Loops and Tuning
Loops and Tuning tests loop checks, controller modes, and systematic tuning. Expect open-loop and closed-loop methods, dead time, process gain, time constant, oscillation diagnosis, and anti-windup.
Core rules
- Proportional action responds to current error; integral removes offset; derivative reacts to rate of change and amplifies noise.
- Increase proportional gain to speed response, but too much causes oscillation. Integral too fast causes cycling; derivative too large causes noisy output.
- Dead time limits achievable speed. A loop with long dead time needs detuning, not aggressive gain.
- In the Ziegler–Nichols closed-loop method, find ultimate gain Ku and ultimate period Pu with integral and derivative off. For PID: Kc = 0.6Ku, Ti = 0.5Pu, Td = 0.125Pu.
- Use anti-windup and output limits; put the controller in manual or use bumpless transfer for maintenance.
Worked example
A loop oscillates continuously with integral and derivative off. The technician finds Ku = 4 and Pu = 20 s. Ziegler–Nichols PID settings are Kc = 0.6 × 4 = 2.4, Ti = 0.5 × 20 = 10 s, and Td = 0.125 × 20 = 2.5 s. These are starting values. If the response is too oscillatory or the valve is noisy, reduce Kc and increase Ti; derivative may be reduced or removed for a noisy flow loop. The reasoning is that Ku and Pu describe the process at the stability limit, and the formulas back away from that limit.
Traps
Tuning with a sticking valve, leaving a controller in manual during a step test, applying derivative to a noisy transmitter, and ignoring dead time are common. Also do not confuse controller gain with proportional band: PB% = 100/Kc for a gain-only controller.
Key points
- Know the effect of P, I, and D.
- Dead time limits loop speed.
- Use Ku and Pu for ZN starting values.
- Detune noisy or long-dead-time loops.
- Prevent integral windup.