Robust and Accurate Closed-Loop Control of McKibben Artificial Muscle Contraction with a Linear Single Integral Action

Bertrand Tondu

doi:10.3390/act3020142

Abstract

We analyze the possibility of taking advantage of artificial muscle’s own stiffness and damping, and substituting it for a classic proportional-integral-derivative controller (PID) controller an I controller. The advantages are that there would only be one parameter to tune and no need for a dynamic model. A stability analysis is proposed from a simple phenomenological artificial muscle model. Step and sinus-wave tracking responses performed with pneumatic McKibben muscles are reported showing the practical efficiency of the method to combine accuracy and load robustness. In the particular case of the McKibben artificial muscle technology, we suggest that the dynamic performances in stability and load robustness would result from the textile nature of its braided sleeve and its internal friction which do not obey Coulomb’s third law, as verified by preliminary reported original friction experiments. Comparisons are reported between three kinds of braided sleeves made of rayon yarns, plastic, and thin metal wires, whose similar closed-loop dynamic performances are highlighted. It is also experimentally shown that a sleeve braided with thin metal wires can give high accuracy performance, in step as in tracking response. This would be due to a low static friction coefficient combined with a kinetic friction exponentially increasing with speed in accordance with hydrodynamic lubrication theory applied to textile physics.

Highlights

Artificial muscles are a special class of soft actuators named because they behave in a phenomenological—as opposed to anatomical—manner and, to some extent, behave like skeletal muscle
Conf. on Mechatronics, in Vicenze, Italy [15], but the present article tries to be more accurate in the closed-loop dynamic analysis including stability conditions and reports original results for the closed-loop control of McKibben artificial muscles with sheaths made of non-fibrous materials); in section three, we report experimental results performed both in step response and sinus-wave tracking, with various embedded loads, on three prototypes of McKibben muscles with sheaths that are braided with three different materials
It is worth noting that this model cannot be considered as an accurate dynamic model of the McKibben artificial pneumatic, due to the fact that the static relationship of Equation (1) is too far away from the real tension-length curve of a McKibben muscle but, we propose that this model is sufficient to put into light the dynamic performances of the artificial muscle contraction resulting from the proposed friction model that we will check experimentally

Summary

Introduction

Artificial muscles are a special class of soft actuators named because they behave in a phenomenological—as opposed to anatomical—manner and, to some extent, behave like skeletal muscle. We try to analyze, in the framework of this paper, the relevance of a one-parameter linear integral action for accurately controlling in a closed-loop the contraction of any artificial muscle characterized by its own stiffness and damping. Artificial muscles are nonlinear complex systems, they globally behave like damped ―active‖ springs whose stiffness can be controlled by some variable mimicking of the neural activation. Our paper is organized as follows: in section two, we first analyze the relevance and efficiency of this idea in a theoretical way from a rectilinear artificial muscle model for which we simulate its closed-loop control with a single linear integral action Conf. on Mechatronics, in Vicenze, Italy [15], but the present article tries to be more accurate in the closed-loop dynamic analysis including stability conditions and reports original results for the closed-loop control of McKibben artificial muscles with sheaths made of non-fibrous materials); in section three, we report experimental results performed both in step response and sinus-wave tracking, with various embedded loads, on three prototypes of McKibben muscles with sheaths that are braided with three different materials

A Simple Artificial Muscle Phenomenological Model with a Damping Component

Closed-Loop Position Control with a Single Integral Action

Closed-Loop Stability Analysis

Findings

Conclusions