Bronkhorst®
Instruction Manual mini CORI-FLOW ML120
9.17.097G
49
5
Batch dosing with CORI-FILL™
This section describes the steps to be taken in order to prepare the instrument for batch dosing. Once the instrument is
configured properly, the
CORI-FILL™
technology controls the dosing process, regardless of the fieldbus system the
instrument is installed in. The digital parameters can be monitored (or even controlled) in the usual way. To this end, each
configuration step is supplemented with a list of the digital parameters involved.
5.1
Dosing principle
At the start of each batch, the flow is initiated by opening a
valve (or starting a pump). The flow meter measures and
totalizes the flow, continuously calculating the current batch
size (Batch size = Flow x Time, see diagram). As soon as the
delivered batch size reaches the configured batch size, the
flow is stopped.
Depending on the importance of accuracy as opposed to
dosing speed, proportional control or on/off control is
advised. Proportional control using a control valve or (gear)
pump is the common method if batch size accuracy is of
critical importance. On/off control using a shut-off valve is
typically used in situations where dosing speed is more
important.
5.1.1
Proportional control
With proportional control, each batch starts with a linear
phase, where dosing is done with the highest possible flow
rate (flow setpoint = 100%). When the configured batch size
is being approached, the flow can be reduced, by gradually
closing the valve (or reducing pump speed), until the flow
rate reaches zero at the end of the batch (control gain
phase).
This proportional optimization method assures accurate
dosing from the very first batch, with minimal overshoot.
To compensate for possible dosing speed reduction by the
control gain phase, the controller response performance can
be enhanced somewhat, by optimizing the PID controller
characteristics of the instrument.
5.1.2
Preventing overshoot
Due to mechanical restrictions, the first dosage in the
process will always have some overshoot (delivered batch
larger than configured size). This goes for both on/off
control and proportional control (even though the control
gain phase of the proportional batch cycle assures minimal
batch size overshoot).
CORI-FILL™
implements an overrun correction algorithm to
prevent batch size overshoot. This automatic self-learning
function optimizes the size of each new batch, based on the
previously delivered one, until the overshoot is eliminated
(typically after 3 batches, assuming the inlet pressure is
stable).
