Second order sliding mode control for an anthropomorphic robot-arm driven with pneumatic artificial muscles

McKibben artificial muscle is one of the most interesting artificial muscle which can actually serves as a biomimetic actuator for human-like robot limbs. However its peculiar non-linearities as the need to use it in direct drive to preserve its human-like compliance make very difficult the control of robots actuated by McKibben muscles. Experimental tests with a second-order sliding mode applied to the control of the regional structure of a 7R-anthropomorphic arm are reported. The so-called twisting algorithm appears particularly relevant for the combined positioning of joints submitted to gravity without inducing a chattering phenomenon. The role of an additional equivalent control term is discussed and it is shown to be efficient for facilitating motion of primary regional joints. In reported experiments with human-size arm and forearm in shoulder and elbow flexion, a response time without and with load between 2 and 3 s is obtained with a steady-state error lower than 0.5 degree.