9
Disturbance Rejection in Information-Poor Systems
External disturbances acting on non-linear systems may be classified into three types: input (or actuator) disturbances, plant disturbances and output disturbances.
In general, an external disturbance 
can affect both the states 
and the measured outputs 
, and the relationship between the states and the control signal 
:
Various approaches to disturbance rejection in non-linear dynamic systems have been suggested (Mukhopadhyay and Narendra, 1993), although simplifying assumptions concerning the plant have to be made in most cases. For example,
1. the disturbance has no direct effect on the measured outputs:
2. the disturbance does not affect the relationship between the states and the control signal:
3. the state is fully observable:
