**5. Conclusions**

22 Will-be-set-by-IN-TECH

whereas the latter limits the amount of input heating power to prevent physical damage when SMAs are overloaded. As a result, the force controller presented in [71] was capable of tracking fast and accurate force references when compared with other works reported in

The improvement in accuracy and speed are due to two factors: i) by avoiding wire entanglement, and ii) by ensuring the passive wire of the antagonistic configuration does not cool completely. Wire entanglement can be produced when the SMA wires extended upon cooling. The passive SMA wire can develop a few millimeters of slack as it cools, which consequently affects the accuracy of the control. This slack-phenomenon is only presented in the antagonistic configuration, due to the two-way shape memory effect produced by each

To avoid the aforementioned issues, the anti-slack mechanism defines a minimum threshold of input heating power that ensures the inactive wire does not cool completely. The improvement in actuation speed is due to the fact that the already-warm SMA wire can begin to pull as soon as the heating current is raised, whereas a cold wire would first need to be raised to its operating temperature. It has been observed from experimental results in [71] and [5] that a suitable minimum threshold of input heating power is about 10% of the power

On the other hand, the anti-overload mechanism is in charge of ensuring that the maximum input power does not increase above an upper limit. This approach avoids overheating the SMAs in case the controller delivers a large amount of power to the wires. As mentioned before, this upper limit can be found using a phenomenological model, or by performing real

Both anti-slack and anti-overload mechanisms are key for improving on SMA performance under a force control architecture. The advantages of using a force-control scheme are twofold: i) high-bandwidth response, and ii) SMA fatigue avoidance. High-bandwidth response requires the use of force sensors capable of providing the force feedback. It has also been demonstrated in [71], that by using high-bandwidth force feedback, limit cycles of SMA

Nevertheless, for some systems, the use of force sensors could be a hardware limitation. In [5], it has been demonstrated that both anti-slack and anti-overload mechanisms can be implemented in a position control scheme. The position feedback can be achieved by measuring the electrical resistance of the SMA wires, which is a linear function of the strain. The key disadvantage of using a position scheme that forces the SMA to behave in overloaded operation mode relies on fatigue. As experimentally observed in [5], overloaded operation mode could be maintained only for about five minutes of SMA continuous operation before decreasing performance to nominal mode. For the application at hand, overloaded mode implied an actuation frequency of 2.5*Hz*, while nominal mode, an actuation frequency of 1.3*Hz*. In this case fatigue issues caused a decrease in actuation speed performance about 56%. Further investigations shall be devoted to quantifying the lifetime of SMAs when subjected to

higher stresses and larger heating currents within a position control scheme.

measurements of SMA temperature and stress on the wires.

[20], [84].

actuator.

applied.

operation are eliminated.

SMA technology allows the development of a wide variety of robotics designs, exploiting their advantages in terms of weight, volume and sensing capabilities. Their particular features open a wide range of possibilities hardly achievable with classical technology: hydrofoils, morphing shapes, and hydrostats are just some of the new concepts that have been made possible to develop thanks to SMAs. The prototypes described in this chapter have been classified first according to the operation environment (water, air and ground) and second according to their application in the robotics system.

Researchers have successfully overcome SMA disadvantages in terms of power consumption, actuation speed and low strain, by:


Biomimetics has emerged as a very promising field together with bio-medical applications. Even if SMAs cannot substitute classical servomotor and hydraulic technology in general, niches can be found where they can effectively compete with, and even outperform standard actuation technology. Clearly, merely replacing servomotors with SMA-based actuators in classical linear-actuation mechanic setups would make little sense. But dedicated mechatronic design, such as embedding SMA fibers in silicone obtaining morphing materials, bending of continuous structures and hydrostats make the best out of this technology, regardless the relatively small strain achievable from the fibers. Finally, cleverly designed control strategies, that exploit the knowledge of the physics of the material and of its behavior over time, coupled with dedicated mechanic setups can dramatically reduce response time.

Our experience with SMA-based actuators has revealed a relatively little considered drawback, which relates to fatigue. In fact, extensive testing with our prototypes has demonstrated that even if SMA performance can be improved by overloading, the effects of overloading disappear quite rapidly. We believe that further investigations shall be devoted to study the behavior of the alloys when exposed to high stresses and large heating currents over a large period of time.

In conclusion, we can say that Shape Memory Alloys have demonstrated great capabilities, as it has been demonstrated by this review of the state of the art. Most importantly, they have the potential to be used in future robotic systems: the more researchers will apply and study them, the more their limitations are smoothed and their employment becomes more effective. After all, they are being extensively investigated only since a short time in comparison with electric motors and hydraulic actuators, that can boast more than a century of history.
