The selection of the appropriate construction of custom industrial cable begins with an understanding of the interaction between conductor stranding, shielding, jacket types and environmental ratings in terms of actual motion profile, electrical load and plant conditions. Although a cable designed correctly for a robotic arm or servo system application may still fail prematurely if the design is not a correct flex cycle and exposure to EMI and chemical contact.
Here, engineers will find out the steps that they need to take when designing a custom industrial cable. It’s getting each tiny component just right in the application to make sure the product delivers the same flex life, signal integrity and long life.
Clarifying the Application Before You Design the Cable
All good custom cables start with a clear definition of the way they will be used. The following list of options is affected by the type of motion, electrical usefulness, environmental stressors and compliance requirements. If you don’t take this precaution, you may end up with cables that don’t function as well as they should or costing more than they need to.
Mechanical Motion and Routing Conditions
Simple stranding and simpler jackets with fixed installations. Medium flex constructions can be occasionally used with flex. If, however, the drag-chain must be continuous, additional features are required to prevent the deterioration of the chain from bending fatigue under constant drag—these include the use of extra-fine stranding, short lay lengths and jackets. The third feature is 3D robotic torsion, which means that the cable needs to twist without breaking conductors or cracking jackets.
Before choosing conductor or jacket be mindful of bend radius, stroke length, cycles per hour and the close routing around corners. The numbers are directly related to the stranding class and the selection of compounds is related to providing acceptable service life.
Electrical Function and Signal Types
The power cores are bigger and stronger with respect to current and magnetic field strength. Analog & control signal cores are more susceptible to noise. High speed data cables such as industrial Ethernet or fieldbus, require controlled impedance and effective shielding.
Power and sensitive signal cores should be separated or use a pair of shielded wires and overall shield for each signal core. They are not always known to produce fault intermittence when mixing without isolation and can be difficult to track in the commissioning process.
Conductor Design – Size, Stranding and Configuration
The conductivity and durability of the conductor are affected by the selection of conductors. The problem is that of selecting the smallest possible cross section that meets the current and voltage drop requirements with enough flexibility to meet the motion profile requirements.
Choosing Conductor Size and Material
Calculate cross-section from load current, acceptable voltage drop and ambient temperature. Oversizing for motion applications may cause the cable to be too stiff or too short of length for tight bend radium and high cycle drag chains.
Tinned copper has slightly greater corrosion resistance and solderability than bare copper as well as an increased cost. For most industrial automation applications, tinned conductors are the best and most dependable conductor available.
Stranding Class and Flexibility Requirements
Only solid conductors are used for fixed installations. Occasional movement is via standard stranded conductors. High flex cables and continuous flex cables also require extra fine flex with as many as class 5 or 6 and an even smaller diameter. Smaller wires distribute bending forces amongst more wires, greatly extending flex life.
In some applications such as a robot wrist, where a cable is twisted over and over, the stranding should also be able to resist such a corkscrew effect. Suppliers who have experience in torsion cable and robot should be able to recommend the optimum strand count and lay direction to ensure that the cable performs well for millions of cycles.
Core Count, Pairing and Layout
Twisted pairs reduce the electromagnetic coupling between the neighbouring signals. The constant pair twisting guarantees signal integrity for the situations of feedback and sensor cables. If high current power cores and low level analog cores are to be in the same cable, then it is recommended that they be separated from each other in the cable, or be shielded from one another to prevent crosstalk.
For some applications, a combination of power, signal and data components in a single jacket may be desirable. Other types of others do better when divided into 2 or 3 separate cables. Selection is made based on the space needed by the connectors, routing needs and sensitivity of the connectors.
Shielding Strategies – When and How to Use Foil, Braid and Hybrid Designs
Electromagnetic interference is one of the most common causes of the “mystery ailments” seen in automated systems. Good shielding helps prevent the damage of sensitive circuits, and keeps the cable flexible.
When Shielding Becomes Critical
The servos feedback cables are nearly always protected if they are parallel to the motor power cable. In industrial applications, variable frequency drives (VFD) or welding equipment require good high frequency coverage. Near contactors or solenoids, individual pair shielding and overall shield benefits sensor lines.
Unshielded cable can be used in quiet control cabinets, but the moment the cable is brought near the cabinet or the power conductors in the cable raceway, then the cable must be shielded.
Comparing Foil, Braided and Hybrid Shielding
Aluminium foil is a good material for relatively static data cables, with very good coverage and low profile. Copper Braid is suitable for continuous flex and torsion applications, and has an excellent low frequency characteristic and strength.
Hybrid designs which use both foil and braid offer broadband shielding and long-term protection from motion. The braid is an additional diameter and expense, so should only be added if the performance demanded by the application really calls for it.
Shield Termination and Grounding Pitfalls
The shield does not really protect if it is not terminated. Long pigtails, floating shields or incorrect grounding at both ends form antennas that radiate noise instead of containing it.
Whenever possible, provide 360 degree shield termination at connectors. When installing field-assembled cables, make sure that installers use the proper backshells and grounding clamps. One of the most common sources of an otherwise well-designed cable failing EMC testing or failing to communicate at all are poor shield termination.
Jacket and Insulation Materials – Matching the Cable to Its Environment
The first line of defense against the plant environment is the jacket. The service life will be dependent on the temperature, oil, chemicals, abrasion, UV and washdown, so what compound will give the correct service life will depend on these.
