On the last page of the article ["Understanding PID Control," CE, June '00, pp. 53-58], the author stated that proportional-only control systems are incapable of hunting. This is only true if the process is a first-order time constant. (A single R-C time constant is a first-order delay.) If the process being controlled is second-order or greater, proportional-only control can result in instability when the gain is too high. Very few industrial processes are first-order only.

Dudley Nye, P.E., Nye Engineering Inc.

[Author's response: You're right. Only first-order processes with proportional-only (P) controllers are entirely immune to closed loop instability. Relative to PI and PID controllers, however, P-only controllers can afford to apply much larger gains, even to higher order processes. That's the point I should have made. Thanks you for pointing out my error.

Vance VanDoren, consulting editor]