Built slim, built tough
By Rod Barber

Tailor-made cables for rugged performance

Slim is in - not only at the beach or the fitness club, but also in high-flex cables put to work in robotics and industrial automation.

Putting cables "on diet" (reducing their overall diameter) is one of the most common challenges to design engineers involved in building tailor-made cables. A significant portion of custom-built cables is specifically designed for applications in robots, where space is extremely limited and cables often must perform in tight cable tracks or in a confined working environment.

What's in it for the user? Tailor-made power and signal cables with smaller diameters than conventional cabling - yet sport identical, or even superior, electrical and mechanical properties - means less space, less weight, more flexibility, longer life and ultimately, improved cost performance.

To endure harsh industrial environments, high-flex automation cables need to be capable of withstanding a wide spectrum of stresses. The challenges range from millions of bending cycles at high speeds in tight cable tracks, to continuous twisting and severe mechanical stresses. Often, cables in robots have to perform in narrow vaults or crammed housings. Space is forever at a premium. And then, there are environmental factors to counter: the presence of high temperatures, wet or moist conditions, chemical contaminants, humidity, etc. The first of these brings to mind an important consideration applicable to welding robots that are perhaps among the most ubiquitous of robots at work in industry.

The harsh landscape and severe challenges of industrial applications have pushed cable manufacturers to develop a host of solutions to combat heat and high-flex stress. To easily follow the 3-D movements of industrial robots, tailor-made automation cables are engineered for superior twisting performance, coping with 360 torsion stress. Besides a high resistance to bending stress, these cables can easily handle the forces caused by high-speed acceleration in robotics, For example, welding robots apply more than 40 spot-welds in a 90 second sequence.

Cables on welding robots must also be able to withstand flying sparks and hot welding splatter. This imposes two major design constraints: welding beads may not create holes in the exterior jacketing; and the cable material, as well as the gases generated in the welding operation, may not ignite. Special jacketing material is used to meet these requirements.

The "diet strategy"
Every tailor-made cable is designed, manufactured and tested to match each customer's specifications. For creating the ideal solution, our engineers have access to an extensive array of choices: design options for the internal architecture and subassembly of the conductors (different stranding and layer designs), internal fillers, unique shielding designs and a variety of jacketing materials. A major part of the design assessment is a thorough check of miniaturization possibilities for any given cable application. High-performance, high-strength materials allow for optimal downsizing. The key to designing a superior product for such applications is to slim down the cable, which either allows for a tighter bending radius or increases the bend radius-to-cable diameter ratio. At elocab, we build high-flex, diameter-reduced cables that can withstand a bending radius as small as five times the outside diameter, designed for a life span of up to 20 million bending cycles or 20,000 operating hours.

A common "diet strategy" applied by elocab consists of hybrid solutions. Three, five or more single cables are integrated into a new hybrid cable. The advantages of compact hybrid designs are instantly recognized. They require less space and are easier to install than conventional cabling.

Design engineers are able to substantially reduce the diameter of tailor-made automation cables - by 30 to 40 per cent - compared to standard cables, while offering identical, if not better, electrical properties. Last but not least, these slim and trim cables come with a bending radius reduced by up to 70 per cent.

EMI-shielding a major consideration
With the appropriate EMI-shielding, hybrid cables can include a complex set of power and control cables. To ensure that the cables are protected against any electric or electromagnetic interference, elocab's engineers weigh a full range of shielding options from the use of served wire shields to foil, and braided shields. Extremely difficult problems are solved by multi-layer designs, which may combine the same or different shielding methods.

"Good shielding is the answer to suppressing the development and transmission of any undesired signals," says electrical engineer Norm Frid, head of elocab's product design department. The choice of material, the diameter of the shielding conductor, the angle of pitch and the number of strands are all factors that determine the density and the specific frequency resistance of a shield. For example, highpermeability alloys (such as Mu-metal), used as a wrapped foil shield, can absorb low-frequency magnetic fields.
Classic EMI "problem sectors", such as machine building, measurement and control systems as well as communications and high-frequency technologies are among the prime users of individually shielded, hybrid cables.

Given their application-specific design, tailor-made cables can combine "contrary" properties that usually are not found in a single standard cable. Despite providing optimal shielding characteristics, they can be designed to offer a high level of flexibility and extensive durability. Perfectly shielded - and slim - tailor-made robotics cables are a good example.

A manufacturer's experience
Failure and downtime are just too costly in today's competitive business environment marked by ever-increasing levels of automated production processes. To confidently cope with these challenges, manufacturers and users of robotic equipment rely increasingly on the dependability of tailor-made cables in punishing work environments.

An important user of trim automation cables made by elocab is robotics manufacturer Cloos Schweisstechnik GmbH, a worldwide leader in welding technology, with headquarters in Germany and production operations in North America. The company, which supplies robots, robot controls, welding units and peripheral equipment such as positioners and safety devices, has relied on elocab's expertise in this area for over ten years.

Working with a high repeatability of 0.1 mm, the company's high-precision welding robots are widely used in production of truck axles, harvesters, heavy-duty construction vehicles, and trains. Operating at speeds of up to 20 ft/mm and processing a maximum of 55 lb. of coldwelding wire per hour, the stability and efficiency offered by Cloos' state-of- the-art tandem-path welding systems are among the highest in the robotics industry.

For years, tailor-made elocab's cables are on duty for Cloos' highly successful series of 6-axes ROMAT robots whose industrial arms are designed as rotating joints. "From the start, it was clear that we needed cables that could withstand the severe torsion stress caused by our new robot," explains Sieghard Thomas, the company's manager of purchasing and quality management.
Two hybrid-round cables, each of which combined six single cables, were installed on the ROMAT series. These tailor-made cables supply the power and signal information for the AC servo drives controlling the robot's precise movements. Both the power and resolver hybrid cable run up to 130 ft, creating the "bulletproof" connection between the switchboard cabinet and the robot. Rated for more than 25 million bending cycles, both hybrid cables require little maintenance.

An interesting feature of the hybrid power cable is the integration of cables with different cross-sections. Three thicker cables powering the strong motors for axes 1-3 are combined with three thinner power cables for the lighter manual axes 4-6. Using special fillers, this "uneven" cable has been transformed into an evenly balanced, round cable.

In the ROMAT base, which can handle rotation of up to 440, both compact hybrid cables are split and fed to the AC servo drives as single resolver and power cables. Here too, in the "bowels" of the robot, the cables are designed for miniaturization and extreme flexibility. To deal with the exceptionally tight spacing in the housing and the traditionally critical passageways from one robot arm to another, the wire gauges of the power cables were slimmed down from AWG 14/AWG 16 respectively to AWG 16/AWG 18.

Using a bundled cable design ensures that all conductors are identical in length, thus making them equally strong and resistant to mechanical stress caused by twisting and flexing. Also, the jacketing is tailor-made to cope with the specific operational challenges in the ROMAT robot. Since the cables automatically make contact with metal while being led through the robot's tight housing, an especially tough TPE (thermo-plastic elastomer) jacketing was selected for its proven abrasion-resistant qualities.

"Ever since we started relying on these cables for our most rigorous applications, we have rarely experienced failures due to cable breaks, which is a great benefit for our customer," says Sieghard Thomas of Cloos, affirming that "slim and sinewy" has its place in the world of automation cables as well.