Abstract:
Devices and related methods of controlling relative movement between a tool of a machine tool and a work piece including dividing a total desired relative tool to workpiece movement in a direction of an axis into a first movement component and a second movement component; controlling the first movement component with a first closed-loop axis control; determining a resulting movement quantity corresponding to the first movement component; feeding a position error from the first closed-loop axis control to the second closed-loop axis control controlling the second movement component in a closed-loop with the second closed-loop axis control, including adjusting for the position error of the first movement component.

Description:
REFERENCE TO RELATED APPLICATIONS 
   This application claims priority under 35 USC §119(a) from European patent application EP 05 002 414.0, filed Feb. 4, 2005. The complete disclosure of this priority application is incorporated herein by reference. 
   TECHNICAL FIELD 
   This disclosure relates to dividing a relative movement between a workpiece and a machine tool in a direction of an axis into at least a first movement component and a second movement component, in which a desired quantity for the total movement is specified. 
   BACKGROUND 
   Methods of moving a workpiece relative to a machine tool, and machine tools which are capable of movement relative to a workpiece are know in the art, for example, from EP 0 594 699 B1. 
   When workpieces are being machined on machine tools, there is the possibility, on the one hand, of moving the workpiece relative to a tool with a suitable drive in order to carry out machining of the workpiece. Alternatively, it is possible to move the tool relative to the workpiece. In some cases, moving the tool relative to the workpiece requires the movement of smaller masses than in the case of moving the workpiece. However, movement of the tool may be subject to limits if, for example, it is attached to a gantry which is moved over the workpiece. Such a gantry can be of a relatively great weight, and therefore the dynamics cannot be too high if there is to be sufficient accuracy. One solution which suggests itself, therefore, is to provide an additional, higher-dynamics drive which acts in the direction of a workpiece drive or a tool drive and which may only have to move very low weights, thereby enabling higher dynamics and greater precision to be achieved. For example, it is possible to provide on a gantry that is displaceable in a Y-direction by one drive a tool holder that is displaceable along the gantry by a relatively shorter distance in the Y-direction by a second drive. A tool held at the tool holder is thereby displaceable in the Y-direction by means of two drives. The first drive, driving the gantry, is frequently referred to as the machine axis and the second, higher-dynamics drive (which drives the tool holder), as the auxiliary axis. A drive that moves the workpiece in an axis direction is frequently referred to as the workpiece axis. The term “axis” is frequently used, therefore, to refer to the drive acting in the axis direction. The auxiliary axis may be a machine axis or a workpiece axis. The open-loop or closed-loop control of two drives acting in the same axis direction may, however, prove difficult. 
   EP 1 366 846 A1 proposes optimising machining of a workpiece with the aid of a computer program by determining, by reference to the contours to be machined, in the case of a laser cutting machine the contours that are to be cut, whether the gantry or only the tool holder is to be moved. In particular, the gantry can be positioned such that a maximum number of contours may be cut without it being necessary to move the gantry. Therefore, the cutting operation can be carried out solely by the auxiliary axis. 
   It is known from EP 0 594 699 B1 to supply a device with a control signal by a CNC control. The control signal indicates the required total movement to be executed. The device splits the signal into movement components of the tool support (lower-dynamics axis) and of an additional device (higher-dynamics axis). Apportionment to the two movement axes is performed by an electronic filter, the control signal components for the tool support being produced by a low-pass filter and the control signal components for the additional device being produced by a high-pass filter. Once the control signal has been apportioned, therefore, independent control of the drive of the tool support and the drive of the additional device takes place. The CNC control is supplied with feedback signals describing the total movement in an axis direction. 
