Abstract:
A method for measuring a position of an element of a structure includes the steps of: attaching a receiving device to the element at a measurement position; attaching a first reference device to the receiving device; determining a first measurement point with a first measurement device adapted to measure a position of the first reference device; removing the first reference device; attaching a second reference device to the receiving device; and determining a second measurement point with a second measurement device adapted to measure a position of the second reference device.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of the filing date of U.S. Provisional Patent Application No. 61/739,783 filed Dec. 20, 2012, the disclosure of which is hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a method for measuring a position of an element of a structure and to a measurement system. 
       BACKGROUND OF THE INVENTION 
       [0003]    When an industrial process like the construction of a large scale structure is supported by a measurement system, measurement points have to be measured on the object coordinate system of the structure. In a first step, known reference points have to be determined and in a second step, the position of an additional element may be determined. 
         [0004]    With laser tracker devices, the measurement points usually are determined one after the other. In this case, wrong measurement points may be determined, when the structure moves, vibrates or deforms a bit during the measurements. 
         [0005]    Further possible measurement techniques are based on photogrammetry, in which the position of a measurement target is determined with the aid of camera images taken from different positions, and on tactile position measurements, in which a touch probe as measurement target is brought into contact with a reference position. The position of the touch probe may be determined by the position and the orientation of an arm fixed to the touch probe. 
         [0006]    As a rule, the reference positions or reference points are based on single points or two point adapters. Via an offset or a vector reduction, the reference points may be determined from these points. For example, specific elements of the structure like holes are used as basis for a reference point. However, these kinds of reference points may only be provided at specific positions and may have the problem that they are aligned in a wrong direction with respect to the measurement device. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    An aspect of the invention provides a simple, fast and cheap measurement method that will produce measurement values with high accuracy. 
         [0008]    An aspect of the invention relates to a method for measuring a position of an element of a structure, for example, the structure may be the primary and/or secondary structure of an aircraft. However, it is also possible that the method may be used with structures like general vehicles (such as ships) or buildings. 
         [0009]    According to an embodiment of the invention, the method comprises the steps of: attaching a receiving device to the element at a measurement position; attaching a first reference device to the receiving device; determining a first measurement point with a first measurement device adapted to measure a position of the first reference device; removing the first reference device; attaching a second reference device to the receiving device; determining a second measurement point with a second measurement device adapted to measure a position of the second reference device. Also the second reference device may be removed from the receiving device. 
         [0010]    A receiving device may be seen as a “nest” for the first and second reference device. Usually not only one but a plurality of receiving devices will be attached to elements of the structure. Each of the receiving devices may receive different types of reference devices with different measurement targets. 
         [0011]    After the receiving device is installed in the structure by attaching it (for example by glueing) to the element of the structure, the (measurement) position of the receiving device is determined with a first measurement device. A first reference device with a first type of measurement target is attached to the receiving device and a first measurement point (i.e. measurement value indicative of the measurement position) is measured with the first measurement device. 
         [0012]    The first measurement device may be very accurate (such as a laser tracker). After the first measurement, the data obtained by the first measurement device (i.e. the first measurement point or a plurality of first measurement points for a plurality of first reference devices) may be used as reference point(s) for the second measurement with the second measurement device. As the receiving device stays at its original position, this reference data is very accurate. 
         [0013]    When necessary, for example for quality control or for moving an element of the structure to its destination position, the second reference device(s) may be attached to the receiving device(s) and a second, third, etc. measurement may be performed with the second, third, etc. measurement device, which may be well suited for the application, resources planning and assembly conditions (stations, blue colours, parallelisation of activities, . . . ) and/or may be faster and more robust to disturbances as the first measurement device, but may be less accurate. 
         [0014]    If at least three second reference devices are recognized by the second measurement device (via their reference targets), a referencing of the second measurement points with the first measurement points may take place. By adjusting the reference data (from a construction plan and/or from the first measurement) with the second measurement points (for example by a best fit transformation), additional information may be gathered, like a thermal deformation of the structure, an offset of a single reference point. In such a way, the measurement space may be scaled and/or local deformations may be detected. 
