Although it is often believed that mysterious software bugs are responsible for intermittent faults in industrial automation servo drives, encoders, sensors and fieldbus networks, the problem usually goes back to failure of custom cables to withstand EMI and noise properly. The kind of shielding you choose (braided, foil, or hybrid) can mean the difference between a stable system and one plagued by jitter, packet loss and expensive troubleshooting. This guide is based on actual experience in designing cables for challenging motion control, robotics, and networking applications to assist engineers in selecting and utilizing the right cable shielding structure.
Understanding EMI in Industrial Automation – Where Noise Comes From
Rapid voltage and current changes, such as those from variable frequency drives (VFDs), servo drives, large motors, contactors and switching power supplies, produce substantial broadband noise. This noise can be coupled to other nearby cables through capacitive (electric fields) or inductive (magnetic fields) channels or radiated channels, particularly in close proximity in dense control panels or on shared cable trays. There are additional transients due to welding equipment and relays.Other transients are generated by welding equipment and relays.
Low level analog signals, high-speed encoder pulses and data lines are most susceptible. If a feedback cable over the span of a few meters runs parallel to a motor power lead then sufficient noise can be picked up to cause encoder miscounts or servo alarm. With the proper cable layout, these risks can be minimised from the outset with the correct shielding.
Shielding Fundamentals – What Foil, Braided and Hybrid Shields Do
Foil shielding is a process employing a thin layer, typically of aluminum or copper with possibly a drain wire, and provides almost 100% optical coverage. Very good at rejecting high-frequency electric field noise typical of data and twisted-pair cables. Foil, however, is susceptible to cracking, or degradation, with repeated flexing, making it less suitable for high motion applications.
Braided shielding is made of interwoven copper wires and it performs well against low frequency magnetic field from motors and drives and has high mechanical strength. It is flexible and durable for dynamic environments, and can provide coverage of 80-95% is generally achieved according to braid angle and density.
Hybrid shielding is a combination of foil and braid (usually foil over braid), providing overall protection. This is a structure that is robust against high frequency and low frequency interference and flexing installations.
Matching Shield Type to Signal and Environment
For the power cables of servo motors, strong overall shielding is required to suppress the motor’s self-radiation, and for the feedback cables (encoder/resolver), the twisted pairs to be individually shielded and the overall shielding are both required. Feedback running close by the power conductors provides best hybrid shielding in high flex applications.
In the case of industrial Ethernet (PROFINET, EtherCAT) and fieldbus (CANopen, Profibus, DeviceNet) adhere to the protocol and shielding specifications. In a quiet, non-dynamic environment, foils might be good enough, but in drag chains or close to drives, braided or hybrid will be necessary to ensure signal integrity.
Analog sensors and mixed bundles (analog and power) require careful design, such as shielding twisted pairs of wires with individual protection from the power groups, typically with individual foils and an overall braid or hybrid shield.
Shield Termination, Grounding and Layout – Where Good Designs Go Wrong
Even the best shielding will be ineffective if the ends are not suitably terminated. Use conductive clamps/connectors that will ensure continuous shielding; avoid pigtails as they become an antenna at higher frequencies and reduce performance.
For grounding, the following should be used as a guide to the manufacturer’s recommendations: Single-end grounding generally results in a loop-free system in many instances and may be appropriate for some instances where low frequencies are used, but both-ends may be appropriate for other low-frequency cases. Document decisions clearly.
The routing of the cables is also important. Keep sensitive signal cables away from high current motor leads, keep as much as possible out of parallel and use separate trays or conduits if possible. A good layout will significantly lessen the shielding burden.
When to Choose Foil, Braided or Hybrid Shielding – Practical Scenarios
For mostly static control cabinets or short sensor runs with low noise, foil shielding (pair or overall) is frequently found to be adequate and cost effective. These requirements can be fulfilled by many of the standard catalog cables.
Braided or hybrid shielding will provide mechanical protection and magnetic field rejection for high-flex drag chains and robotic arms near servo or VFD power leads. Use fine-stranded conductors and flexible jackets for optimum longevity.
Fieldbuses and Ethernet signal transmission through noisy plant floors is aided by hybrid shielding or well-specified shielded twisted pairs (STP) that are fit for protocol.
Common Shielding Mistakes—and How to Avoid Them
Without checking coverage, flex rating or shield construction, field failures often occur when using general purpose “shielded” catalog cables. Always ask for detailed drawings and test data for motion applications.
Shields that are not terminated properly (cut too short, pigtails, etc.) add noise, not subtract it. Ensure the same hardware and procedures are used for project terminations.
One approach fits all signals is ignoring frequency specific needs. Correlate shielding to signal frequencies: foil is good for high frequency data, braid provides strength to low frequency noise from motors.
Practical Shielding Checklist for Custom Cable Projects
Consider the following questions before specifying shielding:
- What are the signals carried by the cable (power, analog, digital, Ethernet/fieldbus, feedback)?
- What are the predominant noise sources close by and their frequencies?
- Are static, flexing, torsion or drag chain motion present in the cable?
- Are there manufacturers/equipment or protocol requirements for shielding?
- What is the plan for installing shields at the end?
With your custom cable manufacturer discuss the following:
- Type, coverage, and foil/braid/hybrid layers of shields recommended.
- Mechanical performance data of flex/torsion with the selected shielding.
- Acceptable termination hardware and connector suggestions.
- Examples of application or test reports from similar applications.
Hulk Electric specializes in the design of customized cables featuring optimized shielding structure that meet your specific servo, robotics, Ethernet and fieldbus requirements. Our in-house testing and real-world experience ensures a reliable EMI and noise control from prototype to full production.