干擾觀測器應用於數位相機的自動對焦系統設計

Abstract

近年來由於數位相機日漸流行，因此諸多能夠提升影像品質的技術正不斷的被開發研究，例如防手震和自動對焦技術。傳統的自動對焦技術多半應用在高階相機上，並且最常見到的是使用步進馬達來當作自動對焦系統的致動器，再搭配簡易的比例積分或比例微分的回授控制器，來實現自動對焦系統的定位控制。對於部份日本的數位相機製造商，針對自動對焦系統的機械結構做精密的加工，使得自動對焦系統機構的摩擦力能均勻化，讓整個控制系統的電子迴路設計上得以簡化設計並能減少摩擦力對控制系統的影響。 使用步進馬達來當作相機的自動對焦系統固然是一個便宜且簡單的方法，但其反應時間高達300至500 毫秒，對於影像拍攝時，往往需較久的時間才能獲得一個清晰的影像品質；而過長對焦時間對於動畫影像的拍攝，是相當不利的一個缺點。 因此本研究，將使用反應快速的音圈馬達來當作自動對焦系統的致動器。由於數位相機的耗電問題有1/3來自於機械控制的部份，如何設計出一個高效率、省電且快速響應速度的音圈馬達是本研究的第一個主題。而摩擦力的問題，常使得自動對焦系統無法達到理想的控制位置，因此在本次研究裡，將導入干擾觀測器的方式配合比例積分控制器，來克服磨擦力問題以及實現整個控制架構，利用其架構亦能降低傾斜角變動所造成的擾動。此外，為了能有效降低自動對焦系統在數位相機裡的成本及體積，本研究採用磁性尺、小型的磁阻式感測器在配合軟體演算法取代使用昂貴的細分割晶片來獲取目前控制系統的位移跟速度。 本次研究的結果，能將自動對焦系統的響應時間減少在100至120毫秒，穩態誤差在10%以內，而這10%穩態誤差來自於磁性尺的先天缺陷，例如像磁阻式感測器量測各個極距的振幅大小不同。而干擾觀測器對於磨擦力和重力改變提供良好的補償。而音圈馬達最大電流僅消耗30毫安培，這對於數位相機的省電也將有莫大的助益。

Digital video cameras (DVC) have be more and more popular in recent years. Many technologies which can improve the qualities of image and video are rapidly been developed, such as anti-shocking (AS) and auto-focusing (AF). Conventionally, auto-focusing system is usually applied to high-level cameras and stepper-motor is mostly used to be the driving plant of the AF system with the simple proportion-integral (PI) controller or proportion-differentiation (PD) controller. As for some manufacturers of DVCs in Japan, they adopt high-precision processes in manufacturing the apparatus of the AF system and that high-precision processes can make the friction of the AF apparatus be more uniform; these methods can simplify the complexity of the control system and reduce the influence of the friction on the AF apparatus.

Stepper motor which is used to be the plant of the AF system is the convenient and simple design. However, the responsed time of the conventional AF system needs 300∼500 millisecond (ms); for taking pictures, it needs more focusing time in order to capture a pellucid image. Moreover, the slow focusing is a very unfavorable defect for motion-video recording.

Therefore, the voice-coil motor (VCM) with faster response time and high energy efficiency is used to this study. Because the electrical apparatus consumes about 30% power energy of the total supplying electric energy of a DVC, so a low-power and high energy efficiency VCM design is the first topic of this study. In the other hand, the friction of the AF apparatus brings many non-ideal control problem such as larger steady-state error and moving vibration. In order to overcome the effect of the friction, the disturbance observer with PI controller feedback system is applied to implement this AF system control. Using this mothod can also reduce the position disturbance which is caused by the change of inclination. Besides, the mini magnetic scale, mini magneto-resistive (MR) sensor and positioning estimating algorithm instead of using expensive interpolation chip are used for cost-down and mini-size; that positioning estimating algorithm can be used to calculate the current position and velocity.

The results of this study, the responsed time needs about 100∼120 ms with the 10% steady-state error; the 10% steady-state error is caused form the defects of the magnetic scale such as the variations of the amplitudes of the MR sensor signals. The disturbance observer offers the better compensation and the effects of the friction and weight force have been abated. The maximum current consumption of the VCM in this study is about 30 milliampere (mA), this advantage can save more battery energy of the DVC.