4WD Turning Difficulties
 

I wanted to bring this helpful "white paper" to the attention of people who are busy designing their robots in the mistaken belief that turning on a slippery surface will not be an issue.

If you build a 4WD skid steer robot in the "traditional" or "narrow" (wheels aligned with the long axis of the robot) orientation there is a very good chance that your robot WILL NOT TURN.

I know this might seem counterintuitive, but run the spreadsheet in the whitepaper with the coefficients of friction for the wheels (KoP list / section 10.2.4.1 of the manual as I recall) and look at the results.

Now keep in mind that in order to spin the trailer about your robot you will need to apply about a 3 lb force (30lb trailer weight estimate x .1 transverse dynamic coefficient of friction) at a distance a couple feet away from your robot's centre of rotation. So, say about 6 lb feet of torque.

In this case even a 6wd robot (with the middle wheels dropped a bit) will have difficulties turning in place... it will do it, but slowly.

And thank you to Mark from 1189 for posting the white paper and spreadsheet.

Jason

Re: 4WD Turning Difficulties
 

I also ran this scenario through a drive train simulator and came up with similar results. Straight was perfectly as expected, but turning was near impossible and took forever. Add the sliding effect to it and your robot drive accuracy will take a nosedive.

Re: 4WD Turning Difficulties
 

what if you double or tripple up wheels?

stack wheels side by side so making a rough drawing using symbols

[ = wheels
| and _ = edges of robot
. =taking up space

________________
| [[[ . . . . . . . [[[ |
| . . . . . . . . . . . .|
| . . . . . . . . . . . .|
| . . . . . . . . . . . .|
| . . . . . . . . . . . .|
| . . . . . . . . . . . .|
| . . . . . . . . . . . .|
| . . . . . . . . . . . .|
| [[[ . . . . . . . [[[ |

shouldn't something like that work? since there would be more surface area?

Re: 4WD Turning Difficulties
 

It's not likely to make any difference at all, since surface area has very little effect on friction with hard materials such as the wheel treads and arena floor.

Re: 4WD Turning Difficulties
 

*sigh* 'ight thanks

---

so much for that idea, lol

Re: 4WD Turning Difficulties
 

Have you crunched the numbers on a wide 4wd chassis? I think it would work a lot better because the wheels have a lareger moment on the robot's cg, providing more turning force, and the trailer has a smaller moment, reducing the friction. I haven't done any of the math, though, and it may turn out that skid steer just isn't the way to go.

Re: 4WD Turning Difficulties
 

A couple members of our team did calculations for the majority of last night and more wheels don't help you at all. I would say 4 is the max you'd need for a drive train this year. We toyed with the concept of a 2 or 3 wheeled drive train but stayed with the 4 wheeled system mainly because of the contact with the side carpet.

I'd love to hear other opinions on this and how many wheels people are planning on using.

Re: 4WD Turning Difficulties
 

Simple skid steer is almost never the correct route to take. If i was going skid steer I would still be tempted to do something like a 6wd with a lowered center wheel to make the control of the robot more consistent (turning around the center) and to make the turning easier.

I would need to try some tests but even if I had the resources to build anything I'm pretty sure I would still go for the 6wd drive over a swerve simple cause the traction is so low a swerve could get excessively hard to control. Then again this isn't really a problem for me considering I'm mentoring a team with very little machining capabilities.

Re: 4WD Turning Difficulties
 

Yes, the numbers for a wide chassis are better, and for six wheel drive (with the middle wheels dropped a bit) are better still. Six wheel drive on a wide chassis, with the centre of gravity over the middle wheels naturally, is the best of the skid steer options. That goes for high friction OR low friction environments.

I suggest everyone who is interested in this download the spreadsheet and try their own numbers in it. You can simulate a six wheel drive with centre drop by assuming it is a four wheel drive with two wheels up in the air. Thus the wheelbase is the distance from the middle wheels to whichever wheels (front or back) happen to be on the ground. The CoG will also move towards the centre axle.

Of course these numbers are calculations and simulations, and need to be confirmed by experimentation. I suspect we'll be seeing some video of the first protobots up on youtube by the end of the week.

Jason

Re: 4WD Turning Difficulties
 

What about wheels like this?

.......__.......
.................
.................
!...............!
.................
.................
.......__.......

When you want to turn, spin the front and back wheels (wheels on short sides). When you want to drive forward spin side wheels.

Another thing we've considered is "toe."

If the wheels are angled slightly at each corner, spinning all of them in the same direction will allow you to spin in place.

It will also take advantage of the superior transverse friction when driving forward. And will provide some braking to prevent the robot from coasting when making a hard stop.

Re: 4WD Turning Difficulties
 

The wheel's tranverse friction is more than twice the inline (apparently).

