Final Design

Microgreen Trimmer

Reciprocating Blades

The final blade was machined out of 304 Stainless Steel for its corrosion resistance, durability, and hardness. The blade serration, shown in Figure 13, was chosen for its ability to be manufactured easily and inexpensively as well as good cutting ability. While shearing clipper blades, shown in Figure 14, provide better safety qualities and cleaner cuts, custom manufacturing a blade proved to be far too expensive. After comparing and assessing all aspects of each blade style, it seemed logical and apparent that serrated blades were the right design choice. The only compromise this choice would have was the safety quality of the blade, which was far outweighed by the monetary cost and can be accounted for with other additions. The serrated blades were also determined to be safe as long as proper use is followed and safety standards and attached warnings are presented.

Dual reciprocating blades were also chosen for the final design due to their ability to dampen the vibration of the system; this is done by balancing the forces on the camshaft. Essentially, the blades are positioned to contact the cams at opposite points, and as the offset cams rotate the blades move in opposite directions at the same time. This causes the blades to exert almost exactly the same force on the camshaft (due to equal masses and inertia), however in opposite directions, shown in Figure 15. This then balances the forces on the camshaft and reduces the vibration of the entire device. Reducing vibration in the system was very important as it makes the trimmer much easier to use as well as allows for precise and accurate cuts. Therefore, the dual blade design proved to be the best option for this harvesting device.

Nozzle

The 25 cm design was chosen in the end because it can cover a larger span than the smaller nozzle size, which allows for more efficient harvesting. Although the 15 cm long nozzle could have a larger optimal width for collecting microgreens, the 25 cm long nozzle could still be made wide enough to prevent the microgreens from jamming. This however, does require more suction power because the nozzle at this size suffers from a lack of airspeed. To combat this issue, the nozzle was designed to allow for a plug. This plug covers part of the nozzle inlet and effectively converts the full 25 cm length down to 15 cm. In doing so, the airspeed in the inlet increases, which aids in the harvesting of heavier microgreens, such as radish stems.

Currently with the 25 cm long nozzle, the Microgreen Harvester converts a 37.57 m/s flow speed from the vacuum to a flow speed of about 20 m/s through the nozzle. By using standard volumetric flow analysis, in which it is assumed that there are no air leaks in the device, it was determined that the nozzle could be made to be 3.8 cm wide. For more details on the analysis of the fluid mechanics and nozzle design, refer to the appendix section A.6. Although it would be ideal to have a wider nozzle, since microgreens with many leaves or big leaves can cover a span up to 5 cm, the width of the nozzle coupled with the added suction power of the plug should be enough to ensure complete collection.

Delrin spacers were machined out of 1.9 cm (¾ in.) round stock to serve as spacers to hold the blades in position and reduce friction (Figure 13). All pieces were then secured down to the Stainless Steel baseplate with #10-32 screws.

Adjustable Mount

The primary goal of the nozzle mount was to hold the nozzle rigidly above the blade as shown in Figure 23. The strength of the system was paramount and had to be able to account for allowing adjustments while maintaining the rigidity. The size of the mechanism was also an incredibly important issue as it had to be as compact and functional as possible as well as strong. Additionally, the main design concerns for the adjustable mount were balancing the weight, the bending moment, the center of mass, the strength, and the location of the nozzle connection interface.

The main frame of the mount serves as the foundation of the mechanisms as well as the location of the height adjustment track, as shown in Figure 24. The track portion extends to a maximum height adjustment of two inches above the lowest location, which would sufficiently account for the different heights of all microgreens. The track was designed to rest on the front flange of the trimmer as shown in Figure 19, and has linearly spaced locations for teeth to hold the height adjuster in position. This tooth locking mechanism is depicted in Figure 25, and employs small compression springs to keep the teeth locked into position while still capable of being adjusted.

