A　parameterized　design　of　composite　nonlinear　feedback　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　unmeasured　state　and　unknown　disturbance.　The　control　scheme　is　applied　to　a　permanent　magnet　synchronous　motor　(PMSM)　position　servo　system,　and　experimental　tests　are　carried　out　using　a　TMS320F2812　DSP　board.　The　results　confirm　that　the　servo　system　can　achieve　fast,　smooth　and　accurate　position　regulation,　and　has　a　certain　degree　of　robustness　with　respect　to　the　load　disturbance　and　system　parameter　perturbations.　©　2014　TCCT,　CAA.