Identification and modelling of the friction-induced hysteresis in pneumatic actuators for biomimetic robots

Pneumatic Artificial Muscle (PAM) is becoming one of the most used actuator technology for the development of biorobotic applications, such as robotic orthoses and wearable exoskeletons, which require the accurate control of the impedance during human-robot interactions. Although the adaptable compliance of PAMs is desirable, the nonlinear and hysteretic relation between contraction length and pulling force, as well as the air pressure within the chamber of the PAM, make difficult the identification and the control of the dynamics of such actuators. After the description of the experimental setup designed for the dynamic identification of PAMs, this paper presents a novel and accurate model of the hysteresis of the mechanical response of PAMs. Some experimental tests have been performed on a real pneumatic muscle in order to reproduce the different features of the hysteretic behavior which are taken into account in the definition of the model. The proposed model, which has been validated through some experiments, provides some advantages in terms of ease of parameter identification and implementation into a control system, thanks to the use of a limited number of parameters.