iGrab Hand Orthosis: Design and Development Using Twisted and Coiled Polymer Muscles
Saharan, Lokesh Kumar
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Four million Americans are suffering from upper limb malformation due to various neurological disorders. Efforts have been made to develop orthosis and prosthesis to improve the quality of life of those individuals. This work primarily focuses on the design, development and analysis of a hand exoskeleton created using novel actuators called twisted and coiled polymer (TCP) muscles, 3D printed structures, and a garment glove for a rehabilitation of the patients with partial or no motor abilities in the hand. The 3D printed exoskeleton incorporates the TCP muscles (380 mm long and 1.25 mm in diameter) wrapped around pulleys to pull the tendons which in turn facilitate the flexion motion of fingers. Extension motion was done using elastic cords on the dorsal side of the hand. A custom made biomimetic hand was developed using a 3D printed hand skeleton and casting Ecoflex 30 silicone to mimic similar stiffness as the natural hand and to test the orthosis device. Eight different orthotic hands were designed and developed, and we showed precise prehensile and non-prehensile hand movements. Further, we analyzed the motions of the robotic and the orthotic devices using Euler-Lagrangian equations. The modeling included the derivation of equation of motion for the three-link under-actuated serial manipulator suitable for numerical simulations. System identification was used to determine the electro-thermo-mechanical model transfer functions of the TCP muscle. These two transfer functions were integrated with the Euler-Lagrange model in the Simulink®. Then, a measured power and force profile of the TCP muscle was used as input to the Simulink model to determine the motion behavior of all three joints of the robotic finger. Errors in the torque and force profile were determined statistically. Also, sensitivity analysis was conducted using key model parameters. The TCP muscles are the most recent revolutionary development in the field of the smart actuators, with high power to weight ratio 5.4 kW/kg, 16 %- 200 % actuation stroke and stress of 1- 35 MPa presented by Carter Haines in 2014 introductory science paper and in a subsequent study in 2016. In addition, the precursor material for the muscles is inexpensive (~$5/kg) compared to a shape memory alloy ($3000/kg) and the muscle fabrication is easier. Also, the hysteresis is minimal for the TCP muscles but the efficiency is one of the limitations of the actuator which is close to 1%. We have developed an experimental setup to fabricate and characterize the TCP muscles under different loading conditions and determine stress, strain, and power, number of cycle and temperature rise. We used silver coated nylon 6,6 multifilament threads and performed actuation tests, microscopy and tensile tests . Different geometries of the TCPs 1-ply, 2-ply and 3-ply muscles were studied for the effect of speed of the rotation during fabrication and natural frequency. Further, the low efficiency of the muscles is addressed with the design and implementation of two different types of the locking mechanism in the prosthetic and orthotic devices. The locking mechanisms have shown significant improvements in the efficiency of the muscle. The novelty of this work lies in the design, development and modeling of the orthotics device, improving the muscle efficiency along with study and characterization of the TCP muscles.