Even though their friction sideways is probably less than the omniwheels we're used to on carpet, that doesn't mean they will slide sideways easily in this case.

It doesn't matter that the transverse friction is really low, it matters that relative to that, the inline friction is substantially lower.

Re: 4WD Turning Difficulties
 

Okay, so here is a crazy thought I had...
If you did a 4WD in the same configuration as a omni-wheel drive...
...(like this) (In the same fashion as "The Pre" :D )
|= Chassis Perimeter
// = Wheels
. = space filler
___________
|//...........\\|
|................|
|................|
|................|
|\\...........//|

Would the low amount of friction on the wheels allow
the robot to move like an omni-drive?

I don't know how slick the flooring is because I was not able to attend a Kick-off. :(
But from what I have heard, it sounds pretty slippery.
Any guesses on how well this would work?

Edit: Hachiban beat me to it!

Re: 4WD Turning Difficulties
 

No problem, although I must give credit where it is due. Chris Hibner put together the white paper upon which my spreadsheet is based. http://www.chiefdelphi.com/media/papers/1443
Chris did all of the heavy lifting.

Re: 4WD Turning Difficulties
 

I'm pretty sure that this would "work" but, without really being a physics person, and not having crunched the numbers, I would think it would be even worse than a 4wd with the wheels on the corners.

Re: 4WD Turning Difficulties
 

Center drop wheels arent the answer this year since because you have the wieght of the trailer attached to you then your always going to be on the back four wheels.

Re: 4WD Turning Difficulties
 

Why is that a problem? You can locate the center wheels towards the front or rear of the robot to take advantage of this.

Re: 4WD Turning Difficulties
 

But you will still most likely only be driving on four wheels due to the weight in the back of the robot. If that is the case why not just put 4 wheels on your robot but place them closer together. You would get the same effect I think. But I am not sure if that will help out the driving situation.

Has anybody out there built up a 4WD chassis and tried it out on the game surface? I would like to hear about the why things handle if so.

Re: 4WD Turning Difficulties
 

The front wheels would just be there to stop the front edge of the chassis from slamming the ground when CG shifts (due to collisions/abrupt deceleration/whatever else), you are correct that the vast majority of the time, you'll be on your wheels closer to the trailer.

Re: 4WD Turning Difficulties
 

Basically in a 6wd this year with a dropped center wheel, the front 2 wheels will be similar in functions to casters, just to keep the robot from slamming forward like has already been mentioned. The advantage of powering is that now when the robot rocks forward, the robot will still be distributing power to 4 wheels, versus if the front wheels were unpowered, only 2 wheels would have power but the friction force on those wheels would be half of the friction force that 4 powered wheels have to work with.

Re: 4WD Turning Difficulties
 

your right wow

i almost forgot the trailer

Re: 4WD Turning Difficulties
 

The trailer wheels are near the center of the trailer, so it won't have a lot of tongue weight. You can position the wheels on your robot wherever you want to, they dont need to be exactly in the center. You can position heavy parts of your robot wherever you want to, so the center of gravity need not be in the center of the robot. What all this means is that you can balance the robot how you want...you can have a 6 wd robot that rides on the front 4 wheels almost all the time, if you so desire.

Re: 4WD Turning Difficulties
 

We slightly modified our fal prototype base so it is 4wd, skid steer. It does not turn as well as we would think, but it does turn, but slowly. If you drive it forwards a bit, then start turning it turns much better. We have not driven it with a trailer as of yet, but on the playing surface it may not be a huge problem, besides the fact that the weight of the trailer may pull the back end of the robot a little bit.

Oh, and the robot drifts. A lot.

Re: 4WD Turning Difficulties
 

Thanks for this post nick! Hmm....now what other designs could we find? :rolleyes:

Re: 4WD Turning Difficulties
 

We had the same result in similar testing. Our base was being driven from a battery, so no speed control. We think some of our initial slowness during turning was because we were "Flooring it" and breaking traction.

Can't wait to get more wheels in so we can test with 6 wheels, and most importantly, with at trailer.

Re: 4WD Turning Difficulties
 

You have not looked closely at the drawings. The wheels are infuriatingly far to the rear of the trailer. This will make it follow without jack-knifing. more importantly, it will increase the arc needed to turn around. Short turning is what we are trying for.
Tongue weight is also important and will increase with the number of rocks in the trailer.

Re: 4WD Turning Difficulties
 

What simulator did you use and is it generally available?
Thanks

Re: 4WD Turning Difficulties
 

yesterday we were driving our 2007 robot with the rover wheels on the regolith, the results were interesting, we found that the inline CoF is higher then .05 inline, more like .12. the robot wieghed 110lbs and had more then 50newtons of pull (it maxed out a force sensor we borrowed from a physics lab) also when we were driving at our max speed we could stop one of the drives so that we would be dragging the left or the right, and we would continue moving in a straight line with very little pull. (newtons first law)

once we got a second force sensor we were reading a pull of approx. 60 newtons.

