July 9, 2007 Robot arms are expensive to build and dangerous to operate. If a robot system malfunctions, people can be injured. This is not the case of ISELLA, a bionic robot arm that is kind on the purse and gentle with people. An elephant’s trunk served as inspiration for its design.
It is long, gray, soft and – endowed with no fewer than 40,000 muscles – extremely agile. An elephant uses its trunk to grasp objects and for drinking. With their trunks, the pachyderms can tear down trees and pull heavy loads, and yet are also capable of performing extremely delicate manipulations. Researchers at the Fraunhofer Institute for Manufacturing Engineering and Automation IPA in Stuttgart have used the elephant’s trunk as a design model. “Its suppleness and agility gave us the idea for a bionic robot arm, ISELLA,” recounts Harald Staab, the IPA researcher who invented and developed the technology.
Robot arms often present a risk to human operators – a technical hitch can provoke wild, uncontrolled movements. Not so ISELLA. Whereas conventional robot arms have only one motor to drive each articulated joint, ISELLA has two, 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, our robot arm has a simple, low-cost muscle, consisting of a small electric motor with a drive shaft and a cord,” explains Staab.
In the same way as a tendon attaches one muscle to another, the cord links two related moving parts. 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. The researchers have dubbed this DOHELIX. “The shaft is no thicker than the cord, but is strong enough to resist breaking.
Consequently, it has a higher transmission ratio than a conventional geared motor,” Staab explains. This has been achieved using elastic materials with a very high tear strength – the type of material used to manufacture yacht sails and hang gliders. As a result, DOHELIX is much cheaper and more energy-efficient than a system of gears. Its tensile force is many orders of magnitude greater than its own weight, and drive systems based on the DOHELIX concept can be used in applications on all scales – from micrometer-scale muscles to cranes in container seaports.
The ISELLA robot arm consists of a total of ten DOHELIX muscles, providing a flexor and an extensor for each articulated joint, four situated in the elbow and six in the upper arm. The robot arm is as flexible as a human arm. “At present we are working on the elbow,” relates Staab. Possible applications for ISELLA include medical rehabilitation, for instance in therapy to restore the use of injured limbs, and low-cost, flexible prosthetic devices. Such devices could be commercially available within about two years, Staab estimates.
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