Compliant Mechanism Research

The Penn State Mechanism Collective Lab
Aug 2024 - Current

SKILLS INVOLVED

Compliant Mechanism Design

MATLAB

Finite Element Analysis

Prototyping & Machining

Experimental Testing

PROJECT OBJECTIVE

Constant force mechanisms (CFM) are a type of device that maintain a constant reaction force as the mechanism displaces, and have many applications in mechanism design. Compliant constant force mechanisms (CCFM) are a specific type of CFM that obtains is functionality through bending parts, resulting in increased lifetime and precision. However, most CCFM function strictly in compression, and can be complicated to design.

My objective is to employ a model for new type of CCFM that employs a simple design framework, and functions both in compression and in tension.

MODELING

The design framework is composed into two modeling components: the Slider-Crank Model, which formulates the desired constant force curve, and the Pseudo-Rigid Body Model, which generates the geometry parameters that will follow that constant force curve.

The Pseudo-Rigid Body Model

Pseudo-rigid bodies (PRB) are used to relate the bending and movement of compliant mechanisms to the forces and displacement of rigid mechanisms.

By utilizing PRB relationships in MATLAB, we can generate a compliant structure with specific dimensions that is "functionally equivalent" to the rigid-link structure.

Finally, we can mirror this mechanism upon itself to eliminate the slider—resulting in a single-part tension-based CCFM, with an associated model.

The Slider-Crank Model

Rigid links are easier to model than bending links. Therefore, the base model of this mechanism is a simple slider-crank, with its sliding end offset from its fixed end.

Using Newtonian methods, we can mathematically derive a system of equations between the displacement of the slider and the required force.

We can optimize the variables of this equation in MATLAB to generate a force-displacement plot that exhibits constant force over a specific range.

FINITE ELEMENT ANALYSIS

Finite Element Analysis provided a rapid method to verify the functionality and accuracy of the model.

The model successfully predicted the FEA results within an 8.48% error—an acceptable range for preliminary testing.

PROTOTYPING

FDM

Excellent for rapid testing and iteration
Lacked the tolerances needed for high-fidelity testing

SLA

Extremely high tolerances and finish
Material properties were unpredictable

Waterjet

Provided high tolerances, and the material properties were consistent
Utilized for high-fidelity testing

EXPERIMENTAL TESTING

Formal, high-precision testing is required to confidently validate that this new model can accurately predict constant force behavior, and formulate the associated compliant mechanism geometry

Testing for Flexural Modulus

The final test specimens will be waterjet from sheets of polypropylene.

To ensure the most accurate results, the flexural modulus and yield strength of this material were experimentally determined following ASTM D790 standards for 3-point bend testing.

Fixture Design and Machining

The final specimen is to be tested on an industry-grade tensile testing apparatus. The system has no compatible test fixtures, so two custom test fixtures were specially designed and machined.

Testing

As of October 20, 2025, the testing phase is still in progress. Final specimens are being generated and waterjet, and precision testing on the tensile tester will immediately follow.

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