a probe head for a coordinate measuring machine and more particularly to a probe head having a replaceable, miniaturized probe system with a rigid stylus that forms a structural unit together with a membrane-type sensor system that deforms in a predetermined way when a force acts on the stylus. 
   In the case of conventional coordinate measuring machines, it is often necessary to replace the stylus so that measurement tasks of different types can be carried out consecutively. These measurement tasks include, for example, the measurement of bores of different depth, the measurement of bores of different inside diameter and the like. It is desirable to have styluses of different lengths and having stylus balls of different diameter at the end. 
   In an embodiment, DE 101 08 774 A1 discloses a new type of a miniaturized probe system for a probe head, wherein the stylus is mounted on a membrane-type arrangement which is square in plan view and which has a thickened periphery edge. Located in the middle of the arrangement is another thickened portion, which supports the stylus. Located between the edge and the middle is a membrane-type zone which is provided with piezoresistive elements or with strain gauges on a side facing away from the stylus. Deflecting the stylus leads, in this case, to the membrane-type zone being distorted or deformed and therefore to a signal being produced via the piezoresistive elements or the strain gauges. According to DE 101 08 774 A1, such a probe system cannot be replaced. Rather, the thickened periphery edge of the probe system is permanently fixed to the probe head. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide a probe head for use with a miniaturized probe system which allows a replacement of the stylus in a simple and reliable manner. It is another object to provide a probe head for use with a miniaturized probe system which allows a automated replacement of the stylus, in particular in connection with a CNC-controlled coordinate measuring machine. 
   According to one aspect of the invention, this object is achieved for a probe head of the type mentioned at the beginning, wherein the structural unit is permanently connected to a probe holder, the structural unit being able to be handled together with the probe holder when the stylus is being replaced. 
   According to another aspect of the invention, this object is achieved by a probe of the type mentioned at the beginning wherein separate means are provided for mechanically fastening the structural unit on the probe, on the one hand, and for electrically connecting the sensor elements of the probe system, on the other hand. 
   In accordance with the first aspect, a larger and easier to handle element is formed by providing the probe holder which is connected to the structural unit. A probe holder can be handled more easily without the risk of the stylus breaking off even in the event of slight inattentiveness. 
   In accordance with the second aspect the structural separation between mechanical fastening, on the one hand, and electrical connection, on the other hand, permits each of these two functions to be optimized in a separate fashion including in a temporal sequence. Thus it is possible, for example, firstly to fasten the structural unit mechanically and then to connect it electrically, or the electrical connection is accomplished automatically when the mechanical fastening is concluded. 
   In a preferred refinement of the probe head according to the invention, the probe holder is constructed like a sleeve. 
   This measure has the advantage that the probe holder can be gripped in a simple way and, for example, can be fixed in a simple way by being pushed onto or being pushed into a corresponding counterpart. 
   It is preferred, furthermore, when a rear section of the probe holder is configured to be connected to a probe receptacle of the coordinate measuring machine. 
   This measure has the advantage that the interface composed of the probe holder and the probe receptacle can be optimized individually for the respective application. 
   This holds, in particular, when in accordance with a refinement of this variant the rear section is configured to be connected to the probe receptacle via a conical fit. 
   This measure has the advantage that it is possible to achieve a perfect equiaxiality with regard to the probe holder and probe receptacle. 
   In a further preferred embodiment of the invention, the sensor system is mounted on a front section of the probe holder. 
   This measure has the advantage that the sensor system is optimally exposed. 
   Also preferred is an embodiment of the invention in which the probe holder is configured to be connected to the probe receptacle in a rotary position defined with reference to an axis of the probe receptacle. 
   This measure also has the advantage of achieving a high reproducibility of the measurement result. Furthermore, it is advantageous that a position of the probe holder and, thus, of the structural unit consisting of the membrane-type sensor system and the miniaturized stylus, defined in a direction of rotation as well, the sensor system is installed in a completely defined position such that the electric contact can be made with the sensor system in a reproducible way. 
   In accordance with a refinement of the this variant of the invention, there is provided a ball for the purpose of defining the rotary position, which ball falls into a groove of the probe receptacle or into a bore on the probe holder in the defined rotary position. 
