5 - Implementation

After verifying the mechanism in the original prototype, there were five major tasks to be completed for the final mechanism:

Improve the mechanism's robustness
Design and implement a grabber
Implement intermittent motion
Electronics and software
Translate the horizontal prototype to a vertical mechanism

Each of these tasks and their iterations will be outlined below.

Improving Mechanism Robustness:

To fabricate our mechanism, most of the components - apart from the electronics, off-the-shelf parts, and, initially, gears - were initially laser cut from a board of 6mm acrylic. Our design had to be modified to provide accurate tolerance to have the laser-cut parts fit other off-the-shelf components. This was achieved by cutting holes with decreasing diameters starting from the nominal value and trying to reach a diameter to determine an apt match for the steel shafts and bearings.

Figure 1. Example of press fit tolerance tests

After the tolerance for the material and machine was determined, we cut the links from 6mm acrylic, while the back plate, and the mechanism platform from 3 mm plywood.

Figure 2. Laser cut features

Additionally, our initial iteration of gears was 3d printed and press fit onto the motor we were provided with. Thus, we also did tolerance prints to ensure proper D-shaft fit.

Figure 3. D-shaft tolerance print

However, after assessing all the compartments and trying the finished mechanism, it was too heavy for the motor to operate. Thus, we decrease the link thickness to 3mm acrylic with the next iteration. Weight reduction holes were cut out of the links to decrease the load.

Figure 4. 3mm link with weight reduction holes

We also 3D printed an Arduino. motor controller, and motor mount to ensure secure mounting of the electronics system.

Figure 5. 3D printed electronics mounts

Grabber

Our goal for the grabber was to be entirely mechanical to minimize our reliance on software. The 4 link grabbing mechanism consisted of six 3mm acrylic pieces that were laser cut to fit the dimensions of a typical cookie when in the closed position. The claw links were each two alternating layers on the screw post which allowed the cookie to be more secure throughout the motion. Four 5mm screw posts of different lengths were used as joints. The grounded pin joint as well as the slot were connected by a steel spring that allowed the mechanism to remain in a closed position throughout the duration of the dipping motion. The grabbing mechanism is actuated when the slotted pin contacts a pinned block cut from two 6mm layers of acrylic, which causes the grabbing mechanism to open.

Figure 6. Claw mechanism

Intermittent Motion

Next, we wanted to add an intermittent motion component to pause the mechanism at the serving and dipping positions. Again, this mechanism was also entirely mechanical to align with the overall goal of simplifying our codebase. Since the primary linkage's dipping and serving positions are 180 degrees crank rotation apart, the intermittent motion was accomplished with two gears. One gear half of its teeth, and the other with all of its teeth. These were initially FDM 3D printed.

Figure 7. Initial 3D printed gears

While these gears did achieve our desired motion, the gears had too coarse of a pitch (i.e. too few teeth at 10). This led them to bind when meshing, causing the motor to stall. To alleviate this issue, the next iteration of gears was designed with a finer pitch with 24 teeth. Each gear was made of two 6mm acrylic sheets glued to each other rather than 3D printing to decrease manufacturing time. This allowed the gears to mesh and the motor and mechanism to run properly.

Figure 8. Acrylic gears

Electronics and Software

Our mechanism was designed to work with a single rotary input from a motor to the input gear. An Arduino Uno ran a code that directed the motor, starting it when a button was clicked and stopping it when clicked again. The Arduino code is based on a previous project in this class but modified to turn the button into a toggle switch. Initially, we used an L298N motor controller with two 9V batteries in series, providing 18V to our system. However, to prevent motor overheating and reduce the need for frequent battery replacement, we switched to a 12V power supply.

Figure 9. Electronics diagram

Translating Horizontal to Vertical

Since the aim of our mechanism was to dip a cookie in a cup of milk, our system was designed to stand upright. In order to move on from a prototype that functioned in a flat plane to an upright mechanism that provided our desired vertical motion, we created a supporting structure. The structure had an additional slider feature to allow the height of the bottom plate to be adjusted accordingly to the level of milk we desired and also had 3d printed compartments to hold the electronics as seen in Figure 5 above. Lastly, a 3d printed motor mount was also screwed onto the structure to hold the motor in position.