**1. Introduction**

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[33] Sittner P, Landa M, Lukas P, Novak V (2006) R-phase transformation phenomena in thermomechanically loaded NiTi polycrystals. Mechanics of Materials 38:475-492

> The chapter aims to provide a review on the basic issues concerning the techniques to handle innovative devices employing materials with hysteresis and the related control strategies.

> Nowadays, the great availability of materials designed to gather different physical characteristics (electric, magnetic, mechanic, etc.) working together to provide improved capabilities, has determined a huge rise of devices able to integrate different "coupled" functionalities into the same device. In this respect, the set of these "smart" or "multi-functional" materials and the devices employing them is really huge in number and quality. With no claim of exhaustivity, we can recall, for sake of example, thermo-electric, [11], or magneto-caloric [33] effects, where a coupling between Entropy/temperature and electric or magnetic fields is observed, and which promise new devices for heat recovery or refrigeration tasks, through the exploitation of new and more effective materials.

> Other materials, conversely, are able to couple mechanical quantities (i.e. stress and strain) to electric or magnetic fields so amplifying the application range for actuation or sensing aims. The most known and widespread are the piezo-electric alloys (Pb[Zr*x*Ti1−*<sup>x</sup>*]O3, PbTiO3), which show increased piezoelectric properties so allowing to make smart devices, otherwise unrealizable, [34], [32], [2]. A similar behavior is shown by the electro-active polymers, [3] which, exploiting the characteristics of polymers, allow to foresee really innovative applications. Materials with magneto-elastic coupling show a complementary behavior with respect to the former and enable to further increase the already huge set of potential smart applications. It is worth to recall the magnetostrictive materials, such as Terfenol-D or Galfenol, [21], exploited for actuation or sensing purposes, or the Ni-Mn-Ga alloys, which presents different magneto-elastic behavior and, even if less assessed than classical magnetostrictives, promise very interesting application, [37].

> Omitting electro- or magneto-rheological materials and many others, we would finally mention Ni-Ti alloys showing a thermo-mechanical coupling by the exploitation of the Austenite-Martensite phase transition that enables to realize further and interesting smart devices, [31], [4]. In summary, the interest and attention of researchers and inventors is so

©2012 Visone et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

focused on these materials and their applications, that everyone can experience, navigating on the web, an impressive "blooming" of ideas and proposals. The key element is the consciousness that smart materials achieve new functionalities going well beyond the "sum" of the individual properties and allowing to develop new and really innovative devices.

Among them, those materials coupling mechanical to other physical quantities (thermal, electric or magnetic) received a special attention since their suitability for sensing or actuation goals. The latter involved many researchers in a multi-disciplinary frame in designing and producing really smart actuators, able to provide high forces, or high precision *micropositioning*, or high speed responses, in dependence of the selected application.

In particular, as we can find in the scientific literature, the employ of Piezo-electric materials, [23], or magnetostrictives, [21], which are able to cover complementary sets of applications, play a primary role and many devices suited for micropositioning, active vibration control, smart actuation, ultrasonic generators, are available.

Similar conclusions can be drawn for Shape Memory Alloys (SMA) which show huge deformations driven by temperature variations and therefore are generally quite slow [45]. The Ni-Mn-Ga materials, also referred to as Ferromagnetic Shape Memory Alloys (FSMA), [40] overcome this limitation still preserving high deformations as SMAs.

However, all of them, share *rate independent* memory properties which strongly affect the global behavior of the device and its performances. Rate independence means that the observed memory behavior doesn't arise from "dynamics" and therefore is still kept also for "quasi-static" input variations, as happens in the well-known behavior of ferromagnetic materials. The common way to refer to it is *hysteresis*.

In the sequel, a strategy to "handle" devices employing smart materials with hysteresis in general working conditions will be outlined and several applications employing magnetostrictives will be discussed to check the validity of the proposed techniques.

**Figure 1.** Elastic and magnetic response of a Terfenol-D sample to a magnetic (a) or mechanical (b) input
