Abstract:
In order to determine spatial coordinates of a multiplicity of measurement points along a contour of a measurement object, a probe head is provided with a probe element that is movably supported on the probe head. During movement of the probe head along the contour, position measuring values of the probe head and deflections of the probe element relative to the probe head are determined. Spatial coordinates for the measurement points along the contour are determined from the position measuring values and deflections. The probe element is kept in contact with the contour during movement of the probe head by using an actuator to produce a defined contact force. The contact force is set as a function of a differential acceleration of the probe element relative to the probe head.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of international patent application PCT/EP2007/009797 filed on Nov. 13, 2007 designating the U.S., which international patent application has been published in German language and claims priority from German patent application DE 10 2006 055 005.6 filed on Nov. 17, 2006. The entire contents of these prior applications are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to a method and an apparatus for determining spatial coordinates at a multiplicity of measurement points along a contour of a measurement object. In particular, the invention relates to a method and an apparatus for the continuous measurement (so-called scanning) of a surface of a measurement object by means of an active tactile probe head that enables the setting of a defined contact force (or sensing force) by means of a preliminary deflection. 
         [0003]    The present invention is applied in the field of so-called coordinate measuring technology. This is understood as measuring workpieces in one, two and preferably even three dimensions by means of measuring tools that enable spatial coordinates to be determined at selected measurement points of the workpieces. A typical coordinate measuring machine has a head part that can be moved relative to the workpiece or measurement object. The head part carries a sensor by means of which the head part can be brought into an exactly defined position with reference to the measurement point. In tactile coordinate measuring machines, the measurement point is contacted by means of a probe element. The head part is therefore usually denoted as probe head in such cases. The probe element is generally a stylus whose free end terminates in a spherical tip. The stylus is movably supported on the probe head such that the stylus is deflected relative to the probe head during contacting. Given a specific deflection and/or contact force, position measuring values are recorded that are representative of the position of the probe head in the measuring volume, and thus of the position of the probe head relative to the measurement object. Moreover, in the case of so-called measuring probe heads the deflection of the stylus relative to the probe head is determined. Spatial coordinates for the contacted measurement point can then be determined from the position measuring values for the probe head and the deflections of the stylus. By contacting a plurality of surface points on a workpiece, it is also possible to detect geometric dimensions and shape profiles, the latter being possible with particular effectiveness by using a continuous (scanning) recording of measurement values. 
         [0004]    In the case of probe heads for tactile coordinate measuring machines, one can distinguish between active and passive probe heads. In the case of passive probe heads, the rest position of the probe element is set solely by means of mechanical springs. A deflection from the rest position takes place only during contacting of a measurement point against the spring force. By contrast, active probe heads have one or more actuators configured to deflect the probe element in a defined way before contact with the measurement object takes place, or without contact taking place. Such active probe heads are particularly suitable for scanning measurements, because the actuator can be used for keeping the probe element in continuous contact with the surface of the measurement object. 
         [0005]    However, the invention is not limited to coordinate measuring machines in the narrower sense. It can, for example, also be used on machine tools or other machines in the case of which a workpiece surface is scanned with a tactile probe element. 
         [0006]    Even in the case of active probe heads, the probe element is supported on the probe head by means of spring elements in order to enable the deflection about the rest position. In the case of active and passive probe heads, the support by means of spring elements renders it possible for the probe element to get into mechanical oscillations because of the movements of the probe head and because of the contacting actions. 
       SUMMARY OF THE INVENTION 
       [0007]    Against this background, it is an object of the present invention to provide a method and a device of the type mentioned at the beginning, wherein oscillations of the probe element are effectively reduced. It is another object to provide a method and a device which are particularly suitable for scanning a workpiece surface in a tactile manner. 
