Load independent trajectory control for an artificial muscle

Abstract

In this study, the hysteretic characteristics of pneumatic McKibben artificial muscle were investigated to develop an alternative trajectory control method to traditional PID (Proportional-Integral-Derivative) controller avoiding feedback delays. Furthermore motion trajectory is intended to be payload independent by developing a physical model that will adapt itself to mass changes. In this study, we focus on only one actuator and evaluate our model experimentally. The contraction of the muscle against different pressure values was measured for several different load masses.The proposed model requires computation of actual forces involved in the motion generation of the muscle. These forces are related to contraction ratio, speed, and acceleration of the actuator. First, the load mass that the muscle lifts is measured by force sensation. The mass assessment is performed by using a friction coefficient model. Next a mathematical model relating actuator pressure with its contraction ratio is established. The coefficients are related to both the load mass and the electrical current speed that controls the servo valve pressure. Because of spring-like characteristics of the muscle, its contraction ratio is different for different loads for the same control signal. To achieve load independent trajectory control, the physical model must contain mass related parameters. In this control system, control signal (input electrical current) and electrical current speed are related to the target trajectory. The control system is open-loop, and has no feedback.