Experimental Characterization and Static Modeling of McKibben Actuators

Abstract

McKibben actuators are pneumatic actuators with very high force to weight ratio. Their ability to match the behavior of biological muscles better than many other actuators has motivated much research into characterization and modeling of these actuators. The purpose of this paper is to experimentally characterize the behavior of McKibben artificial muscles with basic geometric parameters and present a model that is able to predict the static behavior correctly in terms of blocked force and free displacement. A series of experiments aimed at understanding the static behavior of the actuators was conducted. The results for three different lengths (4, 6, 8 in), three diameters (1/8, 1/4, 3/8 in) and two wall thicknesses (1/32 and 1/16 in) at pressures ranging from 10 psi to 60 psi show the expected trends (for example, block force increasing with diameter) as predicted by models presented in the literature. However, these models do not accurately predict static behavior. Corrections to the Gaylord equation are explored in order to obtain a more accurate model. Consideration of elastic energy storage in the rubber tube has been shown to significantly improve the models. Apart from this, the effect of non-cylindrical tips and elastic energy storage in the braid are also considered. To increase model accuracy, another set of experiments was used to characterize the elasticity of the rubber tubes and fibers of the braid. The improved model is able to predict static behavior correctly. Incorporating, various corrections, a model is presented that is more accurate in predicting the static behavior. Finally, in order to possibly obtain larger force output from the McKibben actuators, a series of experiments were performed to study the impact of an applied pre-strain. The results presented show large increases in blocked force with pre-strain. For the largest diameter actuator of 6 inch length, the blocked force at 12% pre-strain is as high as 270 N, while the amplification is higher at lower pressures. The model is tested to predict the pre-strain characteristics. A number of factors are identified that may improve the model and incorporate dynamic behavior.