Abstract:
The invention relates to a method for aligning tracks of a roadwork machine, characterized by using a track alignment detection unit that is attached to a first track unit for allowing detection of an orientation of the first track unit, and frame orientation detection means that are attached to the machine frame for allowing detection of an orientation of the machine frame, the method comprising determining an initial orientation of the machine frame, and determining an initial orientation of the first track unit, determining whether a difference between a most recently determined orientation of the machine frame and a most recently determined orientation of the first track unit is within a predefined threshold value, initializing a pivoting of the first track unit, determining a changed orientation of the first track unit after the pivoting, and determining an orientation of the machine frame after the pivoting.

Description:
FIELD OF THE INVENTION 
     The invention relates to an automated method for aligning the tracks of an automated roadwork machine with the machine orientation which is defined by the alignment of the surface processing unit, e. g. a mould unit. Such tracked automated roadwork machines comprise paving machines, such as mainline pavers or pavers for curb and gutters, as well as milling machines and surface miners. 
     BACKGROUND 
     Particularly, such tracked automated roadwork machines include slipform pavers for producing a surface layer comprising concrete or asphalt material. Slipform pavers are construction machines with a characteristic finishing screed which serves, for example, for the installation of concrete or asphalt. The screed can also be formed with a characteristic profile, for example for the production of rails, channels or water grooves. Screeds are therefore produced for a wide variety of applications, i.e. with different screed profiles and screed widths. 
     The control of such road finishers can be effected by means of reference line scanning devices. A sensor scans the required direction and/or required height of a reference line, such as, for example, a tensioned wire; deviations from the required direction/required height are corrected by a regulating means. DE 101 38 563 discloses a wheel-type road finisher which automatically follows a reference line. In U.S. Pat. No. 5,599,134, scanning of a reference line is effected without contact, by means of ultrasonic sensors. However, this method of controlling a machine requires setting out of the area to be processed before the use of the construction vehicle and is very time-consuming and labor-intensive. 
     The method described in WO 2006/092441 A1 envisages mounting two masts with prisms on the crossbeams of a rigid machine frame formed from longitudinal beams and crossbeams and determining the distance and direction to the prisms by means of one or two tacheometers or total stations, and hence determining the position of the prisms or of the machine. These tacheometers or total stations are advantageously motor-powered and capable of automatically following the reflector. This document does not describe a method for aligning the track units to the machine frame, though. 
     For excellent straight line performance, tracked automated roadwork machines such as slipform pavers require their tracks to be accurately aligned to the surface processing unit (e. g. a roadwork unit such as a mould unit) of the machine. Machine manufactures and operators have developed many methods of aligning the tracks manually and with assistance of laser measurement tools. 
     When aligning tracks in this way, the following difficulties arise:
         Accurately projecting the heading of the mould to the track, e. g. using a string.   Measuring the track against the projected string alignment to millimetre precision on rough ground.   Communicating millimetre rotational movement commands from the person looking at the track to the operator of the machine.   Moving the tracks with the ultra-fine movements that are required to align the track.       

     Also, on several surfaces, when the tracks are pivoted the mould unit will often move slightly. This requires an iterative process of aligning the track to the surface processing unit, then re-checking the surface processing unit&#39;s heading, as it unintentionally might have moved, realigning the track, and so forth. 
     SUMMARY 
     Some embodiments of the invention provide an improved method for aligning the tracks of an automated roadwork machine to its surface processing unit. 
     Some embodiments of the invention provide a system for execution of said method, i.e. for aligning the tracks of an automated roadwork machine to its surface processing unit. 
     The invention relates to an automated method for aligning the tracks of a roadwork machine with the machine orientation. The machine orientation is defined by the alignment of the surface processing unit, e. g. a mould unit. 
     The method according to the invention employs the capabilities of robotic total stations, combined with the machine control abilities of 3D guidance systems already installed: A robotic total station is setup next to the machine, and prisms or other retro-reflective means are arranged at the tracks of the machine and on the surface processing unit, respectively a machine frame the surface processing unit is mounted on. For instance, the prisms can be attached magnetically. A control and evaluation unit is provided on or near the machine and connected to the total station and the machine control system, in particular wireless. Particularly, the control and evaluation unit is a part of the machine control system which is fixedly mounted on the machine. 
