Shape memory materials (SMMs) are strong, lightweight materials that have the ability to recover their original shape after being deformed if a stimulus is applied. They can be programmed to remember their original shape. So if you bend or squeeze them you can get the original shape back again by applying an external temperature, stress, moisture, electric or magnetic field, pH, light or a chemical compound. This ability is known as the shape memory effect. The demand for structures capable of autonomously adapting their shape according to specific conditions has led to the development of NiTi alloys (SMAs) and polymers (SMPs). They possess superelastic behaviour, which allows large deformations with limited or no residual strain and a high power-to-weight ratio. Other properties include biocompatibility and high corrosion resistance, wear resistance and anti-fatigue.
Applications
SMAs are used in couplings and actuators and are particularly suitable for adaptive structures in robotics, aerospace and automotive industries. Systems based on SMA actuators are already in use in valves and drives, where they offer lightweight, solid-state alternatives to normal actuators found in hydraulic, pneumatic and motor-based systems.
Applications and markets for SMMs include actuators, tyres, aerospace, soft robotics and automotive, as well as many other applications such as dampers, ball bearings, sensors, miniature grippers, micro valves, pumps, landing gear and helicopter blades. Other potential applications include structural components that repair themselves, such as car parts in which the dents are repaired by applying temperature. They also have potential for aircraft that morph during flight, such as self-adapting, deformable wings that change shape.
Robotics is another rapidly growing application. Sometimes we need to design unusual robots to reach places that ordinary robots can’t get to. Engineers are now designing self-unfolding robots made from shape memory materials. They start off folded flat; when they need to be activated, an electric current heats them just enough to make them pop out into their preprogrammed permanent shape.
In the oil industry, SMAs can be used in deepwater valves, underwater connectors, seals and self-torquing fasteners. The phase transformation property enables them to withstand erosive stresses. SMA actuators can also be used for low frequency operations due to their high power density, high fatigue life and controllable actuation, as compared to their traditional counterparts, which can be more expensive, less reliable and bulky.
Aerospace
Boeing is applying cutting-edge smart material actuators to next-generation morphing technologies for aircraft. This has led to variable geometry chevrons (VGCs) that utilise compact, lightweight and robust SMA actuators. These actuators morph the shape of chevrons on the trailing edge of a jet engine in order to optimise acoustic and performance objectives. The VGC concept has demonstrated an exciting capability to optimise jet nozzle performance in multiple flight conditions. This variable area fan nozzle design can result in quieter and more efficient jet engines in the future.
SMAs are being explored as vibration dampers for launch vehicles and commercial jet engines. The large amount of hysteresis observed during the superelastic effect allows them to dissipate energy and dampen vibrations. They show promise for reducing the high vibration loads on payloads during launch as well as on fan blades in commercial jet engines, allowing for more lightweight and efficient designs. They also have potential for other high shock applications such as ball bearings and landing gear. There is strong interest in using SMAs for a variety of actuator applications in commercial jet engines that would significantly reduce their weight and boost efficiency.
Automotive
The first high-volume SMA product is an automotive valve used to control low-pressure pneumatic bladders in a car seat that adjusts the contour of the lumbar support. The overall benefits over traditionally used solenoids are lower noise, EMC, weight, form factor and power consumption.
The Chevrolet Corvette was the first vehicle to incorporate SMA actuators, which replaced heavier motorised actuators to open and close the hatch vent that releases air from the boot, making it easier to close. A variety of other applications are also being targeted, including electric generators to generate electricity from exhaust heat and on-demand air dams to optimise aerodynamics at various speeds.
Telecommunications
The second high-volume application is an autofocus actuator for smart-phones. There are currently several companies working on an optical image stabilisation module driven by wires made from SMAs.
