Abstract:
In a method for filling a defined quantity of medium into a container, a shut-off signal for a feed pump and closing signal for a metering valve are set such that, in the closing of the metering valve, the pressure in the section of the pipeline before the metering valve remains essentially constant. In this way, the accuracy of the filling unit can be significantly improved.

Description:
BACKGROUND OF THE INVENTION 
   Field of the Invention 
   The invention concerns a method for filling a defined quantity of medium into a container. 
   In many areas of the foods and pharmaceuticals industries, filling units are used for filling a defined quantity of medium into a containment. An essential requirement of these filling units is to maintain fill quantity constant as filling times become increasingly shorter. 
   SUMMARY OF THE INVENTION 
   In the case of beverage bottling units, up to 65,000 bottles per hour can be filled. A further essential requirement of these filling units is the precision and reproducibility of a single filling. Each overfilling means a financial loss for the plant operator. 
   In the case of known filling units, in order to guarantee the reproducibility of the filled quantity of the medium, a metering valve in a supply line is opened at a given point in time, and then closed again at a precisely determined later point in time. The volume- or mass-flow of the medium, for instance the flow rate, is determined, respectively, by means of a volume- or mass-flow meter. The volume-flow meter can be, for example, a “Promag 53” or a “Dosimag 2.” The mass-flow meter can be, for example, a “Promass 83” or a “Dosimass 2.” Both of these device types are sold by the firm Endress+Hauser. 
   Fill quantity is determined on the basis of the value of flow supplied by the measuring device. As soon as the predetermined quantity of medium is filled into the containment, the metering valve is closed. The pre-run and after-run quantities, that is, the fill quantities filled into the containment when the metering valve is, respectively, not fully open and not fully closed, are determined based on values taken from experience, or on the basis of experimentally obtained values, and are taken into consideration in the filling process. 
   A constant fill quantity can be attained only when the pressure in the supply line remains essentially constant. This is presently achieved by providing a so-called buffer tank, from which the quantity of medium to be filled is taken. The size of the buffer tank is selected such that the quantity of medium for a single filling does not markedly alter the fill level in the buffer tank. 
   In order to keep the fill level in the buffer tank essentially constant, a feed pump is provided, which supplies medium into the buffer tank. Such buffer tanks have a number of disadvantages. For example, they are expensive, must be cleaned, and require a considerable amount of space. 
   An object of the invention is to provide a method for filling a defined quantity of medium into a containment, which method requires no buffer tank, or, alternatively, a considerably smaller buffer tank than conventional filling units. 
   An essential idea of the invention is to keep the pressure in the supply line essentially constant. To do this, the supply position of the feed pump is registered, and the shut-off signal for the feed pump and the closing signal for the metering valve are appropriately coordinated with one another such that the pressure in the supply line is held essentially constant. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in greater detail on the basis of an example of an embodiment illustrated in the drawing, the figures of which show as follows: 
       FIG. 1  schematic illustration of a filling unit suitable for implementation of the method of the invention; 
       FIG. 2  typical plot of a fill curve in the case of a filling instance in a filling unit; 
       FIG. 3  signal for registering the supply position of a feed pump; 
       FIG. 4  schematic illustration of a filling unit as in  FIG. 1 , with multiple filling stations. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a schematic illustration of a filling unit suitable for implementing the method of the invention. Medium is directed/fed to the containment  5  via the supply line  3 . The feed pump  10  drives the medium into the supply line  3  from another component (not shown) of the unit. 
   Flow in supply line  3  is determined with the help of the flow measuring device  20 . 
   Examples of suitable volume-flow measuring devices are, for example, a “Promag 53” or a “Dosimag 2”. Examples of suitable mass-flow measuring devices are, for example, a “Promass 83” or a “Dosimass 2”. These two device types, sold by the firm Endress+Hauser, have already been described above. The flow quantity determined by the flow meter  20  serves for the metering of the fill quantity. As soon as the predetermined quantity of the medium is filled into the containment  5 , the metering valve  30  is closed. 
   In order to assure that a defined fill quantity is filled into containment  6 , a fill quantity control is used, which is implemented as a software program in a microprocessor of the flow meter  20 . The necessary control commands are relayed, respectively, to the feed pump  10  and to the metering valve  30 , via the control lines L 1 , L 2 . By opening the metering valve  30 , the filling instance is initiated. As soon as the desired quantity of medium is filled into the containment  6 , the filling instance is ended by closing the metering valve  30 . 
     FIG. 2  shows a schematic illustration of a typical fill curve. Flow rate is plotted as a function of time. The numbers shown in the drawing represent the points in time of the followings steps of the method: 1. command, open metering valve; 2. metering valve is opened; 3. close metering valve; 4. metering valve is closed. The quantity of medium filled into the containment  5  corresponds to the area under the curve between points  1  and  4 . If the pressure in the supply line  3  changes, then the curve changes to the effect that the approximately horizontal path of the curve between points  2  and  3  is shifted either upward or downward, and the fill quantity would be changed thereby. 
   The influence of the feed pump  10  on the filling process will now be described in greater detail. Feed pump  10  must be controlled such that the horizontal line between points  2  and  3  in the filling diagram has approximately the same height for all filling instances. This is achieved by coordinating the shut-off signal for the feed pump  10  with the closing signal for the metering valve  30 . If the feed pump  10  supplies when the metering valve  30  is closed, then pressure will build in the supply line  3 . At the next opening of the metering valve  30 , a spurting of the medium results. Such spurting is very difficult to register with the known flow measuring devices. In such case, the measurement values delivered by the flow measuring device  20  are subject to considerable inaccuracies, and thus the close command for the metering valve  30  may not come at the correct point in time. Altogether, this would adversely affect the accuracy of the fill quantity. 
   In accordance with the invention, the supply position of the feed pump is registered, and the shut-off signal for the feed pump and the closing command for the metering valve are coordinated with one another such that the pressure in the supply line remains essentially constant. Here it is important that the after-run of the pump, that is, the time lag between the shut-off signal for the feed pump and actual stopping of the pump, is taken into consideration. 
   Therefore, in accordance with the invention, the chamber position of the feed pump  10  is registered, and the shut-off signal for the feed pump  10  and the closing command for the metering valve  30  are coordinated with one another. It is thereby assured that, after the closing of the metering valve  30 , no additional pressure builds in the supply line  3 . In this manner, the pressure in the supply line can be held constant, so that intermediate storage of the medium in a buffer tank is no longer necessary. In order to register the supply position of the feed pump, appropriate sensors are provided at the feed pump. 
   In  FIG. 3   a , the associated signal plot is illustrated. Per revolution of the pump, four pulses are produced. In order to have very accurate information concerning the supply position of the pump, the signal of  FIG. 3   a  is multiplied with appropriate pulse multipliers (e.g. ×100), so that the supply position of the feed pump  10  can be precisely determined at any point in time ( FIG. 3   b ). 
   It has proven to be very advantageous to have the supply quantity of the feed pump following the shut-off signal correspond exactly to the after-run quantity of the metering valve  30 . 
     FIG. 4  shows a filling system in which the feed pump  10  serves multiple filling stations. Each filling station has flow meter  20 ,  40 ,  60  and a metering valve  30 ,  50 ,  70  for each containment  5 ,  5   a ,  5   b . The filling station which is last to receive the close command for its metering valve controls the shut-off of the feed pump  10 , that is, it takes over the master function. By way of example, a filling unit with three filling stations is shown. In principle, more than three filling stations can also be connected in parallel.