Fabricating Structures - MeCO-AUV/MeCO-Documentation GitHub Wiki
Overview
| LoCO | MeCO |
|---|---|
Key Design Changes from LoCO:
- MeCO's final design has one large 6.5" diameter central tube and two 4.5" side tubes arranged in a triangle, compared to LoCO's double-tube design. This configuration provides 96% more internal volume than LoCO.
- The 6.5" tube is essential to fit a Jetson Orin and a preconfigured stereo camera, while the two 4.5" side tubes were chosen to hold the batteries and access them quickly.
- The triangle configuration was chosen to accommodate a stereo camera in the center tube and a camera in each side tube, creating a quad-camera system with wide and narrow baseline stereo vision. However, the side tube cameras are not installed yet.
- MeCO also has 5 (2+2+1) thrusters to provide 6 degrees of freedom, compared to LoCO's 3 (2+1) thrusters.
- MeCO uses an anodized aluminum frame instead of 3D printed brackets as a structural skeleton to hold the tubes. The added weight of the middle tube required a robust frame.
3D Printed Parts
Any decent 3D printer should work for this robot. The original MeCO parts were printed with a Lulzbot TAZ 6. The most difficulty will come with preventing warps and achieving higher precision on the smaller holes.
While any filament material may work, it is recommended to use stronger materials that are stiff, such as PETG. Remember that the plastic must withstand immersion in harsh conditions, such as chlorinated or saltwater, depending on the use of the robot. The original MeCO used PETG plastic for higher temperature resistance, higher chemical resistance, and higher mechanical strength.
General Tips
Bed Adhesion
Make sure you are getting good bed adhesion when you are printing your parts, as warped plastic makes assemblies much more difficult. Depending on the 3D printer you are using, adjusting the bed temperature, adding tape to the bed, or using a gluestick on the bed may help with adhesion. For the Lulzbot TAZ 6 printer with a glass bed, printing PETG worked best with a thin layer of washable Elmer's glue.
Small Holes
Printing holes can be difficult for any FDM 3D printer, no matter how fancy. The easiest way to get good fits for screws and bolts is to print the holes slightly oversized to get close (holes print smaller than the actual dimensions for FDM printers) and then drill the hole out with the exact size you want. Most holes in MeCO are slightly drilled out in the post-processing of the plastic to get good fits.
Print Orientation
The orientation of the part on the printer's bed is important for the strength of the part. Always try to print with the strands of plastic (grain?) along where the force will be applied so that the forces attempt to pull apart or crush the stands lengthwise. The plastic will crack apart between layers, so avoid having major tension forces pulling the layers apart, though compression against layers is fine. Print orientation also affects the support needed to print the part, so make sure to use an orientation that uses less support, for less post-processing.
Post-Processing
The post-processing of the plastic is important to get a clean finish and good tolerances. Drilling out holes to get the best tolerances can be very important. Using a metal file can smooth a part to remove sharp edges and get better fits, though it also roughs up the plastic and changes the look of it.
Laser Cut Parts
MeCO's internal structure is made of 1/4" Medium Density Fiberboard (MDF), which was cut using a laser cutter. While it is possible to make these pieces by hand using drawings, using a laser cutter speeds up the process considerably while also improving precision. The original parts were cut on a Universal Laser Systems laser cutter.
Anodized Frame
The goal of the frame is to securely hold the tubes and thrusters together and provide mounting holes for any external components. Blue Robotics provides anodized aluminum clamps to hold the 4" tubes, which are utilized to secure the side tubes and the angled thrusters. A custom-designed clamp was created to hold the 6" center tube and attach directly to the 4" clamp. Two sets of clamps hold the three tubes together, and struts with mounting holes along the top and bottom connect the two sets. A strut runs along the bottom to attach the two side tube clamps, provide more mounting holes, and protect the tubes when MeCO sits on a rough surface. Another 6" tube clamp attaches the U-shaped bracket that holds the back thrusters to the central tube. This U-shaped bracket is attached to Picatinny rails and can be detached to open the tubes. Moving the back thrusters up and down along Picatinny rails proves useful as MeCO's center of gravity changes. All of the gold parts were machined at the CSE Shop out of aluminum and anodized in a gold finish to prevent corrosion in the water.
The internal structure consists of a long 1/4" MDF board held in the center of the tube by rings of MDF that rest on the inside of the tube. The whole structure is supported by threaded rods that run halfway down the board and connect to standoffs attached to the flange. The entire board is covered with small slots designed to hold 3D-printed parts that snap into place and hold electronics.
The side tubes have three main components: the HREyes, the battery holder, and the ESC board. The HREye mount attaches the Adafruit ring lights to the front of the tube. The battery holder is designed to be quickly removed and is a large 3D-printed part that holds two batteries, up to 4 lbs of ballast weight, and has a channel for the HREye cables to connect to the Teensy. The ESC board follows a similar design to the central internal structure with an MDF board held in the center of the tube by rings of MDF connected to the flange by threaded rods and standoffs. Mounted on the board, there are 3 ESCs, a 6-terminal fuse block, a relay, a Teensy microcontroller, a temperature/humidity/pressure sensor, a status LED, and a leak sensor.
Additional Notes
The manufacturing of the MDF boards is similar to LoCO's base plates, with the addition of slots for the 3D-printed parts. The boards are held in the center of the tubes by MDF rings that rest on the inside of the tube. The whole structure is supported by threaded rods that run halfway down the board and connect to standoffs attached to the flange. Unlike 3D printing, laser cutting produces holes that are slightly oversized to what the original drawing specified, which is why the holes in the drawing are a little smaller than necessary (though this correction is smaller for a laser cutter than for a 3D printer). For getting better precision, use the same technique as 3D printing, and drill the holes out manually to the desired size to fit the bolt or screw.
Additional holes may be added for attaching electronic or other components as necessary.