     In particular, the user starts the alignment process by manually measuring the initial mould and track positions, and then the Automatic Track Alignment Control Kit (ATACK) will automatically calculate, align, and recheck mould unit and track positions until the tracks are in line with the mould unit/machine frame orientation. The mould and track calibration process can be combined and/or processed in a separate process. The track alignments can be performed per track or for all tracks in one process. 
     According to the invention, a method for aligning tracks of a roadwork machine, wherein the roadwork machine is designed for producing and/or processing a surface layer comprising concrete or asphalt material along a predetermined path, and comprises a machine frame with a surface processing unit for carrying out a material processing step, a plurality of track units with tracks for moving the roadwork machine along a first axis and actuator means for pivoting the track units about a second axis with respect to the machine frame, wherein the second axis is basically orthogonal to the first axis, and a machine guidance system for controlling the tracks and the actuator means of the track units, comprises using a track alignment detection unit that is attached to a first track unit for allowing detection of an orientation of the first track unit, and frame orientation detection means that are attached to the machine frame for allowing detection of an orientation of the machine frame. The method further comprises
         determining an initial orientation of the machine frame,   determining an initial orientation of the first track unit,   determining whether a difference between a most recently determined orientation of the machine frame and a most recently determined orientation of the first track unit is within a predefined threshold value,   initializing a pivoting of the first track unit,   determining a changed orientation of the first track unit after the pivoting, and   determining an orientation of the machine frame after the pivoting.       

     In one embodiment, the method comprises the repetition of the following steps until an angular difference between a most recently determined orientation of the machine frame and a most recently determined orientation of the first track unit is within a predefined threshold value:
         reiteratively pivoting the first track unit and determining its orientation after the pivoting, until the angular difference is within the predefined threshold value, and   determining an orientation of the machine frame and/or the surface processing unit.       

     In one embodiment of the method, for pivoting the track unit, a correction angle is calculated in a control and evaluation unit, wherein the track unit is pivoted about the correction angle. 
     In one embodiment of the method, the steps of determining an initial orientation of the machine frame, and determining an initial orientation of the first track unit, are performed by a user using a geodetic surveying instrument, particularly a robotic total station or tacheometer. 
     In one embodiment of the method, the steps of determining a changed orientation of the first track unit after the pivoting and determining an orientation of the machine frame after the pivoting are performed automatically by means of a geodetic surveying instrument, particularly a robotic total station or tacheometer. 
     In a preferred embodiment of the method, the track alignment detection unit comprises at least one reflector or retro-reflector, and a geodetic surveying instrument is used for determining the orientations, by measuring the distance and direction to the at least one (retro-) reflector, the geodetic surveying instrument particularly being a robotic total station or tacheometer. 
     In an alternative embodiment, the second orientation detection means comprise a (retro-)reflector and an optically perceivable pattern, particularly comprising light emitting diodes, and determining the orientations is performed by means of a laser tracker having camera means. 
     In one embodiment of the method, the track alignment detection unit comprises two (retro-)reflectors. In particular, the two (retro-)reflectors are either fixedly connected to each other in a known distance, or the track alignment detection unit comprises two parts, each part providing a (retro)-reflector. 
     In a particular embodiment of the method, the track alignment detection unit comprises two retro-reflectors, wherein the two retro-reflectors are positioned on the track unit at a known distance from each other, and calculating the correction angle is based on the most recently determined orientation of the machine frame, the most recently determined orientation of the track unit and the known distance between the two retro-reflectors. In particular, the correction angle is calculated by the equation 
               α   =       sin   ⁡     (         frame   ⁢           ⁢   orientation     -     track   ⁢           ⁢   orientation       2     )       ·     (     d   2     )     ·   2       ,         
wherein α is the correction angle, “frame orientation” is the most recently detected orientation of the machine frame (or of the surface processing unit, respectively), “track orientation” is the most recently detected orientation of the track unit and d is the known distance between the two retro-reflectors.
 
     The invention also relates to an Automatic Track Alignment Control Kit (ATACK) for aligning tracks of a roadwork machine, the roadwork machine being designed for producing and/or processing a surface layer comprising concrete or asphalt material along a predetermined path, and comprising a machine frame with a surface processing unit for carrying out a material processing step, a plurality of track units with tracks for moving the roadwork machine along a first axis and actuator means for pivoting the track units about a second axis with respect to the machine frame, wherein the second axis is basically orthogonal to the first axis, and a machine guidance system for controlling the tracks and the actuator means of the track units. 
