Artificial Muscles
Artificial Muscles
Dateline: 02/26/00
In the past robot builders have mainly used motors and solenoids as primary actuators. Wheels and tracks were the most straightforward way of making your robot mobile, and most of the legged robots are still powered by servo or stepper motors. Of course, rolling motion is very efficient and simple, but what about more rugged terrains? And what if you need electric motors less than a millimeter in size? There are many cases when alternative actuators are advantageous because of their mechanical simplicity, high strength to weight ratio, silentness, and precise control.
For years, scientists have tested a wide variety of new materials, hoping to find new methods of turning electrical energy into mechanical motion. Pneumatic and hydraulic actuators came first: the McKibben Artificial Muscle is an excellent example of a pneumatic actuator which exhibits many of the properties found in real muscle. Its spring-like characteristics, physical flexibility, and light weight make it ideal for applications such as the Anthroform Biorobotic Arm. One of the best attributes of these devices is very high force to weight ratio (reaching 400:1), which is absolutely essential for real-world mobile robot applications. Compare it to the pneumatic cylinders and DC motors that can attain a ratio of only about 16:1 and you'll get the picture.
The technology was commercialized for robotic applications in the 1990's by the Shadow Robot Group of England. Their Air Muscle consists of a rubber tube covered in tough plastic netting which shortens in length like a human muscle when inflated with compressed air at low pressure. When actuated with a supply of compressed air, it contracts by up to 40% of its original length. It has no "stiction" and have an immediate response, resulting in smooth and neutral movement. According the the company's Web pages, a small Air Muscle, at just 6mm in diameter, has the strength, speed and fine stroke of a finger muscle in a human hand, while an Air Muscle 30mm in diameter is capable of lifting more than 70 Kg at a pressure of four bars.
Shape Memory Alloys (SMA's) are unique composites that undergo a crystaline phase change and shape change when heated or cooled. There are various memory alloys, but Cu-Zn-Al and Cu-Al-Ni have worse chemical, physical and mechanical characteristics when compared with corrosion resistant nickel-titanium alloy (nitinol). It creates direct linear motion by contraction: the material contracts up to the 10% of it's length when heated. Heating is accomplished by passing an electric current through the wire (also known as Ohmic Heating). As the wire cools, it expands back to its original length. The recovery rates can be increased significantly by using various heat sinks. Although they are sometimes considered slow for demanding robotic applications, advanced methods have radically improved the total contraction/relaxation time of nitinol fibers by exposing them to massive electromagnetic fields. A single displacement twitch can now occur more rapidly than the twitch response of a fast human muscle.
Mondo-tronics' Muscle Wire is probably the most popular form of nitinol amongst hobbyists. Recently, Mondo-tronics sold NASA some of its Muscle Wire and reference materials. As it turned out, researchers at the Lewis Research Center used the wire to operate a dust measuring device on the Sojourner Rover that went to Mars. For more down-to-the-Earth applications, the use of instruments and components made of nitinol in the medical technology is additionally promoted by their good body compatibility. Another interesting application of nitinol showcased at Mondotronics site is electric piston, fully integrated actuator that can pull up to a pound of force, contracting 20mm (20% of their length) without the use of compressors or hydraulic fluids. More info on nitinol and other SMAs can also be found at other sites, including Shape Memory Applications, Inc. and TiNi Aerospace, Inc.
Going a step further, scientists at NASA and elsewhere have developed lightweight polymers that move in response to electrical charges. Various polymer gels (also known as intelligent gels) exhibit abrupt volume changes in response to variations in their external conditions shrinking or swelling up to 1000 times their original volume.
Recent research at SRI International used prestrained previously stretched out - material with strain, pressure and response time exceeding those of natural muscle.
Yoseph Bar-Cohen, a researcher at NASAs Jet Propulsion Laboratory who specializes in plastic actuators, maintains one of the best Web resources related to this topic: WorldWide Electroactive Polymer Actuators Web hub.
If you don't find previous research results impressive enough, here's another one: nanotubes, long, thin straws of carbon measuring only one or a few nanometers in diameter, possessing strength and very unusual electrical properties. An international team of researchers created thin sheets of nanotubes (called "Bucky paper") and placed it in a saline solution while applying electric charge: one side expanded more than the other, and as a result the strip curls, thus flexing the "muscle". The first experiment was understandably simplistic, but keep in mind that carbon nanotubes are among the toughest, strongest materials known, having many "diamondlike" mechanical properties.
As for the further reading, I would like to suggest an excellent article on Intelligent Materials by Craig A. Rogers. Another good article focused at the actuation for mobile micro-robots can be found at site of North Carolina State Univeristy. Artificial Muscle Research Institute at University of New Mexico is filled with interesting movies and documents - don't miss it!