Abstract:
Some embodiments of the invention relate to a location determination apparatus for determining a location of two components relative to one another. The apparatus may comprise at least one location encoder having a read head in cooperation with a code for generating first position determination data at a first measurement frequency. According to some embodiments of the invention, at least one inertial measurement unit is arranged for additionally determining translational and/or rotational accelerations of at least one of the two components and, moreover, for generating second position determination data with respect to the location at a second measurement rate.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a location determination apparatus for determining a position of a target object and for use in a measuring machine, in particular in a coordinate measuring machine or a geodetic surveying system, and to a measuring machine, equipped with such a location determination apparatus, and to a measurement method and to an associated computer program product. 
       BACKGROUND 
       [0002]    Determining directions, angles and lengths as locations is required in many fields of application, such as e.g. geodetic surveying and for industrial measurements. Developments in angle metrology have led, via mechanical read processes, to fully automated location measurements, in particular angle or path measurements according to the current prior art. 
         [0003]    Known automated location determination apparatuses generally comprise a code carrier and a scanning apparatus. In the case of angle measurement apparatuses, so-called angle encoders, the code carrier is usually embodied rotatably about an axis relative to a scanning apparatus, wherein an angle location of the code carrier then constitutes the variable to be measured. By way of example, the code carrier can have a division or encoding for determining the position, wherein the encoding may be applied to a surface or lateral face of the code carrier. 
         [0004]    For the purposes of automatically acquiring the location, the code carrier, which is movable relative to the scanning apparatus, is scanned by a read head by means of various techniques. Known scanning methods include electronic-magnetic, electronic and optoelectronic methods (i.e. inductively based, capacitively based and optically based). 
         [0005]    By way of example, location determination apparatuses are used in coordinate measuring devices or coordinate measuring machines (CMM). 
         [0006]    An optical detector of an angle encoder for reading an optically readable code carrier, as described in WO 2008/019835, is, for example, a photodetector, a CCD line array or a CCD area array. In general, the code carrier is embodied as a circular disk or as a circular ring and carries a position code, which can be acquired optically, along the circumference thereof, of which code a section is imaged on the detector. 
         [0007]    As disclosed in e.g. WO 2007/051575, known angle measurement devices generally comprise a so-called circular arc and a scanning apparatus. The circular arc is embodied as code carrier and has a division or encoding for determining the position on the circle. The encoding is applied to a surface, i.e. a circle face or a lateral face, of the code carrier. 
         [0008]    WO 2008/141817 discloses a location determination apparatus with a linear encoder and/or an angle encoder. The code carrier and scanning apparatus are arranged in such a way that relative movement in one degree of freedom, in particular a relative rotational movement or a relative longitudinal movement along one axis, is made possible between code carrier and scanning apparatus. 
         [0009]    The known location determination apparatuses, in particular for coordinate measuring machines or geodetic surveying machines such as e.g. total stations, have very different advantages and disadvantages. 
         [0010]    Known location encoders, such as linear encoders and angle encoders, are characterized by a high measurement accuracy for supplying relative (i.e. incremental) or absolute position determination data, but a relatively low measurement speed, for example of the order of between 1 Hz and 20 Hz, and, as a result thereof, relatively low possible reading speed. Designs with a comparatively increased possible measurement speed are relatively expensive, in particular due to the more complicated mechanics and electronics required for this. 
       SUMMARY 
       [0011]    Some embodiments may include an improved location determination apparatus, in particular for use in coordinate measuring machines or geodetic surveying machines. Specifically in this case, an increased measurement speed is intended to be made possible, wherein, in particular, only small, or no, losses in respect of measurement accuracy have to be accepted. 
         [0012]    Some embodiments may make such an improved location determination apparatus available with the smallest possible production outlay. 
         [0013]    The location determination apparatus, according to the invention, for two components, mobile relative to one another, of a measuring machine, in particular of a coordinate measuring machine or of a geodetic surveying system, is embodied for determining a location of the two components relative to one another. It comprises at least one location encoder having a read head in cooperation with a code. The location encoder is embodied for generating first position determination data with respect to the location at a first measurement rate. 
