Abstract:
A method and device for controlling a movement of a movable machine element of a machine tool or production machine with at least two drive axles are disclosed. At least one mechanical characteristic frequency is determined for each drive axle of the machine, and the lowest mechanical characteristic frequency is selected from the determined mechanical characteristic frequencies. For each drive axle of the machine, desired values are supplied to a control unit associated with the drive axle, whereby the desired values of the drive axles having a mechanical characteristic frequency higher than the lowest mechanical characteristic frequency are time-delayed. The disclosed method and device modify a method referred to as “Input Shaping” so that geometrically linked drive axles can be operated simultaneously.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]     This application claims the priority of German Patent Application, Serial No. 103 59 984.3, filed Dec. 19, 2003, pursuant to 35 U.S.C. 119(a)-(d).  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a method and a device for controlling a movement of a movable machine element of a machine tool or production machine with at least two drive axles.  
         [0003]     Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.  
         [0004]     A mechanical system capable of oscillations (for example, a drive axle of a machine tool or production machine) is typically characterized by at least one characteristic frequency that can be excited during a movement and can adversely affect the actual position value of, for example, a machine element. Excitations the mechanical system of the machine should therefore be prevented.  
         [0005]      FIG. 1  shows in form of a block diagram a prior art electric drive system of a machine tool or production machine, which is depicted as a two-axle machine and includes a controller  1  controlling the two drive axles  6   a  and  6   b  of the machine. The drive axle  6   a  includes a converter  3   a , a drive motor  4   a  and a mechanical system  5   a  connected with the drive motor  4   a . The drive axle  6   b  includes a converter  3   b , a drive motor  4   b  and a mechanical system  5   b  connected with the drive motor  4   b . Each drive axle has a separate control unit  2   a  and  2   b , respectively, with the controller  1  supplying to each of the control units  2   a  and  2   b  a separate desired velocity curve in the form of a corresponding desired load rotation speed n Lsoll1  and n Lsoll2 , which represent a desired movement path of a machine element driven by the drive axles  6   a  and  6   b . Each of the control units  2   a  and  2   b  regulates via the associated converter  3   a ,  3   b  the corresponding actual load rotation speed n List1  and n List2  according to the desired parameters set by the controller, thereby moving the machine element along the predetermined path by way of the mechanical system  5   a ,  5   b  connected to the corresponding drive motor  4   a ,  4   b . The term actual load rotation speed n List1  and n List2  is to be understood as a rotation speed of a spindle driven via a gear, whereby, for example, during each revolution of the spindle a machine element, such as a tool, is moved along an axle of the machine by a predetermined distance so as to change the position of the machine element. The machine element is then moved on the movement path along the two axes of the machine according to the actual position values n List1 , and n List2 . The feedback of the actual regulated valuables required for closed-loop control is not essential for an understanding of the invention and therefore omitted from  FIG. 1  for sake of clarity.  
         [0006]      FIG. 15  shows an exemplary movement path S for the two-axle machine of  FIG. 1 , wherein a machine element  8  implemented as a milling cutter is guided on the movement path S. The drive axle  6   a  in  FIG. 1  is effects the movement in the X 1  direction, whereas the drive axle  6   b  effects the movement in the X 2  direction.  
         [0007]     The excitation of oscillations, in particular of the mechanical components of the drive axles, can be suppressed by employing a so-called jerk-limiter. In this way, the load carried by the individual drive axles of the machine can be reduced, without adversely affecting the program processing time. A jerk-limiter can be used to control the acceleration buildup of a moving machine element, thereby smoothing the desired value so that the mechanical components move with a minimum of oscillations. The term jerk is to be understood as the time-derivative of the acceleration.  
         [0008]     In conventional machines, the jerk and the acceleration values are adapted in the controller according to the defined machine data. Because the jerk and the acceleration values are controlled along the movement path S of the machine element, there exist only a limited number of options for preventing excitation of oscillations, in particular excitation at the respective characteristic frequency of geometrically linked drive axles.  
         [0009]     The present invention has its foundation in the method known as “Input Shaping”, which surmises that an oscillation excitation in a drive axle in response to an input impulse can be compensated by a delayed input impulse with a different amplitude. The mechanical components  5   a  or  5   b  of the drive axles  6   a  and  6   b  can be implemented by way of a so-called second-order proportional delay unit (hereinafter abbreviated as “PT2-unit”), which can be described, for example, for the drive axle  6   a  by the differential equation  
                     ⅆ   2     ⁢     n   List1         ⅆ     t   2         =         ω   0   2     ·     n   Lsoll1       -       ω   0   2     ·     n   List1       -       2   ·   D   ·     ω   0     ·       ⅆ     n   List1         ⅆ   t         ⁢           ⁢   with         ⁢          ⁢       ω   0     =     2   ⁢     π   ·       f   0     .                   (   1   )             
 
