We’re having power draw issues and we have no idea what’s causing them.
Our drive train is four cims, two tough box minis, and belts driving six 8" harbor freight tires. We’re having this weird issue where we’re drawing more power than we seem to be using, so much that the Rio kinda shuts itself down and driving becomes erratic and slow to respond.
We don’t think it’s a code issue, and we’ve replaced the cims, disassembled and reassembled the gearboxes, and checked the talon srx’s we have on the cims multiple times. Someone brought up the idea that our wires were too small of a gauge, but we think they would be melting if that were the issue.
We have been having this problem since a little after we started driving last week. Any ideas would really be appreciated…
The problem, is that you are using wheels that are too large and grippy for the drivetrain you have connected to them. It’s kind of late in the build season to fix this…but suggestions would be to see if you can adjust the height of one pair of end wheels to make it sit up higher than the others, or maybe replace one set of end wheels with something a little smaller, or with less traction.
The basic problem of having not enough gear reduction ratio in the transmissions is harder to fix. If you can get pulleys (or chain sprockets) with different sizes, then you could get more speed reduction (an torque increase) and it would help.
without pictures of your robot, to see how it’s built, it’s not easy to make useful suggestions.
For a torque increase you can swap the standard 10.71:1 ratio in the toughbox mini to a 12.75:1 ratio using a different gear set (if you haven’t done it already). If you have the old toughboxes you can use those gears inside the toughbox mini since they are interchangeable and are geared to 12.75:1
In addition you can swap to a 13 tooth pinion instead of a 14 tooth pinion to get additional torque. I made these recommendations to another team this season.
13 Tooth Pinions: Switch to 13 Tooth pinion gears $8 each, $32 total. Slows down your drive train but gives you more power for climbing, etc, makes your robot more controllable. http://www.vexrobotics.com/vexpro/motion/vexpro-gears/cim-motor-gears.html
Reduce 2nd Stage: If you are using 8” wheels, Switch the 2nd stage of your drivetrain to 14 to 50. You could also use the VEXpro 14 tooth ⅜ hex gear and the 50 tooth ½” hex gear but you would need to mess around with spacing inside the gearbox. $42 Total
Why would adjusting the height work? I was wondering because that’s kinda out of the question, and I want to see if there’s something else I could to to achieve that end.
Adjusting the height reduce the scrub effect of trying to turn with 3 wheels on the ground. Pneumatics really need a larger drop center than the stock chassis has.
With time so short the real question is what parts do you have on hand? Are you using the AM14U3 chassis?
In addition to telling us how much your center wheel is lowered (you really need to be able to see that front wheels off the floor when your rear wheels are on the floor and vice versa). I am still not clear what your ratio is from the CIM to the Wheels, specifically what is the ratio from the Toughbox to the wheels.
Have you used JVN’s spreadsheet to figure out your top speeds and pushing current? If not, you really should make this a standard thing you do going forward.
If you are using chain*, you have some ABS sheets around and access to an Epilog laser cutter, you can make new wheel sprockets in an afternoon. With the exception of really small sprockets (<20 teeth or so), I’ve used nothing but ABS sprockets for years now.
Back to the power draw issues, in the short term you can try adding a little extra air to the middle wheel to increases the rock. You can also do things to reduce the grippiness of the front and back tires (test it with some duct tape).
Dr. Joe J.
*I know that a lot of smart folk really like belts but I am just not there yet because of the freedom to change sprockets without needing new belts. After seeing so many belt drive lifts last year, I think I am sold when it comes to lifts but for drives, I still like chain.
Over inflate your middle wheels and under inflate your outer ones. This alone won’t be enough though, and you may need to wrap your outer wheels in tape - something slippery.
We are using a custom chassis, basically two boxes assembled from 1/8 aluminum with a lexan belly pan in between, reinforced with 1x2 tubing and churros. We have a lot of stock metal, plastic, and wood on hand, and access to a professional metalworking facility.
Also, all of our wheels touch the ground at once, we’re using belts and 3d printed pulleys, and we have a 10.71:1 gear ratio.
We where having verry simmilar issues with our 6wd 4 cim harbor freight wheeled drivetrain geared for 20:1 with a 3/8 wheel drop. The problem was rooted back to poor structural regidity. By simply adding 2 pieces of churo tubbing between the gearboxes and a piece of c channel in the front (standing in for our intake) we fixed the problem.
Yeah, but look at your gear ratio and center drop:
…*** that’s*** why it works.
You could simulate a center drop by inflating your outer wheels less than the center wheel. It’s not ideal but it’s something you can do without having to buy any new parts.
Our team is also having some power issues, the roborio will restart very often, especially when we try turning on carpet, or going over defenses. Our team is wondering if battery age could be a significant issue. The lead acid batteries we were using today are 3 years old, and apparently weren’t taken care of properly the past 3 years (according to an electrical mentor). We’re getting some new batteries, and hopefully that will relieve some of the issues.
Something interesting we noted:
With no load(weight) on the tires decreasing the air pressure can actually increase the diameter. It seems that as we add air pressure, the side walls of the tire bulge out more decreasing the diameter.
- Bochek
We had the same problem, and fixed it by adding some gorilla tape covering our rear wheels (it’s very back heavy). We tried a few kinds of tape, and that worked the best.
We had the same situation and the same problem (two toughboxes, 4 CIMS) and fixed it by resembling the tough boxes with a higher gear ratio. Instead of using the originals that came with it, we went up to the 12.75:1 ratio and that fixed it.