     According to the invention, the ATACK comprises an orientation determining device comprising a laser range finder, particularly a geodetic surveying instrument, a track alignment detection unit comprising at least one (retro-)reflector for use with the laser range finder, and a control and evaluation unit, wherein the track alignment detection unit is designed for being attachable to a track unit of the roadwork machine, the orientation determining device is designed for determining an orientation of the track unit and an orientation of the machine frame, and the control and evaluation unit is designed for determining an orientation of the track unit relative to the machine frame and for sending a command to the machine guidance system to pivot the track unit in order to align the track unit to the surface processing unit. 
     In one embodiment, the ATACK comprises frame orientation detection means for use with the laser range finder for allowing detection of the orientation of the machine frame (or of the surface processing unit), the frame orientation detection means comprising at least one retro-reflector and being attachable to the machine frame, in particular by means of magnets. Particularly, the frame orientation detection means comprise two retro-reflectors, one 6DOF retro-reflector or a retro-reflector and an optically perceivable pattern. 
     In one embodiment of the ATACK, the track alignment detection unit comprises two retro-reflectors that are fixedly connected to each other in a known distance. In another embodiment the track alignment detection unit comprises two parts, each providing a retro-reflector. In a further embodiment the track alignment detection unit comprises a retro-reflector and an optically perceivable pattern, particularly comprising light emitting diodes for allowing determining an orientation of the track alignment detection unit. 
     In one embodiment of the ATACK, the orientation determining device has means for automatically aiming the laser range finder at the at least one retro-reflector of the track alignment detection unit and automatically measuring a distance and direction to the retro-reflector, in particular wherein the orientation determining device is a robotic total station or tacheometer or a laser tracker. 
     In a further embodiment of the ATACK, the control and evaluation unit is designed for performing the following steps:
         initializing a pivoting of the first track unit,   determining the orientation of the first track unit after pivoting,   determining an orientation of the machine frame after pivoting, and   determining whether a difference between a most recently determined orientation of the machine frame and a most recently determined orientation of the first track unit is within a predefined threshold value.       

     The invention also pertains to a track alignment detection unit for use in a method for aligning tracks of a roadwork machine. According to the invention the track alignment detection unit comprises at least one retro-reflector for use with a laser range finder and magnet means for removably attaching the track alignment detection unit to a track unit of the roadwork machine. 
     The invention furthermore pertains to a computer programme product comprising programme code which is stored on a machine-readable medium, or being embodied by an electromagnetic wave comprising a program code segment, having computer-executable instructions for performing the following steps:
         initializing a pivoting of the first track unit,   determining the orientation of the first track unit after pivoting,   determining an orientation of the machine frame after pivoting, and   determining whether a difference between a most recently determined orientation of the machine frame and a most recently determined orientation of the first track unit is within a predefined threshold value.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The method and systems according to the invention are described in more detail below, purely by way of example, with reference to specific embodiments shown schematically in the drawings, further advantages of the invention also being discussed. Specifically: 
         FIG. 1  shows a tracked automated roadwork machine with a first exemplary embodiment of an Automatic Track Alignment Control Kit according to the invention; 
         FIG. 2  shows a flow chart illustrating an exemplary embodiment of the method according to the invention; 
         FIGS. 3 a - d    illustrate single steps of an exemplary embodiment of the method according to the invention, performed with an exemplary embodiment of an Automatic Track Alignment Control Kit according to the invention; 
         FIG. 4  shows a tracked automated roadwork machine with a second exemplary embodiment of an Automatic Track Alignment Control Kit according to the invention; 
         FIG. 5  shows a tracked automated roadwork machine with a third exemplary embodiment of an Automatic Track Alignment Control Kit according to the invention; 
         FIG. 6  shows a tracked automated roadwork machine with a fourth exemplary embodiment of an Automatic Track Alignment Control Kit according to the invention; 
         FIGS. 7 a - c    show a first exemplary embodiment of a track alignment detection unit according to the invention in a front view, in a rear view and attached to a track unit of a tracked automated roadwork machine; 
         FIGS. 8 a - b    show a second exemplary embodiment of a track alignment detection unit according to the invention; 
         FIG. 9  shows a third exemplary embodiment of a track alignment detection unit according to the invention; and 
         FIG. 10  shows a fourth exemplary embodiment of a track alignment detection unit according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a tracked automated roadwork machine  10  being equipped with an Automatic Track Alignment Control Kit (ATACK). A surface processing unit (not shown here) for carrying out a material processing step is mounted to the machine frame  11  of the roadwork machine  10 . The depicted machine  10  comprises four track units (only three of which being visible here) with tracks for moving the roadwork machine  10  in the direction of a first axis  50  and actuator means for pivoting the track units  12 , 12 ′, 12 ″ about a second axis  55  with respect to the machine frame  11 , wherein the second axis  55  is basically orthogonal to the first axis  50 . The machine  10  furthermore comprises a machine guidance system for controlling the tracks and the actuator means of the track units. Obviously, also machines with more or fewer tracks than four can be equipped with the ATACK. 
