Trouble with pneumatic tank drive

[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 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.

[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.

[dradel]

Like I said earlier take a look at your gear ratio.

I think there are going to be more than a few teams not getting the performance they should due to using the kit bot gear boxes without changing the ratio. The ratio selected was based on a 6" wheel not 8-8.5 that I have seen teams say they are using.

[Wayne TenBrink]

Originally Posted by IKE

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

I thought your analysis sounded familiar to something we experienced back in the day. Besides having only the 4 corner wheels on the ground with that chassis, there was a lot of friction in the chain routing/tensioning. You could have fried an egg on those motors by the time we got thru the playoffs at MSC.

[Al Skierkiewicz]

Another thing to think about with this setup is what actually is taking place while you are scrubbing the high friction tires in a turn. You will subject your bearings and other drive parts to exceptional side loads which in turn raise the friction there as well. With all of of this increase in friction, you will send the motors into a near stall condition which will take you to somewhere north of 100 amps per motor. Depending on your electrical design, the electrical loss could be significant and therefore reduce the available current to each motor. If one side has a much longer wire length than the other side, then turning in one direction may give you half the loss that turning in the other direction might produce.
What Dr. Joe is alluding to is shorter wheelbases will reduce this side friction as you turn. You may get a better handle if you were to draw the force vectors encountered in a turn for each of the wheels. The wheels closest to the center of turn will have the least side force which in turn produces the least (relative) electrical load.
A fully charged and new battery can supply up to 600 amps but not at 12 volts. Typical batteries may only produce 500 amps for the beginning of the match but much less towards the end. Your mileage will vary on final ratios, size of tires and where on the motor curve you have chosen to run.

[GeeTwo]

If you're committed to this layout, and you're using one chain per side as is most common, I suggest putting the CoG significantly towards the axle which is not coupled by chain to the center wheel, and note that you will get your best acceleration away from that axle.

Try to find some way to increase the center drop. As I noted above, recall that the center axle is already a dead axle; don't be shy about using cover plates, putting a bolt through them, and mounting your wheels on bearings to accomplish this.

The team had already bought the nano tube for our air cannon before I became seriously involved. Looking at it later, this seems to me to be a great way to mount mecanum if you can get the right gear ratio, but for anything else it leaves much to be desired.