Agile ‘glider’ robot swims without active propulsion system

Autonomous underwater vehicles like Boaty McBoatface are versatile tools for exploring the oceans, sometimes travelling for weeks or months without returning to the surface to aid research, exploration or defence efforts. However, they can be disruptive to the environment and struggle to travel through confined spaces.

The Purdue University researchers have been exploring an alternative to conventional autonomous submarines: a highly manoeuvrable, low-cost underwater glider which operates silently and less disruptively. Components and sensors of the glider can be easily swapped out or added according to a range of mission specifications.

“Our goal is persistent operation of mobile robots in challenging environments,” said Professor Nina Mahmoudian, a mechanical engineer. “Most underwater robots have limited battery life and must return back after just a few hours. For long-endurance operations, an underwater glider can travel for weeks or months between charges, but could benefit from increased deployment opportunities in high-risk areas.”

A glider has no propeller or active propulsion system, instead adjusting its own buoyancy to sink, rise and propel itself forwards. This allows for extremely energy-efficient vehicles, although they are also slow, expensive and not especially manoeuvrable (particularly in shallow water).

Mahmoudian has addressed many of these drawbacks with the development of Roughie (Research Oriented Underwater Glider for Hands-on Investigative Engineering. Roughie, which has been in development since 2012, is roughly 1.2m long and shaped like a streamlined torpedo with no outward propulsion or control surfaces other than a static rear wing.

When deployed, Roughie pumps water into its ballast tanks to change its buoyancy and provide an initial glide path angle. It shifting its mass forward and backward as a pitch (up-down rotation) control mechanism. It controls roll (rotation around longitudinal axis) with a suite of inner components mounted on a rotating rail. The design is modular – the team experimented with various sensors – making it suitable for a variety of applications.

“This is a totally unique approach. Most underwater gliders can only operate in deep oceans and are not agile for confined spaces,” Mahmoudian explained. “Roughie has a turning radius of only about 10 feet, compared to an approximately 33-foot turn radius of other gliders.”

The vehicle is agile enough for testing in the diving well of the university’s aquatic centre. By installing a motion capture system of infrared cameras in the water, Mahmoudian and her team tracked the vehicle’s movements with millimetre accuracy. They programmed Roughie with a variety of flight patterns, including sawtooth (up and down), circular, and S-shaped patterns; it was able to perform all of these tasks within the confined space of a swimming pool.

Roughie’s manoeuvrability opens up the possibility of exploring otherwise inaccessible parts of the ocean which have been eluded by humans and conventional gliders. In addition to its research applications, it could perform tasks such as detecting shipwrecks and underwater munitions.

“It can operate in shallow seas and coastal areas, which is so important for biology or climate studies, and because it’s totally quiet it won’t disturb wildlife or disrupt water currents like motorised vehicles do,” Mahmoudian added.