   SUMMARY 
   According to one aspect, a method of controlling relative movement between a tool of a machine tool and a workpiece includes dividing a total desired relative tool to workpiece movement in a direction of an axis into a first movement component and a second movement component; controlling the first movement component with a first closed-loop axis control; determining a resulting movement quantity corresponding to the first movement component; feeding a position error from the first closed-loop axis control to a second closed-loop axis control, the position error including the difference between the total desired relative tool to workpiece movement and the resulting movement quantity corresponding to the first movement component; and controlling the second movement component in a closed-loop with the second closed-loop axis control, including adjusting for the position error of the first movement component. Therefore, this method allows for closed-loop control of the first and second movement components. The closed-loop control of the first and second movement components are not completely independent of each other. Rather, a value (e.g., the position error) from the first closed-loop axis control is fed to the second closed-loop axis control and is taken into consideration therein for the closed-loop control of the second movement component. The second closed-loop axis control is, as it were, downstream of the first closed-loop axis control. That measure makes it possible to achieve higher dynamics and greater accuracy. The total desired relative tool to workpiece movement can represent a target position, a speed or an acceleration. The total movement can be divided into one or more tool movements and one or more workpiece movements in the same axis direction. It is also possible for one or more machine axes and one auxiliary (machine) axis or one or more workpiece axes and one auxiliary (workpiece) axis to be provided. 
   In some embodiments, the first closed loop axis control is performed by a first position controller configured to control the first movement component, and the second closed-loop axis control includes a second position controller configured to control the second movement component. In this case, an input quantity for the first position controller can be determined from the total desired relative tool to workpiece movement. 
   In some cases, a resulting movement quantity corresponding to the second movement component is determined, and a second input quantity for the second position controller is determined from the resulting movement quantity corresponding to the second movement component and a quantity that is proportional to the position error. 
   In some embodiments, the first closed-loop axis control is configured to control a first axis drive, and the second closed-loop axis control is configured to control a second axis drive. In this case, controlling the first movement component can include controlling a movement of the first axis drive in the axis direction, and controlling the second movement component can include controlling a movement of the second axis drive in said axis direction. 
   In some cases, the input quantity for the first position controller of the first closed-loop axis control is the position error in the closed-loop control of the first axis drive. The first axis drive can be, for example, a lower-dynamics axis. The position error can be applied as a position desired value to the second closed-loop axis control. In the second controlled system, the second axis drive (e.g., a higher-dynamics axis) can be adjusted. Thus, the second axis drive (higher-dynamics axis) can adjust the position error of the first axis drive (lower-dynamics axis) accordingly. This can aid in reducing the deviation of the mechanically in-series axes. 
   Only one desired value for the desired total movement needs to be specified in each case. The division of the movement into the first and second movement component can take place solely in the first and second closed-loop axis controls. 
   Some implementations may include one or more of the following features. The first and second axis drives, which implement the first and second movement components, can be disposed mechanically in series. A resulting movement quantity corresponding to the total relative movement through the first and second movement components can include the sum of resulting movement quantities of the first and second movement components. 
   In another aspect, a method of controlling movement between a tool of a machine tool and a workpiece includes: dividing a total desired relative tool to workpiece movement in a direction of an axis into a first movement component and a second movement component; controlling the first movement component with a first closed-loop axis control; and controlling the second movement component with the second closed-loop axis control, including performing a follow-up control of the second movement component. 
   In some embodiments, the first closed-loop axis control is performed by a position controller configured to control the first movement component, an the second closed-loop axis control is performed by a follow-up controller configured to control the second movement component. In this case, an input quantity for the position controller can be determined from the total desired relative tool to workpiece movement. 
   In some instances, a resulting movement quantity corresponding to the total relative movement through the first and second movement components of a first movement cycle can be determined. In this case, the input quantity for the position controller can include the difference between the total desired relative tool to workpiece movement and the resulting movement quantity corresponding to the total relative movement through the first and second movement components. 
   In some embodiments, the first closed-loop axis control is configured to control a first axis drive, and the second closed-loop axis control is configured to control a second axis drive. 
   In some cases, the first axis drive is configured for higher frequency response than the second axis drive; i.e., the first axis drive is a higher dynamics drive. For example, the first axis drive can be configured to move the tool or workpiece of the machine tool, and the second axis drive (e.g., lower dynamics drive) can be configured to move a gantry of the machine tool. 
