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Casters should be banned!
Posted by James Jones at 03/22/2001 11:22 AM EST
Engineer on team #267, The Demolition Squad, from North Broward & St Andrews and Motorola. After watching the Langley and So. Cal. Regionals it has become apparent to me that FIRST could greatly enhance the success of 1st year teams by taking casters off the additional harware list and making them illegal on a robot. If any of you read my earlier posts you know I am an advocate of loosening the material rules, not tightenning them. But in this case where we all have to depend on other robots to just make it over the ramp and across the stinkin' field, casters should be banned. Caster drive system are a classic rookie team mistake (including me) and are disasterous when there is an inclined surface that must be traversed. The only thing worse than 2 wheel drive with casters in the front is 2 wheel drive with casters in the back! |
Keep Casters Around
Posted by Matt Leese at 03/22/2001 12:13 PM EST
Other on team #73, Tigerbolt, from Edison Technical HS and Alstom & Fiber Technologies & RIT. In Reply to: Casters should be banned! Posted by James Jones on 03/22/2001 11:22 AM EST: I don't think banning casters would solve the drive train problem. You can use casters very effectively. You can also use casters very badly. Our robot this year has front casters and works just fine. We never have problems getting over the bridge (note: we do have four wheel drive in the back). Getting rid of casters because some teams don't use them well would be like getting rid of the plywood because some teams use it badly (yet another thing we used successfully this year). I think the main thing rookie teams need is more mentoring by veteran teams -- not forcing them into a particular design. If you think that rookies are building bad robots, offer to help out a rookie team. Don't make their job more difficult. Rookies need all the help they can get. Matt who was never on a rookie team but his team sort of counts this year because they haven't existed since 1998 |
ISAAC's Casters
Posted by Dodd Stacy at 03/22/2001 1:14 PM EST
Engineer on team #95, Lebanon Robotics Team, from Lebanon High School and CRREL/CREARE. In Reply to: Keep Casters Around Posted by Matt Leese on 03/22/2001 12:13 PM EST: : I don't think banning casters would solve the drive train problem. You can use casters very effectively. You can also use casters very badly. Our robot this year has front casters and works just fine. We never have problems getting over the bridge (note: we do have four wheel drive in the back). Getting rid of casters because some teams don't use them well would be like getting rid of the plywood because some teams use it badly (yet another thing we used successfully this year). I think the main thing rookie teams need is more mentoring by veteran teams -- not forcing them into a particular design. If you think that rookies are building bad robots, offer to help out a rookie team. Don't make their job more difficult. Rookies need all the help they can get. : Matt who was never on a rookie team but his team sort of counts this year because they haven't existed since 1998 Here's my attempt to pass on to new rookie teams what we learned about casters and 2 wheel drive in our rookie year (97) with our first FIRST robot, ISAAC (for Asimov). To my great embarassment, we didn't figure this out till we were practising with ISAAC before the 98 season. Had we grasped the now-obvious physics before the Manchester Regional, our only competition that year, I firmly believe we would have been Regional champs. Secure a turkey (or a concrete block, or some reasonably heavy thing) in the middle of a grocery store shopping cart, and then push it forward and let it go, casters first. Altho' it may curve a bit, it's basically stable in a directional sense. Now push it forward with the casters trailing and let it go. Once the cart begins, for the smallest of reasons, to turn, it will quickly whip around at least 180 and maybe keep on going. If you look at the forces that the shopping cart must exert on the turkey in order to make the turkey deviate from straight line motion, forces which have to be developed by the interaction of the wheels with the ground, you will see the reason this happens. With casters front and 2 drive wheels in the rear, the drive wheels can exert a rotational torque on the ground to maneuver the cart around curves, but it helps to have most of the bot's weight on the drive wheels. You too can try all of this on your next shopping trip, but I don't know you. With casters rear, you're toast. Once your bot begins to spin, very soon you do not have enough traction on the drive wheels to exert enough rotational torque on the ground to fight it, and you spin out. Understand these physics, and you can use casters to great advantage. Ignore them at your peril. The main key with drive plans that include casters, in my opinion and experience (and understanding of the physics) is to keep the drive wheels under the bot's rotational center of gravity or close. 'Course I can't shop at the Coop anymore, so keep my comments in context. Dodd |
Errr... Wha?
Posted by Kevin Sevcik at 03/23/2001 8:53 AM EST
Other on team #57, Leopards, from BT Washington and the High School for Engineering Professions and Exxon, Kellog Brown & Root, Powell Electrical. In Reply to: ISAAC's Casters Posted by Dodd Stacy on 03/22/2001 1:14 PM EST: 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. |
Re: Errr... Wha?
