Although PID con trol has b een widely used in practical engineering, its abilit y to reject external disturbance and to handle severe nonlinearities should b e further enhanced. In this pap er, we presen t a simple robust unknown dynamics estimation, which can be easily incorp orated into PID control to achiev e satisfactory control performance for a rotomagnet plant sub ject to p eriod disturbance. The use of this estimator together with PID control leads to a feedforw ard like composite control framework. Unlike other estimators, only lo w-pass ¿lter op erations on the input and output and simple algebraic op erations are needed to construct our estimator, while exp onen tial conv ergence can be guaranteed. Numerical sim ulations are given to show the validit y of the proposed estimator and comp osite PID control