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We aimed for a cam design capable of pushing links 4 and 5 inducing a transmission angle as close to 180 degrees as possible while swiftly retracting. Initially, we used a geometry based on the linear motion of the joint, crafted using Solidworks. However, this approach did not work. Instead, we adopted a more improvisational method, shrinking the geometry slightly, adjusting for the final length, and progressively increasing the curvature radius in the part file. Iterating through this process, we ensured constant contact in the assembly, refining the design until achieving a promising result, despite the unusual motion graph of the joint. Ultimately, we settled on a design and 3D printed approximately three iterations of the cam. Press fitting secured it to the motor during the final stages of system assembly.
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The linear rail system was designed to constrain our mechanism to 1DOF. We used two 3-foot long 8mm rods from McMaster-Carr as our rails, initially planning to cut them down to a realistic size for our mechanism's jump height before deeming that decision unnecessary. In order to successfully ground our assembly, we needed to construct a sturdy frame that could hold our rails parallel to one another. Initially, our team was planning on building the rail framing assembly out of 1/4-inch laser-cut wood, but we were able to get access to 80/20 aluminum extrusion from the lab of one of our group members. Therefore, we were able to build a significantly studier frame that would be able to easily accommodate the full length of the rails. We did continue to use a wood base, as it allowed for a smooth, uninterrupted surface for the mechanism to jump on; this was manually cut and fit around the frame. The rails were attached to the frame using 3D-printed feet screwed onto the wood base and frame top. The rails were spaced evenly apart such that they lined up with the mounting positions on the mechanism and stayed in line with the 80/20.
Electronics
Motor, Arduino and Motor Driver
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Next, we mounted the pre-built linkage system onto the rails. After this, we attached the motor, with the cam being the final component to be mounted.
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Our initial tests after assembly did not go successfully; we were not able to jump from either electronic actuation or hand actuation. Electronic actuation was limited by the torque of our motor; while our cam geometry was effective, our motor simply was not powerful enough to compress our spring without stalling. However, hand actuation was still a possibility. After some error analysis, we found that our sliding link was binding against the rails, our links were experiencing a large moment that was tilting our leg link, and our leg link was experiencing excessive friction at its base due to the contact of acrylic against wood. To fix these issues, we focused on increasing rigidity and decreasing friction of our system (shown in order in the images below):
- We remounted the rails onto the frame in a way that was more parallel relative to the sliding link.
- We attached a piece of spare acrylic to the base wood to serve as a guide for the leg link while also reducing the friction it would experience.
- We introduced a ~1500g counterweight into our design to counter the increased weight of our assembly from design changes during manufacturing; this was a variant of a suggestion that we had received previously and worked to help keep our system performance more true to our initial analysis.
- We used white lithium grease to lubricate the rails to reduce friction between the linear bearings and the rails.
These changes allowed our mechanism to jump with hand actuation.
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