Large industrial robots have shaped the image of robotics for decades: With their mighty arms, they heave car body parts back and forth behind fences and weld them together. But the image is changing: their successors are getting smaller and smaller, they are no longer fenced in and even work hand in hand with humans. Human-robot collaboration is also one of the major topics at the upcoming Hannover Messe.

The lightweight robots are designed in such a way that they can be quickly trained for new tasks, for example by being manually guided by a person. The aim is to make production so flexible that an armada of motorized arms can assemble a smartphone today and sort eggs and check for cracks in the shell tomorrow. There are already pilot projects in the automotive industry in which robots drive through the factory on processing stations and find their own work. Such robots could one day build cars from the inside out. And all this in batch size 1, i.e. each individual end product manufactured to customer specifications.

So much for the vision. In practice, there are very practical hurdles that robot manufacturers have to overcome. Take cabling, for example: even compact and lightweight robots need cables to supply the drives with power, open and close grippers and transmit data from sensors and cameras. These cables must save space and weight. If you compare a conventional large industrial robot with one of the new lightweight small robots, you usually don't see anything in this respect with the small robot - because the cables are routed entirely inside the arm. If they were on the outside, they would be constantly in the way when working with humans and in confined spaces. However, this does not make cabling any easier. If cables on the outside of the robot already have to withstand tight bending radii, the cables inside the robot arm run even tighter around the axes or are almost kinked.

So what are the specific levers for making cables more compact and even more robust so that they can withstand the tight bending radii? First of all, the cross-sections are determined by the power requirements of the consumers, such as the servomotors - the designer has no leeway here. However, the demand for smaller motors in compact robots is lower, so thinner cables can also be used.

It is therefore important that the designers abandon the principle of 'a lot helps a lot'. Sometimes servo cables are oversized because the same cables are used as in larger robots, whose servo motors are correspondingly more powerful. It is therefore important to select the right components. This also applies to data cables: If only a few sensors with simple binary signals such as position sensors are to be queried, then a fast Cat 6A Ethernet cable is not necessary - a Cat 5 cable is completely sufficient in such cases. Once all these cost-saving measures have been exhausted, there are still a few more levers for lighter cabling:

The insulation

To improve the insulation of data cables, insulation material is foamed during the extrusion process.

© Lapp

The insulation prevents short circuits between the cores and has a major influence on the transmission properties, particularly in the case of data cables for fieldbuses or Ethernet. Data cables with good transmission properties should have a low capacitance. The latter is determined by the distance between the cores and the insulating material between them - the so-called dielectric. The best dielectric would be air. As it is of course not possible to manufacture a cable with this, it is important to use a material with the lowest possible dielectric constant as insulation. A proven material for this is PE (polyethylene).

In order to further improve the data transmission properties of PE, the PE is foamed during the extrusion process for high-quality data cables. By controlling the amount of gas introduced during the production process, properties such as capacitance or impedance can be precisely adjusted. In short: by adjusting the parameters of insulation thickness and gas quantity, a thin, space-optimized cable can be produced - such data cables are definitely recommended for use in compact robots.

Stranding

To understand what is meant by stranding, let's compare it to a plait of hair: the tighter you plait it, the thicker it gets. If you take the same tuft of hair parallel - i.e. as a 'ponytail' - it is noticeably thinner. Something similar happens with the copper strands when stranding: The fine metal wires are twisted because this improves flexibility. If all strands and all cores were to run parallel, the outer copper wires would be stretched and the inner ones compressed every time the cable was bent. The result would be extremely rigid cables.

Thickness and flexibility can be controlled by the lay length - i.e. the distance for one turn of the twist. If it is longer and therefore the twist is smaller, the cable is thinner. With this in mind, Lapp has already developed cores for power transmission with a special, i.e. longer lay length for a manufacturer of compact robots, for example. In the case of data cables, the fact that the wires are stranded in pairs also improves the interference immunity of the data transmission.

