Characterization of Hysteresis in a Pneumatic Muscle Manipulator with Accounting for the Creep Effect

An antagonism actuated by a pair of pneumatic muscle actuators has recently become an alternative basic unit for developing humanoid robots or robotic manipulators. A manipulator may consist of one or more of such basic units. One unit can be ready for a 1-dof ‘soft’ manipulator, which exhibits the advantageous characteristics contributed by the individual muscles such as compliance, high power-weight ratios, high volume-weight ratios, etc. However, hysteresis and muscle creep are the most drawbacks that limit the widespread use of these actuators. This paper presents a new approach to model the hysteresis in a basic manipulator constructed by a pair of Festo fluidic muscles. The experimental results show that the manipulator hysteresis has the same behaviours as those found in the individual muscles, and that this hysteresis is well described by the Maxwell-slip model. The creep effect is factorized and incorporated into the constraint torque model, resulting in a stationary extracted hysteresis loop. The proposed model therefore performs very well as in the prediction of the torque-angle hysteresis not only at an arbitrary angular trajectory but also at any moment of the creep conditions.