US20100269954A1

US20100269954A1 - Method and apparatus for gravimetrically metering pourable or flowable material to be weighed

Info

Publication number: US20100269954A1
Application number: US12/308,419
Authority: US
Inventors: Andreas Kaszkin; Roman Kaszyñski; Przemyslaw Pietrzak; Wolfgang Stiehl
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2006-06-14
Filing date: 2007-06-14
Publication date: 2010-10-28
Also published as: CN101501461A; EP1867965A1; WO2007144402A1

Abstract

An apparatus and a method for gravimetrically metering pourable or flowable material to be weighed are provided. A container holds the material to be weighed and a weighing device detects the weight of the container. A first actuator fills the container located on the weighing device with the material to be weighed and a second actuator empties the container or places the empty container on the weighing device. Further, a control device for generating control signals for the actuators is provided. A signal filter with a variable filter characteristic filters the weighing signal delivered by the weighing device. A filter matching device, controlled by the control device, matches the filter characteristic as a function of the control signals to various predetermined filter characteristics.

Description

The invention relates to a method and apparatus for gravimetrically metering pourable or flowable material to be weighed.
For the gravimetric metering of pourable or flowable material to be weighed, the material is first filled via a first controllable actuator, e.g. a valve or a flap, into a container located on a weighing device, with the filling or metering quantity resulting from the opening time or open position of the actuator. The actual metered amount is determined from the difference between the weight of the filled and empty container and if there is a deviation with respect to the specified desired metering amount the opening time or the open position of the first actuator is corrected during the filling of the container. The container can be part of the weighing device and after each filling it can be emptied via a bottom valve or similar into a package for the metered material to be weighed, or the container can be the package itself which is placed on the weighing device by means of an automated transport system and is removed after filling.
Because the weighing device forms a spring-mass system, the weighing signal supplied by it is subject to oscillations resulting from the dynamics of the filling and emptying process. Furthermore, disturbances, e.g. vibrations, can be superimposed on the weighing signal and therefore it is normally subjected to low-pass or mean-value filtering before further evaluation. With the signal filters presently used, the filter characteristic is normally permanently set so that it remains unchanged during the complete metering process. The setting of the limit frequency of the low-pass filter or the filter width of the mean-value filter therefore represents a compromise for the complete metering operation between the reaction speed to signal changes and the required measuring accuracy. The use of adaptive signal filtering is also known, by means of which the filter characteristic is changed relative to the signal characteristic of the filtered signal.
The object of the invention is therefore to enable fast and precise gravimetric metering by simple means.
According to the invention, the object is achieved by the apparatus specified in claim 1 and the method specified in claim 10.
Advantageous developments of the inventive apparatus or inventive method can be taken from the subclaims.
The object of the invention is therefore
a device for the gravimetric metering of pourable or flowable material to be weighed

- with a container to hold the material to be weighed,
- with a weighing device for detecting the weight of the container,
- with a first actuator for filling the container located on the weighing device with the material to be weighed,
- with a second actuator for emptying the container located on the weighing device or for placing the empty container on the weighing device,
- with a control device for generating control signals for the actuators,
- with a weighing device for detecting the filling weight in the container,
- with a signal filter with a variable filter characteristic for filtering the weighing signal delivered by the weighing device, and
- with a filter matching device, controlled by the control device, for matching the filter characteristic as a function of the control signals to various predetermined filter characteristics,
  or
  a method for the gravimetric metering of pourable or flowable material to be weighed, that is filled by means of a first controllable actuator into a container located on a weighing device, with the container being emptied by means of a second controllable actuator or placed on the weighing device by means of the second controllable actuator, with the weight of the container being detected by means of the weighing device and the weighing signal delivered by the weighing device passing through a signal filter, whose filter characteristic can be changed and adapted to different specified filter characteristics depending on the control inputs to the actuators.

The invention is based on the knowledge that an exact determination of weight or mass is necessary only at specific timepoints of the metering operation and that these timepoints need not be determined from the signal characteristic of the weighing signal, but instead can be specified, and thus known, by controlling the metering process. The dynamic matching of the signal filtering is accordingly controlled by controlling the metering process. Preferably, the weighing signal is subjected to low-pass filtering, with a mean value filtering also being calculated for this purpose. In this process, the filter characteristic can be changed relative to the limit frequency, which in the case of mean-value filtering corresponds to the filter width. The filter width is the number of evaluation points of the weighing signal used to form the mean value. Furthermore, it is also possible to change the filter characteristic relative to the filter type, e.g. Bessel or Butterworth filter and/or the filter arrangement.
The timepoints provided by the control system of the dosing process are, where the container is part of the weighing device and is emptied after each filling:

- the opening of the first actuator at the start of the filling phase in which the container is filled with the material to be weighed,
- if necessary, the changing of the open position of the first actuator for fine metering towards the end of the filling phase,
- the closing of the first actuator at the end of the filling phase,
- the opening of the second actuator at the start of the emptying phase and
- the closing of the second actuator at the end of the emptying phase.

