Assessing the time-domain performance of closed-loop system models is important
because control systems are inherently time-domain systems. The performance of
dynamic systems in the time domain can be defined in terms of the time response to
standard test inputs. One very common input to control systems is the step function.
If the response to a step input is known, it is mathematic possible to compute the
response to any input. Another input of major importance is the sinusoidal function. A
sinusoidal steady-state output is obtained when an asymptotical stable linear system
is subjected to a sinusoidal input. Thus, if we know the response of a linear time invariant
system to sinusoidal of all frequencies, we have a complete description of the
system