Machine Design

Actuators that mimic nature

A new type of pneumatic linear actuator moves by changing shape under pressure.

Allan Poxon
Festo Corp.
Hauppauge, N.Y.

Edited by Victoria Reitz

As compressed air fills Fluidic Muscles from Festo Corp., the muscles contract, bending automotive door frames.

Compared to pneumatic cylinders, Fluidic Muscles from Festo Corp., require only one-third of the cross-sectional area to produce matching forces.

The Fluidic Muscle is connected to a lever. As the muscle contracts, the lever lifts and puts tension on the belt.

Pneumatic actuators are commonly used for linear motion. Unfortunately they depend on moving parts and seals that make them susceptible to leakage and wear. A different type of actuator, the Fluidic Muscle from Festo Corp., does away with seals and moving parts and is suitable for harsh environments. The Fluidic Muscle is a tensile actuator which mimics natural muscles. Compared to normal pneumatic cylinders, it develops high initial tensile forces. The force decreases as contraction progresses, providing strong acceleration and a gentle approach to the desired end position.

The design of the Fluidic Muscle is relatively simple: A length of fiber-reinforced tubing with a strong fiber mesh sleeve is held in place by two connectors. The fiber mesh is arranged in a three-dimensional grid pattern. When compressed air enters the tubing, the grid pattern deforms, creating a bulge which generates a pulling force. The muscles contract axially as internal pressure increases. Fluidic Muscles generate their highest tensile force at the beginning of their stroke, so acceleration is at its peak. As the stroke increases, the displacement velocity slows until the muscle is at its end position.

Contraction reduces the actuator's length by approximately 25% of unloaded tubing length. End and intermediate positions are set by varying operating pressures. The flexible material ensures "even" deformation, with relatively predictable axial stroke and force.

Fluidic Muscles have no rods to guide or pistons to drive, just a hermetically sealed vessel that provides up to 10 times as much force as conventional actuators while consuming only 40% of the energy. They are free of sticking or jerking, even during slow movements.

If a static load is applied to the actuator over a long period of time (more than 500 hr), it becomes longer, and force is slightly reduced, assuming constant internal pressure and unchanged position. Relaxation is less than 5% of the length at room temperature, and is less than or equal to 10% at 60°C.

Continuous use at greater than 60°C is not recommended because the rubber elastomer ages prematurely. However, the actuator can handle temperatures above 60°C for short periods of time (several seconds). It also withstands dynamic operations at temperatures below 5°C, because compressed air warms it up after a few strokes. However, for static loads at temperatures below 5°C, it produces smaller forces because it takes more energy to expand the relatively stiff membrane. Engineers can modify the composition of the rubber elastomer for use above 60°C or below 5°C.

Maximum operating frequency depends on several factors, making it difficult to specify exact values. Frequencies of up to 3 Hz are possible without impairing service life. The required stroke, contraction, load, pressure, temperature, valves, and air-supply lines all influence design. For short cycle times, the actuator should be designed for contractions that do not exceed 10% and it should be equipped with open connectors at both ends for flushing and ventilation.

The "flexing" actuator is suitable for a wide range of applications including simulators, high-speed cutting, and the aeronautical technology, woodworking, metalworking, medical, and mining industries. The Fluidic Muscle's hermetically sealed design protects it in dusty environments. Its light weight also makes it suitable for mobile applications.

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