Anti-reset windup
Controller logic that stops integral action accumulating while the output is at a limit.
Control loop glossary
The vocabulary as it is actually used on a plant.
Bad actor loop
A loop that consistently performs poorly and consumes disproportionate attention.
Bump test
Making a deliberate step change to a loop and observing the response.
Bumpless transfer
Switching a controller between manual and automatic without the output jumping.
Cascade control
Two controllers in series, where the outer loop sets the setpoint of a faster inner loop.
Cohen-Coon tuning
An open-loop reaction-curve tuning method from the same era as Ziegler-Nichols, fitted for setpoint response rather than load.
Dead time
The delay between a change at the output and the first sign of a response in the measurement.
Deadband
A range of signal change that produces no movement at the valve at all.
Derivative on the process variable
Taking derivative action on the measurement rather than the error, so a setpoint change does not produce a derivative kick.
Feedforward control
Acting on a measured disturbance directly, rather than waiting for the process to respond to it.
First-order plus dead time (FOPDT)
A three-number description of a process — gain, time constant and dead time — that most tuning rules start from.
Gain scheduling
Changing the controller tuning automatically as a function of operating point.
Hunting
A control loop cycling continuously around setpoint instead of settling.
Hysteresis
A difference in valve position for the same signal depending on which direction it was approached from.
Integral windup
Integral action accumulating while the output is saturated, causing overshoot when the loop recovers.
Internal model control (IMC)
A model-based tuning framework that reduces the whole design to one knob — the chosen closed-loop time constant.
Lambda tuning
A tuning method that sets the closed-loop response time to a chosen value rather than maximising speed.
Limit cycle
A sustained oscillation of constant amplitude caused by a nonlinearity rather than by tuning.
Measurement filtering
Smoothing the process variable before the controller sees it, trading noise for added lag.
Offset
A steady difference between the process variable and setpoint that does not close.
Override (selector) control
Two or more controllers sharing one output through a selector, so the limit that matters most wins.
Process gain
How much the measurement moves for a given change in controller output.
Proportional band
The error, as a percentage of range, that drives the controller output across its full span. PB% = 100 / gain; raising the band weakens proportional action.
Quarter amplitude damping
A traditional tuning target where each oscillation peak is a quarter the size of the one before.
Ratio control
Two flows held in a fixed proportion, where one flow's setpoint is a ratio times the other flow's measurement.
Sample time
How often the controller executes, a delay that shows up in the loop as additional dead time.
Shrink and swell
The drum level responding the wrong way at first, because pressure changes flash or collapse the steam bubbles in the water.
Smith predictor
A dead-time compensation structure that removes long dead time from the loop by predicting the measurement.
Split-range control
One controller output divided between two valves that share the range between them.
Stiction
Static friction that stops a valve stem moving until the signal change is large enough to break it free.
Three-element control
The boiler drum level scheme that combines drum level, steam flow and feedwater flow.
Time constant
How quickly a process responds once it starts responding — the lag that shapes how fast a loop can settle.
Ultimate gain
The controller gain that just drives a closed loop into a sustained oscillation of constant amplitude.
Ultimate period
The time for one complete cycle of the sustained oscillation used by the closed-loop Ziegler-Nichols method.
Velocity (incremental) PID
A PID form whose output is a change in output, not the output position itself.