28th October 2015

From science fiction to reality – a sonic tractor beam

UK researchers have invented a sonic tractor beam that can move small objects up to 40cm.



Asier Marzo, PHD student and lead author, levitating a polystyrene ball with soundwaves.

Tractor beams are mysterious rays that can grab and lift objects. The concept was created by science-fiction writers, but has since come to fascinate scientists and engineers. A team of researchers at the Universities of Sussex and Bristol, in collaboration with tech firm Ultrahaptics, have demonstrated a working tractor beam that uses high-amplitude soundwaves to generate an "acoustic hologram" able to pick up and move small objects.

This technique, published yesterday in Nature Communications, could be developed for a wide range of applications. For example, a sonic production line could transport delicate objects and assemble them, without any physical contact. Or a miniature version could grip and transport drug capsules or microsurgical instruments through living tissue.

Sriram Subramanian, Professor of Informatics at the University of Sussex and co-founder of Ultrahaptics, explained: "In our device we manipulate objects in mid-air and seemingly defy gravity. We can individually control dozens of loudspeakers to tell us an optimal solution to generate an acoustic hologram that can manipulate multiple objects in real-time without contact."

The researchers used an array of 64 miniature loudspeakers, driven at 40Khz, to create high-pitched and high-intensity sound waves to levitate a spherical bead (4mm in diameter) made of expanded polystyrene. The whole system consumes 9 Watts of power. The tractor beam works by surrounding the object with high-intensity sound, creating a force field that keeps the objects in place. By controlling the output of the loudspeakers with extreme precision, an object can be either held in place, moved or rotated.

Asier Marzo, PhD student and the lead author, said: "It was an incredible experience the first time we saw the object held in place by the tractor beam. All my hard work has paid off. It's brilliant."

Bruce Drinkwater, Professor of Ultrasonics in the University of Bristol's Department of Mechanical Engineering, added: "We all know that soundwaves can have a physical effect. But here we have managed to control the sound to a degree never previously achieved."

The team have shown that three different shapes of acoustic force fields work as tractor beams. The first is an acoustic force field that resembles a pair of fingers or tweezers; the second is an acoustic vortex, with objects becoming trapped at the core; the third is best described as a high-intensity cage that surrounds objects and holds them in place from all directions.

Previous work on acoustic studies had to surround the object with loudspeakers, which limits the extent of movement and restricts many applications. Last year, the University of Dundee presented the concept of a tractor beam, but no objects were held in the ray.

The team is now designing different variations of this system: a much bigger version with a different working principle that aims to levitate a soccer ball from a distance of 10 metres; and a smaller version, targeted at manipulating tiny particles inside the human body.

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