robot tinkering

Over the last week or so I have been resurrecting my from-scratch robot project. I had gathered a number of pieces but for various reasons all this stuff was neglected for a while. After considering where it was left off, a few design changes were in order.

For example, the previous design was to be differential drive, with motors left and right, and passive support (casters) front and back. I had a 12 inch square piece of 3/4" plywood more or less drilled out to support the motors and casters that way, but was never quite happy with it. Finally figured out why. Imagine what happens when the robot comes to the base of a ramp. The front caster goes up a bit, and suddenly one or both drive wheels is off the ground. In other words, said configuration can't climb a ramp. I didn't want to just re-invent the car either, and besides, implementing a pair of articulated steering wheels is probably more than my rudimentary mechanical skills are up for. So here's how it's turned out: the differential drive remains, but the wheels are now moved to the forward edge, and a single caster at the trailing edge completes the setup.

Also dusted off was an L298N dual H-bridge motor controller board I had cobbled together. I wasn't happy with the way that turned out either, which no doubt contributed to the entire project being shelved for too long. Mainly the breadboard-shaped proto-board, with six opto-isolator chips on it, takes up too much room for the amount of functionality provided. After so long on the back burner, I decided that if it removes an obstacle to getting something rolling (it does) then it's good enough.

And in the control department, the ultimate goal is to have wireless operation. I think the way to do this may be to stick an 802.11g wireless router on the robot, and use an SBC to operate the robot hardware. Finding an SBC with built-in wireless is proving difficult, and besides, anything built into the board will be hard to modify, whereas if the SBC just provides a standard RJ45, then eventually replacing the 802.11g with an 802.11n wouldn't be such a big deal. The other advantage of using a wireless router, rather than a simple wireless nic, is that I can follow the robot around with the netbook; if the robot and netbook both were just wireless endpoints, both would have to stay in range of some fixed router. So the robot-as-router simplifies the system. So now the battery is going to have to get bigger, but it's probably worth it. (Unless someone can tell me that it's a piece of cake to get two wireless endpoints talking to one-another *without* a router)

However, before committing resources like a wireless router and an SBC, it seems prudent to target an intermediate stage of development. So for the time being, the plan is to operate in tethered mode with an arduino to provide PWM to the motors. This will be valuable to check out the dynamics of the platform and prove whether that rat's-nest ugly L298N driver board can handle the load. Well, I would be remiss as an engineer if I weren't about 99% certain it would, but it's always reassuring to see a design put through its paces, isn't it? Anyway, this will require getting the arduino to produce PWM outputs. For a tethered controller, a couple of potentiometers will provide analog inputs to the arduino, which will just turn around and drive the PWM with the values from the pots.

And good progress on the arduino front -- tonight I proved out enough code to drive one of the two drive motors. Hooked one potentiometer into the arduino, and an LED on a PWM output, and could vary the LED brightness with the pot. This should easily extend to running a 2nd channel (for the other motor), then it's just a matter of securing the chassis, motor drive board, arduino, and the 1.2A-Hr, 12V sealed lead-acid battery, and seeing how it handles.