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Although our team used the default parallel gripper for our tasks, with a bit of tape to pick up the dollar bill, we considered a few alternative designs:
Our first design involves retractable metal sheets attached to the bottom of the parallel gripper. Ideally, these would slant down at an angle to assist with scooping under flat objects, such as the dollar bill and coin. However, due to the difficulty in designing a retraction mechanism, this idea was promptly scrapped.
Continuing from the previous idea, we designed an extra wide gripper, with two separate grip surfaces. One side is the standard parallel plate gripper, which is sufficient to complete the first two tasks. The other side possess a broom-dustpan structure to sweep up the coin and the dollar bill. This design makes it easy to switch between the grip surfaces by adding a small offset to the EE location; however, it is unwieldy and prone to accidental collisions.
Instead, we switched from a "scooping" concept to an "adhesive" concept, and experimented with double-sided gecko tape:
Instead of using typical glue, gecko tape consists of nanostructures that generate van der Waals and capillary forces to create an attractive force. This tape was to be plastered onto a 3D-printed rectangular surface, with a hole in the middle for a plunger. The linkagefor the plunger directly attaches to the parallel gripper as shown above in the right two images. When the parallel gripper opens, the plunger rises and the adhesive tape lowers; this is the default position. When the parallel gripper closes, the plunger extends past the gecko tape, breaking its adhesive connection with any object being lifted.
Unfortunately, since the gecko tape was not fully double-sided (one side was acrylic adhesive) the acrylic lost most of its adhesive properties in storage, and we were left with one-sided tape. For ease of replicability, we opted for the "Not-so-gecko" option of masking tape, which functionally served the same purpose.
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