Abstract:
A measuring head for a coordinate measuring machine for determining spatial coordinates on a measurement object has a coupling part for detachably receiving a measurement tool. The coupling part has a number of first bearing elements, a magnet and a retaining pin. The measurement tool has a disk with a number of second bearing elements, an anchoring plate and at least one adjustable locking element. The magnet is configured to attract the anchoring plate so as to bring the first and second bearing elements into engagement with one another. The first and second bearing elements, in the engaged state, define a defined position of the measurement tool on the coupling part. The at least one locking element secures the measurement tool to the retaining pin. The anchoring plate is detachably secured to the disk and the at least one locking element retains the anchoring plate on the retaining pin.

Description:
CROSSREFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of international patent application PCT/EP2012/057349 filed on Apr. 23, 2012 designating the U.S., which international patent application has been published in German language and claims priority from German patent application DE 10 2011 100 467.3 filed on May 2, 2011. The entire contents of these priority applications are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a measuring head for a coordinate measuring machine and to a measurement tool for such a measuring head. 
     Coordinate measuring machines are typically used for determining geometric dimensions or even the physical shape of workpieces. A coordinate measuring machine typically has a measuring head, which can be moved relative to the workpiece (called measurement object further down below). Depending on the position of the measuring head relative to the workpiece, spatial coordinates can be determined that represent the position of selected measurement points on the workpiece within a defined measurement volume. If an appropriate plurality of spatial coordinates are determined for a plurality of measurement points, the geometric dimensions and the shape of the measurement object can be determined on the basis of said spatial coordinates. 
     In many cases, the measuring head has a probe tool with which the selected measurement points are touched. Accordingly, such measuring heads are often referred to as touch probe heads. The corresponding probe tool typically has one or more styli having a tip which serves for probing the selected measurement points. However, there are also measuring heads for coordinate measurement machines which operate without contact, such as with lasers and/or cameras. The present invention is particularly suitable for tactile measurement tools (touch probe tools), but can also be used in non-contact measuring heads and corresponding measurement tools. For the sake of simplicity, however, reference is made to the preferred use, i.e. to a touch probe head having a touch probe tool, in the following. 
     In order to reach all measurement points in complex workpieces, it is often desirable for the probe stylus to be held in different orientations relative to the probe head. It is also often desirable to attach probe tools having different styli or stylus combinations to the probe head so as to allow all desired measurement points to be reached in an efficient manner. 
     DE 101 14 126 A1 discloses a probe head having an interchangeable probe tool. The probe tool has a stylus which is angled in the shape of an L and can be secured to the probe head in several rotation angle positions. A rotary disk of the probe tool is arranged above a kinematic three-point bearing at a defined rotation angle position on the probe head. In order to change the rotation angle position, the rotary disk is moved out of the three-point bearing using a pneumatically actuated slider and subsequently rotated on the slider via a rotary drive arranged in the probe head. Subsequently, the rotary disk is moved back into the defined three-point bearing. The rotation angle position of the rotary disk and of the stylus arranged thereon is determined using a sensor, which at the same time also provides signals for the rotary drive. 
     DE 10 2009 008 722 A1 describes a probe head for a coordinate measurement machine, in which the probe tool can be rotated using measurement force generators and what is referred to as roll projection. Measurement force generators in so-called active probe heads usually serve to apply a defined measurement force during the probing of a measurement point. They are present as a matter of principle in active probe heads and according to DE 10 2009 008 722 A1 are also used for rotating the probe tool. 
     In the probe head of DE 10 2009 008 722 A1, the rotary disk of the probe tool is held by a retaining pin, via which the rotary disk can be disengaged from its kinematic mount for the rotational movement. According to a specific exemplary embodiment, the retaining pin can be of a two-part design, such that the front free end of the retaining pin, on which the rotary disk is secured, can detach itself from the shaft of the retaining pin if the probe tool impacts too severely with the measurement object or with another obstacle. The separable implementation of the retaining pin thus allows collision protection, preventing at least major damage to the measuring head, measurement tool and/or obstacle. 
     The same type of collision protection is also described in DE 10 2005 043 454 B3, although for a probe head having a rigid (not rotatable) probe tool. 