         [0010]     In an ideal situation, the transmission characteristics of the controller  2   a  and of the converter  3   a  can be neglected. The following terminology is used in equation (1): 
        D: damping factor of the mechanical components     ω 0 : characteristic angular velocity of the undamped mechanical components     t: time     f 0 =characteristic frequency of the undamped mechanical components.        
 
         [0015]     The impulse response of the PT2-unit can then be derived based on the equation (1) as follows:  
               n   List1     =           ω   0         1   -     D   2           ·     ⅇ       -   D     ⁢           ⁢     ω   0     ⁢   t       ·     sin   ⁡     (       ω   d     ·   t     )         ⁢           ⁢   with             (   2   )                 ω   d     =       2   ⁢     π   ·     f   d         =       ω   0     ·       1   +     D   2                     (   3   )                 T   d     =     1     f   d               (   4   )             
 
 wherein 
        f d : characteristic frequency of the damped mechanical system     T d : oscillation period length of the damped mechanical system.        
 
         [0018]      FIG. 2  shows an exemplary curve of the time-dependence of the actual load rotation speed n List1  (impulse response) of the drive axle  6   a  according to  FIG. 1  after excitation with an impulse for a damping factor of D&lt;1. The conventional method of “Input Shaping” is used to suppress oscillations in the actual load rotation speed n List1  by exciting the PT2-unit with a second impulse which is applied with a time delay of half a period length of the oscillation period T d . The amplitude of the second impulse is selected so that the maximum of the second impulse response produced by this impulse is identical to the minimum of the first impulse response.  
         [0019]      FIG. 3  shows the impulse response n List1  and the delayed impulse response n List1v .  
         [0020]      FIG. 4  shows the summed impulse response of the PT2-unit from both impulses in the form of a summed actual load rotation speed n List1s . The mechanical system with the PT2-unit has an ideal transient response as a result of the addition of the two impulse responses n List1  and n List1v .  
         [0021]     The position X List1  of the machine element along the movement path illustrated in  FIG. 5  is obtained by integrating the summed actual load rotation speed n List1s . The position X List1  is normalized in  FIG. 1  to a value of “1”. As shown in  FIG. 5 , the machine element can be controlled in the afore-described manner so as to attain a predetermined desired position without exciting oscillations. The two amplitudes A 1  and A 2  of the two excitation impulses can be computed as follows from the requirement that the impulse responses cancel out and the steady-state amplification is “1”:  
                 A   1       A   2       =       ⅇ       π   ⁢           ⁢   D         1   -     D   2             ⁢           ⁢   and             (   5   )                   A   1     +     A   2       =   1.           (   6   )             
 
         [0022]     In a practical application of “Input Shaping,” the curve of the desired load rotation speed n Lsoll1  of the drive axle  6   a  is composed of a series of time-discrete impulses having constant amplitudes during the sampling time T s . Because the system is linear, the relationships governing cancellation and steady-state amplification can be superimposed independently for all impulses.  
         [0023]     This situation is illustrated in the example depicted in  FIG. 6  for a step-like change in the desired load rotation speed n Lsoll1  to a value of “1”. In order to change, for example, the actual load rotation speed n List1  to a normalized value of “1”, an infinite number of impulses (indicated by 3 dots in  FIG. 6 ), i.e., an infinite number (sampled values) of the desired load rotation speed n Lsoll1  of length T s  and amplitude A 1  are required, as well as an infinite number of impulses with an amplitude A 2  that are delayed by the delayed time T ver1  (also indicated by 3 dots in  FIG. 6 ). Only five values are shown  FIG. 6  for sake of clarity. The desired load rotation speed n Lsoll1  is obtained by summing all impulses, as shown in  FIG. 6  by a wider solid line. The delay time T ver1  is defined by the equation (7):  
               T   ver1     =         T   d     2     .             (   7   )             
 
