Is it possible to have a pneumatic actuator that gives slow, smooth motion as well as a highly dynamic response? JON SEVERN investigates the Air Muscle

Pneumatic cylinders are commonly used where linear motion is required, but there are many applications where they are unsuitable. Their reliance on moving parts and seals makes pneumatic cylinders liable to leakage and wear, and they are not usually suitable for use in harsh environments.

A novel type of actuator that has a number of advantages over pneumatic cylinders is now being marketed by the West Group. The 'air muscle' is a lightweight single-acting pneumatic linear actuator with a high power-to-weight ratio. Although it is operated by compressed air, the air muscle has little else in common with pneumatic cylinders; for example, applying pressure causes the device to contract instead of extend.

Air muscles have been in continuous development for advanced robotics and animatronics work for several years. In these applications the attractive characteristics of the air muscles are their ability to provide slow, smooth, controlled movement as well as highly dynamic movements with immediate response. Manufactured by Shadow Robotics, the air muscles are now available through the West Group for use in industrial applications as a powerful lightweight actuator. The basic principle used in the air muscle is similar to that of a biological muscle. Consisting of a hermetically sealed rubber tube that is wrapped in a tough plastic woven sheath, the actuator is expanded radially by air filling the tube. The specially-shaped weave deforms in such a manner as to make the tube shorten axially as it expands radially - creating an axial pulling force.

Depending on the size, air muscles weigh as little as 10g, but they can have a power-to-weight ratio as high as 400:1, vastly out-performing both pneumatic cylinders and DC motors that can only attain a ratio of about 16:1. This characteristic makes them particularly useful for applications where weight is critical.

Air muscles behave in a very different manner to pneumatic cylinders; as the air muscle contracts under constant pressure, the pulling force produced between the end points decreases. The maximum possible force at a given pressure is obtained when the Air Muscle is extended as far as possible; if it is not taut, then the full potential will not be yielded. Furthermore, the relationship between pressure and force is linear at constant extensions, allowing the movement to be controlled by regulating the applied pressure.

Because of their simplicity, air muscles are cheaper to buy and install than other actuators and pneumatic cylinders - prices start at around £6. Being flexible, they can be operated when twisted axially or bent around corners, and they do not need to be aligned precisely.

Being sealed, the air muscles are suitable for difficult or dangerous situations, including cleanrooms and explosive atmospheres. And, having no sliding or rotating parts, they are also well-suited to situations where there are fine, abrasive particles such as on woodworking and grinding machines. One recent application employed a series of air muscles to uniformly tension fibreglass hull mouldings during boat construction. It must be remembered, however, that the air muscle is not able to guide the load directly, so this must be catered for in the application's design. Neither is it capable of taking compressive loads.

Available in four standard sizes of 12, 20, 30 and 50mm diameter with active lengths of 150, 210, 290 and 400mm respectively, the air muscles are able to provide up to 160kg pull, and are capable of contracting by as much as 40% of their original length. The strength of the muscle increases with increasing diameter, but changes in length influence only the resulting motion.

The West Group can supply batches of air muscles to different lengths if required, and with alternative end fittings. Meanwhile, plenty of information covering the standard products is available by using the free information card.

• West Group

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