The tasks that cabling has to perform are also changing. In classic robotics, control signals, energy for servomotors and information from simple sensors are often still transmitted using parallel wiring. For faster connections and when larger amounts of data need to be transmitted, serial cabling is increasingly being used - usually via Ethernet Cat 5 cables with data rates of up to 100 Mbps. For compact robots, these cables are designed as a 'star quad'. This means that the two wire pairs are stranded inside the sheath in such a way that they take up less space compared to the aforementioned pairwise stranding and can withstand years of movement such as bending and torsion.

A star in the middle

With the Etherline Torsion Cat 6A, a plastic core with a cross-section is stranded between the four wire pairs, which keeps the wire pairs apart and ensures that they do not change position during movement.

© Lapp

The situation is different if the robot carries many sensors or even a camera, for example to detect parts or for quality control. In this case, a star quad cable is no longer sufficient, as the enormous data rates cannot be managed with just two wire pairs. Ethernet cables according to Cat 6A are the better choice here. They transmit data at up to 10 Gbit/s. These cables contain four pairs of wires that are stranded as pairs. In addition, the four pairs are stranded, which means that the space required is correspondingly greater than with star pairs. A plastic core with a cross-section is also stranded between the four pairs of cores, which keeps the pairs of cores apart and ensures that they do not change position during movement and that crosstalk between the pairs of cores is reduced. Ultimately, this makes the cable suitable for rough use on robots.

Let there be light

Fiber optic cables are still a niche product in robotics. In applications that require very high data rates, they can be an alternative to copper cables. This is particularly the case when strong and pulsed currents flow in the vicinity - for example near welding robots. This can disrupt data transmission on electrical cables and lead to failures. The widespread assumption that fiber optic cables are not suitable for moving applications is not true. Both glass and plastic fibers can be used flexibly, although the minimum bending radii must of course be observed. For small robots, this can be a limiting factor for the use of glass fibers. In this case, plastic fibers are the better choice, although at 100 Mbit/s they are no faster than standard copper data cables.

Evolution of connectors

The robot manufacturers' desire for more compact cables also has an impact on the connectors. Large robots are controlled from the control cabinet with a supply cable, which is usually connected to the base of the robot with an industrial rectangular connector or, in the case of smaller robots, with an M23 circular connector. The main requirement here is flexibility. If, for example, a camera is retrofitted to a robot for quality inspection, it should also be easy to expand the connector. Accordingly, modular solutions such as the easily expandable 'Epic MH' connector system from Lapp are ideal. This can be modularly equipped for cables with a wide range of functions and can accommodate any number of connector modules for power, signals and data. If a function is added, a further module is simply inserted or another one replaced.

However, there are also cases in which a modular solution such as the 'Epic MH' cannot fulfill all user requirements as standard. For example, a manufacturer of compact robots made the following request to the Lapp Group: the connector should be quick to install and blend harmoniously into the design of the robot. In addition, it should have particularly good EMC properties, be qualified for Ethernet data transmission and, last but not least, perform housing functions such as strain relief for the cable - all of this at an 'attractive' price. What sounds like squaring the circle, Lapp finally solved with a modified M23 standard circular connector.

In previous industrial robots, power and data are usually transmitted on or in the robot arm via cables with M23 circular connectors. For smaller robots, this standard has been shrunk to the M17 and M12 formats, with the latter in particular enjoying growing popularity. However, if it needs to be even smaller - for example for robots with purely internal cabling - such connectors are no longer practical. In some cases, the cables are connected via small connectors, such as those used to connect printed circuit boards.

Trend towards ready-made products

When it comes to supply cables from the control cabinet, the trend is clearly towards ready-made assemblies, such as those offered by Lapp in its Ölflex Connect range. When it comes to laying cables on or in the robot, self-laying cables by the meter continue to dominate. But here, too, there have recently been requests for prefabricated assemblies.

Hybrid cables are a general trend in machine cabling. They contain all the necessary cables - sometimes also pneumatic and hydraulic hoses - in a common sheath. However, this concept is not suitable for compact robots. Due to the close sequence of connection points on up to six servomotors and sensors, combining them in one sheath does not make sense.

Authors: Frank Rothermund is Market Manager Robotics at Lapp, Ralf Moebus is Head of Product Management Automation & Networks at Lapp and Joachim Strobel is Product Manager for Connectors at Lapp.