Abstract:
A coordinate measuring machine for the metrological determination of a measurement coordinate on a test object comprises a table for mounting the test object. A first and a second bearing support are arranged on the table. A first and a second carriage, each moveable in a longitudinal direction, are mounted on the bearing supports. According to one aspect of the invention, the carriages are each mounted on the bearing supports via at least three, preferably four separate bearing points, with the bearing points defining a plane that is perpendicular to the plane of the table.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of international patent application PCT/EP2005/003133 filed on Mar. 24, 2005 designating the U.S., which international application has been published in German language as WO 2005/100906 A1 and claims priority of German patent application DE 10 2004 020 996.0 filed on Apr. 19, 2004. The entire contents of these priority applications are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a coordinate measuring machine for determining 2D or 3D coordinates of a test object. More particularly, the invention relates to coordinate measuring machines designed for providing extremely high 2D or 3D measurement accuracies.  
         [0003]     Coordinate measuring machines of this type might be used for determining the object form of a test object with high accuracy. For example, they might be used for checking the object form of machine-produced workpieces for quality control. Typically, a touch probe having a movably mounted stylus is used as a measuring probe, and this touch probe is moved, by means of a suitable traversing mechanism of the coordinate measuring machine, up to the test object until the touch probe stylus touches a desired measurement point on the test object. The spatial coordinate of the sensed measurement point can then be determined from the position of the touch probe and the displacement of the stylus relative to the touch probe. The invention is not, however, restricted to coordinate measuring machines with tactile touch probes, and can also be used in coordinate measuring machines with different measuring probes, such as optical probes.  
         [0004]     Various concepts are known for the traversing mechanisms. In the case of machines for three-dimensional measurement (3D coordinate measuring machines), typical concepts are the portal design, horizontal-arm coordinate measuring machines and coordinate measuring machines having a column design. Coordinate measuring machines of portal design comprise a portal moveable in the X-direction and having a cross beam on which a carriage moveable in the Y-direction is arranged. The Y-carriage carries a quill, which is moveable in the Z-direction. Such a coordinate measuring machine is known for example from DE 101 24 493 A1.  
         [0005]     Horizontal-arm coordinate measuring machines have a pillar that is moveable in the X-direction, on which a carriage moveable in the Z-direction is arranged. The Z-carriage carries an arm that is adjustable in the Y-direction, the free end of the arm carries the touch probe.  
         [0006]     Coordinate measuring machines of column design have a column, which carries the quill that is moveable in the Z-direction. The column itself can be moved in the X- and/or Y-direction. Alternatively, the X- and Y-guide axes can also be provided by a correspondingly moveable table for mounting the test object.  
         [0007]     The known concepts each have design-related advantages and disadvantages, in particular with respect to the accessibility of the measurement volume and susceptibility to static and dynamic deformations and the resultant measurement errors. As will easily be understood, there is for example deformation of the cantilever horizontal arm in a horizontal-arm coordinate measuring machine in relation to the Y-position of the touch probe, under the effect of gravity.  
         [0008]     Another kind of a coordinate measuring machine traversing mechanism is disclosed by the thesis “High-Precision 3D-Coordinate Measuring Machine” by Marc Vermeulen, which is deposited at the Eindhoven Technical University and is available under ISBN number 90-386-2631-2. The thesis can also be found on the Internet at the address http://alexandria.tue.nl/extra2/9902695.pdf.  
         [0009]     In the aforementioned thesis, a coordinate measuring machine having a novel traversing mechanism is proposed, which makes it possible to eliminate at least some sources of errors due to static and dynamic deformations, so that the measurement accuracy can be increased. Basically the proposed coordinate measuring machine has two bearing supports arranged in a V-shape relative to each other and acting as linear guideways for the X- and Y-direction. Each bearing support carries a carriage that can be displaced longitudinally, and a beam that can be displaced perpendicularly to the bearing support is arranged on each carriage. The two beams are once again arranged in a V-shape relative to one another, with the first beam extending substantially parallel to the second bearing support and vice versa. The free ends of the beams, facing one another, are connected to a platform, which is thus moveable in the X- and Y-direction and carries a quill that is moveable in the Z-direction. Practical tests with the proposed coordinate measuring machine have shown that a traversing mechanism of this type allows to achieve far higher measurement accuracy than with the designs known hitherto.  
         [0010]     Although the modified design of the traversing mechanism already eliminates some sources of errors, or reduces their effects, it is still desirable to achieve even greater measurement accuracy.  
       SUMMARY OF THE INVENTION  
       [0011]     Therefore, it is an object of the present invention to provide a coordinate measuring machine of the kind described above, but with an improved design of the traversing mechanism so that even higher measurement accuracies can be achieved.  
