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Anti-delay MFA Controller

Many processes have large time delays due to the delay in the transformation of heat, materials, and signals. No matter what control action is taken, its effect may not be measurable for a period of time. This is equivalent to disabling the feedback, where feedback information is essential to automatic control.

If a PID is used to control a process with significant time delays, the controller output will keep growing during the delay time and cause a large overshoot or even make the system unstable. Typically, a PID has to be de-tuned significantly in order to stay in automatic, but this action sacrifices control performance. Generally speaking, a PID works for the process if its Tau-T Ratio (delay time/time constant) is smaller than 1. For a process with a large Tau-T Ratio, PID must be de-tuned, and when a controller is de-tuned, poor and sluggish control performance occurs. Smith Predictor is a useful control scheme to deal with processes with large time delays. However, a precise process model is usually required to construct a Smith Predictor. Otherwise, its performance may not be satisfactory.

The following graph shows a block diagram for a SISO Anti-delay MFA control system with an Anti-delay MFA controller and a process with large time delays.

Anti-delay MFA control system

A special delay predictor is designed to produce a dynamic signal yc(t) to replace the process variable y(t) as the feedback signal. The idea here is to produce an e(t) signal for the controller and let it "feel" its control action without much delay so that it will keep producing proper control signals. In other words, this artificial dynamic signal yc(t) is able to keep the feedback loop working even when there is a large time delay. Since MFA adapts, the delay predictor can be designed in a simple form.

Compared to a Smith Predictor, the Anti-delay MFA does not need a precise process model. It only needs an estimated delay time as the basic information for its delay predictor. If the estimated delay time has a mismatch with the actual process delay time, the controller is adaptive and robust enough to deal with the difference. Typically, satisfactory performance is achieved in a situation where the delay time is 2 to 5 times larger or smaller than the actual delay time. In addition, there is no real limitation on the size of the process Tau-T Ratio.

The Anti-delay MFA is especially useful in controlling process quality variables since a quality variable is typically measured after it travels to a certain point, cools off, or forms its shape. This controller makes it possible for process industries to achieve Six Sigma or zero defects quality control objectives.

In a semi-continuous production environment, the process line speed may change as many as 100 times or more causing the delay time to change on a similar scale. Since the line speed is measurable, the delay time can be easily calculated and provided to the Anti-delay MFA controller in real-time. In this way, the control performance will not degrade even during large line speed changes.

On the other hand, if the delay time of a process changes on a scale of more than 5 times and the delay time information cannot be provided to the controller, the Time-varying MFA controller will be more suitable.



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