Optical image stabilization principles
Optical image stabilization or OIS is an important element for flawlessly replicating digital reproductions. Without this, capturing still images and recording moving video will result in pixel blur and unwanted artifacts. While digital capture devices such as digital cameras, digital camcorders, mobile phones and tablets have become smaller, their demands for resolution quality and pixel count density have grown exponentially over the past decade.
The market shift toward compact mobile devices with high-megapixel capture capabilities has created a need for advanced stabilization technology. There are two common implementation methods: electronic image stabilization (EIS) and optical image stabilization (OIS).
Figure 1 Optical image stabilization target market
Image stabilization is used to reduce blur associated with movement and/or shake of the camera during exposure of the image sensor to the capture environment. However, it does not prevent motion blur caused by movement of the intended subject or extreme movements of the camera itself, only relatively small shakes of the camera lens (within a few optical angles) by the user. This camera user motion is characterized by its translational and tilt components, where the angular motions are called yaw and pitch, respectively. Camera roll cannot be compensated because "rolling" the lens doesn't actually change/compensate for the rolling motion and therefore has no effect on the image itself relative to the image sensor.
EIS is a digital image compensation technology that uses complex algorithms to compare frame contrast and pixel position for each changing frame. Pixels on the image boundaries provide the buffer required for motion compensation. The EIS algorithm calculates the subtle differences between each frame and then uses the results to insert new frames to reduce the feeling of motion. While this approach has the advantage of being able to create a cheap and compact solution, the final image quality is always reduced due to image scaling and image signal post-processing artifacts, and more power is required for additional image capture and final image processing. Image Processing. The EIS system also functions with fully electronic zoom (long field of view) and in low light conditions.
Figure 2 Comparison of OIS and EIS image quality
OIS is a mechanical technology used in imaging equipment to stably record images by controlling the light path of the image sensor. The two main methods of OIS in compact camera modules are by moving the lens position (lens shift) or the module itself (module tilt).
User movement of the camera may cause misalignment of the optical path between the focusing lens and the center of the image sensor. In OIS systems using the lens shift method, only the lens within the camera module is controlled and used to realign the light path to the center of the image sensor. In contrast, the module tilt method controls the movement of the entire module, including the fixed lens and image sensor. Module tilt allows the OIS system to compensate for a wider range of motion, with the trade-off being increased module height. Since the focal length between the lens and the image sensor is fixed, image distortion can also be achieved by tilting the module. Overall, compared to EIS, OIS systems can reduce image blur without significantly sacrificing image quality, especially for low-light and long-distance image capture.
Figure 3 Main methods of OIS compensation
ROHM's OIS architecture
ROHM offers two OIS controller architectures, including a fully programmable ARM Cortex-M0 processor with custom programmable digital signal processing for "gyro signal processing" and "servo control", and ROHM's custom, fully Programmable RMCU processor with integrated programmable Gyro Signal Processing and Servo Control. All OIS working memory and control calculations are performed on the OIS controller itself, requiring no external host processor computing power or External memory for storing calculation variables. Both products achieve the smallest chip size, lowest external component count and smallest overall mounting area on the market.
Leveraging industry experience in OIS architecture, ROHM's OIS controller series provides comprehensive control of X- and Y-axis voice coil motor (VCM) drivers, Hall amplifiers and current drivers, photoreflector drivers, I 2C interfaces, PLL oscillators , 12-bit ADC, SPI master interface for digital gyroscopes (and support for analog gyroscopes), and many other features. The product range also includes options for integrated driver support for autofocus, neutral density filters or shutter functions. Supports selectable PWM/BTL linear operation to select improved VCM drive power efficiency or improved image quality. Additionally, ROHM's controllers have comparable power consumption due to wafer processing and optimized chip architecture. ROHM's proprietary "Servo Control" and "Gyro Signal Processing" circuits both feature unique digital filter designs that dynamically compensate for gyroscope and actuator temperature drift without eliminating intentional pan and tilt movements by the camera user. These controllers can be applied to lens shift, module tilt, or other less common control systems such as lens tilt. Also included is customizable OIS control software for automatic lens control, automatic gimbal detection, and access to different programmable shooting modes and calibration settings.
Figure 4 General OIS block diagram
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