Shape memory alloy actuators

Sreekumar, M., Nagarajan, T., Singaperumal, M., Zoppi, M., Molfino R., 2007. Critical review of current trends in shape memory alloy actuators for intelligent robots. Industrial Robot An International Journal 34 (4), 285—294. 

Tactile display of spatial patterns on the skin uses three main types of transducers [Kaczmarek et al, 1991 Kaczmarek and Bach-y-Rita, 1995]. Static tactile displays use solenoids, shape-memory alloy actuators, and scanned air or water jets to indent the skin. Vibrotactile displays encode stimulation intensity as the amplitude of a vibrating skin displacement (10-500 Hz) both solenoids and piezoelectric transducers have been used. Electrotactile stimulation uses 1-100 mm -area surface electrodes and careful waveform control to electrically stimulate the afferent nerves responsible for touch, producing a vibrating or tingHng sensation. 

Benard WL, KahnH, Heuer AHetal (1998) Thin-film shape-memory alloy actuated micropumps. J Microelectromech Syst 7 245-251... 

Ma, N. and Song, G. (2003). Control of shape memory alloy actuator using pulse width modulation. Smart Materials and Structures 12, pp. 712-719. 

Shape memory alloys (SMA) undergo solid-to-solid martensitic phase transformations, which allow them to exhibit large, recoverable strains [3]. Nickel-titanium, also known as nitinol (Ni for nickel, Ti for titanium, and nol for Naval Ordnance Lab), are high-performance shape memory alloy actuator materials exhibiting strains of up to 8% by heating the SMA above its phase transformation temperature - a temperature which can be altered by changing the composition of the alloy. 

Lashlee, R. Butler, R. Rao, V. Kern, F. Robust Control of Flexible Structures Using Multiple Shape Memory Alloy Actuators. North American Conf. on Smart Structmes and Materials, Albuquerque, NM (Feb. 1993)... 

Epps, J.J. and Chopra, I. In-flight tracking of helicopter rotor blades vsing shape memory alloy actuators. Smart Mater. Struct., 10 (2001), pp. 104 111... 

Mineta, T., Mitsui, T., Watanabe, Y., Kobayashi, S., Haga, Y., Esashi, M. Batch fabricated flat meandering shape memory alloy actuator for active catheter. Sensors and Actuators A 88, 111-120 (2001)... 

Conventional tactile displays could hardly control delicate tactile sensation, because it was difficult to make fine distributed stimuli on a human skin under the limitations of their actuators, such as magnetic oscillators, piezoelectric actuators, shape memory alloy actuators, pneumatic devices, and so on. EAP materials have many attractive characteristics as a soft and light actuator for such a stimulation device. 

Beebe DJ, Moore JS, Bauer JM et al (2000) Functional hydrogel structures for autonomous flow control inside microfluidic channels. Nature 404 588-590 Benard WL, Kahn H, Heuer AH, Huff MA (1998) Thin-film shape-memory alloy actuated micropumps. J Microelectromech Syst 7 245-251 Benito-Lopez F, Antonana-Diez M, Curto VF et al (2014) Modular microfluidic valve structures based on reversible thermoresponsive ionogel actuators. Lab Chip 14 3530-3538 Chin CD, Linder V, Sia SK (2012) Commercialization of microfluidic point-of-care diagnostic devices. Lab Chip 12 2118-2134... 

The design of smart materials and adaptive stmctures has required the development of constitutive equations that describe the temperature, stress, strain, and percentage of martensite volume transformation of a shape-memory alloy. These equations can be integrated with similar constitutive equations for composite materials to make possible the quantitative design of stmctures having embedded sensors and actuators for vibration control. The constitutive equations for one-dimensional systems as well as a three-dimensional representation have been developed (7). 

Shape-Memory Alloys. Stoeckel defines a shape-memory alloy as the ability of some plastically deformed metals (and plastics) to resume their original shape upon heating. This effect has been observed in numerous metal alloys, notably the Ni—Ti and copper-based alloys, where commercial utilization of this effect lias been exploited. (An example is valve springs that respond automatically to change in transmission-fluid temperature.) Copper-based alloy systems also exhibit this effect. These have been Cu-Zn-Al and Cu-Al-Ni systems. In fact, the first thermal actuator to utilize this effect /a greenhouse window opener) uses a Cu—Zn-Al spring. 

Stoeckel, D. Shape-Memory Alloys Prompt New Actuator Designs, Adv. Mat. Proc., 33 (October 1990). 

The use of shape-memory alloys as actuators depends on their use in the plastic martensitic phase that has been constrained within the structural device. Shape-memory alloys (SMAs) can be divided into three functional groups one-way SMAs, tw o-vvav SMAs, and magnetically controlled SMAs. The magnetically controlled SMAs show great potential as actuator materials for smart structures because they could provide rapid strokes with large amplitudes under precise control. The most extensively used conventional shape-memory alloys are the nickel-titanium- and copper-based alloys (see Shape-Memory Alloys). 

As previously mentioned, the nickel—titanium alloys have been the most widely used shape memory alloys. This family of nickel—titanium alloys is known as Nitinol (Nickel Titanium Naval Ordnance Laboratory in honor of the place where this material behavior was first observed). Nitinol have been used for military, medical, safety, and robotics applications. Specific usages include hydraulic lines capable of F-14 fighter planes, medical tweezers, anchors for attaching tendons to bones, eyeglass frames, underwire brassieres, and antiscalding valves used in water faucets and shower heads (38,39). Nitinol can be used in robotics actuators and micromanipulators that simulate human muscle motion. The ability of Nitinol to exert a smooth, controlled force when activated is a mass advantage of this material family (5). 

As one would expect, there are a number of applications that currently use shape-memory alloy materials many more are projected for the future. The earliest application was for greenhouse window openers, with the metal serving as an actuator to provide temperature-sensitive ventilation. Some commercial faucets/showerheads are already equipped with this material that shuts off the water if a certain temperature is reached, which effectively prevents scalding. An intriguing future application will be for automobile frames as we will see later, some plastics may also be designed with shape memory. Someday soon, your car may reshape itself in front of your eyes within minutes after an accident ... 

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