Abstract:
There is described a method and a device for positioning an element of a machine. A setpoint position variable is generated, said setpoint position variable is reduced by means of a limiting variable and a modified setpoint position variable which is forwarded as a setpoint variable to a controller for positioning the element is formed in this manner. An expected setpoint position variable is determined from the modified setpoint position variable, the braking distance of the element is determined from the expected setpoint position variable, and a braking process of the element is initiated if the braking distance matches a target position which is reduced by the expected setpoint position variable. An element of a machine can so be positioned with accuracy.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is the U.S. National Stage of International Application No. PCT/EP2006/060340, filed Feb. 28, 2006 and claims the benefit thereof. The International Application claims the benefits of German application No. 10 2005 010 089.9 DE filed Mar. 4, 2005, both of the applications are incorporated by reference herein in their entirety. 
   FIELD OF INVENTION 
   The invention relates to a method and facility for positioning an element of a machine. 
   BACKGROUND OF INVENTION 
   In the case of machines, such as robots, machine tools and/or production machines for example, movable elements of the machine, such as tools or workpieces, have to be positioned precisely. To position the element in the case of commercially available machines a positioning command, for example to displace the element to a defined target location, in other words to a defined position, is sent to a reference variable generation unit as an input variable. The reference variable generation unit uses the positioning command to generate a corresponding setpoint position variable, which is forwarded in the form of individual setpoint position values as a setpoint variable to a controller for positioning the element. The controller controls a drive unit of a machine axle, which then displaces the element according to the setpoint position variable and positions it thus. In the case of many machines, for example plastics machines or presses, it is often necessary to limit physical variables that occur during the displacement process, for example the pressure and/or force occurring. A so-called limit controller is generally used for this purpose, which intervenes when a setpoint value (for example maximum permitted pressure and/or force) is exceeded and outputs a limit variable, by which the setpoint position variable generated by the reference variable generation unit is reduced, before the setpoint position variable is forwarded to the controller as the setpoint variable for the controller. Since the controller no longer receives the original setpoint position variable but a thus modified (reduced) setpoint variable as the input variable, precise positioning of the element to be displaced no longer takes place. The element to be displaced is therefore not positioned precisely in the target location defined by the positioning command but the displacement movement of the element is stopped earlier so that the target location is no longer reached. 
   In such an instance until now an operator of the machine generally had to displace the element by manual control into the required position, in other words to the target location, or the displacement speed of the element had to be reduced, so that for example the force occurring, which occurred during displacement of the element, is smaller and the limit controller no longer intervenes and the setpoint position variable is thus not reduced, before the setpoint position variable is forwarded to the controller as a setpoint variable for the controller. However these procedures used until now result in longer processing times and therefore higher costs. 
   SUMMARY OF INVENTION 
   An object of the invention is to allow precise positioning of an element of a machine with the aid of an appropriate method and an appropriate facility. 
   This object is achieved by a method for positioning an element of a machine,
         with a setpoint position variable being generated,   with the setpoint position variable being reduced by a limit variable and a modified setpoint position variable thus being formed, which is forwarded as a setpoint variable to a controller for positioning the element,   with an expected setpoint position variable being determined from the modified setpoint position variable,   with the braking distance of the element being determined from the expected setpoint position variable,   with a braking process of the element being initiated, when the braking distance corresponds to a target location reduced by the expected setpoint position variable.       

   This object is also achieved by a facility for positioning an element of a machine, with the facility having
         means for generating a setpoint position variable,   means for generating a limit variable, with the setpoint position variable being reduced by a limit variable and a modified setpoint position variable thus being formed, which is forwarded as a setpoint variable to a controller for positioning the element,   means for determining an expected setpoint position variable from the modified setpoint position variable,   means for determining a braking distance of the element from the expected setpoint position variable,   means for initiating a braking process of the element, which initiate the braking process, when the braking distance corresponds to a target location reduced by the expected setpoint position variable.       

   It proves to be advantageous that the braking distance of the element is determined from the expected setpoint position variable, in that an expected setpoint speed of the element is determined from the expected setpoint position variable and/or an expected setpoint acceleration is determined and the braking distance is determined based on the expected setpoint position variable and/or the expected setpoint speed and/or the expected setpoint acceleration. This allows precise determination of the braking distance. 
   It also proves to be advantageous that the expected setpoint position variable is determined from the modified setpoint position variable by means of an extrapolation. This measure allows very precise determination of the expected setpoint position variable. 
   It also proves to be advantageous that the limit variable is given in the form of a force and/or pressure-limiting variable. Physical variables such as force and/or pressure are generally the limiting variables in the case of commercially available machines. 
   It also proves to be advantageous that the braking process is initiated by adjusting the generated setpoint position variable according to the braking process. This allows simple control of the braking process. 
