Effectiveness of Casters

[ND4SPDLSC]

My team wants to go with casters and 2 wheel drive. I am one of the few on the team who believe 4 wheel drive or maybe even 6 wheel drive is the best way to go. Has anyone tried using casters with a skid plate in front of the casters and the weight focused over the drive wheels to climb the stairs and the 6" platform? I really don't think it will work, but I'm in the minority and I don't want to be the nag... Any help would be greatly appreciated!! Is there any team near Manchester, CT that would be willing to demonstrate a robot with large casters trying to climb the 6" platform?

[Solace]

although It's possible one could make a casterbot that worked reasonably well, I can tell you from experience that looking for a 2 wheel 2 caster drive train is in general the easiest way to pick out the inexperienced teams from the crowd. The casterbots that manage to retain even a mediocre level of competitiveness are few and far between.

4 wheel drive works fine. 6 wheel drive works better still.
In short, avoid casters AT ALL COSTS. They will be your downfall.

[Tytus Gerrish]

I Never Want to see A caster On A robot Again EVER! There unstable the verry "Wolbbyness" that makes them work is There aqdvantage and There Disadvantage The lack of horisontal stabilty makes it turn when you try to stop it from turning A 130LB robot with a caster is The most frustrating 2 minutes Of your life to drive Of course thatss been my experience To some people who have used a diferent drivetrain Casters may be a Godsend but in my experence I dont like them at all

[Cory]

The only reason that I would ever stick a castor on a robot would be to help turn. I would use a cylinder to engage or retract it, but I would never let two of them be in constant contact with the ground. If you had been around in past years, you would see robots with two driven wheels and two castors like you said. Basically, they would spin around like crazy, with very little control.

Cory

[Alavinus]

Our team is going with two wheels in the back and skids up front. (Gives a new definition to skid steer- doesn't it ) The skids will be made of Delrin and give omnicaster like abilities with the neccessary machining complexities.
You might want to consider this method over the infamous castor, or you will have a story to add to it.

[Will Hanashiro]

i also would not recommend casters. four wheel drive is sufficient, six wheel drive may be going overboard. if you're looking for on-the-dime turning, why not try a trick wheel? for those of you who dont know, a trick wheel is a normal wheel, but with a whole lot of miniature discs going around the perimiter. the miniature discs allow the wheel to slide sideways, yet they provide sufficient traction for rotating foward.

[ryan_f]

When our team used casters on past robots, it worked well. We went in straight lines and didn't have any problems with them. I think the wobbly factor that some of you are talking about is maybe because there is not enough weight on the castor or it is not parallel to the ground. If you are going to go up the steps, then i wouldn't suggest casters, go with 4-6 wheel drive for that

[Cory]

Originally Posted by Will Hanashiro

i also would not recommend casters. four wheel drive is sufficient, six wheel drive may be going overboard. if you're looking for on-the-dime turning, why not try a trick wheel? for those of you who dont know, a trick wheel is a normal wheel, but with a whole lot of miniature discs going around the perimiter. the miniature discs allow the wheel to slide sideways, yet they provide sufficient traction for rotating foward.

Search for "omniwheels" to get more information on such a "trick wheel"

Cory

[Rickertsen2]

Originally Posted by ND4SPDLSC

My team wants to go with casters and 2 wheel drive. I am one of the few on the team who believe 4 wheel drive or maybe even 6 wheel drive is the best way to go. Has anyone tried using casters with a skid plate in front of the casters and the weight focused over the drive wheels to climb the stairs and the 6" platform? I really don't think it will work, but I'm in the minority and I don't want to be the nag... Any help would be greatly appreciated!! Is there any team near Manchester, CT that would be willing to demonstrate a robot with large casters trying to climb the 6" platform?

If there is one piece of advice that i can give, it would be STAY AWAY FROM CASTERS. They thould be avoided at all costs. The problem(besides making it up the 6") is that they wreck menuverability. They are fine for linear motion, but once you turn, the castors must re-align themselves, throwing the robot completely off course as they do so. Instead of Casters, you would be MUCH better off with a skid plate alone. An even beter option would be a skid plate and either a ball castor(picture giant mouse ball) or an omniwheel. The ball castor would probably be better, unless you plan on powering the omniwheel.

[Matt Adams]

Since I don't think that anyone mentioned this, I think it's important to take a look at a bit more than just driving... pushing force is greatly affected by casters.

