Abstract:
A laser based measuring apparatus for measuring a position of a distant target is disclosed. Some embodiments may include a laser tracker for detecting the position and the orientation of a measuring aid The laser-based measuring apparatus may include a base, a support, a telescope unit means, a first bearing apparatus, and a second bearing apparatus. In some embodiments the first bearing apparatus is in the form of a fixed/loose bearing apparatus, having a shaft, the longitudinal axis of which runs coaxially with the tilt axis, a fixed bearing and a loose bearing, and/or the second bearing apparatus is in the form of a fixed/loose bearing apparatus, having a shaft, the longitudinal axis of which runs coaxially with the vertical axis, a fixed bearing and a loose bearing.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a laser-based coordinate measuring device, in particular configured as a laser tracker, for measuring coordinates on target objects, comprising at least one fixed/loose bearing apparatus for a shaft of a rotatable telescope unit and/or for a shaft of a rotatable support, and to the use of a fixed/loose bearing apparatus for such a coordinate measuring device. 
     BACKGROUND 
     A coordinate measuring device of the type mentioned in the introduction comprises a base which defines an upright axis, a support, and a telescope unit for emitting a measurement beam and for receiving at least a part of the measurement radiation reflected at a target. The orientation of the telescope unit is carried out in two axes (upright axis or vertical axis, and inclination axis or tilt axis) by means of motors. The support can be swiveled in a motorized fashion about the upright axis relative to the base, and the telescope unit can be swiveled in a motorized fashion about a tilt axis relative to the support. A measurement axis is defined by an emission direction of the measurement radiation. 
     The telescope unit is equipped with opto-electro-mechanical components and is mounted, rotatably by means of a shaft about the tilt axis, at one or two bearing positions on the support, which is optionally likewise equipped with opto-electro-mechanical components. 
     WO 2007/079600 A1 discloses, with a laser tracker, such a laser-based coordinate measuring device having a telescope unit which can be rotated with respect to at least two axes and in which light emission and light reception optics of the distance measuring apparatus, a measurement camera and a viewfinder camera are arranged. The telescope element is mounted, rotatably about a tilt axis, on a support element, and the support element is mounted, rotatably about an upright axis, on a stationary base. 
     In coordinate measuring devices of the type mentioned in the introduction, which are known from the prior art, a shaft is respectively mounted fixed on both sides along the tilt axis and/or the upright axis. If, as is generally usual, different materials are used for the shaft, bearing or support, the clamping of the bearing varies as a function of the working temperature range. The flow of force which results from the clamping of the bearing of the tilt axis is transmitted via the brace of the support to the tilt axis. Hysteresis effects detrimentally affect the accuracy of the coordinate measuring device. 
     Radial displacement of the axes, for example by bearing air, in this case leads to accuracy losses. 
     SUMMARY 
     Some embodiments of the invention may provide for a bearing concept for a coordinate measuring device of the type mentioned in the introduction, which improves the accuracy of the measurement in comparison with the prior art. 
     Some embodiments of the invention may include the laser-based measuring apparatus, having a fixed/loose bearing apparatus and by the use of a fixed/loose bearing apparatus in a laser-based measuring apparatus. 
     According to the invention, a laser-based coordinate measuring device comprises, along the tilt axis and/or along the upright axis, a shaft mounted by means of a fixed/loose bearing. 
     Preferably, a support, rotatable about the upright axis, of the coordinate measuring device in this case has a shaft mounted in a base by means of a fixed/loose bearing. Furthermore, a telescope unit, rotatable about the tilt axis, of the coordinate measuring device preferably has a shaft mounted on the support by means of a fixed/loose bearing. 
     The advantages of this fixed/loose bearing according to the invention over the prior art are, in particular, as follows:
         no axial and radial play occurs at the fixed bearing;   an expansion of the shaft, for example thermally induced, is non-critical;   the axial position is accurately defined under axial loading, and   the running of the shaft is very precise.       

     In the fixed/loose bearing, according to the invention, of the shaft, the absorption of the axial forces in both directions is undertaken by a single bearing or a bearing group, the so-called fixed bearing. Besides the axial forces, the fixed bearing also absorbs radial forces and transmits these to adjacent components of the support. In this way, the disadvantages of a pure fixed bearing are eliminated without accuracy losses occurring, as for example due to wobble. 
     According to the invention, the clamping of the bearing (or of a bearing group) takes place only on one side. Axial errors due to thermal effects, and the accuracy losses resulting therefrom, are thus minimized. Furthermore, thermally induced expansion of the shaft is non-critical and does not affect clamping of the bearing. The clamping of the bearing therefore remains constant over the entire working temperature range. The connecting parts are touched to the least possible extent by the clamped bearing, so that hysteresis effects are minimized. The overall axis system has a high rigidity. 
