Process Control and Instrumentation
Process control covers dynamics and instrumentation. Key topics: open-loop vs closed-loop control; PID control (proportional, integral, derivative actions); tuning methods (Ziegler-Nichols, Cohen-Coon); first-order plus dead time (FOPDT) models; transfer functions and Laplace transforms; stability (Routh-Hurwitz, Bode and Nyquist criteria); feedback vs feedforward; cascade control; and instrument selection (transmitters, valves, sensors).
Traps: confusing gain and time constant; forgetting that integral action eliminates steady-state offset but can cause instability; and misidentifying the manipulated variable vs controlled variable. Questions on valve fail-safe action often test whether you know that fail-closed vs fail-open depends on the process safety requirement.
Worked example (rule of thumb): for a FOPDT process G(s) = K·e^(−θs)/(τs+1), the Ziegler-Nichols ultimate gain method uses the ultimate gain K_u and ultimate period P_u from a closed-loop test; PI settings are K_c = 0.45K_u and τ_I = P_u/1.2. For a quick sanity check, if the dead time θ is large relative to τ, the process is hard to control and cascade or feedforward may be needed.