Autofocus Machine Vision System for Reading Serial Numbers of Hard Disk Drive Slider Bar

In this work, we concentrate on reading serial numbers of slider bar. Since the serial numbers on a bar are in the scale of tens of microns and their reflectance nature is rather poor, special optics and lighting components are required. As nature of high optical magnification, limited depth of focus (DOF) also causes another issue. Although we may introduce an off-the-shelve autofocus module to the system, the problem remains due to different heights of various elements within the slider in the field of view of the autofocus module. An image processing algorithm with capability of knowing where (in a blur image) to evaluate the focus is then introduced in our autofocus module. We therefore design a special rig to meet all the fore-mentioned requirements. The system has been tested and is currently operated reliably in the slider-bar manufacturing process.