HELP: Voltage Drop

[pujan824]

Originally Posted by billbo911

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?

Our motor controllers are connected via CAN. Our code gives the 2 right cims the value of the y axis of the right joystick on our controller. And same for left side with y axis of the left joystick on our controller.
The code is
Double right = -movementstick.getRawAxis(1);
Double left = movementstick.getRawAxis(5);
Rightbackdrive.set(right);
Rightfrontdrive.set(right);
Leftbackdrive.set(left);
Leftfrontdrive.set(left);

[scca229]

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".

[pujan824]

Originally Posted by scca229

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".

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

[scca229]

Originally Posted by pujan824

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

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

[Joe Johnson]

Originally Posted by pujan824

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

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.

[pujan824]

Originally Posted by scca229

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

Yes we were

[Ichlieberoboter]

It could be an issue with the VRM which we posted about in this thread. Someone might have already said that in this thread, sorry if they did.

[pujan824]

Originally Posted by Joe Johnson

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.

Unfortunately we don't have any of this and local stores don't have any of these. So we can't do anything until our competition in 2 weeks. Our mentors are reluctant to changing the actual wheels since that means we would have to take of all our superstructure, gearbox and down to the chassis. Also taking apart and reassembling the chassis would take a fairly long time. Are there any quick fixes we could do for our competition that would be reliable ?

[Alan Anderson]

Originally Posted by Joe Johnson

Gets some good cloth gaffer's tape...

Originally Posted by pujan824

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

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

[pujan824]

Originally Posted by Alan Anderson

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

We'll check there and see if we can put any on at the competition

[Al Skierkiewicz]

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.

[pujan824]

Originally Posted by Al Skierkiewicz

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.

Thanks for the insight. Would you suggest putting on gaffers tape for the competition?

[Al Skierkiewicz]

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?

[pujan824]

Originally Posted by Al Skierkiewicz

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?

Yes, we do. We have been working with team 1241 throughout the season but were never able to see this problem since we didnt test on carpet until we visited 610 a couple days before stop build. That was when we had this problem coming up. I know this must be a problem in the wheels since it does have alot of friction. We could see the speed difference while driving on the carpet. Since changing the wheels is very difficult this late in the season especially since our robot is bagged up and we don't have a practice bot. We have duct taped our front and rear wheels which did seem to help out a bit. I think that our biggest mistake was swapping the air tubes for pool noodles as mentioned before in the tread. Right now our team is throwing ideas out to see what we could possibly do at our first regional in 2 weeks but this weekend we are going down to 1241 and making extra parts as well as some guidance

[gpetilli]

I agree that there is a definitely a scrub problem given noodle "tubes" and no center drop. The relative wheel traction needs to be changed. Maybe smaller KOP wheels in front? Intentionally offset the CoG?

But back to OP. 1 gearbox per side with 2 CIM per gearbox - front CIM gets hot. No way this should happen. Do you have a gear AND key on each CIM output shaft? Are the gears all the same number of teeth (you need to actually count them). Did we get an answer as to if you are using a Y cable to drive two speed controllers per side (preferred method) - total of 4 speed controllers and 2 roboRIO PWM ports? Have you calibrated the speed controllers (this could actually be an issue if at zero one is forward and one is reverse)? Do the speed controllers on one side light the same color when driving forward?