Back to topEmily Blanchard
Shielding Cables: Eliminating Electromagnetic Interference (EMI) in Motion Systems
12 Aug 2019
If you experience some unintended motion or unexplained drive faults in your motion system, there are several possible causes. For example, motion setup through the drive, a poorly designed program within the control, and motor or feedback commutation phasing 180 degrees out can all cause unintended motion. For this review, we’re going to look at issues created by the contribution of poorly chosen and installed motor cables.
In the overall world of EMI (Electromagnetic Interface), we have some rules we have to play within. The FCC class B rules, also known as the EMC (Electromagnetic Compatibility) rules, forces us to keep the EMI noise that we generate under a certain amplitude within this range. Controllers, drives, and motors together, along with other machine parts, must have minimal EMI emissions and low acceptability. Machines must also show lack of acceptability to outside sources of EMI within the 30-300-megahertz range as well. Because it’s impossible to have an EMI Free environment, a system (drive, cable, motor) must function correctly while being radiated with specific levels of EMI. Part of this resistance is in the product design. Another part is in the integration of the system.
Why do we care about EMI regarding cables though? Because a cable to electro-magnetic frequencies looks a lot like an antenna. Antennas often consist of long metal extensions that enable it to catch and transmit radio waves. Coincidentally, cables are made up of long, thin, metal extensions so, the longer your cable is in your system, the more likely you are to catch a wave. This is one of the reasons to limit the lengths of cables.
If we allow too much signal to noise ratio into the system, the machine will not work as expected. You may see unexpected movement such as, not stopping on position, moving when the machine isn’t supposed to move, or the opposite, stopping when it isn’t supposed to stop. You could also see infrequent faulting of the drive for feedback issues or current loop issues.
Use cables with shielding and twisted pairs. Shielding around a cable is to protect the cable from EMI generated from other areas of the motion system. Utilizing a magnetic shield to prevent radiation from entering and exiting the cable is also highly recommended. (the minimum shield requirement for Kollmorgen motor and drive systems is 85%, Kollmorgen cables are rated at 90%)
Of course, shields do not work if they are not bonded and grounded at both ends.
If you are experiencing system issues, make sure to include inspecting the cables in your troubleshooting checklist. In truth, this might be the easiest troubleshooting aspect, so you might as well go ahead and review. But what are you looking for?
First, check the cable’s connection to the drive. A 360-degree clamp should be around an area of the cable where the outer jacket has been removed and the clamp is directly in contact with the shield. The clamp should be connected to the grounding plate attached to the drive, and, hopefully this is obvious- but the grounding plate should be grounded. Having said that, this might be the trickiest thing to check. Often the grounding plate is grounded to the cabinet. As long as the cabinet grounding connection is touching metal, it should be grounded. However, if there is paint on the cabinet at the grounding point, it’s not grounded. If the cabinet is anodized, it’s probably not fully grounded. If the plate looks like a regular metal plate but you can scratch off a clear coat- it’s not fully grounded. If your grounding plate isn’t properly grounded, then the items you have attached to it aren’t properly grounded. Meaning, the cable wires inside the shields are vulnerable.
Please keep in mind that a ground for high frequency EMI is not the same as a ground required by electrical codes to prevent fires or electrocution. High frequency signals tend to conduct on the skin of a wire and not through the core of the wire. Galvanized screws act as a resistor. Paint or thin layers of material between conductors act more like a capacitor than an insulator. Lack of a low impedance path to ground can force the EMI signal to radiate (transmit, become airborne).Grounding the shielding helps to protect the cables against EMI noise. You may find that the best solution is to eliminate EMI noise from the source. There have been many cases where a cable error on one axis created issues in a completely different axis. The only way to troubleshoot this issue is to disconnect all cables except the cables for the infected (or victim) axis to confirm normal operation. Then add back the secondary axis cable until the issue can be identified.
The ability for a drive to achieve the expected motion and performance from the motor is directly tied to cable function. So, if you’re experiencing unintended motion, check your phasing, check your programming, and check your cable connections.
If you’d like more information we publish our EMI noise checklist as an extended application note PDF through KDN and we’ve published it as a condensed blog as well, and a white paper: Grounding and Shielding Existing Equipment. The article for Eliminating EMI in Motion Systems is available through the Machine Design online publication.
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