A significant portion of our mechanism, motors, and materials were based on materials that were easily available within the lab as well as used with prior builds in the class. The slider crank mechanism created during Build Exercise #2 was heavily used as a reference point for our reusable plastic bag opener’s design, with said slider crank mechanism being seen below:

Figure #2.1: Basic solution for the slider crank mechanism created during Build Exercise #2

Thankfully, the dimensions of a standard 8 ounce reusable plastic bag are roughly 16.5 cm x 14.9 cm, which is a small enough size to be used with materials provided. Every part of our machine was designed with the reusable plastic bag's size in mind. The DC 12V 100RPM Gear Motor used for the joystick controlled robot in Build Exercise #1 was repurposed for the sake of the motor in our bag opening mechanism. A number of elements from previous Build Exercises were repurposed not only due to our group having some familiarity with the pre-existing elements, but reusing pre-existing elements also helped in keeping the overall cost of the mechanism down.

Figure #2.2: Initial claw design sketches

With the basic design concepts established, the actual design work itself had begun. Initially, our very first plan was to create a single claw design that operated using the crank and pin mechanism seen in Figure #1. However, it was quickly discovered that some sort of auxiliary tool was required in order to hold up the reusable plastic bag in addition to the crank being used to open it. The first idea we had was to create a second claw that could be used to pull on the other side of the bag, but problems arose regarding the bag’s orientation, as the bag would need to be very specifically positioned in order for both claws to grab their respective side of the bag.

Instead, we settled on using a wooden box design that would both house the claw and its motor, as well as having a hook that could be used to hold up the reusable plastic bag, allowing it to be opened at numerous different orientations.

Figure #2.3: Initial wooden box design sketches

Using a hook-like mechanism placed within the wooden box, the reusable plastic bag could be first placed onto the hook, without needing to be orientated at any specific angle or direction, and the crank could be activated. At this point, the crank would then slide across the box and latch onto the reusable plastic bag, exerting a shear force on opposing sides of the bag which would result in the bag opening.

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