Re: 4WD Turning Difficulties
 

Yeah, I have to get it from one of my friends but when I do I'll put it up! :)

Re: 4WD Turning Difficulties
 

You're right, I didn't look close enough..the wheels are back a bit. This helps being able to maneuver backwards, but does adversely affect forward turning.

You should still be able to balance the robot the way you want to with clever placement of parts.

Re: 4WD Turning Difficulties
 

We had far more trouble turning from the lack of traction then from too much transverse friction.

http://www.youtube.com/watch?v=u8aqjM1mmEs

The surface we were running on has about double the coefficient of friction as the real surface.

Re: 4WD Turning Difficulties
 

thanks for posting!

Re: 4WD Turning Difficulties
 

I'm more interested in seeing it turn with a trailer.

Re: 4WD Turning Difficulties
 

Hm... what if you had a six-wheel design (skid steer, it won't work so well for arcade style) in which the front and back wheels had motors but the center wheels didn't, and there were sensors in all the wheels. The center wheels would be the actual rotation of the wheels while the front and back would be what the motors are doing, and the controller would adjust the output of these motors based on the actual wheel rotation versus what the motors are trying to do. (if that made any sense at all) I'll try to explain in a diagram below:

| | <motors on these wheels sensor compares rpm to center wheels

| | <no motors on these wheels, sensor gets rpm for actual speed

| | <motors on these wheels, sensor compares rpm to center wheels

Controller will adjust actual output of the motors. It's not fully autonomous, since you're still controlling which direction it is supposed to be going and the speed desired.

If anyone wants me to I'll try and do a sort of block-diagram showing a general flow of possible commands.

Re: 4WD Turning Difficulties
 

much appreciation for the vid. love to see a trailer contributing some drag and then see how little steering happens.

lets see who can put one together and post her up

Re: 4WD Turning Difficulties
 

Joe,

Thanks for posting the video... The problem you had turning at about 15 seconds into the video reinforces a worry I have had... From what I could see, I believe it was caused by having a different COF for the right two wheels as compared to the left two wheels.

If you are using a 4WD (skid steer) and you get two wheels on the carpet at the field perimeter, you may never get back onto the main playing field...

Mike

Re: 4WD Turning Difficulties
 

We were worried about that too. I think in the video was the first time we got into that situation, I don't remember having that much trouble when we did it again. We still do want to do a lot more tests. I think this year more then ever, teams will want to get a base driving as soon as possible. Trying to drive for the first time with 5 other robots on the field will lead to trouble.

Re: 4WD Turning Difficulties
 

Unpowered wheels with significant weight on them means that the powered wheels have just that much less normal force to get traction.

I believe that successful teams are going to optimize their traction. Don't be fooled by past years.

In the past you saw robots jumping as they turned corners, drive motors that cut out from the circuit breakers tripping and sometimes smoke pouring out of the motors and you knew, ah, something is less than optimal there. This year there will be none of that. Teams will think, my robot is not hopping as it turns, my motors are not tripping the breakers or even getting hot... ...everything is right with the world.

No such luck.

Many teams will not discover their lack or RELATIVE mobility until practice day. Too late.

By the way, backing up long distances is going to be impossible for all but the best teams. Think of the mobility issues that is going to cause.

This is a year I regret more than ever being on the sidelines... ...the engineering challenges are fun, not well understood and provide significant advantages...

...now if only I weren't worried about loosing the audience...

Joe J.

Re: 4WD Turning Difficulties
 

What about using 8 wheels, 4 on each side. And there would be one motor for a set of two wheels, or one motor for each side. Would that work better or worse that just a straight 4 wheel drive train?

Re: 4WD Turning Difficulties
 

What about a 4wd car steering system? the only problem i can see is that you could encounter difficulties in backing up if you needed to.

Re: 4WD Turning Difficulties
 

what about a drive train like that, have it set up with duals in the back and put them close together, technically still 4wd, but set up like a semi almost. this is what my team is going to try.


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| [[ . . . . . . . . ]] |
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Re: 4WD Turning Difficulties
 

I was thinking about all this discussion of trailers. Would it be possible to turn the robot without involving the trailer? Like have the hitch on some sort of rotating...I'm not sure what the right word is, "cuff" comes to mind I don't know why...so the robot turns but the trailer stays stationary? Then the robot moves foreword and the trailer is pulled into position behind the robot.

Edit: Also I completely agree with the car steering drive train for a 4wd. It has the advantage of keeping your drive wheels in line with the direction you want to go. Even if you skid, your drive wheels should be able to pull you back on track, like a car on ice. A skid drive means all the wheels face the same direction, giving you less control.

