Showing papers by "James E. Hubbard published in 2014"

Design and Optimization of a Contact-Aided Compliant Mechanism for Passive Bending

Abstract: A contact-aided compliant mechanism (CCM) called a compliant spine (CS) is presented in this paper. It is flexible when bending in one direction and stiff when bending in the opposite direction, giving it a nonlinear bending stiffness. The fundamental element of this mechanism is a compliant joint (CJ), which consists of a compliant hinge (CH) and contact surfaces. The design of the compliant joint and the number of compliant joints in a compliant spine determine its stiffness. This paper presents the design and optimization of such a compliant spine. A multi-objective optimization problem with three objectives is formulated in order to perform the design optimization of the compliant spine. The goal of the optimization is to minimize the peak stress and mass while maximizing the deflection, subject to geometric and other constraints. Flapping wing unmanned air vehicles, also known as ornithopters, are used as a case study in this paper to test the accuracy of the design optimization procedure and to prove the efficacy of the compliant spine design. The optimal compliant spine designs obtained from the optimization procedure are fabricated, integrated into the ornithopter's wing leading edge spar, and flight tested. Results from the flight tests prove the ability of the compliant spine to produce an asymmetry in the ornithopter's wing kinematics during the up and down strokes.

Design optimization of a twist compliant mechanism with nonlinear stiffness

Abstract: A contact-aided compliant mechanism called a twist compliant mechanism (TCM) is presented in this paper. This mechanism has nonlinear stiffness when it is twisted in both directions along its axis. The inner core of the mechanism is primarily responsible for its flexibility in one twisting direction. The contact surfaces of the cross-members and compliant sectors are primarily responsible for its high stiffness in the opposite direction. A desired twist angle in a given direction can be achieved by tailoring the stiffness of a TCM. The stiffness of a compliant twist mechanism can be tailored by varying thickness of its cross-members, thickness of the core and thickness of its sectors. A multi-objective optimization problem with three objective functions is proposed in this paper, and used to design an optimal TCM with desired twist angle. The objective functions are to minimize the mass and maximum von-Mises stress observed, while minimizing or maximizing the twist angles under specific loading conditions. The multi-objective optimization problem proposed in this paper is solved for an ornithopter flight research platform as a case study, with the goal of using the TCM to achieve passive twisting of the wing during upstroke, while keeping the wing fully extended and rigid during the downstroke. Prototype TCMs have been fabricated using 3D printing and tested. Testing results are also presented in this paper.

Optimization of a Bend-Twist-and-Sweep Compliant Mechanism

Abstract: The overall goal of this research is to develop design optimization methodologies for compliant mechanisms that will provide passive shape change. Our previous work has focused on designing two separate contact-aided compliant elements (CCE): one for bend-and-sweep deflections, called the bend-and-sweep compliant element (BSCE), and another for twist deflection, called the twist compliant element (TCE). In the current paper, all three degrees of freedom, namely bending, twist, and sweep, are achieved simultaneously using a single passive contact-aided compliant mechanism. A new objective function for a contact-aided compliant mechanism is introduced and the results of the optimization procedure are presented. A bend-twist-and-sweep compliant element (BTSCE) can be inserted into the leading edge spar of an ornithopter, which is an avian-scale flapping wing un-manned air vehicle. The multiple objective functions of the optimization problem presented in this paper are: for upstroke, maximize tip bending and sweep deflections, maximize twist angle, and minimize the mass and peak von Mises stress in the BTSCE, and for downstroke, minimize tip bending and sweep deflections, minimize twist angle, and minimize the mass and peak von Mises stress in the BTSCE. This allows a designer to select a CCE from a set of optimal designs to accomplish all three displacement goals. The BTSCE was modeled using a commercial finite element program and optimized using NSGA-II, a genetic algorithm. The results for a single angled compliant joint (ACJ) for quasi-static upstroke loading conditions are presented. Two optimal designs are discussed and compared, one with a moderate peak stress and moderate deflections, the other with a high peak stress and large deflections. The optimization results are then compared to the previous results for the two independent CCEs. A design study showed that the angle of the ACJ needs to be obtuse to achieve a positive twist angle during upstroke, and an acute contact angle reduces peak stress. The deflection objective functions were relatively insensitive to eccentricity for upstroke and downstroke compared to the other parameters, and a high stress penalty was paid for any gains in deflection. The downstroke objective functions were relatively insensitive to all parameters compared to the upstroke objective functions, and were much smaller in magnitude. The optimization showed that under simplified upstroke loading conditions, the BTSCE with a single ACJ allowed bending deflection near 30% of the length of the BTSCE, twist angle near 0.14 radians, and sweep deflection near 5% of the length of the BTSCE.Copyright © 2014 by ASME

