Pneumatic artificial muscles
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Pneumatic artificial muscles (PAMs) are contractile or extensional devices operated by pressurized air. Similarly to human muscles, PAMs are usually grouped in pairs (see figure): one agonist and one antagonist.
PAMs were first developed (under the name of McKibben Artificial Muscles) in the 1950s for use in artificial limbs. The Bridgestone rubber company (Japan) commercialized the idea in the 1980s under the name of Rubbertuators.
PAMs are very lightweight because their main element is a thin membrane. This allows them to be directly connected to the structure they power, which is an advantage when considering the replacement of a defective muscle. If a defective muscle has to be substituted, its location will always be known and its substitution becomes easier. This is an important characteristic, since the membrane is connected to rigid endpoints, which introduces tension concentrations and therefore possible membrane ruptures. Another advantage of PAMs is their inherent compliant behaviour: when a force is exerted on the PAM, it "gives in", without increasing the force in the actuation. This is an important feature when the PAM is used as an actuator in a robot that interacts with a human, or when delicate operations have to be carried out.
In PAMs the force is not only dependent on pressure but also on their state of inflation. This is one of the major disadvantages, because the mathematical model that supports the PAMs functionality is a non-linear system, which makes them more difficult to control precisely. However, the relationship between force and extension in PAMs mirrors what is seen in the length-tension relationship in biological muscle systems. There are also other disadvantages: gas is compressible, so a PAM that uses long tubes must have a control system that can deal with a delay between the movement control signal and the effective muscle action. A PAM actuator system needs electric valves and a compressed air generator too, which are neither light nor small.
You can see how a complex configuration of Air Muscles are used in the Shadow Dextrous Hand.