   This measure has the advantage that it is possible to reliably achieve and maintain the defined rotary position when the ball has fallen into the bore together with a simple mounting. 
   In further preferred embodiments of the invention, the probe receptacle extends along an axis and the probe holder is configured to be connected to a front end section of the probe receptacle. 
   This measure has the advantage of enabling a strictly axial design such as can be used, in particular, in conjunction with quills of coordinate measuring machines of gantry design or of free arm design. 
   In the case of further preferred embodiments of the invention, the sensor system is mounted on a radial rear side of the probe holder. 
   This measure has the advantage that the sensor system is accommodated in a largely protected fashion. Furthermore, the sensor system thus has an axial abutment such that the sensor system can be approached from its rear side, for example for the purpose of making electric contact. 
   According to a further exemplary embodiment of the invention, the sensor system is adhered to the probe holder. 
   This measure has the advantage that the unit formed with the probe holder can be prefabricated in a simple way, a permanent connection being produced between the sensor system and probe holder by the bonding. 
   In accordance with a variant of the two above-mentioned exemplary embodiments, the sensor system can be adhered to the rear side of the probe holder, for which purpose it is preferred to provide bonding pockets for holding adhesive in the area of the rear side. 
   These measures have the advantage of enabling a simple mechanical prefabrication that leads to a stable structure. 
   In the case of the above-mentioned exemplary embodiments, it is further preferred when the stylus projects through a central opening in the rear side. 
   This measure has the advantage that a mechanical encapsulation of the sensor system can be implemented as far as possible, only those parts being exposed that must be exposed for mechanical reasons. 
   In a further group of exemplary embodiments, the sensor system is configured to be accessed from its rear side facing from the stylus for the purpose of detecting its deformation. 
   This measure has the advantage that all the measured data can be acquired in the encapsulated region of the probe, and that the actual measurement at the stylus is thereby not impeded. 
   When the sensor system has electric sensor elements for detecting the deformation, it is particularly preferred, when contact can be made with the sensor elements from the rear side. 
   In accordance with a preferred development of the invention, this is achieved by virtue of the fact that spring pins are provided for making contact with the sensor elements. 
   This measure has the important advantage that contact with the sensor elements can be made automatically when the probe holder with the sensor system mounted thereon is mounted on the probe head. The contact force can be set in a desired way by designing the spring pins appropriately. The resilient bearing of the spring pins furthermore has the advantage that it is possible to compensate for fitting errors present in the axial alignment of the spring pins. 
   In the case of this variant of the invention, it is further preferred when a plurality of spring pins are held in a spring pin holder. 
   This measure has the advantage that a multiplicity of electric contacts can be produced at the same time with a single common component, namely the spring pin holder. 
   For this purpose, the spring pin holder is preferably mounted in the front end section of the probe receptacle. This is preferably performed by pushing the probe holder onto the front end section and by plugging the spring pin holder into the front end section. 
   If, in a further refinement, the probe holder is configured to be screwed together with the end section, preferably by means of a union nut, it is possible by means of a few manipulations to mount the inventive probe with all the elements of interest in the present context. 
   In the case of this group of exemplary embodiments, it is further preferred when the spring pin holder is configured to be fastened in a predetermined rotary position relative to an axis of the probe holder. 
   This measure has the advantage already indicated further above according to which it is possible to make exact contact due to the complete definition of the position of the elements, even if it is necessary to make contact with a plurality of sensor elements. 
   Although sensor systems of the most varied type can be used within the scope of the present invention, it is nevertheless preferred to use a miniaturized sensor system that has a thickened edge, in which case the stylus is arranged in the middle of the sensor system, and a membrane area is located between the edge and the middle. 
   In the case of the above-mentioned type of sensor systems, the membrane area can be in the form of a polygonal, preferably rectangular, and even more preferred a square frame, in plan view. 
   However, it is particularly preferred when in plan view the membrane area is of annular construction. 
   This measure has the advantage that notch effects are avoided at the most highly stressed points of the membrane area. Consequently, larger deformations are possible—under the same force conditions—until the breaking point of the membrane area is reached. Furthermore, it is advantageous that symmetrical signal profiles arise in the plane of the membrane area, i.e. usually in the so called X-Y-plane of the coordinate measuring machine. Finally, a greater sensitivity of the sensor system is obtained when the sensor elements are positioned directly on the inner edge of the membrane area, where the maximum deformations occur. 
