日期：2023/02/23 - 2023/02/23
主讲人：Tanzeel ur Rehman, Ph.D. candidate at UM-SJTU Joint Institute
Bifurcating mechanisms are mechanisms that undergo large deformation and bifurcate between various stable structural configurations. Bifurcating mechanisms come in various forms: 1) switching mechanisms, 2) control mechanisms, and 3) morphing mechanisms. These mechanisms find their applications in aerospace applications of morphing airfoils, morphing automobiles, robotic arms, energy harvesters, jumping robots, mechanical switches, deployable structures, origami actuators, and bistable valves to mention a few. Tuning the mechanism’s bifurcation load is achieved in the literature by active control, e.g., using motors or linear actuators, resulting in higher energy requirements. Current methods for active control or passive design result in either complex manufacturing, high energy requirements, design space inefficiencies, and/or optimizations with an excessively large number of design variables and related high computational costs, etc. The objectives of this thesis are to: 1) tune bifurcation in compliant mechanisms semi-actively, 2) exploit the design space more fully for a constant force mechanism (CFM) (i.e., a bifurcating mechanism with a constant force vs. displacement relation) with improved computational cost, and 3) integrate semi-active control with design space exploitation in tuning a CFM. In this dissertation, a semi-active bistable mechanism is designed, analyzed, built, and validated to control the bifurcation load by adjusting component stiffness. The semi-active control is achieved using the structural control method, i.e., tuning stiffness to tune bifurcation load. Next, a new bifurcating mechanism was designed and developed to provide specific constant force vs. displacement relations, i.e., a constant force mechanism, and a new algorithm is introduced to more fully exploit the design space using a modified depth search algorithm to define all the structural topologies using path descriptions, a reduced number of structural topologies in the design space by defining dependent and independent structural member paths, and a FEM based optimization to obtain an optimal structural topology by perturbing the nodal location. The design approach resulted in a 3D printable constant force mechanism that was built and validated to show a larger constant force displacement (90% compared to 75%), and the design approach was computationally efficient because the optimization uses a smaller number of design variables (19 as compared to 70 design variables) when compared with the literature. Lastly, a semi-active tunable constant force mechanism is designed, analyzed, and built with fiber glass reinforced polymer composite laminate which gives high energy density, uses semi-active control to reduce energy requirements, exploits the design space more fully, and accomplishes this with reduced computational costs through optimization with a smaller number of design variables. The design approach resulted in a tunable constant force mechanism with a larger change in constant force (2.86 times compared to 2.32 times) using an optimized mechanism with a more compact design (47% displacement compared to the footprint of the mechanism as compared to 10%) when compared with the tunable constant force mechanisms in the literature. The designs are experimentally validated and compared with values in the literature. The methods developed may be used to improve the designs of a wide variety of compliant mechanisms for various applications.
Tanzeel ur Rehman received his B.S. in Aerospace Engineering from Institute of Space Technology Islamabad Pakistan and M.S. in Space Engineering from Politecnico Di Milano Italy in 2011 and 2015 respectively. Now, he is a Ph.D. candidate student at the UM-SJTU Joint Institute, supervised by Prof. Shane Johnson. His current research focuses on the development and tuning of bifurcating mechanisms.