HELP: Voltage Drop
 

When testing our robot, we were getting a voltage drop to 6V when going over the obstacles. When going over the carpet we have been dropping voltage to 9V.
Our robot has 6 8" pneumatic wheels powered by 2 cims in a toughbox mini each controlled by a talon srx
It has 2 window motors for lifting our arms each controlled by victors
2 mini cims for shooting each controlled by talon srx. (we are planning to swap these out for banebots RS-775 or 775pros )
We also have noticed the front CIMs on our drivetrain getting really hot.
Our drivetrain does not have the center wheel dropped.
Currently we do not have the time to create a center wheel drop.

Re: HELP: Voltage Drop
 

What's your battery voltage when you're not moving (idle)?

Re: HELP: Voltage Drop
 

With all these topics on power draw I am really surprised to not see any current readings from the PDB. With voltage and current it would be more clear what is drawing all that power.

Re: HELP: Voltage Drop
 

It's at 12.5 V

Re: HELP: Voltage Drop
 

We just checked and its from the drivetrain cims. But the front cims are drawing the most power while the back cims aren't anywhere near that

Re: HELP: Voltage Drop
 

So by 'front CIMs' you mean that in if you look at the top of the robot with 2 CIMs in the ToughBox Mini you see this issue only on the CIMs closest to the front of your robot?

Are the wires to the 'rear CIMs' much longer than the 'front CIMs'?

Re: HELP: Voltage Drop
 

Are both SIMs in each Toughbox pair being driven in the same direction? If they are fighting each other, I wouldn't be surprised to see this. We always run one at a time as a first test to make sure they are going the right way.

Ken

Re: HELP: Voltage Drop
 

With what gear ratio?

Please post the actual numbers, and include what command was being given to the CIMs.

Did you guys do any calculations before selecting wheel diameter and gear ratio?

Re: HELP: Voltage Drop
 

I would say the most likely issue is using 6 pneumatic wheels without a drop center. That's a whole lot of scrub when you try to turn, and that will eat your battery like you wouldn't believe.

Re: HELP: Voltage Drop
 

Is there a troubleshooting flow chart for this?

I mean I can easily find 6 topics on related issues.
Maybe it's time someone makes a troubleshooting flow.

Re: HELP: Voltage Drop
 

This happens almost every year right toward the end of build season.

Maybe the kit of parts should contain a big warning notice on day-glo orange card stock about improper drivetrain gear ratios and oversquare skidsteer bots.

Re: HELP: Voltage Drop
 

Maybe if GDC says those are the only types of robots allowed in a team update enough people will read that joke enough to get the idea not to do it :) Reverse design psychology.

In fairness since it happens over and over probably time to crack out Draw.io and make a flow chart.
At least to save myself the CSA size migraine March 4th.

Re: HELP: Voltage Drop
 

Yes. That's the issue. And the wires to the rear cim is about the same length as the front cims

Re: HELP: Voltage Drop
 

We have tried that and found that they aren't fighting each other.

Re: HELP: Voltage Drop
 

No we haven't done any calculations but the lead member for team 1241 told us this should be fine for our purposes.

Re: HELP: Voltage Drop
 

They should definitely do that. Would help a lot of teams that made the same mistake as us

Re: HELP: Voltage Drop
 

You are not clear if you have 4 CIMs or 2 CIMs. I assume 2 CIMs PER SIDE is your configuration.

Are all 3 wheels & all CIM on each side drive train mechanically coupled (chain, belt, gears) such that rotating one wheel rotates all wheels and all motors? Or do you have a different configuration (e.g. front wheel driven by 1 CIM, back & center driven by another CIM)?

All ratios from motors to wheels (and size of wheels -- but we already know that 8") are also needed to answer questions intelligently.

Dr. Joe J.

Re: HELP: Voltage Drop
 

It is 2 CIMs per side. Each side has the 3 wheels coupled using belts. The gearing ration is 12.75:1. Also something important to note is that since the air tubes popped easily, we have replaced those with pool noodles

Re: HELP: Voltage Drop
 

Okay, here is what I think is going on. You have effectively have really really under inflated tires which is giving you an enormous CoF. When you go over defenses, you NEED your tires to slip with respect to the ground (at least some of them) in order to roll because your tires are actually covering different distances but with such strong connection to the ground, the motors are having to work their electrons off just to make the wheels roll at all.

As to the 9V when you are just on carpet, this is not turning right? If it is turning and you have not lowered the center wheels as you have said, then this is just normal behavior, but if it is going straight, then it could be that your pool noodles are not consistent and as a result they give you different circumferences for each wheel. Again, with such a high CoF, your motors are busy turning electrons into heat just due to these variations in wheel circumferences.

