What is a closed-loop motor lens, what are its characteristics, and how does it affect focusing speed and image quality?

June 8, 2022

Latest company news about What is a closed-loop motor lens, what are its characteristics, and how does it affect focusing speed and image quality?

What is a closed-loop motor lens, what are its characteristics, and how does it affect focusing speed and image quality?

1.To clearly explain the advantages of the closed-loop motor AF system, it is necessary to have a preliminary understanding of AF. 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 the photosensitive chip are the main components of imaging, and the motor and the motor driver chip (hereinafter referred to as the driver) are the main components of autofocus.

2. The principle of autofocus

To put it simply, when the object is in the distant view and the close view, the corresponding imaging positions are different, and the distance between the lens and the photosensitive chip needs to be adjusted, so that the photosensitive chip can always obtain a clean imaging effect. For the mobile phone camera module, the position of the lens does not move, and it is mainly realized by the motor to drive the lens to move.

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3. The role of the motor
The main function of the motor is to drive the lens to move. The motor here is the usual term for an 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 represented here, and the other types are not 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 function of each structure is not explained in this answer, but just to give you a preliminary concept: a voice coil is a system composed of coils and magnets. The energized 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 The driver chip of the motor
Corresponding to different types of motors, there are also different drivers. Here, the most common voice coil motor driver chip will be explained first. From the most easily understood point of view, the driver chip is to accept the signal from the control terminal, thereby outputting current to the motor, thereby driving the motor to move. For example, the commonly used DW9714 is 100mA current, and the accuracy is 10bit, which means that 0~1023 can be used to correspond to 0~100mA current.

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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 is given 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 imaged on the chip accordingly, and the chip will output the image to the The control end, the control end judges whether the clearest position is reached by judging the clarity of the picture.

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The process of auto-focusing can be understood as the platform side sends out codes 0~1024 in turn. After passing through the system described above, the platform side can obtain the corresponding picture clarity, so as to establish the code corresponding to the clearest picture. Then as long as the platform sends out the corresponding code, the screen can be in the clearest state and auto-focusing can be achieved. Of course, the actual application will not be a simple search such as 0~1024, which is too slow, but the basic principle is still the same. This will involve factors such as the autofocus algorithm, the stroke of the motor, the starting current, the cutoff current, and the depth of focus of the lens, and it is not appropriate to expand here. The figure below is a simple illustration of the algorithm.

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5. Advantages of closed-loop control
It can be seen from the above description that the advantage of the closed-loop motor is actually the advantage of the closed-loop system relative to the open-loop system. First, the closed-loop motor can use the closed-loop system to control the shaking time and speed up the focusing speed through the feedback of the position; image quality.