     It has been found that the separable implementation of the retaining pin is unfavorable if the retaining pin also serves for rotating the probe tool and therefore has to absorb torques during the rotation. In this case, the retaining pin must ensure a reliable and stable connection not only with respect to the axial loads but also with respect to loads in the radial direction. At the same time, the free end of the retaining pin, on which the probe tool is secured, must be able to be released easily in the event of a collision. A suitable two-part or multipart realization of the retaining pin is complicated and expensive, especially if the probe head additionally needs to be lightweight and compact. 
     SUMMARY OF THE INVENTION 
     Against this background, it is an object of the present invention to provide a measuring head having an alternative collision protection. Preferably, the collision protection should be suitable for a measuring head having a rotatable measurement tool, such as a measurement tool that is rotatable by the method described in DE 10 2009 008 722 A1. 
     According to a first aspect of the invention, there is provided a measuring head comprising a coupling part for receiving a measurement tool, and comprising a measurement tool detachably coupled to the coupling part, wherein the coupling part has a number of first bearing elements, a magnet and a retaining pin, and wherein the measurement tool has a disk with a number of second bearing elements, an anchoring plate and at least one adjustable locking element, wherein the magnet is configured to attract the anchoring plate so as to bring the first and second bearing elements into engagement with one another, wherein the first and second bearing elements, in the engaged state, define a defined position of the measurement tool on the coupling part, wherein the at least one locking element secures the measurement tool to the retaining pin, wherein the anchoring plate is detachably secured to the disk and wherein the at least one locking element retains the anchoring plate on the retaining pin. 
     According to another aspect, there is provided a measurement tool for a measuring head having a coupling part for attaching the measurement tool, the coupling part comprising a number of first bearing elements, a magnet and a retaining pin, and the measurement tool comprising a disk with a number of second bearing elements, which, in engagement with the first bearing elements, define a defined position of the measurement tool on the coupling part, and the measurement tool further comprising an anchoring plate and at least one adjustable locking element configured to secure the measurement tool to the retaining pin, wherein the at least one locking element is arranged on the anchoring plate and the anchoring plate is detachably secured to the disk. 
     According to another aspect, there is provided a measuring head for a coordinate measuring machine, comprising a coupling part and a measurement tool detachably coupled to the coupling part, wherein the coupling part has a number of first bearing elements, a magnet and a retaining pin, and wherein the measurement tool has a disk with a number of second bearing elements, an anchoring plate and at least one adjustable locking element, wherein the magnet is configured to attract the anchoring plate so as to bring the first and second bearing elements into engagement with one another, wherein the first and second bearing elements, in the engaged state, define a defined position of the measurement tool on the coupling part, and wherein the at least one locking element secures the measurement tool to the retaining pin, wherein the anchoring plate is detachably secured to the disk and the at least one locking element retains the anchoring plate on the retaining pin. 
     Preferably, the anchoring plate is attached to the disk such that it is detachable in a destruction-free manner and thus reversibly in principle, i.e. the type of attachment permits repeated detachment and joining of the anchoring plate and the disk. 
     In the novel measuring head, a predetermined breaking point, which allows the measurement tool to reversibly separate from the measuring head in the event of a collision, has been moved into the measurement tool. In contrast, the predetermined breaking point of known measuring heads with collision protection is located in the measuring head. At first glance, the novel measuring head therefore might have the disadvantage that it is not protected by itself against damage in the event of a collision. Rather, collision protection depends on the coupled-on measurement tool. The danger here is that a user deactivates the collision protection when using a measurement tool that does not correspond to the present invention. The novel realization, however, has the advantage that the measuring head and in particular the retaining pin in the measuring head can be produced in a simpler and easier manner while also being more stable. 
     The novel approach further has the advantage that wear, which can be caused by friction on the predetermined breaking point, occurs in the (typically less expensive) measurement tool and not in the relatively expensive measuring head. If the predetermined breaking point for the collision protection exhibits wear due to collisions and/or play in everyday use, all that is needed is to replace the rather inexpensive measurement tool. 
