Rubber Muscle Actuators (RMA) provide high force-to-weight ratios at relatively low costs, but can be hard to manufacture at diameters less than 0.125 inches. The additive manufacturing process (i.e. digital manufacturing or 3D-Printing) was explored to better fabricate such actuators. This work continues development of previous Multi-Material 3D-Printed (MM-3DP) and Hybrid RMAs. Refinement of two closed form models of traditional rubber muscle actuators was also conducted. Previous Texas A&M University-Kingsville MM-3DP RMAs produced little force (7 lbs) or contraction (<1%) and failed prematurely due to poor 3D-printed elastomer tear strength. Two primary objectives of the current work were to develop thinner but stronger actuator walls and replace the 3DP elastomer with a more robust polyurethane. Exploration of various braid configurations and substitution of RMA materials were conducted simultaneously. Three oval braid configurations were developed to have equivalent total cross sectional area and axial load carrying capability. They have aspect ratios of 3:1, 2:1, and 1:1. The 3:1 and 2:1 braids have a thinner wall than the 1:1 (round) braid. These braids were used across 4 different configurations of traditional and digital materials. The first configuration consisted of a fully 3DP braid, elastomer bladder, elastomer skin, and transition region. The second material was a Hybrid configuration which used a 3DP braid, latex rubber bladder, and cast polyurethane skin. The third material configuration used a 3DP braid, skin, and transition region with a latex bladder. The fourth configuration used a stiffer 3DP braid material, latex bladder, polyurethane skin, and reinforced transition region. All 3D Printing was done with a Connex 500. The fully 3DP and Hybrid RMAs were activated with air pressure. Actuator forces as a function of pressure and as a function of contraction were obtained. In the material configurations with 3DP elastomer, the widest oval braid RMAs, with thinner walls, achieved the highest forces and contractions. In the material configurations with polyurethane skins, the RMAs with circular fiber braid configurations, and thicker walls, achieved higher forces and contractions. The Hybrid RMAs with polyurethane skins performed much better than RMAs with 3DP skins, achieving higher pressures, 3X-8X higher actuator forces (up to 40 lbs) and 4X-30X greater contractions (up to 12%). Thus, current RMA architecture requires more bladder/skin elastic elongation and tear strength than currently available 3DP elastomers can provide. A closed form model by Chou, and another by Kothera et. al were refined to better predict actuator force as a function of pressure or as a function of axial contraction. Refined predictions were compared with traditionally manufactured RMA test results. Model refinements include: re-definition of wall thicknesses, inclusion of nonlinear elastomer tensile modulus, effective bladder and skin tensile modulus, and endcap effects. No single model best predicted both force and contraction. The original Chou model was the best predictor of actuator force as a function of pressure. A refined Kothera model with adjusted wall thickness, effective bladder/skin linear modulus, and endcap effects was the best predictor of force as a function of contraction. Although these refinements aligned predictions closer to actual performance, the refined Kothera model is only effective after ~5% contraction, but accurately predicts maximum contraction. Better modeling of RMA activation pressure, spherical end effects, and frictional effects is needed to optimize closed form solutions.
October 17th, 2016
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