Posted by Dodd Stacy at 03/24/2001 8:30 AM EST
Engineer on team #95, Lebanon Robotics Team, from Lebanon High School and CRREL/CREARE.
In Reply to: Errr... Wha?
Posted by Kevin Sevcik on 03/23/2001 8:53 AM EST:
Umm, ahh .. I really didn't explain WHY a shopping cart (or a 2 wheel drive plus casters robot) is directionally stable rolling (on a LEVEL floor) with the casters leading, and unstable (spins out) when the casters are following. I just asserted that as fact and hoped curious people would experiment for themselves or try to visualize the physics and figure it out. And I'll leave it there.
Certainly the physics change on an incline like this year's bridge. I offer this obvious suggestion for teams who think their robot is unstable or difficult to steer on the bridge by virtue of its chassis/drive layout - drive over in reverse. Whatever is hurting you going forward may behave very differently going backwards.
From what I have seen of our team and others so far, I think most of the trouble driving over the bridge comes from too much steering authority in the control program (too "twitchy"

, too little practise time for the drivers, and too little attention in design to the robot's tendency to climb over the side lips/curbs of the bridge and get hung up. Also not thinking through how the bot's steering and drive will behave when the bot is not square to the bridge when making the transition from the floor plane to the bridge deck plane. Lots of stuff to learn from in this year's game.
Dodd
: I can't say I quite follow this explaination. I would think casters would be ok in any situation as long as the center of mass was near the drive wheels. But if it wasn't, I'd think that this year, having casters at the back of the robot would be preferable. Mainly because it's hard to push a shopping cart uphill without it turning to the side.
: Thogh now the I stop to think about this, I think the actual key is to have you center of mass behind your center of rotation. At least when you're going uphill. Alright, I know you center of mass is actually where your robot rotates, I'm talking about where it would rotate if mass wasn't accounted for (ie. the point between the wheels in a 2WD). Of course it's too early for me to really think about the physics of this. All I know is that I saw way too many rear wheel drve bots running into the sides of the ramp at Lone Star.