A Reliable Approach to Modern Harness Systems.
Modern military ground systems today, whether they be communication shelters, air defense systems or main battle tanks,
driven by the need for more technologically advanced intelligent systems, incorporate an increasing amount of complex
and sensitive electronic equipment. The electrical interconnecting harnesses that link these vital pieces of equipment
together are akin to the human nervous system, providing and transmitting essential information, often in extreme environments.
In the past, systems were designed for specific missions and for long service life. The pace of technological advancement
today, as missions and system requirements change to meet new threats, now requires platforms to be adaptable to the possibility
of several upgrades and equipment modifications during it's life time. This presents the harness designer with the challenge of
combining long term reliability and reparability with the ease of adapting the wiring systems to evolve with each phase of the
vehicle or system life.
The harness system - comprising electrical connectors, wiring, screening, grounding components, protection, identification etc.
- typically represents about half to two percent of the total platform cost. Yet despite this low relative cost, it's reliability
is as critical to the platform's performance as the weapons system, the engine or communications. The harness system can not be
the weakest link. A main battle tank with it's electrical system malfunctioning or with unwanted electrical interference disrupting
it's stabilization system, makes a vulnerable target. This article addresses the key aspects of harness design necessary to achieve
dependable interconnection systems and how computer aided design is revolutionizing the time required to complete new harness
designs and modifications.
The Environmentally Sealed, Repairable Harness System.
Totally sealed harness systems have proven themselves in service for over thirty years and thanks to continuous improvements in the
range of components and materials, still represents the state-of-the-art today. Most ground vehicle and system platforms in Europe,
North America and Asia have adopted totally sealed systems. Sealed systems address the areas where harness components are most
vulnerable, corrosion, stress, strain and electromagnetic interference (EMI). let's examine these potential failure modes.
Corrosion
Humidity, moisture, salt and corrosive fluids can corrode conductors and contacts. What is worse, the corrosion can take place well
beyond the point of penetration because of the small tubelike voids -capillaries - between the individual strands of copper that make
up the conductor. Called "capillary action", the penetration of a fluid can "wick" many feet in a relatively short time
(see photo 1), depending on the specific characteristics of the affected wire. As the copper in the conductor is depleted by
corrosion, the conductor can no longer sustain mechanical or electrical loads and the metal will fail. Mechanical failure can thus
occur anywhere in the wiring system.
Even before mechanical failure occurs, electrical performance can be adversely affected by the presence of non-conductive by-products
from galvanic or aqueous corrosion. Moisture within a connector body may cause an impedance mismatch, increase noise in a signal
circuit, or modify the waveform. Even small amounts of corrosion or other contaminants can have a significant impact on contact
surfaces and the efficiency which signals flow through them.
If a chemical solution contacting the electrical connection is itself conductive it can cause a short circuit between conductors.
Pure water, not itself a conductor, can also facilitate a short circuit by providing a medium into which conductive salts can
dissolve. These salts may be the by-products of corrosion or the result of earlier contamination.
High humidity and temperature cycling in some situations cause condensation, the accumulation of which can also result in a short
circuit. Depending on circumstances, the resulting short circuit may be intermittent, significantly complicating the process of
identifying the underlying cause.
Environmentally sealed repairable harnesses incorporating a compatible system of heat-shrinkable molded parts, heat shrinkable
tubing's and adhesives, significantly reduce the risk of corrosion. A variety of materials and components are available to meet
the performance requirements of applicable military and customer specifications.
Stress and Strain.
Wires that are connected to the connector pins need help to withstand stresses and strain from the cable, which could break the
wires from the pins. It is almost always necessary to prevent strain from occurring in a weak spot, such as where wire is attached
to the contact. This is called strain relief and can be provided in a variety of ways, from mechanical devices, such as adapters,
to heat-shrinkable molded parts (see photo 2) and semi-rigid heat-shrinkable tubing's.
EMI (Electromagnetic Interference)
EMI is similar to the noise heard on an AM radio when the radio is close to high-voltage lines. EMI causes the wire or cable to act
like an antenna and pick up electrical signals, which interfere with the signals on the wire and can cause malfunctions in sensitive
electronic equipment.
