Conventional controllers like PID and many advanced
control methods are useful to control linear processes.
In practice, most processes are nonlinear. If a process
is only a little nonlinear, it can be treated as a linear
process. If a process is severely nonlinear, it can
be extremely difficult to control.
CyboSoft, a NI Alliance Partner in Rancho Cordova, California
offers an innovative Model-Free Adaptive (MFA) control
solution for controlling severely nonlinear processes
without having to build process models and retune controller
parameters. It also eliminates the need of traditional
nonlinear characterization allowing the user to easily
configure and launch Nonlinear MFA controllers.
Leveraging National Instruments LabVIEW Real-Time development
environment, the embedded Nonlinear MFA controller can
run in a PC or be downloaded into NI's FieldPoint distributed
I/O or PXI instruments.
Why is Nonlinear Control Difficult?
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 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 Model-Free Adaptive (MFA) Controller
Nonlinear MFA is a general-purpose controller that provides
a more uniform solution to nonlinear control problems.
It is well suited for controlling nonlinear processes
or processes with nonlinear sensors, actuators, and
Nonlinear MFA vs. PID Controller
The following trends in LabVIEW show the Nonlinear MFA
controller (top) is able to control a severely nonlinear
process throughout the entire operating range (linear
and nonlinear). PID works fine in the linear range but
fails to control in the nonlinear range.
Nonlinear MFA Controller Configuration in LabVIEW
Nonlinear MFA has only a few parameters to configure
including Controller Gain, Kc, Process Time Constant,
and Process Nonlinearity Factor. As shown in the following
bitmap, the graph on the menu shows how severe the nonlinear
behavior is between the process input and process output.
The Process Linearity Factor is a number between 0 and
10. A 10 represents an extremely non-linear process,
and 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 a 1 or 2). The Nonlinear
MFA is smart enough to handle the rest.
Nonlinear Control Application Examples
A flow or 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 can also make a linear valve nonlinear.
The variable frequency drive (VFD) has become a popular
flow regulator because it saves energy. However because
of its nonlinear nature, it adds a nonlinear component
to the process.
The dissolved oxygen used to cultivate cells in a bio-tech
micro reactor is another example of a nonlinear process.
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 to be grown can vary significantly,
it is difficult and costly to apply traditional nonlinear-characterization
in solving this nonlinear problem.
Compact FieldPoint Based Nonlinear MFA Control
National Instruments Compact FieldPoint distributed
I/O products provide an ideal hardware platform to implement
a nonlinear control system. The following table lists
all the components and part numbers required. With both
options, at least 2 MFA nonlinear controllers can be
implemented. NI's FieldPoint products can be used to
implement this system as well. See the "pH Control"
solution for detailed product info and part numbers.
Option 1. Remote I/O Option (MFA running
|NI Part Number
||FP-1000 RS232/RS485 Network Interface for FieldPoint
||FP-TB-1 Universal Terminal Base
||FP-AIO-600 8 Channel Combination Analog Input/Output
||PS-1 Power Supply 120 VAC input, 20vdc 0.8 Amps
Option 2. Embedded Real-Time Option. (MFA
running inside FieldPoint)
|NI Part Number
||cFP-2010, LabVIEW Real-Time/Ethernet Network Module
||cFP-BP-4, 4 Slot Backplane
||cFP-DI-300, Digital Input Module
||cFP-AIO-600, 12 bit Analog Input/Output Module
||PS-3 Power Supply 120vac Input, 13.8VDC 4 amp
CyboSoft's Embedded MFA Control Software in
The following table lists the required embedded MFA
control software to be ordered from CyboSoft. A SISO
(single-input-single-output) MFA controller can be configured
as one of the following MFA controllers.
• SISO MFA to replace PID to eliminate manual
• Nonlinear MFA to control nonlinear processes,
• MFA pH controller to control pH processes,
• Feedforward MFA to deal with measurable disturbances,
• Anti-delay MFA to control processes with large
time delays, and
• Robust MFA to protect the process variable from
running outside certain boundaries.
|CyboSoft Part Number
||1 SISO MFA Controller
||2 SISO MFA Controller
||3 SISO MFA Controller
||4 SISO MFA Controller
||5 SISO MFA Controller
||6 SISO MFA Controller
||7 SISO MFA Controller
||8 SISO MFA Controller
||9 SISO MFA Controller
||10 SISO MFA Controller
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.
This general-purpose, powerful solution can be easily
implemented. No special knowledge or experience with
advanced control, nonlinear characterization, or process
modeling is required. Tough nonlinear loops can be controlled
using NI's FieldPoint and CyboSoft's embedded Nonlinear
MFA controllers resulting in smoother operations, longer
actuator life, higher product quality and production
efficiency, and reduced waste in energy and material.