By Mallesham Devasane|Updated : September 12th, 2017


The additional device added in control system to obtain the performance as per desired specification is known as compensator.

Phase lead Compensation: A phase lead compensator improves transient response of the system.

Compensator (1)

Compensator (2)

Compensator (3)

Phase lag Compensation: A phase lag compensator increases the DC gain of the system with negligible phase contribution. This increases the static error constant and improves the steady state performance of the uncompensated system. The addition of a phase lag compensator should not affect the transient response of the system.

Compensator (4)

In other words, there should not be much change in the location of the dominant poles. The pole of a phase lag compensator lies extremely close to the origin and its zero to the left of the pole and very near to it.

Compensator (5)

Compensator (6)

Key Points

  • Objectives of compensation is to design a control system with less peak overshoot, less rise time, small value of settling time and vary small steady state error.
  • A first order compensator is, generally an electric network or operational amplifier circuit the transfer function of which has a zero and a pole that lie on the negative real axis in the s-plane.
  • Compensators can be designed in the time domain using root locus approach or in the frequency domain by using the bode plots.
  • Design specifications are taken into account in terms of a dominant second order complex conjugate poles.


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