Electromagnetic interference (EMI) can cause unintended movement or unexplained drive faults in motion systems. This can be due to motion setup through the drive, a poorly designed program within the control and motor, or because of feedback commutation 180° out of phase.
EMI mitigation is governed by Federal Communications Commission (FCC) Class B rules, also known as the electromagnetic compliance (EMC) rules, to keep electric noise generated below a specified amplitude and range. Controllers, drives, and motors, along with other machine parts, must have minimal EMI emissions and low acceptability.
Machines also must not be influenced by outside EMI sources within the 30MHz to 300MHz 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. Resistance is in the product design and system integration.
To electro-magnetic frequencies, a cable looks a lot like an antenna, which 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 cable the more likely it will catch a wave.
If cables pick up EMI, noise enters the system and the signal-to-noise ratio falls out of balance. The machine may exhibit unexpected movement, such as not stopping on position, and moving or stopping when it isn’t supposed to. The drive may also fault infrequently for feedback or current-loop issues.
Cables with shielding and twisted pairs can help. Shielding protects the cable from EMI generated from other areas of the motion system. Using a magnetic shield to prevent radiation from entering and exiting the cable is also highly recommended (the minimum shielding for working with Kollmorgen motor and drive systems is 85%; Kollmorgen cables are rated at 90%).
Shields don’t work if they are not bonded and grounded at both ends.
If system issues occur, make sure to include cable inspection in troubleshooting checklists. First, check the cable’s connection at the drive. A 360° clamp should be around an area of the cable where the outer jacket has been removed and the clamp is in direct contact with the shield. The clamp should be connected to the grounding plate attached to the drive, and the grounding plate should be grounded.
This might be the trickiest thing to check. Often the grounding plate is grounded to the cabinet. If the cabinet’s 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 the grounding plate isn’t properly grounded, the items attached to it aren’t properly grounded, making the cable wires inside the shields vulnerable
A ground for high frequency EMI isn’t the same as a ground required by electrical codes to prevent fires or electrocution. High-frequency signals conduct on a wire’s skin, not through the core. Galvanized screws act as a resistor. Paint or thin layers of material between conductors act more like capacitors than insulators. Lack of a low impedance path to ground can force the EMI signal to radiate (transmit).
Grounding the shielding protects the cables against EMI noise, but 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 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 secondary axis cables 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 there’s unintended motion, check the phasing, programming, and cable connections.