   Some implementations may include one or more of the following features. The total desired relative tool to workpiece movement can be applied solely to the first closed-loop axis control. Therein, an adjusting signal for the first axis drive (e.g., the higher-dynamics axis or higher-dynamics drive) can be determined. In the second closed-loop axis control, a resulting movement quantity corresponding to the higher-dynamics axis can be adjusted in relation to a specifiable value (i.e., the follow-up offset) by generating an adjusting signal for the drive of the second axis drive (e.g., lower-dynamics axis). The follow-up offset is a desired value for the tracking and/or trailing of the auxiliary axis (higher-dynamics axis). Preferably, in the steady state, the auxiliary axis is (on average) in a position corresponding to the follow-up offset. 
   According to another aspect, a machine tool includes first and second axis drives, each configured to move a tool or a workpiece in respective movement components of a total desired relative tool to workpiece movement along a common axis direction. The machine tool also includes an adjusting system configured to adjust the respective movement components by controlling the second axis drive according to a closed-loop control system (e.g., second closed-loop control) based at least in part on a value provided by another closed-loop control system (e.g., first closed-loop control) controlling the first axis drive. Using such a machine tool, the movements in the direction of an axis can be controlled not only by open-loop control but also by closed-loop control. That results in higher dynamics and greater accuracy. Therefore, a method for apportioning movement components and a machine tool with which greater accuracy and higher dynamics (i.e., higher frequency response) can be achieved in the machining of workpieces. 
   In some embodiments, higher dynamics are implemented by one axis drive than by the other axis drive. For example, the second axis drive can be configured for higher frequency response than the first axis drive. Small, rapid movements can thereby be executed by the axis drive having the higher dynamics (auxiliary axis). In particular, fine adjustment can be carried out by the axis drive having the higher dynamics (e.g., the second axis drive), whereas coarse adjustment can be carried out by the other axis drive (e.g., the first axis drive). The movements of the first and second axis drives, or rather the movements caused by them, can be superposed on each other. 
   In certain embodiments, at least one of the first and second closed-loop axis controls has a position controller. With a position controller it is possible to generate an adjusting signal which can be used to activate an axis drive, especially a controlled system comprising the axis drive. 
   To make closed-loop control possible, measuring devices can be provided for detecting an actual quantity (e.g., resulting movement quantity), for example, the actual position or actual acceleration, of the tool or workpiece. Those actual quantities can be detected for each axis drive, so that, for example, a position value for the position of a drive axis configured to move a gantry of the machine tool, and a position value for the position of another axis drive configured to move the tool or tool holder can be made available. 
   In some embodiments, the first closed-loop axis control has the position controller and the higher-dynamics axis drive, the second closed-loop axis control has a follow-up controller, and an input quantity of the follow-up controller is determined from the detected resulting movement quantity corresponding to the movement of the first axis drive and a specified follow-up offset value. The lower-dynamics axis drive is able to implement a small control bandwidth (i.e., lower frequency response). The higher-dynamics axis drive (auxiliary axis) is able to implement a larger control bandwidth (i.e., higher frequency response) and hence obtain a greater accuracy. The higher-dynamics axis drive is able to implement movements over only relatively short distances but, in return, more accurate adjustment can be performed. A secondary condition of the closed-loop control preferably provides that the higher-dynamics axis is preferably held in a central position (relative to the tool length or the stroke that can be executed by the tool). 
   In an alternative embodiment, the first closed-loop axis control has the position controller and the lower-dynamics axis drive, and the second closed-loop axis control has a second position controller. The adjusting system of the alternative embodiment accordingly has exclusively position controllers. A follow-up controller is not provided. The position error in the closed-loop control of the lower-dynamics axis can be applied as a position desired value to the position controller of the higher-dynamics axis. 
   Preferably, an input quantity of the position controller of the first closed-loop axis control is determined from the total desired relative tool to workpiece movement and the resulting movement quantity corresponding to the first axis drive, and the input quantity of the position controller of the first closed-loop axis control is fed to a determination device for determination of a second input quantity of the position controller of the second closed-loop axis control taking into consideration the resulting movement quantity corresponding to the second axis drive. Preferably, the determination device includes an adaptation device configured to receive the input quantity of the position controller of the first closed-loop axis control. 
