Bill’s electric actuator drive must operate in factory conditions with dust, moisture, heat and vibration. But Bill’s main problem is with electrical noise; there’s enough in his environment to disrupt actuator control signals and cause erratic movements.
Electrical noise–random fluctuation in an electrical signal–is present in all electronic circuits. When the amplitude of these fluctuations is sufficiently high in an electric drive system, communication can be compromised, leading to erratic actuator movements, which can precipitate system failure. The goal is to maximize the signal-to-noise (S/N) ratio, and thereby lower the amplitude of the noise so that it does not interfere with the control signal.
Bill recently returned from a training class on this subject, so he applied what he learned to minimize the effects of electrical noise in this environment and ensure his drive operates smoothly.
Electrical noise cannot be eliminated completely. One may consult a professional electrical industrial engineer for electrical repairs and maintenance of industrial systems. The primary objective during system design is to mitigate the risks associated with each type of electrical interference and make intelligent financial decisions based on the operating environment and costs associated with the system failure modes.
1. First, Bill checked the ground loops. On his installation, several drives are connected to the same communication bus, in this case, Ethernet. Based on tips from the class, Bill ensured that those drives were connected to the same ground, which is also connected to the bus host controller.
He also checked to see if each drive’s power supply was plugged into a grounded power outlet. Unless all of the drives and the host bus controller are plugged into the same outlet, a ground loop could be created.
In large systems where a single-point earth ground is not possible, the grounds can be physically connected. In this case, Bill could install a small resistance between the multiple grounds to limit the amount of current flowing between them, which would reduce the noise and ensure that the ground’s potentials are relatively close to each other. A good starting point for this resistance is 10 ohms.
2. Next, Bill checked for induced electrical noise. In servo drive circuits that use high-current switching signals with sharp edges, sections of the circuitry can act as antennae and induce electrical noise (electromagnetic interference) into nearby conductors or equipment.
Power cables and communication lines can be both transmitters and receivers of noise. EMI noise coupling occurs when electromagnetic noise emitted from the drive is picked up on the power cables or communication lines. Power cables and communication lines can also induce noise into a nearby drive.
Bill measured this noise and certified the level of EMI emissions. He also checked the unit with the motor and communication cables plugged into the drive. The primary noise source on a servo drive is switching noise, which will typically be emitted from the motor cables.
Cable shielding should be grounded on only one end of the cable. Shielding that is grounded at both the drive end and the motor end can generate ground loops. Typically, the cable is grounded at the drive end.
Recommendations for various cable types:
Cable Type Shielding Category Topology Voltage
Ethernet 100 Mb/s Foil Cat 5a or Cat 6, Class D or E Point to Point/ Network 2.5 V Differential
Ethernet 1Gb/s,10Gb/s Foil Cat 6 or Cat6a,Class E Point to Point/ Network 2.5 V Differential
RS-485 (TIA-485) 10Mb/s Foil Cat 5 or 5a Point to Point/ Network 6 V Max Differential
USB 12 Mb/s Full Speed Foil USB Certified Hi-Speed Point to Point/ Network 5
3. Separate motor and encoder cables. Bill also checked to ensure the motor and encoder cables were separate. As a general rule, try to maintain at least 2 inches of separation between the cables to reduce inductive coupling.
Bonus tip–check for noise from the network. Bill learned thatdevice communications are susceptible to external electrical noise. Most industrial communication protocols survive fine in the presence of noise, but control systems can experience error and data loss in device-to-device communication in noisy environments. In addition to the software/firmware that drives the communication protocol, the cabling also must be designed to protect the communication from outside interference.
Bill checked to make sure the cabling was twisted pairs, which cancel out electromagnetic interference; that the cabling was shielded and the shielding grounded; and he checked to make sure the length was as short as possible. Unnecessary coils of wiring add resistance and inductance to the system, which can affect motor tuning. Longer cables also emit and receive more noise.
Source: This material is excerpted from a White Paper, Minimizing electrical noise in actuator drive systems for maximum reliability and performance, by By Patrick Hobart and Jim LaRocque of Tolomatic.