Twist and Flex Cable for Robotic Applications

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Posted On:
June 01, 2020

Robots, with varying levels of integration and human interaction, are becoming more and more commonplace in the industrial manufacturing environment. The labor gap in manufacturing and struggle to hire individuals interested in building their career in manufacturing is no secret. As the price of robots and cobots fall, and labor costs increase with less available workers available on the market, more and more robots will enter the “workforce”.  What does this mean from a cable and wire standpoint? While designing your next robotic application, it is important to consider the connectivity aspect and what types of stressors will be put on the cable and what the environment will be like to ensure optimal performance!

As with any project, a requirements gathering session is imperative to designing the right solution for the needs of your specific robotic application. Having an understanding of the robot’s job including the functions it needs to perform and the environment it has to perform in will help the cable design engineer choose the appropriate components and cable materials.

Requirements Gathering

  • Identify Type of Robot
    • How will it move and how often
  • Robot’s Environment
    • Amount of time of exposure to extreme temps or liquids, etc.
    • Temperature where robot needs to perform
    • Frequency of the exposure
    • Concentration of any liquids or chemicals
  • Critical-to-Quality Characteristics
    • ‘Needs’ vs. ‘Nice to Haves’
  • Flexibility vs. Flex Life Expectations
    • Flexibility – suppleness, a measure of how much movement a cable can tolerate at a given time
    • Flex life – how often the cable will need to flex

If the project you’re working on is to improve an existing design that is underperforming, another important aspect of the initial design consultation will be to get an understanding of any existing pain points or performance failures that are occurring. That way the Cable Design Engineers can evaluate the failure and what may be causing it, which will influence which materials will be chosen to solve the problem and improve performance of the cable solution and the robot!

Extreme Engineering Capabilities

Designing from the inside out

  • Inner Cable Conductors

Stranding configuration can make a big difference on overall flexibility as well as durability of the conductors, contributing to flex life. In general, copper stranding will always be more flexible with a higher strand count. As far as the stranding design goes, a rope lay construction will definitely be the most flexible option, but will also be the most expensive. A rope lay is made up of a number of individual bundles of stranding twisted into groups that make up one larger, solid configuration. Many customers will opt for a concentric configuration with a high strand count, which will still be very flexible and will hold up to repeated flexing demands or a longer flex life. Concentric stranding is made up of a solid conductor running down the center that is surrounded by layers of rotating strands. Each layer is comprised of strands running the same direction, but the direction alternates for each layer.

Aside from the stranding configuration, the material also makes a difference on overall flexibility and flex life. Bare copper is the most commonly used conductor type and is preferred in most applications because it is highly conductive and very cost effective. Offering a pure signal over long distances, bare copper also has natural flexibility and high tensile strength. With a temperature limit of 150°C, it is not the best suited for high temperature applications. If high temperature performance is required, silver or nickel-plated copper may be a better choice.

Alloys are a combination of 2 or more metals. Silver, nickel, copper, beryllium, cadmium, RoHS-compliant chromium are selected for their overall toughness, break strength, and flex-life. Alloys are another great option for high temperature applications.

Finally, the conductor insulation can either offer suppleness or rigidity depending on the material chosen. When looking to optimize flexibility, it is a good idea to choose a material with a high dielectric strength. This feature means that you can extrude a thinner wall, which means a smaller diameter and less material in the way, ultimately making it more flexible. Additionally, the durometer or ‘hardness’ of the material comes into play. A softer material such as a material with a ‘shore a’ durometer of 80 or lower will be easier to bend and flex.

  • Strength Members

Strength members help add support and preserve the integrity of the cable despite being put through different types of movement like pulling, torsion, bending, rolling, etc. Aramid fiber, such as Kevlar® is a strong material ideal for extreme pulling strength while maintaining flexibility and flex life. Aramid fiber can be stranded in a variety of configurations to achieve differing levels of strength. For example, a bundle of four strands provides 450-600 Newtons of pull strength. While it is not rigid, it does offer high compressive strength without adding significant weight. If aramid fiber is used in a cable solution, special attention must be given to the cable’s termination, as incorrect termination can lead to weak points in the fiber.

  • Separator

Separators serve as barriers between the cable’s core components and the outer jacket material so that they do not bond together. Flex facilitating tapes such as PTFE or spun nylon will help increase both flexibility and flex life. These tapes can be wrapped in-line with jacketing and will help the cable flex without compromising the structure of the cable. It will also protect the cable from repeated flexing in a demanding, high flex life application like a robotic arm.

  • Shielding

Shielding provides protection by increasing both structural integrity and signal integrity. Selecting the right shielding materials and configurations can add strength, flexibility, and noise suppression. The primary types of shields include a foil shield, a box weave or basket weave braid shield, and a spiral shield. 

Reduce Interference

The most effective shielding against interference is a combination of braid and foil shields. From an electrical standpoint, foil will guard against high frequency noise and a braid will guard against low frequency noise. A combination of the two is ideal for signal integrity.

Increase Flex Life

In a demanding application like an industrial robot that is constantly moving and flexing on several different axes, a more robust shield like a box weave braid would be recommended. A box weave, while stiffer, will stay together throughout the flex life of the application due to its interlocking nature.

  • Cable Insulation and Cable Jacket Materials

When selecting insulation and jacketing materials for applications requiring cable flexibility such as robotics, it is a good idea to choose materials with a high dielectric strength and low durometer. A higher dielectric strength means you can extrude a thinner wall, which means there’s less material in the way, making it more flexible. It is also best to choose a ‘soft’ material with a ‘shore a’ durometer of 80 or lower. The softer the cable, the easier it will be to bend and flex. A thinner wall and softer material will also increase the cable’s suppleness.

Retractile cords on robot arm
  • Retractile

We see retractiles being utilized in many different types of industrial robotics. For example, the retraction and extension might be beneficial on a tabletop type of robot where the arm will be moving up and down repeatedly. The retractile will allow for the extension and retraction while also keeping the cable out of the way. If a robotic arm will be moving up and down and perhaps rotating at the same time, a small section of retractile in the area of the cable where the majority of the flexing will be happening will help increase flex life. This is because that small section of coil will take on the brunt of the movement, protecting the inner components of the cable from wear and tear and preventing failure. Download our Retractile Cord data sheet to learn more about their key features and benefits as well as custom options for coil cords.

  • Assembly

There are many options when it comes to terminating your cable to a connector especially when flex is a critical factor. An over-molded strain relief is going to give your assembly a much longer flex life than a boot. The over-mold will offer added protection from flexing near the connector or pulling that might occur from flex demands further down the cable. Additionally, a service loop inside the connector will offer a buffer, so that any pull on the cable will not immediately pull the conductors out of the pins.

Whether you need a flexible or high flex life cable solution for your end application, Northwire is ready to partner with you to optimize your cable design to ensure performance. Cable Design Engineers know that industrial robotics in a manufacturing environment are critical to efficiency and maximum output, so they are equipped to make recommendations and choose materials that will prevent any down time or loss of productivity! From napkin sketch to complete specifications, a brand new project or an improvement to an existing design, connect with Northwire to develop a solution specific to your project’s needs.

Connect with a Cable Design Engineer today!