Nonlinear MFA Controller
Nonlinear control is one of the biggest challenges in
modern control theory. While linear control system theory
has been well developed, it is the nonlinear control
problems that present the most headaches. Nonlinear
processes are difficult to control because there can
be so many variations of the nonlinear behavior. Traditionally,
a nonlinear process has to be linearized first before
an automatic controller can be effectively applied.
This is typically achieved by adding a reverse nonlinear
function to compensate for the nonlinear behavior so
that the overall process input-output relationship becomes
somewhat linear. It is usually a tedious job to match
the nonlinear curve; and process uncertainties can easily
ruin the effort.
Nonlinear MFA is a general-purpose controller that provides
a more uniform solution to nonlinear control problems.
It is well suited for nonlinear processes or processes
with nonlinear sensors, actuators, and other elements.
A flow or high-pressure loop is a typical nonlinear
process that can cause the actuator to lose its authority
in different operating conditions. Inevitable wear and
tear on a valve typically makes a linear valve nonlinear.
The dissolved oxygen in a bio-tech micro reactor to
cultivate cells is another nonlinear process example.
As cells grow, they suddenly start to consume much more
oxygen. Since the number of bio-tech experiments is
huge and the types of cells can vary significantly,
applying the traditional nonlinear-characterization
method to deal with this problem can be difficult and
costly. The general-purpose Nonlinear MFA controller
is the appropriate choice for this application.
Nonlinear MFA Configuration
In addition to the parameters used in SISO MFA including
Sample Interval, Time Constant, Controller Gain, and
Acting Type, the Nonlinear MFA has an extra parameter
to enter: the Process Nonlinearity Factor. As shown
in the following graph, the graph on the menu shows
how severe the nonlinear behavior is between the Process
Input and Process Output.
Nonlinear MFA controller configuration menu
The Process Linearity Factor is a number between 0
and 10. A 10 represents an extremely non-linear process
while a 0 represents a linear process. Notice that the
graph shows a nonlinear curve marked with a 10 on both
upper and lower positions. This means, Nonlinear MFA
does not care what the nonlinear characteristics are
for this process. For instance, the valve can be either
"fast open" or "fast close" as represented
by these two convex and concave curves.
When using Nonlinear MFA, the specific characteristics
of the nonlinear curve are not a concern. The curve
can be concave, convex, or S-shaped. Simply advise the
controller whether the process is extremely nonlinear
(enter a 9 or 10), quite nonlinear (enter a 5 or 6),
or somewhat nonlinear (enter an 1 or 2). The Nonlinear
MFA is smart enough to handle the rest.
Simulations and real applications show that Nonlinear
MFA can tightly control a nonlinear process within its
full control range, even when the process gain changes
hundreds of times. In a Nonlinear MFA, there is no linearization
calculation or process model. The MFA controller gain
Kc is simply set at its nominal point and not retuned.
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