Drivetrain freezing mid Teleop


#1

We are having problems with our drivetrain. It begins freezing at roughly 45 seconds to the end of the match - every match.

We have a 6x6, 6.25 inch VEX traction wheel configuration with the center dropped 2.5 mm.
2 CIMs on each side powering a 2 CIM ball shifter with total ratios of 7.5:1 and 17.05:1.

Looking at the DS log we see amperage well over 40 AMPs for short peaks. Eventually we see one side or both crashing for ~6 seconds then coming back.

We suspect that we are tripping the 40 AMP breakers.

Other ideas of what may cause this?


#2

I assume you are at a competition. What you can do is take the bot to the practice field and drive around and check the drivetrain. Often in matches you cannot hear your robot which makes it hard to tell if something is mechanically wrong like chain jumping teeth. Usually when something is seizing the current draw on accel will be abnormally high.


#3

That does sound like tripping breakers. Your high gear is very agressive for a 4-CIM drivetrain.


#4

Maybe a little aggressive, but my back of the napkin calculations say that should be about 20fps free speed in high gear (low gear is about 8fps), meaning something else might be a problem as well. It could be that the OP is trying to accelerate from rest in high gear, which would explain the current issue. Be sure you are in low gear when you accelerate (i.e. make sure low gear is the default in code).

I just ran it through the JVN calculator and saw that you would be pulling about 35 amps accelerating from rest in low gear, but almost 80 if doing that from high gear. (150lbs for the robot was my estimate there-battery and bumpers included)

If you are in low gear when this issue occurs, I would look for a mechanical issue perhaps, just like @JacobD suggested.

If none of the above reasons are the cause, see if you have either current limiting or voltage ramping.

One other thing (and the one that my instinct says is probably the culprit) that may be the cause is if your center wheel has become too worn down from practicing and competing in matches. Your center drop (2.5mm) was about 3/32", which is a little low for the larger wheels, but that shouldn’t be a problem. That is, unless the center wheel has worn down some, meaning there is a lot of scrub when turning. Check to see if turning is when you pull the extra high current. I have seen more than one time where teams with vex traction wheels choose to swap them out between comps because of wear.

Good luck in solving this,
Tomithy


#5

7.5:1 is a bit fast. I would recommend a smaller wheel or different set of high speed gears.

David


#6

Maybe a little aggressive, but my back of the napkin calculations say that should be about 20fps free speed in high gear (low gear is about 8fps), meaning something else might be a problem as well.

We used JVN’s calculator and got more or less the same numbers.

It could be that the OP is trying to accelerate from rest in high gear, which would explain the current issue. Be sure you are in low gear when you accelerate (i.e. make sure low gear is the default in code).

This might actually be the case. We have high gear as default.
Where do you get the current draw for acceleration? Are you considering it as some percentage of the “push current”?

I have seen more than one time where teams with vex traction wheels choose to swap them out between comps because of wear.

We replaced all 6 wheels between day 1 and 2 of the district event we competed in last week. Didn’t seem to make things better. We are also considering replacing the tread.

@JacobD We are not currently on competition and the robot is bagged. The issue I described happened at a district event last week.

Finally, as for @D.Allred’s recommendation, sadly we can’t change to smaller wheels since we have a clearance problem when driving on the HAB. Changing the gearing is an option but we are trying to find other possible causes before doing so.

Thanks for the help guys!


#7

Unless there is current limiting or voltage ramping, the drivetrain will pull almost all of pushing current, albeit only for a split second, while accelerating from rest at full throttle. I always treat it as 100% of pushing current when doing calculations, but in reality it is probably a tad less, like maybe 80-90%, but this is still a substantial amount of current.

This leaves three options:

  1. Make low gear default, and have a button work as a toggle switch between the two ratios. Since this game is only really played offensively in a confined area of the field, you will never really reach top speed. You’ll probably find that using low gear all the time has no negative impact on cycle time whatsoever.
  2. Limit the current at the motors somehow (this could be as simple as a throttle, which is what we do, or it could be current limiting for the Talon SRX, which I assume is the motor controller you are using)
  3. Both of the above, or something I haven’t thought of

Edit: forgot to post this as a reply so it would appear when you sign in, so I will add @Yalib so that it does