So I have the following excercise. Consider a simple feedback system with transferfunction \$G_1(s)=\frac{1.5}{s(s^2+2s+2)}\$. Determine the constants of the phase lag compensator \$G_K=K\frac{s+a}{s+a/M}\$ such that the steady state error when \$u(t)=t\$ is less than \$0.1\$.
My teacher solved it and according to him \$e(\infty)=\lim_{s\to 0}s\frac{1}{1+G_K G_P}U(s)\$ so it should be that \$E(s)=\frac{1}{1+G_K G_P}U(s)\$. But I don't know why that is. From the block diagram I know that \$E(s)=\frac{1}{1+G_K G_P}R(s)\$ but \$E(s)G_K=U(s)\$. So how is it shown that \$E(s)=\frac{1}{1+G_K G_P}U(s)\$ from the block diagram?
