methods have lost the tolerance to the errors at the
frequencies around the disturbance tones. In the
MISO system, all controllers except the IDC method
have very high gain margins. By comparing the DOFC
and IDC methods, it is apparent that the former has
clearly better margins at the low frequencies below the
disturbance. For the frequencies above the disturbances,
the IDC method has better margins.
Another interesting factor is the impact of the tuning
parameter on the closed-loop robustness. For simplicity,
only output model errors in a SISO process are
considered. The main tuning parameters of the IDC,
IHC and DOFC methods are varied, resulting in the
corresponding variations in the robustness margins, as
shown in Figure 27.
Figure 27 shows the impact of the main tuning parameters
of the methods. The robustness to modeling
errors can be adjusted with ease by the manipulation
of a single parameter. The IDC and DOFC methods
converge into approximately the same performance at
the higher frequencies when the robustness is increased,
while at the lower frequencies the DOFC method has
clearly higher robustness margins. Compared to the
two model-based approaches, the filter-based IHC
method has significantly higher obtainable robustness
margins. These increases in robustnesses come with the