Soft-actuators in cyclic motion: Analytical optimization of stiffness and pre-load

Abstract

In this paper, we study the role of soft actuation in the reduction of the energy cost for mechanical systems that perform cyclic tasks. The objective is to determine the optimal stiffness value and spring pre-load such that a given cost functional is minimized. For the analysis, we consider both fully actuated and underactuated mechanical systems using elastic actuators which, depending on how and where the springs are placed w.r.t. the actuator and the load, can be Series Elastic Actuators (SEAs) or Parallel Elastic Actuators (PEAs). The energy consumption depends not only on the actuation parameters but also on the trajectories followed to perform a given cyclic task. We show that the general problem in which both joint trajectories and actuation parameters are the optimization variables, can be cast as a simpler problem in which optimization regards only joint trajectories. Simulations of fully actuated and underactuted compliant robots are reported to demonstrate the effectiveness of the method. Although the stiffness optimization method is analytical in nature, it is directly applicable to existing systems whose model is unknown. A model-free experimental application on a prototype of a hopping robot with SEA is presented.The research leading to these results has received funding from the European Union Seventh Framework Programme under grants FP7 ICT-287513 "SAPHARI" and ICT-611832 "WALKMAN" projects.

Natural Motion for Energy Saving in Robotic and Mechatronic Systems

“…Comparisons between serial and parallel configurations can be found in References [84][85][86][87][88]. T. Verstraten et al in Reference [87], provide a comparison between stiff actuators, parallel elastic actuators and serial elastic actuators in terms of power as well as mechanical and electrical energy consumption.…”

Efficient Walking Gait Generation via Principal Component Representation of Optimal Trajectories: Application to a Planar Biped Robot With Elastic Joints

“…In (10) the gait is constrained to be periodic, by imposing the relation between x red (0) and x red (T h ). The additional inequality constraints, stated in (11), guarantee that the external contact wrench w belongs to the static friction cone, that the contact is unilateral and that the Center of Pressure (CoP) is inside the support area. Moreover, limits on joint positions and velocities, as well as on motor torques, are imposed.…”

“…compliance, purposefully introduced in the mechanical design to guarantee robustness [10], efficiency [11], peak performance [12], is usually neglected or not properly taken into account.…”

An efficient leg with series–parallel and biarticular compliant actuation: design optimization, modeling, and control of the eLeg

“…Some works have compared SEAs and parallel-elastic actuators (PEAs), mainly in torque and power requirements. Cyclic hoppers were considered by Velasco et al (2013) and Yesilevskiy et al (2015), and exoskeleton actuation was considered by Wang et al (2011). Grimmer et al (2012) showed that combining SEA + PEA can yield further benefits.…”