Tuesday, 2 April 2013

Rambling #3 (thoughts on bots)

I recently purchased an Adafruit Ultimate GPS and a cheap ebay compass sensor in persuit of building a serious outdoor UGV. I started a while ago a largish outdoor platform baised on drill motors, you can have a look at that Here but I am incresingly having some doubts that it will ever be finished. I built the bot with two used drill motors, and two new ones, and after some testing, I found that one of the used ones was nearly burnt out, and sucked a HUGE amount of current, it still spins, generally is on it's way out. Normally I would have replaced it, and got on with life, but by the time I found out, I had made that nice steel shell, and riveted it together, so replacing is not an option, as I would have to tear apart the intire bot. Not going to happen. Lessons learned: always design to take apart. I knew this, but I guess I just had to learn it the hard way. Anyway, back to the rant, I think, no problem, I will just put one of my HUGE 35 Ah 12 v SLAs in the bot, but now I am worryed that my 1/6 RC truck wheels will not stand up to the weight. No problem, just buy some 6" lawnmower wheel, and attach those to the drill shafts. Not so fast, the lawnmower wheels have holes in the center that are too big. So here the bot sits, with no suitable wheels, or power supply. Doing a little thinking and I am debating moving to a bigger platform still, using This electric bike wheel (I already have one from building an electric bike, but that is a story for another time.) As a rear/front wheel on a trike (rear or front will depend on weather I am using a conventional or tadpole style trike.) Having some experence with this wheel, I know that the 26" will be a bit much for a robot that weighs a cupple hundred pounds, I do not want to hurt anything/body by crashing into them with a very heavy robot trike going 35 km/h, that would be bad. Anyway, I was thinking I could put either a 20" BMX wheel or a dirt bike wheel of some sort on the electric hub and use that as my main drive system.
The problem is steering, I have weighed a few options, including using two drill motors for the other wheels and having a free swinging "tail" with the bike wheel on it, varing the speed of the drill motors would cause the tail to turn one way or the other. Pros: steering would be simple (just like a differential drive robot), three wheel drive, simplistic mechanical design, very little to go wrong. Cons: lots of math to figure out the size of wheels I need to ballence the speeds of different motors, and figuring out how much to vary the speeds of all three motors.
Another option would be to have the bike wheel on a tail, but have the tail coupled to a motor, and a feedback sensor (probably a variable resistor) so I could actively steer the tail. Pros: fairly easy steering, no math to worry about. Cons: finding a cheap motor that is powerful enough (maybe a windsheild wiper motor from a old truck or something) mechanics for coupling the steering motor to the fork might get tricky, having to hack a servo into a REALLY powerful servo, a bit less sturdy, no three wheel drive.
I could also have kinda the same setup, but have the tail mounted to the chassis and have the non-driven wheels turn. Same pros and cons as above.

Food for thought. I will leave you with these pictures.

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