         [0008]    According to a first aspect of the invention, there is provided a method for determining spatial coordinates at a multiplicity of measurement points along a contour of a measurement object, the method comprising the steps of providing a probe head comprising a probe element movably supported on the probe head and comprising an actuator capable of setting a preliminary deflection of the probe element relative to the probe head, moving the probe head relative to the measurement object in a measuring volume, contacting a first measurement point on the contour with the probe element, recording first position values and first deflections, the first position values being representative of the position of the probe head in the measuring volume, and the first deflections being representative of the position of the probe element relative to the probe head, moving the probe head along the contour and recording further position values and further deflections at the multiplicity of measurement points along the contour, and determining the spatial coordinates as a function of the further position values and the further deflections, wherein the probe element is held in contact with the contour during the step of moving the probe head by producing a defined contact force by means of the actuator, and wherein the defined contact force is determined as a function of a differential acceleration of the probe element relative to the probe head and, furthermore, as a function of a differential speed of the probe element relative to the probe head 
         [0009]    According to another aspect of the invention, there is provided an apparatus for determining spatial coordinates at a multiplicity of measurement points along a contour of a measurement object, the apparatus comprising a probe head comprising a probe head base, a probe element movably supported relative to the probe head base, and comprising an actuator capable of setting a preliminary deflection of the probe element relative to the probe head base, the probe head being movable relative to the measurement object in a measuring volume, first position measuring devices for determining position values that are representative of the position of the probe head in the measuring volume, second position measuring devices for determining deflections that are representative of the position of the probe element relative to the probe head base, an evaluation and control unit for determining the spatial coordinates as a function of the position values and the deflections, and a control device configured to hold the probe element in contact with the contour during movement of the probe head by producing a defined contact force by means of the actuator, wherein the control device is designed to determine the defined contact force as a function of a differential acceleration of the probe element relative to the probe head base and, preferably, also as a function of a differential speed of the probe element relative to the probe head base. 
         [0010]    According to yet another aspect, there is provided a method for determining spatial coordinates of a multiplicity of measurement points in a measurement volume, the method comprising the steps of providing a probe head comprising a probe element movably supported on the probe head and an actuator capable of producing a defined contact force when the probe element makes contact with the measurement object, contacting a first measurement point on the measurement object with the probe element, moving the probe head along a contour on the measurement object, recording position values and deflections, with the position values being representative of the positions of the probe head within the measuring volume, and with the deflections being representative of the positions of the probe element relative to the probe head, determining the spatial coordinates as a function of the position values and the deflections, wherein the probe element is held in contact with the measurement object along the contour during the step of moving by producing the defined contact force by means of the actuator, and wherein the defined contact force is determined as a function of a differential acceleration of the probe element relative to the probe head. 
         [0011]    In preferred embodiments, the new method is implemented by means of a computer program comprising program code designed to execute such a method when the program code is executed in the control unit of such an apparatus. Such computer program renders it possible to easily retrofit embodiments of the new method in older existing coordinate measuring machines. 
         [0012]    The new methods and apparatus are based on the idea of using an acceleration of the probe element relative to the probe head in order to obtain a signal that is in phase opposition to the potential oscillations of the probe element and can be used to compensate the oscillations. The acceleration of the probe element relative to the probe head is a differential acceleration, since the acceleration of the probe head itself is less suitable for effective damping of the disturbing oscillations. By contrast, very good results can be achieved by means of the differential acceleration of the probe element relative to the probe head, because the oscillations to be suppressed result in an approximately sinusoidal position profile of the probe element relative to the probe head. Since the acceleration corresponds to a twofold differentiation of this position profile, the profile of the differential acceleration is likewise sinusoidal, although phase shifted by 180°. The differential acceleration of the probe element relative to the probe head therefore provides a signal that is well suited as a correction signal of opposite phase by means of which the oscillations of the probe element can be compensated for. 
         [0013]    The new methods and apparatus can be implemented very easily and cost-effectively in the case of systems that use active measuring probe heads with an adjustable preliminary deflection. In the ideal case, there is no need for any conversion measures on the hardware. 
