Robot refuses to turn!

[Edoc'sil]

Alright its the last few days of the build season and everything is going fine. Our kicker is looking good, electronics are mounted, and our weight is right at where the CAD model said it would be.

Well almost everything is good, our robot won't turn.

We built a basic six wheel design. There are 6 8in plaction wheels, 3 to a side, all hooked up by a chaindrive, so all 6 wheels are powered. So we take a battery reading 12.5 volts and the thing flies forwards beautifully. Next we switch up the wires so that one side is going forwards and the other back so we can test our turning. It doesn't move at all. We take a look and we realize we made a design mistake, the distance between our front and rear wheels is greater then the distance between the two sets of wheels. Oops. So no big deal we think, we can fix it easily enough. We pop off our front wheels thus halving the distance between our "front" wheels (the old middle ones) and our back wheels. We then go add the power again and it still doesn't turn at all. With a bunch more testing we find out that the only way we can get our bot to turn is if all the weight is supported by one parallel set of wheels.

Our bot has a 24 to 1 gear reduction, so that translates to about 6 feet per second under normal load, and when moving forward our bot does close to that. Perhaps 4 to 5 feet per second. the distance between the one wheel and the next is 12.5 inches and the distance between one drive and the other is 24 inches. Nothing is jamming, the motors are spinning all in the correct direction when we take the weight off the wheels but no matter what we do the bot will not turn under its own power. Are we breaking some kind of cardinal rule here? I would appreciate any input.

We thing the best fix will be to replace the front and back wheels with omnis, leaving just the middle ones being plaction wheels. We hope this will solve our problem, but we do not want to throw $200 bucks at the problem unless we know it will fix it.

Final thought. We were testing this by hooking up the motors directly to the battery. I assume many people will tell me this is a bad idea, maybe for safety reasons, maybe because its hard on the motors etc. I just want to say that I am aware of the risks, but we needed to know if our robot would work at all not two days from now after we have figured out that the problem wasn't an electrical one. If we would have hooked up the motors to the jags and run everything the way it was supposed to would it have any significant improvement in the performance of our bot?

[EricH]

Is is a 6WD "drop center" or "flat"? What motors are you using?

If you're running flat, get 4 omnis and put pairs in diagonal corners. This may help. (Why pairs? it smooths out the ride so you're replacing wheels less often.)

I'd guess that you're experiencing the "scrub" that causes many 4WD robots to bounce around and many >4WD robots to need omnis or a dropped set of wheels to turn.

If you're running one motor per side, you might add a motor to each side.

[Edoc'sil]

I forgot to mention that we are running 4 CIMs. The original configuration was a flat drive, but by removing the front two gears the design should have handled like a drop.

[AustinSchuh]

When you say it doesn't turn, what are the symptoms? Do the motors stall? Do the wheels spin? Is your battery voltage above 10 volts or so when trying to turn?

Also, it might help if you provided a picture of your robot so we can look for potentially other problems.

Last time I ran the numbers, 6 ft/sec with 4 CIMS and roughtop (I think I used a coefficient of friction of 1.5) should be able to spin tires while drawing only about 40 amps per motor. We've also used 6wd long for a couple years, with a dropped center wheel, and not had trouble turning. So, I'm having trouble visualizing what's going wrong with your setup.

[Edoc'sil]

Battery was topped up at 12.5 volts.

Wheels twitch when you add power, you can easily tell they are pushing. If you lift up one side of the bot moves merrily. I am assuming that the bot is at stall amp draw because it is hooked directly to the battery an it is not moving. Also the wires heat up, so significant amps are flowing.

I can post detailed cad models in a minute.

[Tom Line]

A 6 wheel long robot WILL NOT TURN unless you drop the center wheel by 1/8" to 1/4" of an inch.

A skid steer, or tank drive, literally "drags" the front and rear wheels through a turn.

A wide orientation robot with 4 wheel drive will turn if geared around 9-10 fps. This wheel ratio of length to width is necessary to maintain if you want to be able to turn. That means when you go to 6 wheel drive you need 2 of those wheels off the floor to get the same length and width ratios that you have on the wide robot. Essentially by dropping the center wheel you'll be on the middle two wheels all the time, and then either the back or the front wheels depending on your center of gravity.