Edit 2: I wouldn't worry so much about having to back up, this is a small problem compared to getting your entire robot to maneuver correctly.

Re: 4WD Turning Difficulties
 

However, according to
http://www.chiefdelphi.com/forums/sh...07&postcount=7
the coefficents of friction in real life happen to be somewhat different from what FIRST tells us they are.

If you run the spreadsheet with the coefficents of friction being .12 in every direction, there is a good chance that your robot will be able to turn

Quite easily, in fact. Although, you should keep in mind that "turn" and "rotate" aren't entirely synonymous. If you attempt to turn with any sort of speed, you're probably going to instead rotate in place while continuing to slide sideways, so drivers of skid steer 'bots are going to have to practice their four wheel drifts to be able to maneuver effectively this year.

Re: 4WD Turning Difficulties
 

I don't remember where, and it's getting late for me, so I don't want to look it up, but I think, THINK, that there is a rule stating that the place the trailer attaches to has to be stationary or something.

And I would like to bring up the 4WD drive with the steering similar to a car, what's that called, by the way? I think it would definitely give us more control overall. Especially when compared to skid steering or something. One question that bothers me about it though, will the low coefficients of friction render this kind of steering just not worth the time input into designing and making the system?

Re: 4WD Turning Difficulties
 

I believe the FIRST colloquial term would be "swerve drive". edit: apparently not

The major obstacle in your way is probably going to be replacing the chain-drive with shafts and gears. Also, CV joints would likely be involved. Making the steering rack it's self wouldn't be too bad, but the whole system would add a couple layers of complexity to what needs to be the most reliable part of your robot.

While turning, you'd be able to make more efficient use of your traction, and it would be far more controllable at speed. Whether that's worth the extra complexity, weight, and motors depends on what your team finds important in a drivetrain.

edit: come to think of it, you could use a single CIM to drive all four wheels, and a CIM to power the steering rack. that could potentially be a pretty good solution, but you'd need two or three limited-slip differentials for it to handle properly, and I think andymark is fresh out of those...

Re: 4WD Turning Difficulties
 

"Car" style steering is known as Ackermann Steering (front wheels steer), and the variation (like a monster truck, and some luxury cars have) is called 4WS (4 wheel steer)

"Swerve", or "Crab" drive systems are where the wheels are rotated to point in the direction you want to go, (typically they can be turned at least 180deg so you can get the full gamut of translational possibilities).

"Kiwi" drive is using 3 omniwheels spaced 120deg apart traditionally, but in this years game, it would likely be possible with the rover wheels, you drive the front two toward each other to move forward, and steer with the rear one.

"Holonomic" drive is using 4 omniwheels spaced 90deg apart. By varying the speeds, you can achieve translation in any direction. I expect this too would be possible with the rover wheels

"Mecanum" drive is using mecanum wheels (like omniwheels, only the rollers are on 30-45deg angles instead of 90s, and you mount them like a "regular" 4wd setup). These too can be used to achieve translation in any direction. They're not usable this year in any way (well, unless, i suppose if you were to build one using rover wheels as the rollers).

Those, plus 4wd, 6wd, and 6wd-dropcenter are the most common drive systems seen on FIRST robots (well, kiwi isnt exactly popular, but you sometimes see it mentioned). There are others as well, but these form the bulk of whats out there.

Re: 4WD Turning Difficulties
 

Would it be possible to do the car steering with half a swerve drive (the front wheels), then have the rear wheels powered normally?

Also if you wanted to put more weight in the back to reduce the moment the trailer creates, would it not then be a good idea to use rear wheel steering?

Re: 4WD Turning Difficulties
 

If you're referring to what I thought was "swerve drive", but is appearantly known as "ackermann", then

http://en.wikipedia.org/wiki/Ackerma...ering_geometry

The image on that page should give you a good idea of how steering in a car works. The yellow bar moves back and forth using a rack and pinion (hence, steering rack)

http://en.wikipedia.org/wiki/Rack_and_pinion

If you want to have a 4WD drivetrain with car-style steering, you're going to have to have a way for the shafts to the front transmit power while steering, probably through a simple CV joint

http://en.wikipedia.org/wiki/CV_joint

Alternately, you package a wheel and motor assembly together, and steer the whole thing. You'd need 3 motors for that type of drive, though.

Whatever drivetrain you pick, if you put the steering where more weight is, you'll have more available traction to steer with, and have less of a tendancy to understeer. Be careful if you put a steering rack on the rear wheels though, I believe the steering won't try and center it's self like it does in a front wheel steering system, although if it's electronicly controlled, it shouldn't matter.

