[FTC]: Tetrix motor controller weirdness with 4 motors

[aklego]

Greetings All,

My team and I just spent the day chasing a hardware bug that we have not seen before, and we are hopeful that someone can shed some light. We have a skid steer setup with two motors on each side, and all four motors are controlled by a single controller. Motors on each side are mechanically coupled -- a fairly standard set-up. The robot is lightweight, the motors are lightly loaded, and encoders are not used.

After a brief but variable bit of driving, the drive motor controller and every controller further down the daisy-chain would stop responding. The NXT and upstream controllers were unaffected. A power cycle of the Tetrix battery was required to bring everything back to life. We swapped controllers, cables, and changed positions in the dasiy-chain to no avail. We also noted that the fault would occur with a motor direction change, and we were able to get it to fault while the robot was elevated and just spinning its wheels. It was not a loose wire or the like, and it was not software.

Eventually, we unplugged a motor on each side and the problem vanished. We plugged them back in and removed plugs from the other two motors, and again the problem was gone. When all four motors were plugged back in, the problem would return. Our (hopefully temporary) solution was to add an additional controller and split the drive motors between the two controllers.

One thought (and I have no idea as to its validity): The electrical weak link in the old Tetrix motors was an internal inductor that provided some filtering. The new motors feature improved burnout protection, and they are what we are using. Is it possible that the inductor has been removed/changed and that there is a spike in noise when the motors change direction? If so, it seems the controllers are no longer able to handle the noise of two motors on the same channel. Seems pretty far-fetched, but we are at a loss for a definitive explanation.

Any thoughts, or has anyone encountered similar problems?

[DonRotolo]

Absolute speculation: Monitor the battery when this happens, low voltage can do weird things. If the battery is getting on in years, its capacity may be diminshed.

[Ian Curtis]

How did you verify it wasn't a software problem if a hardware swap didn't fix it?

[aklego]

Originally Posted by Ian Curtis

How did you verify it wasn't a software problem if a hardware swap didn't fix it?

The software has no way of knowing or sensing the number of motors attached to a channel. If it was a software problem, the faults would still occur regardless of load.

Once it was faulted, the problem would remain regardless of program running (or not running) on the nxt.

[Al Skierkiewicz]

ak,
This sounds so much like an over current issue with four motors running. When you add up the motors, the reversing as a repeatable cause and reboot to clear the problem, it seems like everything is pointing to the motor loads dragging the battery voltage down. The inductor used in the previous motors was under rated for current such that repeated stall current would eventually burn open the inductor. This does not sound like RF interference as that would produce more problems and generally be not repeatable. RF is at it's highest while the motor is running wide open or under heavy load.

[aklego]

Originally Posted by Al Skierkiewicz

ak,
This sounds so much like an over current issue with four motors running. When you add up the motors, the reversing as a repeatable cause and reboot to clear the problem, it seems like everything is pointing to the motor loads dragging the battery voltage down. The inductor used in the previous motors was under rated for current such that repeated stall current would eventually burn open the inductor. This does not sound like RF interference as that would produce more problems and generally be not repeatable. RF is at it's highest while the motor is running wide open or under heavy load.

I would agree except that it would do it when the motors were unloaded, i.e the robot was up on blocks just spinning it's wheels. What is completely puzzling to me is that this is a pretty standard FTC configuration.

But I would also agree that my explanation, (RF issues) is unlikely.

Not knowing the root cause puts our "fix" in the realm of voodoo and that is particularly unsatisfying.

[Andrew Rudolph]

I would pull the motors out of their mounts and make sure they are all spinning the correct way. As Al said, it sounds like an over current issue. I have seen it where one of the motors on a side is shorting, putting into a break mode, so it causes the other motors to work harder to overcome the motor that's not spinning.

Otherwise I would rewire everything and make sure there are no shorts.

