Effectiveness of Casters

[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!

[Spikey]

It all depends on what kind of competition you are competing in. In 2000 we built a casterbot that won 2 regionals, and in 2002 we built a caster bot that finished in the top 10 in both regionals, casters can be valuable, it all depends on the competition playing field.

[Ryan M.]

If you're thinking about getting onto the platform at all your casters will have to be incredibly strong. Even the small steps will have a tendency to snag on them and the casters will very likely be ripped off.

[Pat Roche]

Our team tested a prototype drive train/robot at River Rage and I drove all the matches and I can say from experience you will have trouble controlling it with two wheel drive and casters. Remember though that casters arent bad when used in combonation with tracks/tread or 4/6 wheel drive. We competed well with a caster to support or robot with tracks last season.

Also remember to acount for what you actually using them for. They do add weight to the bot.

Just some thoughts to dwell upon,

-Pat

[ShadowKnight]

other people have brought up valid concerns that could lead to certain teams deciding to not utilize casters in this year's game. As far as a robot that intends to climb the stairs, casters WILL become a hangup and possibly could cost the team points and time in the pits try to fix their messed up bot because the casters broke off. Casters also are bad bad bad for driving if used as 2. 1 is MUCH better because of how the drive train pivots. so...I personally see casters as a design flaw that will leave a team hanging on a hangup. Having driven bots with 2 casters, I can tell you that they are crazy bad on anything but carpet and are not that easy to control with precision...

[Gary Dillard]

Originally Posted by 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.

At 10 fps, that's 120 ips, I've travelled 3 inches in 1/40th of a second. That's about 120 degrees of rotation on a 3 inch caster - it has already spun around and pushed me off position by then.

If you want to drive all over the field at part power because you're shutting down one motor or the other all the time to compensate for instability in your drive system, please do. We'd love to have you as an opponent.

Spikey, I'm impressed that you won 2 regionals with casters, but I'm betting it's not because of the casters right? You just focused your resources on other parts of the robot rather than on the drive system, so you had a robot that played all aspects of the game and didn't just drive.

The question of this thread is, what do you think of casters. My answer - not much.

[ShadowKnight]

liek alavinus posted, our team is team is using skids...the only thing that 4-wheel drive is good for is added traction...for manuevering stuff liek the steps, it'd be great...6 could work but I'm not entriely sure that many are needed. I'm not entirely sure that you would WANT to get onto the stairs though because if I read your team number correctly, you're a a rookie team...having been on a rookie team myself and knowing a number of people who were rookies at one time...I would focus on a driving box...maybe something that herds balls...and use 1 caster not 2 if your team decides to use casters, though I like many others do not recomend it.

But like I was saying, our team is using skids...so the front of our robot will just slide along the surface of the carpet. you guys might want to look into that...

[Spikey]

Originally Posted by Gary Dillard

Spikey, I'm impressed that you won 2 regionals with casters, but I'm betting it's not because of the casters right? You just focused your resources on other parts of the robot rather than on the drive system, so you had a robot that played all aspects of the game and didn't just drive.

The question of this thread is, what do you think of casters. My answer - not much.

Our robot that year was very fast and maneuverable, we could fly across to the other side of the playing field and steal our opponents black ball, then quickly collect our own giving us a huge advantage. Also we had a very effective scissor jack to deposit balls, and to hang on the bar. Casters actually ended up as a last resort for our team because our 4 wheel drive destroyed the carpet, after we slapped the casters on we went from being dead last to winning two regionals. However this year will not be a good year for casters because they do not do well at all on the polycarbonate(u need traction on there) and the difficulty they add when you have to climb stairs(getting caught on them). This year is a bad year for casters.

[briholton]

Originally Posted by KenWittlief

where are all the REAL engineers in this thread? ...

....you close the loop on steering by having the SW look at what the driver is commanding the bot to do

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

Hey Ken, can a REAL engineer point me in a direction to learn how to use the yaw rate sensor? I'm just a dumb physicist. I'm curious to its use and have seen it for a number of years now and always just piled it up in the dunno pile. Cuz, after all this talk, I have to give up on my casters..... darn it, had some really nice ones ordered too! hmm, perhaps for the goal.

[maxgebhardt]

We used castors our rookie year and had less qualifing points than a robot that didn't even move... they are awful. just imagine pushing a 130lb shopping cart at the supermarket. those castors are also tempermental and jam easily. if anyone on you team still likes castors, then take them to pathmark and you jump in a cart and tell them to push you for 2 minutes, and see how much excess energy it takes.