Dual Optimization of Contact-Aided Compliant Mechanisms and Spatial Distribution for Passive Shape Change
Open Access
- Author:
- Calogero, Joseph Patrick
- Graduate Program:
- Mechanical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 12, 2017
- Committee Members:
- Mary I Frecker, Dissertation Advisor/Co-Advisor
Mary I Frecker, Committee Chair/Co-Chair
Henry Joseph Sommer III, Committee Member
Bo Cheng, Committee Member
Reginald Felix Hamilton, Outside Member - Keywords:
- compliant mechanism
numerical dynamics
finite element analysis
design optimization
ornithopter
motion tracking camera - Abstract:
- Flapping wing unmanned aerial vehicles, known as ornithopters, are of interest for both civilian applications, such as search and rescue, and military applications, such as surveillance. Their designs are inspired by nature: birds have evolved to fly and maneuver at varying speeds, from hovering to high speed flight, while remaining agile, precise, and efficient. Part of their abilities stem from asymmetric wing morphing throughout their flapping cycles. This asymmetry can manifest as flexibility of the wing structure in one direction during part of the flapping cycle, then inflexibility in the opposing direction during another part of the flapping cycle. In some avian gaits, such as the steady level Continuous Vortex Gait, the nonlinear stiffness is primarily located at discrete joints, similar to human elbows and wrists. It is hypothesized that the flight performance of an avian scale ornithopter can be improved by introducing compliant joints with nonlinear stiffness at specific spatial locations, emulating the wing gait found in nature. The overall goal of this research is to develop optimization methodologies for modifying a stiff structure, specifically a wing structure, by means of discrete compliant mechanisms to allow desired passive shape change, i.e., without the assistance of active components, such as motors and actuators, or a control system. Contact-aided compliant mechanisms (CCMs) are mechanisms that allow large localized deformations, typically in thin and/or flexible members, and are designed to come in contact with themselves. Self-contact causes a strong nonlinearity in the stiffness of CCMs. Inserting CCMs in the wing structure of an ornithopter passively introduces asymmetry throughout the flapping cycle, therefore inducing passive shape change. In this work, a dual optimization approach is developed to determine the optimal configuration of flexible compliant joints in a dynamic ornithopter wing structure to increase free flight pitch agility, then to design and optimize a CCM that realizes the optimal configuration. First, a three degree-of-freedom CCM called the Bend-Twist-and-Sweep Compliant Mechanism (BTSCM) is presented, and its design parameters are optimized to maximize flexibility while minimizing peak stress and mass. A large set of optimal designs called the Aggregated Pareto-Optimal Front is solved for using an enhanced multi-objective genetic algorithm. This set of optimal designs can be used by a designer to select a configuration which allows desirable flexibility within chosen constraints. However, several assumptions are made about the boundary conditions in the finite element model, such as quasi-static loading to estimate resultant aerodynamic lift and drag loads. Furthermore, a designer may not know the desired flexibility or spatial distribution of CCMs in a dynamic structure. The Dynamic Spar Numerical Model (DSNM) is a computationally efficient numerical rigid-body dynamics model that models CCMs in the wing structure as compliant joints: spherical joints with distributed mass and three-axis nonlinear torsional spring-dampers. First, the model is developed, then the model is validated using a bench top experiment with high speed motion tracking cameras. Model parameters are tuned using a genetic algorithm to minimize the error between the model and the experiment. Finally, the DSNM is used as an optimization tool to determine the optimal spatial location and stiffness to induce desirable wing morphing that increases free flight pitch agility. The results of the optimization provide the desired stiffness and coupling of the CCM, as well as the optimal spatial location. A new CCM called the Forward-Swept Compliant Mechanism (FSCM) is optimized to induce forward sweep at the wing tip during the ornithopter’s downstroke, thereby increasing its pitch agility. An optimal design was chosen for fabrication and flown in free flight. Motion tracking cameras tracked the wing kinematics of the ornithopter with the FSCM inserted in the spar structure of the ornithopter with an equivalent mass inserted in the same location as the FSCM. The experimental kinematic data shows that the FSCM induces the desired forward sweep and therefore increases the pitch agility. Finally, a finite element model of CCMs in the dynamic flapping wing structure is developed and validated using the benchtop test data. The DSNM and dynamic finite element model with the FSCM inserted are used to determine the sensitivity of the wing morphing to changes in the design parameters. Then, they are compared to the optimization results to show which variables are the most important and which are the most sensitive to changes in the variables. The numerical dynamics and finite element models can be extended in the future to include aerodynamic loading and body motion incurred during free flight.