McGoff-Bethune. Yokogawa Fuzzy Logic Control.

Fuzzy-Based Super Control

When Yokogawa introduced Super Control to the Industrial Automation Industry in 1989, the mention of "Fuzzy" Logic was sure to elicit blank looks and bad jokes, but not much understanding. Only after customers experimented with Yokogawa controllers and observed the dramatic improvement in control with their own applications did the "Fuzzy" jokes subside and real admiration begin.

Fuzzy Logic is the generic name given to non-crisp or human like thought. Crisp thought is Yes or No, 1 or 0, Black or White. This is the way computers or machines have traditionally solved problems. If you have ever tried to explain to your children why they should do something or even give someone directions, you realize, we do not excel in crisp thought. Fuzzy Logic can be applied in many ways, so do not be confused, Fuzzy Logic by itself is no panacea. Yokogawa's Super Control emulates a human's expert thought process to get to setpoint, without overshoot. That's it!

Yokogawa's Super Control eliminates the compromises required by PID controllers. Conventional PID controllers can be tuned to be sensitive to process upsets and recover quickly. Unfortunately, systems tuned this way tend to overshoot badly on start-up, setpoint change, or even recovery from major disturbances. Many people tune controllers to reduce overshoot, which results in "hunting around" setpoint and numbness to upsets or disturbances along with slow response. Most people compromise tuning parameters which results in mediocre control. There is no choice. A conventional controller cannot be tuned to perform perfectly in all these situations, it is fundamentally impossible. To understand how Yokogawa applies Fuzzy Logic to a controller, compare the two (2) block diagrams (left):

Super Control has two (2) basic parts, the "Setpoint Modifier" and the "Setpoint Selector." The setpoint modifier models your process and functions as an "Expert Operator." It looks at a knowledge base about your process, recognizing the limitations of PID control, your systems dynamics, non-linearity of your process (including load changes) and tricks the systems into performing perfectly by feeding artificial target setpoints into the PID block. Results can be expected as shown above.

One common myth about Yokogawa Super Control is that it is a form of continuous automatic tuning. It does not change PID terms. Continuous tuning is initiated by a disturbance or error. Similar to the difference between anti-lock brakes and airbags. Super Control proactively keeps you from getting into trouble while continuous tuning keeps you from killing yourself most of the time.

Super Control has proven itself thousands upon thousands of times in all types of difficult applications. This Super Control approach warrants consideration.

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