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### Author Topic: \$50 robot Caster... Do you really need it?  (Read 3426 times)

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#### bukowski

• Robot Overlord
• Posts: 219
##### \$50 robot Caster... Do you really need it?
« on: November 13, 2007, 01:54:53 PM »
OK, so im neck deep in the \$50 robot, and learning that i dont know anything.
I am wondering if I built a chassy that had two wheels, but moved the board and the battery as far below the centerline of the robot as possible (to increase the moment) if I could do away with the caster. I am still a little worried that the servo motors will start spinning the chassy instead of the wheels, and Ive asked my brother (a mechanical engineer) about it and he started ranting about inertia and friction coefficients. I was just wondering if anyone had tried anything like this.

• Supreme Robot
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##### Re: \$50 robot Caster... Do you really need it?
« Reply #1 on: November 13, 2007, 02:22:16 PM »
so I guess Ill skip the inertia and friction rant . . .

This is near impossible to calculate since it involves friction . . . friction and inertia is what will cause the body to spin with the servos. But I think that if you use very large diameter wheels, and keep the weight as low to the ground as possible, you will maximize the moment arm length. Basically you want:

force from moment >> force from friction and inertia

Use a wheel diameter of perhaps 6" or greater, and keep the batteries/electronics like an inch off the ground, and I think it will work. Would be interesting . . .

Expect it to wobble/swing during stop and start motions, so you may need PID control on it to have the speed slowly ramp up/down.

#### Ro-Bot-X

• Contest Winner
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##### Re: \$50 robot Caster... Do you really need it?
« Reply #2 on: November 14, 2007, 10:50:47 AM »
You may want to tilt the wheels inward or outward. That will greatly increase the stability of the robot. Check out my Lego design:
http://www.mocpages.com/moc.php/5625
Check out the uBotino robot controller!

#### bukowski

• Robot Overlord
• Posts: 219