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SISO MFA Controller

A single-loop MFA control system includes a single-input-single-output (SISO) process, a SISO MFA controller, and a feedback loop.

Single-loop MFA control system

The control objective is for the controller to produce an output u(t) to force the process variable y(t) to track the given trajectory of its setpoint r(t) under variations of setpoint, disturbances, and process dynamics. In other words, the task of the MFA controller is to minimize the error e(t) in an online fashion, where e(t) is the difference between the setpoint r(t) and the process variable y(t). The minimization of error e(t) is achieved by (i) the regulatory control capability of the MFA controller, and (ii) the adjustment of the MFA controller weighting factors that allow the controller to deal with process dynamic changes, disturbances, and other uncertainties.

MFA Controller Architecture

This graph illustrates the core architec¬ture of a SISO MFA controller. Used as a key component, a multilayer perceptron neural network consists of one input layer, one hidden layer with N neurons, and one output layer with one neuron.

Architecture of a SISO MFA controller

Within the neural network there is a group of weighting factors (wij and hi) that can be updated as needed to vary the behavior of the controller. The algorithm for updating the weighting factors is based on the goal of minimizing the error between the setpoint and process variable. Since this effort is the same as the control objective, the adaptation of the weighting factors can assist the controller in minimizing the error while process dynamics are changing. In addition, the artificial neural network based MFA controller "remembers" a portion of the process data providing valuable information for the process dynamics. In comparison, a digital version of the PID controller remembers only the current and previous two samples. In this regard, PID has almost no memory and MFA possesses the memory that is essential to a "smart" controller.

A Comparison of MFA and PID

Most industrial processes are still being controlled by PID (proportional-integral-derivative) controllers. PID is a simple general-purpose automatic controller that is useful for controlling simple processes. However, PID has major problems in controlling complex systems and it also requires frequent manual tuning of its parameters when the process dynamics change. The performance of MFA (top) and PID (bottom) controllers is compared in the following graph to show how MFA adapts when process dynamics change.

Comparison of MFA and PID

Starting from the same oscillating control condition, the system will continue to oscillate under PID control, while the MFA system will quickly adapt to an excellent control condition. If both controllers start from a sluggish situation, MFA will control the process faster and better while PID will remain sluggish.

MFA Control System Requirements

As a feedback control system, MFA requires that the process be (1) controllable, (2) open-loop stable, and (3) either direct or reverse acting (process does not change signs). If the process is not controllable, improvement of the process structure or its variable pairing is required. If the process is not open-loop stable, it is always a good practice to stabilize it first. However, for certain simple open-loop unstable processes such as a non self-regulating level loop, no special treatment is required when using MFA. If a process changes its sign within its operating range, special MFA controllers are required.

SISO MFA Configuration

A SISO MFA controller has only a few parameters to configure: (1) Sample Interval, Ts - the interval between two samples or calculations in seconds. A high speed MFA controller can run at a 1 million second rate; (2) Controller Gain, Kc - use of a default value is recommended, (3) Time Constant, Tc - a rough estimate of the process Time Constant in seconds; and (4) Acting Type - direct or reverse acting of the process. If the process input increases and then its output increases, it is direct acting, and vice versa. However, MFA controllers embedded in various platforms always use the vendor’s definition. Sometimes, controller acting type is used, which is different than the process acting type.

According to the principles in the information theory, it is required that the Sample Interval be smaller than or equal to one third of the Time Constant. That is, Ts<=1/3Tc, where Ts is the Sample Interval, and Tc is the Time Constant.

MFA does not require process identification and is not a dynamic modeling based controller. Once the configuration is done, MFA can be launched at any time, and it will control the process immediately. MFA controllers can be switched between automatic and manual at any time. No specific bumpless transfer procedure is required.

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