2- Initial Concepts and Prototypes

Moving an Oreo cookie from its container, dipping it into milk, and bringing it to the feeding position is fundamentally a position and velocity problem. To solve this we would need to create a linkage that picks up the cookie, rotates 90 degrees, dips the cookie, and then rotates 90 degrees to feed the operator. This must be done slowly enough not to lose the cookie during this process. The challenge in designing the linkage came from obtaining the three stops. Due to the nature of the goal of our mechanism, we established that it would be practical for it to incorporate intermittent motion. Our initial brainstorming sessions were centered around our mechanism's timing and it having three stops. Originally, we wanted to add complexity to a simple slider crank by creating two push stages and two pull stages through the use of a Geneva mechanism. As the slider-crank moved, the slider would push the pin connected arm along the corresponding profile of the guiderail, which would use a rack and pinion to actuate the arm outwards for serving. While this idea worked in theory, the design proved to be complicated due the challenges posed when ensuring the arm stayed on the correct guiderail and quickly went outside the scope of the project. Thus, the team moved to a more planar mechanism to achieve the motion.

Figure 1. Brain-storming: Three Stages of Operation

To address the previous issues encountered, we opted for a mechanism with two stops and a desirable position profile to dip the cookies into the milk. One of the challenges we faced when designing this pick and place mechanism was how to achieve a vertical profile for the picking stage. Our initial ideas all involved motion profiles that followed a parabolic profile downward, which would not work for our desired motion because this type of profile would knock the glass over in the dipping stage. Additionally, we knew that we wanted some way to actuate the end effector to a position where the cookie would be easy for the consumer to get. We initially designed our linkage mechanism in MotionGen due to the program's simplicity. Utilizing MotionGen, we were able to iterate on our design, making adjustments and refinements as needed to achieve the precise position profile required for our mechanism. 

Figure 2. Iterations of mechanism design through MotionGen

Through this process we were able to visualize how increasing the length of the top two grounded links changes the amount that the end effector would rotate in the serving position.  Additionally, we were able to see how changing the bottom right grounded link affected the vertical motion that we desired. Through MotionGen we were able to scale and obtain link lengths and relative angles to accurately CAD our mechanism in SolidWorks.

Figure 3. Linkage Design in Solidworks: Serving Position (Left) and Dipping Position (Right)

Our final step in this process was to assess the real-world feasibility of our mechanism by creating a prototype. We constructed a low-resolution prototype using leftover acrylic from a previous project, optimizing the design by reducing the size of the links to minimize material wastage and using screws as placeholders for joints, as shown in Figure 8. This initial prototype highlighted the importance of spacing between links to prevent overlapping motion. Subsequently, we refined our design, leading to the second iteration of the prototype, which is made up of 3mm thick plywood, ¼ inch wooden dowels, and 6mm ball bearings. This iteration has smooth motion and a desirable position profile, which enables us to feel confident in our linkage design. For our final iteration, we intend to increase its scale and incorporate a grabbing and a timing mechanism.  

    Figure 4. First Iteration of Prototype                                Figure 5. Second Iteration of Prototype

Figure 6. Prototype in motion

Tentative Bill of Materials