A　parameterised　design　of　robust　composite　nonlinear　controller　is　proposed　for　typical　second-order　servo　systems　subject　to　unknown　constant　disturbance　and　control　input　saturation.　The　control　law　consists　of　a　linear　feedback　part　for　achieving　fast　response,　a　nonlinear　feedback　part　for　suppressing　the　overshoot,　and　a　disturbance-compensation　mechanism　for　erasing　the　steady-state　error.　An　extended　state　observer　is　adopted　to　estimate　the　unknown　disturbance.　The　closed-loop　stability　is　analysed　theoretically.　The　control　scheme　is　applied　to　the　speed　regulation　of　permanent　magnet　synchronous　motor,　and　numerical　simulations　are　carried　out.　The　results　confirm　that　the　proposed　control　scheme　can　achieve　fast,　smooth,　and　accurate　speed　regulation,　and　has　a　certain　degree　of　robustness　with　respect　to　the　amplitude　of　disturbances　and　the　perturbations　of　system　parameters.