Abstract:
A machine part is guided along a defined movement path over a workpiece surface. The machine part is held at a defined distance from the workpiece surface during this movement. For that purpose, at least one distance sensor is provided that runs ahead of the machine part with a defined lead. A plurality of distance values indicative of a distance between the distance sensor and the workpiece surface are determined along the movement path. A plurality of control values are determined as a function of the distance values. The defined distance is repeatedly adjusted by means of the control values. In accordance with a first aspect, the distance values are determined at measurement points distributed with a first grid spacing along the movement path, while the control values are determined for actuating points distributed with a second grid spacing along the movement path, the first and the second grid spacings being different. According to a second aspect, the machine part has a linear range of activity on the workpiece surface, and the distance between the machine part and the workpiece surface is controlled by means of a distance control value and an angle control value, which are derived from distance values acquired from at least two distance sensors, which are laterally offset from one another.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of pending U.S. Ser. No. 11/990,299 filed on Feb. 8, 2008, which is a Sec. 371 national stage of international patent application PCT/EP2006/007130 filed on Jul. 20, 2006 designating the U.S., which international patent application has been published in German language and claims priority from German patent application DE 10 2005 039 094.3, filed on Aug. 8, 2005. The entire contents of these priority applications are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to methods and arrangements for guiding a machine part over a workpiece surface along a defined movement path, with the machine part being held at a defined distance from the workpiece surface along the movement path 
         [0003]    DE 33 41 964 A1 discloses a machine having a welding head for welding two plates to one another along an abutting edge. A distance sensor runs ahead of the welding head with a constant lead. The distance sensor serves for determining the course of the abutting edge and the height of the welding head above the surface of the two plates such that the welding head can be guided exactly over the course of the abutting edge. A control circuit for the welding head includes what is called a delay and correction stage, which is fed by output signals of the distance sensor running ahead. The distance sensor is controlled via actuators to the desired height position and lateral position relative to the abutting edge. The delay and correction stage copies the corresponding control signals to the actuators for the welding head with a time delay corresponding to the lead. The purpose of the time delay is to ensure that the welding head assumes at every instant exactly that position which the distance sensor had assumed earlier by the delay time. Since the distance sensor maintains a desired position above the abutting edge owing to the self regulation, the welding torch follows the desired path. 
         [0004]    The known approach has the disadvantage that both the distance sensor and the welding head require drive elements, since the distance sensor is controlled independently of the movement of the welding head. The high number of actuators renders this approach expensive. Moreover, the accuracy with which the welding head follows the distance sensor is limited by the tolerances of the individual actuators. The welding head can follow the self-regulation of the distance sensor only to the extent that the actuators of the welding torch correspond to the actuators of the distance sensor. The known approach is particularly complicated and disadvantageous when, instead of guiding a welding head with a largely punctiform effective range, the aim is to guide on the workpiece surface a machine part that has a linear range of activity on the workpiece surface. 
         [0005]    DE 196 15 069 A1 likewise discloses an arrangement and a method for guiding a tool at a defined distance above a workpiece surface. In an exemplary embodiment, two plates of different size lying on one another are to be welded along the terminating edge of the smaller plate. In this case, the welding head follows a sensing element which acquires the course of the edge in a tactile manner. A control arrangement ensures that the welding head follows the course of the edge, wherein the height position of the welding head above the workpiece surface is also tracked. In contrast to the arrangement of DE 33 41 964 A1, the welding head is here rigidly coupled to the distance sensor. Accordingly, fewer actuators are required. However, the known solution requires an accurately preprogrammed movement path, since the sensing element acquires only a deviation from such a preprogrammed movement path. Moreover, the focus control is exact only for the sensing element, but not for the welding head running behind. 
         [0006]    There are a plurality of other proposals for guiding a machine part at a defined distance above a workpiece surface. According to DE 299 04 097 U1, for instance, a number of running wheels are arranged on the machine part (a laser processing head). The running wheels should be positioned as near as possible to the weld seam of the workpiece to be processed, but this is problematic in the case of welding operations and/or in the case of sensitive surfaces. 
