Experimental investigation of hybrid shape memory alloy pneumatic actuators

Pneumatic actuators are a lightweight, compliant means of exerting force and are especially promising for systems that interact with the human body. These actuators typically consist of an elastomeric bladder and a surrounding sleeve. The sleeve is created using traditional textile manufacturing processes (e.g., wrapping, braiding, and knitting) where the fibers within the textile sleeves are inextensible components that limit the local expansion of the bladder. By varying the fiber angles and applied pressure, different kinematic motions (extension, contraction, bending, and twisting) are achieved. Anti-symmetric fiber angles within the textile sleeve architecture result in axial and radial motion. In this experimental investigation, shape memory alloy (SMA) wires are integrated into two types of passive sleeves (wrapped and knitted) to increase the range of motion of the actuator. Applying pressure to the system pre-strains the martensitic SMA through radial expansion of the actuator. When heated above the austenite finish temperature, the shape memory effect enables contraction of the SMA wire. This fiber contraction causes reorientation of the textile architecture, resulting in additional motion. The effect of the control variables, air pressure and electrical current, on axial displacements and rotations are experimentally characterized using discrete marker tracking. This study explores two basic textile sleeve architectures for hybrid SMA-pneumatic actuators and sets the foundation for increased kinematic tailorability through the design of complex multi-functional actuating textile sleeves.