If a new container is filled each time on each dosing operation, the timepoints are:

- the placing of the empty container on the weighing device,
- the opening of the first actuator at the start of the filling phase,
- also if necessary the changing of the open position of the first actuator for fine metering towards the end of the filling phase,
- the closing of the first actuator at the end of the filling phase and
- the removal of the container from the weighing device.

The exact determination of weight or mass takes place in each case at the end of the filling phase and at the end of the (current or preceding) emptying phase or when the empty container is placed on the weighing device. The weighing signal is here subject to substantial oscillations in each case, which are caused by the filling, emptying or placing of the container and which decay with time. In addition, there is also, for example, the fact that the filling operation does not end at a stroke with the closing of the first actuator because the material to be weighed downstream of the actuator still has to fall into the container. In order to obtain the exact value of the weight or mass corresponding to the mean value of the weighing signal as quickly as possible, the limit frequency of the signal filter, designed as a low-pass filter, is changed, in an advantageous manner, immediately after the filling, emptying or placing of the container, e.g. controlled by the corresponding control signals for the actuators, within a predetermined time, from a predetermined higher value to a predetermined lower value. In the case of a mean-value filter, the filter width is changed from a predetermined lower value to a predetermined higher value. The change advantageously follows a predetermined time function, for example exp −t/τ or exp −(t/τ)², with t being the time and τ a predetermined, e.g. parameterable, time constant. In this way, the output signal of the signal filter is very quickly brought to the mean value of the weighing signal, which precisely reflects the weight or mass to be determined. The waiting time required after filling and emptying, or placing, the container for the exact determination of the weight or mass is therefore correspondingly short so that the phases for filling and emptying the container or for placing, fitting and removing the containers can follow each other in quick succession and therefore more metering operations can be carried out per time unit.

For a further explanation of the invention, reference is made in the following to the drawings. The drawings are as follows:

FIG. 1 An example of the inventive apparatus,

FIG. 2 A further example of the inventive apparatus,

FIG. 3 An example of the weighing signal generated during a metering process,

FIG. 4 An example of the filtering of the weighing signal after the emptying phase, and

