The Air Muscle is an extraordinary actuator that is small, light, simple and 'friendly'. It is soft, has no stiction, is easily controllable and exceptionally powerful. The Air Muscle consists of a rubber tube covered in tough plastic netting which shortens in length like a human muscle when inflated with compressed air at low pressure. An Air Muscle has a power-to-weight ratio as high as 400:1, vastly outperforming both pneumatic cylinders and DC motors that can attain a ratio of only about 16:1. It has been in continuous development for advanced robotics work by Shadow since 1982, and is now available for use in a variety of applications as a powerful, lightweight actuator. Air Muscles are normally operated using compressed air in the 0-70psi (0-5 bar) range. Advantages of the Air Muscle include:

Lightweight - Air Muscles weigh as little as 10 grammes - particularly useful for weight-critical applications.

Lower Cost - Air Muscles are cheaper to buy and install than other actuators and pneumatic cylinders.

Page Index Advantages Range Available Overview Construction Properties Operational - requirements Fixtures and fittings

Smooth - Air Muscles have no 'stiction' and have an immediate response. This results in smooth and natural movement.

Flexible - Air Muscles can be operated when twisted axially, bent round a corner, and need no precise aligning.

Powerful - Air Muscles produce an incredible force especially when fully stretched.

Damped - Air Muscles are self-dampening when contracting (speed of motion tends to zero), and their flexible material makes them inherently cushioned when extending.

Compliant - Being a soft actuator, Air Muscles systems are inherently compliant.

Range of Air Muscles Available The Air Muscles are currently available in three standard sizes. Strength of the muscle increases with increasing diameter; longer muscles give a longer movement. The standard sizes are:

Thumbnails	Diameter	Length (active)	Weight (approx.)	Pull (3.5 bar)	Max. Pull
	6 mm	150 mm	10 g	3 Kg	7 Kg
	20 mm	210 mm	40 g	12 Kg	20 Kg
	30 mm	290 mm	80 g	35 Kg	70 kg

	For ordering information, please see our ordering page. It is relatively easy for us to produce a batch of Air Muscles of a different length - please contact the Shadow Robot Company for more details.
	General Overview The Shadow Air Muscle is a simple yet powerful device for providing a pulling force. It behaves in a very similar way to a biological muscle. When actuated with a supply of compressed air, they contract by up to 40% of its original length. The force it provides decreases as it contracts, and the first few percent of the contraction is very powerful indeed. The simplest use of a muscle is to move a lever. One muscle will pull the lever in one direction, and a spring can return it. Two muscles will allow the lever to be pulled in either direction, with considerable force. Because the muscle contracts over a known distance, it can be used to provide a safe movement: there is no need to ensure that the lever is not going to be rotated beyond its end-stop, because the muscle will only move the lever to its set up angle. Air Muscle Construction The Core of an Air Muscle is a rubber tube.... ....wrapped in a tough plastic weave.... ....which shortens in a scissor action when pulled out, just like a Chinese finger puzzle. As the rubber tube fills with air it is forced to expand. A small Air Muscle, at just 6mm in diameter, has the strength, speed and fine stroke of a finger muscle in a human hand. An Air Muscle 30mm in diameter is capable of lifting more than 70 Kg at a pressure of only four bar, while a large muscle (50mm) has enough power to pull down a brick wall. Most of the robots built by the Shadow Project are driven by Air muscles, and looking at these will provide much insight on the possible uses of the Air Muscle. Properties The Air Muscle behaves in a different manner to the pneumatic cylinder or other actuator. As the Air Muscle contracts under constant pressure, pulling force produced between the endpoints decreases. The maximum possible force at a given pressure is obtained when the Air Muscle is pulled out as far as possible. If the Air Muscle is not taut, then it will not yield its full force.
		At a constant pressure, the curve of the Force against Length is shown on the right. Note that, if the Air Muscle is not fully stretched at the start of the movement, initial force will be lost and the Air Muscle will under perform by a significant margin. The relationship between pressure and force is linear at constant extensions. This allows the movement distance to be set by regulating the pressure in the system. Characteristics of pressure, length and force are given below. These measurements are typical of Air Muscles of the specified size.

Percentage Contraction of Active Length for 6mm Air Muscle

			6mm Air Muscle PRESSURE		LOAD 0.5 Kg		1 Kg		2 Kg		3.2 Kg		4.6 Kg
			0 Bar		N/A		3%		2%		2%		1%
			2 Bar		12%		10%		7%		5%		3%
			4 Bar		23%		20%		20%		17%		11%
			6 Bar		28%		26%		26%		23%		19%

Percentage Contraction of Active Length for 20mm Air Muscle

			20mm Air Muscle PRESSURE		LOAD 0 Kg		5 Kg		10 Kg		15 Kg		20 Kg		25 Kg
			0 Bar		N/A		8%		6%		2%		1%		0%
			2 Bar		35%		29%		23%		17%		12%		10%
			4 Bar		36%		33%		29%		25%		23%		20%
			6 Bar		37%		34%		31%		29%		26%		24%

Percentage Contraction of Active Length for 30mm Air Muscle

			30mm Air Muscle PRESSURE		LOAD 0 Kg		10 Kg		20 Kg		35 Kg		50 Kg		70 Kg
			0 Bar		N/A		2%		1%		0%		0%		0%
			2 Bar		35%		33%		30%		27%		24%		21%
			4 Bar		36%		35%		33%		31%		29%		27%
			6 Bar		36%		35%		24%		32%		31%		30%

Operational Requirements

The Air Muscle is a pneumatic actuator and so the following guidelines must be adhered to:

• Shadow Air Muscles should never be operated above 90 psi (6 Bar).
• When the Air Muscle is not loaded, it must not be inflated above 45 psi (3 Bar)

Over-inflation will cause the life of the Air Muscle to be significantly reduced, and may cause the assembly to burst. The Air Muscle is not rated for use outside the 0-50 Celsius temperature range. Air supplied to the Air Muscle should be clean and oil-free. It is not possible to make a general prediction about the life of the Air Muscle in use; lifespan is dependent on operating conditions and ambient conditions. We therefore do not make any life expectancy statement. (We perform internal tests under uniform criteria to ascertain product consistency with expectations).

Please contact The Shadow Robot Company to discuss your particular application.
For ordering information, please see our ordering page.

Fixtures and Fittings

The 6 and 20mm Air Muscles are supplied with an attached air line of 4mm diameter (Outside) and have looped ends, for easy attachment to hooks or pegs.

The 30 mm Air Muscle has an attached air line of 6mm diameter (Outside). The 30mm muscle are supplied as standard with M6 studding. It is possible to supply them with eyebolts if required, or other fittings according to customer's requirements. Please contact The Shadow Robot Company to discuss your particular application.

	Sources
	All Korean orders Attn. Junghoon Kim Sample Electronics Co 306 Jeshin Buildings 43-22 Shinkey Youngsan Seoul 140-090 Korea Tel: 82 2 707 3882 Fax: 82 2 707 3884 E-mail info@sample.co.kr Web: http://www.shadow.org.uk/cgi-bin/gotoSite.pl?g=www.sample.co.kr		UK Educational (6mm Air Muscles) Middlesex University Teaching Resources Centre Middlesex University Trent Park Bramley Road Oakwood London N14 4XS Tel: 02084470342
	Rest of World Shadow Robot Company 251 Liverpool Road London N1 1LX Tel: +44 (0)207 700 2487

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Document created: 31^th December 1999
Last Modified: Monday, 15-Oct-2001 20:18:14 GMT