help w/ drivetrain

[Jrocklz]

our robot is having issues. When we try driving the wheels dont turn but jerk violently. With much frustration we can get a few laps out but it is not perfoming like it should be. We tried new joysticks, new motors, and even put heat sinks on them. What could be the problem? Is itpossible hat we just dont have enough torque to drive our robot. Could it be programming flaws? If any one could help us out that would be great.

Team 1341
Florida regional

[lukevanoort]

Do you have a backup battery hooked up? If you use main batteries that are a bit low on the juice, not having one can result in the exact behavior you are desribing.

[,4lex S.]

What is your weight, drive motor type and quantity, speed reduction and mass?

It would be easier to determine if you have a mechanical issue if I knew this stuff. However I can't see this being a mechanical issue if it is not readily visible...

[Branden Ghena]

Do you have a four-wheel drive system based on traction wheels?

This sounds a lot like what happened to Team 240 last year. We built the robot with four-wheel drive using four traction wheels, and while our robot was good on the straight paths, it just couldn't turn. We ended up taking off the front wheels for omnis and a two-wheel drive system.

[Alan Anderson]

Originally Posted by Jrocklz

When we try driving the wheels dont turn but jerk violently.

Can you describe your drivetrain?

Something like what you're describing could be caused by having two motors on the same gearbox running in opposite directions. One will occasionally trip its circuit breaker and let the other one move the robot for a while until the breaker resets. Test that possibility by removing all but one breaker to your motors' Victors, and running each motor in isolation to make sure it's turning in the direction you expect.

[Team1341TechMan]

although it's a moot point now that our florida regional is over, our robot had a two wheel drivetrain with caster wheels in the front. We had two large bicycle wheels in the back, each with 1 CIM Motor and 1 AndyMarks Gearbox. When speaking with an engineer from Team 79 (Krunch), he said that our wheels did not have enough torque. We would tether it up and be driving it full speed forward and he could completely stop it by pressing it down. We tried everything from all-or-nothing driving (any push of the joystick results in full speed for the motor) to taping the wheels with electrical tape to reduce traction. Attached is a photo of our robot. This issue was so upsetting because, due to favorable matchups in the qualifying round, we had a real chance of being a top seed. We ended up 5-3 and 16th and not getting picked.

[Mike8519]

Given the diameter of your wheels you needed a higher torque to move your machine, likely adding a 2nd CIM would have solved your issues.

[,4lex S.]

Sounds like a direct drive from a Toughbox... am I not correct?

If you want to grab more torque just increase the reduction with a sprocket setup. A lot of teams seem to be going with a very high speed and it seems to be hurting their acceleration.

[Sam N.]

what battery volatage is your robot controller giving back to your OI?

(it may not seem important but it is important to diagnose IFI errors)

[tennispro9911]

Those look like huge wheels. CIM wheels have 64 oz in torque under "normal load" which is 27 A and 12 V. I've found that at a high current, the battery cannot sustain 12 V even if it is fully charged, so you could use the figure for 40 A, 12 V but I personally doubt you will get that ideal figure.

64 oz in * 12.75 (toughbox speed reduction, torque multiplier) = 816 oz in, although the toughbox is only 80% or 90% effecient so multiply by .8 or .9 depending on your assumptions.

816 oz in * 1lb/ 16 oz = 51 lb in

I don't know the diameter of your wheel but my guess is a 16" bike tire so 8 in radius and 8in lever arm.

51 lb in / 8 in = 6.375 lb force from each wheel/motor. If you had a sprocket drive then you can multiply this number by whatever speed reduction you would use. Of course, you would go slower but thats a small price to pay for being able to turn.

To quote the possibly overused but completely accurate words of an announcer from last year "And this is why we do the math."

So I don't know how much force you need. That depends on the radius that the frictional force is acting on the turning of your robot and at what angle, and at what angle and radius the force of your wheels are acting on the turning of the robot and the frictional force of your caster wheels. I don't even know how you would go about figuring out some of those things, but just a does this number make sense check would tell you that 12.75 lbs from all of your wheels will not sufficiently power a 150 lb robot (15 lb battery and 15 lb bumpers).