Method and device for a pulsation-free, continuous and gravimetric dosing

The gravimetric control of a mass flow from or into a container located on a balance using a pulsating dosing pump, the output signal of the balance is digitally filtered in such a manner that the course in time of the mass flow is cyclically estimated from the filtered wighing signal. This estimated value of the course in time of the mass flow is used to calculate the course in time of an auxiliary mass flow which compensates the pulsation of the mass flow of the pulsating dosing pump and a speed course is calculated from the course in time of the auxiliary mass flow for driving an auxiliary dosing pump.

BACKGROUND OF THE INVENTION 
The invention relates on the one hand to a method for the gravimetric 
control of a mass flow from a or into a container located on a balance 
using an intermittent dosing pump and on the other hand to a device for 
the gravimetric control of a mass flow with a dosing pump which doses the 
material to be doesed from a or into a container located on a balance, 
whereby the delivery force of the dosing pump comprises pulsating 
components; and with a control unit which regulates the delivery force of 
the dosing pump on the basis of the output signal of the balance in such a 
manner that the average time value of the delivery force achieves a set 
theoretical value. 
A method and a device of this type are known from the journal article "Use 
of a Microprocessor-Controlled Dosing Device in Biotechnology" by K. 
Memmert, R. Uhlendorf and C. Wandrey in Chemie-Ingenieur-Technik 59 
(1987), No. 6, pp. 501-504. 
A disadvantage of this known method and of this known device is that fact 
that for a use under sterile conditions, practically only pumps are known 
whose delivery flow is composed of individual delivery impulses. Hose 
pumps and membrane pumps are cited in the above-mentioned article as 
example. This pulsation is not problematic for any applications and it is 
sufficient to maintain the average time value at the set theoretical 
value. However, the pulsating component is problematic for some 
applications, especially in the case of very slight dosing flows, and the 
invention has the problem of indicating a method and a device for 
pulsation-free, gravimetric dosing. 
SUMMARY OF THE INVENTION 
The invention solves this problem in a method for the gravimetric control 
of a mass flow in that the output signal of the balance is digitally 
filtered. The course in time of the mass flow is cyclically estimated from 
the filtered weighing signal. This estimated value of the course in time 
of the mass flow is used to calculate the course in time of an auxiliary 
mass flow which compensates the pulsation of the mass flow of the 
pulsating dosing pump. A rotational speed course is calculated from the 
course in time of the auxiliary mass flow for driving an auxiliary dosing 
pump. 
This is achieved in the device for the gravimetric control of a mass flow 
in that an auxiliary dosing pump is connected in parallel to the dosing 
pump, that the delivery force of the auxiliary dosing pump is regulated by 
the control unit in such a manner that the sum of the delivery forces of 
the two pumps is free of pulsations and that the delivery force of the 
main dosing pump is regulated by the control unit in such a manner that 
the sum of the delivery forces is maintained at the set theoretical value.