   It also proves to be advantageous that an interpolator is provided as the means for generating the setpoint position variable. An interpolator represents a standard means for generating the setpoint position variable. 
   It also proves to be advantageous that a limit controller is provided as the means for generating the limit variable. A limit controller represents a standard means for generating a limit variable. 
   It also proves to be advantageous that an extrapolation unit is provided as the means for determining the expected setpoint position variable from the modified setpoint position variable. It is possible to determine the expected setpoint position variable particularly precisely from the modified setpoint position variable with the aid of an extrapolation unit. 
   It also proves to be advantageous that the means for initiating a braking process of the element initiate the braking process by acting on the means for generating the setpoint position variable, with the means for generating the setpoint position variable adjusting the setpoint position variable according to the braking process. This allows particularly simple initiation of the braking process. 
   Machine tools, production machines and/or robots are the standard machines, in which an element, such as a tool and/or a workpiece for example, has to be positioned precisely. Of course the present invention is however also suitable for positioning elements in other machines. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An exemplary embodiment of the invention is illustrated in the drawing and described in more detail below. In the drawing: 
       FIG. 1  shows an inventive method and facility for positioning an element of a machine and 
       FIG. 2  shows the pattern over time of the setpoint speed and the modified setpoint speed. 
   

   DETAILED DESCRIPTION OF INVENTION 
     FIG. 1  shows the inventive method in the form of a block circuit diagram and the inventive facility in the form of an exemplary embodiment. A target location x Z , in other words the position to which an element  8  is to be displaced, serves as the input variable of a reference variable generation unit  1 . The reference variable generation unit  1  is commercially available in the form of an interpolator. The reference variable generation unit  1  generates a setpoint position variable x soll  as a function of time t according to the target location X Z . The force F ist  occurring during displacement of the element  8  is measured by way of a sensor and supplied to a subtracter  3  as an input variable. The measured force F ist  is subtracted by means of the subtracter  3  from a predetermined setpoint force (generally this is the maximum permitted force for the displacement process) and supplied as an input variable to a limit controller  2 . If the measured force F ist  exceeds the setpoint force F soll , the limit controller  2  generates a limit variable B, which is subtracted from the setpoint position variable x soll  by means of the subtracter  4 . The setpoint position variable x soll  is thus reduced by the limit variable B and a modified setpoint position variable x sollM  is output by the subtracter  4  as the result of the subtraction. In the exemplary embodiment the current location of the element  8  is measured by means of a position sensor, which outputs the current location x ist  of the element and forwards it as an input variable to a subtracter  5 . The modified setpoint position variable x sollM  is forwarded to a controller, which in the exemplary embodiment comprises the subtracter  5  and a position controller  6 . The difference between the modified setpoint position variable x sollM  and the current position x ist  is supplied as an input variable to the position controller  6 , which controls a drive unit  7  to displace a machine axle of the machine. The drive unit  7  is used to displace the machine element  8  to the location, in other words the position, which is predetermined by the modified setpoint position variable x sollM  as the setpoint variable of the controller. 
   As already mentioned above, the problem arises during the positioning of the element  8  that when the limit controller  2  generates the limit variable B, because the setpoint force F soll  is exceeded during the displacement process of the element  8 , the value of the modified setpoint position variable x sollM  no longer corresponds to the setpoint position variable x soll  but is reduced by the value of the limit variable B. In the case of commercially available machines the reference variable generation unit  1  has no knowledge here of whether or not the limit controller  2  is in the process of intervening. Since if the limit controller  2  intervenes, the modified setpoint position variable x sollM  is smaller than the setpoint position variable x soll , the target location x z , in other words the desired position of the element  8  at the end of the displacement process, is no longer reached but the element  8  stops before the desired position. In the case of a plastics machine, this can result in the required product being produced incorrectly. 
     FIG. 2  shows the displacement process of the element  8  graphically yet again, with the setpoint speed v soll  being the setpoint position variable x soll  derived after time t and the modified setpoint speed v sollM  being the variable of the modified setpoint position variable x sollM  after time t. If the limit controller  2  does not intervene during the displacement process, in other words the limit variable B has a value of zero, the solid setpoint curve is followed, in other words there is acceleration in a first segment, displacement at a constant setpoint speed in a further segment and braking with a constant deceleration in a further segment, until the setpoint speed is zero and the element  8  has reached the target location. The surface area resulting below the graph of the setpoint speed corresponds to the displacement distance covered by the element  8 . Since the reference variable generation unit  1  knows the deceleration a v , with which it can slow down the displacement process of the element  8 , it knows that it must initiate the braking process at time t 2 , if the element  8  is to stop at the target position. 
