Session: 12-03-03: General: Mechanics of Solids, Structures and Fluids

Paper Number: 145260

145260 - Synthesis of Large Stroke Constant Torque Mechanisms via Iterative Structural Optimization and Nonsymmetric Design for Enhanced Strain Relief 

Traditional torque control methods mainly rely on rigid actuators and torque sensors requiring rapid feedback control. Compliant Constant Torque Mechanisms (CCTMs) present a new approach to precise torque control with large displacement tolerance and simple rotary sensor feedback, along with negligible wear and friction and a compact design. However, CCTMs suffer from performance limitations such as low stroke and low constant torque quality. Though strain relief has shown to improve stroke in CFM designs, such design principles for strain relief have not yet been applied to improve stroke in CTMs. Moreover, CCTMs exhibit overly constrained domains, including limiting the domain to a small sector for symmetrical arraying purposes, and self-intersection or center intersection constraints. Current CCTM design strategies also face efficiency issues stemming from redundant dependent design variables and excessive geometric descriptions, leading to low computationally efficient designs incapable of fully exploiting the design space. This paper aims to improve efficiency and performance simultaneously by proposing a novel design approach that reduces over-constraints, introduces tortuosity to improve strain relief necessary for increasing stroke, and implements optimization iteration to eliminate redundant design variables. The optimization process employs a graph method with nodal perturbation. The model initially includes 5 compliant components with fixed-fixed boundary conditions and one rigid member to apply rotational displacement loading. Following the initial optimization, the CCTM results are checked to assess constant torque quality and identify redundant components, defined as components with insignificant deformation. Redundant members are replaced by rigid members, followed by further optimization iterations. The CCTM design is fabricated using PLA via 3D printing and subsequently tested utilizing a tension/compression MTS machine with a modified setup to provide torque. The initial optimization yields a design featuring a constant torque level of approximately 90Nmm within a 30-70 degree stroke range. Subsequent re-optimization replaces redundant components, resulting in a constant torque level of 18Nmm with a 40-120 degree stroke range. The total length of compliant members in CTM is around 3 times the radius for both designs, nearly double the tortuosity found in the literature. This novel design method no longer aims to achieve rotational symmetry and adds boundary and loading components in a parallel plane to the compliant geometries, which minimizes the possibility of self-intersection. As a result, the iterative optimization process with expanded design space achieved twice as much stroke as single-stage CTMs in literature and also achieved the same amount of stroke but with much lower variation of constant torque and a flatter slope compared to 2-stage CTMs that are connected in series. While this method has a notable improvement in the performance of CCTMs, there are still significant limitations, such as being unable to define curved beams, a large preload region exists, discrepancies due to stress concentrations, and possible systematic errors from the experiment setup that transfers rotational motion to linear motion. Further improvements are conceivable through stress concentration analysis, promising enhanced CCTM designs. This research underscores the potential for iterative optimization methodologies to advance compliant constant torque mechanisms, paving the way for improved performance and reliability in various engineering application.

Presenting Author: Shun Bi University of Michigan-Shanghai Jiao Tong University Joint Institute

Presenting Author Biography: Shun Bi is currently a Ph.D. student at the University of Michigan-Shanghai Jiao Tong University Joint Institute (UM-SJTU JI). He obtained his Bachelor's degree in Mechanical Engineering from UM-SJTU JI as well. Bi Shun's research interests primarily lie in the field of solid mechanics, with a focus on using computational method to develop compliant mechanism and compliant mechanism's application.

Authors:

Shun Bi University of Michigan-Shanghai Jiao Tong University Joint Institute
Tanzeel Ur Rehman University of Michigan-Shanghai Jiao Tong University Joint Institute
Shane Johnson University of Michigan-Shanghai Jiao Tong University Joint Institute