         [0014]    Moreover, it has emerged in practical tests that the undesired oscillations of the probe element can be very effectively suppressed with the new approach. This is advantageous in the case of measurements in which the contour of a measuring object is continuously measured or scanned. A higher measuring accuracy can be achieved on the basis of the reduced oscillations. 
         [0015]    In a refinement, the differential acceleration of the probe element is measured with at least one acceleration sensor. It is possible to use at least two acceleration sensors of which one is coupled to the movable probe element, while another is coupled to a probe head base that is fixed (relative thereto). 
         [0016]    The detection of the differential acceleration by measurement facilitates an effective suppression of the undesired oscillations in cases where the computing power of the evaluation and control unit is limited such that it is difficult to implement real time suppression. The use of two separate acceleration sensors on, or in connection with the probe element and the probe head base enables a particularly simple and rapid detection of the differential acceleration. 
         [0017]    In another refinement, the differential acceleration of the probe element is estimated with at least one state observer. 
         [0018]    A state observer includes a simulation on the basis of a mathematical model that describes the behavior of the probe element relative to the probe head. The state observer can be implemented by means of software and/or by means of hardware. It receives the same input variables that are also used to control the probe element and to set the contact force. Moreover, the state observer receives the output variables with which the actual position of the probe element is detected. Consequently, the state observer can simulate the behavior of the probe element, and it is then possible to estimate from the model further state variables such as, for example, the acceleration of the probe element. The use of a state observer enables the determination of the (differential) acceleration of the probe element without the need for corresponding acceleration sensors. This is advantageous because commercial acceleration sensors such as, for example, are used to trigger airbags in motor vehicles, respond only given relatively strong accelerations. The accelerations of the probe element that occur in the present case are, however, relatively slight, and this renders detection by measurement technique difficult. The usage of a state observer avoids these difficulties. However, the use of a state observer has the disadvantage that external disturbance variables such as, for example, strong oscillations of the underlying ground on which the coordinate measuring machine stands, cannot be detected in real time, or can be detected only with great difficulty. For this reason, it can be advantageous to combine acceleration sensors and a state observer. 
         [0019]    In another refinement, the acceleration of the probe element is determined differentially from the deflections of the probe element relative to the probe head. 
         [0020]    This refinement is currently viewed with particular preference, because active measuring probe heads usually have one or more position measuring devices that enable the deflections of the probe element relative to the probe head to be determined. The “start value” required for this refinement is therefore available. A twofold differentiation yields the differential acceleration of the probe element relative to the probe head, account being taken of any external disturbance variables. Moreover, this refinement can be used for a multiplicity of various probe heads and stylus combinations, because the refinement is independent of the parameters of the probe head. This is particularly advantageous because the probe heads of coordinate measuring machines are frequently fitted with variable stylus arrangements, and so the behavior of the probe element varies dependent to the respective assembly. 
         [0021]    In another refinement, the contact force is further determined against a speed of the probe element relative to the probe head. 
         [0022]    It has emerged that this refinement yields good results in combination with the acceleration-dependent damping. This holds chiefly when the acceleration of the probe element is determined from the deflections of the probe element by means of twofold differentiation, because the speed of the probe element is then available in any case as an intermediate result. Moreover, the acceleration signal obtained by twofold differentiation can include strong interference components based on noise. A speed-dependent damping enables efficient suppression of the undesired oscillations, chiefly in the cases where strong interferences are present in the acceleration signal. 
         [0023]    In another refinement, the defined contact force is determined periodically as a function of the differential acceleration. 
         [0024]    This refinement is advantageous because it facilitates cost-effective digital implementation by software and can be integrated easily into existing control algorithms. 
         [0025]    In another refinement, the differential acceleration of the probe element is filtered in order to obtain a filtered differential acceleration, wherein the defined contact force is determined as a function of the filtered differential acceleration. 