     The ATACK comprises a robotic total station  8 , a control and evaluation unit  18  and two retro-reflectors  2   a , 2   b  being part of a track alignment detection unit (not shown) that is attached to a first track unit  12  of the machine  10 . A user  9  determines an orientation  60  of the track unit  12  by measuring coordinates of the two retro-reflectors  2   a , 2   b . The control and evaluation unit  18  calculates a difference between the current orientation  60  of the track unit  12  and a known orientation  50  of the surface processing unit. The control and evaluation unit  18  calculates a correction angle α about which the track unit  12  needs to be pivoted in order to be aligned with the surface processing unit. This deviation information is then transmitted to the machine guidance system which can pivot the track unit  12  until it reaches an orientation  60 ′ which is parallel to the orientation  50  of the surface processing unit. 
     The orientation  50  of the surface processing unit or the machine frame to which is surface processing unit is attached can be determined by means of a frame orientation detection means comprising at least one, particularly two, retro-reflectors that are attached to the machine frame, to which direction and distance from the total station are measured. Alternative ways to determine the orientation  50  of the surface processing unit are also possible, for instance reflectorless determination of the orientation of frame part that has a known dimension and orientation with respect to the orientation  50  of the surface processing unit. 
       FIG. 2  shows a flow chart illustrating an exemplary embodiment of the method  100  according to the invention. 
     The “TrackMoved Flag” of the steps  125 ,  155  and  175  is a Boolean flag which is used to describe if a track has been turned since the last mould movement. As track movements can affect the mould heading, the mould alignment must be checked after any track movement. When using the machine reference points as heading reference, a machine calibration has to be done on forehand. In particular, the user is asked what reference should be used. 
     In order to initiate the measurement process, a user sets up a robotic total station next to the machine. This robotic total station need not be accurately positioned via resection, which advantageously allows a very quick setup. The process is then started from the machine guidance system, and the machine put under automatic control. 
     The user is prompted from the machine guidance system and/or the robotic total station to measure the position of the two retro-reflectors on the mould unit or machine frame (step  110 ), and then the two retro-reflectors mounted on the track unit (step  120 ). In step  125 , the “TrackMoved Flag” is then set to “false”. 
     In the next step  130 , the heading of the mould unit and the heading of the track unit are calculated and a difference between the headings is determined. If there is a difference, and particularly if this difference exceeds a threshold value, the track unit needs to be pivoted and the method continues with step  140 . 
     In step  140 , a correction movement is calculated, e.g. the correction angle that the track unit needs to be pivoted in order to bring it into alignment with the mould. Particularly, also the heading and length of the track unit is determined, so that the distance can be calculated which the rear or front end of the track would have to move as the track unit is pivoted for performing the correction movement. 
     In step  150 , the machine guidance system initiates the pivoting of the track unit. Preferably, the pivoting is monitored by the robotic total station, in particular by tracking one of the retro-reflectors attached to the track unit. When the calculated required orientation of the track unit has been reached as monitored by the robotic total station, the machine guidance system stops the pivoting. In step  155 , the “TrackMoved Flag” is set to “true”. 
     In step  160 , the robotic total station determines the orientation of the track unit, by determining the distance and direction to the two retro-reflectors on the track unit. Preferably first the closest end of the track unit, in particular a first retro-reflector, is measured and then the other end, in particular a second retro-reflector. The positions of the retro-reflectors on the track unit are found using their last known location and the robotic total station&#39;s auto-find abilities. 
     Then step  130  is repeated. If the difference still exceeds the threshold value, the track unit needs to be pivoted again, and steps  140  to  160  and  130  are repeated. If there is no difference, or if the difference is within the threshold value, respectively, the procedure continues with step  170 . 
     In step  170 , the current value of the “TrackMoved Flag” is determined. If the value is “false”, meaning that the track unit has not been moved since its heading has last been measured, the procedure finishes with step  190 . If the value is “true”, the “TrackMoved Flag” value is set to “false” in step  175  and the procedure continues with step  180 . 
