Prosthetic Arm: Designing an Under-Actuated Hand
This was built as part of the CMU Graduate-level course "Special Topics in Biomechatronic Systems for Humans." The goal for the class was to develop a complete prosthetic arm for use by an amputee. Our team was in charge of developing an under-actuated robotic hand that interfaced with the lower arm assembly and with objects weighing up to 10 Newtons.
Our team went through 8 iterations before arriving at our final design. The focus was a lightweight, cost-effective, functional hand that would grasp various items with a single pull of a cable. The result was the 3D-printed hand below. Due to the design of the hand mechanism, the fingers could close independently of each other. The hand featured force sensors in the fingers to prevent overloading, interchangeable thumb modules, and lightweight 3D-printed finger linkages.
This project gave me a chance to really flex my creative muscles. Our team was given minimal direction, which allowed us to explore many different possibilities for our design. Our team ran many simulations in SolidWorks, looking to optimize strength and stiffness while minimizing weight and cost. As this was a prototype, we opted to use 3D printing and laser cutting for the plastic pieces. Though it cost more than using traditional machined parts, it allowed us to obtain complex designs with minimal lead time and high accuracy.
Given more time, future improvements to the hand design would be to further strengthen the finger mechanisms (the prototype was unfortunately destroyed when it got caught in a door). The sensor placement could also be improved, as some of the objects that were gripped were not rigid. Despite the shortcomings of the prototype, this is one of the projects I am most proud of from my time at Carnegie Mellon!
Final CAD model of the under-actuated hand, showing the finger closure mechanism, weight-optimized finger joints and customized thumb module.
Comparison of the early version of the fingers (top) and the final version (bottom). Final version was optimized for weight and stiffness. The final version also has limited range of motion, shown in the next picture.
Early prototype (top) shown with too much motion and the final version (bottom) with limited motion.
Final prototype of the 3D printed hand, including force sensors and grip points for extra friction.
Hand closing around a cell phone.
Wearing the fully assembled prototype. Other teams designed the arm/elbow mechanism, body attachment, EMG sensors for sensing muscle activity, and software to control the unit.