     The fixed bearing comprises one or more ball bearings, in particular rolling bearings. Preferably, two rolling bearings are installed pairwise. This may, for example, be achieved by means of a duplex bearing which is composed of two paired rolling bearings with identical tolerance ranges. The clamping of the duplex bearing takes place on one side. In a so-called O arrangement, the construction is carried out by axial prestress on a block of the inner ring, or the outer ring in the case of a so-called X arrangement. With the width of the outer or inner ring, the desired prestress can be defined by the prior processing. 
     As an alternative, the fixed bearing may also comprise two individual rolling bearings. The assembly is carried out by axial prestress of the inner ring in the O arrangement, or the outer ring in the X arrangement. The outer or inner ring may be clamped with an intermediate ring and a spring assembly. The prestress can be adjusted variably with the spring assembly. 
     According to the invention, the loose bearing is intended only to absorb radial forces, the radial load being distributed between the loose bearing and the fixed bearing. The loose bearing does not absorb any axial forces, and is mobile in the axial direction. As an alternative, it is also possible to use a loose bearing which is immobile relative to the support and permits axial movement of the shaft. The running paths are configured cylindrically, so that axial displacement of the shaft is possible. Both running surfaces have a high hardness quality. The loose bearing preferably comprises a ball bearing with a ball cage, which may in particular comprise a plurality of rows of balls slightly offset from one another, so that each ball describes its own running path. This is advantageous inter alia in order to avoid wear, and prevents several or all of the balls from running on a defective running path in the event of shock damage to a running path. 
     The degree of clamping of the loose bearing is carried out by means of the processing of the connecting parts. The rolling body has a slight oversize relative to the inner and outer running surfaces. By assembly of the rolling body, shaft and flange, the bearing is clamped. The clamping is thus selected in such a way that optimal rolling of the rolling body is ensured. 
     In a particularly preferred embodiment, a first fixed/loose bearing is distributed between two braces of the support, in such a way that the shaft is mounted on one side of the telescope unit with a fixed bearing and on the other side with a loose bearing. 
     Since no bearing air can be tolerated because of the very high accuracy requirements in coordinate measuring devices, both the loose bearing and the fixed bearing are preferably prestressed—the loose bearing radially and the fixed bearing both axially and radially. 
     The rolling bearings are preferably not set or clamped directly in adjacent components of the aluminum brace, but in connecting parts made of steel. The desired fit between the bearing and the flange is therefore preserved over the entire working temperature range. The steel connecting parts are connected firmly to the aluminum components. Axial errors due to thermal effects, and the accuracy losses resulting therefrom, are minimized. 
     If the fixed bearing is configured in order to absorb axial forces which, in any orientation of the measuring apparatus, exceed the forces which occur owing to the intrinsic weight of the measuring apparatus, the bearing according to the invention furthermore allows the coordinate measuring device to be set up at an inclination to the vertical, without wobble of the axis occurring. Even an upside-down setup, in which the support hangs from the base, is possible. 
     As rolling bearings, it is also possible to use so-called hybrid bearings, in which the rolling bodies are made of ceramic instead of steel. A main advantage of this solution is a higher possible accuracy by virtue of higher accuracy classes. Irreproducible errors, for example wobble errors or hysteresis effects, are thereby minimized. This has a direct effect on the accuracy of the coordinate measuring device. Furthermore, ceramic has a lower coefficient of friction compared with steel, for which reason the lifetime of such a bearing can be extended, depending on the operating mode. Hybrid bearings furthermore have better operating properties under emergency conditions. 
     Preferably, a motor is furthermore provided, which is accommodated particularly in the brace containing the fixed bearing and which is intended to drive the shaft on the side with fixed bearing. In particular, a direct-drive motor—that is to say a motor without intermediate transmission, for example a piezo motor—may advantageously be used for this, in order to be able to avoid errors due to play in the transmission. 
     Also preferably, an angle measurement functionality for determining an orientation of the telescope unit relative to the base is provided, in particular an angle encoder, which is accommodated in the brace containing the loose bearing and is intended to determine absolute or relative positions of the shaft. 
     In order to save weight, the shaft may preferably be configured as a hollow shaft. This furthermore makes it possible to feed supply lines into the telescope unit inside the shaft. These are, in particular, cables for supplying components of the telescope unit with electrical current or light guides for introducing a light beam into optical components of the telescope unit. The latter is necessary in particular when the distance measuring apparatus is accommodated fully or partially outside the telescope unit, or the laser beam is generated outside the telescope unit, for example by a laser module in the support. 
