Fourier transformed process outputs at some predefined
frequencies, namely those of the disturbances,
are made. The obtained values are then filtered through
the inverse plant dynamics one frequency at a time,
yielding a control signal generating control effort in
the anti-phase of the disturbances, cancelling their
impact thereof. Interestingly, the original frequency
domain control law can be expressed as a discrete
LTI feedback compensator in the time domain.
Because of the formulation of the original control law
in the frequency domain, the time-domain compensator
does not require a process model, but only the phase
and gain information at a frequency. Such property
significantly simplifies the controller implementation
in a process. However, this benefit comes with the
price of the fact that the closed-loop stability cannot
be inherently guaranteed, resulting in stability issues, as
pointed out in Section 4.