Reset windup is a control system issue that occurs when a control effort attempts to correct a large error in a process, but the actuator responsible for implementing that control is too small or limited in its capacity.
This phenomenon is specifically described as follows:
"If a large control effort is used to mitigate an error of significant magnitude for a process whose actuator is too small, the result is reset windup. The actuator gets saturated at a specific value corresponding to its maximum output, unable to affect the process any further." (Reference: The Reset Windup Error, 09-Mar-2020)
Understanding Reset Windup
To further clarify, consider the key elements contributing to reset windup:
- Significant Error: The system experiences a substantial difference between its desired state (setpoint) and its actual state (process variable).
- Large Control Effort: To correct this large error, the controller demands a considerable output, often through its integral (or "reset") action, which continuously accumulates error over time.
- Actuator Limitation: The device that physically influences the process (the actuator, e.g., a valve, motor, or heater) has a maximum output limit. It cannot provide the full control effort requested by the controller.
The Consequence: Actuator Saturation
When these conditions align, the actuator reaches its maximum operational limit. This state is known as saturation. Once saturated, the actuator cannot increase its output any further, regardless of how much more control effort the controller commands.
Consequently, even though the controller continues to "wind up" its internal calculations (particularly the integral term) in an attempt to reduce the persistent error, the actuator is unable to respond to this increased demand. This effectively makes the control system unresponsive to the error, delaying recovery and potentially leading to overshoot when the error finally begins to decrease.
In essence, reset windup leads to a situation where the controller is trying to do more than the physical system (actuator) can deliver, causing an accumulation of internal control effort that becomes problematic when conditions eventually change.
