96 Focus | Cable harnesses Also, the inputs and conditions they are subject to, such as variances between batches of materials or humidity or heat during shipping, will induce minute differences between wires, plastics and other materials that could interfere with the precision a machine would need to produce cable harnesses repeatably. Automation therefore still has many limitations in terms of cable harness production. There are some exceptions to this though. Braiding looms for instance can achieve fast and uniform weaves of protective jacketing material about a set of fastened cables in an automated or semi-automated way. They typically wrap their materials much tighter than human workers can, which leads to the harness being stiffer than manual workers could make it, and which can make routing and integration more challenging. However, that tightness also makes it harder for EMI, heat or anything else to penetrate or escape from the wires. A slip-on braid by contrast could be looser owing to the flexing that often occurs while workers pull or push it over wires. Also, automated machinery is available that can perform consistent and repeatable stripping of sets of wires that are controlled for their cross-sectional diameters, and semiautomated stripping devices will function by performing the cut when it detects that a person has inserted a wire into its aperture. Lastly, semi-automated crimping systems can terminate wires to contacts in a repeatable and less labour-intensive way than conventional hand-crimp tools. These are often pneumatic systems, using pressurised air to drive down the crimping tool to compress an exposed wire conductor (inserted into the tool by a human worker) against the contact terminal. The efficacy of these machines can drop as wires and contacts get smaller though, as it becomes challenging to position wire and contact together for the machine to work. Performing the termination by hand might seem more onerous, but for twisted pairs it is vital, as the pair must first be slightly untwisted, so that the wire can be stripped at the correct length, the conductor can be crimped or soldered onto the contact, and finally the crimped cable can be re-twisted back to the form it started with. Crimping is generally agreed to be more easily performed, more repeatable, easier to train and check for, and safer than soldering, although soldering can be preferred in certain use cases, with some power wires measuring around 10- 14 AWG being designed for soldering only. Also, one advantage of soldering is that crimp tools are more expensive, but most applications nonetheless recommend crimping, with some companies estimating that 75-80% of their cable assemblies are crimped. Either way, that process must be handled carefully, particularly for twisted pairs so that the twist is maintained as the wire terminates at the connector. Any disruption in the twist that makes its way into the final product will lower the impedance and EM performance of the connection. As a final, optional manufacturing step, some suppliers will perform back-potting, for greater stability and protection for the cable assembly against ingress of water and dust through the back of the connector shell. This typically involves applying resin, a curing phase, then a visual inspection. Testing Quality control systems in manufacturing typically ensure consistency in physical and performance characteristics between units and batches of harnesses, with both human inspectors and automated vision systems scanning cables coming off production lines such that tests are limited to initial units of a new design, or to one-off sample tests to gauge the viability of a batch in production. Certification and homologation requirements typically define the type and extent of testing cable harnesses. These can include electrical tests in which they are cycled through different voltages and currents, to observe how they behave and last over extended use and misuse, or what resistance characteristics they exhibit, which becomes increasingly important with longer cable lengths. Also, flexibility tests will put a physical strain on cable assemblies to replicate October/November 2023 | Uncrewed Systems Technology Some semi-automated and automated machinery is available for repeatable production steps such as stripping or crimping (Courtesy of Harwin)
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