Control valve hunts in AUTO but runs smoothly in MANUAL
If a valve cycles in automatic and goes quiet in manual, the feedback path is generating the cycle. What that rules out, what it leaves, and how to finish the diagnosis.
Diagnose a control loop
Start from what the loop is actually doing. Each page works through the causes in the order they usually turn out to be the answer, and says plainly which ones tuning cannot fix.
Valve position oscillates but the process variable barely moves
When the output swings and the process does not follow, the loop is working hard for nothing. The four causes, and how to tell them apart from a trend.
Control valve hunting
A hunting control valve is usually mechanical, not badly tuned. How to tell stiction, oversizing and aggressive tuning apart from the trend, and what to check first.
PID loop oscillating
An oscillating PID loop tells you what is wrong through the shape and period of the cycle. How to read decaying, growing and constant-amplitude oscillation.
Valve stiction symptoms
Stiction produces a distinctive limit cycle: stepped PV against a smoothly ramping output, constant amplitude, gone in manual. How to confirm it.
Control loop will not hold setpoint
A persistent gap between PV and setpoint has a small number of causes: no integral action, output saturation, or a loop that has lost the selection.
Controller output saturated
An output pinned at 0% or 100% means the controller is out of control authority. Why it happens, why windup follows, and what to check first.
Flow loop unstable
Flow loops are fast and noisy, which makes them intolerant of derivative action and of reset set faster than the loop's dead time. What to check.
Overshoot after a setpoint change
Overshoot on setpoint changes but not on load changes points at controller structure rather than tuning. How to tell windup and derivative kick apart.
Sluggish control loop
A slow loop is not always under-tuned. Measurement filtering, drive ramps and output limits all produce sluggishness that tuning cannot fix.
Noisy PV and valve chattering
A chattering valve is usually derivative action amplifying measurement noise. How to tell that apart from real process noise and from an instrument fault.
Pump speed loop hunting
On a VFD-driven loop the drive's own accel and decel ramps are often the dominant lag. What to check before re-tuning a hunting speed loop.
Valve deadband and backlash
Backlash shows as a flat spot on the PV every time the output reverses. How to measure the deadband and what causes it.