Air Muscle
engineering fundamentals Air Muscle
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Introduction  

Nature can be a great source of inspiration. This Air Muscle is an actuator that works very similarly to a human muscle—it contracts by thickening. Inside the black pouch is a balloon. High pressure air pumped through the tube inflates the balloon causing the muscle to shorten by as much as 40%.

Air Muscles can provide substantial pulling force for their small size; they can exert force 400 times their weight. Typical DC motors or pneumatic actuators can exert about 16 times their weight. The largest of the standard Air Muscles is 11 inches long, weights less than 3 ounces, and can lift 154 pounds!


Courtesy: The Shadow Robot Company Ltd.

Air Muscles work when twisted, bent around corners, or under water. It even has a similar power profile to a human muscle: the force exerted decreases as it contracts, just like the strength of your biceps is maximum when your arm is extended and decreases as your arm is bent.


Courtesy: The Shadow Robot Company Ltd.

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Further Information  

The Air Muscle consists of a rubber tube covered in tough plastic netting which shortens in a scissor action when pulled out like a human muscle when inflated with compressed air at low pressure.

The pull force vs. length diagram is illustrated below. The longer the length, the higher the pull force. However, the strain at high pull force region is much smaller than at lower force region.

System Requirements for Air Muscle include:

  • A source of compressed air, air compressor or hand-pump
  • Piping to deliver this air through the system.
  • Fixtures to terminate air lines on other components of the system, and to each other.
  • Air filtering and regulating equipment.
  • Air flow control equipment (Valves).
  • Muscles
  • Mounting points for the muscles

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Applications  

Any place an actuator is needed, such as

  • Robots
  • RC toys
  • Industrial Automation
  • Animatronics/Prosthetics

Since air muscles have nearly the same power curve and ability as human muscles, they have a great ability to mimic human motion. Other possible applications include blood pressure cuff mechanism, jaws of life, and demolition projects.

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Common Specifications  

Common specifications for commercially available air muscles are listed below:

  Weight: 10 ~ 80 g (0.022 ~ 0.18 lb)
  Diameter: 6 ~ 30 mm (0.24 ~ 1.18 in)
  Length (Fully stretched): 150 ~ 300 mm (5.9 ~ 11.8 in)
  Output (Pull force): 30 ~ 350 N (6.6 ~ 77 lbf), normal
70 ~ 700 N (15 ~ 150 lbf), Max.
  Max. inflated pressure : 2 bar (30 psi), unloaded
4 bar (60 psi), loaded

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Pros and Cons  

  • Pros:
  - Light weight (10 ~ 100 g)
  - Low component cost
  - Smooth in operation
  - Flexible in alignment
  - High power-to-weight ratio, up to 400:1
  - Self-Damped
  - Compliant
  • Cons:
  - Limited size availability
  - Long term reliability and maintenance cost unknown

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