Autonomous Army Truck – 1. Mechanics

This is the 6 wheels drive toy army truck that I bought several years ago (for fairly cheap) for one of my kids.

When reading the specs (6wd, 3 differentials, independent suspensions on each wheel, etc. … ) it sounded too good to be true for the price… and indeed, I remember being disappointed when I received it : the rubber of the wheels kept coming off, the battery was really bad, but most importantly both the steering and the main motor were simply on/off not *proportional* !

At the same time, I was impressed by the modularity of the design, how everything was screwed rather then glued in place and how easy everything was to take apart, ready to be modified / upgraded…

And I *knew* I could do better… sooo much better !

Here’s what it can do *now*, after the 2 main mechanical upgrades:

This is how it looked initially:

I started with the steering for which I used a small, 9g hobby servo:

Motivated by how easy and quick it was to design the above, but also by the fact that even with an hobby ESC that added proportional control to the main motor, it was still starting/ stopping pretty abruptly and was using an enormous amount of current (3-4Amps) which was heating the wires and making the LiPo battery protection trip I decided to replace the stock motor with a more powerful / better quality one.

The *problem* with that was that I would have to re-do the gearbox that interfaces the motor to the main axle… and this was far from trivial, in spite of doing what I think was the simplest modification:

After about a day of taking a picture of the existing gearbox, drawing its contour in Inkscape, extracting it as STL then importing and extruding it in OpenSCAD and then printing several test adapters until I found the right position to half a millimetre precision I managed to get a surprisingly good result:

And finally everything put together, ready to be tested:

There’s not much code to all this, but you can of course find the .svg, .stl and OpenSCAD files in my GitHub project.