         [0014]    According to the invention, the location determination apparatus furthermore comprises at least one inertial measurement unit for determining translational and/or rotational accelerations of at least one of the two components and, therefore, for generating second position determination data with respect to the location at a second measurement rate—which is higher than the first measurement rate. 
         [0015]    For determining the location, to the brought about by an evaluation unit, the evaluation unit is now embodied and configured to acquire the first position determination data generated by the at least one location encoder and the second position determination data generated by the at least one inertial measurement unit, correlate said position determination data with one another and determine the location therefrom at a third measurement rate, which is at least higher than the first measurement rate. 
         [0016]    In particular, the evaluation unit in this case is embodied and configured to determine the location on the basis of the second position determination data for times within intermediate time windows which lie between successive measurement times of the first measurement rate. Here, specifically, the second position determination data can be used only in each case for the intermediate time window and the location is determined on the basis of the first position determination data only for measurement times of the first measurement rate. 
         [0017]    Advantageously, the evaluation unit is embodied and configured to reference the acquired second position determination data, in each case with respect to a respectively most current available value for the first position determination data. To this end, for example, the second position determination data, received by the inertial measurement unit, can be corrected at predetermined time intervals to first position determination data, received in advance by the location encoder, as (more precise) reference and therefore it is possible to set a respectively new initial value for further second position determination data to be received over the course of time such that a drift over the second position determination data, generated over time during the intermediate time windows, can be reset to zero, in each case with the clock of the first measurement rate, when determining the location. 
         [0018]    In particular, the evaluation unit can be embodied to correlate the first position determination data and the second position determination data with the aid of a defined algorithm, in particular a Kalman filter or a divided-difference filter, to which the first and second position determination data are fed and which operates clocked at a rate which is at least as high as the third measurement rate, in particular wherein the rate equals the third measurement rate. 
         [0019]    As a result, an improved location determination apparatus is provided, which enables an increased measurement speed at a measurement accuracy which is comparable to that of established location encoders. Here, the high measurement accuracy is ensured by the use of location encoders and the high measurement speed is ensured by the combination of the measurements by means of the location encoder with measurements by means of the inertial measurement unit. A permanently high measurement accuracy is ensured by the inventive, repeated balancing of the data determined by the inertial measurement unit with the measurement values of the location encoder (in particular, wherein the balancing is repeated with the clock of the first measurement rate). 
         [0020]    By way of example, data calculations with the aid of a Kalman filter are suitable for correlating the first and the second position determination data—as mentioned previously. Such an algorithm enables faster data processing. 
         [0021]    The combination according to the invention of first position determination data, gathered at a comparatively low measurement rate, and second position determination data, gathered at a comparatively high measurement rate, enables, as a result of this cooperation, a high measurement rate of the location determination apparatus according to the invention, for example of the order of 1000 Hz and more. 
         [0022]    Advantageously, this renders possible the use, also, of location encoders which operate relatively slowly, e.g. at only 1 Hz, but are more cost-effective since the slow measurement speed thereof is compensated for by the combination with the measurement data from inertial measurement units having a fast mode of operation. 
         [0023]    In accordance with one embodiment of the invention, at least one inertial measurement unit and one location encoder are respectively assigned to one another for correcting and resetting the received second position determination data to first position determination data received previously. 
         [0024]    The location encoder can have different technical embodiments. In one embodiment, the read head can have an optically reading sensor, in particular a sensor line or an area sensor, and the code can be formed by an optically readable pattern made of a multiplicity of optical code elements (e.g. with specific light transmissivity, reflectivity, refraction or diffraction properties, as known to a person skilled in the art per se). In another embodiment, the read head can have a capacitively reading sensor, in particular with one or more capacitors, and the code can be formed by a capacitively readable pattern made of a multiplicity of code elements with specific permittivity, in particular structured plates or plastic strips. In another different embodiment, the read head can have an inductively reading sensor, in particular one or more coils, and the code can be formed by an inductively readable pattern made of a multiplicity of code elements with specific permeability, in particular magnetic strips. 
