Samtec Tech Talk: Connectors in Robotics Design

Abstract/Introduction

Robots have been part of human society for decades, and we’re not talking about the cute or futuristic designs that make headlines or create a stir at tech conferences. They’re the machines that work behind the scenes and are responsible for making many of the products we use in our daily lives.

Difficulties encountered in robot design

These industrial robots are relatively unintelligent by today's standards, but they perform an important function. They are designed to perform the same tasks repeatedly and with great precision. The automotive industry has been using industrial robots for more than half a century. They perform dangerous or dull tasks such as welding, painting and assembly.

Creating connector solutions for industrial robots is a headache for manufacturers, and the key consideration is reliability. When we describe a connector, we often use mating cycles as a key measure of its service life. A mating cycle is defined as the act of mating and unmating a connector once, and most manufacturers publish a minimum expected number of mating cycles for each type as a measure of its reliability.

It is not surprising if there is a wide variation in the actual performance of connectors when they are used in the real world. In a clean and comfortable environment, and when used correctly by an experienced technician, a connector may last far longer than it needs to. Conversely, if a connector is used in harsh field conditions and is frequently unplugged in a hurry, the user may find that its service life is far less than expected. This can easily be seen when we look at the factory floor where industrial robots must perform tasks.

Another problem faced when designing for a robot is motion. A robot moves quickly, often in multiple directions, and for hours or even days at a time. All parts of the robot—its arms and appendages—need to be connected to each other to provide the data, power, and signals they need to work. Every joint that moves is a problem for designers.

Design and test for reliability

When designing for robots, the structure within the connectors they use is central to their performance. Most of us are familiar with connectors that use terminals designed in a traditional plug and socket arrangement. This has been with us for a long time as it provides one of the best ways to connect two circuits.

The concept of traditional terminal and socket design is that a fixed terminal is inserted into a slightly smaller socket. The socket can bend enough to allow the terminal to enter and generate a spring force to ensure contact between the two halves.

One advantage of this design is that pin terminals are relatively easy to manufacture. The terminal can be solid, machined from solid bar stock, or hollow, made from flat sheet metal that is then rolled or formed into a three-dimensional shape. While different manufacturing techniques have their own advantages, the important point in this case is that the terminal itself is fixed.

It is the socket terminal that provides the flexible element in the design. The socket is slightly smaller than the pin, so a certain amount of flexibility is needed to accommodate it. As the socket expands to accommodate the pin, it applies a positive pressure to the pin. This is important to ensure good electrical contact between the mating halves and to reduce the effects of vibration that may occur in normal use.

However, positive spring pressure creates other problems. To overcome the spring resistance, forces are required to engage and disengage the terminals, referred to as insertion and extraction forces, respectively. While the forces required for a small number of terminals may be negligible, this is not the case when we consider a connector with many circuits. In order to successfully mate such a connector, a large amount of force must be applied, so the operator must ensure that the connectors are properly aligned.

Cable risks

Cables also present risks when used in industrial robots. When designed for robots, cables need to be protected, ensuring they can’t bend beyond their built-in limits or twist in cumbersome ways. Even if this can be achieved, cables are still made of metal, albeit very thin wires braided together to make a flexible cable. As we’ve seen many times throughout history, metal can fail. The constant movement of the cable itself can weaken the wires, severely affecting their performance and compromising their reliability, even before the cable physically breaks.

However, with repeated movement, the cable will eventually break, and the connector will become extremely important as the cable will need to be removed and replaced. There are some very impressive cables that offer a long working life, just as there are high reliability connectors. I remember using a multi-conductor cable that was so soft that it felt like a piece of cooked spaghetti. But eventually, even this cable failed from repeated use.

Perhaps the biggest headache when designing for a robot is power. Modern battery technology is now more capable than ever, thanks in large part to the automotive industry. However, batteries need to be charged, and until some clever engineer figures out a way to deliver significant amounts of power over a Bluetooth link, connectors are the way to go.

As long as robots continue to evolve, Samtec will keep coming up with new ways to connect them. For the latest products, visit our Industrial Applications page, where we explore the world of industrial automation and autonomous robots.