   It is furthermore preferred, when the middle is of thickened construction. The thickened middle can be constructed in this case as a pyramidal frustum. 
   However, it is particularly preferred when the thickened middle is constructed as a conical frustum. 
   This measure is a particularly effective supplement to the annular construction of the membrane area in order to achieve the advantages already cited above even more completely. 
   The exemplary embodiments with the annular membrane area and the middle thickened in the form of a conical frustum can also be used independently of the other features of the invention presently being described. 
   It is also preferred when the structural unit can be handled by means of a numerically controlled robot when the stylus is being replaced. 
   This measure has the advantage that it is possible to switch the stylus between two individual measurement tasks in an extremely short time and with the highest reproducibility and lowest risk of damaging the stylus. 
   Further advantages emerge from the description and the attached drawing. 
   It goes without saying that the features mentioned above and those yet to be explained below can be used not only in the respectively 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 
     An exemplary embodiment of the invention is illustrated in the drawing and will be explained in more detail in the following description. In the drawing: 
       FIG. 1  shows a side view, partially cut away, through an exemplary embodiment of a probe head according to the invention; 
       FIG. 2  shows a detail from  FIG. 1 , on an enlarged scale, for the purpose of explaining further details; 
       FIG. 3  shows a perspective view of a probe holder from the probe of  FIGS. 1 and 2 ; 
       FIGS. 4A and 4B  show side views, in section and on an enlarged scale, of a sensor system of the probe head in accordance with  FIGS. 1 and 2 , in two different operating positions; 
       FIGS. 4C and 4E  show plan views of three exemplary embodiments of the sensor system in accordance with  FIG. 4A , in the direction of the arrows IV-IV of  FIG. 4A ; and 
       FIG. 5  shows a perspective illustration, on a greatly enlarged scale, of a spring pin holder of the probe head in accordance with  FIGS. 1 and 2 . 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  shows an exemplary embodiment of a probe head according to the invention with reference numeral  10 . The probe head  10  can be arranged, for example, on the lower end of a quill of a coordinate measuring machine of gantry design or of free arm design. This is indicated in  FIG. 1  with reference numeral  12 . The probe head  10  extends substantially along an axis  11  that is illustrated horizontally in  FIG. 1  and in practice is mostly a vertical axis, the so called Z-axis. 
   The probe head  10  has a probe receptacle  14  at its left-hand end in  FIG. 1 . The probe receptacle  14  has a polygonal head  16  at its left-hand end in  FIG. 1 . Following thereupon on the right is a middle section  18  which terminates in an external thread  20 . This is followed by a conically tapering section  22  whose conical outer surface is denoted by  24 . Finally, there is provided at the right-hand end of the probe receptacle  14  in  FIG. 10  a front end section  26  that tapers radially in the form of an inner annular shoulder  28 . 
   The probe receptacle  14  is provided inside along the axis  11  with a stepped through opening  30 . The through opening  30  is formed at the right-hand end of the probe receptacle  14  in  FIG. 1  by a first inner cylindrical surface  32 . Located in the middle section  18  is a second inner cylindrical surface  34  which has a somewhat larger diameter and terminates on the left in  FIG. 1  in an internal thread  36 . 
   As may now be gathered in further detail from  FIG. 2 , there is plugged into the through opening  30  a substantially cylindrical spring pin holder  40  whose outer cylindrical surface  42  is adapted to the first inner cylindrical surface  32  such that at the front end, that is to say the right-hand one in  FIG. 1 , of the probe receptacle  13  the spring pin holder  40  is held radially and, via its front side  44 , also axially at the inner annular shoulder  28 . A projection  46 , which is to be discerned particularly clearly in  FIG. 5 , engages in a corresponding cutout on the inner annular shoulder  28  such that the spring pin holder  40  is also fixed in its rotary position when seated in its operational position illustrated in  FIGS. 1 and 2 . Alternatively, it is also possible for this purpose to provide a recess  48  on the periphery of the spring pin holder  40 , as is illustrated in  FIG. 5 . 