         [0015]    The method is suited for production environments. The permanently installed receiving device may be installed easily, may be used for a long time period and the reference point or reference position defined by the receiving device does not move with respect of the element of the structure, the receiving device is attached to. Furthermore, the receiving devices may be attached in such a way, that all measurement positions are easily to reach. 
         [0016]    Cost may be saved, when a reference device is provided with more than one measurement target. Such a reference device may have to be calibrated only once to its reference point. 
         [0017]    The reference device may indicate the reference point directly and/or indirectly. In the second case, at least three points on the reference device may define the location of the reference point (for example via mathematic algorithms). Costs may be saved, since such a reference device may be calibrated in such a way that the manufacturing requirements are reduced. 
         [0018]    It is possible that the first and/or second position measurement devices are online measurement devices, i.e. are adapted to acquire a plurality of measurement points of a plurality of reference devices simultaneously or in only a short time. 
         [0019]    With the method, assembling processes of the structure may be supported, for example by visualizing the differences between the measurement points and the positions defined by the construction plan and/or by supporting the movement of an element of the structure towards its destination. Also quality control may be supported, for example by visualizing and/or measuring of mounting positions. 
         [0020]    It has to be noted that the receiving device may be permanently attached to the structure, for example much longer than the reference devices may be attached to the receiving device. For example, when the measurement method is used during the constructing of the structure, the receiving device may be attached to the same point of the structure during the complete construction process. For example, the receiving device may be glued, adhesively attached and/or screwed to the element of the structure. 
         [0021]    On the other hand, the reference devices may only be temporarily attached to the receiving device. For example, they may be attached in such a way, that the connection between the receiving device and the reference device may be loosened without the aid of a tool. 
         [0022]    According to an embodiment of the invention, the receiving device comprises a first bearing element and each of the first reference device and second reference device comprises a second bearing element, such that a reference point of a reference device does not move with respect to the receiving device it is attached to, when the reference device is rotated with respect to the receiving device. A reference device may be rotated freely with respect to its receiving device, such that a measurement target carried by the reference device may be oriented towards the respective position measurement device, which may comprise a measurement sensor adapted for sensing the measurement target. 
         [0023]    The rotation point about the reference device is rotating defines a reference point for the receiving device (which does not move with respect to the structure) and a reference point for the reference device (with respect to a measurement target), which may be determined very exactly for each reference device separately before the measurement of the structure. The reference point of the reference device may be used for the determination of the second measurement point from the measured position of the measurement target(s) attached to the reference device. 
         [0024]    According to an embodiment of the invention, the first bearing element has a bearing surface formed like a truncated cone and/or the second bearing element has a bearing surface formed like a spherical cap. For example, the spherical functional surface of the reference device may be received by a conical functional surface of the receiving device. In such a way, a ring-shaped contact surface is generated between the first bearing and the second bearing, which unambiguously determines the sphere center defined by the spherical surface. 
         [0025]    The sphere center of the second bearing element may determine the reference point of the reference device. When the reference device is attached to the receiving device, the sphere center may be at the reference point of the receiving device. 
         [0026]    It has to be noted that the bearing surfaces of all receiving targets may be equally formed and in particular may have the same cone angle. Also, the bearing surfaces of the reference devices may be formed equally, for example by spheres having the same diameter (such as about 0.5 inch, 1.27 cm). 
         [0027]    According to an embodiment of the invention, the receiving device and the first and/or second reference device are temporarily attached with a magnet. The magnet may be part of the receiving device or the reference device. In such a way, the bearing elements may stick together, when brought into contact and may easily be removed from each other, when a reference device has to be exchanged. 
         [0028]    According to an embodiment of the invention, the first and/or second reference device comprises a plate as a carrier for a measurement target. The second bearing and one or more measurement targets may be fixed to this plate. 
         [0029]    According to an embodiment of the invention, the first and/or second reference device comprises a (metal) sphere as a second bearing element to be attached to a first bearing element of the receiving device. For example, a reference device may comprise a sphere (which may be fixed to a carrier for one or more measurement targets), wherein the sphere provides the second bearing for the reference device. Such a reference device may be targeted in the direction to a sensor of the second measurement device, since a rotation of the reference device in the receiving device does not result in a movement of the reference point. 
