Fixing Motor Lock?

I remember reading in the past that a good indicator for broken motors was to check if they can spin in both directions unpowered.

Recently i replaced two 3 wire motors with two 269 motors and a member noticed that it could spin Clockwise(looking at the screw slots), but not counterclockwise. I eventually tested it with code the 3 wire motor used and noticed one side does not run.
the strange part is that it’s a new motor. Though it was purchased about 8 months ago, it hasn’t been used. I checked the remaining two 269 motors i purchased with the 269 motor and noticed that the other 2 had the same motor lock problem. However, i have a feeling the working 269 motor is spinning slower than usual motors. Batteries were fully charged during the tests and the motors have been opened up prior to the tests. We determined that there are no faults with the internal gears. However, when manually turning the motors, we noticed that the noise is very loud compared to 393s or 3 wires. We assumed that the wires to the robot aren’t faulty because the 3 wire motors spin and that the 269 motor has no visible damage to the head, prongs, cable, or internal electronics. So…

  1. What other issues may we be overlooking?
  2. What causes motor lock?
  3. Can motor lock be fixed?
  4. Is the working 269 motor near the end of it’s lifespan?

Edit: Just realized… this maybe better in the technical discussion

Check your motor controllers.

Also check the port, I had a port on my cortex go bad and have those same symptoms.

I had an IME internal shaft come out of the hole it goes in… I couldn’t turn it by hand but the motor would run…

The teams at our school have also been having this problem, but we cannot seem to find a cause for the lock or a solution to it. Some guesses included a bad batch of 269s, but that seems rather unlikely.
The motors respond to programmed instructions and driver control and whatnot, but simply lock up when they are turned manually. :confused:


mm… yeah i should check that at tomorrow’s meeting… The motors did run for a fraction of a second. But we disregarded it since we tried running it for like 5 minutes

I don’t think it’s the cortex port cause it works with the 3 wire motors.

We don’t use IMEs so thats not a problem :confused: we also checked the axles that ran through the gears and there was very little resistance

Hm… Comparing this situation to my situation it may prove to really be a motor controller problem. The motor did run for like a fraction of a second a few times. But in a time span of like 5 minutes, we disregarded it

Most 269 motors do not have the capability to be turned manually; they just lock up. Some of them will turn manually one direction, and some will be able to to turn if you rotate the shaft back and forth without going past the ‘locking’ limits, but most of the time, they simply will lock. However, they should always be working electronically. Test with the backup battery; it should work.

Last year, my team’s robot arm would not move manually because of this. We did have to turn the robot on just to move the arm up/down for construction, but we turned this around. We used the motor locking to our advantage: it kept the arm up without power. The internal gears on the 269s did break a few teeth at times, which was annoying when 1/2 of the arm would lift, but the other wouldn’t, but we dealt with it.

However, I would not suggest using 269 motors. With 393 motors, you can get higher torque, stronger motor integrity, and increased capabilities (strength vs speed) with them, while you cannot do that with a 269.

(For those of you worried about breakers tripping, use a power expander)

Hm… i should test that out the remaining two motors then.

Unfortunately my team is spread out very thinly and we often have funding issues. These 269s were my personal set and according to the specs, they should be better than 3 wires. Next year we might have a full set of 393s.

Perfectly understandable. 269s are stronger than 3-wires for sure, but they do require motor controllers which can easily go bad (I helped a couple teams that had motors slowly rising without being told to). 3-wire motors also do not stall like 2-wires do, but that’s why they ‘require’ clutches.

Since it seems like the problems seem to fall towards motor controllers, what are the other symptoms of bad motor controllers?

From what I’ve seen, symptoms are either

  1. Motor refuses to turn, but still operates when tested directly (e.g. testing using the 9V backup battery)
  2. Motor slowly turns in one direction. Motor still reacts to signals, but after signals stop, motor still rotates.

There could be others I haven’t seen.

Both untested motors with motor lock was functional. However, the motor which was not functional last meeting ran today with no issues(even with the old motor controller). I have no clue what happened last friday, but i hope it doesn’t happen again.

I have seen other threads that discuss the inability of some motors to be manually turned, but I think the cause of the motors to stop moving under their own power was unrelated. That symptom wasn’t mentioned in the previous threads.

I wonder whether motor lock could be an interesting drivetrain feature? Other teams would be unable to push you if stopped sending power to your drive.

It might work; like I mentioned above before, we used that as a sort of arm lock during Gateway, though the motors were much more prone to teeth chipping off.

Problem is that your motors’ internal gears would break teeth. I don’t think a motor locking up is ever really desirable. Another reason why I don’t really like worm gear brakes.

I suppose that’s true. Plus, it would be impractical to hunt down 4-6 motors that can’t be turned, since it seems to happen somewhat randomly.

My understanding is that:
-A MC#29 motor controller contains a tiny PIC cpu and an H-bridge.
-The PIC takes an RC servo PWM signal and converts it into 4 PWM signals for the H-bridge switches at 0-100% duty cycle.
-The H-bridge switches connect the battery to the motor terminals, either forwards or backwards.

Its always better to test parts in isolation first.
Take a known good CPU and motor, and connect them with a MC29,
show that it can drive the motor forward and backward; that shows that all 4 switches(FET transistors) in the Hbridge can turn on and off.

Symptoms of a bad MC#29:

  • fire; if it is on fire, or looks like it has been in a fire, the motor terminals were probably shorted, and its now burned up inside. Ask Vex if they want them back.
  • known good setup as above fails one direction, but not the other.
    (half of hbridge is broken)
  • known good setup as above fails anything else

Symptoms of failures elsewhere:

  • motor hums or moves when set to not move
    (this is most likely to be bad joystick calibration, or bad programming)