Freedom to Optimize PID Controller Structure – Part 1

There are 8 different choices for controller structures in a modern DCS. Which one is the best for your application and does it take a download to change it?

One choice is for integral only controller (no proportional or derivative action). The most common I-only control is a valve position controller (VPC) which is trying to slowly optimize the position of a valve to save energy or to provide better resolution and rangeability. For example, the furthest open air feed valve position for trains of reactors becomes the PV of a VPC whose set point is the maximum useable throttle position (e.g. 60% for a rotary valve) and whose output sets the air compressor speed or guide vanes. Then there is the classic big and small valve setup where the VPC keeps the small valve in its mid throttle range for fine adjustments by making coarse adjustments to the big valve. Slow I-only tuning is used to minimize the interaction between the VPC and the existing process loops. The VPC is difficult tune and the problem is better solved with model predictive control (see my Advanced Application Note 2).

Proportional-derivative controllers are sometimes used on highly exothermic reactors with a runaway response or for batch temperature or batch pH control where the response in integrating and in only one direction. For example, when heating up a batch with no vaporization or heat loss, the temperature will only rise and cannot drop. Similarly, when adding a base to a batch with no reagent consumption or escape, the pH will only rise and cannot drop. Here integral action causes overshoot from which there is no recovery. Pulse width modulation of a proportional only pH controller output has been successfully used to mimic titration in a batch vessel which can be thought of as very large beaker.

You can get proportional-integral control by simply setting the rate to zero. This still leaves many choices as to structure most of which involve whether you want proportional or derivative action on error.

Since a change in structure require a download, once you have decided you want both proportional and integral action, by choosing the “two degrees of freedom controller” you can tune or write to the set point weights online. Furthermore the transition of the weights can be smooth because you can change them to any value between 0 and 1 whereas a change in structure makes a discrete switch between 0 and 1.

Dinner is waiting on me so we will have to wait till next week for more about weights.