Braid Effects on Contractile Range and Friction Modeling in Pneumatic Muscle Actuators

Within braided pneumatic Muscle Actuators (pMA) the braid structure is vital to the actuator's performance, preventing over-inflation, converting radial expansion into axial contraction and setting limits for both dilation and contraction. This paper seeks to explore the nature of the contractile limit and the hysteresis observed by researchers during the actuation cycle. Maximum actuator dilation occurs when adjacent braid strands are forced against one another. Within this work this is analyzed mathematically and it is shown that by halving the number of strands used to create the braided shell the actuator's contractile range can be increased by approximately 7%. This also results in a simultaneous peak contractile force increases of over 16%. These results are verified experimentally. Hysteresis due to friction between braid strands during muscle operation is also explored. The paper will show how consideration of the deformation of the strands allows the contact area and therefore friction to be calculated without the need for experimentally obtained data as in previous research. A mathematical model is produced and verified experimentally.