Integral windup refers to the situation in a PID controller

PID controller

A proportional–integral–derivative controller is a generic control loop feedback mechanism widely used in industrial control systems – a PID is the most commonly used feedback controller. A PID controller calculates an "error" value as the difference between a measured process variable and a...

 where a large change in setpoint occurs (say a positive change) and the integral terms accumulates a significant error during the rise (windup), thus overshooting

Overshoot (signal)

In signal processing, control theory, electronics, and mathematics, overshoot is when a signal or function exceeds its target. It arises especially in the step response of bandlimited systems such as low-pass filters...

 and continuing to increase as this accumulated error is unwound (offset by errors in the other direction). The specific problem is the excess overshooting.

Solutions

This problem can be addressed by:

• Initialising the controller integral to a desired value
• Increasing the setpoint in a suitable ramp
• Disabling the integral function until the to-be-controlled process variable
  Process variable
  A process variable, process value or process parameter is the current status of a process under control. An example of this would be the temperature of a furnace. The current temperature is called the process variable, while the desired temperature is known as the set-point.Measurement of process...
  
   (PV) has entered the controllable region
• Limiting the time period over which the integral error is calculated
• Preventing the integral term from accumulating above or below pre-determined bounds

Occurrence

Integral windup particularly occurs as a limitation of physical systems, compared with ideal systems, due to the ideal output being physically impossible (process saturation: the output of the process being limited at the top or bottom of its scale, making the error constant). For example the position of a valve cannot be any more open than fully open and also cannot be closed anymore than fully closed. In this case integral windup can actually involve the integrator being turned off for periods of time until the response falls back into an acceptable range.

This usually occurs when the controller's output can no longer affect the controlled variable, or if the controller is part of a selection scheme and it is selected right.

Integral windup was more of a problem in analog controllers. Within modern Distributed Control System

Distributed control system

A distributed control system refers to a control system usually of a manufacturing system, process or any kind of dynamic system, in which the controller elements are not central in location but are distributed throughout the system with each component sub-system controlled by one or more...

s and Programmable Logic Controller

Programmable logic controller

A programmable logic controller or programmable controller is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures. PLCs are used in many industries and machines...

s, it is much easier to prevent integral windup by either limiting the controller output, or by using external reset feedback, which is a means of feeding back the selected output to the integral circuit of all controllers in the selection scheme so that a closed loop

Closed loop

Closed loop may refer to:* A feedback loop, often found in:** Control theory#Closed-loop transfer function, where a closed-loop controller may be used** Electronic feedback loops in electronic circuits** PID controller, a commonly used closed-loop controller...

is maintained.