Project Overview
This project focused on designing a custom test fixture to replicate a critical real-world failure mode in robotic systems known as the “three-wheel jam.” This condition occurs when three wheels remain engaged with their tracks while the fourth loses contact, introducing significant asymmetric loading and torsional stress into the robot frame.
In operation, this failure mode could lead to structural deformation or frame fracture, but it was difficult to reproduce consistently in a controlled environment. The objective was to develop a test system that could reliably simulate worst-case torsional loading conditions, enabling evaluation and improvement of structural reinforcements.
Design & System Implementation
To replicate the failure condition, I designed a fixture that kinematically constrained two wheels of the robot while allowing controlled motion at the other two interface. A custom mounting bracket was developed to secure the robot without over-constraining it, ensuring that the induced loads were representative of real-world conditions rather than artifacts of the fixture.
The other two wheels were replaced with a custom-designed eccentric wheel, which remained unconstrained and rotated on rollers. During operation, this eccentric geometry introduced periodic lifting and displacement, forcing the system to transfer energy into the frame as torsional loading.
Key design considerations included:
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Avoiding over-constraint while maintaining repeatability
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Ensuring accurate load transfer into the frame and brace structures
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Designing components robust enough to withstand repeated high-energy cycles
Validation & Performance Testing
Initial testing revealed unintended failure modes within the fixture itself. Specifically, constraint instability led to unintended contact between components, causing the eccentric wheel to rub against the fixture and resulting in premature failure.
I investigated these issues and identified the root cause as improper constraint behavior and insufficient control of motion paths. In response, I redesigned both:
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The constraint bracket, applying kinematic design principles to ensure stable, repeatable positioning without over-constraining the system
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The eccentric wheel geometry, improving contact conditions and eliminating interference
These components were machined and revalidated, resulting in a significantly more reliable and accurate test setup.
Results & Impact
The final fixture successfully reproduced the torsional loading conditions associated with the three-wheel jam, forcing energy into the robot frame and structural braces in a controlled and repeatable manner. This enabled iterative life testing of brace designs, allowing direct evaluation of structural improvements and failure resistance.
The system provided a reliable platform for validating design changes and guided engineering decisions to mitigate frame failure in real-world operation. By accurately replicating a previously difficult-to-test condition, the fixture played a key role in improving overall system robustness.
Key Takeaways
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Real-world failure modes often require custom test systems to replicate accurately
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Kinematic constraint design is critical for ensuring valid and repeatable test conditions
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Early fixture failures can reveal important insights and drive better system design
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Iterative testing is essential for validating structural improvements under worst-case loading
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Effective validation requires designing both the test method and the hardware