Originally Posted by billbo911 (Post 1545044)

We ran into a similar issue yesterday, but the source of the issue was easy to identify and resolve.
We were dropping the voltage on our battery to <8.5v every time we did a point turn/pivot. Driving straight kept the battery above 12v. This was obviously a scrub issue.

We are running a typical WCD with 8" pneumatic wheels, thus the scrub should not have been too great, but it was. Why???

During early practice session, the students felt it was beneficial to lower the pressure in the center tires to help gain traction going over the Rock Wall.

Lowering the pressure to ~0psi basically renders a WCD to be just a 4 wheel tank steer.

We re-inflated the center tires and our voltage drop on point turns returned to normal, around 11.5v.

With the additional practice time we had put in, our driver also had figured out how to transition the Rock Wall without the need to have "high traction/ low pressure" center tires.


Now what bothers me with the OP's issue is how two motors on the same transmission could have such a huge temperature difference.
Are you controlling the ESC's via "Y" cables, or discrete PWM outputs from the RoboRio?

Originally Posted by scca229 (Post 1545125)

Off topic I know, but I have to.

THANK YOU, Dr. Joe and Ether for the education in this thread. I know that when I'm watching battery numbers on the FMS Monitor screen this season, this thread will come to mind as a way to help explain to teams why they are browning out on the field if their situation is similar, which I expect to see a LOT of.

I actually do have something on topic I guess. When making hub adapters, I played around with air pressure in the 8" Harbor Freight Red tires that we have and it was pretty amazing the change in circumference that a few pounds of air made. I can very easily see just increasing the center tire solving the issue by giving enough "rock".

Originally Posted by pujan824 (Post 1545160)

I agree adding air to the center to get the rock would solve the problem. Except in our case we don't have any tubes in our tires. Rather filled them with pool noodles. So the solution isnt that easy

Originally Posted by pujan824 (Post 1545160)

I agree adding air to the center to get the rock would solve the problem. Except in our case we don't have any tubes in our tires. Rather filled them with pool noodles. So the solution isnt that easy

Originally Posted by scca229 (Post 1545167)

Were you popping tubes in the center tires as well or only the forward/rear?

Originally Posted by Joe Johnson (Post 1545180)

Ok, I have another idea that can probably get you running today. Gets some good cloth gaffer's tape and put about 1/8"-1/4" around the outside of the center wheels. The tape will unwind in one direction, so make it so that your primary pushing direction tends to wind the tape and not unwind it. Good Gaffer's tape may just hold but if it starts to unwind, rewrap the tire and put another piece of tape over the end of the wrapped tape that goes over the sides of the tire and then use screws to attach this tape to the tire (don't look at me like that -- driving screws into wheels is a time honored FIRST tradition ;-)

It is a kluge but it gets you running today. Longer term you want to find a way to get a larger, less grippy tire on the center axle. But this lets you get some driver practice in, lets you test out the rest of your robot, and gets you in the bag with at least a direction that you can implement while you wait for your first competition.

Dr. Joe J.

Originally Posted by Joe Johnson (Post 1545180)

Gets some good cloth gaffer's tape...

Originally Posted by pujan824 (Post 1545402)

Unfortunately we don't have any of this and local stores don't have any of these.

Originally Posted by Alan Anderson (Post 1545809)

I'm pretty sure one of your "local stores" is Uline. They have gaffer's tape.

Originally Posted by Al Skierkiewicz (Post 1545962)

pujan,
Some of the issue you are seeing is also attributable to the natural electrical loss in the wiring and battery. The internal resistance of a fully charged battery is about 11 mohms. Add to that 2 mohms for the #6 wiring between the battery and the PDP and perhaps another 2-4 mohms for the main breaker and terminals and you are now looking at a minimum series resistance of 17mohms. At 400 amps (typical max draw on a four CIM drive at near stall) that resistance will drop ~6.8 volts. That translates to 5.2 volts measured at the input of the PDP. The dedicated outputs at the end of the PDP intended for use with the RoboRio and VRM may drop out when this drop is worse, such as with higher current draw or a less than fully charged battery.
Gaffers Tape is a generic term for a woven cloth tape ~2" wide. It may be called Colored Duct Tape in your area. It is available at big box stores like HomeDepot and Lowes.

Originally Posted by Al Skierkiewicz (Post 1546004)

Gaffers tape will only tell you if the wheels and their interaction with carpet is the issue for high current. What Joe is suggesting should reduce the friction with the carpet. If that works, it still points to an issue with your wheel choice. Tape has a tendency to pull off in competition and for that reason should not be considered a permanent solution. Leaving sticky residue on the carpet is considered "damage" to the field which may cause issues at your events. Everything in our experience is pointing at the wheel choice as being the issue. However, there are several other problems that exhibit the same symptoms. The tape will eliminate the most obvious. If your problems continue then there is likely another issue(s). Don't you have a local team that can come and give you guidance?