         [0012]     According to one aspect of the invention, there is provided a coordinate measuring machine for the metrological determination of a measurement coordinate on a test object, comprising a table for mounting the test object, the table defining a first plane, a probe holder for receiving a measuring probe, a first bearing support, on which a first carriage is mounted moveably in a first longitudinal direction, a second bearing support, on which a second carriage is mounted moveably in a second longitudinal direction, and a first and a second beam, each of which being coupled to the probe holder, wherein the first beam extends parallel to the second bearing support and is arranged on the first carriage such that it can be moved longitudinally in the second direction, and wherein the second beam extends parallel to the first bearing support and is arranged on the second carriage such that it can be moved longitudinally in the first direction, wherein each carriage is mounted on the respective bearing support via at least three vertical bearing points, which define a vertical bearing plane perpendicular to the first plane.  
         [0013]     According to another aspect, there is provided a coordinate measuring machine for the metrological determination of a measurement coordinate on a test object, comprising a table for mounting the test object, the table having a topside and an underside, a probe holder for receiving a measuring probe, a platform on which the probe holder is arranged, a first bearing support, on which a first carriage is mounted moveably in a first longitudinal direction, a second bearing support, on which a second carriage is mounted moveably in a second longitudinal direction, and a first and a second beam, each of which being coupled to platform, wherein the first beam extends parallel to the second bearing support and is arranged on the first carriage such that it can be moved longitudinally in the second direction, wherein the second beam extends parallel to the first bearing support and is arranged on the second carriage such that it can be moved longitudinally in the first direction, wherein the platform is supported on the table via at least three platform bearings which bear against the topside and via at least three platform counter bearings which bear against the underside and cooperate with the platform bearings on the topside.  
         [0014]     The carriages in the known coordinate measuring machines are always mounted on the bearing supports via just two bearing points, with the two bearing points lying on a straight line parallel to the respective direction of movement of the carriage. The provision of a third and preferably a fourth bearing point, which define a plane perpendicular to the plane of the table, gives a relative increase in stability and therefore in the accuracy of guidance. Practical tests have shown that the measurement accuracy can be increased even further in this way, because the probe holder can be positioned even more precisely in the measurement volume. In particular, the repeat accuracy in repeated approach of one and the same measurement point is increased by the modified bearing concept.  
         [0015]     In addition, the known coordinate measuring machine has only one central counter bearing provided on the underside of the table. The design preferred here permits a more precise guidance and therefore greater measurement accuracy. Moreover, this design can be produced with less weight and smaller space requirements, which permits a higher speed of measurement without disadvantages in terms of measurement accuracy.  
         [0016]     In a preferred refinement, each bearing support has a slot-shaped opening, through which one of the beams projects, wherein the bearing points lie outside of a plane defined by the slot-shaped opening.  
         [0017]     This design makes it possible to guide the beams “inside” the bearing supports, as is already known per se from the coordinate measuring machine described above. By moving the bearing points outside of the plane defined by the slot-shaped openings, however, it is possible to achieve a larger amount of travel and in consequence a larger measurement volume, without any perceptible impairment of the attainable measurement accuracy. Therefore even higher relative accuracy can be achieved relative to the measurement volume.  
         [0018]     In a further refinement, at least one bearing point lies on either side of the plane defined by the slot-shaped opening.  
         [0019]     This design means that there is at least one bearing point both above and below each beam, guiding the carriage on the bearing support. This gives even better accuracy of positioning, and especially when the corresponding beam travels a large distance in its traversing direction. In this design the attainable measurement accuracy is even higher, and in fact even with an increased measurement volume relative to the arrangement known hitherto.  
         [0020]     In a further refinement, each bearing support has an upper guide surface, on which the carriage is vertically supported via at least one bearing point, and preferably via at least two separate bearing points.  
         [0021]     This design makes it possible to arrange the carriages so that they are “floating” on the bearing supports, i.e. without further support on the table. In that case the carriages are decoupled from the table. In contrast, the carriages in the known coordinate measuring machine are supported both on the bearing supports and on the table. This preferred embodiment gives at least equally high accuracy of guidance and measurement at a lower production cost.  
         [0022]     In a further refinement, the probe holder is arranged on a platform, which is connected to the first and second beams and is supported on the table via at least one, preferably three platform bearings, wherein for each platform bearing there is a counter bearing which bears against the underside of the table. It is especially preferred if each platform bearing and its respective counter bearing are arranged on the same axis extending perpendicular to the table.  
         [0023]     This preferred design permits even more precise guidance and therefore even greater measurement accuracy. Moreover, this design can be produced with less weight and smaller space requirements, which permits a higher speed of measurement without disadvantages in terms of measurement accuracy.  