     Surprisingly, the predetermined breaking point in the measurement tool does not adversely affect the accuracy of the measurement tool or of the measuring head having the novel measurement tool, since the relative position of the measurement tool on the coupling part continues to be determined by the first and second bearing elements. Even if the anchoring plate should have play with respect to the disk of the measurement tool, this does not decrease the positioning accuracy of the measurement tool on the coupling part, since the position of the measurement tool is determined only by the first and second bearing elements and not by the anchoring plate. 
     As will be explained below with reference to preferred exemplary embodiments, the novel measuring head can advantageously be realized as a probe head without an integrated rotary drive for rotating the probe tool. Rather, the novel measurement head is capable of rotating the probe tool by using one or more measurement force generators which generate a desired rotary movement using a roll projection on the probe head. The retaining pin substantially serves in this case merely for retaining the measurement tool on the probe head during the rotation so that the measurement tool does not fall down when the anchoring plate is removed from the magnet. Owing to the novel arrangement, the retaining pin can efficiently absorb radial loads which result from the rotary movement on the roll projection. 
     Overall, the novel measuring head having the novel measurement tool offers reliable collision protection, which can be realized more simply and more cost-effectively than in known measuring heads by the anchoring plate of the measurement tool detachably attaching in a destruction-free manner to the disk of the measurement tool, wherein the disk may hold a probe stylus, a camera, a laser or another sensor for determining spatial coordinates. The above-mentioned object is thus completely achieved. 
     In a preferred refinement, the magnet generates a defined first retaining force with which the anchoring plate is attracted, wherein the anchoring plate is secured to the disk with a defined second retaining force, and wherein the second retaining force is greater than the first retaining force. In some exemplary embodiments, the second retaining force may be greater than 80 N and preferably greater than 100 N. 
     At first glance, one could assume that it is advantageous for the collision protection if the predetermined breaking point in the measurement tool is more fragile than the connection between the measurement tool and the coupling part, since it is the predetermined breaking point in the measurement tool that is meant to yield in the event of a collision. However, it has been shown that the retaining force with which the magnet attracts the anchoring plate can still be smaller than the second retaining force with which the anchoring plate is held on the disk, since the first and second bearing elements, when engaged, provide additional stabilization of the connection. For this reason, the predetermined breaking point between the anchoring plate and the disk is more likely to detach in operation of the novel measuring head than the connection between the anchoring plate and the magnet, even if the retaining force of the magnet itself is smaller. On the other hand, this configuration has the advantage that the magnet cannot separate the anchoring plate from the disk of the measurement tool. The configuration therefore permits a more stable and reliable changing of the measurement tool. 
     In a further refinement, the at least one locking element is arranged below the anchoring plate if the measurement tool is coupled to the coupling part. 
     In this refinement, the locking element carries or supports the anchoring plate from below against the force of gravity when the anchoring plate is not attracted by the magnet in the coupling part. On the other hand, the anchoring plate retains the disk as long as the (second) retaining force between anchoring plate and disk is not subjected to excess pressure following a collision. One could say that the anchoring plate is suspended on the locking element and for its part retains the measurement tool, which extends downward from the locking element. The refinement permits a cost-effective realization by the locking element being placed in a recess in the disk and subsequently being fixed to the disk using the anchoring plate. In addition, this refinement makes it possible for the anchoring plate to rest against the magnet over a large area and thus makes possible a great first retaining force using a relatively small magnet. 
     In a further refinement, the measurement tool has at least one spring element, which secures the anchoring plate to the disk with the defined second retaining force. 
     Such fixing is a simple and cost-effective variant, which not only offers stable operation in all cases without collisions, but at the same time offers sufficient collision protection. In addition, the user can readily re-join the anchoring plate and the disk if the disk was separated from the anchoring plate in the case of a collision. 
     In a further refinement, the spring element is a helical spring, which extends annularly around the anchoring plate. 
     In this refinement, the spring element is a helical spring, which for its part is bent in the shape of a ring. The “core” of the helical spring preferably extends concentrically with respect to the retaining pin if the measurement tool is secured to the retaining pin. This refinement has proven to be a very reliable connection between the anchoring plate and the disk. The circumferential helical spring offers a highly uniform retaining force around the anchoring part. Point-type loads that could lead to increased wear are minimized. At the same time, the retaining force can be dimensioned effectively with such a spring element so as to achieve the preferred ratio of first and second retaining forces. 
