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Custom Cable for Industrial Automation

By Lisa Eitel

Contributed By Digi-Key's North American Editors

Automation saves labor, energy, and materials while increasing accuracy and quality. But one potential vulnerability is that today’s industrial machinery and operations — which have unprecedented system complexity — are particularly reliant on stable connectivity for the transmission of power, control, and operational data. Any breach in this connectivity can lead to major throughput disruptions and expensive machine or end-product damage.

That’s why the reliability of industrial cable solutions to connect all the controls, sensors, and actuators of automated systems is vital to minimizing unplanned maintenance and maintaining the dependability of automated operations.

Image of ÖLFLEX CONNECT custom cable solutions from Lapp USAFigure 1: Industrial cables in automated settings are used for electrical power distribution as well as the transmission of control signals and data (for data acquisition and operations monitoring). Cable assemblies that combine these functions (such as ÖLFLEX CONNECT custom cable solutions from Lapp USA) are on the rise. (Image source: Lapp USA)

Reliable cables require robust earthing, conduction strands, and connectors; and there are many other considerations. For example, shielding against electromagnetic interference (EMI) is critical to ensuring reliable control and data transfer. On moving axes, cables must also be flexible enough to withstand thousands of cycles of bending. Insulation around each conducting strand must also be robust and abrasion resistant.

So, for industrial-automation applications with particularly demanding requirements, custom cable is increasingly common. New options for shielding, insulation, sheath ruggedness, and form factor are helping engineers tailor cables to their applications for full optimization.

Modular cables in the context of industrial automation

Modular cables and connectors are factory-assembled products that use a standardized set of subcomponents that often clip or clamp together for quick and extremely reliable operation. No wonder that customized cable assemblies based on modular subcomponents are increasingly common in industrial automation — as well as other industries such as construction.

Modular cables can reduce onsite installation efforts by around 60% to 70% compared to traditional cable sets. That’s in large part because modular cable assemblies eliminate the need for plant personnel or integrator technicians to physically connect electrical conductors and execute testing and troubleshooting onsite. In fact, most traditional cable-installation processes require that an electrician cut cable to the required lengths, strip its sheathing, and hand-twist connections. Modular cables eliminate these burdensome tasks even while boosting reliability; because again, all customizing, cutting, connecting, and end finishing are executed with repeatable factory processes. What’s more, cable sourced from suppliers who sell custom assemblies usually undergoes automated testing before the cable ships. That way the application-specific cables are fully specified at the design stage — and onsite installation consists of simply plugging these cables into the machine components.

Image of custom cable assemblies save electricians from onsite wiring and testingFigure 2: Custom cable assemblies can save electricians from the onsite wiring and testing shown here.

Shielding to protect transmission quality

With increasingly complex systems involving the transmission of power, control, and data signals, modern industrial environments are electrically noisy. That’s why sensitive equipment and signals must be protected from EMI. These electrical-circuit disturbances are caused by electromagnetic induction, electrostatic coupling, and conduction. In fact, cables in electrically noisy settings may need to be shielded to prevent the propagation of EMI because:

  • The cables may be subject to disturbances from EMI originating elsewhere
  • The industrial cables themselves may be the source of EMI
  • The cables may otherwise act as an antenna to radiate noise

Motors, generators, transformers, induction heating and power cables can all be sources of high levels of EMI. Control and data cables in close proximity to these sources will require shielding. Very sensitive signals may require shielding even if they are some distance from the source of EMI.

In fact, shielding may be in the form of a cage surrounding an entire automated operation; a metal cabinet or conduit surrounding cables; or (as discussed in detail here) built directly into the cables.

On-cable shielding from EMI can be with a Faraday shield (which is a continuous covering of conductive material around the cable conductors) or a Faraday cage (which is a conductive mesh around the cable conductors). The former is usually constructed of foil and the latter of braided wire.

Foil shields often consist of thin aluminum for providing continuous shielding that is low cost and flexible, but more difficult to ground.

Braided wires are a woven mesh of copper wire that’s easier to connect to ground but in some cases doesn’t provide 100% coverage — as small openings can allow high-frequency signals to penetrate. So, for a Faraday cage to work, all holes or gaps in the mesh must be significantly smaller than the wavelength of the radiation being blocked. Some manufacturers tin their braided shield wires to improve protection from EMI. Others ensure clean signals in very noisy environments with multiple layers of shielding — between individual pairs as well as around the entire cable, for example. Such shielding can be more expensive and less flexible than other options.

Both cages and shields are conductors that reflect electromagnetic radiation to prevent that radiation from reaching inside to the cable’s conducting strands. Shielding in cables is typically placed between layers of insulation surrounding the conductive wires. Then this shielding is usually connected to ground to allow efficient dispersal of the EM energy.

Image of ÖLFLEX CONNECT SERVO cables from Lapp USAFigure 3: ÖLFLEX CONNECT SERVO cables from Lapp USA include power and data connectivity as well as connectors to simplify use with Siemens, Rockwell/AB, Indramat, Lenze, and SEW drives and servomotors. EMC shielding in these cables is done by the manufacturer in an automated process that removes the cable sheathing and spreads the shielding for 360° contact with the connector. (Image source: Lapp USA)

Industrial cable insulation options

Insulation is the nonconductive material surrounding an electrical conductor wire. As well as preventing conduction between wires or to ground, it often also serves to protect against abrasion and the ingress of fluids. It is important to note that insulation alone provides no barrier to EMI with magnetic fields and radiation passing straight through it. The following are some common insulation materials.