         [0007]    DE 32 43 341 A1 proposes to take a photograph with a camera of a slot pattern projected onto the workpiece surface. EP 0 554 523 B1 (=DE 692 19 101 T2) proposes to evaluate the color spectrum in the region of a weld seam, with the welding head likewise being guided on the workpiece surface via rollers. DE 195 16 376 A1 proposes to evaluate the intensity of a laser induced plasma by means of a detector that looks obliquely on to the course of a laser weld seam. All these proposals require complicated signal processing to determine distance. 
         [0008]    Other proposals use a capacitive sensor which should be seated as close as possible on or at the guided machine part (EP 0 743 130 B1, DE 197 27 094 C2, DE 91 17 180 U1, DD 286 887 A5). These proposals attempt to avoid a lead of the distance sensor in front of the guided machine part, or they neglect such a lead. 
         [0009]    DE 37 30 709 A1 proposes to guide a distance sensor over a workpiece surface to be processed in a first operating mode, and to undertake the actual processing operation later in a second operating mode, wherein the measured values from the first pass are used during the second pass for distance control. This approach is time consuming, because the machine part must be guided at least twice over the workpiece surface. 
         [0010]    In addition, it is common to all known approaches that the range of activity of the controlled machine part on the workpiece surface is substantially punctiform. No focus control is provided for a linear range of activity. 
       SUMMARY OF THE INVENTION 
       [0011]    In view of the above, it is an object of the present invention to provide a method and an arrangement that enable simple and cost-effective focus control on a workpiece surface. Preferably, the new method and arrangement should allow a simple and cost-effective focus control in the case of machine parts having a linear range of activity. 
         [0012]    According to a first aspect, there is provided a method for guiding a machine part over a workpiece surface along a defined movement path, the movement path defining a direction of movement, and the machine part being held at a defined distance from the workpiece surface during movement along the movement path, the method comprising the steps of: providing at least two distance sensors each running ahead of the machine part with a defined lead, the at least two distance sensors being offset from one another in a direction transverse to the direction of movement, determining a plurality of distance values along the movement path by means of the distance sensors, with each distance value being indicative of a distance between one of the distance sensors and the workpiece surface, determining a plurality of control values for adjusting the defined distance as a function of the distance values, and moving the machine part along the movement path, while repeatedly adjusting the defined distance using the control values, wherein the machine part has a linear range of activity on the workpiece surface, which range of activity extends transverse to the direction of movement, and wherein the plurality of control values comprise a distance control value and an angle control value. 
         [0013]    According to another aspect, there is provided a method for guiding a machine part along a defined movement path over a workpiece surface, the machine part being held at a defined distance from the workpiece surface along the movement path, the method comprising the steps of: providing a distance sensor that runs ahead of the machine part with a defined lead, determining a plurality of distance values indicative of a distance between the distance sensor and the workpiece surface along the movement path, determining a plurality of control values for adjusting the defined distance as a function of the distance values, and moving the machine part along the movement path and repeatedly adjusting the defined distance using the control values, wherein the distance values are determined at a plurality of measurement positions which are distributed along the movement path with a first grid spacing, wherein the control values are associated with a plurality of actuating positions that are distributed along the movement path with a second grid spacing, and wherein the first and the second grid spacing are different. 
         [0014]    According to yet another aspect, there is provided an arrangement for guiding a machine part over a workpiece surface along a defined movement path, the movement path defining a direction of movement, wherein the machine part is configured to be held at a defined distance from the workpiece surface during movement along the movement path, the arrangement comprising: at least two distance sensors each configured to run ahead of machine part with a defined lead, the at least two distance sensors being offset from one another in a direction transverse to the direction of movement, and each distance sensor being designed for determining a plurality of distance values indicative of a distance between the distance sensor and the workpiece surface along the movement path, a first drive unit for moving the machine part along the movement path, a second drive unit for repeatedly adjusting the defined distance, and a control unit designed for controlling the first and second drive units by determining a plurality of control values as a function of the distance values, wherein the machine part has a linear range of activity on the workpiece surface, which linear range of activity extends transverse to the movement path, and wherein the control unit is designed for determining a distance control value and an angle control value in order to guide the linear range of activity parallel to the workpiece surface. 
