**2. SMA linear actuators**

36 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

SMA is well described by the Clausius-Clapeyron law [1-5].

stabilization of the material behavior can be reached after few thermal loops.

**Figure 2.** Example of thermo-mechanical hysteresis typical of a SMA material.

minutes).

starts at As and finishes at Af.

**Figure 1.** DSC scan of 200μm NiTi wire in the fully annealed state and after aging (500°C for 10

Then, the deformation induced by a state of stress on a SMA element can be recovered heating the material over the austenitic characteristic temperatures: the shape recovery

The transition temperatures are strongly influenced by the applied load, since it stabilizes the martensitic structure causing an increase of all transition temperatures. This behavior of

Furthermore, SMA actuators generally need a training to stabilize their fatigue properties in terms of transformation temperatures and strain recovery. For example, a possible procedure consists in the thermal cycling under a constant load. Figure 2 depicts the evolution of the temperature-stroke relationship during the first thermo-mechanical cycles. It is seen that the

> The first case study is the application of a simple straight wire as base element for actuation systems [18]. The most important functional properties, which characterize shape memory wire for actuators, are the capability to recover high strain values, the characteristic transformation temperatures and the thermo-mechanical cycling stability [1]. These

characteristics of SMA can be tailored acting on chemical composition, thermal and mechanical processing, shape setting treatment and training [1, 20-25].

In recent years, the good ductility of NiTi intermetallic has been exploited for the production of few tens micron wires [21, 26, 27].

Thanks to their small dimensions, such thin wires permit to reach very short cooling time but, obviously, they can be employed just in all those applications in which high loads are not required. The fabrication of these products has led to the miniaturization of SMA components for actuators that are nowadays employed for new kind of devices such as optical image stabilizer and autofocus for small camera [26]

During working life, the SMA wire is subjected to thermo-mechanical cycling and accumulates plastic deformation, elongating irreversibly and reducing its diameter (see Figure 3). This behavior has to be considered in actuator design. Another important point to consider is the unsteadiness of the functional properties of shape memory wires during the working life. The plastic deformation induced during the thermo-mechanical cycling leads to a change of microstructure of SMA wire. Lattice defects, dislocations and nano-scaled precipitates are introduced into the matrix and they cause significant changes in transformation temperature and in the capability of the material to recover a deformation [28-30], as depicted in Figure 4.

**Figure 3.** Thermal cycling of a 80μm wire under constant stress (200MPa). The heating was performed by electrical pulse, and stroke was limited at 3%.

In most cases, SMA wires used as actuator, are heated by means of an electrical pulse (Joule effect) and cooled by natural air convection. So, it is very easy to reach short actuation time (heating time) but the reset time (cooling time) cannot be substantially controlled. Slight improvement in the reduction of reset time is achieved positioning the shape memory wire horizontally [19], as shown in Figure 5. In this figure, it can be noticed that after heating (at 200s) the strain is maintained to two constant values for both the two configurations till time is near to 230s and 250s for horizontal and vertical position respectively. After these points, the two wires start the cooling process which results to be faster for the horizontally positioned wire than the vertical one. As an example, it can be seen that at 275s the horizontally positioned wire recovers half of its maximum strain while at the same time the vertical positioned wire has just started the cooling route.

38 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

optical image stabilizer and autofocus for small camera [26]

of few tens micron wires [21, 26, 27].

[28-30], as depicted in Figure 4.

by electrical pulse, and stroke was limited at 3%.

mechanical processing, shape setting treatment and training [1, 20-25].

characteristics of SMA can be tailored acting on chemical composition, thermal and

In recent years, the good ductility of NiTi intermetallic has been exploited for the production

Thanks to their small dimensions, such thin wires permit to reach very short cooling time but, obviously, they can be employed just in all those applications in which high loads are not required. The fabrication of these products has led to the miniaturization of SMA components for actuators that are nowadays employed for new kind of devices such as

During working life, the SMA wire is subjected to thermo-mechanical cycling and accumulates plastic deformation, elongating irreversibly and reducing its diameter (see Figure 3). This behavior has to be considered in actuator design. Another important point to consider is the unsteadiness of the functional properties of shape memory wires during the working life. The plastic deformation induced during the thermo-mechanical cycling leads to a change of microstructure of SMA wire. Lattice defects, dislocations and nano-scaled precipitates are introduced into the matrix and they cause significant changes in transformation temperature and in the capability of the material to recover a deformation

**Figure 3.** Thermal cycling of a 80μm wire under constant stress (200MPa). The heating was performed

In most cases, SMA wires used as actuator, are heated by means of an electrical pulse (Joule effect) and cooled by natural air convection. So, it is very easy to reach short actuation time (heating time) but the reset time (cooling time) cannot be substantially controlled. Slight improvement in the reduction of reset time is achieved positioning the shape memory wire horizontally [19], as shown in Figure 5. In this figure, it can be noticed that after heating (at

**Figure 4.** Thermal loop under constant stress (200MPa). The heating was performed by a thermal chamber. The test was carried out on a 80μm wire before and after 5x104 thermal cycles under constant stress (200MPa).

**Figure 5.** Comparison between the mechanical performance of horizontal and vertical configuration of 80μm SMA wire used as actuator [19].

As previously stated, the heating process of a SMA actuator can be easily obtained by Joule effect. In this case, it has to be considered that the shape of the electrical pulse used for the actuation strongly affects the functional properties of SMA [27]. As an example, Figure 6 shows the time required by the wires to recover the 3% of deformation (actuation time) when it is heated by two different current pulses. At the 1st cycle ramp and step electrical pulse employed to heat two 80μm wires were designed to have the same electrical efficiency but the actuation time is 400ms by step current pulse while it is 623ms by ramp. After 5·103 cycles the wire heated by step pulse employs 618ms, so 218ms more than the time employed at the first cycle. As opposite, the ramp pulse leads to an increase of just 22ms. After 5·104 cycles the actuation time related to the wire heated by step is even higher than the one related to wire heated by ramp. This behavior leads to a drastic decrease of the step heating method efficiency (from 0.91% to 0.55%), vice versa the efficiency of the ramp heating method does not change so much (from 0.91% to 0.75%).

Then, in order to achieve fatigue performance acceptable for the specific device, the right electrical pulse has to be chosen considering the number of cycles that the SMA actuator has been designed to work.

Recently, the effect of drawing procedure on functional fatigue of thin NiTi wires has also been investigated [21]. Basically, 80μm NiTi wires were produced through two different drawing procedures reaching the same final cold working level before shape setting. These two processes differ for the number of drawing steps carried out to reach a certain cold working level before each heat treatment. After the last thermal treatment the specimen that underwent to a less number of drawing steps shows a narrow thermal hysteresis, even after thermo-mechanical cycling (see Figure 7). It means that adopting a severe drawing procedure leads to improve the actuation performance.

**Figure 6.** Actuation time tests performed by electrical heating (on the left) and by efficiencies (on the right).

**Figure 7.** DSC scans, on the left, and thermal loop under constant tensile stress, on the right, of two 80μm NiTi wires obtained by a severe (SVR) and soft (SFT) drawing procedure.
