Eight soft arms swim well without coordinated timing
To Coordinate or Not to Coordinate? Learning to Swim with Eight Soft Arms
Robotics
Summary
Controlling eight soft robotic arms to swim usually involves coordinating how they move together. The authors tested whether each arm could just change how strongly it moves while sharing the same rhythm, without trying to sync or delay the arms' movements. They trained different controllers with reinforcement learning and found that letting each arm learn its own strength without forcing a timing pattern worked best for swimming speed and direction. This means complicated timing between arms isn’t necessary to swim effectively with many soft arms.
What this means in practice
- •For soft robotics engineers: Develop swimming robots with multiple soft arms that adjust individual arm strength without complex timing coordination.
- •For underwater vehicle designers: Create underwater robots that swim efficiently by controlling arm movement amplitude rather than synchronizing phases.
Authors
Junkai Chen, Dehan Yuan, Yiyuan Zhang, Yu Duan, Fan Shi, Dixia Fan, Cecilia Laschi
Abstract
Does effective swimming with multiple underactuated soft arms require explicit coordination between arms? We hypothesize that an eight-arm swimmer can control its swimming speed and direction by independently learning each arm's actuation amplitude under a shared rhythmic drive, without prescribed inter-arm phase coupling. To test this hypothesis, we compare three reinforcement learning-based control strategies using proximal policy optimization (PPO): (1) learning arm-specific actuation amplitudes without inter-arm phase coupling, (2) learning actuation amplitudes with prescribed inter-arm phase coupling, and (3) directly learning actuator commands through end-to-end PPO without an explicit rhythmic structure. Across 200 episodes with randomized three-dimensional velocity commands, the controller without phase coupling satisfies the tracking criterion in 169 episodes, compared with 117 for the phase-coupled controller and 28 for end-to-end PPO. Straight-line swimming and commanded direction-change trials further demonstrate more accurate velocity tracking without phase coupling. These findings suggest that effective eight-arm swimming does not require prescribed inter-arm phase coupling, but can instead be achieved through learned arm-specific amplitude modulation within a rhythmic control structure.