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Precision Resistors: From the drawing board to reality

Thu, 05/16/2013 - 2:18pm
Yuval Hernik, Director Application Engineering, Vishay Precision Group (VPG), www.vishaypg.com

Commercial and military aircraft manufacturers are promoting the replacement of older hydraulic and pneumatic control systems by lighter and more efficient, electrically-driven systems through a trend known as MEA, or More Electric Aircraft. The electrically-driven systems can be supplemented with multiple distributed redundancy for added reliability and faster response while reducing aircraft weight and increasing fuel efficiency. The systems of cables, pulleys, and hydraulic lines routed through the aircraft are more vulnerable to damage as well as much heavier than their electrical counterparts. Aviation design experts are convinced the newer electrical systems are more reliable and more efficient and are encouraging many countries to invest in this direction and transfer more resources to it.

The shift to MEA is expected to reduce the cost of production, reduce the cost of the final product, improve propulsion efficiency, and contribute to the global green energy efforts by reducing the environmental impact of aviation. By giving a direct input through electrical signals, commands can become sharper and more precise with less noise – providing overall safety to the passengers and more confidence to the pilots and crew.

Component manufacturers as well as universities and research institutes are working on the development and improvement of such electrical systems with the goal of greater safety and reliability. They are investigating electrical, electronic, and mechanical components, and looking for safe and reliable ways to choose, screen, assemble, protect, and monitor these components throughout the operating life of the systems they need to support.

Both hydraulic and pneumatic systems are  subject to replacement. Hydraulic systems can be found in several flight controls such as secondary flight control systems, environmental control systems, landing gears, brakes, steering, doors, and many other actuation functions. Pneumatic methods can be found in systems for flight controls, cabin pressurization, cabin energy recovery, the engine, wing ice protection, and engine ignition.

Another component of MEA is FBW (fly by wire), a class of control systems that are becoming standard and common in many commercial and military aircraft. A FBW system replaces the manual flight controls of an aircraft with electronic interfaces. Fly by wire aircraft controls allows for the replacement of traditional control yokes with computer flight controls that can be easily customized for different aircraft models. But designers must be careful to select passive components that are not sensitive to ESD damage, including the possible latent defects characteristic of some resistor technologies.

With the proper component selection, reliability and flight safety are actually increased with fly by wire and the weight saved by switching to FBW systems can be re-allocated to additional revenue-generating passengers and cargo while reducing maintenance costs and improving passenger satisfaction through fewer repair-related flight delays.

Certainly, technological developments have reached the point where the values of control and system safety are well understood and have been applied to the flight control systems of many popular aircraft by Airbus and Boeing. These companies are working to reduce of aircraft operating costs, reduce fuel costs (a particular concern when pricing in the petroleum market is so volatile) and, hopefully, make air travel an affordable option for passengers who must now rely on slower and possibly less safe means of transportation. 

But additional study is required on the analog side of the MEA/FBW to provide a suitable analog platform based on the most-suitable precision resistors and other components for secure and reliable service. We should also remember that the complexity of this process has increased as designers work to solve conflicting interactions between several equipment systems by enhancing the efficiency of each.

Research in the last several years in the alternative energy and smart grid technologies has shown electrical systems tend to be more energy efficient and environmentally friendly than their hydraulic counterparts. To achieve the right solutions, each application and function on the aircraft must be considered on its own merits. Performance, reliability, and long-term cost reductions all need to be taken into account.

In the component area several resistor technologies are available for use in various flight controls. Different resistor technologies perform at different levels with different consequences for each circuit. For example, electrical systems may be switched on and off as required and that can create power surges in the resistors. Similarly, TCR and TCR tracking (ratio stability through temperature changes) vary considerably by technology.

Using the best-quality precision resistors in the MEA will also contribute to greater efficiency, as the electrical systems will use less energy or waste less energy than their conventional counterparts, and losses in electrical cabling are certainly lower than those in hydraulic or pneumatic piping.

The degree to which environmental stresses can cause temporary or permanent changes in resistance depends on the inherent stability of the resistor, which in turn depends on the technology of its design and manufacture. With precision foil resistors, only a minimal shift in resistance value will occur during the device’s entire lifetime. Most of this shift takes place during the first few hundred hours of operation, and subsequent change is negligible.

Once the allowable end-of-life system error is established, a portion of this error is allocated to the resistors and this, in turn, indicates the allowable end-of-life accuracy limits of the resistors. Working backward from the end-of-life tolerance limits, and allowing for all the shifts to be expected from the effects of materials fabrication, assembly, load-life, and environmental stresses, the necessary purchased tolerance or initial accuracy is established, as shown in Figure 2.

So, where will electric actuation technology move now? Further advances in resistor development will bring incremental performance benefits, but the really important advancements must be derived from system-level studies. Performance analysis and electronic optimization studies for electric actuation will help establish the technical priorities for the next stage of systems development.

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