     In step  180 , the robotic total station determines the orientation of the surface processing unit, particularly by measuring the position of the two retro-reflectors attached to the machine frame, preferably starting at the nearest retro-reflector. The positions of the retro-reflectors are found using their last known location and the robotic total station&#39;s auto-find abilities. Afterwards, step  130  is performed again. 
     When the measuring procedure is finished, the tracks are aligned with the mould unit. In step  190 , the user is then informed about the alignment success. In one embodiment the procedure comprises a SET confirmation of the user in order to store the alignment. 
       FIGS. 3 a - d    illustrate a number of steps of the method described with regard to  FIG. 2 , showing a tracked automated roadwork machine  10  being equipped with an exemplary embodiment of an ATACK. The machine  10  comprises a machine frame  11  to which a surface processing unit  13  and four track units are attached. A first track unit  12  needs to be aligned with the surface processing unit  13 . 
     In this embodiment, the ATACK comprises a robotic total station  8 , a control and evaluation unit  18 , two retro-reflectors  2   a , 2   b  being part of a track alignment detection unit (not shown) that is attached to the first track unit  12  of the machine  10 , and frame orientation detection means  15  comprising two retro-reflectors that are attached to the machine frame  11  for allowing detection of an orientation of the machine frame  11  and, thus, of the surface processing unit  13 . 
     In  FIG. 3 a    step  110  is illustrated: By means of the total station  8  a user (not shown) measures the positions of the two retro-reflectors of the orientation detection means  15 , thus determining the orientation of the surface processing unit  13 . 
     In  FIG. 3 b    step  120  is illustrated: By means of the total station  8  the user measures the positions of the two retro-reflectors  2   a , 2   b , thus determining the orientation of the track unit  12 . 
     The two determined orientations are then compared and if they differ, particularly exceeding a pre-defined threshold value, pivoting of the track unit  12  is initiated. This is shown in  FIG. 3   c.    
       FIG. 3 c    illustrates steps  150  and  160 . In step  150  the machine guidance system initiates the pivoting of the track unit  12 . Preferably, the pivoting is monitored by the robotic total station  8 , in particular by tracking one of the retro-reflectors  2   a , 2   b  attached to the track unit. When the desired orientation of the track unit has been reached, the machine guidance system stops the pivoting. In step  160 , the robotic total station  8  determines the orientation of the track unit  12  by measuring the position of the retro-reflectors  2   a , 2   b , preferably starting with the nearest one, in this case retro-reflector  2   b . The positions of the retro-reflectors  2   a , 2   b  on the track unit  12  are found using their last known location and the robotic total station&#39;s auto-find abilities. 
     The orientation of the track unit  12  is then compared with the initially determined orientation of the surface processing unit  13  (step  110 ) and if they differ, particularly exceeding the pre-defined threshold value, the steps illustrated in  FIG. 3 c    are repeated. 
     Otherwise, step  180  is performed, as illustrated in  FIG. 3 d   . This step is performed because the machine frame  11  and the surface processing unit  13  unintentionally might have moved during the pivoting of the track unit  12 , thus altering the orientation of the surface processing unit  13 . In step  180 , the robotic total station  8  measures the positions of the two retro-reflectors of the frame orientation detection means  15 , preferably starting at the nearest retro-reflector, thus determining the current orientation of the surface processing unit  13 . The positions of the retro-reflectors are found using their last known location and the robotic total station&#39;s auto-find abilities. 
     The most recently detected orientations of track unit  12  and surface processing unit  13  are then again compared. If they differ, particularly exceeding the pre-defined threshold value, the steps illustrated in  FIGS. 3 c  and 3 d    are repeated until the difference is within the threshold value. 
       FIG. 4  shows a slipform paver as an example of a tracked automated roadwork machine  10 . The paver is equipped with a second embodiment of an ATACK according to the invention. The slipform paver  10  comprises a mould unit  13  as a surface processing unit and produces a layer of concrete  19 . Two masts with prisms are mounted on the crossbeams of the machine frame  11  as a frame orientation detection means  15  for allowing detection of an orientation of the mould unit  13 . 
