High force-to-mass ratio: how SMA delivers 10x the force compared to VCM in smartphone cameras

Figure 1: A visualisation showing that if an SMA wire had the same diameter as a human bicep, it could easily lift several double-decker London buses.

One of the main advantages of Shape Memory Alloy (SMA) actuators is their exceptionally high force-to-mass ratio, making them ideal for compact applications where robust force is needed. The high force output of SMA actuators provides a major advantage for AF and OIS in smartphone cameras by enabling precise and efficient movement of larger and heavier lenses and other camera components. Compared to traditional Voice Coil Motor (VCM) technology, which is constrained by weak magnetic fields and energy losses in small spaces, SMA actuators deliver at least 10 times the force in the same volume. This increased force can provide faster and more accurate Autofocus (AF) and Optical Image Stabilisation (OIS), resulting in sharper images and smoother video performance, even in challenging conditions.

Let's examine the properties of SMA wire-based actuators and how they deliver significantly higher force and performance than a VCM actuator of the same size.

How SMA wire-based actuators work

SMA actuators operate by taking advantage of a solid-to-solid phase transition, much like what happens when steel is hardened, only much more easily reversed. Cambridge Mechatronics Ltd. (CML) uses an SMA wire (made from high-grade Nitinol) for its actuators. At its core, the shape memory effect of Nitinol allows the alloy to "remember" its original shape. This means that the material can be deformed or stretched and it can regain its initial shape when subjected to specific heat and stress conditions. This characteristic is a product of the underlying phase transitions that Nitinol undergoes, and forms the basis for how CML’s SMA actuators provide high force-to-mass movement.

At low temperatures, Nitinol is in a Martensitic phase, which means that the crystal structure (how the atoms are arranged in the material) can adopt two stable states: twinned Martensite and detwinned Martensite. When Nitinol is in its unstressed, cold state, the Martensitic structure naturally arranges itself into a twinned configuration. When an external force or load is applied to cold Nitinol, the twinned Martensite rearranges into a detwinned state. In this process, the atomic planes shift relative to each other, allowing the material to deform significantly without breaking atomic bonds. Cooling the material alone (without applying an external force) does not introduce deformation - it simply remains in this twinned arrangement.

The key to harnessing high force-to-mass in SMA wire-based actuators lies in heating the material above its phase transition temperature. This shift prompts a structural change to an Austenite phase, where the material returns to its original shape (or length of wire). It's the high force generated during the heating cycle (the Austenite phase transition) that is used to move the SMA actuator.

Figure 2: Transition phases of Nitinol

Read more about the material properties of SMA (Nitinol) here.

Stress and strain in SMA wires

Before we talk about the force that can be applied in a compact SMA actuator, it is important to understand the concepts of stress and strain in the context of SMA wires.

Stress is a measure of the internal forces that particles of a material exert on each other. It is typically expressed in units of force per area, such as Pascals (Pa) or Megapascals (MPa). In the context of SMA wires, when we apply a force to the wire, it results in stress. This stress alters the wire's internal structure. In the phase transition from Martensite to Austenite (which is key to SMA functionality), stress plays a crucial role. It influences the temperature at which this phase change occurs. The more stress applied, the more the phase transition temperature shifts. Managing this stress is vital for the precise control of the SMA actuator's behaviour.

Strain in SMA wires is the measure of their deformation under stress. It is a dimensionless quantity, often expressed as a percentage. When an SMA wire is under stress, it strains (or elongates) until it reaches a certain point. This strain is reversible in SMAs, which means the material can return to its original shape after the stress is removed, a property crucial for the actuation movements in our applications.

In short, to ensure a long and reliable lifespan for SMA wires, actuators must operate within specific stress and strain limits. The durability of an SMA actuator is determined by a stress-strain balance - higher strain requires lower stress to maintain optimal fatigue performance. Properly managing this trade-off helps maximise actuator longevity and efficiency.