Linear actuators: a new option for positioning
Kinitics Automation has developed linear actuators using bundles of wire made of specialty Ni-Ti alloy for reliable linear motion. Wires made of SMA are bundled together and anchored to an actuator housing. Current through the electrically resistive SMA generates heat; when warmed past a transition temperature, the atoms in the wires realign to another crystalline structure. This results in contraction of the wire when heated and re-extension when cooled to give linear motion output. In some applications these actuators are superior to other linear motion technologies.
Their reversible behaviour makes SMA-wire-based actuators suitable for an array of motion applications, including those on high-cycling machine axes. The rod is the sole moving part, so the actuator exhibits zero backlash or mechanical slop, regardless of how many times it is cycled.
Bundled-wire technology gives linear actuators a high strength-to-size ratio. Plus it retains high wire surface area to allow for forward-stroke times in less than 200 msec and full cycles in less than 5 seconds. When connected to a piston pump, the technology can be applied to fluid power applications.
For design engineers developing machine axes requiring positioning or force control, the Kinitics actuator is a new option. When paired with a controller, an input signal can modulate the actuator gain. Combining this with a feedback sensor and plant controller yields a closed-loop motion system. Programmers can leverage the actuator’s linear responsiveness when developing their control strategy. Usually it is possible to use a PI function block and eliminate the need to develop complex high-order programs.
By adding feedback sensors and a controller to the design, design engineers can even get these actuators to output complex motion profiles and fine positioning and force control. For the latter, a specialised drive-controller regulates current input to control output precise linear motion. Variations on the bundled-wire SMA arrangement for piston-pump operation can deliver fine pressure control, discharge temperature control and volumetric displacement control. When used for positioning, Kinitics bundled-wire linear actuators give repeatable accuracy and precision to within 5 µm. Running off 120 V AC input they can move 725 N over three positions. By limiting the controller gain it is also possible to control the rate of acceleration.
SMA linear actuators avoid some of the electrical noise issues of electromagnetic arrangements and the acoustic noise of mechanical devices such as gearboxes.
The best way to summarise the bundled-wire SMA linear actuator is to describe it as a high-force short-stroke option, scalable to about 2000 N of force. Strokes are to 10 mm. Suitable applications are those that need short strokes with exceptionally precise positioning. A small bundled-wire SMA linear actuator can accurately lift and lower over 70 kg with accuracy and position within 5 mm. Such linear motion is not possible with most other technologies.
Solenoids deliver excellent on-off ‘bang-bang’ strokes, as in poppet valves for fluid process applications, for example. These valves need a lot of force to act on the spring maintaining pressure during the static state. In contrast, SMA linear actuation can make such installations function as proportional valves, because unlike solenoids, controls can drive the actuator to any position to control flow.
Bundled-wire SMA linear actuators can also work as positioners to compete against piezo technologies. Stroke lengths go beyond those of piezos, so design engineers who need more than 0,1 mm to as much as 10 mm may consider bundled-wire SMA actuation as an option. Applications here include fine positioning of machining tables, mirror instruments and other applications that otherwise need the accuracy and precision of piezo actuators but with longer range.
Where stroke lengths are short, SMA actuators compete against traditional linear motion actuator setups that pair a rotary electric motor with a rotary-to-linear mechanical device. Elimination of the lost motion, backlash and wear surfaces of couplings, gearboxes and other mechanical components makes for higher precision as well as longer life.
The direct-drive concept of Kinitics Automation’s bundled-wire SMA technology arose from an automotive application to build an optimised clutch control for motorcycles capable of acting as a master cylinder − essentially a piston pump. “Conventional technologies, in particular, ballscrew-driven actuators, didn’t have enough power density to do the job within the space allotted in the motorcycle footprint,” said president, Dean Pick. “So using SMA technology we began five years of R&D; work on adaptations. At that time, nobody had successfully applied it to a linear motion application.” Once perfected, the team at Kinitics realised the technology might be useful for other linear applications in other industries. Now it has established an entire family of SMA linear actuators and piston pumps.
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