response may have an abrupt phase shift at this frequency.
In addition, the control signal is subject to a
significant amplification, causing possible stability
issues with some approaches. In the second case, the
process is subject to two distinct sinusoidal disturbances
acting below and above the natural frequency
of the system. These kinds of control problems are
likely to have stability issues at the frequencies between
the disturbances, if the applied control law is not based
on a process model. Both of these cases represent the
worst case situation for different kinds of control
approaches, hence making the possible problems
clearly distinguishable.