HELP: Voltage Drop

[pujan824]

Originally Posted by Ether

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.

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

[Joe Johnson]

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.

[pujan824]

Originally Posted by Joe Johnson

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

[Joe Johnson]

Originally Posted by pujan824

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.

[pujan824]

Originally Posted by Joe Johnson

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

[Joe Johnson]

Originally Posted by pujan824

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

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.

[pujan824]

Originally Posted by Joe Johnson

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.

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

[Joe Johnson]

Originally Posted by pujan824

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

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.

[pujan824]

Originally Posted by Joe Johnson

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.

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

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

[Joe Johnson]

Originally Posted by pujan824

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

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

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.

[Ether]

Originally Posted by pujan824

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

Originally Posted by Joe Johnson

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.

You haven't seen their custom hub yet.

[pujan824]

Originally Posted by Joe Johnson

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.

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.

[Joe Johnson]

Originally Posted by Ether

You haven't seen their custom hub yet.

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.

[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?

[Ether]

Originally Posted by billbo911

Now what bothers me with the OP's issue is how two motors on the same transmission could have such a huge temperature difference.

Agreed.

Are you controlling the ESC's via "Y" cables, or discrete PWM outputs from the RoboRio?

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

Originally Posted by Ether

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