         [0024]     When the afore-described method of “Input Shaping” is applied to machine tools or production machines with geometrically linked drive axles that have different mechanical characteristic frequencies, which is practically always the case, then the problem illustrated in  FIG. 7  arises.  
         [0025]     The movement path S illustrated in  FIG. 7  is circular, with X List1  and X List2  representing the corresponding actual position values of the associated drive axles  6   a  and  6   b . Curve  9   a  shows an ideal circular path, whereas curve  10   a  shows a somewhat distorted “circular” path when the machine is operated without “Input Shaping” and inaccuracies are introduced by mechanical oscillations of the drive axles  6   a  and  6   b . When the machine is operated with “Input Shaping”, the curve  11   a  assumes an elliptical shape instead of the ideal circular shape. Although mechanical oscillations are no longer excited with “Input Shaping,” the result is not noticeably improved over a conventional operation without “Input Shaping.” 
         [0026]     The underlying cause for this problem was unclear to this date, and the method of “Input Shaping” was therefore applied only to movements along a single machine axle, when only one axle is driven at a time during operation. “Input Shaping” and other related methods have not been applied to movements along a circular path or in two-dimensional or three-dimensional space, where the individual drive axles are geometrically linked and are operated simultaneously.  
         [0027]     It would therefore be desirable and advantageous to provide an improved method and device for controlling the movement of a movable machine element of a machine tool or production machine with at least two drive axles, which obviates prior art shortcomings and is able to specifically operate multiple geometrically linked drive axles of these machines.  
       SUMMARY OF THE INVENTION  
       [0028]     According to one aspect of the present invention, a method for controlling a movement of a movable machine element of a machine tool or production machine with at least two drive axles includes the steps of determining for each drive axle of the machine at least one mechanical characteristic frequency, selecting from the determined mechanical characteristic frequencies a lowest mechanical characteristic frequency, supplying for each drive axle of the machine desired values to a control unit associated with that drive axle, and time-delaying the desired values of the drive axles having a mechanical characteristic frequency higher than the lowest mechanical characteristic frequency.  
         [0029]     According to another aspect of the invention, a device for controlling a movement of a movable machine element of a machine tool or production machine with at least two drive axles includes a control unit associated with each drive axle in one-to-one correspondence for controlling the movement of the corresponding drive axle, a machine controller that generates for each drive axle of the machine corresponding desired values and supplies the generated desired values to the corresponding control unit associated with each drive axle as desired controlled variable, and a time delay unit that time-delays the desired values of those drive axles that have a mechanical characteristic frequency higher than a lowest mechanical characteristic frequency of the drive axles.  
         [0030]     Machine tools in the context of the present invention can also include, for example, uniaxial or multi-axis lathes, milling machines, as well as drilling or grinding machines. Machine tools can further include processing centers, linear and rotary transfer machines, laser machines, rolling machines and/or gear cutters. These machines have in common that the material is machined along several axes. Production machines in the context of the present invention can include textile, paper, plastic, wood, glass, ceramic or stone processing machines, as well as machines used for forming, packaging, printing, conveying, lifting, pumping, transporting. Furthermore, fans, blowers, wind turbines, lifting gear, cranes, robots, production and assembly lines are also included under the term production machines in the context of the present invention.  
         [0031]     According to an advantageous feature of the invention, the time-delayed desired values of the drive axles can be delayed by a delay time  
           T   v     =     abs   ⁡     (       1     2   ·     f   dv         -     1     2   ·     f   d           )         ,       
 
 wherein f dv  is the mechanical characteristic frequency of a drive axle v with a mechanical characteristic frequency higher than the lowest mechanical characteristic frequency and f d  is the mechanical characteristic frequency of the drive axle with the lowest mechanical characteristic frequency. This approach can significantly reduce distortions in the contour resulting from “Input Shaping.”
 
