Modeling Length Effects of Braided Pneumatic Actuators

Braided pneumatic actuators (BPAs) are attractive for use in bio-robots because they offer many muscle-like properties, especially when compared to most other commercially available robotic actuators. Unfortunately, the same properties that make these actuators similar to muscles make them more difficult to control. One such actuator manufactured by Festo, the MXAM-10-AA, is frequently utilized in robotics because of its commercial availability, durability, and force capability. Although models for BPAs exist, the properties that make this actuator more durable also make its behavior less like other braided pneumatic actuators, especially for shorter actuator lengths. Length specific models that do exist for Festo fluidic muscles have numerous parameters that can only be found experimentally by taking hundreds to thousands of data points and performing a lengthy optimization process to fit parameter values for each actuator in the system. This lack of generalizability makes it difficult to build a new robot and begin testing new control systems without significant startup time and cost. The key contribution of this work is the development of a generalizable actuator model that accounts for the geometry and limitations of the actuator at shorter lengths. This empirical model relates internal pressure, strain, stretching or contracting state, and applied force on the MXAM-10-AA actuator. The model is scalable to different length actuators by measuring their resting length at zero pressure and their minimum contraction length at maximum air pressure, and can be used for feedforward length control. The model is evaluated on a robot leg with three joints and 6 actuators, each with different length. The developed controller, using the actuator model, controls the joints to within ± 3 degrees of the desired position for different desired torques only using internal actuator pressure feedback. We also demonstrate control speed by cycling a joint over 40 degrees of rotation at varying frequencies.