   In the adaptation device, the input quantity of the position controller of the first closed-loop axis control can be modified by an adaptation factor. If the factor is not present (i.e. if it has the value 1), the closed-loop control operates as follows: the lower-dynamics axis (e.g., the first axis drive) is able to comply with the position desired values (i.e., the total desired relative tool to workpiece movement) only to a limited extent. As a result, a position error, or position lag, develops in the lower-dynamics axis. 
   That position error is fed as the desired value into the closed-loop axis control of the higher-dynamics axis (e.g., the second axis drive). Although that axis is similarly able to comply with its desired values only to a limited extent, it does so distinctly better than the lower-dynamics axis. As a result, the higher-dynamics axis reduces the deviations (position error) of the lower-dynamics axis. Therefore, the higher-dynamics axis can be made to compensate solely for errors of the lower-dynamics axis, including deviations that affect the lower-dynamics axis as a result of interfering forces. 
   If the coupled axes travel over position ramps at constant desired speed, a constant position error develops in the lower-dynamics axis (e.g., first axis drive) in the settled state. That position error can be completely compensated for by the higher-dynamics axis (e.g., second axis drive) in the settled state, that is to say, the resulting position error of the coupled axes becomes zero in the steady state. A position error of the coupled axes will, however, be produced during settling, which dwindles and proves to be smaller in its amplitude than the steady-state position error of the lower-dynamics axis. Therefore, a desirable closed-loop control behavior can be obtained in the steady state, with a low amplitude oscillation occurring during settling. 
   If the path behavior of, for example, two Cartesian coordinates, that is to say acting in the X- and Y-direction, is considered, the above-described oscillation in the position error can produce an unfavorable effect on the resulting path movement. The deviations can be greater than when only the lower-dynamics axis is operated on its own, even though the latter&#39;s steady-state position deviation is greater than in the case of the coupled axes. 
   The reason for this is that a settling process of the position deviation of the axes used to generate paths that approximately obeys an e-function produces a very small path error. That holds provided that all the axes involved in the path have identical closed-loop control dynamics. If the settling process deviates from an e-function, a distinctly greater path error can result. 
   Therefore, the closed-loop control of axes in a path combination can benefit from the following: 
   substantially identical bandwidths for the axis/axes for each degree of freedom of the path movement; and a settling behavior of the coupled axes when position ramps are being traveled that as far as possible obeys an e-function. 
   For example, let the Y-axis be of higher dynamics than the X-axis. In that case, it is advisable to improve the X-axis with a further, higher-dynamics axis X 2  acting in the X-direction. Preferably, the X and X 2  axes together have substantially the same closed-loop control behavior as the Y-axis in order to obtain path errors that are as small as possible. 
   The adaptation of the dynamics of one axis to the dynamics of another axis is performed by an adaptation factor of the position error of the lower-dynamics axis (the X-axis in the example), which is between zero and one, to the desired value of the higher-dynamics axis (the X 2  axis in the example). That factor ensures that the higher-dynamics axis is not able to compensate completely for the position error even in the steady state. When the adaptation factor is set correctly, the coupled axes act substantially identically to the other axis/axes in the path combination (the Y-axis in the example), provided that they have a conventional P-position control. In other words, the adaptation factor adjusts the X 2 -axis such that the X-axis and the X 2 -axis together act in the same way as the Y-axis in steady state (i.e., movement at constant velocity). 
   The adaptation factor adjusts for different control factors in the different axis directions (X, Y). For example, if the position control factor of the X 2 -axis is K VX2 , and the position control factor of the Y-axis is K VY , then the adaptation factor can be calculated as A=1−(K VX2 /K VY ), where A is the adaptation factor. 
   The adaptation factor finally reduces the effect of the higher-dynamics axis to the extent required for good path behavior. At the same time, the oscillation in the position deviation of the coupled axes decreases. 
   The adaptation factor allows the axes to satisfy the above-mentioned requirements for a good path behavior. 
