13.2 - Design Process

Preliminary Design

During the prototyping phase of this project, the team brainstormed multiple mechanisms that would provide clean sweeping motions. The team determined that a figure 8 motion path would achieve the best results when cleaning a glass. To achieve this motion the team initially proposed a slider crank mechanism alongside a scissor lift to vertically move the slider crank. This slider crank mechanism would allow sponges to rotate in a circular motion, and the scissor lift will be synced to provide the desired figure 8 motion path.

Figure 1. Preliminary Design

Initial Prototype

While the preliminary design would achieve our desired motion path, the team decided syncing the slider crank to the scissor lift would be too complicated to implement. The team instead decided to opt for a geared five-bar mechanism. This mechanism uses two counter rotating gears couple with two connecting linkages. For our first prototype the team decided it would be best to start by printing the gears and linkages to prove that this mechanism would follow our desired path. A CAD model was designed for a gear with a 6 inch diameter with a 3 inch offset hole, where our linkage would be connected. Another CAD model was made for our linkages with a 6 inch hole distance. The team then laser cut two of each of these CAD models, and put together the first prototype shown in Figure 2 below. To assemble the prototype the team used 3 M3 bolts and multiple M3 nuts to ensure our linkages were properly spaced out. The gears were not initially grounded, but the team was able to prove that this mechanism would achieve our desired figure 8 motion path when the gears were rotated. 

Figure 2. Initial Prototype

Iteration Process 

With our initial prototype we realized our links were too short and had our expected path was over our gears. To prevent any potential interference between the gears and the fasteners connecting the links together we decided to increase out link lengths. We tested out three different link lengths and determined how this affected our path size as well as how far the top of the links were from the gears. Below is a table that includes data from our experiments with different link lengths.

After the experiment we decided to select the 12 inch links due to providing a good sized figure 8. The 16 inch links were also considered; however, we were concerned about the ability for our mechanism to stand upright without the need of extra support. Having too much mass would cause it to fall over which led us to choose the 12 inch links.

Figure 3. Second Design Iteration with 12 inch links

Final Design Model

For our final design we made a CAD model for a base that would hold our gears in place. One gear would be connected to a DC motor, while the other gear would be connected using a rod and a bearing. This base took into account the placement of our arduino and motor driver with holes for them to be mounted to. The base also took into account the front holes of our motor to ensure that it would be mounted tightly. These measurements took a lot of trial and error using individually printed parts, but this ensured our final mount would have correctly spaced mounts. The bore of the spur gears was also changed to ensure that our rod and motor would fit tightly into our gears. Lastly, the links were redesigned with a 12 inch hole distance with built-in spacers to ensure that they would not come into contact with any bolts/nuts as they rotate around the gear.

Figure 4. Final Prototype

Final Design

For our final design we decided to remove the front legs of our base to ensure that the gear connections would print in the first layer and ensure correct dimensions. We also printed an extra spacer that would connect to the bearing gear linkage to ensure a correct offset between both linkages.  The previous bearing gear was pretty lose The bearing gear also had to be redesigned to ensure a press-fit between the rod and the gear to remove any unnecessary movement. Lastly, the team mounted a servo motor to the tip of a linkage to provide a sweeping motion using an attached "squeegee" head. The servo motor and DC motor were connected to a single Arduino to allow their movements to be synced correctly.

Figure 5. Final Design