         [0015]    The novel methods and arrangement thus use at least one distance sensor running ahead of the machine part. Consequently, the methods and arrangement are independent of the technology of the distance sensor used. In principle, it is possible to use any sensor that is capable of supplying a signal by means of which the distance between the machine part and the workpiece surface can be determined. Because of the great variety in the selection of a suitable distance sensor, the novel methods and arrangement can be implemented very cost-effectively. Because of the lead, the distance sensors can further be very well protected against interference and damage by the machine part running behind. Since the distance sensor requires no “visual contact” with the processing site on the workpiece surface, shielding plates can be used for decoupling, for instance. 
         [0016]    The methods and arrangement enable the at least one distance sensor and the machine part to be rigidly connected to one another. Consequently, the number of the required drive elements can be reduced compared to the solution from DE 33 41 964 A1. Moreover, tracking errors caused by tolerance deviations in separate drive elements are avoided. The methods and arrangement therefore enable cost-effective guidance of the machine part with high accuracy. On the other hand, parallax errors owing to tracking of the machine part can be effectively corrected or avoided. 
         [0017]    Embodiments of the methods and the arrangement have the advantage that the steps of recording of distance values (determination of the actual state) and adjusting or setting the desired distance are decoupled as a result of different grid spacing. It is then easily possible to measure and to average a plurality of distance values for determining a control value for one actuating position. This enables a very smooth and accurate control response since short fluctuations are ignored. Conversely, very high movement speeds can be achieved in the case of a flat workpiece surface, because the process of adjusting the defined distance is not “unnecessarily” held up by numerous distance measurements in this case. 
         [0018]    Finally, the recording of distance values and the setting of the defined distance by means of mutually independent grid spacings enable a very simple implementation when a linear or even two-dimensional range of activity is to be optimally set on the workpiece surface, as is illustrated below by means of preferred exemplary embodiments. 
         [0019]    In a preferred refinement, the first grid spacing is smaller than the second grid spacing. 
         [0020]    In this refinement, the distance values are determined with a higher frequency or density than the control values for setting the defined distance. This enables the obtained distance values to be selected, checked for plausibility and preferably averaged. This renders the control response smoother. Moreover, the novel method and the novel arrangement of this refinement are less sensitive to stochastic interference that influences the measurement of the distance values. Consequently, it is possible to achieve a particularly high accuracy of the focus control with this refinement. 
         [0021]    In another refinement, the first grid spacing is greater than the second grid spacing. 
         [0022]    This refinement permits very high feed rates, and it is particularly preferred when the workpiece surface is very flat. Since use is made of more control values in this refinement than measured distance values are available (the density of the control values is higher than the density of the distance values), it is preferred to determine control values without an “assigned” distance value as a function of interpolated distance values. Because of the distance sensor running ahead, it is possible to interpolate by using “future” distance values in this case, that is to say by using distance values of a measurement position that the machine part has not yet reached. Consequently, this refinement enables the defined distance to be accurately observed despite the reduced measurement outlay. 
         [0023]    In a further refinement, each distance value is assigned to or associated with that actuating position which lies nearest the measurement position of the distance value. 
         [0024]    As an alternative, “redundant” or unnecessary distance values could be discarded or serve merely for plausibility checks. However, a more uniform and more accurate control response is achieved if each distance value is assigned to an actuating position and features in the determination of the control value. 
         [0025]    In a further refinement, a number of distance values are determined for each actuating position. 
         [0026]    This refinement likewise contributes to a more uniform and more accurate control response since each control value is a function of a number of measured distance values here. Erroneous measurements and/or interference in the measurement sequence are more effectively suppressed. 
