Development of a low-cost autofocusing probe for profile measurement

In this research, a low-cost and precision autofocus laser probe system was developed. Modified from the pickup head of a commercially available DVD as a basis, it can detect the focus error signal (FES) of the object with a built-in four-quadrant photodiode. The FES will feed back to the developed controller, through which the objective lens can be adjusted in position automatically to remain in focus, driven by a voice coil motor (VCM). The driving current (converted from servo-FES) of the VCM and the objective lens displacement are one to one linearly related within several hundred micrometres. Because of this relationship, the surface profile and the surface roughness of the tested object can be realized. Calibrated results showed that the designed probe has 200 µm linearity range, 0.2 µm accuracy, 0.1 µm resolution and standard deviation of 0.2 µm on average. Some examples are given to show the applicability of this probe system.