Friday, August 19, 2011

The birth of SHORT-E: platform

I decided to name my robot SHORT-E (an obvious reference to WALL-E). Applying the first law of recreational robotics, I wanted to start simple and just build something that moves, avoiding bumping into things or getting stuck, at least mostly.

I purchased my robot platform already years ago from Budget Robotics. It's called Scooterbot and looks (almost) like the image here. Mine's blue and the wheels look a bit different.

The Scooterbot platform came with two GWS S03N R/C servo motors pre-modified for continuous rotation. Normally these servos are used for position control and cannot rotate past a limiter, but the modification makes them suitable for use as robot drive motors.

So, SHORT-E will be a two-wheel differential drive robot, i.e. it will be steered by making the left and right wheels spin at different speeds or even in opposite directions. The Scooterbot base is extremely simple: it doesn't even have a third (freely rotating) wheel, but just a plastic knob that slides across the floor, hopefully with minimal friction.

If I were purchasing a robot platform today, I probably wouldn't choose this type. The problem with Scooterbot (and other similar two-wheel designs) is that it is extremely bad at moving on anything else than hard floor. It can just manage on a (non-furry) carpet, but cannot drive over even the smallest of thresholds or climb from plain floor onto a carpet, even a thin one. I would seriously consider a tracked or a four-wheeled base. But, as I happened to have the Scooterbot base lying around, it is what I will use, at least for the first version of SHORT-E.

Servos as robot drive motors, on the other hand, are perfectly in accordance with the first law of recreational robotics, because they are simple to use and control. They generate plenty of torque and can rotate almost arbitrarily slow (unlike DC motors) as well as go fast, and can be directly controlled by the pulse width modulation (PWM) peripherals that are built-in in many microcontrollers.

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