         [0027]    In a further refinement, a number of distance values are averaged for one actuating position in order to determine the control value for said one actuating position. 
         [0028]    As already explained further above, this refinement is a simple and effective possibility of achieving a smooth and accurate control response. 
         [0029]    In a further refinement, the control values are provided in a rolling memory. The memory positions in the rolling memory preferably correspond to the actuating positions in the second grid spacing, that is to say a memory entry is provided for each actuating position. 
         [0030]    The use of a rolling memory is a very simple and cost-effective possibility of managing the actuating values from the lead of the at least one distance sensor. In particular, this refinement permits the use of a very small memory with a number of memory positions that is equal to or only slightly greater than the number of the control values that must be buffered on the basis of the lead of the at least one distance sensor. 
         [0031]    In a further refinement, the control values for setting the distance are fed to a controller that has a progressive controller gain. 
         [0032]    In this refinement, the controller has a nonlinear controller gain that rises disproportionately in the case of high system deviations. It is preferable for the controller not to react at all in the event of small system deviations, that is to say the controller gain vanishes below a defined threshold value. 
         [0033]    The control operation can be accelerated by means of this refinement, that is to say the defined distance is set more quickly to the desired range in the event of relatively high system deviations. On the other hand, the introduction of “fuzziness” in the event of slight system deviations leads to a smoother response. This enables a higher processing quality. 
         [0034]    In a further refinement, the control values are provided in a memory, and at least two control values of different actuating positions are combined by means of an FIR filter in order to determine a filtered control value. It is particularly preferred when the combination by means of the FIR filter is not performed until the machine part is adjusted, or in other words, upon or after the control values are read out of the memory. Furthermore, it is preferred when at least one of the control values used is a “future” control value, that is to say a control value relating to an actuating position that the machine part running behind has not yet reached. 
         [0035]    This refinement enables a particularly smooth and accurate control response. It utilizes an advantage enabled by the distance sensor running ahead, because “future” distance values can be incorporated in the filtering. It is thereby possible to implement a filter that is true to phase in online operation. It is particularly preferred to undertake the combination of the at least two control values when the control values are read out of the memory, because then a maximum number of “future” distance values can be considered. 
         [0036]    In a further refinement, the machine part has a linear range of activity on the workpiece surface, which range of activity runs transverse to the movement path. 
         [0037]    This refinement is directed to a preferred application of the present invention, where a workpiece surface is scanned with a linear band of light and/or heated. Such an application raises the challenge of keeping not only a point on the workpiece surface in focus but an extended geometric Figure. In order to achieve an optimum focus control here, it is necessary to keep the distances along the linear range of activity in the focus of the machine part, which is not possible with the known approaches or only with s great outlay. The present invention enables a simple focus control for the linear range of activity, as is illustrated below with respect to a preferred exemplary embodiment. 
         [0038]    In a further refinement, at least two distance sensors are provided that each run ahead of the linear range of activity with a defined lead. 
         [0039]    This refinement is a particularly simple and cost-effective possibility of keeping the linear range of activity in focus. In particular, it enables the use of simple distance sensors that measure in punctiform fashion. 
         [0040]    In a further refinement, which also forms an invention per se, a distance control value and an angle control value are determined and provided by means of the at least two distance sensors in order to guide the linear range of activity parallel to the workpiece surface. 
         [0041]    Alternatively, a number of distance control values could be used to this end. By contrast, the preferred refinement enables a very simple and cost-effective setting of a defined distance along a linear effective range. 
         [0042]    In a further refinement, at least three distance sensors are provided that each run ahead of the linear range of activity with a defined lead, with each distance sensor supplying a distance value, and wherein the distance control value and the angle control value are determined as a function of the at least three distance values. 
         [0043]    This refinement enables a very uniform and accurate setting of the defined distance over the entire course of the linear range of activity. In addition, it can be implemented very cost-effectively, as is demonstrated below in connection with a preferred exemplary embodiment. 
