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Research Papers

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

[+] Author and Article Information
Yashwanth Tummala

Graduate Research Assistant
The Pennsylvania State University,
Department of Mechanical & Nuclear Engineering,
University Park, PA 16802
e-mail: yashwanth.tummala@gmail.com

Aimy Wissa

Department of Aerospace Engineering,
University of Maryland,
National Institute of Aerospace,
Hampton, VA
e-mail: aimy.wissa@gmail.com

Mary Frecker

Professor
Department of Mechanical and
Nuclear Engineering,
The Pennsylvania State University,
University Park, PA
e-mail: mxf36@engr.psu.edu

James E. Hubbard

Langley Distinguished Professor
Department of Aerospace Engineering,
University of Maryland,
National Institute of Aerospace,
Hampton, VA
e-mail: james.hubbard@nianet.org

Contributed by the Mechanisms and Robotics Committee of ASME for publication in the JOURNAL OF MECHANISMS AND ROBOTICS. Manuscript received September 10, 2013; final manuscript received April 14, 2014; published online June 17, 2014. Assoc. Editor: Anupam Saxena.

J. Mechanisms Robotics 6(3), 031013 (Jun 17, 2014) (9 pages) Paper No: JMR-13-1179; doi: 10.1115/1.4027702 History: Received September 10, 2013; Revised April 14, 2014

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.

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References

Figures

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Fig. 1

Schematic of a CS with geometric parameters is shown. This mechanism is flexible when it is bent in the flexible direction and is very stiff when it is bent in the stiff direction. Performance of the CJ in the “stiff direction” is dependent on ϕ and gc while performance of the CJ in the “flexible direction” is dependent on Rin, Rout, and e.

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Fig. 2

Stiffness plot of a CS showing nonlinear force-deflection curve

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Fig. 6

Pareto plot comparing deflection and maximum von Mises stress for distributed loads. Size of the circles represents relative mass. All the optimal designs have thin CHs.

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Fig. 5

Different loading conditions used during CS optimization. (a) Distributed loads, (b) tip loads, (c) pure moments. All of these loading conditions were used during the design optimization procedure to understand the effect of different loading conditions on optimal CS designs.

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Fig. 4

Ornithopter research test platform with a wing span of 1.06 m is shown here. The CS is inserted in the leading edge spar at 37% of the half wing span. The CS imitates the function of an avian wrist, thus enabling passive bending during upstroke.

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Fig. 3

Flow diagram showing steps involved in solving the multi-objective optimization problem. NSGA-II (genetic algorithm) is used to solve this optimization problem.

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Fig. 7

Pareto plot comparing deflection and maximum von Mises stress for tip loads. Many of the optimal CSs designs have a very thin CH close to the tip.

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Fig. 8

Pareto plot comparing deflection and maximum von Mises stress for pure moment loads. Optimal designs in this case have the least, maximum possible tip deflections, (only about 2.84*Zreq) amongst the three loading conditions.

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Fig. 9

Optimal CSs that were used for successful flight testing. These designs were obtained from pure moment loading conditions.

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Fig. 10

(a) Compliant spine-spar assembly. Each design consisted of 2.5 in. of CS, 1 in. tab and a Delrin collar on each end. (b) A front view of the left wing, showing the location of the CS at 37% of the wing half span.

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Fig. 11

Test setup schematic showing the Vicon® cameras (representative), high speed cameras, flight path, braking tether, and video capturing area

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Fig. 12

X, Y, and Z positions of the 53 markers mounted on the ornithopter with respect to the inertial frame of reference showing over eight flapping cycles of consistent and repeatable kinematics

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Fig. 13

The Z position of the reflective markers mounted at the right wing leading edge spar versus the normalized span location (a) at mid upstroke and (b) at mid downstroke

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