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Digital archive of theses discussed at the University of Pisa


Thesis etd-06112014-202933

Thesis type
Tesi di dottorato di ricerca
Thesis title
Soft Robotics: from Optimality Principles to Technology Readiness
Academic discipline
Course of study
tutor Prof. Bicchi, Antonio
  • optimal control
  • soft robotics
  • variable impedance actuation
Graduation session start date
For many years robotic researchers have been focused on developing robot sensing and control, while the actuation was left at the level of position controlled servomotors. In the last two decades this trend is changed, and actuators gained a richer dynamical behaviour. From Series Elastic Actuation (SEA) in which an elastic element was interposed between the motor and the link, to Variable
Stiffness (VSA) and variable damping actuators in which the actuation unit can physically change its dynamical parameters, i.e. stiffness and damping. This new actuation paradigm, called Soft Robotics, is perceived as an enabling factor to build robots that are robust, efficient, and have high peak performance. Robot that should be able to perform everyday-life tasks and to safely coexist with humans.

In such a background this work first concerns with: i) to compare different actuation paradigms to effectively evaluate the potential benefit of soft actuation w.r.t. conventional rigid motors; ii) to understand how to manage the additional design and control degrees of freedom of soft robots.

Optimal Control (OC) theory has been chosen as the fundamental tool to accomplish the task. This choice is motivated by two main reasons: i) on one side OC provides an absolute performance reference that factorises the control design out of the equation, hence it furnish a principled basis to compare the performance of different system designs; ii) on the other side, OC is a key element in understanding planning and control methodologies for soft actuators. A careful analysis of results, obtained through either analytic or numerical techniques, allows to distillate laws summarising control policies that can be applied to classes of tasks.

In this thesis, the methodology described above has been applied to show that there exists an optimal linear spring that maximises the peak speed, the energy efficiency, or the force tracking error under robustness constraint of a SEA actuator and its stiffness value depends on motor constraints (e.g. speed and torque), task parameters (e.g. terminal time), joint trajectories, environment parameters (e.g. the stiffness of the environment).
This fact is per se a strong motivation to employ VSA to obtain maximum performances for different kind tasks and environment. Moreover it has been possible to derive a complete analytical solution for the optimal control problem of position and stiffness control of one DoF robot. Through the analysis of these results the stiffness optimal control policy (dependent on the link motion) can be summarised by the law: stiff when speed-up, soft when slow-down.
Experimental tests showed: i)a more than doubled peak speed of the SEA (also tested on a two DoFs series elastic manipulator) w.r.t the rigid motor, and a further 30 % improvement with the VSA, ii) a good agreement between theoretical and experimental cost functionals for the study on the energy efficiency.

Once suitable control laws and field of application for soft Robotics has been defined it is necessary to bring this new actuation paradigm to the proper technology readiness level to be accessible to the whole robotic community not only to researchers focused on the robot design aspect.

In this work this problem has been tackled via: i)the definition of the main functional specifications of a variable impedance actuator that have been collected in a datasheet; ii) the development of a modular open-source and low-cost variable stiffness robotic platform: the qb move.

The datasheet substantially contributed to unify and standardise the language around this topic making this literature (and technology) accessible to a large number of researchers. The qb move (today it is a product commercialised by qb robotics s.r.l.) is intended to be used as a rapid prototyping platform for experimental tests, and it allows to drastically reduce the time to experiment to test new soft robotics applications.