         [0026]    This refinement is advantageous because interference signals present in the acceleration signal have a particularly strong effect, the result being to impair the suppression of the undesired oscillations. In real operating environments, an effective filtering of the acceleration signal is of great importance for the effective depression of the undesired oscillations. 
         [0027]    In another refinement, the acceleration of the probe element is filtered by means of a non-recursive filter in order to obtain the filtered acceleration. 
         [0028]    Non-recursive filters are frequently also denoted as FIR (finite impulse response) filters. These are usually digital filters in the case of which one or more past values of a signal (here, the acceleration signal) are provided with a weighting and added to the current value of the acceleration signal. This provides a type of averaging from the current and past signal values that has proved to be particularly effective for suppressing the undesired oscillations. 
         [0029]    In another refinement, the non-recursive filter has a largely rectangular weighting. 
         [0030]    In this refinement, a number of past values are added to the respectively current acceleration value, all the added acceleration values being largely equally weighted. A sliding average is formed in this way. This refinement yields good results and has the advantage that it can be implemented with a low computational outlay. It is advantageous that such an FIR filter can be implemented without the required computing power being dependent on the filter width or the number of past values. Moreover, this type of FIR filter causes a relatively slight phase shift, and this is advantageous for suppressing the undesired oscillations. 
         [0031]    In another refinement, the non-recursive filter has a largely triangular weighting. 
         [0032]    In this refinement, the weighting is at least approximately inversely proportional to time. The further an acceleration value lies in the past, the less strongly it is weighted. In other words, the acceleration values are rated with a factor that is smaller the further back in time they lie relative to the current acceleration value. Such an FIR filter has a very slight phase shift and is therefore well suited for filtering the acceleration signal within the scope of the present invention. Such an FIR filter can advantageously likewise be implemented with an algorithm independent of the filter width by virtue of the fact that in each filter clock the current acceleration value is added with its associated weighting to the existing weighted sum of the past values and then the sum of the simply weighted values is subtracted. 
         [0033]    In another refinement, the acceleration of the probe element is, furthermore, filtered with a recursive high-pass filter in order to obtain the filtered acceleration. The recursive high-pass filter is advantageously in series with the non-recursive filter. It is advantageous for the recursive high-pass filter to be downstream of the non-recursive filter. 
         [0034]    Recursive filters are frequently also denoted as IIR (infinite impulse response) filters. These are digital filters in the case of which an output value of the filtered output signal is recursively subtracted from the current value of the acceleration signal to be filtered. A recursive filter can be dimensioned quiet simply with known methods such as, for example, the so-called bilinear transformation. The combination of an FIR filter and an IIR high-pass filter has yielded the best results in practical tests by the applicant. 
         [0035]    It is to be understood that the features mentioned above and yet to be explained below may be used not only in the combination respectively indicated, but also in other combinations or separately, without departing from the scope of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]    Exemplary embodiments of the invention are represented in the drawing and will be explained in more detail in the following description. 
           [0037]      FIG. 1  shows a simplified illustration of a coordinate measuring machine according to an exemplary embodiment of the invention; 
           [0038]      FIG. 2  shows a simplified illustration of a probe head from the coordinate measuring machine of  FIG. 1 , 
           [0039]      FIG. 3  shows a block diagram for a first exemplary embodiment of a control device that is used in the coordinate measuring machine of  FIG. 1 , 
           [0040]      FIG. 4  shows a block diagram for a second exemplary embodiment of a control device for the coordinate measuring machine of  FIG. 1 , 
           [0041]      FIG. 5  shows a block diagram for a third exemplary embodiment of a control device for the coordinate measuring machine of  FIG. 1 , 
           [0042]      FIG. 6  shows a measuring curve that shows the deflections of the stylus in the case of the coordinate measuring machine of  FIG. 1  during scanning of a horizontal contour, the new method not being used, and 
           [0043]      FIG. 7  shows a measuring curve similar to that of  FIG. 6 , but with application of the new method. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0044]    In  FIG. 1 , a coordinate measuring machine serving as an exemplary embodiment of a device according to the invention is denoted in an entirety by the reference numeral  10 . The coordinate measuring machine  10  is illustrated here in gantry design, by way of example. However, the invention is not limited to a specific frame structure and can, for example, also be used in the case of coordinate measuring machines of horizontal arm design, and in the case of other machines. Moreover, the invention can also be used for coordinate measuring machines and machines in the case of which a workpiece is moved relative to a fixed head part because it is only the relative movement between the head part and the workpiece that is important in the context of the present invention. 