         [0025]    In accordance with one embodiment of a location determination apparatus according to the invention, the location encoder is embodied as a linear encoder with a read head for reading a linear code. In accordance with another embodiment, the location encoder is embodied as an angle encoder with a read head for reading a code arranged in a substantially circular or circular arc shaped manner. 
         [0026]    Therefore, the location determination apparatus according to the invention can be equipped with a conventional, widely available location encoder. 
         [0027]    In accordance with one embodiment, the inertial measurement unit has acceleration sensors and/or gyroscope sensors, and is embodied as a MEMS microsystem or MOEMS microsystem (i.e. as “micro-electro-mechanical system” or as “micro-opto-electro-mechanical system”). What is essential here is that the inertial measurement unit renders it possible to determine an acceleration in every direction or a rate of rotation about every axis in which the two components, the relative location of which is intended to be determined, are mobile relative to one another (such that the inertial measurement unit can measure position data corresponding to the measurement data which can be generated by the location encoder). 
         [0028]    However, even if this is not mandatory for the invention, the inertial measurement unit can also be embodied to determine acceleration measurement values with respect to three spatial directions, which are, in particular, orthogonal to one another, and to determine rates of rotation about three axes, which are, in particular, orthogonal relative to one another. 
         [0029]    As is known to a person skilled in the art, by the appropriate combination of a plurality of inertial sensors of an inertial measurement unit, the accelerations of the six degrees of freedom can, in the process, generally be measured on the basis of the following types of sensor: 
         [0030]    Three orthogonally arranged acceleration sensors (also referred to as translation sensors) detect the linear acceleration along the x- or y- or z-axis. From this, it is possible to calculate the translational movement (and the relative position). Three orthogonally arranged rate sensors (also referred to as gyroscopic sensors) measure the angular acceleration about the x- or y- or z-axis. From this, it is possible to calculate the rotational movement (and the relative alignment). 
         [0031]    In general, such inertial sensors in this case are not suitable for continuous, precise determinations of the position since they only supply measurement values based on accelerations, but no absolute position determination data. As a result of this, position determination data derived from the measurement values of such sensors is subject to a continuous drift, in particular on the basis of propagating and hence accumulating errors in the measurement of accelerations or location changes. However, as a result of the embedding, according to the invention, into a location determination unit, this disadvantage does not have a substantially negative effect on the overall result of the location determination. 
         [0032]    Such inertial measurement units, which are based on MEMS-based components and which are embodied as miniaturized machines or components, are known from the prior art. 
         [0033]    The subject matter of the present invention also relates to a measuring machine, in particular a coordinate measuring machine CMM (with a portal or articulated arm design, or else embodied as a laser tracker) or a geodetic surveying system (such as a theodolite or a tachymeter), for determining a position of a target object, comprising a location determination apparatus according to the invention according to one of the embodiments described above. The location determination apparatus is then used to determine angles in hinges (see hinged-arm CMM, laser tracker, theodolite or tachymeter) or lengths in linear displacement mechanisms (see e.g. portal CMM). 
         [0034]    In accordance with one preferred embodiment, at least one inertial measurement unit is assigned to each mobile and/or separately mobile element of the measuring machine for measuring accelerations or changes in location. 
         [0035]    The subject matter of the invention also relates to a measurement method for determining a position of a target object using a location determination apparatus according to the invention, which comprises at least one location encoder with a read head in cooperation with a code, and at least one inertial measurement unit. Inter alia, the measurement method comprises the following steps: determining the absolute position of the target object and generating first absolute position determination data at a first measurement frequency by means of the at least one location encoder, and determining translational and/or rotational accelerations and/or changes in location of mobile components of the location determination apparatus and generating second position determination data of the position of the target object at a second measurement frequency by means of the at least one inertial measurement unit. 
