Trouble with pneumatic tank drive

[TimTheGreat]

Our team has decided on using pneumatic tank drive, and we're pretty much done with the robot, but with the weight and all it doesn't turn very well on the carpet, and the battery is draining very rapidly (which we think is due to high current draw to make the wheels resist the fricition) I have heard it is not the greatest idea to use pneumatic wheels in the center of the dropped middle tank drive setup, but it really works for us. What are some things we can do?

[Lil' Lavery]

How large is your center "drop?"
What is your wheelbase and track width?
What pressure are you inflating your wheels to? Is that pressure uniform for all wheels?

[Mike Marandola]

Can you give more information? Like the track width, wheelbase, amount of drop, gearing, what tires you're using, etc.

Edit: Sean beat me!

[Jon Stratis]

Well, it definitely sounds like you need to reduce friction. I can think of a few of ways to do that...

1. you can decrease your wheelbase (the distance between the wheels). That's basically what a drop-center drive train is. So, this breaks up into 1A and 1B - alter your wheel spacing, or add/increase the drop for your center wheel.

2. You can increase the pressure in your wheels (up to the manufacturer recommended max pressure). Increasing pressure will make the wheels less squishy, which reduces the contact area, thus reducing friction. If you look at any websites about hypermiling, you'll see one of the first things they do is get skinnier than normal wheels and over-inflate them. In fact, to increase gas mileage on the Gen 2 Prius, Toyota sold it with skinnier wheels to begin with in the US than they did in some other countries!

3. You can replace your wheels with other ones (normal traction, omni, etc). This could reduce friction quite a bit. Going to a pneumatic/omni setup would have the advantage of pneumatic wheels to hit the defenses with up front (while dragging the omni's through on the back side), and easier turning from being 50% omni.

[TimTheGreat]

So we're using the nano tube transmissions from andymark, as well as the 8" pneumatic wheels from andymark. I don't have an exact number for the PSI, but we pumped them up until they were basically rock hard.

[GeeTwo]

Originally Posted by TimTheGreat

So we're using the nano tube transmissions from andymark, as well as the 8" pneumatic wheels from andymark. I don't have an exact number for the PSI, but we pumped them up until they were basically rock hard.

That's what we have for our air cannon, with the long (31"?) tubes. The drop center is not enough to turn well at low speed. You may find that you can turn better if you're accelerating or braking, as one pair of wheels comes up a bit. If we were using this chassis base for competition, I would definitely increase the center drop. If you want good travel over the obstacles, you may want to decrease the tire inflation, which would require even more center drop. Fortunately, the center is not one of the directly powered axles (unless you've got the one with gears all down the length). If you're driving that with chain, you might do well to go dead non-rotating axle and put bearings in the wheels rather than a hub, which will allow you to add a plate over the stock bearing hole and use a smaller (e.g. 3/8") hole for a bolt.

[TimTheGreat]

Originally Posted by GeeTwo

If we were using this chassis base for competition, I would definitely increase the center drop.

So we ARE driving the middle and rear ones with cims and chain driving the front. Is there still the possibility of dropping the middle further?

[Joe Johnson]

Originally Posted by TimTheGreat

So we ARE driving the middle and rear ones with cims and chain driving the front. Is there still the possibility of dropping the middle further?

Can you give us a little more info?

1. What is the Left/Right distance between the wheels?
2. In the side veiw, how many wheels can we see and what is the distance between axles (sounds like there are 3 but please confirm and also it is possible that the middle wheel is not in the middle of the other two axles so -- share that too if need be)?
3. Are the axles all in a line or are some lower than others (if so how much)?
4. Are all wheels the same or are they different?
5. Are all wheels and CIMs on one side all mechanically connected (rotating one wheel or motors rotates all the wheels and all the motors)? If not explain what wheels and motors are connected to what other wheels and motors.
6. What is the ratio between the Motors and the CIMs?

A side view picture of the chassis would be helpful.

The good news is that you have time. Many teams discover they can't turn when the get in their first practice match. You've got a lot of time to fix this.

Cheers,
Dr. Joe J.

[IKE]

Also how are the Cims driving the wheels. I e what transmission or sprockets are you using. High current can be improved with more ratio too, but the robot will get slower.

[TimTheGreat]

http://www.andymark.com/product-p/am-0691l.htm

This is the transmission

[Joe Johnson]

Originally Posted by TimTheGreat

http://www.andymark.com/product-p/am-0691l.htm

This is the transmission

Ok got it.

The fact that this is a prefab gearbox makes changing things a little more problematic.

So, AM says that the center is lowered by 0.13" I would probably go more in the case of pnuematic tires but you might be able to get by with it if you inflate the middle tire to the high end of the allowed range and have the front and back inflated to the low end. This will effectively make the middle tire a slightly large diameter which can help you get some rock into the robot.

Another problem that could be causing you problems is that you have 2 CIMs per side all the wheels and CIMs are not connected to each other. The front (or the rear depending on how you mounted them) is driven by one CIM and the rear & middle (or front & middle depending again) are driven by the other CIM.

If you actually get enough rock in the robot, then if your CG is such that your robot has the rear and middle wheels on the ground, then the front wheel is in the air. Where is all the power from that front CIM going? Nowhere, that's where.

You really want to sync up all those wheels. You mentioned a chain in another post. I hope that means you have a chain that is doing that sync function.

Next problem. JVN's spreadsheet says that with a 1 CoF, 2CIMs, 12.75:1 and an 8 inch wheel, your top speed is 14.5fps (11.8fps derated) and 84Amps per CIM in pushing match. This is not going to make your breakers happy my man. No sir.

I am strongly asking you to get yourselves more ratio or less grip.

Dr. Joe J.

[Joe Johnson]

Originally Posted by IKE

Also how are the Cims driving the wheels. I e what transmission or sprockets are you using. High current can be improved with more ratio too, but the robot will get slower.

Yes. I typed "What is the ratio between the Motors and the CIMs?"
I intended to type "What is the ratio between the Wheels and the CIMs?

Dr. Joe J.

[Trevor1523]

Simply put, if you drop the centre hole an 1/8th of an inch below the others, your robot with theoretically be turning on only 2 wheels, rather than all 3. So this will solve your friction problem.

[IceStorm]

Correct me if i'm wrong but as a quick test couldn't you inflate the center tires on both sides to have more pressure than the outer wheels. This should give you a similar result to having a greater drop center.

[TimTheGreat]

Originally Posted by IceStorm

Correct me if i'm wrong but as a quick test couldn't you inflate the center tires on both sides to have more pressure than the outer wheels. This should give you a similar result to having a greater drop center.

I was thinking about that and was wondering how much of a difference it would make, because the actual tire is standard across the robot, but we'll give it a try