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.

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

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.

It’s at 12.5 V

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

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’?

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.


With what gear ratio?

Our drivetrain does not have the center wheel dropped.

We also have noticed the front CIMs on our drivetrain getting really hot.

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

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?

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.

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.

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.

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 :slight_smile: Reverse design psychology.

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

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

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

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

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

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.

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

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.

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