As we all know, the most you can push is the coefficient of friction times the robot weight, however, this will be less if not all surfaces of the robot you're using aren't powered- which happens when you use casters or skids.

In addition, casters will give you a sort of weak spot in pushing matches. Assuming you use swivel castors, any robot can hit the end where the casters are located and spin you- there is little you can do to avoid this.

On the plus side, since you know that it's much easier to turn with casters, and since you obviously can't push as much, you can run your robot at much faster speeds. However, since you can turn with much greater ease, you run the risk of spinning all over.

It should be mentioned (for folks like Joe Johnson who love to list exceptions to the standard rule of thumb ) that there have been some GREAT robots that have used casters. Team #47 in 2000 used them, but it should be noted it was done in pair with a crab style drive system. Mobility was the key for them, not pushing power. This was a sound engineer choice.

If you're choosing to go with a standard tank-style drive train, I would definitely power all wheels that are contacting the surface if you can, whether is this 4, 6 or even some really awesome drive train that only uses 3 wheels... hint hint.

However, if you're choosing to couple the FPs, CIMs and Drills to make a light robot that hovers (flys) to keep people off the bar, then you can avoid a lot of the headache associated with wheels. It's something to at least consider- it was a tough choice for us.

I hope this helps, good luck!

Matt

[KenWittlief]

where are all the REAL engineers in this thread? doenst anyone use feedback to control their robot?

yes when you put castors on a bot the steering gets squirrely - thats why you also use a yaw rate sensor that measures how much the bot is turning, and you fly-by-wire!

you close the loop on steering by having the SW look at what the driver is commanding the bot to do (how hard driver wants to turn) and looking at the yaw rate sensor to see how fast the bot actually IS turning

and use a PID closed loop algorythm to MAKE the bot do what its told!

why would you do anything else? this works with 2 wheel skid steering, 2 wheel castor steering - four wheel steering - 6 wheel.... whatever the gearheads throw at you, the sparkies can turn the machine into a PID controlled nice and tight, highly responsive servo like machine

the beauty of this is not only does it make the robot go straight when you want it to go straight, it also allows very precise slow turns. If you want the bot to turn just a tiny bit, the SW will put the necessary power to each motor to make that happen, adjusting the levels 40 times a second.

Try it on ANY bot - feedback is the most powerful tool an engineer has at their disposal - see them F111s flying around, they would be completely unstable, unable to fly AT ALL without fly-by-wire closed loop feedback controlling its motion. And guess what? the same type of yaw rate sensors they use are available to you in the suppliers catalogs.

[Gary Dillard]

That sounds good in theory Ken, but in order to do that you have to command the robot to drive with the caster rolling backwards where it is inherently unstable. Casters are self aligning in one direction - the pivot point is off axis from the contact point with the floor. So if you're driving straight and want to back up straight you have to keep compensating for the moment induced in the caster that wants to push you sideways. Try turning the casters the wrong way on a shopping cart and see how far you can get them to roll that way, even with all the feedback available to your senses. I won't debate whether or not it's possible as an engineer (it's certainly possible), but it doesn't make good sense to fight it.

[Max Lobovsky]

Originally Posted by KenWittlief

Try it on ANY bot - feedback is the most powerful tool an engineer has at their disposal - see them F111s flying around, they would be completely unstable, unable to fly AT ALL without fly-by-wire closed loop feedback controlling its motion. And guess what? the same type of yaw rate sensors they use are available to you in the suppliers catalogs.

I believe you meant the F-117, a small diamond shaped, stealth bomber with no conventional tail developed in the 80's, not the F-111, a large swingwing conventional bomber from the 60's and 70's.

Regardless, I agree with Gary, castors will always align with one direction and this requires them to turn and through your robot off course. A feedback system could align it in the end, but it will take time and the robot wont be in the same place you expected it.

[KenWittlief]

the time it takes is one fourtieth of a second for the feed back loop to see that 'something' is trying to push the robot sideways, when its not suppose to be going sideways, and the SW corrects the power to the motor to force it to do what the driver wants.

This works to stablize castors, it also works if you hit a ball, if a bot hits you, you encounter a change in resistance (like driving on carpet or on the smooth surface)

and it worked last year going up the ramp at an angle.

The PID loop doesnt care whats trying to push the bot off course, it senses the slightest change in yaw rate, and applies an immediate correction.

We used it last year - the bot steered like it was a large servo.

[minic@HYPER69]

All I have to say is DONT DO IT! Casters are like commiting suicide!