II. Mechanism Design Process

Initial Design

Figure 2 depicts the kinematic diagram of our initial idea. The mechanism consists of two robotic arms grounded to a board. The three bar mechanism has an end effector (shown in orange) that will grip the avocado. The end effector will rotate the avocado about the x-axis and the upper link (link 1) of the arm will rotate about the y-axis. While holding a knife at the end effector, the 5 bar mechanism will be able to perform three motions. Links 6 and 7 will be able to rotate about the z-axis with angles θ6 and θ7, respectively. Link 4 will be able to rotate about the x-axis. This link will be fixed to link 5, allowing the entire arm to move along the z-axis.

Figure 2: Initial design sketches

Second Design & Prototype

We quickly changed the design in order to be appropriate for the class. The desire for the class was a closed chain kinematic system while our initial design was two open chain robotic arms. 

Figure 3: CAD of second design

Figure 4: Prototype of second design

As seen in Figure 4, the torque output of the servo was not high enough for the knife to go through the pit. We realized that a high torque servo was required for the final prototype. Additionally, there are motions in two planes required. Typically when one removes a pit from an avocado they cut down into the pit, twist the knife to loosen the pit from the avocado flesh, and then lift the knife and pit out of the avocado. This twisting motion was implemented in our final design.

Figure 5: View of the linkage at the knife handle

Since we already had a hole drilled into the knife in a nonideal plane, we added the two blue linkages to create the twisting motion described above. In the relaxed state, the two linkages remained parallel to the base board (shown). In the activated state, they created a small hill or peak above the base board (not shown).