Modeling and sensorless force control of novel tendon-sheath artificial muscle based on hill muscle model

Abstract

Muscle as the actuator of human body has good biological properties. Various types of mechanical structures called artificial muscles were put forward to simulate muscle characteristics. However, the defects of artificial muscles such as high stiffness, nonlinearity, short stroke and low power density make them not well applied in robotic transmission systems. To overcome these shortages, this paper develops a type of artificial muscle actuated by a motor and tendon-sheath system based on Hill muscle model. The series and parallel elasticity in the model were simplified as linear springs in this structure which can increase the compliance and the abilities of shock-absorbing. Due to the linear series spring, the distal force of the system can be easily achieved by measuring the elongation of the spring with the use of encoders in the joints instead of installing force sensors. A model of the single tendon-sheath artificial muscle is established based on the coulomb friction model. Then, a series of experiments are conducted to validate the transmission model. After that, a method of sensorless force control is proposed based on the muscle's force-elongation model. The results indicate high accuracy of the transmission model (R-square > 0.99, Max error < 1.9 N) and good performance of force control.