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T200 spinning in one direction only

(Harold Scadden) #1

I am working with a high school group on their MATE ROV entry and we have a T200 that is acting weird. There have been some wiring problems that cleared issues with the other motors on the craft; however, we have a motor that only turns in one direction.

All of the wiring from the ESC is color-to-color etc. and we looked at the incoming PWM signal with an O-Scope and we are seeing the pulse width go from full reverse to full forward and when not driven … it is sitting at the null frequency.

Any ideas of what we could check?

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(Adam) #2

Hi @hscadden,

Brushless motors are very simple machines, and the direction one spins is determined purely by the ESC driving it. If it can spin at all, going either direction is identical as far as the motor is concerned, its up to the ESC. Similarly, the ESC commutation is just reversed for the other direction, all the components are doing the same work, just in a different sequence.

What ESC are the students using? Is it a bidirectional ESC, and is it being driven at the appropriate PWM frequencies to get its full range of motion? Electrically, if the thruster/ESC combination is working in one direction, the only thing limiting it from going in the other direction would be an inappropriate signal or software/firmware limitation.

-Adam

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(Harold Scadden) #3

Adam,

It is one of your latest version ESC’s. We are using a VEX controller and differential signal boards to get the signal down the tether etc. and as I said we can observe the PWM signal and we get the correct pulse width shift from the controller for the action that we are trying to do.

What are the “stupid” things that can happen to make it just run in one direction when you have a correct PWM signal?

Also, do you have a sample waveform of what we should see on the ESC outputs? We saw some funky looking wave that kinda of looked like a squared off sinewave for lack of better description. Whatever you have for info that would help us at least see input and a correct output … then I guess it is back to easter egging parts :frowning:

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(Adam) #4

Harold,

An ESC and thruster combination will run as long as it sees a valid signal instructing it to do so. There’s not much to mess up, short of not providing a valid signal between 1100-1900 μs.

The likely explanation for running in one direction and not the other is not receiving a valid reverse signal. When checking the signal , you should be doing so right at the ESC signal input, as long signal wire runs can cause odd issues. RC style PWM signals are designed to travel a few feet at most, any longer than that and your mileage may vary depending on voltage drop and other factors.

Hypothetically, the ESCs may have had the wrong firmware loaded at the factory and only have a unidirectional firmware loaded. This is incredibly unlikely however, and highly doubt is the case especially if all your ESCs are behaving the same way. We have never seen the wrong firmware loaded in many thousands of ESCs. In any case, if you would like to check the firmware, you can do so using BlHeli Suite and an Arduino. Further instructions and the settings file are located here.

You shouldn’t need to monitor the outputs to confirm the ESC is working right. It will either work because it is receiving a valid signal, or not at all because it is not receiving a valid signal, unless there is hardware issue with the ESC. Since it is running fine in one direction, this cannot be the case.

The PWM signal you are sending to the ESC to command it is just a communications method like any other, its arbitrarily associated with a driving duty cycle just by software operation.

In any case, the outputs will look like this at full throttle:

And with some PWM voltage modulation at lower throttle:

This is standard BLDC six step square commutation, and was taken from a T200 running at load underwater with a 12 V input.

-Adam

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(Harold Scadden) #5

Adam,

Thanks for the waveform post. That is what I was seeing on the scope when the ESC was outputing in the one direction. Weird part is multiple units were swapped in … even from the other motors and still the same thing.

I didn’t have much time with the team yesterday to do any solid troubleshooting and the test equipment was laking. I need to take a closer look and track the PWM signal all the way through from the control box, tether etc. to the ESC.

We found one bad terminal connection that was causing a pair of motors to run with a lot of vibration. It was a screw down terminal and we will end up soldering directly to the board now.

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