         [0024]     In a further refinement, the table has recesses under the bearing supports, and each platform bearing is connected to a platform counter bearing by a frame passing through the recesses.  
         [0025]     The design preferred here is less heavy and less spacious, and therefore permits higher measurement speeds with at least equal measurement accuracy.  
         [0026]     In a further embodiment the platform counter bearings are connected together by cross ties.  
         [0027]     This design leads, with little additional weight, to even further improved accuracy of guidance and hence to even higher measurement accuracy.  
         [0028]     In a further embodiment, the new coordinate measuring machine comprises a quill having a drive for longitudinal displacement in a third direction and having a measuring system for determining a longitudinal position of the quill in the third direction, and further comprising a plurality of quill bearings, which define the position of the quill perpendicularly to the third direction, wherein the drive and the measuring system are arranged in the same quadrant of an imaginary coordinate system having its origin at the center of gravity of the quill and having its axes extending parallel to planes that are defined by the quill bearings.  
         [0029]     Whereas the refinements described up to now primarily affect the guidance accuracy in the first two directions of movement, this refinement mainly increases the guidance accuracy in the third coordinate direction. The arrangement of the drive and of the measuring system in the same quadrant of the imaginary coordinate system means that the measuring system can determine the position of the quill more precisely and with better tolerance with respect to static and dynamic effects.  
         [0030]     It goes without saying that the features described so far and those yet to be explained below can be used not only in the respective stated combination, but also in other combinations or on their own, without departing from the scope of the present invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0031]     Embodiments of the invention are shown in the drawings and are explained in more detail in the description given below. In the drawings:  
         [0032]      FIG. 1  is a perspective top view of the new coordinate measuring machine,  
         [0033]      FIG. 2  shows the coordinate measuring machine from  FIG. 1  in an oblique view from below, without showing the supporting feet of the table for sake of clarity,  
         [0034]      FIG. 3  shows the coordinate measuring machine from  FIG. 1  in an oblique view from above, with certain parts not being shown for reasons of greater clarity,  
         [0035]      FIG. 4  shows the bearing supports and carriages of the coordinate measuring machine according to  FIG. 3  in a partially cut-away view, and  
         [0036]      FIG. 5  shows the quill of the coordinate measuring machine from  FIG. 1 , viewed from above.  
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0037]     In the drawings, an embodiment of the new coordinate measuring machine is designated by reference number  10  in its entirety. The coordinate measuring machine  10  has a table  12 , which is preferably made of granite. On the upper side of table  12 , two bearing supports  16  and  18  are arranged in a V-shape relative to one another, and in fact at right angles to one another in the preferred embodiment. The latter is not, however, absolutely necessary for carrying out the present invention. A carriage  20 ,  22  is mounted on each of the bearing supports  16 ,  18 , as can be seen most clearly in  FIGS. 3 and 4 . Carriage  20  can be moved longitudinally in the direction of arrow  24 , and carriage  22  can be moved longitudinally in the direction of arrow  26 . The directions  24 ,  26  are the X- and Y-direction for the coordinate measuring machine  10 .  
         [0038]     Each carriage  20 ,  22  carries a beam  28 ,  30 , as can be seen best in  FIG. 3 . Beam  28  extends parallel to bearing support  18  and can be moved longitudinally in the direction of arrow  26 . Beam  30  extends parallel to bearing support  16  and can be moved in the direction of arrow  24 . Therefore, the two beams  28 ,  30  are also arranged in a V-shape relative to one another, the two V-shaped structures of bearing supports  16 ,  18  and beams  28 ,  30  having their free ends facing one another or “plugged” into one another.  
         [0039]     The free ends of beams  28 ,  30  facing each other are connected to a platform  32 , which in turn is supported on the upper side of table  12  via bearings  34 ,  36 ,  38 . Since carriages  20 ,  22  and beams  28 ,  30  can be moved longitudinally, platform  32  is moveable on table  12  in directions  24 ,  26 .  
         [0040]     On its upper side, platform  32  has a roof-shaped hood  40 , which is above the bearing supports  16 ,  18  and carriages  20 ,  22  and is supported on the outer free ends of beams  28 ,  30 . Hood  40  is made of aluminum plates, which have numerous holes, so as to reduce the weight of hood  40 . It serves for stiffening the arrangement comprising the beams  28 ,  30  and platform  32 .  
         [0041]     The three bearings  34 ,  36 ,  38  of platform  32  form the corners of a substantially equal-sided triangle. Above this triangle, a quill  42  is arranged on platform  32 . Quill  42  can be moved longitudinally in the direction of arrow  44  ( FIG. 3 ), this direction being perpendicular to the plane of the table  12  and thus representing the Z-direction.  
