It's well-known that a control system with integral control can track a constant reference signal (command) with zero steady-state error.
What is needed to generate a controller that can track a ramping reference signal (i.e. one that changes at a constant rate) with zero steady-state error?
Consider a controller $C(s)$ for plant $G(s)$ and feedback $H(s)$.
The closed loop is
$$\frac{Y(s)}{U(s)}=\frac{CG}{1+CGH}$$
The ramp is $U(s)=\frac1{s^2}$
Thus the error is
$$E=Y(s)-\frac{1}{s^2}=(\frac{CG}{1+CGH}-1)\frac1{s^2}$$
According to the final value theorem,
$$\lim_{t\to \infty} e(t)=\lim_{s\to 0} sE(s)=\lim_{s\to 0} (\frac{CG-1-CGH}{1+CGH} \times \frac1{s})=0$$
For a known case, you can find out how many $s$ in the denominator of $C$ will reduce the steady state error to zero.
