A robust composite nonlinear servo controller design was proposed in discretetime domain for typical motor servo systems. The unknown disturbance and uncertainties in the system were lumped into a ramp signal (with a constant rate of change), and a reducedorder linear extended state observer was designed to estimate the unmeasured state and the unknown disturbance. The controller was then applied to a permanent magnet synchronous motor (PMSM) servo system. MATLAB simulation was first conducted, followed by experimental tests based on a TMS320F28335DSC. The results verify that the system, in the face of a variety of disturbances, was capable of tracking the target positions fast, smoothly and accurately, with a good robustness.