3 - Design Process

Pick and Place Linkage

The mechanical design of the linkage has one degree of freedom which is actuated by a press-fit shaft on the input link (A).  We decided to connect the links by press-fitting 6mm ball bearings to each link and connecting them with shafts as they would allow for smooth motion of the linkage system. The Solidworks model of the mechanism proved very helpful when determining the correct spacing and orientation of the links on the shafts. Our system required that all grounded links be below the links that they overlap with. The linkages were laser cut in acrylic to save manufacturing time, which allowed us to make quick changes if necessary. The 6mm shafts were cut using a hacksaw and the spacers were cut from 3mm acrylic. The third iteration of the linkage system was cut in 6mm acrylic, however, the weight of the linkage proved to be a significant issue when transitioning the system from a horizontal to a vertical orientation. To remedy this, the team pivoted to 3mm acrylic and added extra cutouts for further aid in weight reduction. 

Figure 1. Final design of linkage system.

End Effector

The end effector of our mechanism was located on link F and was responsible for holding the cookie as it moved from the consumption to the dipping position. This mechanism uses a scissor linkage connected to a slot and a grounded pin joint. The slotted pin joint allows the claw to open as it is pushed back by a block fixed to the backboard of the mechanism. 5mm binding screw posts were used as the pin joints of the claw mechanism to allow for quick changes to the block height attached to the slotted pin and spacing of the claw. We decided to laser cut four claw links for the end-effector to alternate their position on the screw post to allow for more support of the cookie. 

Figure 2. Claw mechanism in closed (left) and open (right) position

Figure 3. Back view of end effector with block to be actuated by along the path of the mechanism

Intermittent Motion

We used gears to incorporate an intermittent motion component into our linkage system because we wanted to spend more time at the consumption and dipping stage. To achieve this, we designed two gears. The first gear was press-fit to the D-shaft of the motor and had 180° of teeth. The second had 360° of teeth and was press-fit to the 6mm shaft on the input link (A). The design would allow for the mechanism to achieve a half cycle to each stop with every rotation of the motor. The first iteration was 3D printed and it was found that more teeth were needed to provide the appropriate amount of torque. The final iteration was laser cut and used two layer of 3mm acrylic. 

Figure 4: CAD of final gears

Stand and Electronics

The backboard or ground for the linkage system was cut from 6mm plywood and designed with grooves for the rest of the stand to be attached. The input link shaft used a press-fit bearing to allow the shaft to spin the link, while the rest of the grounded joints were press-fit to 6mm shafts on the back plate.  The stand consisted of a backboard, four legs, and a shelf. We decided to add slots to each of the legs and backboard to allow for the distance between the cup and end effector to be adjusted for shorter cups. Each slot had an L-bracket to help stabilize the stand when the baseplate was raised. The shelf was designed to house the electrical components of our mechanism. 

Figure 5. Completed stand to support mechanism (left) and shelf to house electrical components (right)

Electronics

 To support our mechanism we required a 12V brushed DC motor that was driven my the L298N motor controller. A button was added to control the 12V power supply which allows users to turn the mechanism on and off at their command. The electronics were housed on a shelf behind the backplate of the mechanism that held mounts for an Arduino Uno, L298N motor controller, and motor

Figure 6. Wiring Diagram of electronics

Final Design

Figure 7. Final Assembly with elevated baseplate for small cups and grounded baseplate for taller cups.

Figure 8. Final motion of mechanism without claw mechanism