Common Insulation and Jacket Compounds in Industrial Cables
PVC is the most popular and cost-effective, but softens at high temperatures and is sensitive to some oils and is generally used indoors. PUR is also highly abrasion resistant and good for cold weather flexibility and is the preferred choice for drag-chain or robot cables. TPE has a good balance of flexibility and resistance to chemicals at moderate cost.
Designing for Oil, Chemicals, Abrasion and Washdown
Jackets can swell or crack over time when using cutting fluids, hydraulic oil and cleaning agents. Typically, PUR or a specially formulated TPE jacket performs better than PVC in machine-tool applications or in the automotive industry.
Food and beverage lines are usually smooth and non-porous jackets that must resist frequent alkaline or acidic cleaning. Where suitable, identify compounds used for tests for detergent resistance and make sure the cable is not constructed in such a way that it will retain compound residue in pockets.
Halogen-Free and Flame Performance Considerations
In the event of a fire, when cables are located in air-handling spaces, tunnels and public buildings, the low-smoke, halogen-free formulations limit the release of toxic gas and smoke. The compounds generally sacrifice some flexibility or abrasion resistance for increased flame performance and so check the entire property specification, rather than just the flame rating against the actual installation conditions.
Environmental Ratings – Temperature, Voltage, and Installation Categories
Datasheet ratings apply to the cable only when the cable is used under the conditions detailed in the datasheet. Even in real installations, those can be surpassed as the cables may be exposed to thermal cycling or may be expected to carry a higher current than they were tested at or bend the cables to a smaller radius than tested.
Temperature Ratings and Thermal Cycling
This also implies that the cable could be susceptible to failure because it is located near a hot motor casing or because it is subject to hot/cold cycling that could cause the insulation or jacket to become cracked or damaged.
Check for average and maximum temperatures. In high temperature regions near to a stove or strong drives, silicone or cross-linked compounds which have high temperature resistance should be applied.
Voltage Rating and Safety Margins
The voltage rating is dependent upon the thickness of the insulation and dielectric strength of the material. In the case of the power and control cores being on the same cable, the signal core insulation should still be capable of handling the higher voltage that is present on the power core during switching transients and in case of a fault.
Do not exceed specified voltage rating and keep a proper spacing or barrier between power and signal elements.
Fixed vs. Flexible vs. Torsion-Rated Cables
This almost always will create conductor failure with a fixed installation cable in a drag chain. Similarly, a normal flexible cable used in a robot that has a high torsion will show corkscrewing and splitting of the jacket.
If you will be bending or twisting continuously, make sure you want flex or twist ratings. Request flex-life or torsion-cycle test data rather than a general term such as “flexible” from the manufacturer.
Integrating Certifications and Standards into Custom Cable Design
Certifications restrict materials and building parameters in the initial design phase. UL, CE, IEC, industry specifications affects conductor size, thickness of the insulation, jacket compounds and flame performance.
Don’t assume compliance, make it part of the design process. The smart supplier or the involvement of the certification body at the early stage will prevent costly redesigns.
Documentation/traceability of the importance is not to be neglected. OEMs must require test reports for each batch of cable they get, including conductor resistance, integrity of the insulation and the properties of the shield and the jacket.
Common Design Mistakes and How to Avoid Them
There are some common mistakes that are seen throughout the projects that go on to have failures in the field.
If mechanical stress is under estimated, it will result in standard stranding in high-cycle drag chains or inadequate bend radius margins. This results in conductor breakage failure at a young age. Always work out real flex cycles and seek advice from suppliers who have been flex-tested.
Without EMI and shield termination, intermittent faults will occur, which will only be seen in certain states of the machine. The majority of these are avoided by using appropriate 360 degree termination of the shield both within connectors, and by ensuring that EMC aware engineers are engaged into the project as early in the design process as possible.
The jacket is a mismatch with the environment, and will reveal it in the form of cracking, swelling or discoloration within months. Even before production begins, a common environment checkup should be made to ensure that the proper compound is used, such as temperature extremes, oil type, oil concentration, abrasion sources and cleaning chemicals.
Practical Design Checklist for Engineers and Buyers
Use this list to create specifications or an RFQ for custom industrial cables:
- Describe the various types of motion profile (Fixed, occasional flex, continuous drag chain, torsion) and the different cycles and bend radius found with each.
- Explain electrical functions; Take separation of sensitive signal cores and power into account.
- Determine conductor material, cross section, and flex and voltage drop classes of current.
- Identify the location of the shielding for actual EMI sources and EMI sensitivity.
- Consider jacket and insulation compounds based on temperature, chemical and abrasion exposure.
- Establish voltage and temperature ratings with reasonable safety factors for installation.
- Identify necessary certifications, request test reports and traceability documents.
- Specifically ask for flex-life or torsion-cycle test data, if applicable, that apply to the motion profile.
- Have prototype samples and set up acceptance criteria before going to series production.
Questions to Ask Your Custom Cable Manufacturer
- Your custom cable manufacturer has a number of questions you should ask.
- For my particular motion, what is the flex or torsion rating of this construction?
- Which would be the best shield, or shielding configuration, and which the best termination, or termination method, for the above EMI environment?
- Are there chemical resistance test data for the proposed jacket compound on the particular oils or cleaners in question?
- What documentation do you get with each production batch?
- Do you have the willingness to provide prototype samples for test and validation prior to volume?
Tight collaboration with OEMs, machine manufacturers, and automation systems integrators will enable us to turn your application needs into cable designs that will reliably operate in a drag chain, robotic system and extreme industrial environment. We help with specification review, compound selection, shielding design and prototype validation to ensure the cables that we deliver are consistent with the demands of continuous operation.