Astute’s quiet confidence

Advances in sonar capabilities mean that future submarines will need to be even stealthier if they are to go undetected.

This is the drive behind an initiative between BAE Systems Submarine Solutions and Sheffield University that aims to make the Royal Navy’s 7,400-tonne Astute class submarines emit less noise as they roam the world’s oceans. BAE Systems has committed £2.5m over the next five years to the project, which is expected to yield results within two years.

Engineers at Sheffield University’s Centre for Research in Active Control believe their demonstrator platforms will show how noise can be reduced throughout the submarine using a hybrid mix of passive and electronically controlled active technology.

The passive portion of their system would reduce vibration inside the submarine hull by isolating machinery such as the diesel generator with a large block of natural rubber. Noise throughout the rest of the vessel would be further attenuated with computer controls and electrodynamic shakers — devices that look like robust loudspeakers.

The Sheffield research team believes this hybrid technology will supersede earlier developments in this area of submarine stealth technology, such as Project M Mount.

Steve Daley, director of Sheffield’s active control centre, said Project M involved magnetically levitating machinery inside the submarine’s hull. Daley was one of the developers of this prototype system.

‘It was very successful, but it was also very expensive,’ he said. ‘And its failure mode wasn’t very effective because if you lost power, you lost your isolation system. You were no longer levitating machinery.’

Daley’s research team thought of a way to use electronically controlled active systems in addition to a passive component, in this case rubber. ‘If your active system should fail for whatever reason, you are left with your best passive measure,’ he said.

Submarine noise is attenuated with forces from electrodynamic shakers. At the heart of each shaker is coiled wire suspended in a fixed radial magnetic field. When a current is passed through the coil, it will produce a noise-cancelling sound wave that can be applied to a resonating point on the submarine.

The technology works on the principle that sound is a pressure wave that consists of a compression phase and a rarefaction phase. The electrodynamic shaker will emit a sound wave with the same amplitude and the opposite polarity (in antiphase) to the original sound. The waves will then combine to form a new wave, in a process called interference, and effectively cancel each other out.

The Sheffield team was not only concerned about cancelling noise created by the humming machinery inside the submarine’s hull — they also paid special attention to reducing noise from outside machinery. Noise from parts such as the propeller has traditionally been hard to control because it is extremely difficult to mount sensors and actuators on them.

The research team developed a technology called remote selective damping, which reduces vibration at inaccessible points with a remotely located control system. The system applies corrective vibrations at non-resonant parts of the craft connected to the resonant part.

‘It is a big challenge because you are trying to control points you are not measuring,’ said Daley.

The system works by applying forces at a certain point on the submarine and measuring the response with an accelerometer. ‘The acceleration signal then feeds into our computer, which works out with a control algorithm how to drive the force that will minimise vibration at specific points on our structure,’ said Daley. ‘The intellectual part goes into how we design that control algorithm.’

Vibrations throughout the rest of the submarine will be measured with on-the-spot sensors. Sensors will detect the frequency of vibrating parts of the submarine and feed that information into digital signal processors, said Daley.

The microprocessor then works out with a control algorithm the correct waveform to cancel the vibration at various points on the submarine.

Daley said his team was currently testing their technology out in the laboratory but they also had the opportunity to try it out full-scale on decommissioned submarines. It could be ready to install in current and future submarines in two years’ time.

‘The idea is that it’s retrofit,’ he said. ‘We don’t only want to address future submarines, but our current fleet too.’

Sheffield University’s remote selective damping has further applications outside submarines. Daley said these could include remote selective damping for bridge or building resonance. The technology could also be used for helicopters.

‘The transmission of vibration from helicopter blades goes down the rotating shaft into the thrust bearing system and into the fuselage of the helicopter,’ he said. ‘It is not so much a noise problem, but a comfort problem for passengers and crew.’

Siobhan Wagner