This contribution distinguishes between two conceptually different windup p
henomenons, namely controller and plant windup. Even in the absence of dyna
mic controller elements, saturating nonlinearities can trigger undesired os
cillations. This effect is obviously due to inappropriate plant states, and
is therefore called plant windup. Based on the frequency response of the l
inear part, a design aid for plant windup prevention is presented. It facil
itates the search for suitable closed-loop dynamics, or for an appropriate
additional dynamic network. State and disturbance observers add dynamic ele
ments to the compensator, and in the presence of very slow or unstable cont
roller modes, the well-known reset windup or integral windup occurs. This e
ffect, the controller windup, is related to mismatched controller states. B
y feeding the limited plant input signal into the observer it is completely
removed, so that the remaining effects of input saturation are solely attr
ibutable to plant windup. Various known windup prevention techniques can be
explained within the presented framework. (C) 1999 Elsevier Science Ltd. A
ll rights reserved.