         [0030]    According to an embodiment of the invention, the first reference device is a laser target and the first measurement device is a laser tracker device. For example, the first reference device may comprise a retro reflector. The retro reflector may be positioned inside an at least partially spherical carrier such that it is aligned with the center of the spherical carrier. A laser tracker device may be very accurate but may be very sensitive to disturbances like vibrations or persons walking around in the vicinity of the receiving device, which may cause small deformations. 
         [0031]    According to an embodiment of the invention, the second reference device comprises a carrier for a light source (as measurement target), for example an infrared LED, and the second measurement device is a light tracker device. For example, the light tracker device may be a device that is adapted for measuring all positions of all reference devices simultaneously. 
         [0032]    According to an embodiment of the invention, the second reference device comprises at least three light sources, for example infrared LEDs, that are attached to a carrier of the second reference device. 
         [0033]    In general, a reference device may comprise a spherical bearing surface and at least three measurement targets that are aligned not singularly and that are fixed together. In a calibration process, the position of the sphere center of the spherical bearing surface (and therefore the reference point) may be determined from the arrangement of the measurement targets. 
         [0034]    Furthermore, it is possible to use more than one type of second reference devices. It also is possible, that a third measurement is made with a third reference device attached to the receiving device and a third position measurement device. 
         [0035]    According to an embodiment of the invention, the second (or third) reference device comprises a carrier with a high-contrast target image (as measurement target) and the second (or third) measurement device comprises at least one camera. 
         [0036]    For example, the high-contrast target image may comprise one or more circles or other geometries painted on the carrier plate. Different reference devices may have different target images, such that the reference device may be identified by the measurement device. 
         [0037]    It is possible that the second reference device is a photogrammetry device. The second measurement device may comprise more than one camera. In this case new reference devices may be oriented in space (and the photogrammetry device may be fixed with respect to the structure). 
         [0038]    According to an embodiment of the invention, the second measurement device is adapted for determining its own position and/or orientation in space by detecting reference targets fixed to the structure, for example with a camera. In this case, the second measurement device may comprise only one camera. The second measurement device may be connected to an object that may be oriented in space with the aid of the second measurement device detecting reference devices fixed to the structure. It also possible that a mounting device or a holder device comprises such a second measurement device that is fixed to the mounting device or to the holder device for orienting and/or positioning the mounting device or the holder device. 
         [0039]    It has to be noted that more than one type of measurement target may be fixed to one reference device. For example, a reference device may carry light sources and a target image. 
         [0040]    According to an embodiment of the invention, the second (or third) reference device is a touch probe and the second (or third) measurement device is a tactile position measurement device. For example, the tip of the touch probe may be provided with a sphere, which sphere center provides the reference point for the tactile position measurement device. 
         [0041]    A further aspect of the invention relates to a measurement system for measuring the structure. In general, such a measurement system may comprise one or more of the above and below mentioned position measurement devices, a computer for evaluating the sensor data of the position measurement devices. Furthermore, the measurement system may comprise a visualisation tool (computer with screen) for showing the results of the measurements. 
         [0042]    Furthermore, the measurement system may comprise the above and below mentioned receiving devices and/or at least some of the above and below mentioned reference devices. 
         [0043]    According to an embodiment of the invention, the measurement system comprises a plurality of receiving devices; a plurality of first reference devices (for example laser targets); and a plurality of second reference devices (for example with a carrier for light sources and/or target images). Each of the plurality of receiving devices comprises a first bearing element for temporarily attaching a second bearing element of one of the first and second reference devices, such that a reference point of a reference device does not move with respect to a receiving device, it is attached to, when the reference device is rotated with respect to the receiving device. Each of the first reference devices carries a first measurement target (like a laser target) for a first position measurement device and each of the second reference devices carries a second measurement target for a second position measurement device. 
         [0044]    These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0045]    The subject-matter of the invention will be explained in more detail in the following text with reference to exemplary embodiments which are illustrated in the attached drawings. 
