Effectiveness of Casters
 

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?

Re: Effectiveness of Casters
 

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.

Re: Effectiveness of Casters
 

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

Re: Effectiveness of Casters
 

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

Re: Effectiveness of Casters
 

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.

Re: Effectiveness of Casters
 

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.

Re: Effectiveness of Casters
 

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

Re: Effectiveness of Casters
 

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

Cory

Re: Effectiveness of Casters
 

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.

Re: Effectiveness of Casters
 

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

Re: Effectiveness of Casters
 

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

Re: Effectiveness of Casters
 

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.

Re: Effectiveness of Casters
 

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.

Re: Effectiveness of Casters
 

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.

Re: Effectiveness of Casters
 

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

Re: Effectiveness of Casters
 

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.

Re: Effectiveness of Casters
 

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.

Re: Effectiveness of Casters
 

I dont know if I can give enough info in a post to explain how to use them if your are not familiar with PID control loops.

The idea behind it is simple. If you are using single joystick control then the X axis is indicating how fast your driver wants to turn

if you just feed this into the single joystick equation that comes with the default code - then the stick postition is mapped directly to a PWM output, which ends up telling the motor how much force to apply (you can think of this as how much voltage is on the motor)

problem is - there is friction in your drivetrain that will resist movement - so putting a small pwm command on the motor will not be enough to make it move - once you have moved the stick enough to make it move, you have overcome the static friction and now you are dealing with kinetic friction, which is less, so once the motor moves a little, that pwm command is enough to make it move noticeably more

to make matters worse, the left and right drivetrains will not have exactly the same amount of static friction, so one will start sooner than the other. If you are telling the bot to go straight, it will goto one side a little.

The yaw rate sensor detects how fast the robot is turning - if you mount it with its bolt holes up and down.

the way a PID control loop works is, instead of taking the signal from the joystick and driving the motors pwm with that, you take the joystick X axis signal and you subtract the yaw rate sensor signal.

so if the driver is commanding a slight turn, and the yaw rate sensor says "im not seeing any turning going on here!" you end up with an error signal - the error is the difference between what the driver wants, and what the robot is doing.

the simplist form of closing the loop would be to use the error signal to drive the motors - replace the joystick X axis signal with the error signal and feed it into the one joystick equation. You have to adjust the error signal for 127 being zero, something like this:

yaw_error = (((2127 + p1_x - yaw) Min 2000 Max 2254)-2000)
yaw_command =(((2063 + yaw_error/2) Min 2000 Max 2254) -2000)

BTW - this is in basic from last year - wont take much to change it to C

In PID lingo this is a Proportional loop. The output to the motors is proportional to the difference between the commanded value and the actual (the feedback) value. how responsive the system will be depends on the gain. in the code example above, the error is divided by 2, so the gain is 0.5

thats not a bad place to start with a machine that is using two wheels and castors, esp if your weight is balanced towards the two drive wheels. if you try the code and the bot is sluggish, you can increase the gain ( and you have to adjust the 2063 number so that a yaw error of 127 (zero) stays zero

if you have a bot with 4 wheels, you might need more gain, like 2 maybe - in that case you would adjust the 2063 down to 1873.

if you crank the gain up too high, what will happen is, when the loop corrects itself it will over correct, and overshoot the turn rate - then the yaw rate sensor will say "you went too far" and try to correct it back, which will overshoot again, and the bot will twist back and forth. If the gain is just slightly too high, the bot will respond like its nervious and twitchy - if its way too high the bot will swing back and forth all by itself.

one thing you have to be carefull here - make sure your yaw rate sensor is mounted in the right orientation - for the code above we had our sensor mounted upside down, the holes were upward (simply because that was a convient place to put it on our frame). Also the sensor has to be solidly attached to the frame. If you tie wrap it on, and it can move around, it will go nuts on you.

so what are the I and D parts of PID? I is intergral - if you move the stick a little bit, the error signal might not be enough to get the motors to move - so to add an intergral part you take part of the error signal and you sum it, or accumulate it over many cycles - so if the driver is telling it to move, and its not moving, the error signal keeps increasing until it does.

The D is differential. sometimes you want the bot to respond quickly to changes in the joystick position, so you can either keep track of where the joystick was last time, and subtract where it is now, to tell when it has been moved, and apply a gain to that signal to sort of give the motors a 'kick' when the joystick has been moved - or you can use D on the error signal in the same way.