An interesting way to power a robot with wheels that can steer would be to have the motor in the center of the car, with shafts going to the front and rear which then split to the left and right sides. To distribute the torque at 90* angles, you could have simple locked ring and pinion setups and live with a little wheelspin on cornering, or go with a differential setup, either electrically or mechanically controlled.

edit: seriously consider a traditional 4WD skid steer setup first, though. if your COG is going to be shifted significantly to the rear, then a conventional drivetrain will likely be perfectly adequate. run the numbers through that spreadsheet with coefficents of friction being .12 in every direction, and you'll find that a square wheelbase with a rear-based COG is going to turn easily, without resorting to complex steering systems

Re: 4WD Turning Difficulties
 

I'm of the opinion that any ackermann based steering system will experience major understeer and oversteer. I don't really know what the answer is as I think there isnt any traditional FRC drive system (that can be done with the rover wheels) that is particularly good, and I think thats the point.

Re: 4WD Turning Difficulties
 

With this little traction, any drive system is going to have issues with under- or over-steer, right? Skid drive is going to slide around as well.

Re: 4WD Turning Difficulties
 

Absolutely it will. I don't think anything will be particularly good, but I know that cars on ice dont steer worth crap, and tanks steer better than cars, so, by extension I would think similar would apply to FRC bots.

Re: 4WD Turning Difficulties
 

But what happens when you put the same tires on the car and the tank?

Re: 4WD Turning Difficulties
 

Being on a team that built an Ackerman steering robot for last year's game (see: http://www.chiefdelphi.com/media/photos/29780) and as a drivetrain guy myself, I can say that the steering robot was a very, very high traction system. In the picture you can see that we used dualie 10" rubber wheels on the differential and 8" rubber wheels in the front. It needed a lot of traction to work.

We used those same 8" wheels on a tank drive in the Aim High game, and the robot had entirely too much traction and essentially "danced" while trying to turn (we have since learned..) So if you extrapolate you can conclude that the high traction environment was needed on the car steering, while completely overkill on the tank drive.

I can't conclusively say that the Ackermann steering requires significantly more traction to work, but from my experience it seems as if it does - and I would have a hard time seeing it working with a lack of traction on the front steering wheels. Take it as you may.

-Greg

Re: 4WD Turning Difficulties
 

That sounds simillar to an idea I had about traction control. In cars its called ASR(Anti-Slip Reduction), the controller would have a set of known speeds that would equal to a set of rpms. For Example:

5fps=50rpm
10fps=100rpm
15fps=150rpm

Now while the robot is driving on the slick surface, the wheels start to slip and spin out(the rpms increase above the set known speeds)

5fps is not equal to 300rpms acorrding to the controller.

Therefore, the controller would reduce power to the motors so that the speed(fps) and revolutions of the wheel (rpm) meet the rules set by the knowns. This system in effect doesn't increase traction but keeps the wheels from spining out.

Re: 4WD Turning Difficulties
 

I can attest to this in an empirical setting. Last night we ran four types of robots: Light (100 lbs or so) 6WD, Heavy (150 lbs) 6WD, Light 4WD (long & skinny), and Heavy 4WD (long & skinny)

The 6WD turned with relative ease (relative being the key word). It would make zero-turning-radius turns (just spinning around) and arcing turns. Of course, it slid around a bit, but not as much as we thought it would.

The 4WD also turned (which kind of suprised me, based on the math), but looking closer at the wheels as it turned, it was actually skipping across the surface, not sliding like the 6WD had. Our driver (3rd year, I believe) also mentioned it was harder for him to control the turn with the 4WD set up and it was turning slower, which confirmed that it was a skip-turn and not a true skid-turn like the 6WD.

We (234) have a lot of experience with the "dancing robot". Our 2003 robot looked like it was doing the jitterbug while trying to turn, which was scary the first time we saw it, but we learned to drive with it...we just had to take our time.

Re: 4WD Turning Difficulties
 

Did you have the trailer hooked up too?

Re: 4WD Turning Difficulties
 

Awww...you didn't do the one we're interested in, the 4 WD short & wide

Re: 4WD Turning Difficulties
 

Yeah I'm disappointed too... :)

It would seem that the 4 WD "Wide Configuration" would be the closest thing to the "Ideal Configuration" which would be to have the trailer at the center of your turning radius, so that it would produce no torque on the system.

Re: 4WD Turning Difficulties
 

I know, sorry. That's the one I'm really interested in, too. But, we had a 6WD chassis available for testing, so that's what we used. Hopefully, we can use the kit-bot chassis to make a wide robot and see how it handles.

We did not have a trailer available. We are making that today. Hopefully we'll have a video available sometime soon.

Re: 4WD Turning Difficulties
 

On this surface, I think you will spin out no matter what you do. If you don't spin out, you will still slide around.

Think you can put this year's wheels on your robot and give us a demo?

Re: 4WD Turning Difficulties
 

Could you use a 2 WD hooked to a sulky and rigidly couple the trailer to that?

