My First Robot part 3 – Drivetrain
Friday, 25 April 2014
Once I decided a custom tank tread wasn’t going to work for me, I went back to searching the internet for an off-the-shelf solution. Someone recommended looking at Lego Technic parts, and sure enough I found this on Ebay (EDIT: a helpful commenter on hackaday pointed out this much cheaper option). I don’t know what set this came from, but they are just about the right size (a little on the small side, maybe) and the price was right. Once they arrived I got them into CAD and quickly pulled together a basic design.
Aside from the treads this design is driven entirely by a few key components. First are two small SM-S4303R servos, which offer continuous rotation without modification. After much debate I decided on this lithium ion battery pack from Adafruit for two reasons: it has 6600mAh capacity which should provide hours of use between charging, and it is known to work well with this multi-source charger, also from Adafruit. I have plans to include solar charging in the future, which will impose some challenges, but I’ll get into that later.
The rear tank tread wheel is mounted to a sliding part that allows the tread to be tensioned just right. The front wheels are attached directly to the servos, via a modified servo horn that I turned down to size on the lathe and glued into the wheel. Shown below (from left to right) is the original servo horn, the same part after turning its diameter down, and the wheel it is about to be glued into.
Once glued together, the screw that holds the wheel onto the servo is trapped in the assembly:
The chassis is 3D-printed, and I used small brass threaded inserts everywhere, considering how much I expect to be disassembling/reconfiguring things…
…which made assembly nice and smooth.
The top part is a slab of polycarbonate, meant to be easily removed and re-drilled, etc. as things change and evolve. Here it is, ready for electronics!