Lens Position Control System Using Limited Pole Placement Method

Abstract

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.

Vibration Suppression in Position Control System by Pole Zero Cancellation using Limited Pole Placement Method

“…In such systems, the lens position is controlled to focus on the image sensor. Figure 4 shows a diagram of a lens positioning system (18) , and Fig. 5 shows the frequency characteristics of a plant with a resonance frequency of 60 Hz.…”

“…Therefore, the poles of the system should be assigned accurately, even in a digital system. The limited pole placement (LPP) method (13) (18) , which has been proposed by the author, can assign representative poles precisely. In this study, PZC with the LPP method is investigated.…”