In controller tuning, what best describes the Ziegler–Nichols method and one of its common uses?

Introduction / Context:
The Ziegler–Nichols method is a classic heuristic for tuning feedback controllers. It provides rules to set proportional, integral, and derivative parameters based on either the process reaction curve or the ultimate gain and period obtained from sustained oscillations.

Given Data / Assumptions:

• We are tuning P/PI/PID controllers, not redesigning process dynamics.
• The method outputs numerical settings (Kp, Ti, Td) from empirical measurements.
• Goal is to achieve a standard compromise between responsiveness and robustness.

Concept / Approach:
Two familiar variants exist: the step-response (reaction curve) method and the ultimate gain method. Both yield formula-based tables to compute controller gains. These rules can be applied to PI controllers by omitting D and using the prescribed relationships for Kp and Ti.

Step-by-Step Solution:
1) Identify what Ziegler–Nichols is: a tuning heuristic, not a new control structure. 2) Recall that it provides gains for P, PI, or PID from measured process data. 3) Conclude that it can determine PI gains specifically, satisfying the prompt. 4) Evaluate other options: graphical-only is inaccurate; it is formula-rule based from measured parameters.

Verification / Alternative check:
Tuning tables list Kp and Ti for PI when given ultimate gain Ku and period Pu (or reaction curve constants), validating option B.

Why Other Options Are Wrong:
Option A: Not purely graphical; numeric rules dominate usage.
Option C: It tunes existing controllers; it is not a control method by itself.
Option D: Fails because A is incorrect.
Option E: Invalid because B is correct.

Common Pitfalls:
Applying Ziegler–Nichols blindly to noisy or integrating processes; always confirm stability margins and refine gains afterward.

Final Answer:
A procedure that can determine gain factors for a PI loop