Process Control
Scope
Loop hardware, PID behavior, tuning, and control strategy selection.
Concepts to own
- Loop elements: sensor, transmitter, controller, final element, process. Total loop gain is the product of the individual gains.
- PID: u = Kc[e + (1/Ti)∫e dt + Td de/dt]. Proportional band PB% = 100/Kc for a dimensionless gain. Proportional-only control leaves offset; integral removes offset but can wind up; derivative anticipates but amplifies noise.
- Process dynamics: first-order lag, dead time, second order. Dead time dominates controllability.
- Tuning: Ziegler-Nichols, Cohen-Coon, IMC/lambda tuning; quarter-amplitude decay as an acceptance criterion.
- Stability: Bode and Nyquist plots, gain and phase margin. The loop is unstable if the open-loop gain exceeds 1 at 180° of phase lag.
- Strategies: feedback, feedforward, cascade, ratio, split range, selective control.
- Valves: air-to-open is fail-closed and air-to-close is fail-open; equal-percentage versus linear characteristics; cavitation and flashing.
Traps
- Getting controller action backwards for a fail-safe valve.
- Confusing reset rate (repeats per minute) with reset time (minutes per repeat).
- Forgetting that dead time adds pure phase lag without changing gain.
Worked example
A proportional controller with Kc = 5 has a 20% proportional band. With a process gain of 2 and unity transmitter and valve gains, open-loop gain is 10, so a 1% setpoint step leaves a steady-state offset of about 1/(1+10) = 0.09% of span.
Rule of thumb: start with low gain, tune integral next, and add derivative only when the signal is clean.