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After building one of the basic models in the set and playing with it for a while, I decided to strike out on my own and see what sort of vehicle I could build from scratch. I had noticed a gear housing that could be used to make a differential in the pile of parts, so I decided to try to construct a truck that used a differential to power the rear wheels. While I was at it, I decided I'd try to rig up a pair of front wheels that turned together, just like a regular truck.

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Here are a couple of side views of the truck. The linkage between the rear drive section and the front steering section is pretty bulky, but it works.

Here's the rear drive train, with differential.

Initially, I had put the large gear directly on the motor shaft, using an intermediate gear to drive the differential cage. That approach worked, but the gear ratio was too high - the truck would zip along pretty quickly once it got underway, but it had very little torque. Later, I built the drive train as you see it here, with a tiny gear on the motor shaft to give me a nice gear reduction. Top speed is much less than before, but I now have much more torque. The differential drive works like a charm!

Here's what I came up with for the front steering gear. I went through quite a number of combinations before settling on this one - most of my other efforts were either too fragile, too bulky, or both. This geometry worked out pretty well - once I had the idea of driving the whole gear train from the end instead of the middle.

As before with the drive train, I initially built the steering unit with much too high a gear ratio. I was able to program it that way (just barely), but by redoing the steering drive with a high gear reduction I got much more precise control of the steering and an easier programming task. The steering unit could actually stand an even greater gear reduction, in fact - it's something I'm thinking about.

Here's a view of the steering mechanism that shows the position sensor. The vertical shaft that drives the steering gears continues up out of the frame, and I've got a cam attached that presses the sensor switch when the wheels are aligned straight ahead. This is a pretty simple arrangement, but it lets me write code to center the wheels properly even if I don't know their starting position.

This position sensing is a crude substitute for a radial position sensor, I know - I'm probably going to replace this later with a sensor that tells me the actual angular displacement.