18.2 Design Process
Project Prototype, Iteration Documentation, BOM
Prototype in MotionGen:
To visualize the complex motion of our mechanism, we began with an initial design in MotionGen. This tool allowed us to experiment with different linkage configurations and better understand the motion paths. During this stage, we discovered that adding a second four-bar linkage could be an effective way to push the cherries into the drink, adding functionality to our system.
Prototype in CAD:
Using our MotionGen design as a reference, we created a CAD model of the linkage system. We tested our linkage lengths against the Grashof condition to ensure full rotation where needed. To enable adjustability during physical assembly, we added multiple holes in each linkage. Additionally, we defined key dimensions—such as thickness, lengths, and hole sizes—using variables, which made it easy to iterate quickly. This CAD phase also helped us determine the stacking order of linkages and identify potential interference issues early on.
Physical Prototype:
We laser cut all the linkage components out of wood and assembled the mechanism as modeled in CAD. However, when testing the motion by rotating the input link, we noticed it couldn’t complete a full rotation due to restrictions. After revisiting the CAD model, we discovered that some links had been flipped in the physical assembly, breaking the Grashof condition. Once we corrected the orientation, the mechanism functioned as expected!
There are still improvements to be made—for instance, we plan to add two side plates to reduce play in the system. We also realized that our current motion resembles more of a scooping action than a precise stabbing motion. To address this, we will refine the design using further kinematic analysis in the next iteration.
Iteration Documentation:
There were many issues with the first design prototype- the links were interfering due to a lack of spacer, the two links on the bottom left corner were actually one ternary link and the links were flipped during assembly. The 12 hole pin design on the ends of the link were added to support fast iteration after we printed our parts, since the real world is not as perfect as CAD!
This assembly fixed the interference issue between links by using spacers between layers of stacking, as well as fit the Grashof condition.
This assembly referenced the first CAD prototype and therefore would not allow for a full range of motion. This is when we realized that the linkages in our CAD were flipped.
This was our fixed assembly that worked with a full rotation of the crank (Link 14). We found that more adjustments that need to be made for our position profile for a stronger jab. This will be done with our software models of the mechanism.
Bill of Materials:
Item Name | Vendor | Price | Quantity | Notes |
|---|---|---|---|---|
12” x 24” Acrylic Sheet: 6mm | TIW | $ 13.82 | 1 | Front & back mounting plates |
12" x 20" Birch Sheet: 6mm | TIW | $ 5.62 | 1 | Material for linkages |
Bearings | Already Own | $ 9.99 | 20 |
|
56 RPM Econ Gear Motor | Servo City | $ 14.99 | 1 | High torque motor |
Arduino Nano | Already Own | $ 24.90 | 1 | Motor & sensor control |
11.1V 2200 mAh LiPo Battery | Already Own | $ 16.65 | 1 | Power |
8mm x 400mm Steel Shafts | Home Depot | $ 11.99 | 1 | Axles |
Thin Film Pressure Sensor | TIW | $ 11.99 | 2 | We'll place cup on the button to act as a power switch |
Limit Switch | TIW | $ 7.99 | 1 | Use to zero the position after each loop |
Full Assembly Development, Electronics & Circuit Design, Software Design
Full Assembly Development:
For our final design, we decided to scale all of our links down in order to minimize the slop in our final linkage. We designed our CAD in a way that let us easily adjust the scaling of our links by utilizing global variables linked to a single scaling factor. After some iteration we decided on a scaling factor of 0.85, which resulted in the shortest links possible without causing interference.
The lower four-bar linkage that was meant to remove the cherry had to be cut from the design. Due to the realities of the connection to the motor, we were unable to design a configuration of the links that had not interference. We replaced this mechanism with a stationary part that pushes the cherry off of the toothpick at a set point.
We designed all of our major connections besides the linkages to be adjustable. This allowed for some error in the placement of components to not require a full reconstruction. It also gave us the freedom to tune the placement of the cherry hopper and toothpick holder, which turned out to be a necessity.
One design feature we decided to include is a button that activates the mechanism. The glass is placed on top and depresses the button, activating the mechanism. The motor runs continuously until the glass is taken off, so multiple cherries can be added to the drink by leaving it on the button. The button consists of a small button on a breadboard, a lower button assembly, and an upper button assembly. The upper button assembly rests on the small button and depresses it when the glass is placed on it.
Our design requires the use of replaceable toothpicks, so we need a part to facilitate this. We decided on a 3D printed part that tightens onto a toothpick using an M3 bolt. This design allows easy replacement of the toothpick, as well as length adjustment. The single mounting hole made it simple to adjust the angle of the toothpick for tuning of our stabbing motion.