Adapting opto technology from automotive use to military applications
By Steve Coble and Sergio Dela Garza
As the military electronics industry seeks to adapt to changing technology requirements in communications, avionics, and weapon systems, the automotive industry may have an opportunity to offer new ways to use optoelectronics technology.
While military optoelectronics and automotive electronics have evolved differently over the years, a convergence of sorts is possible, even likely. Although military and automotive systems designers approach testing differently, changes in each industry may make it possible for automotive technology to be put to use in various types of military systems.
During the early development history of the military and aerospace electronics industry, the emergence of optoelectronic technology-at the time still in its own developmental phase-came at a time when military design engineers were seeking alternatives to mechanical switches, especially for avionics and the aerospace program. While relatively expensive devices for their day, those first optoelectronic switches in “metal-can” packages soon demonstrated their superiority over relays and mechanical switches; they were lighter, more reliable, and took up far less space in the cramped confines of early satellites and guided missiles.
Advances in optoelectronic technology soon brought new options for the aerospace systems designers. In addition to optoelectronic switches, there came optocouplers and optoisolators, fiber optics, light-emitting-diode technology, and a wide variety of optoelectronic sensors. Each new development brought increased reliability and performance to the systems into which they were designed.
At first slow to adopt the “newer” optoelectronic technology, automotive designers have quickly come to appreciate the benefits of lightweight and high reliability for applications such as position sensors. There is also a growing acceptance of fiber optics for multiplexed communications in vehicle bus systems, such as the MOST (media-oriented system transport) for vehicle entertainment systems, where plastic optical fiber can replace literally miles of copper cabling required for automotive systems.
Automotive design engineers have steadily increased the electronic content of their new vehicles-now approaching 40 percent of a car’s value. Many automakers have already begun to deploy optoelectronics into their automotive systems in sensors, dashboard displays, motion and position sensing.
Military and automotive design engineers have similar requirements for optoelectronic components-long-term performance and bulletproof reliability over a wide range of environmental conditions. In automotive systems, optoelectronic components must perform reliably and consistently through an entire range of extreme temperatures. Because most automakers manufacture cars for a world market, each system they design could be subject to temperatures as low as -55 degrees Celsius in very cold climates like Scandinavia and Alaska, to more than 125°C for optoelectronic components in underhood applications.
Electronic systems in automobiles must perform consistently over the life of the vehicle for both safety and economic reasons. First and foremost, a failure in a critical system such as steering control or antilock braking could result in a fatal accident. And even a failure in a non-safety system can result in an expensive recall and replacement program.
Military and aerospace design engineers are faced with the same reliability and performance issues, although their environmental extremes are even more extreme-temperatures that optoelectronic components encounter in a deep-space satellite system can range from -55°C to well over 125°C over the lifetime of the satellite’s operation. Similarly, an optoelectronic sensor for a military jet might have to operate at temperatures that quickly changes from more than 100°C at sea level to -55°C at 40,000 feet as it carries out its mission. Like automotive design engineers, military designers must develop systems that are reliable in every part of the world, with widely varying temperature, humidity, and weather conditions.
Long-term reliability is also critical for military systems from a safety standpoint. Many weapons systems rely on optoelectronic components for mission-critical operations like guidance or arming systems, and these systems can spend months or even years in storage before being put to use. Failure of a mission-critical system in a missile or other weapons system is quite literally a life-or-death situation.
Differences in test procedures
While sharing the common goals of performance and reliability, automotive and military design engineers have developed different approaches to meeting those goals, owing to the unique requirements each industry faces. However, in recent years there has been something of a convergence between the two approaches.
Historically, automotive design engineers have established specific performance parameters for various systems, and then sought out component suppliers to meet these specific requirements. Because of their concern for cost-efficiency and the economies of scale, the goal is often to develop systems that can be used in the widest possible range of vehicles. For example, sensors for a common fuel pump design may be specified so that they can be used on millions of engines in production.
Because of this need for high-volume production with a focus on reliability and lowest cost, automotive design engineers typically subject prototype designs to exhaustive testing prior to beginning production. Design engineers will release orders for production quantities of necessary components only after setting and testing a design. The component manufacturer bears much of the cost of meeting these early tests, with the payoff being a long-term production order for high volumes for a specific component that could be used over several model years.
Because of the unique nature of military and aerospace systems, design engineers in this industry have developed a different approach to achieving quality and reliability. Industry leaders have established an open specification for individual component performance under the auspices of the Defense Supply Center Columbus (DSSC) in Columbus, Ohio. Component manufacturers are obligated to produce parts that comply with these test procedures.
To ensure the ultimate level of reliability, experts perform an extensive testing regime on individual components, including burn-in, extended life testing, accelerated life testing, thermal shock, salt-spray exposure, and other types of environmental tests, all designed to simulate conditions in the field. Careful documentation procedures are part of the certification process.
Typically, industry produces military and aerospace electronics systems in much lower volumes, and at more irregular schedules than automotive production, so the cost of component testing is part of the unit prices of the components. While resulting in higher component costs for the military, this has ensured a continuing supply chain of parts with proven reliability. Nevertheless, it has required component manufacturers to maintain separate production lines and extensive test capabilities to meet these requirements.
COTS and convergence
The advent of the COTS (commercial-off-the-shelf) program for military component procurement has started to bridge the gap between the two approaches to component quality. For many military and aerospace applications, it has become possible for design engineers to specify promising commercial technologies like optoelectronics into many military systems. Companies typically build COTS-level components on the same production lines as commercial components, but then put them through a battery of test procedures to ensure long-term reliability.
While COTS-level components will not completely replace DSSC testing for the most mission-critical applications like satellites and weapons systems, COTS has proven to be a means of introducing newer optoelectronic technologies into military electronics systems while maintaining MIL-STD process controls. For many of these systems, the use of COTS-level components has meant lower costs and shorter lead times.
Industry officials are considering establishing a formal specification or testing procedure for COTS-level components that have undergone additional testing to meet specific requirements (referred to now as “COTS-plus” or “ruggedized COTS”).
Likely optoelectronic technologies
One optoelectronic technology for automotive applications that is promising for military applications is visible LEDs (VLEDs) used in place of incandescent or fluorescent lighting systems. Recent increases in light output for VLEDs have made it possible to use them for illumination in cockpit displays as well as aircraft and vehicle interior and exterior lighting systems.
IRLED (infrared LED) technology in automotive sensing applications is another area that shows promise to provide high-reliability sensing for applications such as aircraft doors, armored-vehicle hatches, and cockpit closure sensors. A natural extension of this technology would be for bomb-bay doors and missile-release indicators in aircraft. Infrared technology can transmit audio signals over short ranges, such as in wireless headsets for cockpit voice communications systems.
Another area in which automotive optoelectronics technology is likely to move to military and aerospace systems is in position sensing, both rotary position (detecting position and speed in motor shafts) and in proximity sensing (detecting the presence of an object). Recent advances in optoelectronics, including VCSELs (vertical-cavity surface-emitting lasers) have improved the distance and accuracy of these types of sensing systems.
Sergio De La Garza is product manager of Optek Technology in Carrollton, Texas, where he focuses on high-reliability military and aerospace products. Steve Coble is engineer manager at Optek Technology, and has an extensive background in quality systems, product design, and testing processes for high-reliability products.