         [0045]    The coordinate measuring machine  10  has a base  12  on which a gantry  14  with a drive  15  is arranged. The gantry  14  can be moved by means of the drive  15  along an axial direction that is usually denoted as y-axis. 
         [0046]    Arranged on the upper transverse mount of the gantry  14  is a carriage that can be moved in x-direction. The carriage  16  carries a quill  18  that can be moved in z-direction. Located on the lower free end of the quill  18  is a probe head  20  with a stylus  22 . On its free end, the stylus  22  has a contacting sphere  23  ( FIG. 2 ) that serves to contact a surface point  24  on a workpiece  26 . In order to explain the subsequent exemplary embodiments, it may be assumed that the surface point  24  is a measurement point within a contour  25  that runs on a surface of the workpiece or measurement object  26 . 
         [0047]    The reference numerals  28 ,  30 ,  32  denote linear scales that are arranged parallel to the axial directions of the coordinate measuring machine  10 . By way of example, here these are glass scales that can be read off by means of suitable sensors (not illustrated here), in order to determine the moving positions of the gantry  14 , the carriage  16  and the quill  18 . By means of these measuring values, it is possible to determine the position of the probe head  20  in the measuring volume of the coordinate measuring machine  10 . The spatial coordinate of a contacted surface point  24  can then be determined from the position of the probe head. 
         [0048]    The reference numeral  34  denotes an evaluation and control unit that is connected via lines  36 ,  38  to the drives and sensors of the coordinate measuring machine  10 . Furthermore, here the evaluation and control unit  34  is connected to a control console  40  and a keyboard  42 . The control console  40  enables manual control of the coordinate measuring machine  10 . The keyboard  42  enables the input of operating parameters, and the selection of measurement programs etc. 
         [0049]    The control unit  34  has here a display  44  on which measurement results, parameter values, inter alia, can be outputted. Furthermore it has a processor  46  and a memory  48  that is illustrated with a plurality of memory areas  48   a ,  48   b . The memory  48  is denoted here as RAM, but can also include a ROM, the ROM serving chiefly to store the so-called firmware of the coordinate measuring machine  10 . In exemplary embodiments, the firmware includes program code (not illustrated here) that, inter alia, implements a control device such as is explained below by means of  FIGS. 3 to 5  in various exemplary embodiments. 
         [0050]      FIG. 2  shows the probe head  20  of the coordinate measuring machine  10  with further details, although in a greatly simplified schematic representation. 
         [0051]    The stylus  22  is fastened on a movable part  50  that is connected by two leaf springs  52 ,  54  to a probe head base  56 . Owing to the leaf springs  52 ,  54 , the movable part  50  can move with the stylus  22  relative to the probe head base  56 , the two mutually opposite movement directions being indicated here by the arrows  58 ,  60 . The movement directions of the stylus  22  are typically parallel to the movement directions x, y, z in which the probe head  20  can be moved. 
         [0052]    Persons skilled in this field will see that the probe head  20  illustrated in  FIG. 2  enables a deflection of the stylus  22  in only one axial direction  58 ,  60 , and this is to be ascribed to the simplified illustration. Further leaf springs  52 ,  54  can be present for deflecting the stylus  22  in the two further axial directions, as is known from the relevant probe heads of the applicant. 