         [0032]     According to another advantageous feature of the invention, the desired values can represent desired velocity values, desired position values, desired acceleration values and/or desired jerk values. The method and device of the invention can be used for these and other types of desired values.  
         [0033]     According to yet another advantageous feature of the invention, the delay unit can be an integral component of the controller or of a closed-loop controller. The delay units can easily be integrated in an already existing controller and/or in existing closed-loop controllers or in the control units without requiring additional hardware, in particular when the machine tool or production machine already includes a numerical controller. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0034]     Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which:  
         [0035]      FIG. 1  is a block diagram of a two-axle machine tool or production machine;  
         [0036]      FIG. 2  shows an impulse response of a PT2-unit with a damping factor &lt;1;  
         [0037]      FIG. 3  shows impulse responses of a PT2-unit to two input impulses having a time offset;  
         [0038]      FIG. 4  shows the summed impulse response of a PT2-unit to two input impulses with the time offset of  FIG. 3 ;  
         [0039]      FIG. 5  shows a normalized curve of the position of a machine element;  
         [0040]      FIG. 6  shows time-discrete impulses of desired position values;  
         [0041]      FIG. 7  shows ideal movement paths during conventional operation and with “Input shaping”;  
         [0042]      FIG. 8  shows a curve of the actual load rotation speed values after a jump in the desired values;  
         [0043]      FIG. 9  shows a curve of the actual position values after a jump in the desired values;  
         [0044]      FIG. 10  shows a time delay of the actual load rotation speed values according to the invention;  
         [0045]      FIG. 11  shows curves of actual position values obtained with the method of the invention;  
         [0046]      FIG. 12  shows a time delay of an actual position value according to the invention;  
         [0047]      FIG. 13  shows a device according to the invention for controlling the movement of a machine element;  
         [0048]      FIG. 14  shows an exemplary circular movement path obtained with the method of the invention; and  
         [0049]      FIG. 15  shows the movement path of a machine element. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0050]     Throughout all the Figures, same or corresponding elements are generally indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the drawings are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted.  
         [0051]     As mentioned above, the method of “Input Shaping” has not been applied to machines with drive axles that are geometrically linked and have different characteristic oscillation frequencies, because the results are unsatisfactory, as indicated in  FIG. 7 . The inventor has investigated the underlying causes for these unsatisfactory results, which were not known to date.  
         [0052]     For the following discussion, it will be assumed that the method known as “Input Shaping” is already implemented in the controller  1  of  FIG. 1 , so that the two desired load rotation speed values n Lsoll1  and n Lsoll2  and  FIG. 1  have each already been evaluated using “Input Shaping.” 
         [0053]     Turning now to  FIG. 8 , there are shown curves of two actual load rotation speed values n List1  and n List2  for the two drive axles  6   a  and  6   b  depicted in  FIG. 1 . It is evident that the actual load rotation speed value n List1  of the drive axle  6   a  and the actual load rotation speed value n List2  of the drive axle  6   b  do not exhibit transient oscillations, because the controller  1  applies “Input Shaping” to the outputted desired values.  
         [0054]      FIG. 9  shows the corresponding position values X List1  and X List2  obtained by integrating the respective actual load rotation speed values n List1  and n List2 . As seen in  FIG. 9 , the position values X List1  and X List2  have different delays, which distort the contour of the circles of  FIG. 7 .  
         [0055]     According to the present invention, the desired rotation speed values are provided by the controller  1 , with “Input Shaping” applied in the controller  1 , to each drive axle in the form of two time-delayed input impulse sequences, with  FIG. 6  showing an exemplary input impulse sequence for the drive axle  6   a . The desired rotation speed values are symmetrized so as to minimize the contour errors of the movement path S caused by “Input Shaping.” According to the invention, the desired values of the drive axles v having a mechanical characteristic frequency f dv  that is not the lowest mechanical characteristic frequency are time-delayed. The index v is an index of a drive axle and runs from 1 to (n-1), whereby n is the number of drive axles of the machine.  
         [0056]     The required delay time T v  is illustrated in  FIG. 10 , which shows again the two actual load rotation speeds n List1  and n List2  from  FIG. 8 . According to the invention, the actual load rotation speed n List1 , which in the embodiment represents the drive axle with a mechanical characteristic frequency f d1  that is not the lowest characteristic frequency, is time-delayed by a correction value corresponding to the actual load rotation speed of the drive axle with the lowest mechanical characteristic frequency, which in the illustrated embodiment is the drive axle  6   b . In the illustrated embodiment, the drive axle  6   a  (v=1) has a characteristic frequency of 50 Hz, whereas the drive axle  6   b  (v=2) has a characteristic frequency of 25 Hz. The time-delayed actual load rotation speed n List1  with the time delay T v  is labeled in  FIG. 10  with n List1d . It should be noted that the amplitude of the time-delayed actual load rotation speed n List1d  and of the actual load rotation speed n List1  of the second drive axle, i.e., the drive axle with the lowest mechanical characteristic frequency f d , must be identical over time.  
         [0057]     The corresponding position values X List1 , X List2 , and X List1d  shown in  FIG. 11  are obtained by integrating the actual load rotation speeds n List1 , n List2 , and n List1d . It is evident that with the delay according to the invention, the integrated position values are identical for the two drive axles  6   a  and  6   b , thereby minimizing contour errors.  
         [0058]     In order to ensure that the time delay of the actual load rotation values of the drive axle with the lowest mechanical characteristic frequency is always used, the desired values, i.e., the desired load rotation values n Lsoll1  of the drive axle with a mechanical characteristic frequency that is not the lowest characteristic frequency are time-delayed. In the depicted embodiment, the drive axle  6   a  has a characteristic frequency that is not the lowest mechanical characteristic frequency. Particularly advantageous results are obtained, when the desired values of the drive axles v with a mechanical frequency f dv  that is not the lowest mechanical characteristic frequency have an identical or at least similar delay time of  
                 T   v     =     abs   ⁡     (       1     2   ·     f   dv         -     1     2   ·     f   d           )         ⁢     
     ⁢     v   =     1   ⁢           ⁢   to   ⁢           ⁢     (     n   -   1     )                 (   8   )             
 