     Since the shaft extends along the tilt axis, it intersects a measurement axis—preferably extending orthogonally to the tilt axis. In a preferred embodiment, the shaft therefore has an opening at this position for a beam path of the optical distance measuring unit. 
     As an alternative, the shaft may also consist of two parts, which connect the telescope unit to the support on both sides. A first part of the shaft is then mounted on the fixed bearing and connected to the facing side of the telescope unit, while a second part of the shaft is mounted on the loose bearing and connected to the other side of the telescope unit. The shaft is then accommodated inside the telescope unit, which may be advantageous in particular for reasons of space. The stability and rigidity of the bearing must, however, be ensured by components of the telescope unit. 
     In an alternative embodiment, the loose bearing may also be configured as a sliding, air or magnetic bearing. In another alternative embodiment, the loose bearing may also be omitted. In this case, the shaft is mounted exclusively on one side with a single fixed bearing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages and characteristics of the invention may be found in the following description of currently preferred embodiments in connection with the appended figures. In the figures: 
         FIG. 1  schematically shows a coordinate measuring device according to the invention and a measurement aid; 
         FIG. 2  schematically shows a coordinate measuring device according to the invention in a front view; 
         FIG. 3 a    schematically shows a first embodiment of a coordinate measuring device according to the invention in cross section with a representation of two fixed/loose bearing apparatuses; 
         FIG. 3 b    schematically shows a second embodiment of a coordinate measuring device according to the invention in cross section with a representation of one fixed/loose bearing apparatus; 
         FIG. 4 a    schematically shows a cross section through the shaft and the fixed bearing; 
         FIG. 4 b    schematically shows a cross section through the shaft and the loose bearing; 
         FIGS. 5 a - c    schematically show two embodiments of the ball cage of a loose bearing according to the invention; 
         FIGS. 6 a - b    schematically show two embodiments of a fixed bearing according to the invention; and 
         FIG. 7  schematically shows the structure of a vertical fixed/loose bearing for mounting the support on the base. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a coordinate measuring device according to the invention, configured as a laser tracker  1 , comprising a base  40 , a support  20 , which is fitted thereon and has a handle  21 , and a telescope unit  10  mounted on two braces (not represented) of the support  20 . The laser tracker  1  shown is arranged on a stand  45  and, by means of a laser beam  36 , measures the distance to a retroreflector  81  located on a measurement aid  80 . The measurement aid  80 —configured here, by way of example, as a measurement probe—furthermore comprises a number of target markings  82 , for example in the form of reflective or self-illuminating light points, and a measurement head  83  for placement on a target point to be measured on a target object  85 . 
     The laser tracker  1  contains a measurement camera, which is configured in particular as a focusable vario camera system with variable magnification, in order to acquire the target markings  82  arranged on the measurement aid  80 . With the aid of the positions, acquired with the aid of the measurement camera, of the target markings  82 , the spatial orientation of the measurement aid  80  can be determined. 
     In order to be able to acquire and track movements of the measurement aid  80 , so that the laser beam  36  remains aligned with the retroreflector  81 , the laser tracker  1  has a position-sensitive detector (PSD), in particular a 2D tracking sensor, as disclosed for example in WO 2007/079600 A1. 
     The PSD is preferably arranged in the telescope unit  10 , and by acquiring the orientation of the laser beam reflected by a target, in particular the retroreflector  81 , makes it possible to track the alignment of the laser beam  36 . By tracking the laser beam alignment, continuous target tracking of the target point can be carried out, and the distance and position of the target point relative to the measuring device can be determined continuously. 
       FIG. 2  shows an exemplary embodiment of a coordinate measuring device according to the invention, configured as a laser tracker  1 , in a front view. The laser tracker  1  comprises a base  40 , which can be fastened on a holding apparatus, here represented in the form of a stand  45 . A support  20  is fitted, mounted rotatably about the vertical axis  9 , on the base  40 . The support  20  comprises a first brace  26  and a second brace  27 , which project upward from the support  20  and on which a telescope unit  10  is mounted, tiltably about the horizontal axis  8 , by means of a shaft  63 . A handle  21  for transport and handling of the laser tracker  1  is fitted on the two braces  26 ,  27 . The handle  21  may be connected firmly to the braces  26 ,  27 , for example produced from casting therewith, or welded, so that it serves as an additionally stabilizing element for the braces  26 ,  27 , particularly in respect of bending. 