Wiring systems are susceptible to two types of EMI:
-
Radiated emissions (the
electromagnetic energy a wiring system radiates to it's surrounding
environment), such as the EMI a high-voltage line radiates to it's
surroundings.
-
External radiated emissions (the electromagnetic energy in the environment) such as the EMI an AM radio picks up from
a high-voltage line, causing distortions in the conducted signal. Conducted EMI is noise carried by the cable into the receiving
circuit.
To reduce susceptibility to radiated emissions from the cable or from external sources, the harness must be shielded, grounded and/or
filtered. Before the appropriate shielding system can be designed, the following must be determined: a) anticipated level of
intrference, b) type of threat and c) required level of shielding.
To design a cable harness to meet a particular level of shielding, the entire system -from connector to connector- must be
considered. This can involve the cable braid design and termination, the connectors, the shielding of individual components within
the cable, and the splices, feedthroughs, and cable breakouts. Simply increasing the visual coverage of a braid does not necessarily
lead to higher levels of shielding. However, computer-aided braid design (also called optimization) has been developed and can
increase screening efficiency by as much as 20 decibels (dB) without increasing the size or weight of the braid. Using two optimized
braids increases shielding efficiency further; adding a high-permeability tape between the two layers of optimized braid produces even
higher levels of shielding. The shielding effectiveness of cable braids at high frequencies is measured as surface transfer impedance
(Zt) . The lower the impedance the higher the shielding effectiveness. It is important to note that shielding effectiveness is also a
function of length, so this is particularly relevant when designing a cable harness.
When terminating braids to connectors, good practices allow for 360 degree shielding between the back of the connector and overall
cable shield, leaving no "windows" for EMI to enter, available connector termination's provide shielding levels from 40dB (low
interference level) to 80dB (high interference level), when tested at 30 MHz. When tested at the same frequency standard braids will
offer shielding performance of 40dB/meter to 50dB/meter and up to 60dB when optimized, with higher levels of screening going up to
135dB/meter.
Repairability
Once installed, cables are often difficult to remove. Complex branched harnesses are frequently only removed by dismantling the vehicle
or system. A key feature of using heat-shrinkable molded back shells on connectors is that this allows repairs or changes to be made at
the connector without removing or replacing the harness. Using a repairable harness system simplifies logistics as fewer spares need to
be held in the depot and it reduces down time for the vehicle or system.
Computerized Custom Designs.
Often the design, prototyping and installation of the harness is left to a late stage in the process of developing a new platform, which
adds to the challenge of timely harness design and making an efficient equipment fit. Larger systems can have 200 or more individual
harnesses. In order to speed up the design, specialized CAD software is being developed which can reduce design time by up to 90%.
One such PC based design system has been developed by Tyco Electronics Corporation. Called
HarnWare it contains a library of more than 200 intelligent harness drawing shapes and
catalogues more than sixty thousand wiring harness products.
Once the designer has made decisions about wire size, routing, connector selection, shielding requirements and operating environment,
HarnWare will use this information to select the right sized components to protect the
harness and its environment. Shapes representing harnessing products are dragged and dropped into the drawing, automatically snapping and
gluing together. Dimensions and connector references are entered by clicking a shape and typing in the relevant information. The system
automatically traces on screen the route of each wire in the point-to-point wire list through the harness geometry, calculates wire lengths,
determines the best positioning of mixed-diameter wires in each harness leg and outputs bundle diameters.
In about an hour a complex harness design can be finalized and documented, including a drawing (see photo 3), bill of materials, labor
estimate and wire list. HarnWare can generate harness documentation in several
languages including Chinese, English, Japanese and Korean.
Summary
The proven performance and adaptability of environmentally sealed heat-shrinkable harness allow the designer to reliably select the
optimum performance for mechanical, electrical, EMI and chemical environments. Ease of reparability in the field as well as the
incorporation of modifications and upgrades is becoming increasingly important. With the aid of new PC based software programs the
time to design and modify these harnesses, as well as the cost of design, is substantially reduced.
Raychem Corporation was acquired by Tyco Electronics in 1999 and is now part of Tyco Electronics Corporation
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