What is the principle of mobile phone camera focusing?

March 16, 2023

1. Overview To clearly explain the advantages of the closed-loop motor autofocus system, it is necessary to have a preliminary understanding of autofocus. The autofocus system of the mobile phone camera module is mainly composed of the following components: 1. lens; 2. motor; 3. motor drive chip; 4. photosensitive chip. Among them, the lens and photosensitive chip are the main components of imaging, and the motor and motor driver chip (hereinafter referred to as the driver) are the main components of autofocus.
2. The principle of autofocus is simply that when the object is in the distant view and the close view, the corresponding imaging positions are different. It is necessary to adjust the distance between the lens and the photosensitive chip so that the photosensitive chip can always obtain a clean imaging effect. For the camera module of a mobile phone, the position of the lens does not move, mainly relying on the motor to drive the lens to move.

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3. Function of the motor
The main function of the motor is to drive the lens to move. The motor here is the usual term for the Auto Focus Actuator. In fact, there are many kinds of AF brakes, such as voice coil motor (VCM), piezoelectric motor (Piezo) and so on. VCM has shrapnel type, ball type and other differences. The most common shrapnel-type VCM is used here as a representative, and other types will not be explained one by one.
3.1 Structure of shrapnel voice coil motor
The structure of a common voice coil motor is shown in the figure below. The specific functions of each structure will not be explained in this answer, but just to give everyone a preliminary concept: a voice coil is a system composed of a coil and a magnet. The electrified coil will be subjected to ampere force in the magnetic field, thereby driving the lens to move in the direction of the optical axis.

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3.2 Motor driver chip
Corresponding to different types of motors, there are also different drivers. Here, the most common voice coil motor driver chip will be used to illustrate. From the most convenient point of view, the driver chip is to receive the signal from the control terminal, so as to output current to the motor, so as to drive the motor to move. For example, the commonly used DW9714 has a current of 100mA and an accuracy of 10bit, which means that 0~1023 can be used to correspond to a current of 0~100mA.

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4. Open loop motor autofocus system
4.1 Control method of autofocus system
Through the above introduction, we can already have a preliminary understanding of the principle of the autofocus system. The whole control process can be understood as: the platform gives a code (such as 0~1024), the driver chip will output the corresponding current, the motor will make the corresponding displacement, the lens will be correspondingly imaged on the chip, and the chip will output the image to the The control terminal, the control terminal judges whether to reach the clearest position by judging the clarity
of the picture.