Accurate force function approximation for pneumatic artificial muscles

Many papers exist in the literature dealing with the force function approximation for pneumatic artificial muscles (PAM) with different approaches. The aim of this paper was to create a more precise and simple transfer function for the braided fiber reinforced type PAM actuator. For evaluation purpose, 340 measurements were performed on a testbed using the FESTO DMSP-20-400 type artificial muscle. As a result, a new static model was described with 6.44N root-mean-square error (RMSE) in the whole operating range that consist of only six unknown parameters. By using the inverse of this function, it is possible to express the pressure required to maintain a given contraction with a given force. The compactness of the function results in high evaluation speed necessary for fast controllers and for modeling complex systems equipped with PAM actuator. The proposed model can be used in case of a limited number of measurements that is desirable in a practical calibration procedure. This capability can be used to develop a system with real-time calibration inducing longer operating time.