Modelling in Modelica and position control of a 1-DoF set-up powered by pneumatic muscles

The characteristics of pneumatic artificial muscles – or McKibben muscles – make them of great interest for the development of robotic applications such as orthoses or certain wearable robots. In order to research the applicability of these actuators in marketable applications, an experimental one-degree-of-freedom set-up based on pneumatic muscles manufactured by Festo was built at Ikerlan. After the detailed description of the experimental set-up, the paper presents the modelling of a pneumatic muscle in Modelica as a new component totally compatible with objects from commercial libraries, thus enabling any mechatronic device that contains pneumatic muscles to be modelled. It then offers a description of the experiments performed to identify the model in the case of real pneumatic muscles. With a view to adjusting the static model to the experimental tests, the inclusion of a new polynomial term depending on muscle contraction is proposed by the authors. The paper then shows the complete model of the experimental set-up in Dymola/Modelica. The part related to the modelling ends with a validation of the model with experimental data. The experimental set-up is very non-linear and very difficult to control properly. As a reference, an enhanced PID controller was designed, and at the same time, a robust controller H∞ and a sliding-mode controller based on an observer were designed and implemented. After this, a position controller based on an internal pressure loop for each pneumatic muscle was tuned up. The paper goes into detail regarding each of the four position controllers designed, and finally, a comparison is made by means of experimental results.