Need help problems at North Star regional

We have an issue, we have bot with skid steer ran on lab view from Pwm on jaguars tried 2 motors added two motors for a total of 4 can maneuver really well off carpet get on carpet we lose one side plus have a lack of power. And can’t turn forward and back work but no real load. Once we have load we have issues. We have jags that blink really fast while some stay solid also seem to be drawing a lot of power. Any help, is it program issue or do we have short, we have a bot bot weight of 109 lbs.

“we have bot with skid steer…can maneuver really well off carpet get on carpet we lose one side plus have a lack of power. And can’t turn…”

I’m assuming that you have 4WD, “long” wheelbase, reasonable traction on your wheels. That type of drivetrain will almost always have this type of problem. It’s due to the fairly high “scrub” (sideways friction) when turning, where the robot will bounce a bit if it turns. A bit more detail on drive type might be quite helpful.

Your best solution is to reduce the traction on one end. Omni wheels, zipties around the tread, lower-traction wheels (say, the 2009 KOP wheels) or add a wheel in the middle of each side slightly lowered and powered to make a 6WD drop-center drivetrain are your main options at this point.

As for the power loss: If it were a short, it’d probably also show up on non-carpet flooring. I’d be willing to bet that something’s pulling a lot of current, causing breakers to trip or causing much less current to go elsewhere.

I really don’t think it’s programming or a short. Just a 4WD being a 4WD. (A 6WD flat would normally have the same problem, unless you’re FRC25.)

We have the big slick wheels on front. Two chains each side driving the wheels from simple box looking at going to swivel casters on front to eliminate friction. We are long and as wide as we can be thinking of shortening the axel to axel base what do you think.

Also we are using the compressor.

Rule #1 of FRC drivetrains: NO swivel casters.
Rule #2 of FRC drivetrains: See Rule #1.

If you *must *use casters, use ball casters. They’re easier to work with, as they don’t quite have the tendency to force your robot in the wrong direction that swivel casters do. Omni wheels are better still–but it’s kind of late to get some if you don’t have them.

“Big slick wheels”: I’m not sure I understand that. AndyMark 8" wheels with no added roughtop tread? A bicycle wheel without a tire? The 2009 6" KOP wheels? Help me out here.

The compressor shouldn’t be a problem–ideally, your tanks are charged when you go onto the carpet.

You pretty much have two options right now that can be implemented easily:

  1. Slicker wheels on the front. That’s dependent on what you have available; a large box of zip ties or a roll of duct tape can make a wheel less sticky in a hurry.
  2. Shorter wheelbase. There’s a couple ways to do that; either move the axles closer together or use the lower hole on the chassis (if you’re using the KOP chassis) to add another wheel, and power that. How short you need it to be… well, that’s something I’m not sure about.

I’d go with option 1 right now; option 2 will take more time, more parts, and is more complex.

We have the large kop slick white wheels on the front rubber covered ones same size on back, still having trouble with turning here, we grabbed two swivel casters on way to hotel tonight, could grab two ball casters if we need too just don’t see how we are losing so much torque that we are stalling out on the carpet when turning. When are batteries are fully charged we manage to turn, but seems to drop voltage fast then stuck with forward and back.

Are you sure all your jags are working and working in the right directions? Easy way to check is to rraise the robot off the ground and have just one jag plugged in at a time and run the robot foward backward, left and right. Make sure they’re all going in the directions they should. You might have something wired backwards or a half dead jaguar.

Second, what’s your weight distribution? Are you putting more weight on the sticky wheels or the sliick wheels? If you’re putting most of your weight on the slick wheels, that would also explain this.

Most of our weight is on the back two grip tires but we are long in our axel design, we had the Texas instrument guy go through our jags he said they were good, could we be having an issue with the program and it interfering with the code to drive, just trying to trouble shoot, we do have good movement when it is off the ground, could it be an issue with firmware on the two gray jags they are older. Not sure with our program as I seem to remember something about it can stutter and shut down drive motors when checking the pressure switch from the compressor.

It’s probably the scrub I mentioned earlier. It’s a common 4WD problem. The 2009 slicks are only a little bit slicker than normal wheels on carpet, I’d think. There’s quite a few posts on why it happens on CD, including someone calling them Mexican jumping beans.