The rotary angle adjustment mechanism was designed in order to position the nozzle opening nearly on top of the leaves of the microgreens. To do this, the locking rotary gear was designed to position the nozzle angle at various 15 degree angle displacements, the most important two angles being 45 and 60 degrees. The teeth on the gear were also designed to be as thick and strong as possible in order to prevent any fatigue or breaking that would render the mechanisms useless. The rotary gear is then locked in place with compression springs in order to maintain strength as well as adjustability. It was designed to be adjusted by easily pulling the gear along its axis then rotated until the next next teeth are above hole then the gear can be depressed back into the locking positions. Extruding from the rotary gear is the cylinder in which the nozzle attachment bar can be inserted to hold the nozzle rigid, and a pinhole slot to lock the nozzle arm in place.The entire mechanism and its functionality are depicted in Figure 26.

Thein Baffle

As displayed in Table 3 above, both a Thien Baffle and a Conical Cyclone Separator are capable of separating the microgreens from the vacuum cyclone and reducing the vacuum pressure in the lower container. The primary motivation behind choosing the Thien Baffle however was twofold; the first reason was the time the greens spend spinning around getting damaged, and the second was the potential for clogging as the greens fell through a small cross-sectional area. The reason to employ one of these devices was to limit the damage the microgreens sustain upon collection. Therefore it is absolutely essential that the greens are separated from the cyclone and come to rest as quickly as possible (Thien example shown in Figure 29). This system is also needed to operate for an extended period of time to harvest multiple trays of microgreens, which cannot occur if it gets clogged. For all of these reasons, the Thien Baffle proved to be the better choice and therefore was selected for this system.

In order to better direct the greens through the separation slot in the Thien Baffle and prevent circulation, a 10 degree inclination angle was designed into the hose attachment as shown in Figure 30. This angle should help to prevent damage the microgreens could sustain if they were to circulate in the vacuum cyclone. Additionally, a dampening feature was built into the Baffle in order to prevent the greens from hitting the inner wall with high velocity and obtaining damage. This feature is similar to a curtain/backstop and utilizes a soft cloth material and an air pocket to reduce the velocity of the microgreens and soften the impact to prevent damage, shown in Figure 31.

The top and bottom of the Baffle were made of 0.635 cm (¼ in.) transparent acrylic with 0.159 cm (1/16 in.) clear polycarbonate walls to allow for monitoring of the microgreens. Structural support columns were precision machined out of 0.953 cm (⅜ in.) aluminum rod to provide rigidity and strength. The hose attachments were 3D printed with PLA for precision fittings and unique shapes in order to keep costs as low as possible. FDA food grade gasket material, 0.318 cm (⅛ in.) O-ring cord stock, and silicone were then used to seal the entire system, meanwhile everything was held together with #8-32 screws.

Final Performance Results

The first round of testing involved using the Ryobi trimmer to cut a tray of small microgreens,vacuum the greens using the nozzle, run the greens through the Thein Baffle, and collect the green in the collection container. This first round of testing was very successful. The tray of greens was cut within a minute with very little damage to the greens as determined by our sponsor.

A recommendation to resolve the nozzle problem is changing the shape of the nozzle opening to be half as wide (12.7 cm) but twice as high (6.35cm) . This would allow more room for the larger greens to be collected while maintaining the same flow rate into the nozzle. This would also involve cutting the length of the blade in half (6.35 cm) to prevent excess cutting. This would still fall within the sponsors parameters of cutting the tray in two passes.

The second round of testing had a few small problems. This round used larger microgreens with more leaves. These larger microgreens had a tendency to clump up which sometimes impeded the flow of the nozzle. This was accounted for by increasing the height of the nozzle opening by 0.635 cm and adding a plug that could be put in the entrance of the nozzle to reduce area, but increase flow rate. These two changes improved the performance of the prototype, but greens were still getting stuck in the nozzle albeit less often. This issue only pertained to the larger greens while the smaller greens were unaffected.

To reduce the damage of the greens even further, a mesh screen was added on the inside of the thine baffle. The screen provided a softer wall to hit than the acrylic wall that the greens were colliding with before. After testing the prototype again with the addition of the screen, no greens were found to be damaged.

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