It was discovered early in the process of finding good weights for the controllers that a typical failing of a classic PID controller was a significant issue: integrator saturation and/or wind-up. Since the integrator function of the PID is constantly accumulating the system error in an effort to force the controller to respond to small errors that persist over time, it acts as if it has a long memory. This can cause problems as a large but temporary error is not forgotten or neglected by the integrator and causes its output to skew heavily in one direction for a long period of time, even past the point where such a skew is beneficial. Similarly, if conditions exist where a small error persists over a long period of time (such as an actuator at its mechanical limit or other control weights dominating the response of the system), the integrator will
accumulate this error and its output will attempt to correct all
of this error even if the conditions that originated in the error
have passed. If it is not possible to keep the error signal into
the integrator at a low value, the integrator can easily come
to have a significant negative impact on the overall control
effectiveness.