**2.1. Principle of operation**

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Although SMAs are mostly used as actuators, they also have sensing capabilities. Despite most of the SMA physical parameters are strongly related in a nonlinear hysteresis fashion, the electrical resistance varies linearly with the strain of the alloy. Because strain is kinematically related to the motion of the actuator (either linear motion or rotational), the electrical resistance and the motion produced by the actuator are both linearly related. This linear relationship between resistance variation and motion is achieved because the martensite fraction is kinematically coupled to the motion, and the martensite fraction is what drives the resistance changes. This issue is an advantage for developing closed-loop position controllers that regulate the SMA actuation. In fact, most of the applications involving position linear control of SMAs, feedback electrical resistance measurements to estimate the motion generated by the actuator. This avoids the inclusion of external position sensors for

SMAs are used in a variety of applications [46],[40],[56],[29],[27],[80]. Their special properties have aroused great expectations in various technologies and industries; it can be used to generate a movement or storing energy. In addition, its scope covers many sectors ranging from the use in deployable satellite antennas for different sensors to machinery, to materials for the construction of suspension bridges or anti-seismic devices. In general, all applications somehow depend on the effect of action-reaction of the material and the conditions under which particular application takes place, which make the SMAs a functional material.

For instance, they are being used in many non-invasive surgery devices [45],[21],[62],[23],[43] and biomedicine, taking advantage of their large strains and their capability to recover the shape when the load is removed. This property allows applications in devices such as stents, tubular prosthetic devices, because it restores the ability of flow of any bodily duct affected by

In classical robotic systems, linear actuation systems have been proposed using SMAs. The focus of this chapter is on bio-inspired robotics. SMA-based actuators provide a suitable technology as muscle-like actuation mechanisms, which resemble the mechanics of muscles in biological systems. For this reason in the last years a number of bio-inspired robots have been

closing the control loop.

a narrowing.

**Figure 1.** Microscopic viewpoint of the Shape Memory Effect

Shape Memory Alloys are metallic materials with the ability to "remember" a determined shape, even after a severe deformation produced by a thermal stimulus. In the case of metallic alloys, the shape memory effect consists on a transition that occurs between two solid phases, one of low temperature or martensitic and other of high temperature or austenitic. The material is deformed in the martensitic phase and retrieves, reversibly, its original dimensions by heating above a critical transition temperature. The terms martensite and austenite originally referred only to the steel phases, however these terms have been extended referring not only to the material but also the kind of transformation. Thereby, the martensite steel involves a change of volume and shape, while the SMA has basically a change of length.

In general, NiTi (Nickel-Titanium) SMAs are the most common alloys used. This is basically because these materials are intrinsically susceptible of use both as sensors and actuators, which makes them suitable for integration in smart structures. NiTi SMAs work based on the shape memory effect, which essentially takes place by the influence of temperature change of the material; i.e. the temperatures at which the martensitic and austenite phase transformations begin and end. Figure 1 depicts how these changes occur at the microscopic level of the material. The phase transition occurs when the material is heated or cooled. In general, there is a certain temperature range for the transition, which is mainly defined by the manufacturer.

SMAs normally exhibit one-way shape memory effect, also called memory effect in a simple manner. The alloy deforms upon heating but cooling does not change the shape unless it is stressed again. The percentage of deformation of NiTi alloys (% of strain) is about five percent, a range considerably higher if one considers that the deformation of common steel allows only an average of two percent. Currently, SMAs that exhibit two-way shape memory effect are also manufactured. In this case, the alloy expands by heating above the range of transition temperature and spontaneously contract when cooled again below this temperature [47]. To produce the double shape memory effect, the material is subjected to heat treatment, also called training. This training-phase forces the material to remember both heating and cooling states.

From the microscopic viewpoint (Figure 1), all the physical properties of the alloy vary depending on the phase, i.e. from cooling to heating and vice versa. Some of these properties refer to corrosion resistance, elasticity, damping capacity, strain, stress, electrical resistance, and temperature. Therefore, shape memory alloys behave in a thermo-mechanical way, with all these variables strongly coupled within a nonlinear hysteresis fashion.

Table 1 shows the commercial characteristics of SMAs depending on the diameter of the wires (NiTiNol®). From the table it can be noticed their high electrical power consumption. In robotics applications, power consumption is a critical issue due to the level autonomy of the robotic system is fully dependent on the capacity of the onboard batteries.


4 Will-be-set-by-IN-TECH 56 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

**Table 1.** Characteristics of NiTinol®SMA wires [6].
