1.3 - Design Process
Prototyping:
Our first “iteration” was a simple cardboard cutout to get a feel for how the Geneva mechanism worked. While whiteboarding the design, we were initially planning to have a sort of ratcheting mechanism to hold the pulleys in place, but this iteration showed us that the Geneva mechanism itself acts as a lock while not engaged.
Additionally, we still had the original slider-crank from Build Assignment 1 that was very effective, and thus we decided to reuse a very similar design for the wiper.
Next, we designed in CAD, which was pretty much a straight shot to the current physical prototype. Even though it was first fully designed in CAD to allow us to visualize the packaging, we first printed the Geneva mechanism (not pictured, though it is the same one on the final prototype). This allowed us to effectively verify that not only would the driver mesh with the follower when engaged, thus allowing for intermittent rotation, but also that the Geneva mechanism itself acts as a lock while not engaged. Afterwards, we 3D printed the parts, laser cut the acrylic base, and assembled the components (excluding electronics), building our demo prototype as shown above, which is representative of Figure 4, our first CAD prototype.
Figure 2: First CAD Prototype | Figure 3: V2 Added Idlers |
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F4: Back of window washer | F5: Front of window washer | F6: Added cleaning pad |
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Above is the mechanism we showed on demo day. In Figures 4 and 5, the front and back sides of the mechanism are shown, illustrating the overall geometry of the system. We had not yet added the belts, but this allowed us to test the Geneva mechanism itself. Figure 6 shows the fully assembled mechanism, where we added a cleaning pad at the end of the slider-crank to act as the wiper, made from a portion of a foam mat and a cloth. As demonstrated in the video, turning the main driver produces continuous reciprocating motion of the slider-crank and intermittent motion of the pulleys. Additionally, we show that the gears cannot be rotated from the back side when the Geneva is not engaged, and thus the Geneva mechanism prevents backdriving when not engaged, effectively holding the mechanism in place.
Further Iterations:
Figure 7: First iteration with motor | Figure 8: Video of testing with motor |
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This was the first time we added the motor mount to attempt to drive the mechanism. However, we quickly realized that not only did we not have enough tension on the belt, the yellow motor also did not have enough torque to lift the mechanism. Therefore, we made the decision to switch to a motor that we believed would have more torque. However, as shown in Figure 9, while the mechanism could climb, some of the belts to the pulleys started slipping, so we finally installed the idlers. However, this created just enough resistance that the motor no longer had sufficient torque to climb, but since windows are cleaned from top down, we decided to adapt and have the mechanism descend.
Further Additions and Final Iteration
We added a case to conceal electronics, and made pulleys thicker for consistency, as the strings were slipping off on occasion.
Figure 10: CAD of electronics bay | Figure 11: Final CAD |
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We also slightly modified the Geneva Mechanism to have tighter tolerances and improved geometry. This helped with consistency where it had previously got caught and stalled on occasion.
Figure 11: Original Geneva Cross
| Figure 12: Filleted Geneva Cross |
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Figure 14: Final prototype front view
| Figure 15: Final prototype side view
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