         [0044]    It goes without saying that the features mentioned above and those still to be explained below can be used not only in the respectively specified combination, but also in other combinations or on their own without departing from the scope of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0045]    Exemplary embodiments of the invention are illustrated in the drawing and explained in more detail in the following description. In the drawing: 
           [0046]      FIG. 1  shows a simplified schematic of an exemplary embodiment of a novel arrangement, 
           [0047]      FIGS. 2-4  show the arrangement from  FIG. 1  in three different operating positions, 
           [0048]      FIG. 5  shows a simplified flowchart illustrating the recordation of distance values in accordance with an exemplary embodiment, 
           [0049]      FIG. 6  shows a simplified flowchart for further illustration of an exemplary embodiment of the invention, 
           [0050]      FIG. 7  shows a schematic of an embodiment where the machine part has a linear range of activity on the workpiece surface, and 
           [0051]      FIG. 8  shows a graph illustrating a preferred exemplary embodiment for an arrangement in accordance with  FIG. 7 . 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0052]    An exemplary embodiment of a novel arrangement is denoted in its entirety by reference numeral  10  in  FIG. 1 . The arrangement  10  includes a machine part  12  and at least one distance sensor  14  which are arranged here jointly on a support  16 . The distance sensor  14  is fastened on the support  16 , with a lateral offset  18  from the machine part  12 . The offset  18  is the lead by which the distance sensor  14  runs ahead of the machine part  12  when the support  16  is moved relative to a workpiece. 
         [0053]    The reference numeral  20  denotes a table on which a workpiece  22  is arranged. The workpiece  22  can be, for example, a multilayer element whose surface is to be heated in a specific way in order to interconnect the near-surface layers. Such an application arises, in particular when producing liquid crystal displays (LCDs). In this preferred case, the machine part  12  is a laser that must be guided at an optimum focal distance from the workpiece surface  23  of the workpiece  22 . 
         [0054]    The height of the table  20  can be adjusted in this exemplary embodiment as is indicated by a hydraulic cylinder  24  and an arrow  26 . Alternatively, or as a supplement hereto, the height of the support  16  could also be adjustable. Moreover, in this exemplary embodiment the table  20  can be moved in the direction of the arrow  28 , thus producing a relative movement of the machine part  12  over the workpiece surface  23  in an opposite direction. The table  20  is therefore provided with a drive  30 , which is illustrated here only schematically. Alternatively, or as a supplement hereto, it could also be possible to move the support  16  parallel to the arrow  28 . The arrow  28  therefore specifies a general movement axis of the arrangement  10 . This movement axis is also denoted below as Y axis. 
         [0055]    The reference numeral  32  denotes a control unit that controls the movement of the table  20 . The control unit  32  includes a memory  34  that is designed in this exemplary embodiment as a rolling memory. The memory  34  has a number of memory locations that are written to and read from cyclically in sequence. The oldest entry in the memory locations is respectively overwritten by the newest entry. The number of memory positions corresponds to the lead  18  between the distance sensor  14  and the machine part  12 . It is at least so large that a distance value read in by the distance sensor  14  at a position Y=Y 0  (or a control value based thereon) is still present in the memory  34  when the machine part  12  reaches the position Y 0 . 
         [0056]    The control unit  32  has an input circuit  36 . The input circuit  36  serves to record the distance values or distance signals of the distance sensor  14 . Moreover, the input circuit  36  is fed by the output signal of a sensor  38  by means of which the height of the table  20  can be determined in the direction of the arrow  26  (Z axis). The input circuit  36  is designed for conditioning the received distance and height values such that they can be stored at a memory position of the memory  34 . It goes without saying that this memory position can comprise a number of bytes in order to record the data. The number of the memory positions preferably corresponds in the rolling memory  34  to the number of Y-positions that can be resolved along the movement axis  28  over the lead  18 . 