FIG. 5 An example of the filtered weighing signal during a metering process

FIG. 1 shows a very simplified schematic of a metering apparatus with a container 1, which is mounted on a weighing device 2 with one or more weighing cells 3. A feeder device 4 for the pourable or flowable metering material to be weighed is arranged above the container 1. The feeder device 4 has an actuator 6 which is opened for filling the container 1 with the material to be weighed 5 and is then closed. During the filling phase, the amount of flow can be controlled by the different positions of the actuator 6, so that a coarse and subsequently fine metering of the material to be weighed 5 is possible. The container 1 has a second actuator 7 in its bottom area, through which it can be emptied to fill the metered material to be weighed 5, for example into packages (not shown here).
The actuators 6 and 7 are automatically controlled by a control device 8, which for this purpose generates the opening or closing control signals s₆or s₇at predetermined timepoints. The weighing device 2 generates a weighing signal m, which is prepared for further processing in a signal filter 9 with a variable filter characteristic. The weighing signal m can, e.g. be formed by summing the signals supplied by the individual weighing cells 3 and amplified and digitized as necessary (not shown here). A filter matching device 10, which can be controlled by the control device 8, enables the filter characteristic of the signal filter 9 to be matched to different predetermined filter characteristics as a function of the control signals s₆or s₇.
The exemplary embodiment shown in FIG. 2 differs from that in FIG. 1 in that the package for the material to be weighed 5 forms the container 1, with the filled container 1 being removed from the weighing device 2 after each metering operation and a new, empty container 1 being placed on the weighing device 2. The second actuator 11 is in this case designed as an automated transport or handling system, which is controlled by control signals s₁₁of the control device 8 and automatically performs the container changeover.
FIG. 3 shows an example of the time characteristic of the device, shown in FIG. 1, during a weighing signal m generated during a metering process.
At a timepoint t₁, which signals the start of the filling phase, the first actuator 6 opens and the container 1 is filled with the material to be weighed 5.
At a later timepoint t₂in the filling phase, the open position of the first actuator 6 is changed in order to enable fine metering.
At a timepoint t₃, the first actuator 6 is closed and the filling phase is thus ended.
At a timepoint t₄, which signals the start of the emptying phase, the second actuator 7 opens and the container 1 is emptied.
Finally, at a timepoint t₅, the second actuator 7 is again closed.
During the succeeding metering operations, Timepoints t₁to t₅are cyclically run through.
At the end of the filling phase, i.e. between timepoints t₃and t₄, as well as at the end of the emptying phase of the current or preceding metering cycle, i.e. after timepoint t₅(possibly also before timepoint t₅if the actuator 7 is not closed again until immediately before the next filling phase), the weight of the container 1 is measured and the actual metered amount (mass) is determined from the difference in weight between the filled and empty container 1.
During the filling phase and the emptying phase, the weighing signal m is very heavily influenced by the material to be weighed 5 falling into the container 1 or falling out of said container 1. After the ending of the filling phase and emptying phase, the weighing signal m initially has substantial oscillations, which decay with time, as a result of the preceding filling and/or emptying operation. FIG. 3 shows clearly that the characteristic of the weighing signal m is very pronounced in each case between the different timepoints t₁to t₅. Timepoints t₁to t₅are predetermined by the control device and therefore used for the optimum matching of the filter characteristic of the signal filter 9 to the characteristic of the weighing signal m between timepoints t₁to t₅. The reaction speed of the signal filter 9 must therefore be greater at a higher dynamic of the weighing signal m, i.e. during the filling and emptying phase, than in the interim times when the full and then the empty weights of the container 1 are to be determined and accordingly the signal filter 9 should not react quickly but instead should filter precisely, especially precisely form the mean value m′ of the weighing signal m. The control of the filter characteristic of the signal filter 9 using the control signals s₆and s₇or timepoints t₁to t₅delivered by the control device 8 thus enable a fast matching of the filter characteristic of the signal filter 9 to the characteristic of the weighing signal m.
As shown in the example of the emptying phase in FIG. 4, the weighing signal m is initially influenced mainly by the material to be weighed 5 falling from the container 1. The spring-mass system formed by the container 1 and the weighing device 2 then oscillates at its natural frequency about the value of the empty weight of the container 1, with the initially high oscillation amplitude reducing over time. Further interference signals, such as those from external vibration, are superimposed on the weighing signal m. In order to obtain the exact weight of the container 1, which corresponds to the mean value of the weighing signal, as quickly as possible, at timepoint t₄, if the control device 8 generates the control signal S₇to open the second actuator 7, the limit frequency f₀of the signal filter 9, in this case formed as a low-pass filter, is changed within a predetermined time from a predetermined higher value f₀₁to a predetermined lower value f₀₂. The change in the limit frequency f₀thereby takes place with a predetermined time function, in this case, for example, ō(t)=f₀₂[1+((f₀₁−f₀₂)/f₀₂)exp−(t/τ)²]. The output signal m′ of the signal filter 9 is thus very quickly brought to the mean value of the weighing signal m, which precisely reflects the empty weight of the container 1 to be determined. The waiting time required for the exact determination of the weight after emptying the container 1 is thus very short, so that the next filling phase can take place correspondingly early.
The matching of the filter characteristic of the signal filter 9 after the filling phase for measuring the weight of the filled container 1 takes place in a corresponding manner, with the frequency level being lower due to the higher mass of the spring-mass system.
In the case of the exemplary embodiment according to FIG. 2, the characteristic of the weighing signal m up to timepoint t₄is practically identical to the signal characteristic shown in FIG. 3. At timepoint t₄, the filled container 1 is then raised from the weighing device 2 and then, before timepoint t₁, a new empty container 1 is placed in position. The weight of empty container 1 is first measured and then that of the filled container 1, in order to determine the metering amount from the increase in weight. The weight measurements of the empty and filled container 1 are therefore influenced by the oscillations resulting from the placing of the empty container 1 on the weighing device 2 and/or from the filling of the container 1, which is why in a similar manner, as described with reference to FIG. 4, the output signal m′ of the signal filter 9 is quickly brought to the mean value of the weighing signal m by tracking the limit frequency f₀of the signal filter 9.
The parameters for the different filter characteristics of the signal filter 9 are stored in the filter matching device 10 e.g. in tabular form, and in addition to the various values for the limit frequency can also include various filter functions, e.g. Bessel or Butterworth filters or different filter arrangements.
The filling of the container 1 can be time-controlled, with timepoint t₂, after which fine metering takes place, and timepoint t₃at which the filling process is ended, being predetermined by the control device 8 and corrected as a function of the weight measurement. But these timepoints can also, as shown in the very simplified and idealized representation in FIG. 5, be determined from the filtered weighing signal m′. By matching the filter characteristic of the signal filter 9 to the dynamics of the metering process, a characteristic of the filtered weighing signal m′ can be obtained, which enables the determination of timepoints t₂and t₃from the signal characteristic m′ which is sufficiently accurate for metering purposes, for example by comparison with a threshold value sw₂or by differentiation and subsequent threshold value comparison. In the case of the example mentioned, the threshold value sw₂can be corrected as a function of the result of the weight measurement of the filled and empty container 1.

