Bistable SMA-driven engine for pulse-jet locomotion in soft aquatic robots

TL;DR

The paper addresses efficient underwater propulsion for soft robots by using a bistable SMA-driven engine inside a silicone bell to achieve pulse-jet locomotion. It introduces antagonistic SMA springs encapsulated as SpIRAl actuators to slowly store energy and rapidly release it for thrust, eliminating the need for a dedicated reset stroke. Experimental tests with the DilBot demonstrate a top speed of 158 mm/s and peak thrust of 5.59 N, with relatively symmetric impulses between strokes and jellyfish-like flow structures observed. These findings advance soft robotics for underwater environmental monitoring and motivate future work on drag reduction, swarm deployments, and quantitative flow/force analysis.

Abstract

This paper presents the design and experimental validation of a bio-inspired soft aquatic robot, the DilBot, which uses a bistable shape memory alloy-driven engine for pulse-jet locomotion. Drawing inspiration from the efficient swimming mechanisms of box jellyfish, the DilBot incorporates antagonistic shape memory alloy springs encapsulated in silicone insulation to achieve high-power propulsion. The innovative bistable mechanism allows continuous swimming cycles by storing and releasing energy in a controlled manner. Through free-swimming experiments and force characterization tests, we evaluated the DilBot's performance, achieving a peak speed of 158 mm/s and generating a maximum thrust of 5.59 N. This work demonstrates a novel approach to enhancing the efficiency of shape memory alloy actuators in aquatic environments. It presents a promising pathway for future applications in underwater environmental monitoring using robotic swarms.

Figures (8)

• Figure 1: The morphology and function of a biological box jellyfish are reflected in the DilBot, highlighted in the circle on the right. It consists mainly of a silicone bell attached to an SMA-driven engine. The robot and animal both achieve swimming by cyclic opening and closing of their bells to generate water jet thrust for propulsion. Water is slowly drawn into the body's interior when the robot bell expands. The water is then forced outwards upon rapid bell contraction to produce a reaction force that facilitates the net forward motion.
• Figure 2: Mechanical overview of the DilBot with (a) main body components and (b) concept model of the Bistable SMA-driven engine during actuation of the top pair of springs. Once one pair of SpIRAl actuators reaches the activation temperature, their state switches from OFF to ON, pulling the central hub from an equilibrium position through the central rod with force $F_{SMA}$ while linkages push the $k$ springs, slowly storing energy on them. As the central rod reaches the unstable peak energy position, the $k$ springs quickly release the stored energy with force $F_{k}$, causing the central hub to snap to reach another equilibrium position. The used actuators return to the OFF state, and the other pair can be readily activated to start the next stroke in the opposite direction.
• Figure 3: Theoretical energy plot of a bistable system, which presents two stable equilibrium states. The upper and lower curved arrows showcase what happens in terms of energy in our proposed system for both actuation directions, where an unstable central position causes the central hub to "snap through."
• Figure 4: Thrust force characterization experimental setup.
• Figure 5: Flow visualization experimental setup.