Charlie Duncheon
Artificial Muscle, Inc.
Menlo Park, Calif.

Courtesy SRI Intl.
This robot uses EPAMs to move— a technology that may one day allow more efficient robots, light enough to be carried by hand.

In the last 20 years or so, researchers have developed several classes of electroactive polymers— plastics that can change shape when one applies a charge to them. Because this mechanism resembles an animal's muscle, that's what actuators based on this technology are often called. Electroactive polymer artificial muscles (EPAMs) consist of a thin layer of polymer film between two compliant electrodes. When a voltage potential is applied across the electrodes (by Maxwellian forces) the two electrodes attract each other, forcing the incompressible film to contract in thickness and expand in area. When mechanical constraints and output linkages are fixed to the film, the expansion and contraction of the film can be harnessed for useful work. The universal muscle actuator configuration — generally the building block for more complicated arrangements — is constructed of two independent stacks of film, attached at their centers and separated by a lightweight spacer. Each stack of film is then attached to a frame, which gives structure to the film, and is usually used for mounting. Also attached to each stack is an output disc, by which force and stroke are transmitted from the expanding film to a load.

Designers can also layer EPAMs in multiple planar configurations or in linear rolls for additional displacement, stroke, or force — to 20 to 30% displacement in some cases. In contrast, shape memory alloys and piezoelectric technologies might get only 1% direct displacement. And compared to conventional electromagnetic motors, EPAM has a significant advantage in power density.

The polymer (here in green) acts as an incompressible fluid, flattening from electrode pressure when voltage is applied — creating mechanical motion.

For economic and technical reasons, robotic applications are not on the immediate horizon for EPAMs just yet. Even so, other valve, small actuator, and gripper applications are. Automatic focus and zoom motion on lens positioners already can be accomplished with silent EPAMs. As actuator position is a function of voltage, actuator resolution is in the microns.

In off-road industries, EPAMs replace conventional hydraulic valves for drive-by-wire applications. The conventional design includes a motor, gearhead, belt drive, and ball screw — over 100 parts. In contrast, the entire EPAM system is comprised of an expanding and contracting linear-roll actuator, controllable with variable-voltage input. It has significantly lower cost and weighs less than a tenth of the conventional electric motor actuator.

Actuator rundown

On low-speed linear motion applications, instead of converting highspeed rotary motion (commonly required from conventional rotary motors) universal muscle actuators can directly drive linear positioning devices at frequencies from dc power to several hundred Hz. This allows designers to remove complicated, fatigue-prone gear reduction and motion conversion power trains.

Researchers continue work to bring mechanical performance variability (currently at 30% from -40 to 120°C) to evertighter tolerances. Siliconebased polymers are most hearty.

EPAM technology can also be used for "infinite stroke" applications, such as rotary motors and pumps. These applications convert the reciprocating motion of universal muscle actuators to single-direction motion by way of clutches or check-valves, respectively.

As rotary motors, units consist of a clutch mounted to a universal muscle actuator configuration, converting its motion from linear to rotational.

As a closed-loop pneumatic system, a universal muscle actuator (connected to valves and pressure sensors) serves as an inflatable bladder system. Reading inputs from user-controlled switches, an integrated microcontroller processes pressure levels from the sensors, directing the EPAM valves to release air from or add air to the bladders (in conjunction with the operation of the EPAM pump) until the bladders fill to the desired level.