Re: 4WD Turning Difficulties
 

Exactly right on both points. I'm really pumped about the new robot building challenges, but as for the game as a whole, my gut says it will be boring to watch. Of course, for examples of my gut being wrong about such things, one needs only look at the games from 2004 and 2008, so who knows.

Re: 4WD Turning Difficulties
 

It's a safe bet the game will be boring for the spectators, considering how slowly the robots will be moving, and how much distance they have to cover.

Re: 4WD Turning Difficulties
 

Were debating between 2 kinds of drive systems...tank style and car steering (back wheels are powered)...which do u think would be better on this lack of traction situation? (keeping the trailer in mind)

Re: 4WD Turning Difficulties
 

I really like the car style because it doesn't require you to whip around the trailer every time you turn. I also like that you can keep your wheels pointed (at least as close as possible) to the direction you want to move in. The only disadvantage I can see is the robot can't turn on a dime.

Also if you do the car style, make the turning wheels powered. It will be easier, maybe the only way, to turn in the direction you want, since the surface is so slick your unpowered turning wheels may have little effect.

Re: 4WD Turning Difficulties
 

What about having a 2wd? Are the two wheels without motors easier to turn than wheels with motors? Or is it just as ineffective as the 4wd?

Re: 4WD Turning Difficulties
 

Well we hitched the trailer up to our 4WD "long" bot (wheels on long sides)...

It sucked a** trying to steer... :(

Our driver came up with a novel solution of jack-knifing the bot back and forth which worked pretty well.

We'll have to try out some other configs to see how they do.

Re: 4WD Turning Difficulties
 

One wheel and two wheel drives are definite possibilities. Whatever you plan to do, understanding the physics behind turning and towing are going to be critical. The trailer is both a field element that you cannot damage and an integral part of the robot. With some designs there me quite an interesting set of forces at work on the hitch.

Re: 4WD Turning Difficulties
 

For all those that are making early design decisions based on driving experiments involving driving Robots on the "Regolith" flooring without a Trailer attached, I would urge exercising a LOT of caution. The driving/handling characteristics of a Robot with a Trailer are COMPLETELY different than a Robot without a Trailer. Learning how the Trailer affects the performance and agility of the entire vehicle system is critically important. And once you understand those effects, learning how to use them to your advantage is equally important. And don't make a decision too early - a driver with about an hour of practice will learn how to use the Trailer to spin the Robot through turns and pivots with a lot more agility than may have originally been anticipated.

As one benchmark, after a bit of practice I was able to take a standard kit-bot system with trailer attached and run it from one end of the Crater (starting with the Trailer touching the Alliance Station Wall) to the far end, execute a 180-degree turn and make contact with the far wall, and run back to the starting wall in about 12 seconds. This was repeatable several times.

-dave



.

Re: 4WD Turning Difficulties
 

That was my first thought, but the setup of the robot doesn't allow the wheels to fit on the differential shaft, and since the differential wheels and steering wheels are different sizes - it would be uneven and just bad.

If only we could..

Re: 4WD Turning Difficulties
 

Think of being on ice. The smaller the tire foot print, the higher the psi, and thus.... better the traction. Let's say the wheel provided has a footprint of 1 square inch, your robot comes in at 120 lbs and you have 8 wheels with the weight evenly distributed. Then you have 15 psi. Go to 6 wheels and your at 20 psi. Go to 4 wheels and you are at 30 psi, 2 wheels.... I think you get the idea.

Ideally, one wheel would be best. Let me know how to make that work.

I'm still in search of a very large bucket! :ahh:

Re: 4WD Turning Difficulties
 

[off topic] My dad had a Gravely! Cool machine. [/off topic]

Was that standard "wide" kitbot or standard "long" kitbot?

Re: 4WD Turning Difficulties
 

Travis, I'm not kidding about the Gravely form factor. That aside, they are interesting machines.

Re: 4WD Turning Difficulties
 

So.. Our team was debating over what drive system to use. Last year was out first, and we chose to go with a 4-wheel tank system. This year, we are not so sure about using the same system because of what is being said here; the friction will just be too little. That brought us to active steering in the front or rear, not including all of the other crazy suggestions. We were wondering, would rear-wheel turning on a 4-wheel drive pose any significant handling issues in comparison to front-wheel steering. Our current design ideas leans in favor of a rear-wheel system, but we don't know exactly how that will impact our performance. Any input would be greatly appreciated.

- Team 2496

Re: 4WD Turning Difficulties
 

I think a rear steering system will have drastically less turning ability than a front steering system. With a front steering system, all the non-steering wheels are behind the steering wheels, so they will follow them. In a rear steer system, I think that the non steering wheels being in front and behind the steering wheels will reduce the turning ability, and lead to awkward handling characteristics. Take this with a grain of salt though, I am having a little difficulty imaging a rear steering robot with a trailer turning, so I'm probably missing something.

