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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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