One of the objectives of a control system is to achieve good plant performance in the face of uncertainty. The major sources of uncertainty in control are:
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Unmeasurable perturbations produce output deviations. With a controller in place, the achieved deviations must be below a user-defined bound.
Modelling errors can be classified into three categories:
Parameter uncertainty originated by uncertainty in physical parameters. Parameter uncertainty may be accommodated with high-gain regulators in some instances.
Unmodelled dynamics i.e. neglected delays and fast time constants. The order of real plants is infinite (partial differential equations) and lumped parameter models are only approximations. In most cases, these approximations are intentionally made to keep the problem traceable. For example:
Linear regulators must be at least robust to smooth non-linearities on the process. As a matter of fact, practical requirements usually require appropriate behaviour in the face of non-smooth non-linearities like hysteresis or backlash that occur mostly with valves and other mechanical actuators.
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Take note of the fact that, modelling errors can change with time, due to plant aging or component replacement; significant modelling errors need to be considered in time ahead irrespective of the initial effort devoted to modelling.
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