     In this exemplary embodiment, a plurality of optics are provided on the telescope unit  10 , in particular optics  52  of a measurement camera, as well as laser emission and reception optics  51  of an optical distance measuring apparatus. The telescope unit  10  furthermore preferably comprises optics of a localization camera  54  for approximate localization of the measurement aid  80  and optics of a viewfinder camera  56  to provide images for a user. 
       FIG. 3 a    shows, in a cross section through the laser tracker  1  of  FIG. 2 , a view of a first embodiment of a first fixed/loose bearing apparatus  60  according to the invention on the suspension of the telescope unit  10 , and a second fixed/loose bearing apparatus  70  according to the invention on the support  20  and base  40 . The telescope unit  10  comprises various optical components inside it, inter alia a measurement camera  12  for acquiring a spatial orientation of the measurement aid  80  and an optical distance measuring apparatus with an interferometer  13  and an absolute distance meter  14  for measuring the distance to the measurement aid  80 . The measurement camera  12  is configured as a focusable vario camera system with variable magnification. 
     The first fixed/loose bearing apparatus  60  allows rotatability of the telescope unit  10  about the tilt axis  8  and contains a shaft  63  mounted in the two lateral braces  26 ,  27 , a fixed bearing  61  being provided in the first brace  26  and a loose bearing  62  being provided in the second brace  27 . A direct-drive motor  65  is also provided in the first brace  26  in order to drive the shaft  63  in rotation. An angle encoder  66  is provided in the second brace  27  in order to acquire relative and/or absolute positions of the shaft  63 , so as to determine a current orientation of the telescope unit  10 . 
     The shaft  63  is preferably made of steel, brass or ceramic, and is essentially cylindrical, in particular having cylindrical running surfaces on the bearings  61 ,  62 . The shaft  63  is hollow, so that it is suitable for receiving supply lines such as cables or light guides  31 ,  32 . In the direction of a measurement axis defined by an emission direction of the measurement radiation, in particular extending orthogonally to the tilt axis  8  and to the upright axis  9 , the shaft  63  has a vertical opening  69 , in particular for a beam path of the optical distance measuring apparatus. 
     A laser module  30  is integrated into the support  20 , or into one of the braces  26 ,  27 , preferably a helium-neon laser module, here represented in the second brace  27 . A light guide system, comprising a first fiber  31  and a second fiber  32 , leads from this laser module  30  through the shaft  63  into the telescope unit  10 , as far as a collimator  34  of the interferometer  13 . The first fiber  31 , extending in the first brace  27 , of the light guide system is connected rotation-free via a jack connection  33 , preferably provided in the first brace  27 , to the second fiber  32 , extending in the telescope unit  10 , of the light guide system. Arranging the jack connection  33  in the vicinity of the laser module  30  in the support  20  has the advantage that the laser module together with the first fiber  31  can be replaced more easily. 
     The second fixed/loose bearing apparatus  70  allows rotatability of the support  20  about the upright axis  9  and contains a shaft  73  mounted in the base  40  and fastened on the support, a fixed bearing  71  being provided in the upper part, facing toward the support  20 , of the base  40 , and a loose bearing  72  being provided in the lower part. A direct-drive motor  75  is provided at the loose bearing  72  in order to drive the shaft  73  in rotation. An angle encoder  76  is provided the fixed bearing  71  in order to acquire relative and/or absolute positions of the shaft  73 , so as to determine a current orientation of the support  20 . 
     Although this embodiment contains two fixed/loose bearing apparatuses, it is likewise conceivable for only one bearing apparatus to be configured as a fixed/loose bearing apparatus. 
       FIG. 3 b    represents a second embodiment according to the invention of the first fixed/loose bearing apparatus  60  according to the invention in a cross section through a laser tracker  1 . In contrast to the embodiment represented in  FIG. 3 a   , the shaft consists of a first part  63   a  and a second part  63   b . Radial and axial forces in this case need to be transmitted via components of the telescope unit  10 . These components are therefore preferably formed particularly stably and rigidly. In particular, they may consist of the same material as the shaft. A second fixed/loose bearing apparatus  70  is not represented in this exemplary embodiment, but is optionally possible. 
       FIGS. 4 a  and 4 b    respectively represent in cross section a part of the shaft  63  of the first fixed/loose bearing apparatus  60  with its respective mounting in the braces  26 ,  27 .  FIG. 4 a    shows the mounting on the first brace  26  with the fixed bearing  61 , and  FIG. 4 b    shows the mounting on the second brace  27  with the loose bearing  62 . The shaft  63  is hollow and has a cylindrical shape at the mountings. 