     In a further refinement, the disk has a recess, in which the anchoring plate is held detachably. The anchoring plate preferably sits in the recess with accurate fit, with the spring element being arranged between the external circumference of the anchoring plate and the inner jacket of the recess. 
     This refinement permits highly reliable and yet detachable connection between the anchoring plate and the disk. It minimizes or even avoids the risk that the anchoring plate can become separated from the disk if the measurement tool is not on the retaining pin, for example if the measurement tool is placed in a tool magazine. The arrangement of the anchoring plate in the recess avoids impact locations or points of attack where the anchoring plate can become disengaged against the second retaining force. 
     In a further refinement, the retaining pin has a longitudinal axis and is displaceable axially along the longitudinal axis. 
     This refinement is advantageous in order to separate (disengage) the two bearing elements on the measurement tool from the first bearing elements on the coupling part, without the measurement tool and the measuring head completely disconnecting from each other. This refinement therefore simplifies the preferred rotation of the measurement tool relative to the measuring head. 
     In a further refinement, the retaining pin is rotatable about the longitudinal axis, wherein the first and second bearing elements define a plurality of defined rotation angle positions. 
     This refinement is based on the previously mentioned refinement by way of the retaining pin enabling a rotation of the measurement tool on the measuring head. The novel type of collision protection is very advantageous especially in those cases where the retaining pin must absorb various loads in different directions. 
     In a further refinement, the measuring head has a detector and the at least one locking element has a locking position in which it secures the anchoring plate to the retaining pin, and at least one release position, in which it releases the anchoring plate, wherein the detector generates a signal that is representative of the locking position and/or the release position. 
     In this refinement, the measuring head has a detector which is used to monitor the function of the locking mechanism. The detector generates a signal that is representative of at least one of the positions of the locking element (locking position and/or release position). Accordingly, the signal is configured to indicate the respective position of the locking element. The signal of the detector is preferably evaluated in the measuring head and/or a controller connected to the measuring head so as to early identify insufficient attachment of the measurement tool to the retaining pin and to output, in dependence thereon, a warning signal to the operator of the machine and/or trigger an operational stop. Furthermore preferred is that the control unit prevents, in dependence on the signal of the detector, disengagement of the rotary disk and, if appropriate, an associated change in the rotation angle position. 
     Alternatively or in addition, the locking mechanism could be of a fail-safe design, for example by way of a mechanical construction that rules out attachment of the probe tool with insufficient locking of the locking element. The use of a detector for generating a specific monitoring signal, however, simplifies the mechanical construction of the interface between measurement tool and coupling part. Moreover, the detector makes it possible to take into account changes in the mechanical interface between rotary disk and coupling part, such as by contamination or wear. 
     In a further refinement, the disk has at least one identification circuit and the coupling part has a sensor for reading the at least one identification circuit. 
     An identification circuit in the context of this refinement is a—preferably electronic—circuit containing coding that identifies the measurement tool. A preferred identification circuit includes a memory in which the coding is digitally stored. In principle, the identification circuit could also have mechanical coding, which is read mechanically, electrically and/or optically using a suitable sensor in the coupling part. The refinement has the advantage that the probe head can recognize the identity and properties of the probe tool simply and in an automated fashion, for example so as to determine the number of possible rotation angle positions and/or the presence of the novel detector. 
     In a further refinement, the detector is configured to prevent the identification circuit from being read by the sensor in dependence on the locking position and/or the release position. 
     This refinement makes possible a very simple, cost-effective and space-saving realization of the detector, by the detector using the coding which is already supplied by the identification circuit to generate the monitoring signal for the locking mechanism. In one preferred exemplary embodiment, the detector prevents the identification circuit being read if the locking element is not in its locking position. The “signal” of the detector in this case consists in the fact that the sensor receives no signal from the identification circuit. The detector can therefore be a passive element, which is advantageous for minimizing heating of the measuring head during the measurement operation. 
