Fishing for information

Robotic surveillance fish are to be given artificial fins to enhance their manoeuvrability, as part of a project for the US Navy.

Once fully developed, the fish are designed to swim with no visible wake and without producing propeller noise which alerts enemy sensors to a conventional vessel’s presence.

The development of the pectoral fins is the latest stage in a project – funded by the US Navy’s Office of Naval research since 1997 – which has already produced a 1m-long working prototype tail.

Like the tail, the fins will be powered by shape memory alloys (SMAs), or wires that stretch when heated to between 60 degrees C and 70 degrees C. This extension and subsequent retraction allows the tail and fin to replicate a fish’s natural motion.

Fish use their fins for manoeuvrability more than most people realise, said Dr Othan Rediniotis of Texas A&M University’s aerospace department, who, with colleague Professor Dimitris Lagoudas, has been involved in the project since its inception.

‘Our fins could be made of a passive material and have shape memory alloys at their base to move them. But a fin could also be made of an electroactive polymer, which is a plastic that acts like an SMA,’ said Rediniotis.

The team is now planning water tunnel tests, to allow them to measure the forces the fins will have to cope with, and Rediniotis is hoping to receive a further grant of £2.5m from the Office of Naval Research for the next stage in the project.

Having developed the tail, which consists of titanium nickel sections through which the SMA wires are threaded, the team will work on improving the heating system. At present the wires are heated electrically by battery, but the efficiency is only between 15 and 20 per cent.

One possible solution is to use hot liquids, as these produce more energy output per gram compared to batteries. A second cooler liquid would then be used to cool and shorten the wires after they have lengthened.

The team is also developing neural networks to enable the fish to navigate autonomously.

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