Exploiting Natural Dynamics to Reduce Energy Consumption by Controlling the Compliance of Soft Actuators

Abstract

Exploiting natural dynamics for bipedal locomotion, or passive walking, is gaining interest because of its energy efficiency. However, the natural trajectories of a passive walker are fixed during the design, thus limiting its mobility. A possible solution to this problem is creating a "semi-passive walker" equipped with actuators with adaptable compliance, which allows the natural dynamics to be changed according to the situation. This paper proposes a compliance controller, a strategy for continuously changing the compliance in such a way as to adapt the natural motion of the system to a desired trajectory. This opens up the possibility of following a range of different trajectories with a relatively low energy consumption. The idea is to fit the controllable actuator compliance to the "natural" compliance of the desired trajectory, and combine that with trajectory tracking control. This strategy was implemented and tested on a 1-DOF pendulum setup actuated by an antagonistic pair of pleated pneumatic artificial muscles. Both simulations and measurements show that the proposed strategy for choosing actuator compliance can significantly reduce the amount of control activity and energy consumption without harming tracking precision.

Exploiting Variable Stiffness in Explosive Movement Tasks

“…The optimisation of the passive system properties has also been discussed under more dynamic conditions. In this context, [22,23], demonstrated that tracking a small amplitude oscillatory motion of a pendulum can be made efficient by matching the (constant) stiffness of the actuators with the natural stiffness of the reference trajectory. With a similar objective in [20], the authors combined trajectory tracking with adaptation of the constant joint stiffness to its optimal value.…”

Kick it with elasticity: Safety and performance in human–robot soccer

“…The idea is to implement joint compliance not by means of control but via adjustable intrinsically compliant joints, inspired by the unquestionably successful design of human and animal muscles. The design and control of such systems were addressed in numerous publications [45,8,44,63,53,65]. Our paper follows this general line in clearly deriving why elastic joints are necessary for human-robot soccer.…”

“…In WL-14 [66,67], a sophisticated nonlinear spring mechanism was used for stiffness adjustment. More recently in Lucy [63], a biped that is able to walk in the sagittal plane, approaches were made to utilize adjustable passive compliance for high energy efficiency during walking. The robot Flame [29] uses constant compliance (Series Elastic Actuation) in the hip, knee, and ankle pitch joint.…”

“…The former has received larger attention especially for biped walking [66,67,63]. Our focus lies on the latter as it allows to considerably increase the link speed [56,54,51,24,65] above motor level.…”

“…Joint elasticity has long been addressed in lightweight robot construction, however more as an undesired consequence which the control has to handle [28,1]. An interesting and promising paradigm currently re-arising in robotics design is antagonism [61,63], or more generally variable stiffness/impedance actuation (VSA/VIA). The idea is to implement joint compliance not by means of control but via adjustable intrinsically compliant joints, inspired by the unquestionably successful design of human and animal muscles.…”