Analysis , Design and Control of a new Generation of Compliant Actuators for Safe Human - Robot Interaction

Recent studies demonstrate that flexible (compliant) structures are safer with respect to conventional stiff actuation systems [1-2] and are even capable of being more energy-efficient [3] or achieve better performances [4] if controlled properly. However, despite the proved benefits of compliance, the integration of actuators with passive fixed or variable stiffness within multi-DOF (MDOF) robotic structures still remains a challenging task because of the strict volume and/or weight requirements often present in these type of structures. This work presents new generation class of compliant actuators which offer a potential solution to be used within MDOF robotic systems cooperating or working nearly with humans and their control. The work is structured as follows: a preliminary analysis is used to determine which category of compliant actuators (between series or antagonistic elastic actuators) offers lower energy consumption levels [5] in order to choose the best candidate for the realization of a compliant actuator unit. As a consequence of this analysis, a series elastic actuator (SEA) prototype has been designed and built [6], paying particular attention to the dimensions and the modularity of the resulting actuator in order to make the integration in compact MDOF robotic systems feasible. This SEA has the possibility of further tuning the impedance perceived from the human/environment interacting with it to a value imposed by the user by means of an admittancebased control law. The last phase is based on the design and realization of a variable physical damping actuator (VPDA) [7] which has the objective of assisting the compliant MDOF system control task by providing variable damping levels on demand. This design is motivated by the fact that the compliance embedded in these actuators introduces dynamics which can induce oscillations with a Matteo Laffranchi is with the Italian institute of Technology (IIT), Genova 16163, Italy (phone: +39 010 71781 418; e-mail: matteo.laffranchi@ iit.it), and within the University of Sheffield, Sheffield S10 2TN, UK. Nikolaos G. Tsagarakis is with the Italian institute of Technology (IIT), Genova 16163, Italy (phone: +39 010 71781 428; e-mail: nikos.tsagarakis@ iit.it). Darwin G.Caldwell is with the Italian institute of Technology (IIT), Genova 16163, Italy ( e-mail: darwin.caldwell@ iit.it). frequency that depend on the stiffness of the joint and the inertia of the link. This complicates the dynamics of the system, especially if a multiple DOF system is considered resulting in a system which is difficult to control. The capability of regulating the damping levels could be necessary whenever there are variations on the configuration of the robot and/or the trajectory and loads. Moreover, the VPDA system has the further possibility of locking the relative motion of the compliant joint and thus effectively managing the potential energy storage and release in the compliant element of the joint or be disconnected when required. Future work will include the analysis of the relationship between the different physical variables of the robotic system (stiffness, inertia, velocity among others) in order to introduce safe-oriented design criteria. Another work that is currently in preparation includes the integration of the variable physical damping actuator (VPDA) [7] in the SEA of [6] in order to develop a compliant actuator with variable physical damping to be used in compact MDOF systems.