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.

[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

[Orion.DeYoe]

Can you specify whether this is 4 or 6 wheel drive?

[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.

[dradel]

You may want to take a look at your gear ratio as well

[Joe Johnson]

Aspect ratio is all.

Wider is better. Longer is less good.

The reason is that when you turn a WIDE robot the wheels don't have to move normal to their roll direction (i.e. scrubbing) very much, they just roll. When you turn a LONG robot, the wheels practically do nothing BUT scrub along the carpet. It's this scrubbing along with grippiness that kills you (and your battery).

That the reason that you lower the middle wheels; It effectively halves the Width/Length Ratio because either the front two axles are on the ground or the rear two, but not both. So the distance in the length direction halves while the width stays the same. Less scrubbing so you can keep your grippy wheels.

What do to?

Well, if you're 4 wheels in the corners, you need to either get a third axle and lower that center wheel or you need to get rid of some of that grippiness on at least one set of tires (also you might want to read Dr. Joe's Seven Stages of Dealing with a 4WD Robot with Grippy Tires)

Dr. Joe J.

[IKE]

Originally Posted by Joe Johnson

Aspect ratio is all.

Wider is better. Longer is less good.
...snip... Dr. Joe J.

Gulp, I can't believe I am going to do this, but I somewhat disagree with Dr. Joe.....

While aspect ratio will dictate whether or not you can turn, the overall length and weight distribution on the wheels touching dictates how much power it takes to turn at a given turn rate. What is turn rate you ask? It is angle of the turn divided by the time to execute the turn.

For instance, lets compare two scenarios of wide and long for those "back in the old days" of 28x38 dimensions.

Long bot was a 6x6 drop, and on 8" wheels, the point of contacts with the carpet were at least 4 inches from each edge and then of course the center. total length touching was 38-4-4 or 30 inches. If you dropped the center, they you were riding on an effective wheelbase of 15" while the width was typically 1" inside each for frame, and the crown of the tire was inboard another inch each side (2" wide pneumatic tires) for a effective width of 24". This robot turns well. Assum that the CG is right in the middle so all four touching wheels have the same contact. In this scenario, the lateral forces are the same as the longitudinal and for a COF of 1.3, the resultants work out to about 0.91. The longitudinal forces when pivoting act about a 12 inch radius (24/2), where as the laterals act a a 7.5" radius (15/2). To turn 90 degrees in 1 second, the tires end up traveling around a 14" arc, and 90 degrees of that arch would be 22 inches of travel. 22 inches of scrub in 1 second on a 120 lbs robot is about 2400 in*lbs/second or about 268 watts (also 268 J).
For the old wide bot, the wheel base was 28-4-4 or 20 inches, and the width was 38-2-2= 34 inches. This robot will turn well, but to turn the same 90 degrees in 1 second it will have the tires traveling about a 19.7" radius for a total travel of 31 inches which for a 120 lbs robot is 3700 inch*lbs/sec or 420 Watts of power (also since 1 second 420 J)

So to perform the same turning action, the wide bot actually required about 56% more energy to execute the action.

While it will turn just fine, this will cause your motors to get hot. Especially if chasing down balls.

*I did this math back in 2010 after watching 1918 have hot motor problems with their awesome wide bot that year.

[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 IKE

Gulp, I can't believe I am going to do this, but I somewhat disagree with Dr. Joe.....

<snip>

It's okay I can take it.

The wider is better statement is just a notional idea not a hard and fast rule.

It is not as sexy to put in all the qualifiers: wider (with respect to length) matters and that it is the groups of tires that actually touch the ground matters and that location of CG matters (though often if you have an arm or other moving subsystem you don't always get control of the exact location of your CG with respect to the tires that are on the ground) and that it CoF for tires is often different for scrubbing vs. the direction of rolling and and and.

Dr. Joe J.