   In order to fix the spring pin holder  40  in the position illustrated in  FIGS. 1 and 2 , use is made of a hold down sleeve  50  that strikes in  FIGS. 1 and 2  in the axial direction against a rear side  51  of the spring pin holder  40 . The hold down sleeve  50  is provided for this purpose with a tapered front end  52 . 
   The hold down sleeve  50  runs with its outer cylindrical surface  53  in the second inner cylindrical surface  34  of the through opening  30 . A rear end  54  of the hold down sleeve  50  is provided with a peripheral slot  56 , which is illustrated in  FIG. 1 . 
   The purpose of fixing the hold down sleeve  50  in the position illustrated in  FIGS. 1 and 2  is served by a hold down nut  60  that runs with its outer cylindrical surface  61  in the second inner cylindrical surface  34  of the through opening  30 . The hold down nut  60  is provided on its front side with an annular projection  62  located on its periphery and which engages in the peripheral slot  56  of the hold down sleeve  50 . 
   The hold down nut  60  is provided at the rearward end with an external thread  64  that runs in the internal thread  36  of the probe receptacle  14 . The hold down nut  60  can therefore be screwed into the through opening  30  by means of suitable peripheral grooves and the like (not illustrated) until it presses the hold down sleeve  50  against the spring pin holder  40 , and the latter bears permanently with its front side  44  against the inner annular shoulder  28  in an axial fashion. 
   A replaceable, miniaturized probe or feeler system in the form of a sleeve-shaped probe holder  70  can be pushed onto the front end section  26  of the probe receptacle  14 . The probe holder  70  has a rear section, that is to say a left-hand one in  FIGS. 1 and 2 . This section is provided with a conical inner surface  74  that is constructed to be complementary to the conical outer surface  24  on the conically tapered section  22  of the probe receptacle  14 . The probe holder  70  is thereby centered on this section  22  of the probe receptacle  14 . 
   A middle section  76  of the probe holder  70  runs with an inner cylindrical surface  78  on the front end section  26  of the probe receptacle  14 . 
   When the probe holder  70  is pushed onto the front end of the probe receptacle  14  in the way described, it can be fixed in this position by means of a union nut  79  that can be screwed onto the external thread  20  of the probe receptacle  14 . 
   A front section  80  of the probe holder  70  extends downward substantially in the radial direction and exposes the central opening  82  in the region of the axis  11 . This and further details are to be discerned particularly well from the perspective illustration in  FIG. 3 . 
   At a peripheral position, the middle section  76  of the probe holder  70  is provided with a radial bore  84  into which a ball  86  is inserted and is prestressed radially inward, preferably by means of a spring (not illustrated). The bore  84  merges into an axial slot  88  in the front end section  26  of the probe receptacle  14  that is likewise provided only at a peripheral position of the probe receptacle  14 . Consequently, when the probe holder  70  is pushed, in the way described and illustrated in  FIGS. 1 and 2 , onto the front end of the probe receptacle  14  and then rotated about the axis  11 , a defined rotary position is additionally reached when the ball  86  partially enters the axial slot  88  from the bore  84 . 
   As may be discerned particularly well from the perspective illustration in  FIG. 3 , but also from  FIG. 2 , the front section  80  of the probe holder  70  has a rear side  90  that extends substantially in a radial plane relative to the axis  11 . A cutout  91  of square shape, for example, is located in the rear side  90 . Bonding pockets  92  are located at the corners of the cutout  91 . 
   This arrangement serves for mounting a sensor system  100 , e.g. for adhering it. When being mounted, the sensor system  100  is inserted into the cutout  91  and then fixed on the rear side  90  by filling adhesive into the bonding pockets  92  and by subsequently hardening the adhesive. 
   The sensor system  100  fixed in this way is mechanically extremely sensitive, and so a protective cap  101  can be slipped onto the middle section  76  of the probe holder  70  in order to prevent damage to the sensor system  100  when the latter is not being operated. 
   The sensor system  100  that is known per se from DE 101 08 774 A1 cited at the beginning has either the square shape in plan view that emerges from  FIG. 3 , or a circular shape. 