High force actuation for smartphone cameras and other compact devices

Currently, most smartphone actuators rely on VCM technology. However, space constraints in smartphones limit the size of magnets and the number of coils, resulting in weaker magnetic fields. Additionally, the use of thinner copper wires increases electrical resistance, leading to significant energy losses. As a result, VCM actuators generate just enough force to lift the lens against gravity and any flexure resistance, typically ranging from 1mN (100mg) to 10mN (able to move a 1g mass).

SMA actuator technology, developed by CML, is increasingly being adopted in the smartphone camera market. SMA actuators play a key role in delivering AF and OIS, offering a compact and efficient alternative to VCM actuator technologies. The SMA wires used in CML’s actuators are extremely thin, typically measuring between 25-30 microns in diameter.

Figure 3: Close-up of a 4-wire SMA actuator used in an OIS actuator

For smartphone camera actuators requiring a reliable lifespan lasting millions of full-stroke, full-force cycles, SMA actuators must operate within defined stress and strain limits (as described earlier). In typical daily use, an SMA actuator extending to 2% strain can safely generate a force equivalent to 200 MPa of stress. For a 25-micron diameter SMA wire, this translates to a tension of about 100mN, capable of lifting or moving a mass of approximately 10 grams. This means that the force that can be delivered by an SMA actuator in a compact space like a smartphone is at least 10 times larger than that of a typical VCM for the same volume.

Currently, the lens masses in smartphone cameras are typically less than a gram, meaning that both VCM and SMA actuators can handle moving these small lenses. However, as the industry shifts toward larger, heavier camera components such as lenses and variable apertures, SMA actuators are becoming the preferred choice for handling these weightier components. While VCM actuators may be able to move current lens masses, they might lack the force needed to accelerate them quickly enough for effective OIS.

Additionally, the low stiffness of a magnetic field means that VCM-based actuators are more susceptible to external forces. This can negatively impact image stability, particularly in high-motion environments. For example, to compare the performance of SMA and VCM OIS under real-world external vibrations, leading smartphone cameras were mounted on the handlebars of a moving scooter. The results, captured in the video below, show a clear advantage for SMA-based OIS, with the SMA-equipped camera consistently delivering sharper, more stable footage while VCM-powered cameras struggled with blurriness. This is despite the fact that the iPhone has gone to considerable expense of using sensor shift VCM OIS instead of lens shift VCM OIS to reduce the moving mass for the purpose of addressing this issue.

Video 1: External vibration test: S24 Ultra, iPhone 15 Pro Max, Xiaomi 14 v HONOR Magic 6 Pro
For users in China, watch the video here.

This superior performance is driven by SMA’s stronger hold force and higher stiffness, which help keep the lens steady, even during rapid movements. The advantage is especially clear in scenarios involving intense vibrations - like filming on a moving vehicle - where SMA effectively minimises shake-induced distortion, resulting in smoother, clearer footage.

Beyond smartphone cameras, CML’s SMA technology is proving valuable across various industries due to its precision, compact size, low power consumption, and high force output:

• AR/VR Headsets: SMA actuators offer a compact, low-power, high-force solution for controlling optics and other moving components without adding bulk, helping to enhance the user experience.
• Medical Devices: In applications like insulin patch pumps, where precise, miniature actuation is critical, SMA actuators provide a reliable and energy-efficient solution that meets strict medical standards. The high force-to-mass ratio of SMA wire is enabling CML to design much smaller insulin patch pumps than are currently available. Such miniaturisation advantages of SMA-based patch pumps make them smaller and lighter than traditional pumps, enhancing patient comfort and discretion.

As demand grows for compact, high-performance actuation in small electronic devices, SMA actuator technology is well-positioned to play a key role in advancing smartphone cameras, wearable technology, and more...

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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. ACTUATOR SOLUTIONS based on SMA wire (thinner than human hair) can be controlled to submicron accuracy. These actuators are particularly suited to applications requiring high precision and force levels, in a fast, compact, and lightweight design. 

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

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