           [0046]      FIG. 1  shows a three-dimensional view of a primary structure of an aircraft with a measurement system according to an embodiment of the invention. 
           [0047]      FIG. 2  shows a three-dimensional view of a receiving device for a measurement system according to an embodiment of the invention. 
           [0048]      FIG. 3  shows a three-dimensional view of a reference device for a measurement system according to an embodiment of the invention. 
           [0049]      FIG. 4  shows a three-dimensional view of the other side of the reference device of  FIG. 3 . 
           [0050]      FIG. 5  shows a three-dimensional view of the reference device of  FIG. 3  attached to the receiving device of  FIG. 2 . 
           [0051]      FIG. 6  shows a three-dimensional view of a reference device for a measurement system according to an embodiment of the invention. 
           [0052]      FIG. 7  shows a three-dimensional view of the reference device of  FIG. 6  attached to the receiving device of  FIG. 2 . 
           [0053]      FIG. 8  shows a three-dimensional view of a reference device for a measurement system according to an embodiment of the invention. 
           [0054]      FIG. 9  shows a three-dimensional view of a reference device for a measurement system according to an embodiment of the invention. 
           [0055]      FIG. 10  shows a flow diagram for a measuring method according to an embodiment of the invention. 
       
    
    
       [0056]    In principle, identical parts are provided with the same reference symbols in the figures. 
       DETAILED DESCRIPTION 
       [0057]      FIG. 1  shows an exemplary primary structure  10  of an aircraft  12  together with a measurement system  14 . The primary structure comprises struts, stringers and a skin as elements  16 . 
         [0058]    The measurement system  14  comprises a number of receiving devices  18  that are attached to the elements  16  and a number of reference devices  20  that may be attached to the receiving devices  18 . 
         [0059]    Furthermore, the measurement system  14  comprises a number of measurement devices  22 , like a light tracker device  22   a , a laser tracker device  22   b , a photogrammetry device  22   c  and a tactile position measurement device  22   d . Each of the measurement devices  22  is adapted for determining the position of a corresponding reference device  20 , which will be explained in the following in more detail. 
         [0060]      FIG. 2  shows a receiving device  18 , which comprises a base plate  24  and a bearing element  26  that is attached to the center of the base plate  24 . The base plate has a rectangular shape, for example about 20 mm to about 40 mm. The base plate  24  is adapted for being attached to an element  16  of the structure  10 . For example, the base plate  24  may be glued or adhesively attached to the element  16 . 
         [0061]    The bearing element  26  has a conical bearing surface  28  and a magnet  30  positioned in the middle of the conical bearing surface  28 . 
         [0062]      FIGS. 3 and 4  show a reference device  20   a  from different sides. The reference device  20   a  has a carrier plate  32  with a substantially trapezoid shape. At the longer edge of the trapezoid, the carrier plate  32  has a protrusion  34  onto which a bearing element  36  in the form of a sphere is fixed. The bearing element  36  provides a bearing surface  38  that fits onto the bearing surface  28  of the bearing element  26 . 
         [0063]    On the side opposite to the side the sphere  36  is attached to, the carrier plate  32  carries four measurement targets  40  in the form of infrared LEDs  40   a . The infrared LEDs  40   a  are fixed to the corners of the carrier plate  32 . 
         [0064]    The reference device  20   a  may be used together with a light tracker device  22   a  that comprises a sensor beam  42  with three infrared cameras  44  as sensors. The light tracker device  22   a  may take pictures of the reference devices  20   a  attached to the receiving devices  18  and may determine the position of the reference devices  20   a.    
         [0065]      FIG. 5  shows the reference device  20   a  attached to the receiving device  18 . The magnet  30  pulls the sphere  36  onto the bearing element  26  and the reference device  20   a  may be rotated freely about the center of the sphere  36 . 
         [0066]      FIG. 6  shows a reference device  20   b  that may be used with a laser tracker device  22   b . The reference device  20   b  comprises a bearing element  36  with an outer surface  38  in the form of a (partial) sphere. A measurement target  40  in the form of a retro reflector  40   b  is centred in the bearing element  36 . 
