Up in arms

The muscle structure of an elephant’s trunk inspired researchers to create a new type of robotic arm, and the technology could have wide uses. Siobhan Wagner reports.

A new robotic arm that draws inspiration from an elephant’s trunk is claimed to be less expensive and safer to operate than previous designs. The device, called Isella, uses multiple systems of motors, drive shafts and cords to work like the muscles in a human arm or an elephant’s trunk.

The researchers behind the development, from the Fraunhofer Institute for Manufacturing Engineering and Automation (IPA) in Stuttgart, believe the concept could be applied to a variety of devices, such as factory robots or cranes in container seaports.

Harald Staab, the IPA researcher who invented and developed the technology, said his research team had originally used an elephant’s trunk as a model because of its strength and agility. A trunk contains about 40,000 muscles, and is used to grasp objects and to drink.

With their trunks, elephants can tear down trees and pull heavy loads but are also capable of performing extremely delicate manipulations.

Staab said he thought such delicacy would be useful for robotic arms, which can pose a risk to human operators if technical glitches cause wild, uncontrolled movements. Staab and his team designed Isella to make such problems less likely by giving the arm 10 muscle-like systems, which he admits is significantly less than the number in an elephant’s trunk.

‘An elephant has a huge number of muscles and, from a technical point of view, it’s very difficult to control every muscle,’ he said. ‘So we simplified the design with 10 muscles and came to something that looked more like a human arm.’

Conventional robotic arms have only one motor to drive each articulated joint, but Isella has two. They are grouped in pairs so that if one motor control should fail, the second takes over to prevent uncontrolled movements.

Unlike pneumatic or hydraulic actuation systems, the Isella arm has a simple muscle consisting of a small electric motor with a drive shaft and a cord. The cord links two related moving parts and the drive shaft is attached to the midpoint of the cord. When the shaft turns, the cord wraps around it in both directions, forming a kind of double helix. This pulls the parts towards each other, causing the arm’s joint to flex.

Although the shaft is no thicker than the cord, it is strong enough to resist breaking. So each turn of the motor exerts more power than it would if it were working through a conventional gearbox.

Staab said this was achieved by using a highly flexible and strong polyethylene-based fibre called Dyneema, from DSM. As a result, the ‘muscle’ is much cheaper and more energy-efficient than a system of gears.

The tensile force of the muscles is greater than their own weight, said Staab, so the arm could theoretically be used in heavy load applications.

‘The concept of the muscle is scalable with respect to any size, quality, performance speed or force,’ he said.

The Isella robotic arm is equipped with four muscles in the elbow and six in the upper arm, providing a flexor and an extensor for each articulated joint.

Staab said it was almost as flexible as a human arm, but there is still room for improvement.

‘At present we are working on the control of the motors that move the elbow so that it doesn’t oscillate or slip,’ Staab said.

Outside of industry, the researchers believe the concept behind the robotic arm has many medical applications, such as medical rehabilitation in therapy to restore the use of injured limbs, and low-cost, flexible prosthetic devices.

Staab said any commercial applications were still two years away.