Re: 4WD Turning Difficulties
 

Yeah.. We figured that maybe because we have the trailer attatched it may not be too much of a problem. Technically it would be steering from the middle with a pair of stationary wheels still behind the steering wheels. I am not quite familiar with the characteristics of a rear-steer (Ackermann), so I'm afriad there will be some crucial factors I am leaving out.

Re: 4WD Turning Difficulties
 

I think you would get better performance with the front steering wheels. If you drove the back wheels like a tank drive (not opposite directions though, I expect you would want to program in a motor differential) and power the front wheels in the direction of the turn you should have greatly reduced resistance when turning.

Re: 4WD Turning Difficulties
 

Give it a little bit to finish clearing the upload, but:

http://www.youtube.com/watch?v=03tYPM1Bw4c

Videographic evidence of what "standard" 4wd bots will turn like. Bot is weighted down (with about 60-80lbs on the bot and some unknown amount on the trailer).

Before anybody asks, yes, our tile flooring is similar to the regolith. We had a mentor who works for 3M bring in their version of Regolith and we did traction testing (tile floor vs. 3m "regolith" and we got the same results, 10lb breakaway force, 8lb dynamic pull @60lb 2wd robot).


(What you don't see, after the video: The robot's wheels just about fell off and it took a good hour or so to get them working completely again. Oh, the joys of thrown-together prototype bots)

Re: 4WD Turning Difficulties
 

Dave,
Is it possible you have a career ahead of you in some kind of R/C drifting competitions????? ;)

Re: 4WD Turning Difficulties
 

To be completely honest, this didn't look as bad as I thought it would...

Re: 4WD Turning Difficulties
 

Is that video on real Regolith?

Our testing (with a stock kitbot weighted 150 lbs and a stock goal) shows it is worse than this video would lead you to believe.

Re: 4WD Turning Difficulties
 

Yeah, we got the same results agreeing with John. We had to spin our wheels for a good 5 seconds to get our bot to rotate (without trailer), on Glasteel FPR.

Edit: To be fair, our robot wasn't loaded to full weight, it was at about 35-40 lbs.

Re: 4WD Turning Difficulties
 

this video appears to be on a school type tile floor, which is different from the field material.

Re: 4WD Turning Difficulties
 

This discussion has helped me think through more on our plans. Any videos or real world experience would be helpful.

Re: 4WD Turning Difficulties
 

While we didn't use exact weights, we drove a 4 CIM AM Shifter in Low Gear (with a long drive base) at about 30 pounds and about 180 pounds (yes, I know this is 30 pounds more than an actual robot!). While noticeable, the difference in turning ability was nothing to write home about.

Re: 4WD Turning Difficulties
 

Four-wheel tank drive on a ~140lb robot seems about as easy to turn and maneuver with as a car towing a small trailer on a very icy road... Unreliable at anything approaching speed, but something you can definitely live with -- especially if there are no ditches into which one might slide, and the intactness of one's body is not on the line.

I didn't notice that skid steering was any better than the steering of an actual car on 1/4" of ice... and I had a chance to test that out in the school parking lot just this past Wednesday! (We had a snow day because of an ice storm, so I did a little 'research' with my car.)

Patrick

Re: 4WD Turning Difficulties
 

I took a few minutes to update the spreadsheet from the beginning of the thread to include the forces from the trailer in the FBD. At present I still do not have the physical properties (i.e. an adequate trailer) to validate the spreadsheet, but it does seem to agree with what I have seen in videos from other teams. If some folks concur that the physics are correct I'll put it up in the white papers. Let me know if it is worthwhile.

Re: 4WD Turning Difficulties
 

a = f/m
friction force = normal force * mu
normal force = mass * gravity

a = ( mass * gravity * mu ) / mass

a = mu * gravity


so, a 30lb robot and a 180lb robot should be able to change direction equally fast.

perhaps the 30lbs and the 180lbs were distributed differently, changing the moment of the center of gravity?

Re: 4WD Turning Difficulties
 

Has anyone seriously considered a robot with two wheels in the middle as the drive wheels and having it pivot? We're thinking it would get good traction, as all the weight (well most of it) would be on the two drive wheels, and the trailer would help balance out the wheels to keep the center of gravity in the middle. Also, the simplicity factor would allow us to spend more time making a way to score.

Re: 4WD Turning Difficulties
 

I've seen that discussed on here before, I think that the concensus was that the hitch allows too much vertical movement for that to be feasible. You'd probably rock the hitch up and down enough for your team to get called on the rule that states the hitch needs to be a certain height

Re: 4WD Turning Difficulties
 

This would give you no more traction than a 4WD robot as the downward force (max ~150lbs) is the same.

