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Research Papers: Design Innovation and Devices

Ring-Based Stiffening Flexure Applied as a Load Cell With High Resolution and Large Force Range

[+] Author and Article Information
Jocelyn M. Kluger

Department of Mechanical Engineering,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: jociek@mit.edu

Alexander H. Slocum

Professor
Department of Mechanical Engineering,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: slocum@mit.edu

Themistoklis P. Sapsis

Department of Mechanical
and Ocean Engineering,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: sapsis@mit.edu

1Corresponding author.

Contributed by the Design Innovation and Devices of ASME for publication in the JOURNAL OF MECHANICAL DESIGN. Manuscript received September 12, 2016; final manuscript received June 7, 2017; published online August 30, 2017. Assoc. Editor: David Myszka.

J. Mech. Des 139(10), 103501 (Aug 30, 2017) (8 pages) Paper No: MD-16-1634; doi: 10.1115/1.4037243 History: Received September 12, 2016; Revised June 07, 2017

This paper applies linear elastic theory and Castigliano's first theorem to design nonlinear (stiffening) flexures used as load cells with both large force range and large resolution. Low stiffness at small forces causes high sensitivity, while high stiffness at large forces prevents over-straining. With a standard 0.1 μm deflection sensor, the nonlinear load cell may detect 1% changes in force over five orders of force magnitude. In comparison, a traditional linear load cell functions over only three orders of magnitude. We physically implement the nonlinear flexure as a ring that increasingly contacts rigid surfaces with carefully chosen curvatures as more force is applied. We analytically describe the load cell performance as a function of its geometry. We describe methods for manufacturing the flexure from a monolithic part or multiple parts. We experimentally verify the theory for two load cells with different parameters.

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Figures

Grahic Jump Location
Fig. 1

A nonlinear load cell flexure cut from a flat plate with an applied load P: (a) entire load cell with a linear encoder and (b) free body diagram of one symmetric flexure quadrant for compression mode

Grahic Jump Location
Fig. 2

Fabricated load cells: (a) multipart assembled load cell experiment during maximum tension, (b) monolithic load cell with tapered ring thickness and root inserts in compression, and (c) close-up view of root inserts

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Fig. 3

Load cell performance sensitivity to 1% increase in surface radius: (a) force versus deflection, (b) force versus contact angle, (c) force versus stiffness, and (d) stress along the ring inner radius when force P = 1000 N. Monolithic load cell: black. Multipart load cell: gray. Experimental parameters: solid line. Inner surface radius Si increased by 1%: dashed line.

Grahic Jump Location
Fig. 4

Experimental force, stiffness, and deflection behavior for load cells shown in Figs. 2: (a) and (b) multipart load cell, (c) and (d) monolithic load cell. The horizontal and vertical bars indicate measurement uncertainty.

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