         [0036]    According to the invention, the measurement method comprises the following further steps: reading, with the aid of the evaluation unit, the first position determination data generated by the at least one location encoder and the second position determination data generated by the at least one inertial measurement unit, correlating, with the aid of the evaluation unit, the read first position determination data and second position determination data in time, correcting the second position determination data received by the inertial measurement unit at predetermined time intervals to absolute first position determination data received in advance by the location encoder and, thereby, setting a respectively new initial value for further second position determination data to be received over the course of time. 
         [0037]    Preferably, the first and second position determination data are correlated using a Kalman filter in this case. 
         [0038]    Advantageously, the measurement method according to the invention is carried out at a measurement rate of 1000 Hz and more. 
         [0039]    The subject matter of the invention moreover relates to a computer program product with program code, which is stored on a machine-readable medium, for carrying out the measurement method according to the invention, in particular if the program code is executed on an evaluation unit of a location determination apparatus according to the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0040]    The location determination apparatus according to the invention, a coordinate measuring machine or geodetic surveying system equipped with such a location determination apparatus, and an associated measurement method according to the invention are described in more detail below in a purely exemplary manner on the basis of specific exemplary embodiments schematically depicted in the drawings, with further advantages of the invention also being discussed. In detail: 
           [0041]      FIG. 1   a / 1   b / 1   c  show an illustration of a measurement method which can be carried out using a location determination apparatus according to the invention; 
           [0042]      FIG. 2  shows a coordinate measuring machine according to the invention, embodied in an exemplary manner as a portal coordinate measuring machine and equipped with a location determination apparatus according to the invention; 
           [0043]      FIG. 3  shows, in a magnified illustration corresponding to the portal coordinate measuring machine from  FIG. 2 , an illustration of the cooperation between the linear encoder and an inertial measurement unit of a location determination apparatus according to the invention; 
           [0044]      FIG. 4  shows a robot arm, which is equipped with a location determination apparatus according to the invention; 
           [0045]      FIG. 5  shows a geodetic surveying machine according to the invention, embodied as a theodolite in an illustrative manner, which is equipped with a location determination apparatus according to the invention; and 
           [0046]      FIG. 6  shows the cooperation between an angle encoder and inertial measurement units of a location determination apparatus according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0047]      FIGS. 1   a ,  1   b ,  1   c  illustrate a measurement method which can be carried out with the location determination apparatus according to the invention. 
         [0048]    Here,  FIG. 1   a  schematically shows first position determination data  110 , acquired during a course of time t, which are measured by a location encoder according to the prior art, such as e.g. a linear encoder or an angle encoder, as absolute first determination values  111  at time intervals  112 . The acquired determination values  111  usually correspond with a desired maximum accuracy  135  and exceed a predetermined minimum accuracy  136 , which is defined by a difference  137  between the maximum accuracy  135  and the minimum accuracy  136 . Here, the time intervals  112  between sequentially acquired absolute first determination values are relatively large, or the corresponding data acquisition rate is relatively low, typically corresponding to a measurement rate of the order of 1 Hz to 20 Hz. 
         [0049]      FIG. 1   b  schematically shows a typical time profile t of second position determination data  120 , which are generated from acceleration or position change data of mobile components of the location determination apparatus, as measured by an inertial measurement unit. Assigned second determination values  121  are recorded sequentially at relatively short time intervals  122  or with a relatively high corresponding data acquisition rate. 
         [0050]    At an initial time t 0 , the determination values  121  of the second position determination data correspond to a value of corresponding maximum accuracy  135 , for example due to an initial calibration. However, the further determination values  121 , recorded in sequence exhibit, a temporal drift, as depicted schematically in  FIG. 1   b , with a pronounced drop below the predefined minimum accuracy  136 . This is due to the fact that sensors of an inertial measurement unit only supply measurement values on the basis of accelerations or changes in position, but no absolute position determination data. As a result of this, position determination data derived from the measurement values of such sensors are subject to a continuous drift, in particular on the basis of propagating and hence accumulating errors in the measurement of accelerations or changes in position. 
