Morphological Control of Cilia-Inspired Asymmetric Movements Using Nonlinear Soft Inflatable Actuators

Abstract

Soft robotic systems typically follow conventional control schemes, where actuators are supplied with dedicated inputs that are regulated through software. However, in recent years an alternative trend is being explored, where the control architecture can be simplified by harnessing the passive mechanical characteristics of the soft robotic system. This approach is named “morphological control”, and it can be used to decrease the number of components (tubing, valves and regulators) required by the controller. In this paper, we demonstrate morphological control of bio-inspired asymmetric motions for systems of soft bending actuators that are interconnected with passive flow restrictors. We introduce bending actuators consisting out of a cylindrical latex balloon in a flexible PVC shell. By tuning the radii of the tube and the shell, we obtain a nonlinear relation between internal pressure and volume in the actuator with a peak and valley in pressure. Because of the nonlinear characteristics of the actuators, they can be assembled in a system with a single pressure input where they bend in a discrete, preprogrammed sequence. We design and analyze two such systems inspired by the asymmetric movements of biological cilia. The first replicates the swept area of individual cilia, having a different forward and backward stroke, and the second generates a travelling wave across an array of cilia.

Artificial Cilia–Bridging the Gap with Nature

“…In an attempt to limit the amount of fluidic valves, the same research group harnessed nonlinearities in a new design of bistable inflatable cilia to create both spatial asymmetry and metachronal asymmetry with only one pressure supply line, at a larger scale. [106] However, no fluid propulsion tests were carried out. Becker et al [28] used a dip molding process to create large-scale and high aspect ratio inflatable bending actuators, featuring the possibility of a metachronal asymmetry.…”

Nonlinear Inflatable Actuators for Distributed Control in Soft Robots

“…To design underactuated systems for practical applications, it must be possible to design nonlinear actuators for a desired set of p max and p min where these snapping thresholds: i) show a large variety over the design space, ii) are robust to imperfections in the manufacturing process, and iii) remain constant over a large numbers of actuation cycles. In the case of inflatable actuators with hysteron characteristics, literature [22,23,26,33] offers no designs that meet these requirements. For our conical shell actuator however, it is possible to accurately predict its p max and p min in all these cases by means of a Finite Element Model (see Section S4, Supporting Information).…”

“…This means that each actuator has two stable states and that discrete transitions between both states are triggered when the control signal passes a critical threshold. [ 20,21 ] In inflatable soft robotics, several actuators have been proposed with hysteron characteristics that stem from the used hyperelastic materials [ 20,22,23 ] or from nonlinear geometric effects in the buckling of beams, [ 24 ] flat sheets, [ 25 ] singly curved shells, [ 26–28 ] or doubly curved shells. [ 5,29 ] However the critical thresholds of these solutions are not separately tunable.…”

Soft robotics for infrastructure protection

“…Alternatively, a reduction of the number of tethers can be achieved by mechanically programming simple control tasks (e.g. a pattern of actuation sequences) in the hardware of the system, by harnessing the nonlinear response of soft actuators ( Gorissen et al, 2019 ; Milana et al, 2022 ) or soft valves ( Rothemund et al, 2018 ; van Laake et al, 2022 ).…”