with an average margin of 30 dB, followed by the
DOFC method with a similar performance. The IHC,
SFSCF and FSCF methods have high margins in all
processes in the presence of a single disturbance tone.
However, in the presence of multiple tonal disturbances,
these margins are completely lost. This is a
direct consequence of their design approach, which
does not take the process dynamics into the account,
hence enabling the possibility for stability problems at
frequencies outside the actual disturbance frequency.
The preceding stability analysis guarantees nominal
stability; however, due to the existence of model errors,
another analysis is done in order to determine the
robust stability and the tolerance to model errors.
The robustness analysis is divided into two separate
parts, as for nonSISO systems the error can be located
either at the input or at the output, having completely
different impacts on the stability. For SISO systems, the
location of the error is interchangeable between the
input and output, hence a single analysis is sufficient.
The model error considered here is either an outputmultiplicative
error, given as