   If coupled axes are to be used in all the directions of movement (that is to say, for example, in the X-direction and in the Y-direction, the coupled axes then being in the example the X and X 2  axis (=auxiliary axis in the X-direction) and the Y and Y 2  axis (=auxiliary axis in the Y direction)), optimum parameterisation can be obtained when the adaptation factor is set such that the position error when position ramps are travelled in one direction of movement falls just short of having an overshoot. In this case also, it is necessary to create identical closed-loop control behaviour in the X-direction and the Y-direction, that is to say, the “better” axes must be adapted to the “poorer” axes by way of the described factor. 
   The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description and drawings, and from the claims. 

   
     DESCRIPTION OF DRAWINGS 
       FIG. 1  shows a first machine tool with a workpiece movable in two axis directions; 
       FIG. 2  shows a machine tool with a tool movable in two axis directions; 
       FIG. 3  shows a machine tool with five axis directions; 
       FIG. 4  shows a machine tool with six axis directions; 
       FIG. 5  shows a machine tool with four axis directions and a movable workpiece; 
       FIG. 6  shows a first embodiment of an adjusting device of a machine tool; and 
       FIG. 7  shows a second embodiment of an adjusting device of a machine tool. 
   

   Like reference symbols in the various drawings indicate like elements. 
   DETAILED DESCRIPTION 
     FIG. 1  shows a machine tool  1  in which a workpiece  2  is movable in the axis directions  3 ,  4 . A tool holder  5 , which is capable of receiving a tool not shown, is also movable in axis direction  4 . Drives for the workpiece, which act in the axis directions  3 ,  4 , may be referred to as workpiece axes. The drive of the tool holder  5 , which acts in axis direction  4 , may be referred to as the machine axis. 
     FIG. 2  shows a further machine tool  10 , in which the workpiece  11  is arranged to be stationary. A gantry  12  is movable in the axis direction  13 . Arranged on the gantry  12  is a device  14  on which a tool holder  14 ′ capable of holding a tool is in turn arranged. The device  14  (and therewith the tool) is movable in axis direction  15 . The tool holder  14 ′ is movable relative to the device  14  in axis direction  13 . It will be apparent from  FIG. 2  that the device  14  has a lower mass than the gantry  12 . Consequently, movements in axis direction  15  can be carried out more rapidly than movements of the gantry in axis direction  13 . Highly dynamic movements in axis direction  13  can, however, be executed by the tool holder  14 ′. 
     FIG. 3  shows a machine tool  20  in which an arm  21  is movable in the axis direction  22 . A device  23  is movable along the arm  21  in axis direction  24 . The device  23  is additionally movable in axis direction  25 . The axis directions  22 ,  24 ,  25  define the X-, Y-, Z-direction of a Cartesian coordinate system. An extension arm  26  is rotatable in axis direction  27 . Arranged on the extension arm  26  is a laser cutting head  28 , for example, which in turn is rotatable in axis direction  29 . 
   The illustration in  FIG. 4  resembles that in  FIG. 3  inasmuch as the same axis directions  22 ,  24 ,  25 ,  27 ,  29  are provided. In addition, an axis direction  30  is provided, about which a workpiece can be turned. 
   A further embodiment of a machine tool is shown in  FIG. 5 . A device  42  is movable on a gantry  41  in axis direction  43 . The device  42  is in turn vertically displaceable in axis direction  44 . The extension arm  45  is rotatable in axis direction  46  and the laser cutting head  47  is rotatable in axis direction  48 . Drives acting in the axis directions  43 ,  44 ,  46 ,  48  may be referred to as machine axes, while a drive acting in axis direction  49  may be referred to as a workpiece axis since the workpiece  50  is movable along that axis. In the case of a machine tool, a second displacement can be possible relative to an axis direction. In  FIG. 5 , this could be achieved by means of the gantry  41  also being movable in axis direction  49 . 