If you do not want to drop your middle wheels, then you can put a single omni in the front and in the back. If you are still having difficulty turning, you can dump the traction wheel for something with a little less side-traction, like the old skyways, a traction wheel with the tread worn smooth, or last year's ABS wheels.

[Joe Johnson]

What are your dimensions?

Distance between axles also your width.

24:1 between your wheels and the CIM's may be a bit light for an 8" wheel with that much traction but I would think it should be able to turn if you get the right ratio between width and center distance.

Give the dims and let's talk.

Joe J.

[Edoc'sil]

http://www.flickr.com/photos/47595136@N03/4364660600/
http://www.flickr.com/photos/47595136@N03/4364660576/
http://www.flickr.com/photos/47595136@N03/4364660548/
http://www.flickr.com/photos/47595136@N03/4363918531/

As you can see a simple 6 wheeler, undropped.

While we were testing we removed the wheels up by the kicker to simulate a dropped drive to see if that solved our issues. Because the majority of our weight is in the back it rested nicely on just 4 wheels. Sill didn't turn with only 12.5 inches between the back wheel and our "front wheel"

*One sec the pics are not appearing
*It's poor form but I just hyper-linked the photos, not going to sort out why the images didn't want to load.

[Edoc'sil]

@ Joe

Between axles 12.5 in apiece, 25 total
Width between wheels side to side, 24 in, measured from the middle of the wheel.

Those specs were in my first post, not surprised it was passed by, I have handed in shorter essays to my English teachers then that post.

* found a typo in my first post, I see why the specs were missed when vs wheel changes the meaning of a sentence a bit.

[Big Ideas]

Any chance you dont have the motor polarities switched properly, so that pairs are working against each other? Or, shorting the battery?
Just a thought.

[Edoc'sil]

Checked polarities, we thought for sure that was the issue, but to no avail. the motors run fine when the wheels are of the ground, ie. one side (2 motors) is going forwards, one is going back (other two motors).

If by shorting the battery you me crossing leads some where so the energy doesn't get to the battery, then no we are not. Everything runs fine in Forwards/Reverse or when there is no load on the wheels.

[EricH]

Better go with the omnis, or grab the slicks from the KOP if they'll fit.

[Edoc'sil]

Actually we at one point replaced the front wheel with a slick, but nothing changed at all.

slick in the back would have been good, but axle is this odd hex shaft that is a real pain to switch out a wheel on it. looks like a project for tomorrow.

This whole thing is bugging me because one we switched our wheel base to just 12.5 in that thing should have spun like a top. I am just agonizing that I am doing something dumb here, similar in caliber attaching the motors wrong, but I just can't find anything wrong.

Btw are omnis the same diameter and width as the plaction wheels? Andymarks specs have left me feeling a little wanting.

we have these http://andymark.biz/am-0514.html
and i think these are basically the same form factor http://andymark.biz/am-0048.html, barring the .5 in difference in the hub width.

[Tom Line]

Something is wrong.

A 24inch wide 12.5 inch long wheelbase with two cims on each side geared at 24:1 (the low gear on the supershifter ratio) should be able to turn. I'm taking this dimension from your test where you removed both front wheels.

You say you're directly powering the cims on both sides from the battery, with no breakers involved.

You say you've taken a battery with 12.5 volts. That is not a charged battery. A fully charged battery will usually read between 12.8 and 13.1 Volts. A 3/4 discharged battery can still read 12.5 until it is under load, then it can drop to 7 and 8 volts.

#1 Confirm that the battery is fully charged.
#2 Confirm to us that you are running on regulation carpet WITH NO PADDING under it. Carpet with padding is death to turning.

Edit: Can you please confirm that you're running dual cims on each side, and not singles?

[Vikesrock]

Originally Posted by Edoc'sil

I forgot to mention that we are running 4 CIMs. The original configuration was a flat drive, but by removing the front two gears the design should have handled like a drop.

Did you remove the two front wheels or just the sprockets on them? Removing the sprockets doesn't do much of anything for you, the wheels still have to drag across the carpet.

If you removed the wheels, then as Tom suggested, that robot definitely seems like it should turn.