A Modular Geometrical Framework for Modelling the Force-Contraction Profile of Vacuum-Powered Soft Actuators

Abstract

In this paper, we present a generalized modeling tool for predicting the output force profile of vacuum-powered soft actuators using a simplified geometrical approach and the principle of virtual work. Previous work has derived analytical formulas to model the force-contraction profile of specific actuators. To enhance the versatility and the efficiency of the modelling process we propose a generalized numerical algorithm based purely on geometrical inputs, which can be tailored to the desired actuator, to estimate its force-contraction profile quickly and for any combination of varying geometrical parameters. We identify a class of linearly contracting vacuum actuators that consists of a polymeric skin guided by a rigid skeleton and apply our model to two such actuators-vacuum bellows and Fluid-driven Origami-inspired Artificial Muscles-to demonstrate the versatility of our model. We perform experiments to validate that our model can predict the force profile of the actuators using its geometric principles, modularly combined with design-specific external adjustment factors. Our framework can be used as a versatile design tool that allows users to perform parametric studies and rapidly and efficiently tune actuator dimensions to produce a force-contraction profile to meet their needs, and as a pre-screening tool to obviate the need for multiple rounds of time-intensive actuator fabrication and testing.

A Vacuum-Powered Artificial Muscle Designed for Infant Rehabilitation

“…One important point that this model demonstrates is the value of the skeleton in this low-profile actuator design. If the actuator was built like a bellows actuator (no skeleton), the short cross-sectional dimension of the actuator would dominate, and it would have the output of a bellows actuator with a characteristic diameter of 10 mm and ring spacing of 56 mm, meaning its total contraction would be limited to a maximum of 10 mm with a highly variable force output (high at the beginning and low at the end of the contraction) [ 41 , 50 ].…”

“…LP-VPAM can be employed for rehabilitative treatment in babies, since it can produce low force above the baby’s leg weight. The force output can be higher than the required value, however it can be controlled as it is proposed in [ 50 ].…”

“…To simulate these two phases, and better predict the actuator’s force output and contraction numerically, we developed a model of the actuator using the virtual work principle, following the process previously described [ 50 ]. Briefly, the actuator is approximated as a 2D geometry and the volume loss over its contraction is geometrically extracted in a numerical algorithm.…”

“…Briefly, the actuator is approximated as a 2D geometry and the volume loss over its contraction is geometrically extracted in a numerical algorithm. The derivative of the volume loss throughout contraction is applied to Equation (1) [ 50 ]: where is the actuator’s cross-sectional area, is its actuating pressure, is the actuator’s volume loss, is the variable symbolizing contraction, and D is the characteristic diameter, in this case, the width of the actuator (30 mm). This equation calculates the actuator’s quasi-static force output for all given points through its contraction, called a force-contraction profile (FCP).…”

“…Finally, once the model is calculated, a shifting and scaling factor are applied to capture the system losses that are not included in the idealized model. The shift used was 2% and the scaling factor followed the equation previously described [ 50 ]. Using a Young’s modulus of 110 MPa and thickness of 0.1 mm for the skin (manufacturer values), the magnitude scaling factor was 0.48.…”