An auto-focusing system that consists of a compact disk (CD) optical head and a piezoelectric translator (PZT) is developed for measuring the displacement or profile of a tested surface. The modified CD optical head provides the defocusing signal from the tested surface. The defocusing signal is then used to drive the PZT via a feedback controller. The focus spot of the optical head follows the displacement of the tested surface through the movement of the PZT. The displacement or the profile of the tested surface can be obtained through three ways with different accuracy: the integration of the focus error signal (FES), the curve of driving voltage to expansion distance of the PZT or a secondary sensing device of displacement. The resolution of the system can reach 10 nm and the accuracy is 0.05 /spl mu/m. Some design considerations such as the control algorithm of focus servo and error analysis of the system are addressed in this paper. The measurement system and the sensor based on the CD pickup can also be applied to position sensing for robot as well as the noncontact probe for localization.