DETAILED DESCRIPTION OF THE INVENTION 
The processing container which is to receive the dosing is designated by 
reference numeral 2 in the schematic view of the dosing device in FIG. 1. 
This processing container 2 stands on a balance 1 whose output signal is 
fed to a control unit 3. This control unit 3 regulates dosing peristaltic 
pump 5 via lead 7, which dosing pump delivers the medium to be dosed from 
a storage container 6 into processing container 2. Control unit 3 also 
controls an additional auxiliary dosing pump 4 via lead 8. 
The interplay of these components during dosing results from the flow chart 
in FIG. 2. Processing container 2 and balance 1 are shown in the middle. 
The balance output signal is proportional to the total mass of processing 
container 2 with contents. The change in mass per unit of time and 
therewith the instantaneous inflow of mass is determined by means of 
differentiation in functional module 10. This value is filtered in 
functional module 11 by a deep pass, which suppresses momentary 
disturbances in the output signal of balance 1 and the pulsating 
components in the mass flow. This average value for the inflow of mass is 
compared in functional module 12 with the set theoretical value and, 
depending on the result of this comparison, the speed of peristaltic pump 
5 is increased, lowered or maintained constant. This control loop which 
has just been described and is shown on the left in FIG. 2 is already 
known. 
In addition, FIG. 2 shows a second loop on the right which drives auxiliary 
dosing pump 4. In this loop, the output signal of the balance is first 
filtered in digital filter 13. This digital filter 13 can be e.g. a triple 
polynomial filter, as is described in more detail further below for FIG. 
5. This filtered signal is fed to a non-linear estimator 14 in which the 
course in time of an auxiliary mass flow is estimated in such a manner 
that the sum of the auxiliary mass flow and the mass flow of dosing pump 5 
is time-independent, that is, pulsation-free. Auxiliary dosing pump 4 is 
then driven via pump control 15 in such a manner that it generates the 
estimated auxiliary mass flow. The loop shown on the right in FIG. 2 with 
auxiliary dosing pump 4 therefore serves only to smooth the pulsation of 
dosing pump 5 and the adjustment to the required theoretical value of the 
mass flow to be dosed and the long-time constant maintenance of this 
theoretical value takes place in an unchanged manner by means of the 
control of the speed of dosing pump 5 via the control loop shown on the 
left in FIG. 2. In order to synchronize the rotary motion of auxiliary 
dosing pump 4 with the rotary motion of dosing pump 5, pump control 15 
requires information about the current angular positions of auxiliary 
dosing pump 4 and of dosing pump 5. It obtains this information via leads 
16 and 17. 
FIG. 3 shows an example for the time-dependent course of the delivery force 
of a peristaltic pump. The time t is entered horizontally and the delivery 
force m is entered vertically. A sharp break occurs after an approximately 
constant delivery force during the greatest part of a rotation which break 
can extend to negative values of the delivery force (reversal of the 
direction of delivery) at the point in time at which the squeezing roller 
has concluded the squeezing phase and the hose returns back into its round 
cross-sectional form. The time from t.sub.1 to t.sub.2 corresponds in the 
case of a pump head with two squeezing rollers to one half a rotation of 
the pump head. The average time value of the delivery force is sketched in 
dotted lines in FIG. 3. 
In order to supplement the course of the delivery force of dosing pump 5 
shown in FIG. 3 by means of the delivery force of an auxiliary dosing pump 
4 to a pulsation-free total delivery force, the delivery force of 
auxiliary dosing pump 4 must exhibit the course shown in FIG. 4. This 
course is estimated by estimator 14 (FIG. 2) in accordance with 
mathematical methods from the filtered output signal of balance 1 and 
converted by pump control 15 (FIG. 2) into corresponding adjustment 
commands for the drive of auxiliary dosing pump 4. The drive of auxiliary 
dosing pump 4 can take place e.g. by means of a stepping motor, which 
results in the possibility of a simple control both forwards and 
backwards. Auxiliary dosing pump 4 can also be e.g. a peristaltic pump 
like dosing pump 5. In FIG. 4, the average time value of the delivery 
force m.sub.4 of the auxiliary dosing pump is zero. Auxiliary dosing pump 
4 can therefore always be operated in a range of approximately constant 
delivery force per angle of rotation and does not need to reach the range 
in the vicinity of times t.sub.1 and t.sub. 2 in FIG. 3. As a result 
thereof, the conversion factor between angle of rotation and delivery 
force for auxiliary dosing pump 4 is practically constant and the 
mathematics for the conversion correspondingly simple. This operation of 
auxiliary dosing pump 4 with an average delivery force of zero is 
especially advantageous but not absolutely necessary. 
It is best if estimator 14 receives its information via the course of the 
delivery force of auxiliary dosing pump 4 in that dosing pump 5 stands 
still in a learning phase and auxiliary dosing pump 4 executes a few 
revolutions. Estimator 14 can then obtain the course of the delivery force 
of auxiliary dosing pump 4 from the output signal of balance 1 under 
processing container 2. 
Estimator 14 receives its information via the course of the delivery force 
of dosing pump 5 in that it estimates the course in time of massflow m 
which represents the sum of mass flow m.sub.5 of dosing pump 5 and of 
auxiliary mass flow m.sub.4 of auxiliary dosing pump 4, from the output 
signal of balance 1 under processing container 2. If the auxiliary mass 
flow m.sub.4 of auxiliary dosing pump 4, which can be calculated with the 
information from the learning phase, is subtracted from the estimated mass 
flow m, one has the course in time of mass flow m.sub.5 of dosing pump 5. 
Estimating device 14 supplies the information about the current mass flow m 
with a constant delay conditioned by digital filtration 13 so that he 
control profile for auxiliary dosing pump 4 is not used for compensation 
until the next-following cycle. 
In order to suppress small disturbances in the output signal of balance 1 
which stem e.g. from vibrations or the impact of the dosed medium without 
reducing too greatly the reaction speed of the balance and of the dosing 
control, it is advantageous if only one analog prefiltration with an upper 
limiting frequency of approximately 20 Hz is built into balance 1 and if 
the digital filter 13 shown in FIG. 2 exhibits the filter structure shown 
in FIG. 5. This so-called triple polynomial filter consists of three 
polynomial filters 20, 21 and 22 by means of which the digital output 
signal of balance 1 is freed in accordance with mathematical methods of 
disturbances not suppressed by the analog prefiltration. The output 
signals of the individual polynomial filters are composed via a selector 
device 23 to the final filter signal (filter output y). 
All filters, estimators and control groups shown in the flow chart of FIG. 
2 are combined in FIG. 1 to control unit 3. Essential parts of this 
control unit can be realized e.g. by a microprocessor or a PC. 
A device described above involving a dosing into a processing container can 
naturally also be used in the inverse direction of flow for a 
pulsation-free removal of a substance from a processing container. The 
directions of flow and of rotation of the pumps shown in FIG. 1 are 
reversed in this instance.