This is a schematic of the wiring. Check the link below for the most updated version and read ahead for an explanation of the components.
Important Information:
The Kill Switch kills the power to both of the servos and (unfortunately) the telemetry module. It was necessary in the case that the SPARTA would "go rogue" and make it impossible to stop until the battery would die.
The Telemetry module draws power from the Servo Rail instead of the Pixhawk because we thought that our GPS issues may have been due to the power draw from the Telemetry module. In hindsight, maybe they were to some extent, but we don't know for certain. A consequence of this, however, is that when the servos draw too much power, we have telemetry brownouts. Solution? Bigger battery (won't fit in the current design so we'll make use without it).
The BEC heats up like crazy so don't touch it. I swear I could make boiled eggs with it.
This is the current (as of 3/14/2021) drop mechanism for SPARTA (and hopefully future UGVs too). For a detailed view of the inner workings, check the CAD file and play around with the animation.
Basics:
An angular servo manipulates two opposing steel bars in a linear fashion.
These bars, when in the closed position, stick into the hole of the sled tip seen in SPARTA.
This tip is held in by the frictional forces of the pin (also seen in SPARTA).
The tip also holds the parachute strings.
When the UGV touches the ground, the sled will swing backward when it starts driving, so the pin will then be pushed out of the hole and SPARTA will ditch its parachute (otherwise, the high winds at the competition will drag the UGV like Mary Poppins and her umbrella)
This mechanism box will be mounted directly onto the wing spars.
Testing: So far, the pin has a 100% success rate at holding the parachute under normal conditions, a 100% success rate at popping out on a relatively even surface (grass testing needed), and the box has a 100% success rate for holding and dropping the UGV.
Link to the holy grail of UGVs (images & videos): https://docs.google.com/presentation/d/1VWA0Pe8AOZyd7vzRCUjkAXJih7SJy8Na3AizVNVS-Tg/edit#slide=id.p
Servo Powered Autonomous Rough Terrain Automobile
This is SPARTA (as of 3/14/2021), a heavy, 3D-printed Unmanned Ground Vehicle and dropping mechanism. If you desire a more in-depth view of the design, check out the CAD latest CAD file (make sure you turn the visibility on for the correct parts).
Design Rationale:
Big wheels spanning the whole UGV cushion falls and make driving over rough terrain easier (it still has trouble because of how small and comparatively light it is).
Sled (and pin) in the back acts as a parachute attachment point and a stabilizer so that the inner body doesn't rotate instead of the wheels.
ABS plastic allows for non-brittle impact resistance and wear-proof parts.
Velcro permits the simple reorientation of any components while achieving a strong bond nevertheless.
Heavy-duty metallic servos provide a huge amount of torque and a strong connection point for the wheels.
Small screws provide strong connection points while allowing the simple disassembly of SPARTA.
The back wing (not shown in the picture) is an addition to the sled that prevents it from sinking into tall grass and mud by increasing the surface area.
Previous Versions: Check the old CAD files to see the progression of wheel designs, all of which ultimately failed. Or check out the Google Slideshow linked here.
The Speed Car Super Speed (SCSS) is was a four-wheeled prototype UGV which took 10 minutes to build and 50 hours to figure out how to use and debug. Much of the Mission Planner documentation and this page are a testament to the tough work of getting things to function as expected. Some key takeaways are described at the bottom of this page.
Takeaways:
4 wheels are not the move because it's not space effective, prone to bottoming out, low to the ground, and more!
The placement of the GPS in such a tight space is super important, but the location also matters. Some places at certain times just don't have a good connection. Always place the GPS away from other components and as high as possible.
The battery and the bottle are the heaviest objects and almost entirely influence the COM.
For small vehicles, grass is the main enemy. Even short grass can create a soft bed on which it is impossible to progress. Mesh wheels do not work. Compliant wheels do not work. Hard wheels do not work. See the SPARTA page to learn more about how we partially solve this.
The wiring is basically standard, so you shouldn't need to change anything between one UGV and another.
The transmitter/receiver is very strong, so you shouldn't worry about losing connection with it.
Things that interfere with GPS signals include rough terrain, buildings, anything that visually blocks your line of sight of the sky. This GPS module has peaked around 17 or 18 satellites, with 10 being the bare minimum to have the accuracy we desire.
