NEW BRUNSWICK SCIENTIFIC BioFlo 415, Manual To Operations

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New Brunswick Scientific User’s Guide
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Cascades are control schemes in which the Output % of one process control loop influences
the setpoint of one or more other loops. In other words, it uses feedback from one parameter
to influence others. In New Brunswick Scientific’s BioFlo 415 bioreactors, the output % value
is mathematically determined by evaluating the error between measured present values and
desired setpoints, and integrating these values into a PID-based control algorithm.
The BioFlo 415’s RPC controller allows cascading from any loop to as many as five other
loops. DO and pH are the most commonly
cascaded-from
loops; oxygen and nitrogen
commonly receive the cascade from DO, and CO
2
and Base pump usually receive the cascade
from pH, altering their respective setpoints to correct errors in DO. Systems with 2 or more
TMFCs require the writing of cascades for pH and DO control, where systems with 1 TMFC
can be set to automatically adjust the mixtures of the sparge gas depending on the need to
make adjustments.
When more than one loop is configured as the recipient of a cascaded loop, they may respond
in parallel, at the same time, or in series, one after the other, depending on how the cascade
has been set up. Cascades set up to run in series generally give more predictable control
responses. Sometimes a small region of overlap, where two loop setpoints vary
simultaneously, is used to smooth the transition from one loop to another.
11.1 Creating a Cascade
Figure 18a below shows the headers from the CASCADE screen (set to “Cascade From
DO”), with an explanation of the settings those headers represent:
Figure18a: Cascade Screen
End Setpoint
is the loop value the user
defines as the maximum allowable value
when the
DO End Out%
is reached.
Typically this value will also be the same
as the system’s maximum allowable
setpoint for the loop.
@DO End Out%
represents the DO output % where the
user wants the cascade to stop. This value can be set to any
integer from 0 to 100% as long as it is greater than the Start
Out%. The greater it is than the Start Out%, the smoother
the increase in setpoints.
@DO Start Out%
represents the DO output % value
where the user wants the cascade to begin. When this
output % is reached, the setpoint of the Cascade To loop
will change to the value entered as Start Setpoint (
see
below
). The current DO output % can be found on the
SUMMARY
screen at the intersection of the Output%
column and the DO loop row. This value is calculated
by using the integrated PI values. It is essentially a
mathematical calculation of setpoint “error” from PV
(current process value), “error” meaning any readings
that are above or below the programmed setpoint. As
the “error” discrepancy increases, or as the duration of
such a discrepancy remaining static increases, the
Output% also increases.
Start Setpoint
is the loop value the
user defines for the system to be at
when initial DO Start Out% is
reached. Typically this value will
be close to the normal operating
setpoint.
See important NOTE on the next page.