         [0051]    Therefore, an inertial measurement unit is advantageous for data acquisition at a high measurement rate, for example of the order of 50 Hz to 2000 Hz, but it is not suitable as only a device for generating position determination data with a continuously sufficient accuracy. 
         [0052]    The approach according to the invention is illustrated on the basis of  FIG. 1   c . According to the invention, an evaluation unit  140  is assigned to the location determination apparatus, which evaluation unit is configured to read the first position determination data  110 , generated by the at least one location encoder, and the second position determination data  120 , generated by the at least one inertial measurement unit, together with the corresponding determination values  111 ,  121  thereof, to correlate these determination data with one another in time and, at predetermined time intervals  132 , to correct the second position determination data  120  or second determination values  121  received by the inertial measurement unit to absolute first position determination data  110  previously received by the location encoder or first determination values  111  and, thereby, in each case set a new initial value  133  for second position determination data  120  or second determination values  121  continuing to be received over the course of time. 
         [0053]    As a result, combined position determination data  130  are generated at very short time intervals  122  corresponding to the time intervals  122  with an inertial measurement unit, which position determination data are, however, due to the corrections of the second position determination data undertaken at the time intervals  132  in each case, fixed within a predefined deviation  137  between the maximum accuracy  135  and the predefined sufficient accuracy  136 . 
         [0054]    In accordance with one embodiment, the evaluation unit  140  is configured to bring about corrections and resettings of the received second position determination data  120  at a time interval  132 , which can be defined by a user of the location determination apparatus by an interval to a pre-definable deviation  137  between an absolute value, generated by the first position determination data, and a deviating second determination value  121 , generated by the second position determination data. 
         [0055]      FIG. 2  shows a coordinate measuring machine  1  according to the invention, embodied in an exemplary manner as a portal coordinate measuring machine  1 . 
         [0056]    The coordinate measuring machine  1  has a base  10 , on which a portal  14  is arranged in such a way that it can be moved in a longitudinal direction (Y-direction). The portal  14  has two portal supports  11 , which are connected to one another at their upper ends by a bridge  17 . 
         [0057]    An X-carriage  12  is arranged on the bridge  17 , which X-carriage can be moved along the bridge  17 , i.e. in a spatial direction (X-direction) connecting the two portal supports  11 . A rod or Z-column  13  can be moved along a third spatial direction (Z-direction) and it is guided in a receptacle of the X-carriage  12 . For this movement in the Z-direction, the Z-column  13  is guided by bearings which are components of the carriage  12 . The three spatial directions X, Y and Z are preferably aligned perpendicular to one another, even if this is not a precondition for the present invention. 
         [0058]    The coordinate measuring machine  1  is provided for determining one or many measurement points  6  on an object  5  and therefore has three linear guide mechanisms for enabling the movement of a measurement head  15 , which is arranged at the lower, free end of the Z-column  13  facing the base  10  and for example embodied for tactile measurements, in the three spatial directions X, Y and Z relative to the base  10 . 
         [0059]    Each linear guide mechanism has an associated guide (in X-, Y- and Z-direction). Moreover, each one of the linear guide mechanisms has an assigned location encoder, embodied as a linear coder in this example, for determining the position in the assigned guide direction, wherein the respective linear encoders cooperate with associated codes  61 ,  62 ,  63  embodied as measurement scales  62 ,  61 ,  63  for determining the positions in X-, Y- and Z-direction. 
         [0060]    According to the invention, the coordinate measuring machine moreover has inertial measurement units  51  for measuring accelerations and changes in position of the portal  14  with the portal supports  11  in the Y-direction,  52  for measuring accelerations and changes in position of the carriage  12  in the X-direction and  53  for measuring accelerations and changes in position of the Z-column  13  with the sample head  15  in the Z-direction. 
