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.