the first step in designing compensation for any plant is to observe the closed loop unity feedback response to check for stability. Many systems are unstable in open loop but stable in closed loop configuration. The other way round is also possible that the system is stable in the open loop but unstable in closed loop,although this case is rare.
The closed loop uncompensated system can be studied by viewing the root locus plot of the system. 3.4 shows the root locus plot of the system
the plot reveals that the system cannot be controlled by a simple unity feedback loop. Whatever be the value of loop gain,k,one branch of the locus remains on RHS (in unstable region) of s-plane.
This makes control impossible by unity feedback. The root locus has a branch on the right hand side of the imaginary axis, which indicates that the system is unstable in closed loop for all values of K.
From the above analysis,it is concluded that using only the gain compensation in closed loop cannot control the system. Reshaping of the system roots is necessary so that for certain range of gains the system has all its root in the left plane (stable region) of the s-plane.