Tuesday, December 12, 2017

Margin of Safety 3

This version of Margin of Safety sought to solve three problems: 1) reduce thickness of front armor, to extend blade reach, 2) make the belt drive less annoying and more reliable, and 3) use brushed motors again.
Final render.
The holes make it faster.
Machining progress shot.
Bandsawing UHMW with the wrong type of blade.
To reduce the thickness of the front armor the hub motor holder was redesigned yet again. This time I went for a spline shape instead of fish shape.

The spline teeth prevent rotation and provide strength around the screw holes. The inverse teeth from the plastic are clamped between a bottom flange that is integrated in the holder and a top flange that is clamped on with screws. The screws also hold the main hub bearing in. This way the plastic is clamped between metal pieces with a hard endstop, so the plastic can deform if it needs to without compromising the overall structure.
Spline assembly assembled with bearing.
It is one of my prouder designs. The same spline assembly is still used after four events, with nothing more than scuffs.

The drive axle bearings were clamped between two T shaped pieces of aluminum. The aluminum then clamps onto the plastic with screws running through to keep it in place.
Each half had a flange only on one side, to make it easily machined.
The idea was the foam wheels and soft plastic body would allow me to get away with cantilevered drive wheels. This assumption was false, however it did limit deformations before they affected drive performance. It was also successful in decoupling the side armor from the drive axles.
Fourteen half-inch holes were drilled because I used slightly thicker aluminum plates than what I modeled.
This board is called Calabash.
A new control board was made, using TI's DRV8821 current limiting brushed motor drivers, Allegro A4960 brushless controller, STM32F3 microcontroller, and L3GD20 digital gyro. This is to support using a bunch of 13 mm diameter Maxon gear motors I got off eBay. The motors did great, until I burned out the brushes due to overvolting them from 12 V to 24 V. Luckily they made audible noise before failure, so they could be preemptively replaced.
Showing packaging of motors, batteries, and electronics.
This robot won first place at Sparkfun AVC 2016, but not before breaking every weapon disk I had on hand.
All the pieces.

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