     In a further refinement, the detector comprises a passive electric switch, which is arranged electrically in series with the at least one identification circuit. This refinement makes possible a very simple, cost-effective and reliable realization of the detector. The switch is preferably closed only if the locking element is in its locking position, so that the identification circuit can only be read if the locking mechanism is closed. The electric switch is preferably a mechanically actuated microswitch. A switch of this type can be integrated well in the small installation space of a rotary disk. 
     In another refinement, the locking mechanism includes two—preferably spring-loaded—sliders, which are displaceable in mutually opposing directions in order to bring the at least one locking element into the release position. 
     Two opposed sliders facilitate simple and reliable opening and closing of the locking mechanism. Moreover, a locking mechanism having at least two opposed locking elements is robust and tolerant with respect to slight positioning inaccuracies when inserting the rotary disk. Such inaccuracies are compensated by opposed elements. Spring-loaded sliders have the advantage that they ensure a defined resting position in which the rotary disk is preferably locked on the retaining pin. 
     The preferred sliders each have a free end, wherein the free ends in the resting position are located diametrically with respect to one another and project radially beyond the rotary disk&#39;s edge. This refinement makes possible simple manual actuation and effectively reproducible machine actuation of the locking mechanism. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       It is to be understood that the previously mentioned features and the features explained below are applicable not only in the respectively specified combination but also in other combinations or alone, without deviating from the scope of the present invention. 
       Exemplary embodiments of the invention are illustrated in the drawing and will be explained in more detail in the following description. In the figures: 
         FIG. 1  shows a coordinate measuring machine having a probe head according to an exemplary embodiment of the invention, 
         FIG. 2  shows a highly simplified illustration of the probe head having a probe head sensor system and a measurement force generator, 
         FIG. 3  shows a preferred exemplary embodiment of the probe head with a view onto the coupling part from below, 
         FIG. 4  shows the coupling part of  FIG. 3  in a sectional view along the line IV-IV, 
         FIG. 5  shows an exemplary embodiment of a probe tool with a view onto the disk which can be coupled to the coupling part of  FIG. 3 , 
         FIG. 6  shows a simplified illustration of the locking mechanism of the probe tool of  FIG. 5 , 
         FIG. 7  shows a simplified illustration of the probe tool of  FIG. 5  on the coupling part of  FIGS. 3 and 4 , and 
         FIG. 8  shows the coupling part and the probe tool of  FIG. 7  in the event of a collision. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In  FIG. 1 , a coordinate measuring machine having the novel probe head is designated overall with the reference numeral  10 . The coordinate measuring machine  10  here has a base  12 , on which a portal  14  is arranged so as to be moveable in the longitudinal direction. The movement direction of the portal  14  relative to the base  12  is generally referred to as the Y axis. Arranged on the upper cross member of the portal  14  is a carriage  16 , which is displaceable in the transverse direction. The transverse direction is usually referred to as the X axis. The carriage  16  carries a quill  18 , which can be moved in the Z direction, i.e. perpendicular to the base  12 . Reference numerals  20 ,  22 ,  24  designate measurement scales, which can be used to determine the position of the portal  14 , of the carriage  16  and of the spindle  18 . The measurement scales  20 ,  22 ,  24  are typically glass measurement scales which are read using suitable sensors. 
     Arranged on the lower free end of the quill  18  is a probe head  26 , which holds a probe tool  27 . The probe tool  27  here has three styli  28 , which at their respective free ends each have a ball  29 . The ball is used to touch a measurement point on a measurement object  30 . The measurement scales  20 ,  22 ,  24  can be used to determine the position of the probe head  26  within the measurement volume during the probing of the measurement point. In dependence thereon, spatial coordinates of the probed measurement point within the measurement volume, which is defined by the movement axes of the measuring head, can be determined. 
     Reference  32  designates an evaluation and control unit which is connected to the drives and sensors on the portal via lines  34  and  36 . The control unit  32  serves to drive the drives for the movements of the probe head  26  along the three coordinate axes X, Y and Z. In addition, the evaluation and control unit  32  reads in the measurement values from the measurement devices  20 ,  22 ,  24 , and it determines, in dependence thereon and in dependence on the deflections of the probe tool  27 , the current spatial coordinates of the probed measurement point. 
