Direct Digital Controls and Lab Practice Test

In this method, you start by applying proportional control to the process and gradually increasing the proportional gain until the closed-loop output shows sustained oscillations. The gain at that threshold is called the ultimate gain, and the period of those oscillations is the ultimate period. These two values—ultimate gain and ultimate period—are then used with the Ziegler–Nichols formulas to set the PID (or PI) parameters. This starting point matters because it captures the system’s critical dynamic behavior at the verge of instability, providing real, model-free measurements of how fast the process responds and how much gain it can tolerate. The other choices don’t fit because Ziegler–Nichols relies on this P-only test to obtain Ku and Pu first, rather than broad trial-and-error, model-based optimization, or starting with a fast PI controller with zero derivative.