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Precision air muscle rivals servos and cylinders

01 January, 2004

Precision air muscle rivals servos and cylinders

A British company has developed a range of servo-controlled "air muscle" actuators which, it says, offer a simple, low-cost alternative to conventional pneumatic cylinders, and to rotary and linear servo-motor actuators. Plymouth-based Merlin Actuators says that its Humaniform actuators use closed-loop position control to ensure that target positions are reached, even if the external load or other factors change.

Although the air muscle is not a new idea - Festo, for example, produces a range of "fluidic muscles" - Merlin claims it is the first to couple these devices to position sensors and proportional valves, opening up precision positioning applications that were previously restricted to costly, complex servo motors.

The air muscle (above) consists of a rubber inner tube surrounded by a braided mesh. As air is fed in, the braid expands radially and contracts axially, shortening the muscle. By adding a patented optical feedback sensor, proportional valves and a control system, Merlin says it can deliver accurate repeatable positioning over a wide range of air pressures.

The company is offering a choice of three types of control interface:

. a servo version which acts as a plug-in replacement for standard rotary or linear servo motors and has on-board motion control software;

. an ASCII version which uses a simple two-byte code carried on a bus to specify the required length; and

. a version that uses a reference voltage, which is suitable for automation applications using controls such as PLCs, or any motion controller with a command reference voltage and encoder feedback.

Merlin is developing versions that will be controlled via CANbus and other fieldbuses.

The air muscles come in standard lengths from 20-50cm and typically contract by 30% of their length with a 4 bar pressure. They can deliver peak forces of more than 200N and force-to-weight ratios of more than 800:1. Merlin claims a positional accuracy and repeatability equivalent to 3% of the travel length.

The muscles produce their peak outputs at maximum extension, and consume power only when changing length. In typical automation and robotics applications, the muscles form the prime mover in a lever system and work against a spring force. Alternatively, they can work in pairs against each other.

Merlin, which was formed in 1998, has a team of engineers working on artificial muscle and robotics applications.

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