Smartphone cameras are adopting many of the features of state-of-the-art digital stills cameras and are being marketed as enabling Hollywood quality movie making. One such feature used in leading smartphone cameras is optical image stabilization (OIS). OIS reduces blur in stills photography, enhancing image quality especially in low-light conditions and enables smoother videos to be recorded when on the move.
The current trend in smartphones is for the adoption of larger format image sensors which require large, heavy optics. Substituting light-weight plastic elements for glass lenses or the use of an additional variable aperture further adds to the lens mass. Traditionally, OIS is achieved by moving the lowest mass component, the lens – hence lens shift OIS. However, the balance of lowest moving mass has now shifted from the lens to the image sensor.
Sensor shift OIS moves the image sensor in the X and Y directions, responding to handset handshake motion. In addition, rotational motion can now be stabilized, not possible with lens shift OIS, enabling 5-axis stabilization.
The main challenge in sensor shift is the requirement to move the image sensor whilst simultaneously connecting it to the handset – an interconnect is required. The interconnect must transmit electrical current and data whilst in motion when the camera OIS is in use and must do so reliably over many years of use. The actuator needs to be able to control the position of the image sensor very fast and accurately, overcoming all resistive forces from the interconnect.
One solution to enable sensor-shift OIS inside a smartphone is a traditional voice coil motor (VCM) actuator using magnets and electromagnetic coils, which is limited in the force it can generate. This limitation can be overcome by using highly complex interconnect components and assembly processes, available to only a few companies. Furthermore, the requirement to move both the image sensor and interconnect through the use of VCM magnets and coils has led to an increase in camera footprint and camera bump-out size.
The design compromises made in a currently available VCM solution do not allow for the 5th axis of motion, roll, and as the designs are protected by one smartphone brand’s patent portfolio, the technology is not available to the wider market.
Cambridge Mechatronics (CML) have developed a sensor shift OIS actuator design controlled using Shape Memory Alloy (SMA). The actuator uses a simple design of 4 SMA wires of 25um diameter in a square configuration around the image sensor. These SMA wires contract upon heating above a transition temperature - the image sensor position and motion is therefore achieved by controlling the wire lengths thermally. CML’s designs ensure no compromise is made to the camera performance.
This approach means that CML’s solution does not have many of the drawbacks you get with a VCM design:
CML’s SMA sensor shift actuator designs for a range of image sensor formats are now widely available for sampling from proven and experienced supply chains. CML’s 4-wire designs are reliable and have shipped in >45 million handsets to date, routinely with top camera quality scores from independent testing. The first smartphone camera incorporating CML’s new SMA sensor shift technology was released in the flagship handset of a major brand early in 2022.
Sensor shift OIS is very well suited to the next generation of smartphone cameras which utilise increasing image sensor sizes with increasing heavy lenses and variable apertures. CML and its use of SMA technology provides cutting edge solutions to its customers that are miniature and more flexible in design than VCM solutions. CML’s licensees and partnered CCMs have samples available now for testing.
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