         [0057]    On the output side, the control unit  32  has a controller  40  that serves for setting the height and the feed movement of the table  20 . In a preferred exemplary embodiment, the controller  40  has a nonlinear controller gain, which is illustrated by the characteristic curve in  FIG. 1 . It is preferably a PID controller that is used, but it may also be a PI, a PD or a P controller. Moreover, it is particularly preferred when the controller  40  does not react in the event of very small system deviations. In other words, the controller  40  does not begin to correct the system deviation until there is a system deviation lying above a defined threshold value. 
         [0058]    A scale  42  is illustrated below the arrangement  10 . The scale  42  has a relatively coarse grid  44  and a finer grid  46 . The relatively coarse grid  44  here specifies the Y-positions, which can be resolved in the movement direction  28  of the table  20 . In the preferred exemplary embodiment, a control value is determined for each Y-position  48 , the height of the table  20  and thus the distance  50  between the machine part  12  and the workpiece  23  is adjusted by means of said control value. 
         [0059]    The grid  46  has grid spacings that are smaller than the grid spacings of the grid  44 . Each grid point  52  of the grid  46  denotes a measurement position at which the distance sensor  14  measures the distance from the workpiece surface  23 . These measured values are transmitted as distance values to the control unit  32 , and they are not always identical to the distance  50  between the machine part  12  and the workpiece surface  23 , as follows from the illustration in  FIG. 1 . 
         [0060]    The relatively high grid density of the first grid  46  can also be a consequence of the fact that the distance sensor  14  determines the distance from the workpiece surface  23  continuously, wherein the continuous distance values are then preferably converted by an A/D converter, in order to obtain digital distance values. 
         [0061]    The grid points of the first grid  46  and of the second grid  44  coincide at the Y-positions of the second grid  44  which are illustrated with reference numeral  48 . The Y-positions (grid points)  48  of the second grid are read in here, for example, by means of a glass scale in a way as is known per se from machine tools and coordinate measuring machines. The resolution of the glass scale determines the grid spacings  44  of the second grid. 
         [0062]      FIGS. 2 to 4  show the arrangement  10  in three operating positions, with identical reference symbols denoting the same elements as before. 
         [0063]    It may be assumed that the table  20  in  FIG. 2  is located at the position Y Y 0 , and that the lead between the distance sensor and the machine part is 50 mm. The height of the table  20  may be, for example, 5 μm with reference to a table zero point (not illustrated here). The distance sensor  14  measures, for example, a distance value of −3 μm relative to the workpiece surface  23 . The value of −3 μm is referred to a zero point (not illustrated here). The zero points for the table  20  and the distance sensor  14  are selected such that the workpiece surface  23  is located at the focus of the machine part  12  when both values equal zero. 
         [0064]    It may be assumed in  FIG. 3  that the table  20  is located at a Y-position of Y=25 mm. In other words, table  20  has moved to the right by 25 mm. The distance sensor  14  supplies, for example, a distance value of 2 μm, while the height of the table  20  may be 7 μm here. 
         [0065]    It may be assumed in the operating position in accordance with  FIG. 4  that the table  20  is at y=50 mm. The height of the table  20  is 6 μm, while the measured distance value of sensor  14  may (accidently) be 2 μm. All specified values are summarized again in the following table: 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
               
               
                   
                 Table 
                 Distance 
                   
                   
               
               
                 Y-position 
                 height T 
                 value S 
                 ΔTS = T − S 
                 CV = ΔTS(i) − T(i + V) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0 
                 5 μm 
                 −3 μm   
                 8 μm 
                 ? 
               
               
                 25 
                 7 μm 
                 2 μm 
                 5 μm 
                 ? 