Polyvinyl chloride (PVC) is an inexpensive and commonly used insulation. It has a temperature range of about -55° to +105°C and is resistant to common solvents and fuels. Capacitance and attenuation lead to some power loss.

Semi-rigid PVC (SR-PVC) has higher abrasion resistance than other options; the similar plenum polyvinyl chloride (plenum PVC) material also boasts superior flame resistance.

Polyethylene (PE) has low-capacitance, making it well suited to high-speed data transmission. It is inflexible and flammable with a temperature range of -65° to +80°C.

Chlorinated polyethylene (CPE) has excellent heat and fire resistance and is often employed in industrial power and control cables.

Silicone is highly heat-resistant (even to 180°C) as well as flame-retardant and flexible.

Fiberglass is commonly used for applications requiring extreme heat resistance such as foundries and metal processing. It can be used at sustained temperatures of up to 480°C.

Note that operating conditions are given as a rough guide only. Design engineers should always refer to cable-manufacturer specifications before using a cable for a particular application.

Industrial cable sheaths and jacketing

In some cables, the function of electrical insulation and external protection are separated, with optimized materials being used for each function. In this case, the inner layer providing electrical insulation is referred to as insulation, while the outer layer providing protection is referred to as the jacket. This can improve both ruggedness and flexibility.

Different jacket materials may be used to provide specialized protection from threats such as abrasion, fluids, heat, chemicals or microbes. Some common jacket materials include:

Polyurethane (PUR) which has high toughness and flexibility as well as chemical, water and abrasion resistance. However, polyurethane is flammable. Poor electrical properties mean it is not suitable for use as insulation.

Nylon which has excellent toughness, flexibility, and abrasion and chemical resistance.

Neoprene which is a synthetic thermoset rubber with excellent abrasion, cut-through, oil and solvent resistance. It has a long service life and is often used in military applications.

Styrene butadiene rubber (SBR) which is a thermoset with excellent abrasion, oil and solvent resistance. It is used in Mil-C-55668 cables.

Strand arrangements in industrial cable

Individual strands within a cable may be arranged in numbers of different ways to impart distinct flexing properties.

Solid conductors consist of a single thick wire which is low cost but stiff and less robust.

Bunched stranding is relatively simple with all the strands twisted together in the same direction but is more robust than solid cable.

Concentric stranding has a single wire in the center with a layer of wires twisted around it; any successive layers are twisted in alternating directions. This produces smooth cables suitable for use in automation applications.

Rope stranding has bundles of cables stranded in a variety of other ways — also a design meant to yield a flexible cable.

Final note on connectors for industrial cable

Cable connectors for industrial automation are just as customizable as the cables themselves. This topic will be covered in another article and will discuss the various permutations of these connectors (as well as gland and grip options). However, it’s worth noting that connectors are increasingly specialized to particular components — such as the servomotor cables mentioned above, for example.

In addition, cable connectors today offer highly specialized designs to maintain resistance to environmental conditions. Connector resistance against ingress is rated in the same way as that for enclosures, using an Ingress Protection (IP) code. These codes consist of two digits — with the first digit indicating the level of protection against foreign bodies and dust, and the second digit indicating the level of protection from fluids. The first digit ranges from 0 for no protection to 6 for dust tight. The second digit ranges from 0 for no protection through to 8 for continuous protection against continuous submersion at a depth of 1 meter.

Elsewhere, standardized geometries for cable connectors have come to simplify specification. For example, modular RJ connectors are increasingly common for cable employed in data transmission. Note that the term RJ in this context stands for registered jack and originates from its telephone-system applications. Although technically speaking, modern modular connectors aren’t actually RJ connectors at all. These connectors can be rapidly and reliably fitted to terminate cables using special purpose crimping tools which fix the connector and make the electrical contacts in a single operation. This allows efficient and convenient assembly of cables onsite, although factory-assembled cables are more reliable. This type of connector plug usually has a tab to hold the connector securely in a socket and often has a clear plastic body enabling the internal contacts to be visually inspected.

Image of second and third cable connectors shown here are RJ connectorsFigure 4: The second and third cable connectors shown here are RJ connectors. (Image source: Lapp USA)

For more information on this topic, be sure to read the Digi-Key article, “The Right Cable for an Industrial Application: How to Choose and Use for Design Success” for some guidance on choosing industrial cable.

Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.

About this author

Lisa Eitel

Lisa Eitel has worked in the motion industry since 2001. Her areas of focus include motors, drives, motion control, power transmission, linear motion, and sensing and feedback technologies. She has a B.S. in Mechanical Engineering and is an inductee of Tau Beta Pi engineering honor society; a member of the Society of Women Engineers; and a judge for the FIRST Robotics Buckeye Regionals. Besides her motioncontroltips.com contributions, Lisa also leads the production of the quarterly motion issues of Design World.

About this publisher

Digi-Key's North American Editors