I would advise not daisy chaining the electronics on the robot, I have all my teams use Anderson Power Poles or Wago Levernuts, so if there is a wiring issue, everything down line doesn't go down. Especially since the voltage sensitive Samantha module is always the end of the line in the daisy chain wiring scheme.

[Al Skierkiewicz]

ak,
I went back and read over your post again. Am I correct that you have one controller feeding all four motors? Is that one controller output or two motors on one output and two on the other output? The motor controller is intended to power only one Tetrix motor per output. The controller has over current sense and will shut down if the current is too high or the temperature climbs above a certain point. If the controller is shutting down for a fault, it may fail to pass I2C signals on to the following devices. The specification sheet lists the current output for each motor circuit at 4 amps which is below the stall current for the motors.

[orangemoore]

I do have a question,

What is the purpose of running four motors off one controller?
There is no limitations to how many DC motor controllers you may have, the limitation is being only allowed to have 8 motors.

[CougarRobot]

http://ftcforum.usfirst.org/showthre...ull=1#post7575

2 motors per port has always worked just fine and is allowed. However, we have not yet done it with the new motors.

Is anyone else using 2 motors per port with the new motors?

[Scott_4140]

4 motors on one controller worked fine for us with the new motors at our first Qualifier.

Don't confuse FRC and FTC. There is nothing in the FTC rules preventing 4 motors on one controller.

Also, from the LEGO Education site:

http://www.legoeducation.us/eng/prod...or_tetrix/1648

The HiTechnic DC Motor Controller for TETRIX® connects to an NXT Intelligent Brick sensor port and will enable you to control powerful DC gear motors for use with TETRIX robots. The controller has two H-bridge outputs to control the speed and direction of two DC gear motors and is designed to connect to the TETRIX hole pattern. A four-motor robot can also be built by paralleling two motors to one channel. A total of four DC Motor Controllers for TETRIX and/or Servo Controllers for TETRIX can be daisy-chained to provide additional motor and/or servo outputs on each port.

--Scott

[aklego]

Originally Posted by Scott_4140

4 motors on one controller worked fine for us with the new motors at our first Qualifier.

...

This is good to know although I'll admit that I was hoping other teams w/a similar setup could reproduce the problem.

We have our qualifier this weekend that we'll run with our current setup and then return to troubleshooting after the event.

Lots of suggestions of an over-current fault but that seems inconsistent with the applied load. When we return to troubleshooting I'll instrument the controller and see what our current actually is.

[Al Skierkiewicz]

ak,
I received some info that the I2C passes through the controller in the event of a fault. So if you are having issues down stream from a particular controller, that device could have an issue or you could have a defective cable. However, there is still the off chance that one of the drive motors has excessive friction causing some high motor currents. You need to isolate the motors and see if you can find that issue as well. You mentioned in your first post that the motors are mechanically coupled, so it is possible that one of the motor issues is being coupled into the other. If you remove one electrically the mechanical problem still exists. Good luck this weekend.

[PhilBot]

Originally Posted by aklego

I would agree except that it would do it when the motors were unloaded, i.e the robot was up on blocks just spinning it's wheels. What is completely puzzling to me is that this is a pretty standard FTC configuration..

My experience is that rapidly reversing motors causes worse than Stall-like power consumption. Instead of the Back EMF apposing current flow (like in normal operation), when you rapidly reverse, it augment the applied voltage with no way for the motor to instantly reverse rotation, so for a moment you have twice the voltage applied to the pure resistance of the coils. So this action can invoke stall-like issues in motor controllers (I've seen it blow out a motor controller chip on another system, with no load on the motors).

So, continue down the overload diagnostic path.

I'd consider swapping out that motor controller. It's possible that it has a partial failure that prevents if from dealing with a full over-load.

Phil.

[wilsonmw04]

Are you making your two motors per channel connection in the controller itself? IF so, having eight wires in such a crammed space can be your problem. It does sound like excessive current. If the motors are not under load, and it's still happening, make sure you are not shorting the wires right at the controller. We have had that happen several times.