               
               
                 50 
                 6 μm 
                 2 μm 
                 4 μm 
                 8 μm − 6 μm = −2 μm 
               
               
                   
               
             
          
         
       
     
         [0066]    The rows of the table correspond to the memory positions in the rolling memory  34 . Each Y-position is assigned a memory position=table row. Stored in each memory position are the table height T(y) and the distance values S(m). In this exemplary embodiment of the invention, the control operation cannot begin until the table  20  has reached the Y-position Y=50 mm. Available at this instant are both the current table height T (50 mm)=6 μm, and the information as to which table height T (0)=5 μm and which distance value S (0)=−3 μm were present when the distance sensor  14  had been located at the Y-position y=0. In other words, the machine part  12  must initially be moved by the lead  18  in relation to the workpiece surface  23  so that the control process can start. 
         [0067]    In accordance with the fifth column, it is now possible to determine the instantaneous system deviation CV from the difference between the two table heights at the Y-positions y=0 and y=50 and the distance value S(0) at the Y-position y=0. In the exemplary embodiment illustrated, the result is a system deviation of −2 μm with respect to the reference zero point. This system deviation is fed to the controller  40  in order to correct for the system deviation. In other words, the controller  40  controls the table height such that the system deviation of −2 μm vanishes. This operation is repeated cyclically for each further Y-position. 
         [0068]      FIG. 5  shows a preferred exemplary embodiment for reading the table heights and distance values into the memory  34 . 
         [0069]    In accordance with step  60 , the height T(i) of the table  20  at the Y-position y=i is read in first. A counter that corresponds to the grid spacings  46  is set to zero in step  62 . The counter m=m+1 is incremented in step  64 . In accordance with step  66 , the distance value S(m) is then read in. The difference ΔTS(i) between the table height T(i) read in and the distance value S(m) is determined in accordance with step  68 . This difference is stored in memory  34  in accordance with the table illustrated above. Furthermore the table height T(i) is stored in relation to the difference value. A determination of an angle can be performed in accordance with step  70 , as is explained in more detail below. An inquiry as to whether the next Y-position has already been reached is performed in accordance with step  72 . If this is not the case, then method returns to step  64  in accordance with step  74 . A further distance value is read in for the grid position (measurement position) m=m+1. Since the Y-position y=i is the same (or at least the measurement resolution indicates no change), the distance values S(m) and S(m+1) are averaged and subtracted in step  68  from the table height T(i). This produces a smoothing of the distance values that leads to a smoother and more accurate control response. 
         [0070]    Only when the interrogation  72  indicates that the next Y-position y=i+1 has been reached, the counting variable m is set to zero again. The distance values that are assigned to the Y-position y=i+1 are now read in, averaged and stored. 
         [0071]    With this method, the distance values at the measurement positions m (recorded in the grid  46 ) each are assigned to that Y-position (=actuating position) to which they lie closest. This is symbolically indicated in  FIG. 2  by reference numeral  77 . 
         [0072]    It is assumed in the exemplary embodiments thus far that the grid spacings  46  which specify the measurement positions of the distance sensor  14  are smaller than the grid spacings  44  which specify the Y-positions of the table  20 . The opposite case is also possible. It can occur here that a new Y-position is read in but no new distance value is available. In contrast from the previous explanation, no distance value is then read in step  66 , but a distance value is formed by extrapolation—or in the case of a later post-processing—by interpolation. In this case, as well, at least one distance value is thus assigned to each Y-position. 
         [0073]      FIG. 6  illustrates the control operation for setting the table height by means of a simplified flowchart. Here, as well, a counting variable that specifies the Y-position of the table  20  is first set at zero in step  80 . The counting variable i is incremented in step  82 . The actual table height T(i) is read in step  84 . In the table given above, this table height was, for example, 6 μm (see lowermost table row). 
         [0074]    The difference ΔTS(i−V) between the table height and distance value at the Y-position y=i−V is retrieved from the memory  34  in step  86 . Subsequently, the system deviation CV is determined in step  88  from the difference between the values read in: 
         [0000]        CV=ΔTS ( i−V )− T ( i ). 
         [0075]    The system deviation CV is fed in step  90  to the controller  40 , which adjusts the table height correspondingly. Subsequently, a further program run is performed for the next actuation position i=i+1 in accordance with step  90 . 
