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Research Papers

Design and Testing of a Thin-Flexure Bistable Mechanism Suitable for Stamping From Metal Sheets

[+] Author and Article Information
Benjamin Todd

 Weatherford International Ltd., Houston, TX 77041ben.l.todd@gmail.com

Brian D. Jensen1

Department of Mechanical Engineering, Brigham Young University, Provo, UT 84602bdjensen@byu.edu

Stephen M. Schultz

Department of Electrical and Computer Engineering, Brigham Young University, Provo, UT 84602schultz@ee.byu.edu

Aaron R. Hawkins

Department of Electrical and Computer Engineering, Brigham Young University, Provo, UT 84602aaronrhawkins@gmail.com

1

Corresponding author.

J. Mech. Des 132(7), 071011 (Jul 07, 2010) (7 pages) doi:10.1115/1.4001876 History: Received September 08, 2009; Revised May 20, 2010; Published July 07, 2010; Online July 07, 2010

We present a new technique for fabricating compliant mechanisms from stamped metal sheets. The concept works by providing thinned segments to allow rotation of flexural beams 90 deg about their long axis, effectively providing a flexure as wide as the sheet’s thickness. The method is demonstrated with the design and fabrication of a metal bistable mechanism for use as a threshold accelerometer. A new model based on elliptic integral solutions is presented for bistable mechanisms incorporating long, thin flexures. The resulting metal bistable mechanisms are tested for acceleration threshold sensing using a drop test and a vibration test. The mechanisms demonstrate very little variation due to stress relaxation or temperature effects. The force-displacement behavior of a mechanism is also measured. The mechanisms’ switching force is less than the designed value because of out-of-plane motion and dynamic effects.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Diagram showing a thin sheet rotated out of plane 90 deg to create a compliant flexure

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Figure 2

Bistable design showing the first and second stable positions, proof mass, compliant legs, and anchored center shuttle

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Figure 3

The bistable design showing the compliant members bent about the center of axis

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Figure 4

An illustration of a SLBM, with both stable positions shown

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Figure 5

A diagram showing one flexible beam being deflected by end forces and moments. To model the motion of a SLBM, the beam’s end must traverse a line at an angle γ with respect to the vertical.

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Figure 6

A plot showing the basic shape of the first and second modes of bending

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Figure 7

A sample model force-displacement curve for a single beam with S=123.7 in a SLBM with γ=6 deg

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Figure 8

Bistable design showing the von Mises stresses in megapascals from the ANSYS model for yielding at the rotation points

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Figure 9

Bistable prototype shown before yielding the flexures

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Figure 10

Bistable prototype shown after yielding the flexures by twisting about the center of axis. Metal constraining bands have also been added to resist out-of-plane motion.

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Figure 11

Drop test setup showing the bistable device, the shuttle, the accelerometer, and the impact platform

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Figure 12

The measured average switching G’s for the (a) 20.5 mm and (b) 25 mm bistable devices

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Figure 13

The measured frequency responses for the 20.5 mm and 25 mm bistable devices

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Figure 14

The predicted and measured force versus displacement curve for the 25 mm bistable devices at 4.33 deg

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