Pumping iron

July 27, 2006
Artificial muscles mimic the action of their human counterparts to synthesize novel kinds of motion.

Leland Teschler Editor

The Humanoid robot can synthesize movements to about 1° of precision. Ten Festo Fluidic muscles go into each side of its torso and 16 more small prototype muscles in each wrist let the robot pick up and hold a 1-kg object in each of its hands.

Electronics and pneumatic valves sit inside the waterproof head of the Airacuda devised by Festo AG as a demonstration. The 100-cm-long fish weighs about 4 kg. Its internal ribs are lasersintered polyamide and the skin is silicone. Lithiumpolymer batteries provide electrical power for the valves controlling two Fluidic muscles for the tail and two more for steering.

An artificial pneumatic muscle gets shorter and contracts against a load if the pneumatic pressure increases. Exerted force is proportional to the change in actuator volume divided by the change in actuator length. Artificial muscles are generally set up to work in antagonistic pairs. Typical configurations either rotate a pulley or move a load linearly. In the case of the Airacuda fish developed by Festo AG, a pair of muscles deflect a tail fin to propel the device through the water.

Servomotors and hydraulics are great when you need precision and large amounts of rigid force. These technologies are less desirable for situations that demand a soft touch and some compliance. The latter needs have motivated research now underway on artificial devices that mimic the action of human muscles.

Many of the artificial muscles that researchers are experimenting with today are pneumatic. They work like inverse bellows, contracting as they inflate. The force they apply depends on their degree of inflation and operating pressure. They can be quite lightweight yet they can transfer as much energy as a pneumatic cylinder of the same volume operating at the same pressure.

These attributes make pneumatic muscles candidates for mobile robotics. Also, the fact they operate with air makes them relatively safe, and they can be replaced easily. All in all, they have attracted the interest of several research groups.

The pneumatic muscles made commercially available so far all have the same basic structure. Their main element is some kind of flexible reinforced membrane with fittings at each end. Application of gas pressure forces the membrane to bulge outward and thus shortens the muscle. The resulting action pulls on a load attached to the end of the muscle.

In operation, artificial muscles typically generate motion the same way the real ones do, by working in what's called an antagonistic setup. In other words, as one muscle moves a load, another acts as a brake to stop the load at the right position.

The classic example of an antagonistic setup consists of two artificial muscles each with one end connected to a rope. The other end of the muscles connect to a common base. The rope is strung over a pulley. Then a contraction of one muscle and a relaxation of the other rotates the pulley to synthesize rotary motion. Similarly, two muscles can mount opposite each other to move a platform back and forth in a linear motion.

The force that each muscle generates is proportional to the pressure of the gas in its membrane. Thus the position of the effector that the antagonistically coupled muscles drive will be determined by the ratio of their two gage pressures.

There have been several different kinds of pneumatic muscles developed over the years. But the type most widely used today is called a McKibben muscle, named for a researcher who introduced the design as an orthotic actuator in the late 1950s. It consists of a gas-tight elastic tube or bladder surrounded by braided sleeving. Braid fibers run helically around the long axis of the device at some angle.

Both braid and sleeving terminate in fittings at both ends of the muscle. The braid works against the inflated tube to make the muscle contract as inflation pressure rises. McKibben-type muscles were originally conceived as components for prosthetic devices. But they are now more likely to be used in robotic manipulators. One recent project in this area uses McKibben-type muscles made by Festo AG in Germany.-Dubbed Fluidic muscles, they go into a Humanoid robot that is a joint project with Evo-Logics GmbH and the Bionics and Evolution Technology Dept. of the Technical University of Berlin. Starting with a first functional study of a simple robot arm in 2000, the project has now developed into a torso with two anthropomorphic arms and fivefingered hands.

The Festo muscles attach to cables made of tough Dyneema fibers, material that is quite lightweight, keeping the mass of moved parts to a minimum. Researchers built a controller that switches two actuators together as an antagonistic muscle pair. The Humanoid robot can bend, stretch, and turn in much the same way as a human with a total of 48 degrees of freedom.

The Humanoid has almost the same radius of action as a human with similar dimensions. It can grasp objects thanks to a set of small Fluidic muscles in its wrists. The finger muscles for gripping are actually return springs that the Fluidic wrist muscles control through Bowden cables. (The second finger joints are not powered in this version of the Humanoid.)

Magnetic sensors resolve the motion of each joint to about 1°. Sensor data go to one of two microprocessors in each arm and hand. One reads pressure and angle sensors and calculates the next move for the actuators. The other directly controls valves which manage the air supply to the body and hands. The valves are fast-acting devices that are separate from the muscles they control. Festo says its next-generation Fluidic muscles will combine the valves and actuators into single units.

The Fluidic muscles in the Humanoid are driven by an air compressor generating 8 bar. The amount of air the Humanoid consumes depends on the intensity of its movements. When demonstrated at the recent Hannover Fair, the apparatus used air at about 60 lpm.

The robot can either follow preprogrammed motions or be controlled in real time by a human operator wearing a data suit and data glove. A PC relays the movement instructions to the microprocessors in the Humanoid. The robot can follow the data suit movements with about a half-second delay. Festo envisions future applications for bionic stand-ins based on Fluid muscles in places which are either inaccessible or too dangerous for humans.

Another example of how McKibben-type muscles can come in handy is the Airacuda, a demonstration device also devised by Festo. It is a remote-controlled, pneumatically driven fish. Its mode of movement is analogous to that of fish classified as ray fins. These animals contain thin, long rays of endoskeletal bone and swim by using muscles inside their trunk to move their tail. The Airacuda's tail muscles consist of two Festo Fluidic devices operating in antagonism. When inflated with compressed air at 6 bar the muscles contract by about 20% to make the tail flap. (The Airacuda Fluidic muscles are super-small 5-mm-diameter devices that aren't yet commercially available.)

Two additional muscles facilitate steering. The hull contains a cavity which floods with water or fills with air to make the Airacuda sink or rise. A pressure sensor evaluates depth and sends a corresponding signal to the electronic controls, which then regulate valves sending compressed air into the chamber.

The fin itself contains a flank connected to a rib structure. Diagonals in the structure are what the Fluidic muscles alternately shorten and release to move the tail back and forth. A human operator controls the tail and steers the fish via wireless link using a simple joystick controller.

The air accumulator in the fish was adapted from a paint ball gun. It provides about 400 L of compressed air at 300 bar, enough to operate the fish for about 35 min.

Festo says the Airacuda isn't just a demonstration platform for Fluidic muscles: Fin drives can have numerous advantages over ordinary propellers. Perhaps the most compelling benefit is that a greater proportion of the motion generated gets converted into thrust.

Festo AG, festo.com

About the Author

Leland Teschler

Lee Teschler served as Editor-in-Chief of Machine Design until 2014. He holds a B.S. Engineering from the University of Michigan; a B.S. Electrical Engineering from the University of Michigan; and an MBA from Cleveland State University. Prior to joining Penton, Lee worked as a Communications design engineer for the U.S. Government.

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