Good movement off the ground is another classic symptom of the 4WD scrub; it’ll act right on the blocks and as soon as it tries to turn on the ground, friction takes over.

Quick sanity check: Are you talking 6" wheels or 8" wheels in the front? The 6" wheels have the truly white surface and are from the 2009 KOP; the 8" wheels came in this year’s KOP. There are also 8" plastic wheels that look similar to this year’s KOP wheels; I believe they came in the 2010 KOP. The white (and I do mean white) wheels should have the lowest coefficient of friction (and can generate a lot of static).

Here’s how I’d start out:

  1. Seeing as you already have swivel casters, get them onto the robot right away. Get to the practice field and try driving. Have team members kick the robot to simulate impacts–or have another robot out there do the same. What’ll probably happen is that while your turning problems go away, you get a different set, caused by the casters needing to be turned. You’ll also lose a little pushing force. There’s a physicsy explanation for that; I can give that if you’re interested.

If the casters are acceptable, you’re done. If not (and I’d be willing to bet that after a match or two, they won’t be), on to phase 2, or get ball casters and repeat phase 1.

  1. Wrap the front wheels in zip ties all over. Duct tape may also work, but make sure it doesn’t stick to the carpet. This is something that you can be doing while you’re testing the casters. This should reduce the scrub by giving less friction.

If neither 1 or 2 work, phase 3:

  1. Figure out where your CG is. Move the slicker wheels to the middle of the robot, on the opposite side of the CG from the grippier wheels. Put casters on the other end. Whether you drop the middle wheels is up to you; it may be helpful (keeps the casters mostly out of trouble). Pseudo-6WD can help with wheelbase reduction.

Ideally, of course, you find two extra wheels and enough sprockets and chain to build a true 6WD drop-center, but that may be tricky to do fast enough.

Oh, and one other thing: Make room for some carpet on the way back. Inform the FTA that you’d like some field carpet after the regional is done; he’ll either point you to the team(s) that have claimed it already or ask, “how much can you take?” Take the carpet home and use it for next year’s testing.

Thanks so much I’ll try in the morning then go from there I’ll let you know. If you have any other suggestions they would be appreciated i would love the extra info explanation as well, as far as sanity slowly leaving the realm of sanity this season.

One physics explanation coming right up!

Normal force, coefficient of friction, and torque exerted by the wheels all come into play here. Normal force is applied at each point of contact the robot has with the ground. The normal force * the coefficient of friction dictates the maximum force that can be applied horizontally at that point before something slips. (You’ve probably heard something like this in your physics classes–if you haven’t, you will.)

Under the current setup, all 4 wheels are exerting a torque, which can be translated into a force tangent to the wheel (we’ll call that tractive force for now) at the point of contact with the ground. This is why the robot moves in the first place.

But, when you remove two power sources, you remove two powered points of contact. The contact remains, which means that some friction remains (to be dealt with either by overwhelming force or by bearings on the robot). However, now all the available power runs through only two points of contact. The maximum possible force remains the same, however, due to the normal force not changing, so the maximum tractive force per wheel remains the same.

But wait! You just removed two places to apply power! Now you have less available tractive force; the two wheels left have to drive the whole robot; and friction hasn’t changed.

The physics behind the “scrubbing” phenomenon are kind of similar: force at a distance. Let’s figure that you turn around a point on your back axle line, for now; I’m sure that you can figure out what would happen if you moved that point forwards towards the middle.

You have two drive forces, one in each direction, at ~ 13" from the pivot point (I’m assuming it’s in the center, and counting both wheels on the same side together). You have a moment (F*distance, in this case times 2). But, on the other end of the robot, ~36" away, you have the side friction forces of the wheels. Those also exert a moment, in the opposite direction of motion (because that’s the way friction always goes). What often happens with a 4WD is that that moment due to friction is very close to the moment caused by the drive… and life gets very interesting until the robot starts bouncing and sliding.

A 6WD drop-center puts the center of rotation close to the center of the robot and cuts that long distance in half, while at the same time reducing the normal force (and therefore the friction force) at one end. Omni wheels can really cut down on the sideways friction force; casters can as well, but add in turning resistance (though ball casters add less than swivel casters).

Now, why does the robot work fine on non-carpet surfaces? Carpet has a rather high coefficient of friction with rubber. Like I said before, get some of the event carpet on Saturday and use it to test on next year.