     The fixed bearing  61  represented in  FIG. 4 a    has two pairwise arranged rolling bearings, in particular a duplex spindle bearing pair or a UKF® spindle bearing with spacer balls. An X arrangement of the pairwise arranged rolling bearings is represented by way of example. 
     The loose bearing  62  represented in  FIG. 4 b    has a ball bearing with a ball cage, which is intended to absorb radial forces. The loose bearing  62  is fixed on the shaft  63  and arranged movably in the axial direction in the brace  27 , in order to be able to move stress-free with the shaft  63  in the event of thermally induced expansion variations of the latter. The rolling body of the loose bearing  62  has a certain oversize relative to the inner and outer running surfaces. Both running surfaces have a high hardness quality. 
     The bearings  61 ,  62  are not mounted directly in adjacent lightweight components, in particular consisting of aluminum, of the braces  26 ,  27 , but in connecting parts  67 ,  68  made of steel. The desired fit between the bearing and the flange is therefore maintained over the entire working temperature range. The steel connecting parts  67 ,  68  are connected firmly to components  22  of the braces  26 ,  27 . Axial errors due to thermal effects, and accuracy losses resulting therefrom, are thus minimized. 
     An optional light guide system, fed through the hollow shaft  63 , is also represented in  FIG. 4 b   . It comprises a second fiber  32 , which leads into the telescope unit, a first fiber  31 , which leads to a laser module in the second brace  27 , and a jack connection  33  for rotation-free connection of the two fibers. The jack connection  33  is arranged inside the shaft  63  in this exemplary embodiment. 
       FIGS. 5 a  to 5 c    each represent an exemplary embodiment of a ball cage of the loose bearing  62 ,  72  in a side view. The balls of the ball cage consist of steel or ceramic, and are respectively arranged slightly offset from one another so that each ball describes its own running path. This is advantageous in order to avoid wear, and prevents several or all of the balls from running on a defective running path in the event of shock damage to a running path. 
     Two exemplary embodiments of the fixed bearing  61  of the first fixed/loose bearing apparatus  60  are represented in  FIGS. 6 a  and 6 b   . A detail around the fixed bearing  61  with a section of the shaft  63  and the surrounding first brace  26  are respectively shown. The tilt axis  8  is likewise represented. The same applies for the fixed bearing  71  of the second fixed/loose bearing apparatus  70 . 
       FIG. 6 a    shows a first embodiment of the fixed bearing  61 , with two paired rolling bearings in a so-called O arrangement. The connecting lines of the ball contact points diverge in the direction of the shaft  63 . A greater support width is therefore obtained, which makes the unit very rigid. The O arrangement allows reversible axial and radial loads, and ensures less tilting play. 
       FIG. 6 b    shows a second embodiment of the fixed bearing  61 , with two paired rolling bearings in a so-called X arrangement. The connecting lines of the ball contact points converge in the direction of the shaft  63 . A smaller support width is therefore obtained, which leads to reduced angular rigidity of the unit. The X arrangement permits greater alignment deviations, and likewise allows reversible axial and radial loads. 
       FIG. 7  shows the arrangement of a fixed bearing  71  and loose bearing  72  of a second fixed/loose bearing apparatus  70  of a coordinate measuring device according to the invention, configured as a laser tracker  1 , by means of which the support  20  is fitted rotatably about the upright axis  9  on the base  40  (see also  FIG. 3 a   ). A shaft  73  fastened on the support  20  is mounted with a fixed bearing  71  and a loose bearing  72  on the base  40 . The shaft  73  is, as represented, preferably a hollow shaft. The fixed bearing  71  is preferably located on the part of the shaft  73  facing toward the support  20 , and the loose bearing  72  on the part facing away from the support. The loose bearing  72  is fixed on the shaft  73  and is arranged movably in the axial direction in the base  40 , in order to be able to move stress-free with the shaft  73  in the event of thermally induced expansion variations of the latter. The fixed bearing is configured in such a way that it can absorb radial and axial forces. The fixed bearing  71  is preferably also configured in order to be able to absorb forces, in particular axial forces, occurring because of the intrinsic weight of the device—or because of the intrinsic weight of the device parts which can be rotated relative to the base  40  about the upright axis  9 —so that the device can also be used “upside down”—that is to say with the support  20  hanging below the base  40 . 
     The second fixed/loose bearing apparatus  70  may be both provided in addition to the first fixed/loose bearing apparatus  60  and combined with another bearing apparatus. 
     It is to be understood that these represented figures only schematically represent possible exemplary embodiments. The various approaches may likewise be combined with one another and with methods and devices of the prior art.