         [0061]    The functionality and method of operation of the location determination apparatus, which in this example consists of the combination of the location encoders, which have the measurement scales  61 ,  62 ,  63 , with the inertial measurement units  51 ,  52 ,  53 , is analogous to what was described above on the basis of  FIGS. 1   a  to  1   c.    
         [0062]    The invention is not restricted to portal coordinate measuring machines as depicted in  FIG. 2 . Rather, any known type of coordinate measuring machine, in particular e.g. also a so-called “articulated arm coordinate measuring machine”, which enables an object surface measurement by means of a suitable measurement head, is suitable for the invention. 
         [0063]    In a magnified illustration corresponding to the portal coordinate measuring machine  1  from  FIG. 2 ,  FIG. 3  illustrates the cooperation between a linear encoder  60  and an inertial measurement unit  51  of a location determination apparatus  50  according to the invention. The linear encoder  60  comprises a read head  71  for reading a code  61  of a code carrier  64 . The read head  71  is arranged at the lower end of a portal support  11  in a suitable vicinity to the code carrier  64  attached to the base part  10  so as to be able to read the code  61  thereof. An inertial measurement unit  51  for measuring accelerations or changes in position of the portal support  11  with respect to the base part  10  is arranged on the portal support  11 . 
         [0064]    A location determination apparatus according to the invention can also be used for checking or monitoring the changes in position of movable components of a robot arm  2  with a tool  25 , guided thereby, for machining an object  7 , as is depicted in an exemplary manner in  FIG. 4 . In this example, the object  7  to be machined is an automobile door. The robot arm  2 , depicted in a schematic and exemplary manner, comprises a plurality of sections  20 ,  21 ,  22 ,  23 ,  24 , which can be adjusted or rotated relative to one another by means of hinges  26 ,  27 ,  28 . Typically, such a robot arm, for example as a component of a machine tool, is remotely controlled by an evaluation, monitoring and control unit (not depicted here). The hinges  26 ,  27 ,  28  are equipped with location encoders (likewise not depicted here), embodied as angle encoders for, in this case, determining rotations of the sections  20 ,  21 ,  22 ,  23 ,  24  with respect to one another. In order to determine accelerations and enable a high measurement rate, inertial measurement units  54 ,  55 ,  56  and  57  are attached to the robot arm sections  21 ,  22 ,  23  and  24  in each case. 
         [0065]    As a further example for use,  FIG. 5  shows a geodetic surveying machine, embodied illustratively as a total station or as a theodolite  3 , which is equipped with a location determination apparatus according to the invention. 
         [0066]    In this example, the theodolite  3  comprises a base  41 , embodied as a three-legged stand, with an upper part  42  mounted thereon in a rotatable manner. A sighting unit  43 , pivotably mounted on the upper part  42 , is typically equipped with a laser source embodied to emit a laser beam and with a laser light detector, and therefore provides a ranging functionality for determining the position of a target object. 
         [0067]    The spatial alignment of the sighting unit  43  with respect to the base  41  can be acquired by means of two angle measuring units or angle encoders (not depicted here). 
         [0068]    In order to determine accelerations and enable a high measurement rate, inertial measurement units  57 ,  58  and  59  are respectively attached to the base  41 , to the upper part  42  and to the sighting unit  43 . 
         [0069]      FIG. 6  illustrates the cooperation between an angle encoder  80  and inertial measurement units  51  of a location determination apparatus  50  according to the invention. Two arm-like sections  31  and  32  are connected to one another by means of a hinge  29  and can be pivoted about this hinge  29  against one another in the direction of the arrow  83 . The angle settings between the two sections  31  and  32  are measured by an angle encoder  81 . The angle encoder  80  comprises a read head  84  for reading a code  81  with a plurality of concentric code tracks on a code carrier  81 . In order to determine accelerations and enable a high measurement rate, inertial measurement units  59 ′ and  59 ″ are arranged on the two sections  31  and  32  that can be pivoted with respect to one another.