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Underwater Hydraulic Stiffness Control for Soft Swimming Robots

Updated: Apr 17, 2023

Previous work

This work is based on the tunable-stiffness principles outlined in the IROS2022 paper 'Energy-efficient tunable-stiffness soft robots using second moment of area actuation'. The foil, shown above, is comprised of a rigid nose connected to a soft tail. Embedded within the tail are two inflatable elastic tubes. The tail has a base stiffness provided by the silicone, and the tubes can be pressurised to increase the second moment of area and stiffness. In simple terms, pressurising the tubes causes an increase in the second moment of area of the tubes which adds to the overall stiffness of the system. The ability to tune stiffness has multiple benefits, including the ability to have soft underwater control surfaces and the ability to expand the range of efficient swimming speeds for a soft underwater propulsor. Up to this point, this work has been conducted with air as the working fluid and the tests have been conducted in air.

Under Water Disturbance Rejection

The system outlined above has so far been tested in air but to challenge the system underwater, we must switch the working fluid to water. The idea is to use to tunable-stiffness as a means of disturbance for a soft control surface. As can be seen in the figure below, the lift force of a rigid foil would be dramatically increased with an increase in angle of attack but if the foil was allowed to soften then the effect of the angle of attack disturbance change could be mitigated. The faster the response time, then the more a disturbance could be mitigated for.

Intended FlowIO Use

To avoid having to use a large, bulky linear actuator and syringe to stiffen the foil, FlowIO would allow for the system to be pressurised and de-pressurised in a compact, swift, efficient, and measured manner. FlowIO would also allow the system to be used for efficient propulsion generation. This would then allow for the system to be inserted within a swimming robot for a truly soft swimming robot with propulsion and heading control as well as disturbance rejection. The system operates in the pressure range of 0-0.8 bar and for this application the goal is to reject disturbances of duration of 10 convective cycles (the time it takes a fluid particle to travel a distance of 10 chord lengths). The change in volume required is approximately 30ml per tube, so 60ml overall. The desired flowrate is therefore 12-30 ml/s. Please check back here for future updates on assembly progress, any issues and debugging in the use of FlowIO for hydraulic applications!

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