Problem with Robot Drive...

Hello all,
My team is currently facing a problem with the drive stopping after the robot is ran for a certain time (~1 minute). Everything is tightly secured - the metals, motors and standoffs…etc. However we came up with multiple reasons why the robot is acting like this:

  • Drawing too much current from the microcontroller
  • Motors are heating up/cannot handle the stress
  • Defective motors

We haven’t placed weight as a factor because we have tested the robot running on the ground and off from a surface; and this wasn’t the first time we’ve encountered this problem but this also happened during a competition where we actually assembled a robot. Right now, it is currently a chassis and the problem still remains…

The only change we made from the first design was that we swapped out two of the four 393 motors (for drive) for 269 motors, in favor of trying to draw less power; but the problem still occurs. Currently, we’re powering a 60-teeth HS gear onto a 36-teeth HS gear, in favor of a little more speed. Battery was in the higher-range of 7 volts so didn’t believe low battery was a problem.

Needs help :eek:

Hey, we’ve had problems similar to this earlier in the year. What we found is that simply put, the motors aren’t powerful enough to drive your robot on that ratio. We found that even though our robots drove fine for >10 minutes before going to tournaments, they moment they were on the foam tiles, they froze in around 15-30 seconds.

You can fix this problem by:
-Making a lighter robot (aluminium)
-Adding more motors to the drive
-Tone down the ratio to 1:1 and see if it helps.

I’m not sure what is technically happening. It may be that it is drawing too much current, or the motors may be overheating. Either way, the only way we found to fix it was to add more motors and build a lighter robot.

Our robot is currently reasonably light (I don’t know exactly how heavy) and we are driving with 4 393 motors geared 2:1 for speed. Even with this, after around two minutes of driving backwards and forwards with sudden jerks, the drive motors shut off, however this is an extreme situation and doesn’t happen in game, so it’s alright. If I were you, I would try and do a 4 motor drive, depends on the weight of your robot if you will need 393’s on the drive or not at that ratio.

Hope this helped,

You can solve this problem by
-Check if the delrin bearing are loose. Make sure its free rolling.
-Checking all motors are of equal speed/str .

I would suggest you to have 4 393 motor in hi-speed mode which will give you 1.6x speed .

Something I’ve heard before that might help with the motors drawing too much current and tripping breakers in the cortex is to split the motor ports out on different sides of the cpu’s 8 motor outputs. I believe this works because there are two separate circuit breakers along those ports, and if half of your drive is on each, it is less likely that in a high load situation that you will trip the breakers. Someone posted this before, but I do not recall which ports have a separate breaker. Or, if I am totally wrong about the breakers in the cortex, you could always add a power expander and split some of your drive motors to that.

I agree with Edward, it is almost necessary to make sure to split the motors that are drawing the most current (the drive train) across the two breakers in the cortex. The first breaker is on ports 1-5 while the second breaker is on ports 6-10. Additionally, you can look to add a power expander for another breaker that will help to lighten the load.

I also agree that your ratio is a little bit too high. Looking for a 1:1 might be better.

Unfortunately, I don’t believe weight is a factor in this problem since we tested the robot above the ground (so the wheels aren’t touching anything); and we have previously used 4 HS motors in our old design - but the same problem still occurred.

But I will take in consideration of gearing down because the ratio haven’t changed at all.

We tried that previously by splitting it with one side of the motors on the first breaker and the other side on the second breaker - to no avail, it only gave us an extra 30 seconds of run time (which comes to a total run time of 1 minute). Even tried it with a power expander also, but same problem occurs.

Ok, it looks like you have a different problem to us.
You can try and check for excessive friction by removing the motor(s) and see if the wheel spins entirely freely or not. If it doesn’t, consider reassembling your whole drive and see if the problem is fixed.

If this problem does actually happen when the robot is OFF the ground I am really confused… Have you tried attaching other motors to the robot? If so, do all the motors stop, or just the drive motors? Do both drive motors stop at the same time?

if you are using HS chain, maybe they are a bit too tight?

pictures would be nice for us to troubleshoot the problem

Friction is my first thought.

There’s a lot of experienced VEX builders in this thread…

We ran the drive motors continuously OFF the ground until we have seen a sign that they are shut off - which is after running it for a good minute. The other motors (like for the lift and manipulator) still works, it’s just the drive motors.

For further info, the drive is being run by HS gears mainly; and usually one side (mainly the left side) would gradually to die out, then the other side.

Ok, If I were you I would take the motors off. This way you can:

-Try and run the motors with NO LOAD on them, and see if they cut off. If they do, it is likely the motors are broken/faulty. If so, replace the motors. If they don’t cut out:

-Check to see if the wheel is spinning completely freely or not. A completely free spinning wheel should be able to continue spinning for around 20 seconds with a good spin (off the ground). If it is not, disassemble the drive chain, and reassemble it, this time paying attention to whether the wheels are spinning freely, as ANY friction is likely to cause the motors to seize up. (I hope that made sense)

Also, as a side note, I would recommend using high strength chain to build your drive chain instead of gears (if you have any). If I recall correctly, chain is more power efficient than gears, and also gives you more freedom on where you mount your motors. From what I have seen, most competitive teams use chain drives. Then again, whatever works for you is fine.

The disadvantage to chain is that there is a tendency to make it so tight that it introduces friction in the drive train. My preference is to use chain only when it is necessary for some other reason, such as freedom to mount the drive motors far from the drive wheels. The small theoretical efficiency advantage of chain over spur gears is not really relevant in the VEX world, especially since I don’t know if the pin-jointed VEX chains are as efficient as metal roller chain.

If you DO use chain in your drive train, make sure it is loose enough not to introduce additional friction on the driving axles.

Yeah, I agree. A little slack, but not enough to allow slip, is ideal with high strength chain.
Gears are sometimes better because the vex metal seems to be specifically made to make sure the gears mesh but dont add additional friction, which is good.

I’ll give it a try with chain then. Thanks guy!

Going off of the possibility of a friction issue others have pointed out…
Make sure ur axle is supported by atleast 2 sleeves, otherwise the axle might grind against the metal holes and slowly enlarge the hole. And ofcourse it could also be causing the motors to stop.