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?