We will go through a whole bundle of tuning methods. We only need to “pick” three numbers for a PID controller, but this is one of the most confusing parts of learning control. Different tuning techniques give similar but not identical results. There are no “best” or “absolutely correct” answers. The methods all have pros and cons and, working together, they complement each other. We need to make proper selection and sound judgment – very true to the act (and art) of design.

What Are We Up to?

• Tuning a controller with empirical relations

• Tuning a controller with internal model control relations

Tuning Controllers with Empirical Relations

Let's presume that we have selected the valves and the transducers and even installed a controller. We now need to determine the controller settings – a practice that is called tuning a controller. Trial-and-error tuning can be extremely time consuming (and dumb!), to the extent that it may not be done. A large distillation column can take hours to reach steady state. A chemical reactor may not reach steady state at all if you have reactor “runaway.” Some systems are unstable at high and low feedback gains; they are stable only in some intermediate range. These are reasons why we have to go through all the theories to learn how to design and tune a controller with well-educated (or so we hope) guesses.