       FIG. 2  shows, on the basis of a highly simplified, schematic illustration, the basic function of the probe head  26 . The probe head  26  has a body part  38  and a coupling part  40 , which in this case are connected via two leaf springs  42  and  44 . The leaf springs  42 ,  44  form a spring parallelogram which allows the coupling part  40  to move in the direction of the arrow  46  (and back in the direction of the arrow  46 ′). The probe tool  27  with the styli  28  can thus be deflected from its resting position by a distance D. 
     There is a leg  48 ,  50  arranged on each of the body part  38  and the movable part  40 , respectively. The legs  48 ,  50  are parallel to the leaf springs  42 ,  44  here. A deflection detector  52  (having a plunger coil  53  and a plunger body  54  in this case) and a measurement force generator  56  are arranged between the legs  48 ,  50 . The plunger coil  53  generates an electrical signal in dependence on the plunging movement of the plunger body  54 . Alternatively or in addition, Hall sensors, piezoresistive sensors, magnetoresistive sensors or any other sensor (such as optical sensors) which can be used to determine the spatial deflection of the probe tool  27  relative to the body part  38  are conceivable deflection detectors  52 . The measurement force generator  56  is in this case likewise configured as a plunger coil. The measurement force generator  56  can be used to pull together the two legs  42  and  50  or to push them apart. 
     In the highly simplified illustration in  FIG. 2 , the probe head  26  allows the probe tool  27  to be deflected merely in the direction of the arrow  46 . A person skilled in the art will know, however, that a probe head  26  typically allows corresponding deflection in two other, orthogonal spatial directions. This can be realized for example using further spring parallelograms and/or a diaphragm spring. However, the invention is not limited to this specific realization and can also be used in other types of measuring heads. 
       FIG. 3  shows a preferred exemplary embodiment of the probe head  26  with a view onto the change interface from below (that is to say without probe tool  27 ).  FIG. 4  shows a simplified section of the probe head of  FIG. 3  along the section line IV-IV. 
     The body part  38  holds the coupling part  40 , which is movable on the body part  38  in preferably three orthogonal spatial directions. For the sake of simplicity,  FIG. 4  shows only two spring elements  42  which make possible the three orthogonal movement directions. The coupling part  40  here has a pin  57 , which is guided in the coupling part  40  to move axially, i.e. along its longitudinal axis. (The axial movement is described in detail in DE 10 2009 008 722 A1 mentioned in the introduction, which is incorporated here by reference). In the edge region of the coupling part  40 , three ball pairs  58  are arranged which form first bearing elements for a kinematic mount of the probe tool  27 . The coupling part  40  furthermore has a magnet  60 , here in the form of an annular electromagnet. Alternatively, the magnet may be a permanent magnet, which is strengthened or weakened by an additional electromagnet. The magnet  60  is arranged here concentrically with respect to the pin  57  on the coupling part  40 . The coupling part  40  in this exemplary embodiment further has a first sensor  62  with two contacts  64  and a second sensor  66 . Finally, a cylinder sleeve  68  having an inner jacket  70  is formed on the body part  38 . The cylinder sleeve  68  here extends concentrically with respect to the pin  57  and forms a roll projection  68 , which can be used in preferred exemplary embodiments for rotating the probe tool  27 . The rotation of the probe tool  27  using the roll projection  68  is described in detail in DE 10 2009 008 722 A1 already mentioned, which again is incorporated here by reference insofar. 
