Faraday tape/ copper tape for EMI shielding encoder cable

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rk9268vc
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Faraday tape/ copper tape for EMI shielding encoder cable

Post by rk9268vc »

im having issues with my encoder cable reading values slightly off
basically command spindle servo (NOT vfd) to go a speed, spindle goes exactly that speed, but from the encoder passthrough to acorn it says a slightly different speed (up to 100rpm different)

i know it is the cable and not the encoder because if i move the cable around, it reads differently. (not just wiggling connectors, only moving cable around other cables)

tldr. my encoder cable is picking up EMI/ EMF and i need to shield it better.
it is already a shielded cable with a drain wire, and properly grounded only on one side.

i am thinking of getting something to help double shield the cable. I found copper or nickel conductive tape as an answer
has any one tried this, and is this a good idea, or just snake oil?



thanks
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Re: Faraday tape/ copper tape for EMI shielding encoder cable

Post by eng199 »

I would look for cable with braid and foil shield to improve shielding.
Maybe something like this: https://www.digikey.com/en/products/det ... 0/12147913
The cable will be too big to fit common DE9 plastic hoods. Look for the die cast hoods with a selection of rubber grommets included for different cable diameters.

I have seen the foil tape make an improvement on unshielded flat ribbon cable. I would expect the improvement to be minimal on cable that is already shielded.
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Re: Faraday tape/ copper tape for EMI shielding encoder cable

Post by tblough »

It would also improve noise rejection if the cable was composed of twisted pairs as well. Each pair would go an individual encoder channel.

https://www.mcmaster.com/ethernet-cable ... net-cable/
https://www.digikey.com/en/products/det ... 04/8321935
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I have CDO. It's like OCD, but the letters are where they should be.
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Re: Faraday tape/ copper tape for EMI shielding encoder cable

Post by eng199 »

tblough wrote: Thu Jun 30, 2022 9:55 am It would also improve noise rejection if the cable was composed of twisted pairs as well. Each pair would go an individual encoder channel.
Good point. If the cable is not twisted pair, that should be top of the list of things to upgrade.
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Re: Faraday tape/ copper tape for EMI shielding encoder cable

Post by rk9268vc »

i am using twisted pair foil shielded cable with a drain wire already. i ordered some copper foil tape, ill see how well it works tomorrow
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Re: Faraday tape/ copper tape for EMI shielding encoder cable

Post by rk9268vc »

got the copper foil tape in. It is a 1mil copper on a 1 mil conductive adhesive.
I wrapped the encoder cable in the foil tape and soldered on a drain wire at the end and connected to ground. I then wrapped the cable in gorilla tape so it doesnt short anything.
This improved my encoder inaccuracy by about half. I was getting error of around 120rpm off, now it is about 50. It measures 50 rpm high in fwd, and 50 rpm low in rev
This means i still have some interference, but i ordered some nickel plated tape made specifically for RF interference, so ill try that out

hindsight, i probably should have just ordered some better cable, but, as seen in other threads, i have had great difficulty with this encoder pinout since it is different than acorn's. And now that i got it working i really dont want to screw with it haha
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Re: Faraday tape/ copper tape for EMI shielding encoder cable

Post by Spartan117 »

I just talked with an electronics-pro today about this topic.
He told me in this case (Encoder-cable, differential signals, only 5V level) the twisted-pair is by FAR the most important attribute.
Even more important than the shielding. And for sure, every pair of twisted cables tranfers one channel, e.g. A+ and A-

Second important thing is, there should not be a connection between machine-ground and the shielding.
The shield of the cable should only be connected to one side, the db-connector on the Centroid-board and it's hood.
If the encoder-housing has contact to the machine-ground (if it touches metal-surfaces), it should not be connected to the shield of the cable.

Some japanese manufacturers (like Tamagawa) connect the shield to the encoder-housing, too, but therefor they isolate the encoder from the machine completely. Not that easy, because you need to isolate the mounting AND the axle, but at the same time free of mechanical clearance, even in long-term-usage.
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Re: Faraday tape/ copper tape for EMI shielding encoder cable

Post by Gary Campbell »

Have you used a scope to try and find the origin of the EMI? That's where I would start
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Re: Faraday tape/ copper tape for EMI shielding encoder cable

Post by Muzzer »

Spartan117 wrote: Sat Jul 02, 2022 7:03 pm ....the twisted-pair is by FAR the most important attribute....there should not be a connection between machine-ground and the shielding.
The shield of the cable should only be connected to one side, the db-connector on the Centroid-board and it's hood.
If the encoder-housing has contact to the machine-ground (if it touches metal-surfaces), it should not be connected to the shield of the cable.
It's often claimed there is one and only one method but there are examples of each that work best. Beware of people offering the definitive solution. It really isn't a perfect science in quite the way you may believe. For instance, Yaskawa recommend connecting both ends of the screens on power cables, while other manufacturers recommend only one end. Yaskawa also recommend connecting the control cable shield to the VFD end (not Acorn). Having carried out EMC certification testing of high power VFDs, I can attest that the route to success is to keep all techniques open as options and find out what works on your installation.
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Re: Faraday tape/ copper tape for EMI shielding encoder cable

Post by tblough »

Connecting one end or both of the shield borders on religion for some. Crusades and jihads have been waged on this topic and a gathering of 10 experts will get three different majority decisions.

Personally I connect the shield at the end where the signal originates for control signals (I.e. currents that are affected by noise) and both ends of power leads (I.e. noise generators). Your mileage may vary.

However, as previously mentioned an ounce of prevention is worth a pound of cure. Find the cause of the interference and fix that.
Cheers,

Tom
Confidence is the feeling you have before you fully understand the situation.
I have CDO. It's like OCD, but the letters are where they should be.
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