MFA vs PID Control Comparison

PID Control

Most industrial processes are still being controlled manually or by 60-year-old PID controllers. PID is a simple general-purpose automatic controller that is useful for controlling simple processes. However, PID has significant limitations:

1. PID works for the process that is basically linear and time-invariant and cannot effectively control complex processes that are nonlinear, time-variant, coupled, and have large time delays, major disturbances, and uncertainties. Industrial processes with changing fuels and operating conditions are complex processes for which PID control is insufficient.

2. PID parameters have to be tuned properly. If the process dynamics vary due to fuel changes or load changes, PID needs to be re-tuned. Tuning PID is often a frustrating and time-consuming experience.

3. PID is a fixed controller, which cannot be used as the core for a smart control system.

PID Self-Tuning Methods

To deal with the PID tuning problems, PID auto-tuning and self-tuning have been developed. These special PIDs work well in some cases, but they also have some fundamental problems.

If the self-tuning is model-based, finding and keeping a good process model for re-tuning the PID is a major challenge. Insertion of a test signal into the process is often required. Since this will cause a bump to the process, operators find model-based PID self-tuning awkward to use.

If the self-tuning is rule-based, it is difficult to distinguish between the effects of load disturbances and genuine changes in the process dynamics. The controller may thus overreact to a disturbance and create an unnecessary adaptation transition. Also, the reliability of the tuning may be questionable since there are no mature stability criteria available for rule-based systems.

Most importantly, if the PID controller is unable to deal with the complex process, no matter how it is tuned, the system will not work. For this reason, self-tuning or auto-tuning PIDs are not good candidates for a smart control system.

Comparison of MFA and PID

The adaptive capability of MFA is illustrated when comparing with a PID. The performance of MFA (top) and PID (bottom) is compared to show how MFA adapts when process dynamics change.

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. When the setpoint is changed again, MFA does not oscillate anymore. If both controllers start from a sluggish situation, MFA will control the process faster and better while PID will remain sluggish.