     As has already been mentioned in the explanation of  FIG. 2 , the position of the coupling part  40  relative to the body part  38  can be changed using measurement force generators  56 . This is usually done to generate a defined measurement force when probing a measurement point. In order to additionally permit advantageous movement of the pin  57  relative to the coupling part  40 , a stop  72  is provided in the illustrated exemplary embodiment, which stop  72  is in this case formed on the body part  38  or is at least rigidly connected thereto. The stop  72  interacts with a counterpiece  73 , which is formed on the upper end of the pin  57 . In  FIG. 4 , the upper end of the pin  57  projects upwardly beyond the stop  72 , and the counterpiece  73  is arranged above the stop  72 . If the coupling part  40  is pressed down using the measurement force generator  56 , the pin  57  follows this movement until the counterpiece  73  abuts the stop  72  from above. From this position, the pin  57  is blocked against any further downward movement. The coupling part  40 , on the other hand, can be pressed down further using the measurement force generator  56 . From the point at which the pin  57  is blocked on the stop  72  by the counterpiece  73 , the measurement force generator  56  only moves the coupling part  40  down, and no longer the pin  57 . In other words, the measurement force generator  56  pushes the coupling part  40  downward relative to the pin  57 . Since the pin  57 , at its bottom free end, is configured to retain the probe tool  27  (see  FIGS. 7 and 8 ), the measurement force generator  56  and the stop  72  can be used to vary the distance between the coupling part  40  and the probe tool  27 . In exemplary embodiments, this is advantageously used to “gently” move the coupling part  40  toward the probe tool  27  and subsequently attract it using the magnet  60 . 
       FIG. 5  shows an exemplary embodiment of the probe tool  27  with a view onto the interface by which the probe tool  27  is coupled to the coupling part  40 . The probe tool  27  has a rotary disk  74 , which in this case is in the shape of a circle. A traction element  76  is arranged on the external circumference of the rotary disk  74 . The traction element may be a rubber ring or an external tooth system, which interacts with a corresponding tooth system on the inner jacket  70  (not illustrated here) of the roll projection  68 . The rotary disk  74  in this case has a plurality of rollers  78 , which interact as bearing elements with the ball pairs  58  on the coupling part  40  so as to effect a reproducible, kinematically determined mounting of the probe tool  27  on the coupling part  40 . Other bearing elements which effect a kinematic mounting of the probe tool  27  on the coupling part  40 , such as a Hirth tooth system, can also be used instead of the bearing elements that are illustrated here in the shape of rollers and ball pairs. Furthermore, the rollers  78  can in principle be the bearing elements on the coupling part  40 , while ball pairs  58  are formed on the rotary disk  74 . 
     The rollers  78  are arranged here in the circumferential direction of the rotary disk  74  with equal spacings between them. Two contacts  80 , which interact with the contacts  66  on the coupling part  40  if the rotary disk  74  on the coupling part  40  is secured in a rotation position that is defined by the rollers  78  and ball pairs  58 , are arranged radially inwards with respect to each roller  78 . Reference numeral  82  designates an identification circuit, for example in the form of a memory chip, on which individual coding is stored. Each identification circuit therefore contains unique information. The sensor  64  can in each case read only one identification circuit  82  via the contacts  66  and recognize, on the basis of the read coding, the rotation angle position of the probe tool  27  relative to the coupling part  40  and possibly further properties of the probe tool, such as the length of the probe stylus. 
     An anchoring plate  83  having a holder in the form of a circular opening  84  is arranged in the center of the rotary disk  74 . The anchoring plate consists of a magnetizable material, such that it can be attracted by the magnet  60  on the coupling part  40 . Two locking elements  86 , which can be used to additionally secure the rotary disk  74  on the bottom free end of the pin  57 , are arranged here on the anchoring plate  83 . This allows, in the preferred exemplary embodiments, the rotation of the probe tool  27  via the pin  57  while the rollers  78  are disengaged from the ball pairs  58 . In the illustrated exemplary embodiment, the locking elements  92  are two rods which are configured to engage in a groove at the bottom free end of the pin  57  (see  FIGS. 7 and 8 ). 
       FIG. 6  shows a locking mechanism with which the locking elements  86  can be opened or closed in order to secure the rotary disk  74  on the pin  57 . In this exemplary embodiment, the locking mechanism has two sliders  87   a,    87   b,  which are displaceable in mutually opposing directions. Each slider  87   a,    87   b  is pre-tensioned via a spring element  88  into a resting position. In the preferred exemplary embodiments, the spring elements  88  pretension the sliders  87  into a resting position in which the locking elements  86  clamp the rotary disk  74  in place on the pin  57 . In the preferred exemplary embodiment, each slider  87  is connected to in each case one clamping piece  89   a,    89   b.  A locking pin  86  is arranged on each clamping piece  89   a,    89   b.    
