Controlling a MEMS Deformable Mirror in a Miniature Auto-Focusing Imaging System

Conventional auto-focusing camera modules adjust their optical focus by moving lenses. Although motors, such as stepping motors or voice coil motors, can change the lens position with great precision and thus vary the image sharpness, large volume and excess power consumption remain critical issues for mobile device applications. This paper discusses an auto-focusing camera module that adopts a microelectromechanical system (MEMS) deformable mirror (DM) with variable focusing capability to replace mechanical motors. The DM used in this paper can provide 20 diopters of focusing capability, which is one order of magnitude higher than silicon-based DMs, so the autofocus module can be compact enough for mobile devices. Because of the characteristics of MEMS DMs, we proposed a “percentage-drop” searching algorithm to find a sharp image based on a focus value calculation. The average auto-focusing time is 45% faster than the global searching method.