Electrical noise and linear actuators
By Tolomatic on May 5, 2015
Electrical noise is a common occurrence. Random fluctuations in electrical signals are known as noise, and if the amplitude of the fluctuations becomes high enough, the noise can interfere dramatically with the operation of industrial automation devices like linear actuators. The results can be anything from erratic movements to complete system failure.
Industrial control system design engineers seek ways to keep electrical noise levels down. This white paper explains some best practices for mitigating electrical noise in motion control systems.
Sources of electrical noise
Here are the two major sources of electrical noise and how to control them:
Ground Loop Noise
Ground loops happen when sections of a system are connected to different earth grounds. If the earth grounds have different electrical potentials, an unwanted current flows between them causing disruptive electrical noise.
In industrial control systems, drives that are connected to the same communications bus also should be connected to the same ground as that controller.
Minimizing ground loop noise
- Use a single-point earth ground for the entire system.
- If a single-point ground isn’t possible, connect the grounds physically with a wire that has low resistance – 10 ohms is a good starting point. This limits the current flowing between the grounds, reduces the chances of a ground loop occurring and minimizes noise.
Induced Noise or Electromagnetic Interference (EMI)
Sections of electrical circuits may act like antennas, picking up electrical noise and transmitting it to nearby equipment like linear actuators. This electromagnetic interference (EMI) can happen in linear actuator servo drive systems when electrical noise from switching signals is induced into communications lines.
On the other hand, electrical noise emitted by communications lines and power cables may be picked up by the linear actuator servo drive. In either case, system performance is disrupted.
Minimizing induced electrical noise
- Keep cables as short as possible. Longer cables emit and receive more noise.
- Avoid coils of wire and cable.
- Mechanically secure all cables to keep them separated.
- Separate motor and encoder cables. Minimize the chance of induced electrical noise by keeping at least 2” between them throughout their path from drive to motor.
- Use properly shielded motor and communications cables
- Ground the cable shielding on only one end of the cable.
Electrical noise can be especially troublesome for device-to-device communication (including drive-to-electric linear actuator). Here are recommendations on minimizing electrical noise and its harmful effects on communication in factory automation systems.
Minimizing electrical noise in communications cables
- Use twisted pair cable to cancel out electromagnetic interference (EMI).
- Use shielded cable that’s certified for use in an industrial environment.
- Ground the cable shielding at one end only. Grounding on both ends could create a potentially noise-emitting ground loop.
- Keep communications cables as short as possible and avoid coils.
- Secure cables to minimize wire fatigue. Too much movement can result in broken wires.
Best practices for automation system set-up
The communications links between an electric linear actuator, the drive and the PLC are vital to the motion control system. If communication is blocked, motion stops so the right system set-up is essential.
- Use switches and hubs that can stand up to the challenges of the environment. Example: For better installation, network monitoring and survivability options, use a fully-managed, industrially-rated Ethernet switch for Ethernet communication.
- Daisy-chain drives intelligently. Daisy-chaining can reduce the number of network switches and the amount of cabling needed in a system. However, daisy-chaining can slow communications speed. Also if a failure occurs, all the drives in a daisy-chained system that fall after the failure point will stop communicating. Industrial motion control systems designers seek to strike a balance between cost, communication speed, and system survivability.
- Eliminate “network storms” by avoiding ring configurations in daisy-chained networks that do not support ring topography. Network storms occur when messages are forwarded back and forth through a ring. They can be extremely disruptive and can halt communication completely.