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Compliant Mechanismas an

Automotive Seat Cushion

Christine Vehar Jutte

Faculty sponsor: Sridhar KotaCompliant Systems Design Laboratory (CSDL)

University of MichiganSeptember 5, 2007

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Original Design

Rigid Seat Pan Multi-piece stamped and welded steel pan(No foam or cover shown)

4-inch foam cushion(Expensive to store and ship)

Reduce foam thickness from 4” to 2” without compromising passenger’s comfort and safety

Anti-submarine geometry

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Passenger Comfort

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Automotive company force-displacement data measured at the center of the seat cushion. [1]

4-inch foam cushion

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New Design Model

Nonlinear spring: A compliant mechanism where the input force and the measured output displacement share the same location and have a nonlinear relationship.

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Problem Definition

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Target Points

Spring in Parallel

Spring in series with foam

Design a nonlinear spring:1) Match load-displacement

function2) Fit within prescribed

design space

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Functional Description

In-plane thickness = 0.027in (0.69mm) Out-of-plane thickness = 12in (304.8mm) Material = MartINsite M130 (E = 200 GPa)Max stress = 605MPa (< yield 930 MPa)Safety factor =1.5Disp. = 29% of largest footprint dimension

Final Spring Design

Spring within new assembly

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Functional Description

Final Spring Design’s Assembly in Prototype

Nonlinear springs

Connecting Plate

Anti-Submarine feature

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Functional Description

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Nonlinear Spring Target Points

Final Spring Design’s Load-Displacement Function

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Functional Design

Validation

Instron 8516

Nonlinear spring assembly (No foam included)

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Functional Design

Validation (Instron 8516)

Nonlinear spring assembly (Foam included)

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Novel Features

Long, slender curvilinear shape(Enables large geometric nonlinearities with low stress)

Horizontal constraint at input (Activates the axial mode of the spring for stiffening effect)

Free rotation at input(Enables large rotations with low bending stress)

1) Generates nonlinear response by varying effective stiffnesses of beam elements

- Geometric nonlinearities (large displacements and rotations)- Boundary conditions

Tradeoff between bending (flexible) and axial (stiff) modes

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Novel Features

2) Matches a prescribed load-displacement function at one point on the mechanism

(Separate input and output point)

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Novel Features

3) Distributed compliance - Large displacement relative to footprint (29%) - Easier manufacturing

Various large-displacement compliant mechanism synthesis methodologies

Nonlinear load-displacement relationship at single point (lumped compliance)

• Howell (2001) – Constant-force mechanism/spring• Pedersen, Fleck, and Ananthasuresh (2006) – Compliant mechanism and actuator system with constant-force output

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Procedure

• Objective function (0.98): - Minimizes error relative to prescribed load-displacement function- Includes penalty functions for displacement, stress, and buckling

Developed a generalized nonlinear spring synthesis methodology for matching any prescribed load displacement function

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Procedure

• Shape and size optimization of spline determines final design(Full scale nonlinear spring synthesis methodology includes a network of 9 splines for topology optimization)

• Genetic algorithm (optimization)

• ABAQUS’s modified Riks method (nonlinear finite element analysis)• Hybrid beam elements (for increased efficiency)

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Benefits

• Nonlinear spring design– Reduces storage and shipping costs of seat cushion

• Nonlinear spring synthesis methodology– Reduces design process cost & time

• New seat assembly– Recommendations made to reduce manufacturing costs [1]

• Stamp springs using permanent dye

• Use lightweight, high-strength composite for connecting plate

• Integrate the hinges into connecting plate

• Integrate the support bars into either (i) seat frame or (ii) nonlinear spring via a modified stamping process.

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Possible Applications

Freedom Innovations Otto Bock

Statically Balanced Mechanisms

Nature’s Nonlinear Compliance

MEMS Devices

Artificial Implants and Prosthetics

- Structures with synthesized nonlinear elasticities

- Mimic nonlinear and viscoelastic materials

Constant-Force Springs

Shock absorbers

Design for crashworthiness

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Acknowledgements

• Senior design team: Trevor Knauf, Rachel LaValley, Ben Sivulka, and Joe Thomas[1] Knauf, T., LaValley, R., Sivulka, B., and Thomas, J. “Compliant Seat Cushion Pan for an Automotive Seat.” University of Michigan. December 2006.

• Faculty sponsor: Sridhar Kota

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THANK YOU

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