A link to the live CAD is below (another benefit of OnShape!)
https://cad.onshape.com/documents/adb172d8f79c73e7c1618ddb/w/9408f9d5912cf2206b553a06/e/480036dd0f42900a96338610?renderMode=0&uiState=656cb49ed0c5750e7057788d
A good portion of the CAD design for the robot was initially completed in SolidWorks as a natural follow-up from completing the FEA simulation in the same software. The preliminary CAD done in SolidWorks can be seen below.
However, due to issues with collaboration and access, the CAD was eventually transferred to OnShape instead, which allowed multiple team members to work on and access the CAD without much worry about version conflicts. The CAD as of December 3, 2023 is shown below.
The design of the frame involves 4 drive motors, 3 linear mechanisms to allow for three degrees of freedom for the end effector, and a sturdy frame to keep everything rigid while the robot is moving.
The FEA analysis done earlier in the month proved that the 20 mm x 20 mm extrusions were suitable for the frame. All connections to the frame are done using M5 screw and T-slot nuts. To reduce overall mass and cost, some parts (like the large cross members) will be made with Lexan (polycarbonate), while other parts like the corner brackets will be made from 1/8” aluminum. Parts will be machined by team members at the engineering student machine shop in E5. Additional extrusions were also added as structural members in the x and y axes where possible. An example of the cross members being used is shown below.
For the movement in the x and y-axis, a belt drive mechanism was chosen because its balance between cost, reliability, complexity, mass and speed was rated the highest among other proposed linear mechanisms. The decision matrix is shown below.
| Mechanism | Cost | Reliability | Complexity | Mass | Speed | Total |
|---|---|---|---|---|---|---|
| Ball Screw | 2 | 3 | 1 | 1 | 1 | 35 |
| Rack and Pinion | 1 | 1 | 2 | 3 | 3 | 32 |
| Belt Drive | 3 | 2 | 3 | 2 | 2 | 41 |
The belt drive mechanism was then designed for the two axes, which are driven by stepper motors. The x-axis movement will use two extrusions, each one supporting one side of the y-axis extrusion. To sync the motion between the two x-axis mechanisms, a longer shaft will be coupled with the motor shaft on one side, which will drive the motion on the other x-axis mechanism. As for the y-axis a single belt drive mechanism was needed. All three axes use a gantry cart, with the belt tied off at a notch on the cart. GT2 belts were used as there weren’t any significant loads in the system to warrant a thicker belt or a larger pitch. An animation of the motion in the x-y plane is shown below.
Cartesian Mechanism 2 Cropped.mp4
For the z-axis mechanism, a solution was needed that ensured there was no interference in the y-z plane when the mechanism was fully retracted. This is because the robot will be moving along rows of plants, which will be going through the y-z plane. If there was a part of the mechanism (say a lead screw) that extended too far down, it would hit and possibly damage the plants. The x-axis will travel around 400 mm while the y-axis will travel around 600 mm.
The solution chosen was a scissor mechanism, which is driven through a much smaller lead screw connected to a stepper motor. This requires some overhead distance but otherwise allows for the 500 mm end effector travel distance while ensuring there are no obstructions in that 500 mm path when retracted. The additional attachments were needed to ensure the paintbrush at the end remained vertical and in the same x-y position as the mechanism extended. The video below shows an animation of the mechanism in action.