New Robotic Arms Gain Flexibility of Elephant’s Trunk with Artificial “Muscles”

In contrast to traditional robot arms with their hinged and swivel joints, the flexible arms being created by Professor Stefan Seelecke and his research team at Saarland University are built using “muscles” made from shape-memory wires that possess the potential to twist in nearly any direction and to coil themselves around corners.

The flexible arms are power-driven electrically and hence can function without the standard pneumatic equipment or other large accessories. Since the shape-memory alloy itself has sensor properties, there is no need for any extra sensors to control the arms. The innovative technology can be used to construct large robotic arms with the flexibility of ultrafine tentacles or an elephant’s trunk for use in endoscopic operations.

The research group will be at Hannover Messe from the April 1^st to 5^th, 2019, where they will be presenting the potentials of the shape memory arms using prototypes at the Saarland Research and Innovation Stand (Hall 2, Stand B46). Seelecke’s team is searching for partners who have an interest in creating technology for practical applications.

There are restrictions to the flexibility of limb regardless of whether it is a human arm or robotic arm. Usually, the joints are quite large and link rigid bones or mechanical assemblies. Motion is normally limited to certain spatial directions. On the contrary, an octopus’s tentacle or an elephant’s trunk provides far greater agility.

The presence of tens of thousands of muscles allows these creatures to move the tentacle or trunk in all directions, to bend it to just the appropriate degree, and to hold objects with great power. These natural models inspired the engineers at Saarland University. They are creating robotic arms that eliminate the need for rigid skeletons, or frameworks or joints, developing structures that are lightweight as well as very agile.

Professor Stefan Seelecke and his group are joining hands with scientists from Darmstadt Technical University on a project funded by the German Research Foundation (DFG) where they are creating thin, precisely controlled artificial tentacles. In the future, the system could find application as a guide wire in cardiac surgery or as an endoscope in colonoscopic and gastroscopic procedures.

The scientists are thus equipping the artificial tentacles with supplementary functions such as a gripper or a tip with modifiable stiffness that offers an enhanced pushing force. However, the technology can also be improved successfully to produce large robotic arms analogous to an elephant’s trunk.

The flexibility of these new robotic arms is due to the artificial “muscles” employed by the Saarbrücken research group. These muscles are made up of ultrafine nickel-titanium (nitinol) wires that can shrink and expand in a controlled manner. The ultrafine nitinol wires shrink like real muscles, based on whether or not an electric current is passing.

Nickel-titanium is what is known as a ‘shape memory’ alloy, which means that it is able to ‘remember’ its shape and to return to that original shape after it being deformed. If an electric current flows through a nitinol wire, the material heats up, causing it to adopt a different crystal structure with the result that the wire becomes shorter. If the current is switched off, the wire cools down and lengthens again.

Professor Stefan Seelecke, Intelligent Material Systems Lab, Saarland University.

His group at the Intelligent Material Systems Lab at Saarland University develops bundles of these wires that function as artificial muscle fibers. “Multiple ultrathin wires provide a large surface area through which they can transfer heat, which means they contract more rapidly. The wires have the highest energy density of all known drive mechanisms. And they can exert a very high tensile force over a short distance,” explains Seelecke, who also carries out research at ZeMA—the Center for Mechatronics and Automation Technology in Saarbrücken. The team of scientists at ZeMA is developing a variety of applications for these wires, from innovative cooling systems to new types of pumps and valves.

For the robot arms, the scientists connect the wire bundles so that they serve as flexor or extensor muscles, which produce a flowing motion when they work together.

The tentacles that could be used in future as medical catheters or in endoscopic procedures have diameters of only around 300 to 400 micrometers. No other drive system is of comparable size. Previous systems used for catheter procedures were significantly larger and this tended to limit their capabilities.

Paul Motzki, Research Assistant, Saarland University.

Paul Motzki also wrote his doctoral thesis on the shape memory wires.

It is possible to develop multifunctional tools and very precisely control the new tentacles. For instance, the distal end of the tentacle can be made to do a pushing movement. The correct pattern of movement needed is designed by the scientists and then programmed on a semiconductor chip. Moreover, the system does not require any additional sensors. The wires themselves offer all the required information. “The material from which the wires are made has sensory properties. The controller unit is able to interpret the electrical resistance data so that it knows the exact position and orientation of the wires at any one time,” stated Paul Motzki.

In contrast to traditional robotic arms that need power from an electric motor or from a pneumatic or hydraulic system, the arms being created in Saarbrücken do not require any such bulky equipment. Electric current is all that the wires need. “This makes the system light, highly adaptable and quiet to operate, and it means that production costs are relatively low,” stated Professor Seelecke. The research team will be presenting the system prototypes at Hannover Messe and will be showing the capability of these new continuum robotic arms.