Dissertation 02 — MSc Mechanical Engineering

Bio-Inspired Pneumatic Soft Actuators for Underwater Robots Sea Life Exploration and Conservation

University College London · MSc Thesis AY24-25
Dated 01/09/2025 · Supervisor: Dr Adam Wojcik

Rayang holding the finished soft underwater robot prototype at UCL — FIG. 01 — SHOWCASING THE PROTOTYPE AT UCL

In a nutshell

I designed soft, squishy robot limbs — borrowed from manta rays and trilobites — that bend when water is pumped into them, so an underwater robot can study coral reefs without breaking them.

2sea-creature limb designs

11–20%more bend from 30° tilted chambers

00-30Ecoflex silicone — very soft

≈2.2 kgfull prototype mass

From screen to bench

3D render of the soft underwater robot with blue silicone limbs — FIG. 02 — THE DESIGN, RENDERED

The built prototype on a table: blue printed chassis, bellows actuators, breadboard electronics and translucent silicone limbs — FIG. 03 — THE BUILT PROTOTYPE

The cast silicone limbs lined up on the bench with their servos — FIG. 04 — CAST SILICONE LIMBS

01

The abstract, simply

Coral reefs are some of the richest — and most threatened — places on Earth, so we need to keep a close eye on them. Human divers can only go so far, it’s risky, and they can disturb what they’re studying. Most underwater robots are rigid boxes with propellers: clumsy around something as fragile as coral.

Sea animals solve this by being soft. This project builds a robot the same way: limbs made of silicone that move by being pumped full of fluid, so it can swim, crawl and gently grab — getting close to the reef without damaging it.

02

How it was done

Manta-ray inspired wing limb with segmented chambers — Design A — Manta rayA gliding wingA fin shaped like an aerofoil (78 mm chord, 143 mm span) with a row of small air chambers, so it can flap and glide efficiently like a manta.

Cut-away render of the trilobite-inspired limb showing its internal chambers — Design B — TrilobiteA walking legA 120 mm segmented leg inspired by the ancient sea arthropod — built to crawl steadily over an uneven seabed without being flipped by currents.

The muscle
A servo-driven bellows
A small motor turns a worm gear that squeezes a bellows, pushing water into the limb. Uneven walls inside make the limb curl one way as it fills.
The maths
Simulate before building
Each limb was modelled as a smooth arc and simulated in ANSYS with a rubber-like (Yeoh hyperelastic) material model, to predict how far it bends at a given pressure.
The build
Cast in silicone
Ecoflex silicone was poured into 3D-printed PLA moulds and vacuum-degassed to remove bubbles. An ESP32 board runs six servos, with pressure, tilt and camera sensors.

Cut-away render of the servo-driven bellows actuator — FIG. 05 — BELLOWS ACTUATOR

3D-printed two-part mould for the silicone limb — FIG. 06 — PRINTED MOULD

Wiring schematic: ESP32, PWM driver, sensors and servos — FIG. 07 — CONTROL ELECTRONICS

03

What happened

FEA result: the trilobite limb bending under pressure, coloured by deformation — FIG. 08 — SIMULATED BENDING (RED = MOST MOVEMENT)

Chart comparing simulated and experimental actuator tip displacement over time — FIG. 09 — SIMULATION VS EXPERIMENT

Angles matter. Tilting the chamber walls about 30° toward the bend gave 11–20% more movement for the same pressure.
More pressure, more bend — but not in a straight line. Going from 15 to 20 kPa increased bending unevenly, a sign of the rubber’s non-linear behaviour.
A smooth walking cycle. The limb tip traced a clean, repeatable path through each step.
Simulation matched reality. The MATLAB prediction closely tracked the measured tip movement; small wobbles came from real-world lag, stretch and sensor noise.
Getting the casting right. Vacuum degassing, warm moulds and slow pouring gave bubble-free, airtight limbs.

04

So what?

A working prototype was built and tested, showing soft, fluid-powered limbs really can move a robot underwater. There were small manufacturing flaws (sealing, limb consistency), and one limb design can’t do everything — so the next version splits the jobs.

One limb to grab, one to swim A buoyancy system Onboard camera object detection Remote control Ship-hull inspection