Motor Question: High-strength/regular motors

This is a really simple question, but I’ve gotten conflicting answers about it in the past. We have a robot which is having motion problems in the base: after a certain amount of time (about 20 seconds), one of the sides of the robot (the right) dies, and the robot is forced to go in circles. After about 5 seconds of waiting, the side becomes operable again, and we can drive normally for another 20 seconds or so. We believed this to be a power issue, so we moved the two problem motors to the to the power expander, along with the two intake motors, which don’t need to exert much force. This has improved driving significantly, but the right side of the base still dies after about 40 seconds.

So anyway, we’re thinking of moving two high-strength motors from our lift down to the back side of our base, to increase the power on the base. Our lift (a linear-slide fork-lift design) works great, but can you gear high strength and regular-strength motors together? We would need to do this on our lift if we were to go ahead with the base motor replacement.

Thanks, and sorry for the long description!

Sounds like a friction problem to me. Disconnect the motors on the flaky side and spin the wheels by hand. If they don’t spin freely (like 20-30 seconds on one spin) you have a friction problem. An easier way to check is to wiggle the wheels from side to side on their axles. If they don’t move a little, you have a friction problem.

I agree that this could be a friction problem, although we had a similar problem with our wheel base and friction was not the problem. If you add the high strength motors, make sure they are geared for strength instead of speed. If they’re geared for speed, they will run faster than the regular motors and that would be bad.

There are lots of threads with advice for the cut part about drive train death.

Regarding your actual question (with ?):
Yes. if you use the default high-torque (100rpm) gear set in the 393 motor, and not the optional high-speed (160rpm) gear set.

Motors have a integrated resettable thermal fuse. It heats up based on current flow and when it exceeds its internal limit it opens the circuit. Once it has cooled down the circuit is ‘closed’ again. Your symptoms sound like this may be happening. The cut-off limit is intended to be a current limit higher than full power but still moving vs the much higher current from when a motor is stalled. If this is the problem then it seems like you’re putting too much strain/load on the motor; you may need a reduction gear to reduce the strain.

NOTE: the VEX Cortex also has two similar but higher current capacity resettable fuses. One is for motors 1-5 and the other is motors 6-10.

The resettable fuse is probably cutting power due to a combination of friction and a gear ratio that is too high (not enough torque).

If you want a non-mechanical fix, in your code you can limit the torque (the current) of motors by making the maximum voltage supplied a function of the rotational velocity of the motor, so that you aren’t running at full torque (current) ever. You wouldn’t be winning any pushing battles, but it seems like you’re more interested in maneuvering than pushing.

If you’re still having issues this weekend I could look at it.

That would be helpful. We have a 1-1 gear ratio on the base: I think our robot’s too heavy for starters, and we’re only powering it with 4 regular motors, not high-strengths, so it probably is a torque problem

Thanks for all the helpful post so far: we’ve been able to narrow down our problems quite a bit!

Our robot last year was extremely heavy, and we also used 4 regular strength motors with no gear ratio. Our wheel base would stop just like yours, but we fixed this problem by adding the green motor clutches to our wheel motors. Once we did that, the wheel motors never stopped working again. Even with the clutches on, the robot won all kinds of pushing battles.

We had 5 matches at the World Championships without the clutches. 4 of these we lost, and the fifth we were disqualified because we overheated while pinning another robot. Then we had 4 more matches there, this time with the clutches on our wheel motors and we won all 4 of them.