 wherein 
        n: number of drive axles     abs(u): absolute value of the function u.        
 
         [0061]     f dv  represents here the mechanical characteristic frequency of a drive axle v that does not have the lowest mechanical characteristic frequency, whereas f d  represents the drive axle with lowest mechanical characteristic frequency. In the illustrated embodiment, the drive axle  6   a  has a mechanical characteristic frequency f d1  that is not the lowest mechanical characteristic frequency, so that the desired load rotation speed n Lsoll1  of the drive axle  6   a  is time-delayed with the delay time T v .  
         [0062]      FIG. 12  shows the time-delayed desired load rotation speed values n Lsoll1  according to the invention which are time-delayed with the delay time T v . Unlike  FIG. 6 , the desired load rotation speed values n Lsoll1  of the drive axle  6   a  are now time-delayed with the delay time T v  according to equation (8).  
         [0063]      FIG. 13  shows a device according to the invention for controlling the movement of a movable element of a machine tool or production machine, which unlike the embodiment depicted in  FIG. 1 , includes an additional time delay unit  10 . In all other aspects, the components of the device of  FIG. 13  and their operation are identical to those of the device of  FIG. 1  and will therefore not be described again. According to the invention, the desired load rotation values n Lsoll1  of the drive axle  6   a  are delayed by the delay unit  10  by the delay time T v  before being transmitted to the control unit  2   a  as a desired controlled variable.  
         [0064]      FIG. 14  illustrates the improvement in the movement paths achieved with the invention over the circular movement paths described before with reference to  FIG. 7 . Curve  9   b  represents an ideal circular path, curve  10   b  represents the circular path in a conventional operation without “Input Shaping”, whereas curve  11   b  illustrates the circular path attained when both “Input Shaping” and the method of the invention are employed. As seen in  FIG. 14 , the circular path  11   b  is almost identical to the ideal circular path  9   b . In particular, a significant improvement is obtained over the circular path  11   a,  which included “Input Shaping”, but did not employ the method of the invention.  
         [0065]     In the exemplary embodiment, the invention was described with reference to a two-axle machine. However, the invention can also be applied to a machine with three or more drive axles. In this way, a high-quality three-dimensional movement of machine elements can be implemented by using the method and the device of the invention.  
         [0066]     The mechanical characteristic frequency of the damped oscillation of the individual drive axles v can be determined by conventional measurement procedures, such as a frequency-dependent excitation of the drive axle and measurement of the resulting oscillations, so that the desired load rotation speed values of the individual drive axles v that do not have the lowest mechanical characteristic frequency f dv  can be time-delayed by using a delay unit associated with each drive axle.  
         [0067]     In the illustrated embodiments, desired speed values are transmitted from the controller  1  to the control units  2   a  and  2   b  that control the movement of the machine element. It will be understood, however, that the desired values can also be provided between the controller  1  and the various other control units for the drive axles as desired position values, desired acceleration values or desired jerk values. The desired position values, acceleration values or jerk values can also be delayed with the method and device according to the invention.  
         [0068]     It should also be noted that the delay unit  10  can be an integral component of a controller  12  or of a closed-loop controller  13  and can be easily integrated, for example, in already existing numerical controls without the need for additional hardware.  
         [0069]     Moreover, drive axles can have several characteristic frequencies, and the method of the invention can be separately applied to each of these characteristic frequencies or to a combination or superposition of these characteristic frequencies.  
         [0070]     While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.  
         [0071]     What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and includes equivalents of the elements recited therein: