Precision control with shape memory alloy actuators in embedded systems

Shape memory alloy (SMA) is typically comprised of nickel and titanium (AKA Nitinol) and has some unique material properties due to its crystal structure. When the material is deformed below a given temperature, it will return to its original shape by heating. In other words, when heated above their phase transition, the SMA crystal structure rearranges causing the material to contract. When the SMA is then allowed to cool, the material returns to its original shape. The contraction upon heating (shape memory effect) allows for application in a wide range of embedded systems.

^{Figure 1: Temperature control of SMA when heating and cooling is applied.}

SMA is used in different form factors for a variety of applications including medical, aviation, automotive, fluidics, and robotic systems. For some applications, where miniaturization is critical, actuator designs utilizing SMA in the form of very thin wires (20-30µm diameter) have been developed. In smartphones, such wire-based SMA technology is being used for autofocus (AF) and optical image stabilization (OIS). By managing the temperature of the wire, the length can be controlled, which in turn allows for precise position control of the lens or image sensor in the smartphone camera module.

Accurate control of SMA actuators is challenging due to the non-linear and hysteretic behaviour of the material. Cambridge Mechatronics Ltd (CML) has successfully harnessed the reversible phase transformation to develop algorithms that provide accurate control with full integration into existing embedded handset environments. The control algorithm works by regularly measuring the wire resistance, which correlates to the wire length. Correspondingly, current is applied to or removed from the wire to effect the required motion. Using resistance control avoids the need for dedicated Hall position sensors, which are used by voice coil motor (VCM) OIS and AF technology. The control algorithm resides on a dedicated driver chip, which is integrated into the system architecture as shown in the diagram below.

^{Figure 2: Comparison between all-in-one architecture and split architecture, note FETs (field effect transistor) and FW (firmware).}

Gyroscopes measure the user shake, whose signal is interpreted by a motion processing function and the actuator control algorithm to deliver the correct power to the SMA wires to enable image stabilization. There are two architectures commonly employed in smartphone cameras, where the split architecture incorporates the motion processing function on the handset application processor (AP).

Overcoming Challenges

When integrating SMA camera actuator hardware and software into an embedded environment such as a smartphone, some key challenges need to be addressed.

The performance requirements are hugely dependent on the application. These may comprise moving heavy payloads, such as heavy lenses or variable apertures. Alternatively, these may relate to a high response frequency, which is enabled by the use of opposing (heated and cooling) wire designs. Typically, SMA actuators are able to achieve fast response and transition times to meet smartphone user case shake conditions, up to 3 degrees of amplitude and frequencies exceeding 10Hz.

The actuator and control solution must be designed to perform optimally in the product environmental conditions. Challenges including managing vibration and mechanical shock need to be overcome as they have a direct impact on mechanical performance. This means that reliability and robustness must be achieved, usually through a combination of hardware design and software features that take in handset data such as temperature and gyroscope motion. Additionally, the material stiffness of SMA means that designs are very rigid, resulting in high resonance frequencies which are far away from typical application conditions. This makes SMA actuators significantly more robust and tolerant to environments with high vibration.

As a new technology, SMA actuators and associated control software need to readily embed into existing system architectures. This has been achieved by working closely with the existing supply chain to ensure seamless and supported integration. For example, SMA actuator solutions use voltages, such as 2.8V or 3.1V, which are already commonly used in the embedded systems they are integrated into.

As the application demands for actuator performance increase, management of power consumption becomes critical, especially in battery-powered products. Unlike VCM, where power consumption increases as the actuator operates at the edge of the performance window, SMA’s power consumption stays unchanged and is lower. In applications with intermittent position changes, such as focus correction, utilizing architectures that only require power to move to new positions can further reduce average power consumption. Other low-power designs and control features are under development in response to customer requests.

Advantages of using SMA

SMA actuator solutions are fundamentally low profile, allowing easy integration into thickness-critical products. Modern actuator designs and control systems are highly scalable in footprint for a flexible approach to adoption by OEMs. A key advantage of SMA technology in smartphones is the high force that the SMA phase transition generates, accommodating a wide range of payloads from 1mg to 5000mg. This is in line with trends to larger image sensors and subsequently heavier lenses, and incorporation of variable apertures. Furthermore, SMA technology is free of electromagnetic interference (EMI), allowing for easier system design, especially in a multi-camera setup, proximal 5G antennae and microphone electronics and magnetic hinges in folding phones. To date, >50 million SMA-based actuators have shipped in leading smartphone cameras with top rankings from independent testers such as DxOMark.

In summary, we have introduced shape memory alloy material and its application challenges in smartphone cameras. When combined with sophisticated control algorithms, there are benefits to adoption which also extend to other product markets such as AR/VR, Haptics and fluidics.

This article was originally published on Embedded.com

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About CML: Cambridge Mechatronics Limited (CML) is a world-leading developer of mechanical, optical, electrical, silicon and software designs for system-level solutions using its Shape Memory Alloy (SMA) platform technology. Solutions such as ACTUATORS based on SMA wire (as thin as a human hair) can be controlled to the accuracy of the wavelength of light. These actuators are particularly suited to applications that require high levels of precision and force, in a fast, compact and lightweight design.

For more details about SMA technology and Cambridge Mechatronics, please GET IN TOUCH.

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