Sepios is a bionics project mimicking the undulating side fins of squid and cuttlefish. These animals feature a very high agility when moving under water. A technical implementation of their propulsion mechanism is practically silent compared to conventional propellers. A submersible robot with cuttlefish inspired fins enables it to move omni directionally.

ETH Zurich


Length70 cm
Wingspan95 cm
Weight22.7 kg
Parts> 4700
Max. Depth10 m
Speed1.8 km/h
Time1 h 30 min
9 per fin
Battery7.4 V (2S)
20'000 mAh
Current2 to 40 A
Power<300 W


A technical implementation of their propulsion mechanism is practically silent compared to conventional propellers. A submersible robot with cuttlefish inspired fins enables it to move omni directionally.

For this purpose four undulating fins were developed symmetrically around one central cylinder. The number and arrangement of the fins can be varied modularly. They are actuated by servomotors which are placed within waterproof enclosures. This allows for a high degree of flexibility as each fin segment can be steered individually. 


The four fins are arranged symmetrically around one base unit and can be interchanged modularly. Each fin contains nine servomotors. These are aligned inside a watertight fin case. There, each servo actuates one rotatable ray via two small shafts and a bevel gear. The rays are placed outside in the water. A latex foil glued to the rays constitutes the actual fin. By actuating the servos in the right order this foil performs the desired undulating movements.

The driving force behind the fin propulsion system is the servomotor HS- 5646WP produced by Hitec. Position feedback is integrated in every common servomotor. 

The fins consist of a cone shaped fabric glued to nine small carbon sticks. These sticks are designed to be the weakest link in case a fin collides with an object. This prevents the servos or bevel gears from getting destroyed during such calamities. Because of this and also for more flexibility, the rays
are fastened to the shafts with only a single screw each, allowing their quick attachment or detachment. 

There is extra space between the rays so that they can be operated at a reasonable phase shift without stretching the material. Three fins are blue and one fin is golden. This characteristic allows the operator to recognize the robot’s attitude, which would otherwise be very challenging as the robot is almost perfectly symmetric.

Base structure

The base unit is the central structure of Sepios. It is a cylinder made out of polycarbonate with two aluminium covers at each end. A carbon fibre construct keeps all the interior electronics in their place and is easily retrieved for maintenance. 

To tare the robot there is a swim bladder in the base unit. The cylindrical shape ensures to keep potential invading water from reaching the electronics. The cylinder is enclosed by two aluminium covers sealed with O-ring seals. These covers are screwed onto two octagonal aluminium rings imposed onto the polycarbonate cylinder. The octagonal rings allow for a modular attachment of the fin cases.

The interior structure is attached to the back cover by a metal thread for easy adjustment of the centre of gravity. Furthermore this connection to the back cover allows for an easy extraction of the interior structure by pulling on the cover. All the cables leading outside pass through this cover, no interior cables ever have to be detached to extract the structure. Next to all the plugs for cables leading outside, the back cover features holes for the swim bladder pump and for the pressure sensor. On the front cover, a big acrylic glass acts as a porthole for the built in camera to see through. 

Swim Bladder

This mechanism enables the robot to sink and lift without using its fins. This mechanism consists of a hydraulic cylinder and a gear pump which can fill or empty the cylinder with water from outside the robot. It is located inside the base unit. The pump is controlled by a motor driver in combination with a micro-controller.


Final report on the project, describing the technology, the design process, mechanical design, electronics, modelling & control. Describes the software, and an evaluation.

Martin Peter Moller, Allessandro Schappi, Pascal Buholzer, et al.

Contains the software libraries used in the Sepios project.

Wevolver 2023