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SMALLER, LIGHTER ROBOTS DEMAND DIFFERENT CABLING

19-06-2019
by 
in 
The Italian made Comau Racer is a fine example of a compact lightweight robot

The trend towards smaller and lighter collaborative robots poses major challenges in terms of cabling and connectors, says Lapp’s Simon Pullinger.

Large industrial robots have shaped the image of robotics for decades, but today’s cobots are rewriting the rulebook. These lightweight robots are designed to be quickly trained for new tasks, making production so flexible that an armada of motorised arms can assemble a smartphone one day and sort eggs and check them for cracks the next.

However, even compact and light robots need cables to supply their drives with power, open and close their grippers and transfer data from sensors and cameras. But these cables need to be both space- and weight-saving.

If you compare a conventional, large industrial robot with a modern cobot, in most cases you won't be able to see anything. That's because the cables are housed inside the arms.

So while one of the key properties for the cables on the outside of a robot is the ability to handle narrow bending radii, this criterion is even more important when the cables are on the inside as the bends are even tighter.

While there are a number of ways to make cables more compact and more robust, the required copper cross-section of any cable is determined by the specifications of the servomotor or tool that has to be powered, so there is no leeway in reducing the cross-section.

However, careful selection of the appropriate cables can save some space.

The motors are smaller for compact robots, so they require less power and therefore smaller conductor cross-sections.

It is similar for data cables: if only a few sensors with simple binary signals like position sensors are to be scanned, then a fast CAT 6A Ethernet cable is not required - a slimmer CAT 5 cable will be sufficient.

Once these simple saving measures have been exhausted, there are still a few other options for improved cabling.

For example, polyethylene (PE) is commonly used as a dielectric in data cables, and in the case of high-quality data transmission cables, the PE is foamed during the extrusion process. Regulating the amount of gas introduced into the production process allows cable manufacturers to optimise properties such as capacitance or impedance, enabling the production of thinner space-optimised cables.

In order to improve insulation and reduce the size of data cables, insulation material is foamed during the extrusion process

The lay of the copper strands within the conductors also has a part to play. Typically the strands are twisted because this improves the flexibility of the cable. (If all the strands and all the wires were parallel, the outer copper wires would be stretched at each bend in the cable and the inner ones would be compressed, leading to an extremely rigid cable.)

So the thickness and flexibility can be controlled by the length of lay (ie the distance for a round of twisting). If it is longer there is less twist and the cable turns out thinner.

Lapp has developed wires for power transmission with special (longer) lengths of lay for a manufacturer of compact robots. And in data cables, twisted pairing of the wires also improves fault-free transmission of data.

There are also changes to the tasks that the cabling has to do.

In classic robotics, control signals, energy and sensor data are often transmitted by parallel wiring. But when larger amounts of data have to be transferred serial cabling is used, usually Ethernet CAT 5 cables with datarates up to 100Mbit/s.

In compact robots, these lines are structured as star quads. The two wire pairs are twisted in the interior of the sheath so they occupy less space yet still tolerate long-term bending and torsion.

The situation is different when the robot has multiple sensors or even a camera as the two pairs of wires cannot cope with the datarates required. In this case, CAT 6A Ethernet cables can transfer data at up to 10Gbit/s. These cables contain four pairs of wires, which are stranded as pairs, and the four pairs are then twisted - which unfortunately means they take up more space than star quads.

Fibre optic cables are still a niche product in robotics. However, they might be a viable alternative in applications that require very high datarates, and they are also attractive in applications where electromagnetic interference might be present – such as in welding robots.

Contrary to common belief, neither glass nor plastic optical fibres are inflexible, although there are limitations on their minimum bending radii. This does rule out the use of glass fibres in compact robots. And with plastic fibres generally only capable of 100Mbit/s, they are no faster than copper data cables. But they do have the advantage of being immune to EMI.

The demand for more compact robot cables has also impacted connectors. Large robots are controlled from the control cabinet by a supply line connected to the robot's foot with an industrial rectangular connector. Smaller robots us an M23 circular connector.

Applications flexibility is a key factor. For example, if a robot is retrofitted with a camera for quality inspection, the connector should be able to accommodate the additional data transmission.

Modular connectors, such as the easily extendible Lapp Epic MH connector system, accommodate all kinds of plug-in modules for energy, signals and data. If a function is added, another module is simply inserted or replaced.

Lapp Group has recently created a special connector for an innovative manufacturer of compact robots. The connector had to be easy to fit, harmonise with the design of the robot, have exemplary EMC performance, incorporate strain relief, be qualified for Ethernet data transfer, and be attractively priced. Lapp finally solved the conundrum with a modified M23 circular connector.

The general industry trend towards ready-made cable sets is having limited impact in the world of robotics.

For supply cables from the control cabinet, there is a clear trend towards ready-made products such as the Lapp Ölflex Connect range. And that trend looks like expanding into the construction of the robots themselves.

However, while hybrid cables are becoming popular with other machine builders, they are really not suitable for compact robots.

Lapp Australia
1800 931 559
www.lappaustralia.com.au

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