We feature a four bar on our robot. Up until yesterday our four bar had been working at the same capacity it had been for around the last month. However, it has began to give out when raising and lowering the bar near the top and bottom of it’s range.
Our first instinct was to analyze for any friction and discovered nothing. The next step was to unplug the motors and see if each one ran separately. Each motor had the same problem as if both of the motor were plugged in. We also noticed that the moment the single motor began to slow down, the unplugged motor (still connected to the axle) blinked the red light one time. The exact same thing happened on the other side.
We proceeded to take the motors off and put an axle with a gear on the end and turned it manually with a great rate of speed and noticed the light blinked once and the motor began to slow down even though it was not powered. Testing the motors with the brain showed that once they came to speed they began to scream. We have had this problem in the past and had just replaced the motors (but at a point we will just run out of spare motors).
One thing that comes to mind is the Cortex 393 Motor PTC, which sets a limit on the amperage a motor can handle. The V5 11W motors are also the same. As described in their description, “Stall current is limited to 2.5A to keep heat under control without affecting peak power output. Limiting stall current eliminates the need for automatic resetting fuses (PTC devices) in the motor, which can cause unintended motor outages. The 2.5A limit essentially removes the undesirable region of the motor’s performance curve, ensuring users do not unintentionally create stall situations.”
Has anyone had any experience with this issue and can provide us with a way to fix the motors?
If the motor screams a high speed it’s likely a hardware issue.
I have had problems with screaming motors in the past. I found that something was rubbing inside the motor housing. Can’t remember exactly what was rubbing but taking it apart and reseating everything fixed it.
You could be having a completely different problem but it’s worth a shot.
An easier test to do first would be taking out the gearbox and running the motor to see if it’s causing the screaming.
You could also open up a working motor to compare with the broken ones to see if there is any difference.
If this only happens when you take the cartridge out, there is nothing to worry about. Check the wattage drawn when the motor isn’t connected to anything.
The stall current will not affect peak power output.
I am convinced it is your mechanism that is the problem. It sounds like something is binding at the top and bottom of the range of motion. Investigate this, or gear the mechanism down more. A higher gear ratio will give you more torque.
Let’s use what we have at our fingertips to dig into this a bit.
I would want to see data from the motor(s) involved.
Change code, display to brain.
motor temp
motor watts
you can also capture motor peak, average watts with some code
The screaming is the internal gear(s) undergoing precession as well as rotation around the shaft(s). A careful cleaning and regreasing with some vex-approved grease will help this. But once it happens… the damage is usually done (the gear’s hole is microscopically enlarged). If it stops when a gear cartridge is installed… then run with it and don’t worry unless amps/watts are too high.
We have identified the problem to be within the motor. The two potential problems would have been the circuit board or the bearings of the motor. Since the motor still works it is not the circuit board so we have given our motors to our mentor for him to experiment with the bearings.
