Drive stalling


#1

Hello, our robot currently keeps stalling on the drive. We use the cortex system. We used to have a 4m torque drive(100 rpm) but we changed to a 6m torque drive sprocketed for speed (150 rpm) which has the exact same stall torque. For some reason the bot kept stalling out even after we changed to a 6m torque drive(100 rpm). We have checked for friction and made sure everything is smooth. Could someone help us out with this?


#2

Hm, pictures would help, your gear ratio isn’t aggressive enough to where it would burn out.


#3

I can get pictures on Tuesday. I think its weight but last year the bots that we built were of nearly equivalent weight when carrying the mogos which is why this issue is confusing.


#4

I know it seems obvious but check for friction.
Also, and this seems dumb but make sure that all the motors have the same internal gearing (aka high speed ,turbo, tourqe


#5

Try running it with new motor controllers. Whenever we have a problem with motors not behaving we always try that first. Over the past two years, motor controllers have caused our robots to attack us and to drive away.


#6

Do you have the front wheels chained to the back? This causes some friction. Also are you using all omni wheels?


#7
  1. Too much friction
  2. Over-geared <and/or> overweight
  3. Bad motor controller
  4. Bad motor
  5. Chains that are too tight (leads to high friction (see #1)
  6. Other weird stuff (motor not plugged in)

Listed in order of occurrence after watching VEX robots for 12 years.


#8

Do you have your drive hooked up to your Cortex or Power Expander. After over a year of wondering why my students drives would have major stalling issues, we found out that you should never, ever hook your drive up to a power expander.


#9

Using a power expander for drive motors is just fine, just not ALL of the drive motors.

OP didn’t specify if there is an expander on the robot. If there is the best wiring configuration will be to have two motors on each PTC channel. Otherwise split the motors evenly between the two PTCs in the cortex.

Cortex 1-5 is one channel
Cortex 6-10 is one channel
Expander is one channel

@AryanG how is the bot wired?


#10

From your description, I would check to make sure each motor is functional and spinning at the same speed and then check the sprocket chain for friction when on the ground with the weight of the robot pressing down

When two motors are chained and one of them is underperforming, it creates more issues than if there wasn’t even one there


#11

We really can’t change motor controllers as vex has stopped shipping them and we only have around 12 all of which are on the bot.

We do have a power expander specifically for the drive but it is wired on the correct cortex channels to split them between the PTC’s on the cortex.

We have checked through 1,5 and 6 we are strongly suspecting it to be bad motors or MC23’s. We shouldn’t be too overweight as the bot is approximately the same weight as a bot with a mogo on it from itz.


#12

I would talk to teams in your area that have made the switch to v5, they could possibly be able to hook you up with some motor controllers that they no longer need.


#13

You mean that your entire drive is going through the power expander? That would be your issue. It doesn’t matter where the control wires are hooked up because power expander motors do not draw from the cortex; it’s the whole point of the power expander. They instead will load the lone PTC in the power expander and trip it much more easily.


#14

Speaking from experience at the beginning of the season we had a 4 motor high-speed drive and all 4 motors were on the power expander and we never had problems with it burning out due to it all being in the power expander.


#15

Power expanders vary depending on how well they have been taken care of, I had some trip easily and others not trip under a 4 motor load, also battery quality can have a bug big effect.


#16

Our power expander has only 2 of the 6 drive motors on it.


#17

When I hear that you guys have presumably around a 2:3 speed ratio, that’s a bit concerning because normally you don’t want to have a ratio in your chassis. Direct drive or 1:1 chains are enough.

Other than that my second guess would be your chain tensioning or lack thereof. If you have a speed ratio + some standoff tensioning your chain, it’s probably that issue because that standoff creates a lot of friction.


#18

You can also try implementing jpearman’s smartMotor library.

It attempts to predict when the PCTs are going to trip and dials back the motor power. I think you’ll find your robot will need to run quite slow to keep from burning out with the way it’s currently geared.


#19

Pictures will definitely help - when you check for friction, make sure your include the motors - these being misaligned can cause a lot of friction. Check to make sure that you aren’t overloading the motors, and try double checking with different motors and controllers.


#20

We found out that some of the motors were misaligned and the gears in the gearbox were causing a problem. We fixed it now and it works fine so far.