Trends in avionics connectors
After World War II, aviation electronics evolved away from electromechanical components toward electronic and digital technologies, as seen in the development of fly-by-wire systems. That evolution continues today, as both military and commercial aircraft use increasing amounts of computing power and generate huge reams of data.
Avionics designers continue to look for ways to put more computing power into embedded systems that can handle navigation, communications, and other critical functions – thereby reducing weight, increasing data transmission rates and bandwidth, while boosting robustness and security. To this end, military and commercial avionics standards are rapidly co-evolving to the point that it’s worth taking a moment to focus on how advancing standards are improving military avionics connectors.
Commercial airline manufacturers were the first to kick off the development of standards for aviation electronics by forming Aeronautics Radio INC (ARINC) in 1929. The initial focus was standards for ground-based communications. The scope was soon enlarged to include standards for communications inside the aircraft as well.
Relevant ARINC standards include:
- ARINC 429: In 1977, defined the two-wire serial bus standard for point-to-point networks
- ARINC 629: In 1992, defined a multitransmitter data bus protocol allowing multiple devices to access the same bus (implemented in Boeing 777 aircraft)
- ARINC 664, Part 7 with Avionics Full-Duplex Switched Ethernet (AFDX) data network implementation: In 1998, defined full-duplex switched Ethernet for aircraft data networks, which supports the adoption of commercial off-the-shelf (COTS) networking technology
- ARINC 653: In 1993 and updated in 2003, defined a real-time operating system (RTOS) interface for space- and time-partitioned computer resources (implemented in Boeing 787 aircraft and Airbus A350 aircraft)
- ARINC 836A: A soon-to-be issued standard, defines enclosures for miniature modular racks
Accompanying the development of ARINC standards were the U.S. military’s two own initiatives. The first is Integrated Modular Avionics (IMA). More of a concept than a standard, IMA grew out of the F-22 Joint Integrated Avionics Working Group (JIAWG) formed in 1987. The benefit for designers is that IMA enables the same part or card to be used between different computer modules, helping reduce weight and maintenance issues.
The second of the military initiatives is Open Systems Architecture (OSA). The result of a 1994 Department of Defense (DoD) directive, OSA is also not a standard but instead a strategy that relies on defined and published standards-based interfaces and module designs instead of proprietary technologies. Many OSA modules are defined by standards developed by VITA working groups, such as VPX module standards for embedded systems.
While IMA and OSA share similarities, IMA is leading the march to the future with vigorous development of standards being driven by two factors critical to avionics designers: First, reducing weight by enabling a robust platform that puts more computing power in one box requiring far fewer nodes; and second, expanding data-transmission speed and bandwidth.
Developments in packaging
Given the coevolution of standards and application requirements, military avionics designers and connector OEMs face a number of challenges.
The first challenge is to implement modular integration with a compact, lightweight packaging system that it is rugged enough for the aerospace environment. At first glance, it looks like it’s simply a matter of the OEMs leveraging COTS technologies and making them more robust.
OEMs can take two different paths when designing more rugged connectors. Path 1, while it offers the most freedom, uses a proprietary design approach. As such, the connectors will meet demanding defense and aerospace requirements, but will not be an OSA component.
Path 2 follows an OSA approach implementing open ARINC and VITA standards. An example of Path 2 is the composite miniature modular rack principle (MiniMRP) enclosure, defined in the upcoming ARINC 836A (Figure 1). The MiniMRP exemplifies commercial/military coevolution. Initially, the product was aimed at commercial cabin systems. The standardized module is similar to the established MRP systems defined in ARINC 836, but MiniMRP modules are 40 percent smaller, which translates into as much as 60 percent weight savings. Moreover, MiniMRP modules can replace traditional metal enclosures used in military/aerospace applications. Composite enclosures are not only sturdy, but they also can be easily customized with shielding, circuit traces, embedded antennas, and other features. Designers get the flexibility of customizing a manufacturable module or selecting a module in a catalog-like fashion from suppliers, similar to a COTS part.
