Today's machines are largely supplied with energy and data via energy chains. However, caution is required here, especially with Ethernet-based communication technology. Up to now, network technicians have been able to fall back on relatively simple mechanical cables (which also function reliably in the office), but these do not offer the necessary security in moving industrial applications. This is because the relatively inexpensive Ethernet or fiber optic cables intended for fixed installation are not designed for moving applications and therefore only have a very limited service life under these conditions. Special solutions are therefore required in the area of energy supply and special design principles for movement in the energy chain.

Worth a closer look

Specially designed Ethernet cables are required to ensure that the robots can communicate perfectly between the axes, the controller and higher-level systems.

© Igus

If Ethernet cables are permanently installed in the machines and systems, inexpensive cables with a solid conductor or a flexible conductor consisting of seven-wire strands can be selected. For applications in the energy chain, it is necessary to use Ethernet cables with fine stranded conductors. In addition, a cable structure that is adapted to the movement in the energy chain is important in order to ensure reliable data transmission over many years. The fact that there are clear differences between moving and fixed cables is particularly evident in applications with long travel distances, many double strokes or high dynamics: the mechanical effect of the movement changes the cable structure slightly and data transmission suffers. The longer the cable, the lower the data transmission rate of the bus system.

With highly flexible Ethernet cables for moving energy chains, a data transmission length of up to 70 m between two Ethernet subscribers is possible. This length also depends on the cable category (CAT5, CAT5e, CAT6, CAT6A, CAT7) and the electrical ambient conditions. An alternative for long distances are fiber optic cables, which allow cable lengths of several 100 m up to 2 km.

The materials are crucial

The insulation and jacket materials are a decisive factor for movement in the energy chain. Igus has gained important insights into the materials and their interdependencies during many series of tests on Ethernet cables. The plastics of the cables and energy chains, combined with external factors such as temperature, media and any radiation that may occur, are constantly in contact with each other and influence the durability of the cable in the chain.

No national or even international standard and no associated test is actually suitable for testing cables in energy chain applications, although there are certainly regulations or standards that relate to the subject of moving cables. Igus, for example, has developed its own standards that deal with the testing and evaluation of structures and materials.

Specifically, there are well-known and very good test procedures from various standardization institutes in the cable industry. However, these are too general and do not take into account the special requirements of continuous movement in the energy chain. Neither the VDE bending cycle tests nor the abrasion test meet the requirements of a chain-cable combination. In the bending fatigue test, for example, the cable has a completely different movement sequence compared to the movement in the energy chain. Many cable designs that have reliably met these requirements have failed in energy chain movement tests within a very short time.

Sandpaper and razor blade tests

Standard abrasion tests, which determine the abrasion of a material using sandpaper, a needle or a razor blade test, are certainly also very good in general comparison. In these tests, the sheath material of the pipe is rubbed with a defined pressure, either with sandpaper, a razor blade or a needle. This creates abrasion with a defined amount of movement. However, this test is not meaningful for determining the durability of the sheath material in the energy chain, as there is normally neither sandpaper nor razor blades inside a chain. It is much more important to test and match the two sliding partners - i.e. chain and cable material - to each other.

Media and different temperature influences also play a central role in the development and testing of sheath materials for permanently moving cables. The 'cold winding test' in accordance with EN 60811-504 is usually used to determine the cold flexibility of moving cables. In this method, test cables are wound onto a mandrel and cooled down according to the test temperature. The diameter of the mandrel is adapted to the cable diameter to be tested. Once the cable to be tested has reached the corresponding test temperature, the cable is unwound again. If no sheath fractures are visible, the test has been passed and the sheath material is considered flexible at the tested temperature.

Realistic test procedures

These tests do not simulate the realistic conditions in an energy chain. This is why Igus operates its own 3800 m2 test laboratory, in which practical tests are developed and carried out that can reliably simulate reality in the applications. The products are tested for their resilience in continuous operation on over 60 different test stands and run through over two billion cycles a year. As it is important to accurately reproduce real working conditions, test axes with a wide variety of travel paths and accelerations or weather conditions are available. There is an outdoor test facility with a travel distance of 400 m for testing large energy chain systems, such as those used in crane systems.