         [0042]     At the lower free end of quill  42 , a probe holder  46  is arranged for receiving a measuring probe. In the embodiment shown here, the coordinate measuring machine  10  even has two probe holders  46 , namely a first probe holder  46   a  for a tactile touch probe and a second probe holder  46   b  for an optical measuring system. Quill  42  is mounted on a total of 8 air bearings  48 , which bear against three external sides, each flat, of quill  42  (cf.  FIG. 5 ). The carriage of a linear drive  50  is located on the fourth external side, as is explained in more detail below with reference to  FIG. 5 .  
         [0043]     As can be seen in the partially cut-away view in  FIG. 4 , each bearing support  16 ,  18  has a parallelepiped-shaped portion, along which the respective carriage  20 ,  22  slides. Each carriage  20 ,  22  wraps round the parallelepiped-shaped portion like a sleeve and is supported on an upper guide surface  52  in each case via two bearings  54 ,  56  arranged separately from one another. There are two counter bearings  58 ,  60  on the parallel bottom-sided guide surface of the parallelepiped-shaped portion, the two counter bearings lying on the same axis below bearings  54 ,  56 .  
         [0044]     Moreover, each carriage  20 ,  22  is supported via four bearings  62 ,  64 ,  66 ,  68  and four counter bearings, in each case on the same axis, on the vertical guide surfaces of the respective parallelepiped-shaped portion. All the bearings mentioned so far are air bearings, and bearing  66  is shown with its respective cable connection in the partially cut-away view of  FIG. 4 . In the preferred embodiment, bearings  62 ,  64 ,  66 ,  68  of each carriage  20 ,  22  are designed as solid bearings, whereas the counter bearings, which cannot be seen in  FIG. 4 , are moveable bearings.  
         [0045]      FIG. 4  also shows that the parallelepiped-shaped portions of bearing supports  16 ,  18  each have a horizontal, slot-shaped opening  70 , which can, however, only be seen for bearing support  18  in the view given in  FIG. 4 . Beams  28 ,  30  pass through the slot-shaped openings  70 , so that opening  70  permits displacement of carriages  20 ,  22  perpendicularly to the respective beam  28 ,  30 .  
         [0046]     The slot-shaped openings  70  define a horizontal plane  72  (or a horizontal space volume with the height of the openings), and bearings  62 ,  64 ,  66 ,  68  are arranged in such a way that they are outside of plane  72  or outside of the space volume. In fact bearings  62 ,  64  (with their respective counter bearings) are arranged above plane  72 , whereas bearings  66 ,  68  are below plane  72 . Bearings  62 - 68  (and their corresponding counter bearings) thus define a bearing plane  74 , which lies perpendicularly to plane  72  and perpendicularly to the plane of the table  12 .  
         [0047]     Beams  28 ,  30  are also supported by air bearings in carriages  20 ,  22 , one being designated with reference number  76  in  FIG. 4 .  
         [0048]     Beneath bearing support  16 , table  12  has a recess  78 , and two further recesses  80 ,  82  are arranged beneath bearing support  18 . C-shaped frames  84  pass through the recesses  78 ,  80 ,  82 , and wrap around table  12  above and below. The upper free end of each frame  84  is connected to platform  32 , namely in the region of bearings  34 ,  36 ,  38 . The lower free end of each frame  84  carries a counter bearing  86 ,  88  or  90 . Counter bearing  86  is arranged beneath bearing  34  on the same axis, whereas counter bearings  88 ,  90  are each located on the same axis under the bearings  36 ,  38 . In addition, the lower free ends of frames  84  are joined together by means of three cross ties  92 ,  94 ,  96 .  
         [0049]     The coordinate measuring machine  10  thus has a different bearing concept than the coordinate measuring machine from the thesis mentioned at the beginning, both with respect to the mounting of the carriages  20 ,  22  and with respect to the mounting of platform  32 . As a result the new coordinate measuring machine  10  can be produced more economically but its maximum attainable measurement accuracy is higher.  
         [0050]     A further improvement over the known coordinate measuring machine can be seen from  FIG. 5 . This shows quill  42 , moveable in the Z-direction, as viewed from above. In addition, a coordinate system is shown with two axes  98 ,  100 , with its origin at the center of gravity of quill  42 . The coordinate system forms four quadrants, and the arrangement of quill  42  is selected so that the linear drive  50  and the measuring system for determination of the vertical Z-position (designated here by reference number  104 ) are arranged in the same quadrant IV. As already mentioned above, this allows to achieve a higher measurement accuracy.