Unconventional actuators for robotics

A robot is a mechatronic system in which the combination of a mechanism, sensors and actuators results in a device that performs tasks autonomously or in collaboration with humans. Actuation in robotics is, of course, a central issue: an actuator is the element providing the mechanical energy that drives a robot's movements. Although robotics was initially focused on specific industrial contexts, with the realization of repetitive tasks, it is now becoming ubiquitous and its application contexts are therefore numerous: home automation, health, transport, etc. Various and sometimes very strong integration constraints for actuation are emerging from these new application fields. In medical robotics, for example, the need for compactness is extremely acute when it comes to developing millimeter-sized robotic surgical tools. Safety imperatives are also major, and we want to be able to materially limit tool travel to ensure the existence of guarded movements. In the context of mobile robotics, the weight/power ratio of actuators is a critical characteristic for reasons of autonomy. In bio-inspired robotics, we are also led to consider solutions that do not exploit simple rotational or translational movements, but rather movements corresponding to distributed deformations of structural elements, which need to be actuated. What's more, the compliant, i.e. non-rigid, nature of actuators can be an asset for robotic tasks. This is the case in mobile robotics, but also, for example, in collaborative robotics, where human-robot interaction can be enhanced by the presence of compliances. The association of an electromagnetic actuator and a compliant (i.e. flexible) element, in series with the latter, can be envisaged , but with a level of integration that remains limited. Finally, in all these contexts, conventional actuation strategies are finding their limits. Alternative solutions are essential. Three of these are presented in this article: piezoelectric, flexible fluidic and shape-memory alloy actuators. They are the most widely exploited if we compare them with other technologies such as thermal, phase-change or magnetostrictive actuation, whose use is rarer and more specific to small scales (≤ 1 mm). For each technology, the basic principle is first introduced, followed by a state-of-the-art review of the actuators developed....