In the 40-foot climate containers at Igus, test cables are subjected to a real-life test at temperatures of -40 to +60 °C in energy chains.

© Igus

The cold tests for energy chain cables are also carried out more realistically than described in the VDE requirements. In contrast to the cold winding test, the Igus test does not involve winding the cable once around a mandrel, cooling it down to the temperature to be tested and moving it once. Instead, the cables are continuously moved several million times at the test temperatures to be achieved under realistic conditions in the chain. The test cables are located in a 40-foot sea container in energy chains. Depending on the test objective, temperatures from -40 to +60 °C can be achieved. In addition to the temperature load, the cables are also subjected to constant bending stress. A cable is considered to have passed the test if it shows no sheath fractures.

Large-scale series of tests in the cold chamber with many materials and commercially available chain-compatible cables have shown that, in principle, no commercially available compound - not even PUR - can withstand the temperatures specified in the catalogs or data sheets during continuous movement in the energy chain. Only real endurance tests under realistic conditions provide reliable information about the service life of cables in energy chains.

Movable in the third dimension

Twistable cables and 3D energy chains are tested directly on the robot in the 3800 square meter test laboratory.

© Igus

The increasing number of robots in the factory is worth a special look. In order to ensure that robots are supplied with data and energy without disruption, cables are required that can 'keep up' with the 3D movement of the robot in continuous motion.

Robot cables for torsion applications have to be designed and manufactured in a fundamentally different way to cables for linear movement. The latter are as compact as possible, tightly stranded and have outer sheaths extruded at high pressure. This particularly 'hard' construction allows the cable to follow the movement of the energy chain. Robotic cables, on the other hand, require force compensation and loose stranding elements, various gliding levels and completely different shielding concepts to ensure a long service life even after several million torsional movements. This is because in robot technology, the cables are exposed to a wide variety of directions of movement. For example, the diameter of the cable structure can change depending on the angle of torsion. In order to compensate for the forces acting on the cores, Igus uses damping elements and torsion-absorbing fleece to construct the core assemblies, which are specially designed for use in dynamic torsion applications.

The requirements for the shielded versions are particularly high: Sliding elements are inserted under and over the shields to prevent the forces acting on the shield wires from becoming too great. They ensure freedom of movement of the shield in relation to the overall stranding as well as to the outer sheath. The shield structure is designed as a folded structure with damping elements in the direction of the fold. This 'soft' design gives the entire cable construction the necessary freedom of movement, reduces tensile and compressive forces and prevents machine downtime caused by premature wire breakage.

In many applications, the robot axes perform rotary movements of up to ±360° per meter. If the cycle times are short, a six-digit number of cycles is quickly reached. As space is usually at a premium in highly automated systems and workstations, the cables need the smallest possible outer diameter and a small bending radius. With these requirements in mind, Igus has developed a new twistable bus cable for Ethernet, Profinet and Profibus. The development work was based on the 'CFRobot8' series, for which the manufacturer guarantees five million operating cycles and a maximum twistability of ±180° per meter.

Anyone developing twistable bus cables must always resolve a conflict of objectives: On the one hand, the cables should transmit high data rates with great reliability. This means that the shielding must be effective and as tight as possible; additional elements in the cable structure are necessary to enable compensating movements to be carried out. On the other hand, it is important to keep the cables extremely flexible and extremely compact - a tightly braided shielding is therefore rather unfavorable.

To meet these requirements, Igus designed and tested a cable with special characteristics over a period of five years: the bus cable family for Ethernet, Profinet and Profibus - CFRobot8.Plus - is guaranteed to achieve ten million cycles and can be twisted 360° in both directions with a cable length of one meter. This increase in performance was achieved thanks to a special braiding of the shielding structure: the braiding pattern is similar to a knitting pattern that 'carefully' omits a few stitches, but still achieves an identical shielding effect with significantly greater mobility of the cable. This design has also made it possible to reduce the outer diameter of Ethernet and Profinet: to 7.5 mm for Ethernet and 7 mm for Profinet. The cables are therefore not only suitable for particularly mobile applications, but also for small installation spaces.

Author:
Rainer Rössel is head of the Chainflex cables division at Igus in Cologne.