3. Need of controller (P, PD, PI & PID)
Every practical system takes finite time to reach its steady state and during this period , it
oscillates or increases exponentially.The behaviour of the system gets decided by the type
and location of closed loop poles in s–plane. The closed loop poles are dependent on
selection of the parmeters of the system. Every system has a tendency to oppose the
oscillatory behaviour of the system which is called as damping. Now this tendency controls
the type of closed loop poles and hence the nature of the response. This damping is
measured by a factor called damping ratio of the system. Damping ratio indicates how
much dominant the opposition from the system is to the oscillations in the output. In some
systems it will be low where system will oscillate but slowly i.e. with damped frequency. If
damping ratio is high, system output will not oscillate at all and not only it will be
exponential, but also so slow that it will take a very long time to reach a steady state. That is
why all practical systems are designed for the damping ratio less than 1 i.e. underdamped.