         [0076]    The flowchart in  FIG. 6  shows a modification of this preferred method sequence. Here, not only the difference ΔTS(i−V) is retrieved from the memory  34 . Rather, the corresponding values ΔTS(i±1−V), ΔTS(i±2−V) of the neighboring Y-positions are also read out from the memory. Subsequently, all values are combined with one another in a FIR filtering (Finite Impulse Response filtering) in order to obtain a filtered value ΔTS filt (i−V). The filtered value is then used in step  88  in order to determine the system deviation CV. The FIR filtering leads to a smoother control response. Since it is also possible to incorporate “future” Y-positions in the filtering as a result of the distance sensor  14  running ahead, a FIR filter that is true to phase and enables a particularly high control accuracy is obtained. 
         [0077]      FIG. 7  shows a schematic plan view of the workpiece surface  23  in a preferred exemplary embodiment. In this exemplary embodiment, the machine part  12  is a laser that generates a laser line  98  on the workpiece surface  23 , which laser line is intended to be kept in focus over the entire length L by means of the novel method. A preferred exemplary embodiment is the heating of a workpiece surface that passes through below the laser line  98  in the direction of the Y-axis. The laser line  98  runs transverse to the movement direction of the workpiece surface  23 . In the exemplary embodiment illustrated in  FIG. 7 , the laser line  98  is aligned in a fashion orthogonal to the Y-axis. 
         [0078]    In the preferred exemplary embodiment, three distance sensors  14   a ,  14   b ,  14   c  run ahead of the laser line  98 . The distance sensors  14   a ,  14   b ,  14   c  are arranged next to one another and have the same lead  18  relative to the machine part  12  or the laser line  98 . By means of this arrangement, it is possible to determine a rolling movement  100  of the workpiece surface  23  above the Y axis. In this case, the arrangement  10  is preferably designed such that the table  20  can be pivoted about the Y axis such that the laser line  98  can be focused on to the workpiece surface  23  over the entire length. 
         [0079]    In a particularly preferred exemplary embodiment, the workpiece surface  23  is adjusted around the Y axis by using the distance values from at least two distance sensors  14   a ,  14   b ,  14   c  to determine a distance control value and an angle control value. This is shown in step  70  in the flowchart of  FIG. 5 . Indices “1” and “2” denote the at least two measured distance values of the at least two distance sensors  14   a ,  14   b ,  14   c.    
         [0080]    In a further preferred exemplary embodiment, it is contemplated that an angle offset value and a distance offset value can be entered into the control unit  32 . The controller  40  considers the offset values during setting of the table position. By inputting suitable offset values, it is possible to specifically remove the laser line  98  from the focus in order, for example, to carry out test series. Inputting an angle and distance offset values of zero results in keeping the laser line  98  in focus over the entire length. 
         [0081]    It would be sufficient to have two distance values from two distance sensors  14   a ,  14   c  for the focus control of the laser line  98 . The use of three or more distance sensors  14   a ,  14   b ,  14   c  leads to a higher number of distance values than required for determining the two control variables of distance and angle. 
         [0082]    In other words, the system of distance and angle control is overdefined with three and more distance sensors. The overdefinition can, however, be advantageously used when a mean straight line is determined that is then used to determine the system deviations. Such a mean straight line is illustrated in  FIG. 8  by reference numeral  102 . In this case, the straight line  102  is a mean straight line in accordance with the method of least squares between the distance values of the distance sensors  14   a ,  14   b ,  14   c . The offset  104  of the straight line  102  (the point of intersection of the straight line  102  with the Z axis) can advantageously be used as system deviation for the distance control. The gradient of the straight line, that is to say the angle  106 , then serves as a system deviation for adjusting the table inclination around the Y axis. 
         [0083]    It is contemplated in further exemplary embodiments (not illustrated here) that the controller  40  is limited to the maximum permissible dynamics (maximum acceleration and maximum speed) of the arrangement  10 . Damage to the arrangement  10  is thereby avoided in the case of large system deviations.