What to do?

Well, put your robot up on blocks and see what the current draw is. It might be that you have losses generally that are causing you to start in a current hole to begin with. Look for tight belts, sticky bearings, mis-aligned axles, something rubbing against the wrong bit of a bearing (you want your wheel's thrust load to go into the inner race of the bearing not the outer race or the shield).

Assuming you can get the off the ground current to something manageable, then it is time to get real about wheels. 6 wheel drive with these tires and no rock from lowering the center wheel is a slow motion train wreck. You have to fix that. Go back to air on the center tire and over inflate that tire, put some tape over the front or rear (or both) wheels) to make them less grippy, switch to another wheel style for the front or back (or both) wheel, figure out SOMEWAY to give yourself some rock in your chassis.

Good luck...

Dr. Joe J.

Re: HELP: Voltage Drop
 

Thanks so much. We will be busy all day fixing this right before bag and tag tomorrow. We are thinking of replacing one of the wheels for a smaller one to get the rock and taping them. But putting the tube back in and inflating is not an option. The tubes we have pop way too easily. We already popped 3 when going over defences

Re: HELP: Voltage Drop
 

What tires are you using? The AndyMark style 200x50mm tire? A LOT of teams are using those tires and there have not been a lot of reports of popped tubes.

How much pressure did you have them inflated to? Maybe it was too high or too low?

Also, there are tubes and there are tubes. Not all are created equal. If you search around a bit (or even ask ChiefDelphi) you can find thicker tubes that will be less susceptible to popping.

Good luck,
Dr. Joe J.

Re: HELP: Voltage Drop
 

No. We are using ones from princess auto. Team 1241 and 1285 had the same problem as us too. I believe they were at 25 psi but I'm not exactly sure

Re: HELP: Voltage Drop
 

Looked around a bit for tires on the Princess Auto website and wasn't able to find 8" pneumatic tires that seem like something you'd be using. Can you share a link to your tire?

Also, can you get us some photos of the hub and tire you are using (get close ups of the tire so we can read the make & model numbers)? Perhaps you can switch to a tougher tire that will fit on the hubs you are using?

Dr. Joe J.

Re: HELP: Voltage Drop
 

http://www.princessauto.com/en/detai...el/A-p8005916e

We have custom hubs made. I will upload a picture later today

Re: HELP: Voltage Drop
 

I am not sure I understand the history here because I don't see how you had popping tubes with this tire but I guess I don't need to.

Here's what I think you might want to do: Take advantage of the fact that not all 8" wheels are actually 8" in diameter to give yourself some rock and also to replace one of the sets of wheels in contact with the carpet with something less grippy.

Path 1:

Buy tires like this one from AndyMark or this one from VexPro and replace the center tires. If these tires are a little bigger in diameter than the Princess Auto tires, then hopefully it will give you enough rock.

Path 2:

Buy Omni Wheels like this one from AM or this one for Vex. Replace the middle wheels if the diameter is a little bigger than the Princess Auto Tires, replace either the front or the back if they are smaller. As to front or back, experiment a little, it depends on where your CG is located and how you climb. You are trying to get only two adjacent Princess Auto Tires to touch the floor at the same time.

Good luck.

Dr. Joe J.

Re: HELP: Voltage Drop
 

You haven't seen their custom hub yet.

Re: HELP: Voltage Drop
 

We'll look into those. Although our mentors are heavly against having omni wheels since they are more prone to breaking while going over defences. We were thinking about having a 6" pneumatic wheel replace the rear wheel. Although we aren't sure if that's a good idea.

Re: HELP: Voltage Drop
 

You are right.

Let me vent a little here and just say that making custom hubs is probably something teams should think long and hard about before deciding to go down that path. There are so many really great wheel/hub options to choose from. Unless you are getting a lot of bang for the buck (broadly construed not actual $s), I just don't think this is the path for 95% of teams out there.

This from a guy who has made robots with many many custom hubs and custom wheels and custom tracks and custom FILL IN THE BLANK. But I'm older and wiser these days. There are so many places to innovate in FIRST. Spend your creativity bucks in areas where it's going to get you significant advantages. For the vast majority of teams that isn't in making custom hubs for 8" wheels.

Calling 'em as I sees 'em.

Dr. Joe J.

Re: HELP: Voltage Drop
 

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?

Re: HELP: Voltage Drop
 

Agreed.

