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Product category: Electric Linear Actuators
News Release from: Merlin Systems Corp (Actuators Division) | Subject: Humaniform actuators
Edited by the Engineeringtalk Editorial Team on 12 December 2003

Servo control adds precision to air
muscles

Humaniform servo-controlled air muscle actuators feature closed loop position control to provide both lower cost and simpler installation than traditional rotary and linear servomotor based solutions.

The Humaniform range of servo-controlled air muscle actuators feature closed loop position control to provide both lower cost and simpler installation than traditional rotary and linear servomotor based solutions The range is designed to replace pneumatic cylinders and is available with a range of interfaces to suit a majority of machine control technologies

Although air muscles have been available for simple, fixed-length applications for many years, Merlin is the first company to launch a standard range of servo-controlled actuators complete with their own patented Lex optical position sensors and smart proportional valves.

Such closed loop position control ensures that the target position is reached, even if external load or other factors change.

By adapting the air muscle in this way, applications previously restricted to servomotors, with their inherent high cost and complexity, are now possible.

The basic air muscle construction combines a high durability rubber inner tube surrounded by a tough braided mesh.

The muscle is mounted and secured by clamp blocks at each end.

As air is introduced into the muscle via standard 4mm push-fit connectors, the braid expands radially and contracts axially causing the muscle to shorten in length.

Merlin has taken this basic concept and added a feedback sensor, proportional valves and a choice of three control systems to realise position control.

Position feedback is provided by Merlin's patented Lex optical sensor, which has been designed for stand-alone applications as an LVDT replacement system.

The Lex sensor is robust yet light and compact with fully integrated onboard position processing and is intrinsically immune to electromagnetic fields.

The Merlin smart valves are fully proportional microminiature air valves using their latest smart material technology and in combination with the control system, provide consistent positioning accuracy and repeatability over a wide range of applied air pressure.

Three different control interfaces are available, the "Servo", the "ASCII" and the "Voltage", all of which use the same Merlin developed system components.

The Servo interface is designed to be a plug in replacement for a standard rotary or linear servomotor.

It includes onboard control electronics and motion control software and is similar in concept to an integrated or intelligent servo drive.

The ASCII interface is driven from an ASCII input command interface where a simple 2byte code allows muscles to be addressed on a bus, and driven to maintain a required length.

The Voltage interface is driven from a reference voltage level input.

This interface is ideal for automation type applications where a machine control system such as a PLC is used or could be used with any manufacturer's motion controller with a command reference voltage and encoder feedback.

CANBus and other fieldbus control options are in development.

The new Humaniform range of precision air muscles is available in standard lengths of 20, 30, 40 and 50cm and typically contract by 30% of their length with a pressure rating of 4bar (5bar maximum).

They have a 12mm minimum diameter and a 25mm contracted diameter and deliver peak forces in excess of 200N.

Muscles become increasingly powerful with diameter, and force/weight ratios in excess of 800:1 are achievable.

By designing the complete system using its own technology, Merlin can maintain positional accuracy and repeatability to 3% of travel length.

The Humaniform muscles are inherently self-damping when contracting and their flexible construction allows cushioning when extending.

Muscles produce their peak force output at maximum extension, and only consume power when changing length making them energy efficient for many applications.

For maximum contraction muscles should be installed in a taut position and it is usual to design them into applications where they form the prime mover in a lever system and work against a spring return force or have muscle pairs working against each other.

Typical applications for this exciting new range of products will include robotics, animatronics and automation systems where their low cost and compact design combined with flexible installation possibilities will challenge designers to consider their use to replace traditional pneumatic cylinder, servo and stepper motor driven systems.

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