         [0053]    The reference numeral  62  denotes an actuator by means of which the part  50  can be deflected relative to the probe head base  56 . In the exemplary embodiment illustrated, the actuator  62  is, for example, a plunger coil that is arranged between two limbs  64 ,  66 . The limb  64  is connected to the movable part  50 , while the limb  66  is connected to the probe head base  56 . The actuator  62  is capable of pressing the limbs  64 ,  66  apart, or pulling them together, the result being that the stylus  22  with the part  50  is deflected in the spatial direction  58  or in the spatial direction  60 . Such a deflection produced by means of the actuator  62  serves, inter alia, to set a defined measuring or contact force, respectively. Moreover, within the scope of the present invention the actuator  62  is used for the purpose of reducing oscillations of the stylus  22  relative to the stationary probe head base  56 , by setting a defined contact force against an instantaneous differential acceleration of the stylus  22  relative to the probe head base  56 . 
         [0054]    The reference numeral  68  denotes a sensor that is likewise arranged between the two limbs  64 ,  66 . The sensor  68  is illustrated here with a scale  70  that enables a current deflection X of the stylus  22  (illustrated at the reference numeral  22 ′) to be acquired by measurement technique. By way of example, the sensor  68  can be a plunge coil, a Hall sensor, an optical sensor or another position sensor or length sensor. 
         [0055]    Here, the reference numerals  72  and  74  denote two acceleration sensors. The acceleration sensor  72  is arranged on the movable part  50  of the probe head that is connected to the stylus  22 . The acceleration sensor  74  is seated on the stationary base  56  of the probe head  20 . By means of the two acceleration sensors  72 ,  74  it is possible to determine a differential acceleration of the stylus  22  relative to the probe head base  56 . Since this differential acceleration represents in the ideal case a signal that is in phase opposition to the oscillations of the stylus  22  about its rest position, the differential acceleration is suitable as a correction signal for suppressing these oscillations. However, some exemplary embodiments of the invention manage without such acceleration sensors  72 ,  74  and so the accelerating sensors  72 ,  74  are to be regarded here as optional. 
         [0056]      FIG. 3  shows one exemplary embodiment of a control device  80  by means of which a defined contact force of the stylus  22  is set. In exemplary embodiments, the defined contact force is set such that the contacting sphere  23  of the stylus is held continuously in contact with the contour  25  during movement of the probe head  20  along the contour  25 . 
         [0057]    The control device  80  receives as input variable a desired value  82  for the deflection of the stylus  22 . An actual deflection  84  of the stylus is subtracted from the desired deflection  82 . The difference yields the system deviation  86 . During scanning of a contour  25  on a workpiece  26 , the desired deflection  82  of the stylus  22  is advantageously set to zero. The actual deflection  84  can, for example, be determined by means of the position measuring device  68 . The system deviation  86  is amplified via a P element  88 . 
         [0058]    In an exemplary embodiment, the actual deflection  84  of the stylus  22  is, moreover, fed to a D element  92 , that is to say a differentiator. The output signal of the D element  92  is the deflection rate vACT of the stylus  22 . It is denoted here by the reference numeral  96 . The deflection rate vACT is amplified via a further P element  94  and subtracted from the amplified system deviation  86  at a summation point  98 . This branch of the control device  80  forms the behavior of a fluid damper, since the preliminary deflection of the stylus  22  is the more strongly damped the higher the deflection rate vACT. Practical tests by the applicant have, however, shown that such a simulation of a fluid damper does not yield an optimum result in all instances. Consequently, in exemplary embodiments the control device  80  has a further branch with a further D element  100 , an FIR filter  102  and a further P element  104  that are arranged in series with one another. On the input side, the further D element  100  receives the deflection rate  96  from the output of the D element  92 . The further D element  100  supplies the deflection acceleration  105  of the stylus  22 , and thus a signal that specifies a differential acceleration of the stylus  22  relative to the stationary probe head base  56 . Since the oscillations of the stylus  22  are typically sinusoidal, the deflection acceleration is likewise sinusoidal, but shifted in phase by 180°. In a ideal case, the subtraction of the deflection acceleration  105  leads to an optimum damping of the oscillations. 