   As may be gathered in further detail from  FIGS. 4A and 4B , the sensor system  100  has a thickened edge  102  and a raised middle  104 , which is called boss, a membrane area  106  being located between the edge  102  and middle  104 . The thickness of the edge  102  is denoted by D in  FIG. 4A , and the thickness of the membrane area  106  is denoted by d. The sensor system  100  is preferably of silicon based construction and can, for example, have an edge length of approximately 6 mm. In this exemplary embodiment, the thickness D is, for example, 0.5 mm, and the thickness d is approximately 30 μm. 
   Extending from the middle  104  in the direction of the axis  11  there is a stylus  108  at whose free end a ball  110  is located. In this exemplary embodiment, the stylus  108  can have a length of 8 mm and a diameter of 0.2 mm, while the stylus ball  110  can have a diameter of 0.3 mm. 
   Sensor elements  114  are provided on the rear side  112  of the sensor system  100 . The sensor elements  114  operate electrically. They can, for example, be constructed as strain gauges or as piezoelectric elements. The sensor system  100  with the stylus  108  and the sensor elements  114  forms an overall structural unit  116  that is adhered, that is to say premounted, in the way described above on the rear side  90  of the front section  80  of the probe holder  70 . 
     FIG. 4C  shows a first embodiment of a sensor system  100  according to the invention. In this sensor system, the membrane area  106  is constructed—in plan view—in the form of a rectangular, preferably square frame, and the raised middle  104  is constructed as a four-sided pyramidal frustum in the center of the membrane area  106  with a basic surface that is likewise square, as is known per se from DE 101 08 774 A1. The membrane area  106  has, for example, an outer edge length of 3 mm, and the pyramidal frustum a lower edge length of 1.6 mm. 
   In a second embodiment in accordance with  FIG. 4D , the membrane area  106  of a sensor system  100 ′ is likewise of square construction, whereas the raised middle  104 ′ is constructed in the form of a conical frustum. The lower diameter of the conical frustum is 1.6 mm, for example. 
   In a third embodiment, the membrane area  106 ′ of a sensor system  100 ″ is of circular construction in plan view and the raised middle  104 ′ is constructed, in turn, in the form of a conical frustum. The annular ring has, for example, an outside diameter of 3 mm. 
   In the case of the second and third embodiments, the site of maximum deformation at the inner edge is located inside the membrane area  106  or  106 ′. In these cases, it is therefore preferred to fit the sensor elements  114  there. 
   The second and third embodiments can also be used independently of the other features of the invention presently being described. 
   The electric contact with the sensor elements  114  is achieved with the aid of a plurality of spring pins  120 . The exemplary embodiment illustrated in  FIG. 5  provides four groups of in each case four spring pins offset in each case by 90° to one another and distributed around the axis  11 . The spring pins  120  each have a front section  122  that can be applied with the aid of a front tip  124  to an associated contact platelet (not illustrated) of a sensor element  114 . The front sections  122  of the spring pins  120  are mounted in an axially resilient fashion in rear sections  126  of the spring pins  120 . 
   In the case of the probe head  10  described above, the procedure for switching the stylus  108  between two measurement operations is as follows: 
   Firstly, the union nut  79  is screwed off from the external thread  20 . Next, the probe holder  70  is removed, with or without the protective cap  101  slipped on, from the front end of the probe receptacle  14  in the direction of the axis  11 . In this process, the sensor system  100  is automatically detached from the spring pins  120 , which are lifted from the respectively associated contact platelets of the sensor elements  114  during this withdrawal. 
   The next step now requires a new probe holder  70  with the desired other stylus  108  to be pushed in the axial direction onto the front end of the probe receptacle  14 , it being ensured by the rotation in the peripheral direction that the ball  86  passes into the position illustrated in  FIG. 2  in which the predetermined rotary position is reached. The probe holder  70  need now only be fixed in the axial direction by screwing the union nut  79  onto the external thread  20 . The conical fit 24/74 in this case automatically centers the probe holder  70  relative to the axis  11 . 
   The above mentioned mounting operations can easily be automated, in particular with the aid of an NC-controlled robot, in particular of the coordinate measuring machine itself.