ABSTRACT
We have described the relationship between control signal and output in transfer operator form as
y(t) = G(p)u(t) (5.1) In practice we are never so fortunate that the measured signal exactly follows (5.1). It will deviate from G(p)u(t) in different ways. These deviations depend on disturbances, and the real output will be written
y(t) = G(p}u(t) + w(t) (5.2) where w(t) is called the disturbance signal. Prom a system perspective, w is of course nothing but yet another input, to the system - it is the fact that it is not at our disposal to control the plant that leads to a distinction.