Inertial effects due to passive wing morphing in Ornithopters

Abstract: Margaret Northrup University of Maryland, National Institute of Aerospace, Hampton, VA, 23666 Abstract Passive wing morphing has proved to be beneficial to the performance of small flapping wing un-manned air vehicles or ornithopters. Previous work has shown that passive morphing, achieved by inserting a compliant mechanism called a compliant spine into the leading edge wing spar of a test ornithopter, reduced the power consumption by 45% and increased the mean lift by 16% without incurring any thrust penalties during straight and level flight. The focus of this paper is to isolate the inertial and aerodynamic effects that occur due to the presence of the compliant spine. Isolating the inertial and aerodynamics effect enables a better understanding of the reason behind the aforementioned force benefits. In order to isolate the inertial effects from the aerodynamics, the ornithopter was placed inside a 5 foot x 5 foot vacuum chamber at NASA Langley Research Center and it was tested at vacuum (1 Torr) and at ambient pressure (760 Torr). The ornithopter was mounted on a 6 degrees of freedom load cell to measure the lift and thrust forces produced by the ornithopter at various flapping frequencies. Also the pitching moment is measured using the same load cell. During the test, four wing configurations are tested. The first configuration is the ornithopter with a uniform, solid, carbon fiber wing spar. The remaining three configurations tested are the ornithopter with various compliant spine designs, inserted in the leading edge spar at 37% of the wing half-span to mimic the function of an avian wrist. The results presented in this paper compare the effect of the presence of several compliant spine designs on the ornithopter's inertia and aerodynamics. Lift, thrust, and pitching moment are used as the comparison metrics. Results show that all of the benefits observed in the vehicle’s performance due to the presence of the compliant spines are due to aerodynamic and not inertial effects.

Stability Analysis of the Wing Leading Edge Spar of a Passively Morphing Ornithopter

Abstract: This paper presents a stability model for the wing leading edge spar of a test ornithopter. The long-term goal of this research effort is to passively improve the performance of ornithopters during steady level flight by implementing a set of wing kinematics found in natural flyers. The desired kinematics is achieved by inserting a compliant mechanism called a compliant spine into the wing leading edge spar to mimic the function of an avian wrist. The stiffness of the compliant spine is time varying and given the nature of flapping flight, it is periodic. Introducing a variable stiffness compliant mechanism into the leading edge spar of the ornithopter affects its structural stability. Therefore, a stability analysis is required. In order to start the stability analysis, an analytical model of the ornithopter wing leading edge spar with a compliant spine inserted in is necessary. In the model, the compliant spine is modeled as a torsional spring with a sinusoidal stiffness function. Moreover, the equations of motion of the wing leading edge spar-spine system can be written in the form of non-homogeneous Mathieu’s equations, which has well-known stability criteria. The analytical system response is then validated using experimental data taken at NASA Langley Research Center. Results show that the analytical spine angular deflection agrees with the experimental angular deflection data within 11%. Stability was then demonstrated using both analytical and graphical proving that the response of leading edge spar with a compliant spine design inserted at 37% of the wing half span is bounded.Copyright © 2014 by ASME