How Miniaturized Distributed Modular Architecture Advances Avionics Design
Most of today’s collision-avoidance, in-flight-entertainment (IFE), air-to-ground-communications, and other avionics systems employ electronics packaging based on the Aeronautics Radio INC (ARINC) 600 standard. Compared to the older ARINC 404 standard dating from the 1970s that defined “black box” enclosures and racks within aircraft, ARINC 600 specified a Modular Concept Unit (MCU) – the basic building block module for avionics. An ARINC 600 metal enclosure can hold up to 12 MCUs, allowing a lot of computing power to be placed in a centralized “box.” By making it possible to run numerous applications over a real-time network, ARINC 600 enabled “next generation” integrated modular avionics (IMA).
A centralized IMA approach offers several advantages: reduced size and weight, easier maintenance with standardized cards that are easily replaceable, and expanded data transmission speed and bandwidth. However, a centralized big box has some significant limitations, which inspired the development of ARINC 800 series standards at the turn of this century.
ARINC 800 standards respond to the general technological trend toward embedded computing on miniaturized printed circuit boards (PCBs) deployed locally. This trend got started in other industries looking for ways to reduce weight and size, handle higher data loads due to the proliferation of sensors, improve thermal management, and push intelligence closer to the point where control decisions are made. For example, automotive designers now employ a myriad of electronic control units (ECU) in highly engineered cars, and consumer product designers embed numerous intelligent controls in “connected” consumer appliances.
This trend is now moving into avionics. Embedded computing and distributed architecture are combining to push IMA to the next level. Designers can now implement modular and distributed avionics throughout the aircraft using miniaturized electronics packaging to perform navigation, communications, the gathering of various sensor information, and other intelligent functions locally without being wired back to a central microcomputer.
To this end, a family of new ARINC 800 standards was developed, which include:
ARINC 801 through 807 that advance the use of fiber optics in avionics systems;
ARINC 836 that defines modular, standardized rack-style enclosures, cabling, connectors, and grounding methods for aircraft cabins;
ARINC 836A that updates the original ARINC 836 standard to establish a mini modular rack principle (MiniMRP) for avionics packaging. (Initially aimed at commercial cabin systems, ARINC 836A MiniMRP is also finding use in military aerospace.)
For the designer, ARINC 836A MiniMRP makes it possible to realize the full potential of integrated modular and distributed avionics. The basic technical goal of ARINC 836A was to define standardized cabin-system-module form factors for weights ranging from a few ounces to a maximum of six pounds.
The advantages of MiniMRP implemented in TE Connectivity (TE) technologies include:
Significantly reduced size with a 40% smaller package and up to 60% weight savings;
Enhancing flexibility and simplifying configuration with a less costly commercial-off-the-shelf (COTS) selection approach;
Securing modules and boards with robust latches and interconnects;
Increasing network speed and bandwidth by supporting 10-Gigabit Ethernet over fiber optics and/or high-speed copper, with a fiber optic backbone that can support 40G and even 100G links.
By using miniaturized, standardized modules that can be mixed and matched within a high-speed network, designers enjoy far greater design flexibility in avionics placement and cabling than with centralized architectures. It's worth examining how TE’s MiniMRP delivers these design advantages.
ARINC 836A MiniMRP modules are available in four compact size combinations: single-width (42 mm/1.6 inches) or double-width (84 mm/3.3 inches), and single-height (112.3 mm/4.4 inches) or double-height (224.8 mm/8.8 inches) variations. Lightweight composite materials replace traditional heavy metal enclosures. Advanced composite formulations — including base materials and fillers — can be selected according to specific application needs. Fillers range from carbon fibers to microsphere and nanotubes. Composites can be selectively plated to add shielding, circuit elements, and other features, such as embedded antennas. Sometimes considered an expensive, exotic solution, composite enclosures are now more cost-effective thanks to advanced manufacturing techniques.
Flexibility and Configuration
With MiniMRP avionics, a big box in the avionics bay can be replaced with many small boxes distributed throughout the aircraft. Modules can be used singly or combined as needed for specific functionality and external environmental factors. MiniMRP packaging encompasses connecting hardware, including bus-structured modules, interfaces, and power supplies. Standardization allows designers to take advantage of COTS components to lower costs and speed up the design cycle. Modules are designed for quick and easy tool-less installation. Changes, maintenance, and upgrades can be accomplished by simply swapping out modules.
Robust Latches and Interconnects
Avionics designers often face tight constraints with PCBs; design flexibility is accounted for by using European Standard EN4165-mateable interconnects for modular racks. With a fully-integrated MiniMRP design, PCB connector modules and boxes can be securely latched using techniques to protect against pull-out and torsion. The preferred connector for MiniMRP modules is the classic DEUTSCH DMC-M series aircraft connector. This design conforms to demanding avionics specifications, including EN4165. The DMC-M family offers many contact arrangements and insert layouts in both multi-cavity and singlemodule configurations. Sizes include 8, 12, 16, 20, and 22 gauges. Contacts can be crimped on copper wire, aluminum wire, or PCB mounting.