         [0067]    The laser tracker device  22   b  may direct a laser beam towards the reference device  20   b , which is reflected by the retro reflector  40   b  back to the laser tracker device  22   b . From the reflected laser beam, the laser tracker device  22   b  can determine the position of the reference device  20   b.    
         [0068]      FIG. 7  shows the reference device  20   b  attached to the receiving device  18 . The magnet  30  pulls the sphere  36  onto the bearing element  26  and the reference device  20   b  may be rotated freely about the center of the sphere  36 . 
         [0069]      FIG. 8  shows a reference device  20   c  that may be used with a photogrammetry device  22   c . The reference device  20   c  may have the same components like the reference device  20   a . However, on the side opposite to the side with the bearing element  36 , the reference device  20   c  comprises a high-contrast target image  40   c  as measurement target  40 . The target image  40   c  may have more than one circle or, more general, one or more coded marks that may be used for identifying the reference device. With at least three marks, the position and/or orientation of the reference device may be determined exactly. 
         [0070]    The photogrammetry device  22   c  may have two cameras and may be adapted for determining the position of the reference device  20   c  via the target  40   c  photographed from two different directions with the cameras. 
         [0071]      FIG. 9  shows a reference device  20   d  that may be used with a tactile position measurement device  22   d . The reference device  22   d  may comprise an arm  20   d  as reference device  20  and may comprise a sphere  36  at the tip of the arm. When the sphere  36  is positioned in the bearing element of the receiving device  18 , the tactile position measurement device  22   d  is adapted for determining the position of the sphere  36  by way of the position and orientation of the arm  20   d.    
         [0072]      FIG. 10  shows a flow diagram for performing a measurement with the structure  10 . 
         [0073]    In step  50 , a number of receiving devices  10  is attached to elements  16  at specific measurement positions. For example, the receiving devices  10  may be glued to the elements  16 . The measurement positions may be chosen in such a way, that a geometrical arrangement of the elements  16  may be determined. For example, the measurement positions may be such that the geometrical arrangement of the elements  16  may be compared with a construction plan. 
         [0074]    In step  52 , first reference devices  20  are attached to the receiving device  18  and first measurement points are determined with a first measurement device  22  that is adapted to measure a position of first reference devices  20 . 
         [0075]    Step  52  may be used for determining the exact positions of the receiving devices  18 , in particular for exactly determining the reference points of the respective receiving device  18 . The reference point may be the sphere center of the bearing element  36  of the reference device  20 . 
         [0076]    For example, step  52  may be performed with a laser target  20   b  as reference device  20  and with a laser tracker device  22   b  as position measurement device  22 . With a laser tracker, the first measurement points may be determined with high accuracy. The reference points may be determined from the first measurement points and thus also may have a high accuracy. 
         [0077]    In step  54 , the first reference devices  20  are removed. It has to be noted that the receiving devices  18  stay at their position and may be not removed until the end of the process that is supported with the measurement system  14 . Such a process, for example, may be the construction of the structure  10 . 
         [0078]    Since a laser tracker device  22   b  may be very sensible with respect to disturbances like vibrations and it may not be optimally adapted for measurements during the complete process, for example at times when a lot of persons are inside the structure  12  or when machines are working on the structure. 
         [0079]    Thus, in step  56 , second reference devices  20  (such as reference devices  20   a ,  20   c  or  20   d ) are attached to the receiving devices  18  and second measurement points may be determined with a second measurement device  22  (such as  22   a ,  22   c ,  22   d ) adapted to measure a position of the second reference device  20 . 
         [0080]    The second measurement points may be compared to the first measurement points or to the reference points derived from the first measurement points. For example, with the second measurements point it may be determined, whether the elements  16  of the structure  10  are at the correct positions, whether the elements  14  have deformed and/or whether the elements conform to a construction plan. 
         [0081]    It is possible that during the supported process, step  56  is repeated with a different kind of reference device  20  corresponding to a different type of position measurement device  22 . 
         [0082]    While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art and practising the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or controller or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.