The effect of scale on fluid-filled flexible composite actuators

Morphing aircraft and other shape-changing structures are well suited to McKibben-like flexible composite actuators. These actuators, made from fiber-reinforced elastomeric composites, are extremely efficient in converting potential energy (pressurized air) into mechanical energy. Such actuators are promising for use in micro air vehicles, prosthetics and robotics because they offer excellent force-to-weight ratios and behave similar to biological muscle. Use of an incompressible pressurizing fluid instead of compressible air may also offer higher actuator stiffness, better control, and compatibility with existing actuation systems. Using incompressible fluids also allows the actuator to serve as a variable stiffness element which can be modulated by opening and closing valves that constrain or allow fluid flow. The effect of an incompressible fluid (water) on the performance of Rubber Muscle Actuators (RMA), with varying diameters, lengths and segment lengths, was experimentally investigated in the current work. Upon pressurization with air or water, past an activation threshold, overall force and stroke increased with increasing actuation length and diameter. Actuation force when pressurized with water is slightly greater than with air. Both air and water-pressurized actuation force and strain decrease significantly when segment length is less than a minimum critical length. Closed valve actuator stiffness (modulus) of actuators at full length, when pressurized with an incompressible fluid is up to 60X greater than the open valve stiffness of the same actuator. Air-filled RMAs with equal parameters only see a 10X increase. Incompressible fluid-filled RMAs have great potential to provide needed high actuation forces within adaptive material systems. Design guidelines are given to aid additional RMA use.