Content Comparison

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The first iteration was a basic proof-of-concept used to test the physical construction and motion of the mechanism. In this version, a TPU finger was fixed to a PLA base, and a strip of denim was sewn to the finger and routed through the design to act as the tendon. The finger was fastened into an a PLA component attached to a simple spool, which we manually rotated to mimic motor-driven actuation.

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Here is a visual of the manual rotation. This first prototype allowed us to validate the general tendon-driven curling motion, but it also showed that the system was fairly stiff and that the denim tendon was, as you would imagine, not ideal for smooth motion. Also, this design was not made to be automatically actuated, or scaled.

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With this next design iteration, we redesigned the pulley to fix directly into the motor, which makes actuation for this finger possible. The interior was made more robust to allow for more torque than the previous iteration.

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Although the second iteration improved compactness and actuator integration, it also revealed a new problem. Because the flexures were thinner, the TPU was less stiff, and the tendon slots were narrower, friction became too high and the finger became overly compliant. As a result, the finger did not reliably return to its resting position after curling. This iteration was still valuable because it highlighted the tradeoff between compliance and recovery. Based on this result, the next iteration should increase stiffness by either using a stiffer TPU, increasing flexure thickness, widening the tendon slots, or combining those changes. Overall, these iterations helped us move from a simple manually driven concept to a more integrated motor-actuated prototype while identifying the main design changes needed for the next build.

Final Iteration

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The final implementation expanded the design from a single compliant finger into a two-finger pinch gripper actuated by a shared motor-driven pulley system. Rather than treating each finger independently, the final design used a redesigned spool that allowed both fingers to be connected and actuated simultaneously, creating a more realistic gripping mechanism for object manipulation. This required the overall system to become more compact, modular, and structurally stable so both fingers could operate consistently without excessive friction or misalignment.

Additional iterations focused on improving the finger’s ability to return to its neutral 90° resting position, reducing tendon binding, and matching the compliance of the TPU flexures to the available motor torque. Tendon slot dimensions were adjusted to reduce friction while maintaining accurate tendon routing, and nylon straps were sewn into closed loops at the distal end of each finger to create a cleaner and more reliable attachment to the pulley system. Once the mechanical connections were validated, a PLA housing and handle assembly were designed to improve system stability, protect the electronics, and create a cleaner final presentation.

This final version demonstrated the transition from a proof-of-concept compliant mechanism into a functional motor-actuated two-finger gripper capable of controlled pinching motion. While further refinement is still needed for full repeatability and long-term durability, the final prototype successfully validated the feasibility of the tendon-driven compliant gripper design.

Bill of Materials

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