Block diagram of a control system is shown in the figure.
The condition that must be satisfied to remove the effect of disturbance on the reponse of the system.
C(s)=D(s)G3(1+G1H2)1+G1G2G3H1+G1H2
Response due to disturbance,
C(s)=D(s)G3(1+G1H2)1+G1G2G3H1+G1H2
C(s) should be zero for D(s)≠0
∴G1H2=−1