Network Speed and Bandwidth
In distributed avionics, a large number of links are shorter than 83 meters and are suitable for 10 Gigabit Ethernet. In these cases, Cat 6a copper cabling can be used for flight control, avionics, and cabin-management systems. For links less than 60m, 26 AWG cables may be used, resulting in smaller and lighter harnesses. Cat 6a cable can be terminated with small, high-speed ARINC-compliant circular connectors.
Two TE connector families are relevant:
CeeLok FAS-T circular connectors use a true 100-ohm impedance design that is compatible with Cat 6A cable. The compact size 8 shell reduces SWaP (size, weight, and power) requirements. Crimp-snap contacts allow easy termination and field repairability. An integral backshell enables easy 360-degree shield termination. A T-shaped contact pattern provides noise cancellation and decoupling to minimize crosstalk and increase signal integrity.
CeeLok FAS-X circular connectors come in a small size 11 shell in a M38999 profile for one 10 Gb/s Ethernet channel (size 25 shell for four channels). CeeLok FAS-X contacts employ a proven AS39029 design for rugged environments. A patented shielding arrangement shields each pair through the connector to provide improved impedance matching and also eliminates crosstalk. TE’s CeeLok FAS-X Connector is qualified to a new Military Standard MIL-DTL-32546.
To provide higher speeds over longer distances, MiniMRP accommodates fiber optic cabling. A multimode fiber can transmit 10 Gigabit/sec up to 550 meters. Optical fiber can also be used for avionics backbones that can support 40G and even 100G links. Moreover, compared to a Cat 6a counterpart, fiber optic cable is 78 percent lighter. Optical fibers also excel in noise immunity. They neither emit nor receive electromagnetic interference (EMI). They are made of dielectric materials and cable shielding is not required.
Despite its benefits, fiber optic cable has a reputation for being fragile and hard to use. Once again, technological advances have developed fiber optic cables that are crush and pinch resistant during installation. Fiber preparation during termination is now simplified in ways that significantly reduces labor and installation time.
Designers specifying optical connectors have two main choices:
Physical Contact (PC) types with mating termini that physically touch. PC termini are further distinguished by ceramic ferrules for single fibers and MT ferrules of multiple fibers. Ceramic ferrules yield the highest performance as well as lowest insertion loss and return loss. Multifiber MT ferrules offer the highest fiber density.
Expanded Beam (EB) types with a non-contacting interface for the termini. By avoiding physical contact, EB is more tolerant of vibration, shock, and other mechanical hazards. Wear and tear on the fiber/ferrule face is practically non-existent during vibration. Lensed MT ferrules leverage the EB benefits along with offering increased density.
Functionally, EB connectors expand and refocus light at the fiber end-faces and allow an air gap in the optical pathway. The EB concept uses optical lenses to expand and collimate the beam emitted from the launch fiber. The expanded beam remains collimated across the mechanical interface until the receiving lens focuses the beam onto the receiving fiber. Because the ferrule end-face is enclosed and protected behind the lens, the fiber does not require cleaning. Although EB exhibits higher insertion loss than PC, its longevity and consistency are superior.
In MiniMRP applications, copper and fiber can easily coexist. Each medium brings specific advantages, from the comfortable familiarity of copper to the high-bandwidth capabilities of fiber over longer distances. Avionics designers who are challenged to handle demanding data, IFE, and other bandwidth-hungry processes can employ both optical fiber and copper for an array of high-speed connectivity needs, from box to box, box to backbone, and box to server in the electronics bay.
For an end-to-end optical solution, the TE ParaByte transceiver uses a robust parallel optical design capable of achieving 10+ GB/s while also meeting MIL-SPEC standards for robustness. The small and dense design allows multiple transceivers to fit easily inside a single MiniMRP module.
A relatively new development that can push heavy data loads through active optical fiber networks is technology that features extremely small, low cost 10 GbE transceivers for MiniMRP units that are the end or drop nodes of these large networks. These transceivers are packed with GbE switches internal to the MiniMRP that require 10G optical ports.
In an end-to-end solution, the MiniMRP concept empowers designers with standardized, thin boxes packed with embedded computing power that can be distributed as intelligent nodes on an ultra-high-speed network within an airframe. As a result, designers, systems integrators, and avionics OEMs can quickly add and remove capabilities throughout the cabin, using MiniMRP to realize the full potential of IMA today.
This article was written by Russ Graves, Global Aerospace Business Development Manager, TE Connectivity (Berwyn, PA). For more information, visit here .