Not too thin!

In the skin-foam-skin process, three extruders apply the wire insulation in a single work step. This means that the insulation can be particularly thin to enable cables with small cross-sections.

© Lapp

Suppliers of connection systems have various levers to make cables more compact and at the same time more robust. However, there is hardly any leeway when it comes to the conductor, as the power requirement of the servomotors determines how thick the conductor has to be. However, motors are also becoming smaller and their output lower, so that they require less current and therefore smaller conductor cross-sections. In some cases, designers are wasting space because they are in the habit of using the same thick cables as in larger robots with more powerful drives.

In high-quality data cables, the PE is foamed during extrusion. Properties such as capacitance or impedance can be precisely adjusted by regulating the amount of gas. This allows a thinner cable to be manufactured.

© Lapp

If space is still tight despite the optimum choice of conductor, the cable manufacturers have to come up with something else:

• Insulation prevents short circuits between the cores. In the case of data cables for fieldbuses or Ethernet, it also influences the data transmission properties. Low capacitance is important. It is determined by the distance between the cores and the insulating material between them, the so-called dielectric. Polyethylene (PE) has proven itself as a material. This is foamed on high-quality data cables during extrusion. By regulating the amount of gas, properties such as capacitance or impedance can be precisely adjusted and thinner cables can be produced.
• During stranding, the fine metal wires of the conductor are twisted because this increases flexibility. However, this increases the diameter. Thickness and flexibility can be controlled by the lay length - the distance for one turn of the twist. If it is longer and therefore the twist is smaller, the cable is thinner. However, the cables are still flexible.
• Ethernet is increasingly being used for fast connections and higher data volumes. Cat. 5 cables with data rates of up to 100 Mbit/s have four cores, which are constructed as a star quad for compact robots. The two pairs of cores are stranded inside the sheath in such a way that they take up less space compared to normal pair stranding - the diameter can thus be 30% smaller - and still withstand years of movement such as bending and torsion.
• So-called single-pair cables even have only one pair of wires. They can transmit 1 GBit/s - which is fast enough for many applications. The cables take up little space, require little installation effort and are low cost. However, cables for single-pair Ethernet are not yet available for use in industry, as the necessary standards are still outstanding. Newly established working groups are working on this, so Lapp expects the first series products for single-pair Ethernet to be available in two to three years.

If the robot is equipped with many sensors or even a high-resolution camera, a star quad or even single pair cable is no longer sufficient. Instead, Ethernet cables according to Cat. 6A or Cat. 7 with up to 10 Gbit/s are used. They contain eight cores that are stranded as pairs. These four pairs are also twisted together to save space. Nevertheless, the space requirement is naturally greater than with the star quad. If the cable is moved a lot - which is unavoidable in robotics - the four pairs of wires must be kept apart so that they do not touch each other during bending. This is done by a plastic core with a cross-section that is also stranded. It ensures that the wire pairs do not change position during movement. At the same time, it reduces crosstalk between the wire pairs.