   If however the limit controller  2  intervenes at time t 1 , because the force occurring during the displacement process is too great, the setpoint position variable x soll  is reduced by the limit variable B, so that the modified setpoint position variable x sollM  is smaller than the setpoint position variable x soll , as a result of which the modified setpoint speed v sollM  (derivation of the setpoint position variable x soll  over time) is similarly smaller than the setpoint speed v soll  from time t 1 . If the braking process is again initiated at time t 2 , as is standard for commercially available machines, the desired position of the element  8 , in other words the target location x z , is not reached and the element  8  comes to a stop before it. The present invention is applicable now. 
   The modified setpoint position variable x sollM  is supplied as an input variable to an extrapolation unit  21 , which determines an expected setpoint position variable x sollE , which preferably corresponds to the setpoint position variable x soll , which would be generated by the reference variable generation unit  1  during the next interpolation clock pulse. (The interpolation clock pulse period T of the interpolation clock pulse is preferably the time period between two consecutively generated setpoint position values of the setpoint position variable x soll . The interpolation clock pulse period T can be a millisecond for example). The modified setpoint position variable x sollM  is differentiated to this end within the extrapolation unit  21  first consecutively twice by means of the two differentiators  10  and  11  (corresponding to two derivations after time t) and a modified setpoint acceleration a sollM  is thus calculated. This is then supplied as an input variable to a multiplier  12 , which carries out a multiplication of the modified setpoint acceleration a sollM  and the interpolation clock pulse period T. The output signal of the multiplier  12  is then supplied as an input variable to a multiplier  13  and this carries out a further multiplication using the interpolation clock pulse period T. The result of the multiplication is supplied as an input variable to an adder  17 . The modified setpoint position variable x sollM  is also differentiated once by means of the differentiator  9  (corresponding to a derivation after time t) and a modified setpoint speed v sollM  is thus calculated, which is supplied as an input variable to a multiplier  15 , which carries out a multiplication of the modified setpoint speed v sollM  and the interpolation clock pulse period T. The result of the multiplication is supplied as an input variable to an adder  17 . The modified setpoint position variable x sollM  is also supplied as an input variable directly to the adder  17 . As a result of the addition of the three signals the adder  17  generates the expected setpoint position variable x sollE  on the output side. The expected setpoint position variable x sollE  is thus determined from the modified setpoint position variable x sollM  by extrapolation. 
   The expected setpoint position variable x sollE  is supplied as an input variable to a subtracter  18 , which subtracts the target location x z  from the expected setpoint position variable x sollE  and thus reduces the expected setpoint position variable x sollE . The result of the subtraction is supplied to a comparator  19 . The current braking distance x B  is also determined from the expected setpoint position variable x sollE  using means for determining a braking distance, which are present in the exemplary embodiment in the form of a braking distance calculation unit  20 . In the simplest instance the expected setpoint position variable x sollE  is differentiated to this end once by means of the differentiator  21 , in other words derived after time t, as shown with a solid line, and the expected setpoint speed v sollE  is thus determined and supplied as an input variable to the braking distance calculation unit  20 . In the simplest instance this then calculates the braking distance x B  according to the equation 
                   x   B     =       v   sollE   2       2   ·     a   v                 (   1   )               
taking into account the predetermined deceleration a v . It is of course also possible to calculate the braking distance x B  in a much more complex manner, by for example also calculating an expected setpoint acceleration a sollE  by means of the two differentiators  22  and  23  shown with a broken line and supplying this together with the expected setpoint position variable x sollE  to the braking calculation unit  20 , with this then determining the braking distance x B  taking into account all three input variables. The deceleration a v  also does not have to be constant as in the exemplary embodiment but can also be a function of one or more of the input variables of the braking distance calculation unit  20 .
 
   If the braking distance x B  corresponds to the target location x z  reduced by the expected setpoint position variable x sollE , the braking process is initiated in that a signal is sent from means for initiating a braking process, which are present in the exemplary embodiment in the form of a comparator  19 , to the reference variable generation unit  1 , which in the exemplary embodiment represents a means for generating a setpoint position variable. When the reference variable generation unit  1  receives the signal from the comparator  19 , the setpoint position variable x soll  to be generated is adjusted according to the braking process. The setpoint position variable x soll  is hereby generated by the reference variable generation unit  1  such that the element  8  is slowed down with the predetermined deceleration a z . 
   The element  8  then stops precisely at the required target location x z , in other words at the required position. 
   In  FIG. 2  at time t 3  the braking distance x B  corresponds to the difference between the expected setpoint position variable x sollE  and the target location x z , so that the comparator  19  sends a signal to the reference variable generation unit  1 , which then initiates the braking process with the constant deceleration a v , so that the speed graph shown in  FIG. 1  with a broken line results. The surface area content of the surface areas A 1  and A 2  shown hatched in  FIG. 2  is identical here. 
   It is also possible, for example to save computation capacity, not to have the extrapolation unit  21 , the comparator  19 , the subtracter  18  and the braking calculation unit  20 , as well as the differentiators  21 ,  22  and  23  running all the time but just when the limit controller  2  intervenes.