Machine automation: a dynamic challenge for cables
01 May 2010
Giuseppe Di Lorenzo outlines some of the key design challenges faced by his company when it embarked on the development of a new range of machine automation cables
Cables that deliver power and control services in machine automation applications usually have a very different - and much harder - life than static cables. They must deliver high performance and absolute reliability across a very broad spectrum of chain, bus, sensor, robotic and control applications, and they must be capable of withstanding the extreme dynamic loads experienced at high operating speeds.
To achieve the desired high levels of industrial productivity and throughput, automated equipment now has to move at a considerably faster pace than heretofore. For example, robotic spot-welding machines now routinely achieve repetition rates of several hundreds of welds per minute.
To cope with this, automation cables not only need to be lighter in weight, to keep their inertia as low as possible, they must also resist the stresses due to rapid acceleration and offer long fatigue life when subjected to repeated bending and stretching cycles. And in the case of 6-axis (3D) machinery, they must also cope with torsional cycling.
The lowest acceleration standard normally applied in the industry is the (Schleppfahigkeits-Klassen) SFK2 chain capability, which defines acceleration at less than 0.5g and speeds of up to 3m/s. A more common criterion is SKF4, a category that calls for accelerations of 0.5g and speeds of 3m/s.
Latest generation equipment is now considerably more demanding, often calling for cables that can withstand 5g acceleration and speeds of 5m/s - laser machine tools being a prime example. And this trend has not stopped there. Shorter chain applications and pick-and-place equipment has raised the bar even higher and acceleration testing at rates some ten times greater than previous levels has become the norm. At the same time, the industry now expects cables to achieve a life of 20 million bending cycles - double the 10 million cycles that used to be the accepted standard.
To ensure that its automation cables meet this ‘need for speed’ Nexans has made substantial investments in a dedicated Automation Application Centre, which forms part of the company’s research facility in Nuremberg. Here, cables are exposed to dynamic operating loads that simulate realistic, in-service conditions, ensuring that they achieve the required combination of bending, tension and torsional strength demanded by their intended applications.
One of the earlier design features adopted to meet these extremes of duty was an inner jacket that acted to stabilise the cores and protect the twisted configuration. However, this configuration priced the cable beyond that of the competition and the design was subsequently changed. Now, the cable features a double wrapping construction that meets the same dynamic standards, but at lower cost. Moreover, this construction enables the cable diameter and weight to be reduced and introduces additional benefits such as easier stripping.
Nexans has recently been working in a niche market area to develop miniature cables for a limited number of applications involving the use of miniaturised robotic systems. These reduced operating dimensions means that developers of automation cables now have to cater for correspondingly smaller bending radii and provide a much higher degree of flexibility. Where a bending radius of ten to twelve times the cable diameter was once considered normal, these applications now call for a bending radius as small as seven times the cable diameter or even less.
Clearly, the construction of the cable has a significant impact on the bending radius that can be achieved and every element of the construction is affected, including the conductors, fillers, insulation and jacketing. As well as paying specific attention to the construction, a general reduction in diameter of the cable is also required.
While the design and engineering of the cable are important factors, Nexans emphasises that a successful outcome is also heavily dependent on consistent process control during cable manufacture. Only when this is achieved, can it ensure that the cable construction remains within tightly controlled parameters.
Automated systems must frequently tolerate difficult environmental conditions - particularly extremes of temperature - which can adversely affect the behaviour of cables. Cables are now routinely specified for both high temperatures (up to +80oC) as well as low temperatures (down to –40oC) when they are likely to be exposed to the elements. Dockside crane applications are good examples.
It is also important that cable conductor and insulation materials provide the fire and heat resistance demanded for safety and performance, as well as offering resistance to oils and other chemicals if they are to meet long service life requirements. Some important developments in new material technologies, such as TPM (Thermoplastic Modified) insulation and flame retardant PUR Medoxprotect-S jacket material, are tackling these challenges.
An interesting trend in the automation industry is towards hybrid cables that combine power, control and data cores within a single cable. While these cable types are very useful for implementing decentralised control systems, they also make installation easier and achieve a much more elegant result.
The move towards hybrid cables started in the late 1990s, and early manifestations were usually cables that had copper cores (for power supply) and polymer optical fibre elements (for data transmission). Currently though, hybrid cables are normally all copper. For example, Nexans recently designed a cable with three 2.5mm2 cores (power), two 2.5mm2 cores (50V), six 0.34mm2 cores (control) and four AWG22 cores (Ethernet) for a major industrial drives manufacturer.
However, the higher cost of cables and connectors with hybrid design has resulted in a slower uptake of the concept in recent years.
Nexans offers a very wide scope of cable products for automation under its Motionline brand, which conforms to all major international safety standards, such as German (VDE), Canadian (CSA), Chinese (CCC) and UL worldwide. However, the catalogue range is sometimes only a starting point, since the company is frequently called upon to design and engineer a special cable that is capable of delivering the vital combination of high-performance, reliability and long life required for a specific application. This is particularly the case for miniaturised installations where a cable often needs to be tailor-made to fit the available space envelope.
Giuseppe Di Lorenzo is global segment manager and commercial director,
Automation, at Nexans
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