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. |
Physics
Posted by Matt Leese at 03/24/2001 9:50 PM EST
Other on team #73, Tigerbolt, from Edison Technical HS and Alstom & Fiber Technologies & RIT. In Reply to: Re: Errr... Wha? Posted by Dodd Stacy on 03/24/2001 8:30 AM EST: Ok, I'll try and explain a bit of the physics of why this happens (note, the example robot is two-wheel drive in the back and two casters in the front): When traveling forwards, the pivot point of the robot is about the back drive wheels. When the back wheels stop, the back of the robot stops and the momentum of the front of the robot just drags the back with it slightly. When traveling backwards, the pivot point is still about the drive wheels but this is now in the front. When the robot stops, the back part of the robot still has momentum but it can't just pull on the drive wheels. Instead, the momentum causes the robot to pivot about the drive wheels. This happens in the opposite way going up an incline for the same reason. When going up an incline, there is a downward force of gravity applied to the robot that causes it to want to slide down the incline. When the robot goes up forwards, the pivot point is in the back. The back is also being propelled and is pushing the front. The front is pulled down by the gravity and instead of just pushing on the back, the gravity causes the front to pivot about the back causes the robot to slide. The robot doesn't slide when going up backwards because the casters are in the back and the force is pushing them away from the drive wheels so there's no pivoting. Basically when there's momentum or a force applied towards the drive wheels on the casters, the robot pivot. When it's applied away, it will not pivot. The difference between the casters and the drive wheels is that it's much harder to move the drive wheels in a direction perpendicular to the normal motion of the robot. This is why four-wheel drive robots don't suffer this (including our robot with four drive wheels in the back and casters in the front). Matt who should qualify this statement with the fact that he's a CE and in Physics 2 |
Teams learn over time
Posted by Jason Leslie at 03/22/2001 4:53 PM EST
Other on team #157, Aztechs, from Assabet Valley RTHS Alumni (Class of 1998) and . In Reply to: Casters should be banned! Posted by James Jones on 03/22/2001 11:22 AM EST: Yes it is a mistake to use caster SOME of the time. If you design your robot correctly to use casters then it is fine if you don't then you lose. Why should every team lose because someone didn't design there robot correctly. Everyone knows the field and everyone has an equal chance in useing a field. I know some teams may not have the money to spend to have there own field and goal etc... but if you want to complain about this then the team should go out there and pound the pavement like other teams and get the money. OR Ask a nearby team if you can use there field. I think going to the extreme of banning this would be a bad decission. |
Re: Casters should be banned!
Posted by Patrick Dingle at 03/22/2001 10:14 PM EST
Coach on team #639, Red B^2, from Ithaca High School and Cornell University. In Reply to: Casters should be banned! Posted by James Jones on 03/22/2001 11:22 AM EST: No!!! Casters are quite handy for the robot wagon, along with the 8" wheels! Patrick : After watching the Langley and So. Cal. Regionals it has become apparent to me that FIRST could greatly enhance the success of 1st year teams by taking casters off the additional harware list and making them illegal on a robot. : If any of you read my earlier posts you know I am an advocate of loosening the material rules, not tightenning them. But in this case where we all have to depend on other robots to just make it over the ramp and across the stinkin' field, casters should be banned. Caster drive system are a classic rookie team mistake (including me) and are disasterous when there is an inclined surface that must be traversed. The only thing worse than 2 wheel drive with casters in the front is 2 wheel drive with casters in the back! |
Re: Casters should be banned!noooooooo
Posted by nick237 at 03/23/2001 8:54 AM EST
Engineer on team #237, sie h2o bots, from Watertown high school ct and sieman co. In Reply to: Casters should be banned! Posted by James Jones on 03/22/2001 11:22 AM EST: Hey James. we love casters, we used them last year and had no problem with the ramp. In fact we loved them so much we won the UTC regionals with them. How ever we did have a "great" driver ( my son ) who when he reads this will be blushing, and the machine worked flawlesly. On another note we did for the first time this year change to four wheel drive and now I wonder if we would have won last year with the same system as we love the new drive. Its awsome....... I dont think we will ever go back to casters but they did work great for us. nick237 : After watching the Langley and So. Cal. Regionals it has become apparent to me that FIRST could greatly enhance the success of 1st year teams by taking casters off the additional harware list and making them illegal on a robot. : If any of you read my earlier posts you know I am an advocate of loosening the material rules, not tightenning them. But in this case where we all have to depend on other robots to just make it over the ramp and across the stinkin' field, casters should be banned. Caster drive system are a classic rookie team mistake (including me) and are disasterous when there is an inclined surface that must be traversed. The only thing worse than 2 wheel drive with casters in the front is 2 wheel drive with casters in the back! |
Re: Casters should be banned! - No Way!
Posted by Thomas A. Frank at 03/23/2001 3:32 PM EST
Engineer on team #121, The Islanders/Rhode Warrior, from Middletown (RI) High School and Naval Undersea Warfare Center. In Reply to: Casters should be banned! Posted by James Jones on 03/22/2001 11:22 AM EST: : After watching the Langley and So. Cal. Regionals it has become apparent to me that FIRST could greatly enhance the success of 1st year teams by taking casters off the additional harware list and making them illegal on a robot. ============= Ban casters? Never! We wouldn't have been able to build our absolutely unique steering/suspension system without them. Stop by our pits at the Nationals (Team 121) and you'll see how we used the casters...it is not how you typically use them, but then, the "misapplication" of technology is half the fun. TAF |
Re: Casters should be banned!
Posted by ChrisH at 03/27/2001 12:36 AM EST
Engineer on team #330, Beach 'Bots, from Hope Chapel Academy and NASA JPL, J & F Machine, Raytheon, et al. In Reply to: Casters should be banned! Posted by James Jones on 03/22/2001 11:22 AM EST: That's why Team 692's robot, which was designed and built AT the Silicon Valley regional with the help of Teams 192, 22 and some from 330 among others, used skids instead of casters. Having greater friction with the carpet they were less likely to swap ends with the drive wheels. Why the robot was built there is a long sad story, but at least it only missed one round of practice. The joke was that 692 stands for 6 hours, 9 minutes, 2 seconds, the time it took to get it operational. I think that beats the record set at the Langley regional by quite a margin. Chris Husmann Team 330 the Beach 'Bots |
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