Analysis of Pneumatic Artificial Muscles in Anchoring, Torsion, and Weight-Limited Robotic Applications

Abstract

Pneumatic artificial muscles (PAMs) are studied in two novel actuation methods: radial expansion and torsional response. In addition, PAMs utilizing novel material for light weight scenarios are studied and applied to a sweeping wing mechanism in an unmanned aerial system (UAS).

For the radial expansion model, modeling was performed using a force balance approach to capture the effects that bladder strain and applied axial load have on the anchoring force. Radial expansion testing was performed to validate the model. Force due to anchoring was recorded using force transducers attached to sections of aluminum pipe using an MTS servo-hydraulic testing machine. Data from the test was compared to the predicted anchoring force. Radial expansion in large diameter (over 2 inches) PAMs were then used in worm-like robots to create anchoring forces that allow for a peristaltic wave which creates locomotion through tunnels and pipes. Modeling of the torsional response was also performed using a force balance approach, and multiple model variations were considered, such as St. Venant's torsion, bladder buckling, and asymmetrical braid loading. Torsional testing was performed to validate the model using a custom torsional testing system. Data from the tests was compared to the predicted torsional response.

In addition to alternative forms of actuation, alternative materials for end fittings were also tested. Two materials were tested, stand alone Acrylonitrile Butadiene Styrene (ABS-R) and ABS embedded with carbon fiber. These materials were chosen to reduce the overall weight of the PAM and increase the specific axial force output. Multiple iterations were designed and tested to achieve an operable pressure range of zero to one hundred psi with a minimum safety factor of 1.5. The PAMs also underwent endurance testing to determine the number of cycles before fatigue failure. The final iteration of the PAM design for both materials were compared to traditional metallic end fitting PAMs in specific force production as well as fatigue life.

Once design work was complete, the 3D-printed end fitting PAM was utilized in a sweeping wing mechanism for a UAS. Due to their small size, UAS possess the ability to fly through confined areas that full-sized aircraft would not be able to. This, however, presents the challenge of an increased risk of collisions with obstacles present in these areas. An active wing sweeping mechanism was proposed to both mitigate the possibility of, and damage associated with, collisions. The mechanism was capable of sweeping the wings of an E-flite V900 BNF Basic remote controlled plane by 45 degrees. Multiple bench top tests were performed on the system, such as a simple actuation test, a response time test, and a hardware-in-the-loop (HITL) test. The gear system was found to be fully capable of sweeping the wings from the default 35.4 inch wingspan to a 24 inch wingspan and clearing a 30 inch gap at speeds up to 55 mph in the HITL tests.