2017 Volume 6 Issue 2 Pages 117-123
Recently, image sensors for mobile phones have significantly improved, and lens position control is needed for the autofocus actuator of the mobile camera module to realize a robust and precise focus system. The lens position control system is a highly resonant system because the lenses are held by small springs in narrow spaces. In this paper, a digital servo controller for the mobile camera module is discussed. First, continuous control systems are studied for various servo bandwidths with respect to the resonant frequency. Next, three pole assignment methods, 1) multiple poles, 2) a coefficient diagram method, and 3) an optimal controller, are discussed. Third, the limited pole placement method is applied to design a digital controller to suppress the vibration that comes from delay of a system. Finally, a feedforward controller is also investigated to obtain a smooth response for the reference signal. We confirmed that the highly resonant system can be controlled smoothly enough for autofocusing, using the limited pole placement method and the feedforward controller.