     The ATACK comprises a robotic total station  8 , a single 6DOF retro-reflector  3  which is mounted on a first track unit  12  and a control and evaluation unit  18  which is provided on the slipform paver  10  as a part of the machine control system. The 6DOF retro-reflector  3  is adapted for allowing determining the orientation of the track unit  12 . A position and orientation measurement device using such a retro-reflector is disclosed in U.S. Pat. No. 5,267,014. The robotic total station  8  is adapted for determining the distance and direction to the 6DOF retro-reflector  3  and the frame orientation detection means and hence for determining the orientation of the machine frame  11  and of the track unit  12 . The robotic total station  8  is furthermore adapted to communicate position data of the retro-reflectors and/or orientation data of the machine frame  11  and of the track unit  12  to the control and evaluation unit  18 . The control and evaluation unit  18  is adapted for calculating a correction angle and for initiating pivoting of the track unit  12 . 
       FIG. 5  shows the slipform paver  10  of  FIG. 4 , being equipped with a third embodiment of an ATACK according to the invention. In contrast to the embodiment shown in  FIG. 4 , this embodiment of the ATACK comprises two robotic total stations  8 , 8 ′. A first total station  8  for determining the orientation of the track unit  12  by determining the positions of the two retro-reflectors  2   a , 2   b , and a second total station  8 ′ for determining the orientation of the mould unit  13 . Furthermore, in this embodiment, the control and evaluation unit  20  is part of a mobile device, e. g. a laptop computer having input means  21  and output means  22 . The control and evaluation unit  20  has a wireless connection with the total stations  8 , 8 ′ for receiving measurement data and sending measuring commands, and with a machine guidance unit of the paver  10  for sending pivoting commands. 
       FIG. 6  shows a further tracked automated roadwork machine  10 , being equipped with a fourth embodiment of an ATACK according to the invention. 
     The ATACK comprises two robotic total stations  8 , 8 ′, a first pair of retro-reflectors  2   a , 2   b  attached to a first track unit  12  and a second pair of retro-reflectors (not shown) attached to a second track unit  12 ′. The ATACK furthermore comprises frame orientation detection means  15  attached to the machine frame  11  and a control and evaluation unit being integrated into the first total station  8 . The first total station  8  is used for determining the orientation of the first track unit  12  and of the machine frame  11 —and, thus, of the surface processing unit  13 . The second total station  8 ′ is used for determining the orientation of the second track unit  12 ′. 
       FIGS. 7 a - c    show a first embodiment of a track alignment detection unit  1  according to the invention. In  FIG. 7 a    the track alignment detection unit  1  is shown in a front view. It has two retro-reflectors  2   a , 2   b  that are positioned on the track alignment detection unit  1  with a known distance d. In  FIG. 7 b    the backside of the track alignment detection unit  1  is shown. It comprises a number of magnets  6  for attaching the track alignment detection unit  1  to a track unit  12 . In  FIG. 7 c    the track alignment detection unit  1  is depicted being attached to a track unit  12 . By means of the two retro-reflectors  2   a , 2   b  and using a total station (not shown) an orientation of the track alignment detection unit  1  and, thus, of the track unit  12  is determinable. 
       FIGS. 8 a  and 8 b    show a second embodiment of a track alignment detection unit  1  consisting of two parts  1   a , 1   b , each part comprising a retro-reflector  2   a , 2   b . In  FIG. 8 a    the two parts are attached to a track unit  12 . By means of the two retro-reflectors  2   a , 2   b  and using a total station (not shown) an orientation of the track alignment detection unit  1  and, thus, of the track unit  12  is determinable.  FIG. 8 b    shows the two parts  1   a , 1   b  of the unit  1  in a rear view. Each part  1   a , 1   b  comprises a magnet for attachment to the track unit  12 . 
     In  FIGS. 9 and 10  two alternative embodiments of a track alignment detection unit  1 ′, 1 ″ according to the invention are depicted. 
       FIG. 9  shows a second embodiment of a track alignment detection unit  1 ′, comprising a single 6DOF retro-reflector  3 , which is adapted for allowing determining the orientation of the track alignment detection unit  1 ′. 
       FIG. 10  shows a third embodiment of a track alignment detection unit  1 ″, comprising a single retro-reflector  4  and a number of light emitting diodes  5  forming a defined pattern, from which the orientation of the track alignment detection unit  1 ″ can be deduced by means of a camera. This embodiment e. g. can be used together with a laser tracker having a camera. 
     Although the invention is illustrated above, partly with reference to some preferred embodiments, it must be understood that numerous modifications and combinations of different features of the embodiments can be made. All of these modifications lie within the scope of the appended claims.