Claims

1.-18. (canceled)

19. An apparatus for gravimetrically metering pourable or flowable material to be weighed, comprising:

a container for holding the material to be weighed;

a weighing device for detecting the weight of the container;

a first actuator for filling the container located on the weighing device with the material to be weighed;

a second actuator for emptying the container located on the weighing device or for placing the empty container on the weighing device;

a control device for generating control signals for the actuators;

a signal filter with a variable filter characteristic for filtering the weighing signal delivered by the weighing device; and

a filter matching device, controlled by the control device, for matching the filter characteristic as a function of the control signals to various predetermined filter characteristics.

20. The apparatus as claimed in claim 19, wherein the signal filter is a low-pass filter.

21. The apparatus as claimed in claim 19, wherein the filter characteristic is varied relative to the limit frequency.

22. The apparatus as claimed in claim 20, wherein the filter characteristic is varied relative to the limit frequency.

23. The apparatus as claimed in claim 19, wherein the signal filter is a mean-value filter with a filter characteristic and a filter width, the filter characteristic being varied relative to the filter width.

24. The apparatus as claimed in claim 19, wherein the filter characteristic is changed relative to a filter type and/or filter arrangement.

25. The apparatus as claimed in claim 19, wherein the filter matching device is configured to change a limit frequency of the signal filter, designed as a low-pass filter, from a predetermined higher value to a predetermined lower value within a predetermined time immediately after the filling of the container with the material to be weighed.

26. The apparatus as claimed in claim 19, wherein the filter matching device is configured to change a limit frequency of the signal filter, designed as a low-pass filter, from a predetermined higher value to a predetermined lower value within a predetermined time immediately after emptying the container or after placing the empty container on the weighing device.

27. The apparatus as claimed in claim 19, wherein the filter matching device is configured to change a filter width of the signal filter, designed as a mean-value filter, from a predetermined lower value to a predetermined higher value within a predetermined time immediately after the filling of the container with the material to be weighed.

28. The apparatus as claimed in claim 19, wherein the filter matching device is configured to change a filter width of the signal filter, designed as a mean-value filter, from a predetermined lower value to predetermined higher value within a predetermined time immediately after the emptying of the container or after placing the empty container on the weighing device.

29. A method for gravimetrically metering pourable or flowable material to be weighed, comprising:

providing a container located on a weighing device;

filling the material to be weighed into the container by a first controllable actuator;

emptying the container by a second controllable actuator or placing the container on the weighing device by the second controllable actuator;

determining a weight of the container by the weighing device;

delivering a weighing signal by the weighing device;

passing the weighing signal through a signal filter,

wherein a filter characteristic of the signal filer is changed and matched to different predetermined filter characteristics as a function of control inputs of the actuators.

30. The method as claimed in claim 29, wherein the weighing signal is low-pass filtered in the signal filter.

31. The method as claimed in claim 29, wherein the filter characteristic is changed relative to the limit frequency.

32. The method as claimed in claim 30, wherein the filter characteristic is changed relative to the limit frequency.

33. The method as claimed in claim 29, wherein the weighing signal is averaged in the signal filter designed as a mean-value filter, and the filter characteristic is changed relative to a filter width of the signal filter.

34. The method as claimed in claim 29, wherein the filter characteristic is changed relative to a filter type and/or filter arrangement.

35. The method as claimed in claim 29, wherein, immediately after filling the container with the material to be weighed, a limit frequency of the signal filter designed as a low-pass filter is changed from a predetermined higher value to a predetermined lower value within a predetermined time.

36. The method as claimed in claim 29, wherein, immediately after the emptying of the container or after the placing of the empty container on the weighing device, a limit frequency of the signal filter designed as a low-pass filter is changed from a predetermined higher value to a predetermined lower value within a predetermined time.

37. The method as claimed in claim 29, wherein, immediately after the filling of the container with the material to be weighed, a filter width of the signal filter designed as a mean-value filter is changed from a predetermined lower value to a predetermined higher value within a predetermined time.

38. The method as claimed in claim 29, wherein, immediately after the emptying of the container or after the placing of the empty container on the weighing device, a filter width of the signal filter designed as a mean-value filter is changed from a predetermined lower value to a predetermined higher value within a predetermined time.