Woven Active Textiles and E-Skin Enable Controllable Tunable-Stiffness Flapping Foils

Flapping foils represents the natural mode of propulsion for the vast majority of aerial and aquatic organisms. Despite its supposed simplicity, this type of propulsion manifests a subtle complexity which has fascinated academics for decades. In recent times, the role of structural flexibility of fins and tails in determining the overall propulsive efficiency of self-propelled organisms and robotics artefacts has attracted attention because of its implications for the optimal design of bioinspired vehicles as well as energy harvesting devices. In particular, the opportunity to actively adjust the stiffness of an otherwise passive-elastic, man-made flapping foil offers unprecedented potential in attempting to match the propulsive efficiency of the biological counterpart. On one hand, tuning the structural flexibility of a flapping foil permits to align the natural frequency of the structure with that of the flapping motion, enabling to exploit the benefits of resonance in magnifying power output; on the other hand, control over the temporal pattern of stiffness variation enables enhanced capability for manoeuvrability and disturbance mitigation. To achieve this, flapping foil systems will require actuators which enable active stiffness tuning along with dedicated sensors for online estimation of the foil spatial configuration.<br/>Here we present the design of new variable-stiffness flexible foils by means of active textiles. These consist of bundles of thin fluidic artificial muscles which can be woven into planar arrangements with a periodic pattern, thus giving rise to a fabric with adjustable, anisotropic stiffness. The foil is studied under dynamic actuation resemblant of the cyclic mode employed during flapping routines, ultimately identifying the range of flapping frequencies where resonant modes can be triggered. In parallel, we develop a stretchable, capacitive electric skin embedded with a capacitance-to-deformation transformer which provides realtime, millimeter-accuracy estimation of the foil camber during actuation.<br/>The combination of active textiles and e-skin offers the chance to develop advanced flapping-based propulsors with fine, closed-loop control over the stiffness and spatial configuration of the foil, enabling unprecedented performances and further narrowing the gap between biological and man-made systems.