Pneumatic artificial muscles
Encyclopedia
Compressed air
Compressed air is air which is kept under a certain pressure, usually greater than that of the atmosphere. In Europe, 10 percent of all electricity used by industry is used to produce compressed air, amounting to 80 terawatt hours consumption per year....
filling a pneumatic bladder. In a vague approximation of human muscles, PAMs are usually grouped in pairs: one agonist
Agonist (muscle)
Agonist is a classification used to describe a muscle that causes specific movement or possibly several movements to occur through the process of its own contraction. This is typically a term designated for skeletal muscles...
and one antagonist
Antagonist (muscle)
Most muscles work in pairs, and when a muscle works it needs to have an agonist and an antagonist, unless the muscle's natural state is opposite to that which is produced by the muscle, example Sphincter ani externus muscle....
.
PAMs were first developed (under the name of McKibben Artificial Muscles) in the 1950s for use in artificial limbs. The Bridgestone
Bridgestone
The is a multinational rubber conglomerate founded in 1931 by in the city of Kurume, Fukuoka, Japan. The name Bridgestone comes from a calque translation and transposition of ishibashi, meaning "stone bridge" in Japanese....
rubber
Rubber
Natural rubber, also called India rubber or caoutchouc, is an elastomer that was originally derived from latex, a milky colloid produced by some plants. The plants would be ‘tapped’, that is, an incision made into the bark of the tree and the sticky, milk colored latex sap collected and refined...
company (Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
) commercialized the idea in the 1980s under the name of Rubbertuators.
The retraction strength of the PAM is limited by the sum total strength of individual fibers in the woven shell. The exertion distance is limited by the tightness of the weave; a very loose weave allows greater bulging, which further twists individual fibers in the weave.
Examples of complex configurations of Air Muscles are used in the Shadow Dexterous Hand
Shadow Hand
The Shadow Dexterous Hand is a humaniform robot hand system developed by The Shadow Robot Company in London. The hand is comparable to a human hand in size and shape, and reproduces all of its degrees of freedom...
.
Advantages
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 muscleMuscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
. 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
Robot
A robot is a mechanical or virtual intelligent agent that can perform tasks automatically or with guidance, typically by remote control. In practice a robot is usually an electro-mechanical machine that is guided by computer and electronic programming. Robots can be autonomous, semi-autonomous or...
that interacts with a human, or when delicate operations have to be carried out.
Disadvantages
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 controlControl theory
Control theory is an interdisciplinary branch of engineering and mathematics that deals with the behavior of dynamical systems. The desired output of a system is called the reference...
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.
Due to the loose-weave nature of the outer fiber shell, it is important for the surface fibers to stay evenly distributed and undisturbed across the internal bladder. If a gap is created by externally pushing an object into the loose weave, this gap allows nonuniform swelling of the internal bladder, which may inflate through the gap and rupture the internal bladder.