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Jon Lawton
01-17-2002, 02:35 PM
Okay... This year, I've been taking a more active role in the construction of my team's drivetrain. All of the sudden I have found myself in need of a *lot* more physics quickly! We have adult mentors, but they all seem to have different opinions and different ideas and even different formulas! What it comes down to is: I need to lock myself in my room with a physics-learning-tool and assimilate it. SO, finally, my question is: "What have you, or your team, found useful in learning about the physics behind the robot [drive train, in particular]? Thanks for your input!

Jordan A.
01-17-2002, 02:53 PM
Not sure if will help, but my OAC physics text book has some great information on forces.

More specifically components and resultants.

Using that you can get some great ideas for CG

Also covered is centripital force and something else regarding circles that while I know, cant name.

So you might want to try the high school physics book. But it may come up short.

I however dont know much about the detailed physics of a motor. I know the basic motor principal, I have no problem understanding how it turns and such. Just some things I havnt learnt and would like to, such as:

-How to determine torque (preferably in metric) and what factors go into it.
-Why a stalled motor draws more amps.

So as a tag along question if anyone has any info in that area...

*once again, eratic post, I apologize*

gniticxe
01-17-2002, 04:12 PM
I've been wondering the same, Jordan.
Today was my Physics midterm, and while i should be basking in knowing that its done, we have a robotics meeting tonight :) Anyway, I'll try to help with what I can:
Torque (tao) = R x F (cross product)
that is T = R*F*sin(theta)
R being the radius in meters from the folcrum to where the force is applied. F being the force applied in Newtons and theta is the angle between the Force and the plane of the radius.
This seems to work and be happy for all the problems in my physics book dealing with rotation/translation and such, but I don't know how to apply it to gearing :( any thoughts on this?
Also, to determine gearing speeds (in metric SI of course),
V = omega R
V is tangential velocity (meters/s), omega is the rotational speed (in radians/s), and R is radius (meters). When you're going from gear to gear, either omega or V will stay the same. V stays the same when connected directly tooth to tooth, or by a chain. Omega stays the same when two gears are attached and rotate around the same axis.
I may have screwed some things up in there, so if you find something wrong, please correct me - and does anyone know an equation to relate the initial rotational speed to the final speed to get the increased torque? there's gotta be one :)

s_alaniz
01-17-2002, 05:19 PM
What's that you say? You're down because you need the straight scoop on physics? You say you need help getting your drive in drive? DUDE! Just scroll down to the bottom of the page and go see the Doctor! Dr. Joe Johnson's White Pages Motor Selection workshop lecture notes!

Seriously these are a MUST read if you havn't already done so on page 2 of the white pages titled WRRF Motor Selection Workshop Lecture notes. I'd also check out the PDF on the Dual Shifting just below that for some inspiration.

Just my two cents... I try to stick to electronics myself...

Good luck

Steve Alaniz

"What good is technology if you can't abuse it?" - Ted Forth

Anton Abaya
01-17-2002, 05:36 PM
Physics? Who needs physics!?

Try building a bot that defies physics! (cough like the one i'm trying to make)

-anton

Jeff Waegelin
01-17-2002, 07:21 PM
Originally posted by Anton Abaya
Physics? Who needs physics!?

Try building a bot that defies physics! (cough like the one i'm trying to make)

Heh. We try that every year. Never quite works too well.....

Physics must be broken.

GregT
01-18-2002, 08:06 PM
Have you tried a physics teacher (in your school)?

Al Skierkiewicz
01-18-2002, 11:08 PM
I can only answer your question with this observation. I have seen many teams run too low a gear ratio in an attempt to have a high speed robot. In operations where the robot must do some real work, the motors are running near to stall which has the highest current demand. This will always mean tripping breakers and burned motors and speed controllers. Determine a realistic top speed (we could cover 48 ft in about 10-12 seconds although I have heard some teams clocking as high as 12 ft./sec) calculate your drive wheel RPM for that speed using the circumference of the wheel you intend to use. Decide which motor to use for drive system and then calculate the gear ratio needed to achieve that. Many teams use either the drill motors or Fisher Price motors for drive.
If you plan on using the drill motors, remember there is a locking pawl in the transmission you may want to remove. This locks the output shaft when you remove drive power to the motor.(part of the screwdriver mechanism) This results in almost certain failure of transmission housing.
If you plan on using the Fisher Price trasmission, remember it was intended for kiddie cars to not go fast enough to damage a small child. It also needs some modification that has been discussed elsewhere on the forums to beef up one of the shafts and improve efficiency.
I hope this gets you started, since I am an electrical guy and can't help much more than that.
Al

Anton Abaya
01-19-2002, 01:44 AM
Last year, I calculated our robot to a gear ratio of 13ft/sec. Realistically it went aroun 10-12ft/sec.

It was a 2.66:1 gear ratio, or 48:18 gear tooth ratio. We were ~75lbs only that year.

If you want a fast robot that you do not expect to be carrying a load, feel free to use our gear ratio and gain from our experience using it. We had the drills at high gear that year, btw.

-anton

ps. http://www.chiefdelphi.com/forums/pictures.php?s=&action=single&picid=329&direction=DESC&sort=date&perrow=4&trows=3

pps. http://www.chiefdelphi.com/forums/pictures.php?s=&action=single&picid=335&direction=DESC&sort=date&perrow=4&trows=3