         [0059]    However, there is the problem that existing interference signals (noise, external disturbances, inter alia) are disproportionately amplified by the twofold differentiation. In order to suppress these disturbances, use is made of the FIR filter  102 , which in the present exemplary embodiment has a largely rectangular weighting. In other words, the FIR filter  102  forms a sliding average from the current acceleration value and past acceleration values. The disturbances are reduced by the averaging. The filtered acceleration signal  107  is amplified by means of the further P element  104  and subtracted at the summation point  106  from the amplified system deviation  86 . This provides an actuating variable  108  by means of which the defined preliminary deflection of the stylus  22  is set. In exemplary embodiments, the actuating variable  108  is a control current by means of which the actuator  62  is actuated. 
         [0060]    The reference numeral  110  denotes a clock signal which indicates that the closed control loop  80  is traversed periodically. In other words, with each stroke of the clock signal  110  a desired value/actual value comparison is carried out in order to determine the system deviation  86 , and the manipulated variable for setting the contact force is determined by means of the elements  88  to  106 . 
         [0061]      FIG. 4  shows another exemplary embodiment for a control device that is used in the coordinate measuring machine  10  from  FIG. 1 . The basic design of the control device  120  corresponds to the control device  80  from  FIG. 3 . Identical reference symbols therefore denote identical elements in each case. 
         [0062]    By contrast with the control device  80  from  FIG. 3 , the control device  120  has, however, an FIR filter  122  that has a largely triangular weighting. Past acceleration values are weighted less in the FIR filter  122  the further back they lie in the past. In other words, acceleration values lying further in the past feature less strongly in the weighted filter sum. By contrast with the FIR filter  102  with a largely rectangular weighting, such an FIR filter has the advantage that the phase shift of the filtered acceleration signal  107  is even less conspicuous than the unfiltered acceleration signal  105 . 
         [0063]    Moreover, the control device  120  has an additional IIR filter  124  that is arranged between the FIR filter  122  and the further P element  104 . The IIR filter  124  is designed as a high-pass filter in order to suppress high-frequency disturbances even further. Such disturbances can be, in particular, the consequence of ground oscillations that are transmitted to the coordinate measuring machine  10 . Such ground oscillations can, for example, already occur (if only to a slight extent) when someone passes the coordinate measuring machine  10  during the scanning measurement. 
         [0064]      FIG. 5  shows another embodiment for a control device  130  that can be applied in the coordinate measuring machine  10 . In the case of the control device  130 , the acceleration of the stylus  22  relative to the probe head  20  is determined not by twofold differentiation, but by means of a state observer  132 . The state observer  132  is a model or a mathematical simulation of the probe head  20 . The state observer  132  is fed both the actuating variable  108  for setting the defined preliminary deflection, and the actual deflection  84 . The state observer  132  can model the system behavior of the probe head  20  by means of these input and output variables. The differential acceleration  134  can be determined in a known way from the modeled system behavior. The differential acceleration  134  is amplified again via a P element  104  and subtracted from the amplified system deviation  86 . 
         [0065]      FIG. 6  shows a measurement profile  140  that was recorded without the new method. The measurement profile  140  exhibits the deflections of the stylus  22  relative to the probe head  20  during scanning of a horizontal contour  25  on a measurement object  26 . The oscillations of the stylus  22  are clearly to be recognized. 
         [0066]      FIG. 7  shows a comparable measurement profile  142  that was, however, recorded by means of the new method. As may be seen, the oscillations of the stylus  22  are clearly reduced. (For the sake of completeness, it may be noted that here the measurement profile  142  seems to include a linearly rising component. However, this component is not actually present. The linear rise is to be ascribed to the possibilities of representation in the measurement setup used.)