Numerical and Experimental Analysis of McKibben Actuators and Dielectric Elastomer Sensors

Abstract

Fiber-reinforced pneumatic actuators have the potential to enable the continually evolving technological fields of flexible control surfaces for aircrafts or aquatic vehicles, compliant limbs and graspers for robots, prosthetics, orthotics, and other human-augmenting systems. The McKibben actuator is a pneumatically actuated cylindrical construct consisting of a flexible rubber bladder sheathed in a fiber network, which garners its impressive contracting force from the inextensible fibers that prevent axial extension when an inflation pressure is applied to the internal bladder. The relationship between the axial deformation and contraction force can be accurately modeled using a large deformation continuum model. Our approach is based on the work of Kydoniefs and Matsikoudi-Iliopoulou. Specifically, numerical solutions are obtained by assuming an Ogden strain energy function for the actuator. The deformed shape of the membrane, the fiber angle and the stress components of the membrane and the fibers are predicted by using the initial shape and elastic material parameters. In this paper, a comparison between experimental and numerical modeling results are compared for various actuator geometries. Furthermore, placing a cylindrical dielectric elastomer sensor in direct contact with the inner surface of the McKibben actuator could facilitate in situ monitoring of actuator strains. To illustrate this principle, an experimental setup was developed to measure the changing capacitance of a cylindrical dielectric elastomer sensor as a function of axial extension. A comparison of the predicted and the measured response provides validation of a proposed numerical model. The effects of the dimensions of the sensor on the sensitivity of capacitance measurements are investigated in this paper.