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 180 degrees out of phase can all cause unintended motion. For this review, we’re going to consider how poorly chosen or poorly installed motor cables can contribute to these issues.
In the overall world of EMI (electromagnetic interference), we have some rules we have to play within. The FCC Class A and B rules, also known as EMC (electromagnetic compatibility), force 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 susceptibility. Machines must also show lack of susceptibility to outside sources of EMI within the 30–300 MHz 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 with regard to cables? Because, to electromagnetic frequencies, a cable looks a lot like an antenna. Antennas often consist of long metal extensions that catch and transmit radio waves. Coincidentally, cables are made up of long, thin, metal extensions—so the longer the cables are in your system, the more likely they 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 stopping when it isn’t supposed to stop. You could also see infrequent faulting of the drive for feedback issues or current loop issues.
To prevent these issues, use cables with twisted pairs and shielding. Shielding around a cable serves 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 installed 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) 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 wires inside the cable shield 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 is essential to protect the cables against EMI noise. But you may find that the best solution is to eliminate EMI noise at the source. There have been many cases where a cable error on one axis created issues with a completely different axis. The only way to troubleshoot this is to disconnect all cables except the cables for the infected (or victim) axis to confirm normal operation. Then add back the secondary axis cables one by one 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, access our EMI noise checklist and see the related topics below.