I asked a similar question yesterday, but pujan824 hasn't had a chance to answer it yet:

Re: HELP: Voltage Drop
 

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);

Re: HELP: Voltage Drop
 

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".

Re: HELP: Voltage Drop
 

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

Re: HELP: Voltage Drop
 

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

Re: HELP: Voltage Drop
 

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.

Re: HELP: Voltage Drop
 

Yes we were

Re: HELP: Voltage Drop
 

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.

Re: HELP: Voltage Drop
 

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 ?

Re: HELP: Voltage Drop
 

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

Re: HELP: Voltage Drop
 

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

Re: HELP: Voltage Drop
 

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.

Re: HELP: Voltage Drop
 

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

Re: HELP: Voltage Drop
 

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?

Re: HELP: Voltage Drop
 

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

Re: HELP: Voltage Drop
 

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?

Re: HELP: Voltage Drop
 

Resolve the issue of whether or not the front and rear motors are actually getting the same voltage. Measure it. This is not difficult to do.

Re: HELP: Voltage Drop
 

Yes, they are getting the same voltage

Re: HELP: Voltage Drop
 

Please tell: How did you measure it?

Re: HELP: Voltage Drop
 

I'm going to talk about just one gearbox for now.

Both motor controllers in a gearbox are getting the same control signal according to your code.
Both motors are receiving the same voltage from their respective controllers.
One motor is hot while the other is room temperature.

You can see why this is so difficult to address. The illogical is happening.

At this point, I would have to say there is a mechanical difference between these to motors. Something like: one is using a 12 tooth pinon while the other is using an 11 tooth.

Sorry, without more details, I really am stumped by this issue.

Re: HELP: Voltage Drop
 

That could be it. While practicing on a floor we could run 20 minutes or more until the battery dies and the motors don't get hot at all despite going over some obstacles. I'm stumped on this issue too. This has happened on our robot from last year as well. Unfortunately, our robot is all bagged up and we won't be able to check until our competition

Re: HELP: Voltage Drop
 

pujan,
If you are simply using joystick code to control the motors then you need to calibrate the controllers to be sure they are being fed the current input. If you are sure you are sending the exact same coded input to each motor controller then it is possible you have a mechanical issue in the transmission. This is often a motor misalignment or the keeper that holds on the pinion gear has been pushed against the transmission case. About 100% of the time, the transmission makes noise to tell you something is wrong. The only way to be sure is to disassemble the transmission. I know that sound like bad news, but you have to get to the bottom of your problems and correct all.
While there is nothing you can do right now on the robot, you can plan your work for your competition. Work efficiently and plan exhaustively. I think by now you know you will need a wheel change for at least the outside tires. If nothing else, you could run the current style wheel on a lathe and turn down the outer diameter. That is the same as dropping the center wheels when you think of it. Remember you are able to bring in 30 pounds of fabricated items for your withholding. Ask you close teams for assistance, the veterans may have wheels laying around they could let you have.
Also check all electrical connections when you take the robot out of the bag. You want all #6 wiring tight, secure and insulated. That means checking the hardware on the batteries, the main breaker and the PDP. Ask you LRI for assistance, they have lot's of experience and can usually spot something in a few seconds that you have been scratching your head over for weeks.

Re: HELP: Voltage Drop
 

I will be even bolder and say there is a key missing on one of the CIM gears.

Re: HELP: Voltage Drop
 

Majority of that 30 pounds we can have off is used up by keeping our shooting mechanism off and improving it. I doubt we have enough weight to have anything else

Re: HELP: Voltage Drop
 

Do you have the budget to purchase one or two new gearboxes? Are the gearboxes in your robot accessible such that you can swap them out with new ones?

The Toughbox Mini gearboxes are considered COTS parts. If the gearboxes come assembled, you can bring them in as is. If they come disassembled, you would have to bring them in to your tournament in the disassembled state to count as COTS. If that is the case, make contact, ahead of time, with one of the many veteran teams that will be attending GTRE. Ask if they can provide an experienced team member help your team assemble the gearboxes at the tournament. Preferably, find a team with someone who has direct experience with the gearboxes you are using. It may also be possible (maybe easier) to find a team NOT competing at GTRE to send someone to help you for a day.

Best of luck.

Re: HELP: Voltage Drop
 

Getting to the toughbox is going to be a challenge. It will make us have to disassemble our entire robot just to get to it. Our mentors are skeptical to the problem and do not want to disassemble the robot mainly due to the time it takes to re-assemble it. But we have a couple extra toughbox minis and cims that we know work properly. The only problem we would have is getting to the gearbox and replacing it.