     Owing to the sliders  87   a,    87   b  being pushed together in the mutually opposing directions of the arrows shown in  FIG. 6 , the clamping pieces  89  can be pushed apart. Letting go of the sliders  87  results in the spring elements  88  pushing the locking pins  86  back together. For opening the locking mechanism, each slider  87  has a free end  91   a,    91   b,  which projects outwardly beyond the external circumference of the rotary disk  74 . In the preferred exemplary embodiments, the free ends  91   a,    91   b  of the sliders  87   a,    87   b  are located diametrically with respect to one another on the external circumference of the rotary disk  74 . 
     In the preferred exemplary embodiments, the locking mechanism furthermore includes a detector  93  which is configured to detect the locking position and/or the release position of the locking elements  86 . In the exemplary embodiment according to  FIG. 6 , the detector  93  is a microswitch with a switch contact which is arranged electrically in series with all contact pairs  80  of the rotary disk  74 . The switch contact can be a mechanical contact or an electronic switch, such as in the form of a transistor. The switching position of the switch  93  is influenced by the slider  87   a.  In the position shown in  FIG. 6 , a lug  94  of the slider  87   a  touches the switch  93 . The switch contact  95  is closed by the lug  94 . In this position, the sensor  62  in the probe head  27  can read the identification circuit  82  whose contacts  80  are in contact with the contacts  64  on the coupling part. However, if the lug  94  does not press onto the switch  93 , the sensor  62  cannot read any of the identification circuits  82 . The absence of an identification signal in one of the identification circuits  82  is a signal that is used by the detector  93  to indicate that the locking elements  86  are not properly closed. 
       FIGS. 7 and 8  show a sectional view of the coupling part  40  with the rotary disk  74  in an operating position, in which the rollers  78  are disengaged from the ball pairs  58  such that the rotary disk  74  can be rotated on the roll projection  68 . As can be seen in  FIG. 8 , the rotary disk  74  has a recess  96 , in which the magnetizable anchoring plate  83  is held with an accurate fit. In this exemplary embodiment, the anchoring plate  83  is held in the recess  96  by a spring element  98 . The spring element  98  in this case is a helical spring, which is arranged in the shape of a ring around the (in this case circular) anchoring plate  83  on said anchoring plate. Alternatively or in supplementation, the spring element  98  could be arranged on the rotary disk  74 , for example in the recess  96 . In one preferred exemplary embodiment, the spring element  98  is a spiral spring which is arranged to form a ring, as is available for example from Bal Seal Engineering, Inc., 19650 Pauling, Foothill Ranch, Calif. 92610-2610, USA. 
     In other exemplary embodiments, the anchoring plate can be secured in the recess  96  using a retaining ring, using tension wires, using spring-loaded retaining pins and/or using further magnets (not illustrated here). 
     It is preferred in all exemplary embodiments if the retaining force with which the anchoring plate  83  is secured to the rotary disk  74  is greater than the retaining force with which the anchoring plate  83  is attracted by the magnet  60  in the coupling part  40 , such that the magnet  60  cannot tear the anchoring plate  83  off the rotary disk  74 . At the same time, the retaining force with which the anchoring plate  83  is secured to the rotary disk  74  should be only somewhat greater than the retaining force with which the anchoring plate  83  is attracted by the magnet  60  in the coupling part  40 , so that the rotary disk  74  can easily pull away from the anchoring plate  83  and the coupling part  40  in the event of a collision with an obstacle, without a damaging introduction of force into the coupling part  40  occurring. 
     As can be seen in the illustration in  FIG. 8 , the anchoring plate  83  with the locking mechanism remains on the retaining pin  57  in the event of a collision, while the rotary disk  74  pulls away from the anchoring plate  83 . In some exemplary embodiments, the anchoring plate  83  can additionally be connected via a wire or another flexible element (not illustrated here) to the disk  74  of the measurement tool, so as to prevent the disk  74  from falling onto the workpiece or the base of the coordinate measuring machine in an uncontrolled manner when it pulls away from the anchoring plate. Such a safety wire is known for example from DE 10 2009 008 722 A1 mentioned in the introduction, which to this extent is also incorporated here by reference.