Air Muscles

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Air Muscles

Introduction

Pneumatic actuators, usually cylinders, are widely used in factory floor automation. Lately, robotics as well is starting to use pneumatics as a main motion power source. One of the major attractions about pneumatics is the low weight and the inherent compliant behavior of its actuators. Compliance is due to the compressibility of air and, as such, can be influenced by controlling the operating pressure. This is an important feature whenever there is an interaction between man and machine or when delicate operations have to be carried out (e.g. handling of fragile objects). Thanks to compliance a soft touch and safe interaction can be easily guaranteed. Hydraulic and electric drives, in contrast, have a very rigid behavior and can only be made to act in a compliant manner through the use of relatively complex feedback control strategies. Several types of pneumatic actuators—e.g. cylinders, bellows, pneumatic engines and even pneumatic stepper motors—are commonly used to date. A less well-known type is that of the so-called Pneumatic Artificial Muscles (PAMs). These are in fact inverse bellows, i.e. they contract on inflation. Their force is not only dependent on pressure but also on their state of inflation, which makes for a second source of spring-like behavior. They are extremely lightweight because their core element is but a membrane, and yet, they can transfer the same amount of energy as cylinders do, since they operate at the same pressure ranges and volumes. For these reasons they carry a great potential to be used to power mobile robots, where they have additional advantages, such as direct connection, easy replacement and safe operation, as will be seen later.

What does a natural muscle do?

It is a contractile organ.
It consists of fibers which “actuate” force and motion in response to nervous stimulation. Muscles are simply transducers
They change the chemo-electric signal from nerves to mechanical energy How does it work?
Muscles contract by the chemo-mechanical action of the proteins actin and myosin. Joints of the body are arrayed such that they comprise muscles which oppose each other.

Air Muscle General Overview

• 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.

• 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.

2 Concept and operation

PAMs are contractile and linear motion engines operated by gas pressure....
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