Damping ratio
is defined to conveniently divide
the underdamped, critically damped, and overdamped conditions at unity
for a second-order system.
The damping ratio
(zeta) is defined by
where
As shown in Eq.(E.9), a damping factor of 1 is critically
damped, while a damping factor less than 1 is underdamped, and
is overdamped.