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Unread 09-09-2017, 05:07 AM
ProfessorAlekM ProfessorAlekM is offline
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CIM Motor Interference with PWM and CAN?

Hello!

I am planning on moving my roboRIO to the center of my bot. However it will be surrounded by four CIM motors. All of my PWM wires and CAN bus wires will have to get 1.5in close to the CIM motors. Is this a safe move or do the CIM motors create a lot of interference?

Thanks,
Alek
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Unread 09-09-2017, 08:10 AM
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Re: CIM Motor Interference with PWM and CAN?

The strongest magnetic field in a motor is just that, inside, and it is much weaker outside and the farther you go.
Reference image: https://cdn.miniphysics.com/wp-conte...etic-field.jpg

Our team hasn't had issues even when having the wires right on the motor (you may want to watch out for heat issues though, possibly), so you should be fine.
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Unread 09-09-2017, 08:12 AM
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Re: CIM Motor Interference with PWM and CAN?

We've had a similar set up in 2013 (though it was cRIO) with no difficulties. If there is any interference, I would expect it to be from the wires and controller rather than the motor itself, as CIMs are pretty well shielded. Also, the motor controller switching speeds are below 20kHz, so a foot or two of wire isn't going to make a very good antenna, especially if you keep the black and red wires close to each other (or better yet, twist them).

I don't recall reading or hearing about this being a problem.
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Unread 09-09-2017, 09:42 AM
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Re: CIM Motor Interference with PWM and CAN?

Tin foil. Cover everything in tin foil. Protect your RoboRIO from the aliens.

I kid... you should be fine running it pretty close. I'd be more worried about heat than signal degradation but you don't really need to worry about that too much for most FRC applications.
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Unread 09-09-2017, 09:57 PM
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Re: CIM Motor Interference with PWM and CAN?

Thanks for all the help!
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Unread 09-09-2017, 10:13 PM
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Re: CIM Motor Interference with PWM and CAN?

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Originally Posted by marshall View Post
Tin foil. Cover everything in tin foil. Protect your RoboRIO from the aliens.
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Unread 09-10-2017, 01:41 AM
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Re: CIM Motor Interference with PWM and CAN?

We have had wires in similar proximity without any issues.



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Tin foil. Cover everything in tin foil. Protect your RoboRIO from the aliens.
So what you're saying is that when the robots take over it's actually aliens?
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Unread 09-10-2017, 02:19 AM
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Re: CIM Motor Interference with PWM and CAN?

As others have mentioned, you'll be hardpressed to find any sort of interference issues short of wrapping your CIM with a coil of signal wire.

For CAN, the twisted pair is designed specifically to cancel out any outside interference, since the signal is equal but opposite in the pair of wires. Ethernet uses the same twisted pair mechanism.

For PWM, the logic level is between 0-5V, with the trigger level being between 0-0.25V for low and 4.75-5.25V for high. The kind of interference you'll find near CIM motors won't be enough to trigger a change here. You can find these details in the RoboRIO Specifications document. http://www.ni.com/pdf/manuals/374661a.pdf

Finally, be aware that there are still some issues with interference. On page 3 of the RoboRIO User Manual (http://www.ni.com/pdf/manuals/374474a.pdf), it explicitly states: "[the roborio] product is intended for use in industrial locations. However, harmful interference may occur in some installations". It warns that PWM, CAN and other signal lines should be kept under 2m of length. It's not a hard limit, but it is what the roborio is rated for.
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Unread 09-10-2017, 12:43 PM
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Re: CIM Motor Interference with PWM and CAN?

I heard from one team whose power cable (the one that runs into the PDP, I mean) got moved out of place after a fall and ended up close to their Talon SRX's CAN. When trying to accelerate fast from standstill the wire's electromagnetic field killed the CAN. The cable was re-secured and they had no more issues.

Other than that, I would agree with what everybody else has said above... uh, not the tinfoil stuff.
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Unread 09-10-2017, 02:52 PM
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Re: CIM Motor Interference with PWM and CAN?

Aluminum foil is more available. Works almost as well.
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Unread 09-10-2017, 03:49 PM
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Re: CIM Motor Interference with PWM and CAN?

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Aluminum foil is more available. Works almost as well.
Doesn't protect from the aliens though... only the g-men.
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Unread 09-10-2017, 07:01 PM
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Re: CIM Motor Interference with PWM and CAN?

The practice of twisting wires - pairs, triplets, or quads - will minimize both susceptibility as well as radiation.

Note that if you have wires with silicone insulation - e.g. new talons or victors - do not tightly twist the wires; the wires will cut through the insulation!

If you are concerned about susceptibility on PWM cables, you might try making your own twisted PWM cables.
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Unread 09-10-2017, 07:02 PM
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Quote:
Originally Posted by Peter Salisbury View Post
I heard from one team whose power cable (the one that runs into the PDP, I mean) got moved out of place after a fall and ended up close to their Talon SRX's CAN. When trying to accelerate fast from standstill the wire's electromagnetic field killed the CAN. The cable was re-secured and they had no more issues.


I suspect the improvement was because of tightening a loose connection, not because of interference.
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Unread 09-10-2017, 11:06 PM
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Re: CIM Motor Interference with PWM and CAN?

Quote:
Originally Posted by phargo#1018 View Post
The practice of twisting wires - pairs, triplets, or quads - will minimize both susceptibility as well as radiation.

Note that if you have wires with silicone insulation - e.g. new talons or victors - do not tightly twist the wires; the wires will cut through the insulation!

If you are concerned about susceptibility on PWM cables, you might try making your own twisted PWM cables.
Twisted pairs only work for equal but opposite signals. Twisting PWM cables won't do anything notable.
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Unread 09-11-2017, 05:06 AM
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Re: CIM Motor Interference with PWM and CAN?

You might be accurate to state that twisting PWM lines will not provide a significant result. Twisting wires minimizes the loop area between the conductors. Since these conductors are normally run in parallel, there is a loop formed between the conductors. For the PWM lines the concern is susceptability. Twisting the lines will minimize the loop area. In addition, both supply lines form an AC ground; by twisting the lines the supply and return lines form a coarse shield for the PWM command line.
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