Starquad layout for latest cables

25 January 2013

Horst Messerer of Nexans explains why a starquad configuration is key to the success of the company's new Type C Profinet cables targeted at dynamic industrial automation installations

Horst Messerer of Nexans explains why a starquad configuration is key to the success of the company's new Type C Profinet cables targeted at dynamic industrial automation installations

Automated equipment is expected to move faster and faster to achieve the desired high levels of industrial productivity and throughput ? for example, robotic spot-welding machines routinely achieve repetition rates of several hundreds of welds per minute. To cope with this, automation power and data cables not only need to be lighter in weight, to keep their inertia as low as possible, they must resist the high stresses due to acceleration and ensure a long fatigue life under repeated bending and stretching cycles.

The need for high data performance and reliability were particular challenges in the development of the latest addition to Nexans Motionline range of automation cables ? a Profinet Type C trailing data cable with UL approval Style 20236 80?C 30 V.

Profinet is a leading industrial ethernet standard, with 100 Mbit/second data transmission, and Type C cables are identified for special applications (eg highly flexible, permanent movement, vibration or torsion). In contrast to traditional ethernet cables that comprise two twisted pairs, it was decided to base Motionline Profinet on a starquad configuration.

The diameter has been reduced by around 20%. This enables a reduction in both the weight of the cable ? hence reducing inertia and the bending radius (which is usually defined as around 15 times the cable diameter for continuous motion). A conventional twisted pair cable is designed with two different twist lengths between each pair in order to reduce the effects of electromagnetic interference between each pair ? known as near-end cross-talk (NEXT).

This creates different delay times between each pair ? in the region of 20ns/100m. Even this very small delay can have significant implications for RT (Real Time) and IRT (Isochronous Real Time) applications.

Nexans modern cable twisting machines have enabled the company to adopt a starquad configuration, with four cores twisted in one step with the same twist length. This ensures identical data transmission delays between each pair. A further advantage of the starquad design is that it matches the round 4-pole M8/M12 connectors often used in industrial automation installations.

Motion application centre
The Profinet Type C cables were exhaustively tested within Nexans' own dedicated Motion Application Centre (MAC), which is part of the Nexans Research Centre (NRC) in Nuremburg.

This facility enables cables to be exposed to dynamic operating loads that simulate realistic, in-service, conditions, thus ensuring that they offer the combination of bending, tension and torsional strength and vibration resistance required for their intended application. It provides proof that an automation cable will perform throughout its expected lifecycle as described in the relevant standard.

The Profinet Type C cable test programme covered drag chain applications with a variety of combinations of speeds, accelerations and bending radius and in lengths of 0.5, 1.5, 3, 5 and 13m.

Dynamic test regime
It is common practice to measure the data transmission capability of cables under static conditions 'on the drum'. In view of the demanding applications in which the Type C Profinet cable is employed, data integrity testing was regarded as integral to the dynamic test regime. So throughout the tests, not only was the physical condition of the cable monitored, data protocols were transmitted constantly and any bit failures logged to prove that data integrity will be maintained even in extreme conditions.

The Motionline Profinet cable features four stranded bare-copper conductors arranged around an optional centre filler with polyolefin insulation. On top of this is a covering layer of a halogen free polymer compound followed by a wrapped screening layer of plastic/aluminium film. The outer sheath is manufactured from flame-retardant and chemical resistant, halogen free PUR.

The Profinet cable is a catalogue product available off-the-shelf. However in many cases, the extreme demands of automation applications call for a specially engineered cable to ensure the optimum combination of performance, reliability and long-life. This is especially true for miniaturised installations where a cable often needs to be tailored to fit the available space envelope.