     FIG. 6  shows a first embodiment of an adjusting system  60 . A desired quantity, x desired  is supplied to the adjusting system  60 . An actual quantity (e.g., a resulting movement quantity corresponding to a total relative movement through first and second controlled systems  65 ,  69  of an initial movement cycle) x actual  is available as the starting quantity. As a further input quantity, the adjusting system  60  has a follow-up offset at the point  61 . An input quantity  71  for a position controller  64  of a first closed-loop axis control  63  is determined from the values x desired  and x actual  in the comparator  62  of the first closed-loop axis control  63 . The position controller  64  determines an adjusting signal  72  for a controlled system  65  comprising a high-dynamics auxiliary axis. The resulting movement quantity adjusted in that manner is determined by a measuring device, not shown, as quantity x 1 . That quantity is adjusted in relation to the follow-up offset  61  by subtracting the follow-up offset  61  from it in the comparator  66  of the second closed-loop axis control  67  and passing the result as the input quantity  73  to a follow-up controller  68  of the second closed-loop axis control  67 . The follow-up controller  68  generates an adjusting signal  74  for one or more machine or workpiece axes arranged in a second controlled system  69 . At the output of the second closed-loop axis control  67 , an actual quantity x 2  is determined. The latter is added to the resulting movement quantity x 1  at the point  70 , yielding the value x actual . It should be noted here that two of the quantities x 1 , x 2 , x actual  have to be measured or detected and the third quantity can be determined therefrom. The quantities x 1 , x 2 , x actual  can, for example, be determined by position sensing of the axes or by distance measurement, for example measurement of the distance between workpiece and tool. Such distance measurement is frequently implemented in any case for distance control in a machine tool. 
   If, for example, the axis position x 1  is above the follow-up offset  61 , the follow-up controller  68  and the controlled system  69  generate an increasing axis position x 2 . As a result, the sum of x 1 +x 2  increases. For that reason, the position controller  64  reduces the axis position x 1 , since it adjusts the sum x 1 +x 2  in relation to the desired value x desired . In the steady state, the auxiliary axis (higher-dynamics axis) of the first controlled system  65  thus reaches the position specified by the follow-up offset  61 . 
     FIG. 7  shows an alternative embodiment of an adjusting system  80 . A specified desired value x desired  is once again applied to that adjusting system  80 . At the output, the actual value x actual  (e.g., the resulting movement quantity corresponding to a total relative movement through first and second controlled systems  84 ,  89  of an initial movement cycle) is available. In contrast to the illustrative embodiment shown in  FIG. 6 , in the first closed-loop axis control  81  an input quantity  91  for a position controller  83  is determined by subtracting the actual quantity x 1  of the first controlled system  84  of the first closed-loop axis control  81  from the desired quantity xdesired in the comparator  82 . The first closed-loop axis control  81  includes the position controller  83  which generates an adjusting signal for the controlled system  84  which comprises one or more machine or workpiece axes, in this case of lower dynamics. The input quantity  91  of the position controller  83  corresponds to a position error in the closed-loop control of the axis/axes of the controlled system  84 . That position error is passed to an adaptation device  85  which is arranged in a determination device  85   a . In the adaptation device  85 , the position error can be multiplied by a factor having a value between 0 and 1. If the factor is 1, the position error passes directly into the closed-loop control of the second closed-loop axis control  86  as the desired value. In the comparator  87 , the actual quantity x 2  at the output of the second controlled system  89  is subtracted from the output of the adaptation device  85 . In that manner, the second input quantity  92  of the position controller  88  of the second closed-loop axis control  86  is produced. The position controller  88  determines a control quantity for an auxiliary axis of the controlled system  89 , in this case the higher-dynamics axis (i.e., the axis configured for higher frequency response). At the point  90 , the actual quantities x 1  and x 2  are added to obtain the value x actual . If the position error of the first closed-loop axis control  81  enters the second closed-loop axis control  86  as the desired value, the position error of the axis/axes of the controlled system  84  is adjusted accordingly by the auxiliary axis of the controlled system  89 . This reduces the deviation in the mechanically in-series first and second axes of the controlled systems  84 ,  89 . 
   A number of embodiments have been described. Nevertheless, it will be understood that various modifications can be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.