For our bot last year, all we needed was the P part of the PID loop - the equation I listed above. We ended up with the best response with the gain at 2, but I would try it at lower values and work your way up.

If you want to learn more about PID control loops, try searching the web - lots of companies make equipement that is used professionally, and I have seen some excellent websites that explain the concept better than I have here.

PID loops can also be used for thing like postioning an arm on the bot, or using pots to tell where your wheels are pointing if you use crab steering, or your front wheels turn left and right to steer.

hope this has been useful.

Re: Effectiveness of Casters
 

I broused through a few web sites, if you search yahoo on

PID control feedback

you will get many links

this one gives a good overall explaintion without going into the math too much:

http://www.netrino.com/Publications/Glossary/PID.html

I found others that use fequency domain equations, if you can get past that, they are very informative - ie, skip over the math you dont understand and look at the graphs and you will get some insite.

Re: Effectiveness of Casters
 

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.

Re: Effectiveness of Casters
 

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.

Re: Effectiveness of Casters
 

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...

Re: Effectiveness of Casters
 

if you dont like castors then the next best thing is to have steerable wheels, like a car or wagon.

4 wheel drive with tank/skid steer is hidious - the amount of power and energy you waste dragging a drivewheel sideway is tremendous - in this years game, as other have pointed out, if you dont want to use castors, use a couple blocks of nylon instead, and have most of your weight balanced over your drive wheels.

Re: Effectiveness of Casters
 

I have to agree with minic@HYPER69, castors are not a good idea. Imagine having a 130 lb robot hit you when only 2 wheels will resist turning. Last year there were a few robots that had castors that tried to make it up the ramp, and the only ones there were able to do it were the ones that were smart enough to drive so their powered wheels went first up the ramp. A 4 wheel design works just fine, and a 6 wheel can get a bit tricky if you don't know what you're doing. As it's been said before, having castors as part of your drive train is the easiest way to identify a rookie team.

This team tried to make it up the steps using a 2 drive/2 castor robot, it didn't work
Here

Re: Effectiveness of Casters
 

sorry if this was mentioned in this thread before...

one good reason that you dont want to put casters on because during autonomous period you will lose your track. i have real bad experience with casters and i would like to suggest to not to use them as everybody did in this thread...

Re: Effectiveness of Casters
 

Although we have used casters in the past, we have decided that for most applications, a caster design will be considered last. If you are moving in one direction and want to reverse, the casters have a mind of their own as to which way to turn and rarely will they allow you to back up in a straight line. For last year's competition we had four wheel crab but did find it neccesary to turn the robot at times. For this we had a pnuematic cylinder drop a "foot" to lift two wheels off the ground. This allowed a change in attitude and moved the center of turn to one side of the robot. It is tricky for drivers to learn but with practice it was very effective.

Re: Effectiveness of Casters
 

Let me add to that. We had casters in 1999 when we had 2-wheel crab. It was terribly hard to drive straight when the casters were trailing. We will NEVER use casters again. Omni-wheels are much more stable!

Re: Effectiveness of Casters
 

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.

Re: Effectiveness of Casters
 

I disagree ........

We are a very experienced team (34), we will be utilizing some very cool casters, we will be on top of the platform, and we WILL be competitive.

You just have to think outside that norm a little ........

Re: Effectiveness of Casters
 

Oh Al, you master of the understatement. Your robot ROCKED last year!!! As they do every year.

Re: Effectiveness of Casters
 

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.

Re: Effectiveness of Casters
 

Hah. Our robot already has 3 PI controllers and we Havn't even started codign for THE MANIPULATOR yet. :D Feedback control rules!!

We still wouldn't touch castors again after our last experience with them.

Please!! If you are considering casters, at least use skid plates instead.

Re: Effectiveness of Casters
 

Yeah, castors that extend and retract are great for a robot with four wheel and too much traction. We were tripping breakers when we turned until we put the casters on. Just a tip, get sturdy casters and if you use pneumatic cyclinders do everything you can to protect the rod. They don't like side loads and the machine shop guys get tired of bending them back. Also, only lift so that the turning wheels touch and the others barely touch.

Re: Effectiveness of Casters
 

Team 103, the Cybersonics, used two casters in their 2003 robot. That machine gave more or less every robot a run for their money. I'm not too certain of the specifics, or how it worked, but you most certainly can build a competetive and successful robot using casters.

Re: Effectiveness of Casters
 

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

Re: Effectiveness of Casters
 

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...