It would reduce your force required to turn (scrubbing) significantly.

I would be concerned that when you are fully turned (60+degrees difference between your robot and your trailer) you could possibly tip enough to touch the floor (breaking the rules) or tiping outside the envelope.

Also, I believe the robot needs to fit within the sizing box without being supported by the sizing box, and a 2 wheel bot may not do that on it's own.

Re: 4WD Turning Difficulties
 

We considred it for awhile but figured we could not use the hitch to keep us balanced and it was too risky. But we are trying to keep the CG over the rear wheels as much as possible and trying to achieve about the same thing.

Re: 4WD Turning Difficulties
 

we were thinking of using more wheels to support our robot at the front and back when it pivots. The mentors were thinking that even though the robot may be resting on an un-powered wheel at some point, the amount of force that is placed on those wheels will be negligible, as most of the force will still be on the center drive wheels. I guess it is more like the 6 wheel drive with the lowered center drive wheels, only we'll use 5 (2 drive, 2 in front on each side of our ball gatherer, and 1 in the back)

EDIT: So... the trailer hitch can never change from that height? Well that could be a problem. I suppose we could have the rear supporting wheel powered and at the same height as the main drive wheels, and have it programmed to run only when moving foreword and backword. I really want to keep the number of drive motors as low as possible.

I'll bring up this idea that was really good, but ultimately shot down because of complexity.

One of the mentors brought up an idea of a three wheeled system. it would work similar to a crab drive, but has a huge difference. The base that the wheels are attached to would be able to rotate, or rather, the robot would rotate around it. The wheels move, then the entire body moves around the base to face the direction of movement. It seemed really stable and a great way to keep the same weight distribution on all wheels, but it was rather complicated and could interfere with a ball gathering mechanism.

Re: 4WD Turning Difficulties
 

To this general topic about 4WD: I designed the wheel base to be wider than it is long, the closest being... make sure the insides of the wheels make a square. This way the wheels don't have to slip along an arc across a surface while it's turning.

Re: 4WD Turning Difficulties
 

From the drawings of the swivel and other parts it appears that the design intent is to create a rigid attachment without freedom to rotate about the trailer wheels.

The hitch pin is a close fit. The close fit requires that the hole in the swivel have a vertical axis. The same is true for the holes in the c-channel. The 0.385 of gap is, IMHO, intended to allow for the tolerances in the height of the hitch swivel (when level) and the robot c channel (when level). I would not expect the swivel to ride much up and down on the pin. Of course I could be wrong and only testing will reveal how rigid the attachment is in practice.

Re: 4WD Turning Difficulties
 

I was thinking the same thing.

Also, I talked with the mentors today, and one of them brought up a really good point. If this was illegal, in theory, then suspension could be called illegal, since it changes the height of the robot, and could potentially make the hitch higher or lower than 2-13/16 inches from the ground. So I think we're sticking with our current plan. We're only going to have the height of the drive wheels and the wheels it will pivot onto on the front and back a 1/16 of an inch or less difference.

Re: 4WD Turning Difficulties
 

You are correct. This will work, however, the math proves you are not getting the full force of straight in line wheels and mechanums have a tendency to be difficult to steer straight plus a tendency for the operator to over correct. That's why so many teams experiment with a gyro feedback to help drive straight and side to side. The low friction of the surface means that the gyro would try to stop a skid but only manage to spin the wheels really fast... the exact opposite of what you need. Slower speeds seem to give more control but they're... slower. speed will be a factor in this game almost as much as friction.

Just my 2 cents

Steve

Re: 4WD Turning Difficulties
 

Purely anecdotal, but tonight we put our bot on regolith, and managed to steer without too much difficulty. It is a 4WD in standard ("portrait") orientation, with the difference being we incorporate a shorter wheelbase (to increase the space for the gullet). To use the now-standard way of drawing:

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________________

While I can't say I've read every word of this thread (and especially not all the links off of it), I have to wonder what we did "wrong" to get it to steer. Could it be the shortened wheelbase?
(Yes, pictures coming)

Re: 4WD Turning Difficulties
 

The shortened wheelbase will make a big difference.

We got our new wood chassis rolling tonight

http://www.youtube.com/watch?v=Yb8IPQYp_M0

The concrete floor is pretty smooth, and it probably has a bit more friction than the proper material (which we need to buy soon!). Handling is pretty good...all things considered...but having it light, no trailer, wrong floor, everything will be different in the game.

Re: 4WD Turning Difficulties
 

Mr. I,

Did you have the trailer attached during your driving? We noticed a considerable difference with and without the trailer. Driving without the trailer was quite fun, but driving with the trailer was just painful.

Paul

Re: 4WD Turning Difficulties
 

Good point. Tonight, maybe?