Lecture Notes 5
Lecture 5 - Proportional-Only Control
A FOPDT model can be used to obtain initial tuning constants for a Proportional-Integral-Derivative (PID) controller.
For today's lecture, we'll cover the Proportional-only controller (simplest form). Two following lectures will cover the Integral and Derivative portions of PID control.
Discussion Questions
Test the lecture's ideas before or after class - commit to an answer first, then check it with a classmate or an AI tutor.
- A P-only controller on a heated tank never quite reaches the new setpoint. Is that a tuning mistake, a broken controller, or expected behavior? What single equation defends your answer?
- Doubling Kc cuts the offset roughly in half. Why not just keep doubling it? Name two different things that go wrong, and which one appears first on a process with dead time.
- A level controller on a surge tank is deliberately left P-only with modest gain. Why might zero offset be the wrong objective for that tank?
- For the FOPDT parameters Kp, taup, thetap: which one most limits how aggressive Kc can be, and what is the physical reason?
"Quiz me with 4 questions, one at a time, on proportional-only control: the controller equation u = u-bar + Kc e, why offset exists at steady state and the formula offset = Delta-SP/(1+Kc Kp), how the controller action (sign of Kc) is chosen from the process gain, and how the FOPDT parameters set a first tuning for Kc. Grade my answers and list my misconceptions."
App: See offset live in the Tank Level Control Studio - run P-only control at two gains, read the offset from the plot, and compare it with the offset formula's prediction.
