27 May 2022

Fast precise actuation with SMA

SMA actuators are inherently high force and compact, but must also provide accurate, fast movement in order to meet the most demanding customer applications. In this article, we explain how CML’s patented technology delivers unrivalled precision with no compromise on actuator speed.

The unique properties of Shape Memory Alloy (SMA) material lend themselves well to produce actuators for applications requiring high force in a miniature form factor. The ongoing trend for large format image sensors with large, heavy optics in smartphone cameras has fuelled the success of SMA to displace conventional electromagnetic actuators in high-end handsets.

Other benefits of SMA include silent operation and low power consumption in a lightweight package. CML’s SMA actuators cause no magnetic interference, which allow them it to be assembled in close proximity to magnetic components such as antennae and speakers. This gives designers the ability to create high packing density component layouts which maximises design freedom. These features are all highly valued in the wearables space. It is expected that SMA actuators will proliferate in products including VR headsets, AR glasses, smartwatches and hearables.

Despite these advantages, there are still industry myths about the limitations of SMA, particularly speed. CML’s R&D Director, Dr. Ben Brown has set out to dispel this viewpoint:

“The fundamental operating principle of SMA actuators is to heat the material above a phase transition temperature and so cause the material to contract. When the source of heat such as electrical current is removed, the material cools and returns to its original length. By using thin wire with a low heat capacity and low latent heat and a carefully designed electronic drive, heating the wire through the phase transition and the resulting contraction can be almost instantaneous.

It is cooling the material to cause motion in the reverse direction where more ingenious solutions are necessary. The cooling rate can be increased by maximising the ratio of surface area to volume of the SMA material. This is another advantage of using thin wire, but practical wire diameters impose a limit to the improvement. In addition, CML use a patented coating on the wire which further enhances the thermal conduction between the wire and the surrounding air further increasing cooling rates.

Additional increases in the motion speed are achieved in CML’s patented actuator designs by arranging SMA wires so that they pull in opposite directions to one another. This means that the very high heating speeds can be achieved in both directions and there is no requirement to wait for the material to fully cool to generate the desired motion.

However, this apparently simple approach complicates the material behaviour and adds numerous variables and unknowns to the system. CML has overcome these additional control and performance challenges and encoded algorithms in encrypted firmware that is supplied with CML’s SMA controller ICs. These algorithms themselves can be adjusted and tuned for individual customer and project requirements to deliver the most compelling product performance.”

CML’s SMA actuators meet the speed requirements of even the most demanding AF and OIS applications. With its most recent technology developments, CML has been able to provide 1 millisecond settling time for super-resolution and depth camera resolution enhancement applications.

CML has engineering solutions to the perceived limitations of SMA actuators, and is now developing actuators to address markets beyond smartphone cameras and wearables, including precision fluid management, drug delivery devices and micro robotics.