Facilitating system connectivity
The second challenge for avionics connectors is handling increased data speeds and bandwidth. The digital battlefield is outpacing the data speed/bandwidth requirements of commercial applications. This puts more demands on box-to-box connectivity in military applications.
In distributed avionics, a large number of links are not long distances and range from 100 megabit Ethernet to 10 Gigabit Ethernet. In these cases, copper cabling, specifically Cat 6a, is suitable for flight-control, avionics, and cabin-management systems. Cat 6a can support 10 Gigabit Ethernet at 83 meters, versus 36 meters for a Cat 6 cable. For ruggedness, Cat 6a cable is constructed with fluoropolymers that support ANSI/TIA-568-C specifications for stability in extreme conditions over long lengths and is available in size 24 AWG or smaller 26 AWG to minimize size and weight.
For connectors, Cat 6a cable can be terminated with ARINC high-speed connectors. (Figure 2.) They can handle the more stringent insertion loss, crosstalk, and other signal-degrading factors, as well as the increased bandwidth return loss requirements that result from faster I/O. Proprietary, circular connectors are also available that are designed for extreme environments, offer speed and size benefits, and come in metallic or composite shells.
To provide higher speeds over longer distances, optical fiber is gaining ground in backbone applications, especially as 100 G links (Ten 10 Gigabit/sec fibers per transmit to receive link) become more prevalent. The metrics clearly show the fiber-optics advantage:
- Speed and distance: A twisted pair cable can carry a 1 Gigabit/sec signal over a distance of 100 meters; a multimode fiber can transmit 10 Gigabit/sec up to 550 meters; and a single-mode fiber, an order of magnitude farther.
- Weight: A generic Cat 6a Ethernet cable weighs 45 pounds per 1,000 feet, while a fiber-optic counterpart is 78 percent lighter, weighing just 10 pounds.
- Noise immunity: Optical fibers are made of dielectric materials; they neither emit nor receive EMI. Cable shielding is not required.
As military avionics systems continue to carry heavier data loads, an end-to-end fiber-optic solution is attractive. Optical fiber used to have a reputation as being fragile and hard-to-use. That’s not true now: Today’s optical fiber constructions resist crushing and pinching during installation. No-epoxy/no-polish connectors significantly speed up termination.
Optical connectors come in two main categories: 1) Physical contact (PC), in which the mating termini physically touch using either ceramic ferrules for single fibers and MT ferrules for multiple; and 2) expanded beam (EB), which is the most tolerant of vibration, shock, and other mechanical hazards. The higher insertion loss of an EB connector is often outweighed by the long life, reliability, and consistency of EB performance over time.
A relatively new component that can push heavy loads through active optical fiber networks is the video transceiver. The MiniCube video transceiver from TE can handle bandwidth-intensive video applications for commercial or military video displays. MiniCube products include electro-optical devices to extend distance and to transmit discrete individual signals or support multiplexing of multiple video.
Where are packaging and connectivity headed in the future?
As far as packaging is concerned, previous ARINC standards are being eclipsed by new standards, such as ARINC 836A. The same smaller sizes and higher density seen in consumer and industrial electronics are now in play for avionics. Generic packaging for aviation electronics will keep shrinking to accommodate smaller boxes and significantly lower weights. As far as connectivity is concerned, copper and fiber will continue to coexist, each lending their specific advantages.
Additional sensor processing for the digital battlefield’s reconnaissance, surveillance, and target acquisition (RSTA) tasks puts huge demands on data exchange, which will require more innovation in avionics connectivity and computing platforms.
The Internet of Things (IoT) is also driving the sensor revolution. With IoT, multiple systems within the aircraft – such as engines, tires, structural components, and even seats – can self-report their status to multiple hubs. While the aircraft is en route, IoT-enabled devices can schedule maintenance, order parts, and report on performance to improve reliability and minimize downtime.
The good news: Avionics connectivity continues to evolve. Designers will benefit from a wide range of new options to meet ever-increasing data loads. Due to the proliferation of standards and products, avionics designers will need to consider all sides, from physical layer components, cabling, and assemblies for IMA solutions.
TE Connectivity www.te.com