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There's much to consider when sourcing I/O connectors for harsh environments
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By: Fred Kozlof, tech support, Conec Corp.
2012-01-18
Despite all the warnings to avoid use of electrical devices in wet areas, it seems that designers are working at a fast pace to create devices that play well with water. Traditionally, most electronic boxes have been located in a sheltered, controlled environment. However, the proliferation of electronics into every industry has resulted in the need for designers to give serious consideration to enclosure and connector design elements that ensure a reliable product function for use in every environmental condition.
The most common harsh environment applications are found in the factory automation market, outdoor communications, portable or vehicle-mounted instrumentation or navigation systems, and security/surveillance equipment. The most common I/O connectors are D-subs, M12 and M8, RJ45 for Ethernet, USB 2.0, and LC fibre optic.
Sealing against liquid ingress
Just how ‘sealed' is a sealed connector? The industry has defined a range of Ingress Protection (IP) ratings, represented by a two-digit number. The first digit indicates the ability to shield against intrusion of foreign objects or particles, and the second digit indicates the strength of which the connection's ability to withstand various levels of pressure. Within the context of this article, the main ratings all start with a 6, which represents sealing against liquid:
* IP65, the ability to seal out low-pressure hose-directed liquids
* IP67, the ability to withstand light, temporary immersion in liquid (30 minutes@1 metre)
* IP68, the ability to withstand liquid immersion for a specified time and pressure
* IP69k, the ability to withstand very high-pressure liquid jets, directed at specified angles
Most I/O connectors are not sealed to IP65 specifications. Design enhancements are required to achieve the most common harsh environment rating, IP67. This usually means component and assembly changes to incorporate potting materials/solder, adhesives or ultrasonic welding, extra gaskets, and reliable means to compress these elastomeric seals. This is seldom achieved through the simple addition of room temperature curing sealants. IP67-sealed D-subs are readily available, and they offer the enhancements of a panel-to-connector seal and an elastomeric interfacial seal on one of the connectors.
IP68 tends to be an application-specific rating, so there is no universal set of tests. Thus, details are unique to the application and specified by the designer. This can range from a continuous low static pressure (e.g. a pot filled with dielectric oil), to long-term submersion of an instrument at the bottom of a tank. Hermetic-rated connectors are often specified for these conditions, with all the costs and complexities of glass seals and special metals for bonding.
IP69k is a special category that tends to be used in factory automation applications, and requires all components to withstand ‘pressure washer' type cleaning operations. Circular connectors, like the M12 and M8 families of I/O and sensor connections, may require such a rating. These types of connectors are typically used in the food and beverage packing industry.
Materials compatibility
A wet environment immediately brings concerns about corrosion of metal parts. The most common tin-plated steel I/O connectors, like D-subs, do not fare well in wet applications. As a result, designers should consider various plated copper alloy shells, nickel- or tin-plated zinc die-casting, or even stainless steel. For example, CONEC offers a wide variety of D-subs that are available in these more exotic materials
In addition to retaining integrity in water, components also must withstand various lubricants, fuels, test chemical agents, or gases. Thus, typical connector materials might not be suitable for this type of application. Design engineers need to calibrate their selection of component materials to account for all chemical aspects in the operating environment including where the device will be located geographically, since cold climates pose different environmental challenges than warm locations.
Ethernet RJ45 connectors and USB ports are available in sealed configurations and for harsh environments, including factory automation, outdoor instrumentation, navigation, or communications equipment. The connector bodies are available in a variety of plastics, or even die-cast zinc.
With connectors, there is no ‘one size fits all' recommendation for material compatibility. The designer must optimize the product selection based on all aspects of the application environment.
Temperature
Any time a device is used outside of a controlled environment, temperature extremes are sure to be a concern. Because of this, the normal temperature range of -25C to +85C is extended down to -40C or -55C, and up to 105C or 150C. Many connectors can withstand these extremes, but unique challenges should be considered. For example, thermal shock requirements can challenge even the most stringent MIL-spec standards, let alone commercial I/O connectors. Factoring in the need for sealing, plus the thermal shock parameters, choices quickly become limited. Most catalogue specifications do not detail thermal shock, so if this is important, designers must ensure the selected connector product testing matches these needs.
In the factory automation area, for example, using the circular family of connectors (e.g. M12 and M8) is optimal. Some are designed to withstand long-term exposure to 125C and 150C for 2,000 to 8,000 hours but most are rated from -30C to +85C, which is adequate for the typical sensor application. Again, selected connectors must be evaluated to the specific environmental needs of the application.
Don't Forget the Cables
Since I/O connections will involve a cable, the cable jackets need to be specified as well. Some performance parameters to consider include: UV and sunlight resistance, low temperature flex rating, chemical- or oil-resistance, weld slag tolerance, and high flex needs. It must also be able to withstand millions of cycles.
Cables may need a special hood/backshell to withstand the harsh environment. In the world of D-subs, there are liquid-sealed hoods available, which are also optimal for field installation. Overmoulded hoods are often a good choice, if implemented properly.
In the M12 and M8 families, overmoulded cable ends are extremely common. Overmoulding can improve durability, streamline size, and enhance the appearance of a cable connection. The increased durability is a result of the extra strain relief for wire terminations resulting from the overmould material bonding to the cable jacket. The overall assembly gives a high-reliability connector on each end with customer-specified cable properties in between.
Overmoulded connectors are available in right angle or straight, and shielded or unshielded options. They can also come with LEDs embedded in the overmould material for easy monitoring of signal integrity, or to indicate presence of power. These connectors are all IP rated as well.
RJ45, USB 2.0, and fibre optic LC connections, with their limited range of sizes and circular connector formats, are well suited to the use of field installed plug & boot kits. Factory-made cable sets are also available. Of course, shop pre-assembly is more typical in the case of LC connectors. The end result is an IP-rated connector system.
In summary, with proper design and careful component selection, the common connector formats for I/O connections can often be used in applications that require ruggedized components. With careful evaluation of the specifications, design engineers may be able to avoid expensive connector formats and utilize the cost savings of the various families of ruggedized commercial I/O connectors.
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