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