Humaniform Actuators

From Merlin Systems Wiki

Humaniform Actuators

Soft actuator technology

The Humaniform Actuators is an attempt to build an artificial muscle actuator that matches human muscles as closely as possible. The key parameters are the degree of compliance and elastic properties (particularly important for walking because the muscle is able to store energy during heel strike and release during toe-off), the range of contraction (approx 35%) the force range and physical dimensions.

media:forceextensionimperial.jpg Typical Force Extension - Imperial
media:forceextensionmetric.jpg Typical Force Extension - Metric

Because an air muscle is pneumatic the first challenge was to select suitable valves. Typically, with McKibben style actuators a solenoid style valve is used and mounted remotely. However, this introduced control lag as the pipe between the valve and the muscle is a column of air that needs to be physically moved for each command change. We decided to develop our own proportional air valve technology called the Merlin Air Valve (MAV) which is much smaller, and can be embedded into the dead-space of a muscle. So a Humaniform Twin-Valve muscle has single acting MAV on the inlet side and a single acting MAV on the outlet side. This approach was adopted to allow for recovery of energy on the outlet side via a two-stage compressor like a circulartory system. However, the twin valve muscle only solves part of the problem. A human muscle has a number of other sensors, the human stretch sensor measures length and our first attempt to achieve the same thing was the Merlin Stretch Sensor (developed 2001). The Merlin Stretch Sensor uses a conduction process known as percolation, but after two years of research we were unable to use the sensing technology as an absolute sensor but did achieve accpetable performance as a relative sensor. However, the way the Merlin Stretch Sensor worked did not make it an ideal candidate for measuring stretch within a muscle (because it also needed to be held in tension). We then spent a further two years developing an new absolute optical sensor technology which we call LEX (Linear EXtension sensor - developed 2003-2004). This approach proved considerably more succesful and a flexible non-contacting optical sensor was suspended within the inner-space of the Humaniform muscle (we subsequently developed a high precision variant of the LEX sensor which could be used to measure sub-micron displacements to a precision of 0.01% using relatively cheap solid-state opto-eletrical technique). The final step was to close the loop, which we did by embedding all the electronics into an end-block to create a full servo-controlled muscle. You can control the muscle by specifying a length command via a bus-addressable ASCII interface (PWM, voltage, current and I2C interfaces also possible). The PID controller (implemented on a tiny micro within the end-blocks) converts this into a series of drive signals for the valves and takes account of the LEX sensor feedback.

01/09/2005 Merlin Develop Leaky Air Muscle (Single inlet valve with variable output leak - gives a fully compliant control mode) for use in an interactive exhibit in Wolfsburg, Germany.

31/01/2006 Update -> we have now developed a method of incorporating a force measurement into the end-blocks.

The Humaniform Air Muscle is still being developed as part of the Soft Arm Project.

media:servomusclepressrelease.jpg
media:Humaniform040113.pdf Servo Air Muscle Flyer

These images below show a full Humaniform servo-air muscle as developed in 2005.

media:muscleinstallation.PDF Muscle Installation Notes
media:musclespecification.pdf Muscle Specification
media:endblock4.jpg End Block Diagram

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