Passive Wireless Sensors Application for NASA. Department of ECE.

1) INTRODUCTIONThe environment of aeronautical vehicles is typically harsh, with temperature extremes ranging from cryogenic to above 1,500 C. Future hypersonic vehicles, for example, will require high-temperature sensors mounted on the structure, as well as cryogenic sensors for monitoring fuel tanks. Sensors are typically located in internal structures with limited access, making the periodic changing of batteries prohibitively costly and time consuming. Furthermore, batteries do not work well at temperature extremes. In contrast to current wireless systems, passive wireless sensor systems do not require batteries. One of the main challenges for wireless sensors is power. Often, batteries cannot be used due to inaccessible locations or exposure to large temperature extremes. Energy- harvesting systems that rely on batteries for energy storage are eliminated for the same reasons. Thus, passive wireless sensing systems that do not include batteries should be developed. Several passive wireless technologies may be adapted to meet the needs of the extreme environment community. MEMS, RFID, SAW, and backscatter techniques have all shown potential for passive wireless and extreme environment operation. As a result, NASA is investigating the use of passive wireless technology for aeronautical applications. From ground tests to the operation of high-altitude long - duration aircraft, many applications could benefit from small, passive, wireless sensors that can operate in extremely harsh environments.

2) GENERAL REQUIREMENTSAircraft sensing equipment must operate in harsh environments that include temperatures ranging from cryogenic to extremely high temperatures (greater than 1,500 C). Often, the temperature extremes preclude the use of batteries for sensor applications. Besides temperature extremes, issues of vibration, humidity, and even ionizing radiation must be addressed. In addition to the listed environmental challenges, the radio frequency (RF) environment can also pose a challenge when wireless RF sensors are placed within enclosed metallic structures, such as the interior of wings.

3) GROUND TESTING APPLICATIONSNASA aeronautical researchers perform tests on components and systems on the ground in conjunction with flight testing. These tests require the placement of a large number of sensors on a test article. To date, very few of the sensors have been connected wirelessly. Frequently, these tests are performed on models and test articles in NASAs wind tunnels. Many of the wind tunnels emulate extreme environments. The National Transonic Facility wind tunnel uses nitrogen as the gas and operates at temperatures from -157 C to -101 C, and at pressures from 103 kPa to 896 kPa . Another transonic tunnel, the 0.3 Meter Cryogenic Tunnel, operates down to -195 C. The Glenn Research Center operates an icing tunnel. The temperatures only get down to -40 C, but the liquid water content can range from 0.2~3 g/m3, and the droplets can vary from 15 to 50 m. The Glenn Research Center also operates the Cryogenic Test Complex (CTC), which houses a 7.6 m diameter test chamber that can accommodate a cold wall that can reach -252 C. The facility can handle both liquid and gaseous storage of hydrogen, oxygen, nitrogen, and helium.The Hypersonic Tunnel Facility (HTF) achieves Mach 5, 6 and 7, while reaching temperatures of 1,893 C and pressures of 8,375 kPa. The 8-ft high temperature tunnel is a hypersonic tunnel that can achieve speeds of Mach 3, 4, 5, and 7. The temperatures range from 482 C to 1,927 C, while the pressure ranges from 345 kPa to 27,579 kPa. The Arc Heated Scramjet Facility can reach temperatures of 2,616 C while achieving speeds up to Mach 8, and the 20 Mach 6 Wind Tunnel can operate at pressures up to 13,780 kPa. These tunnels use air, nitrogen, CF4, hydrogen-combusted air with oxygen, combusted methane/liquid oxygen, and R-134a as the gaseous medium. Wind tunnel tests that routinely operate with extreme temperatures and pressures could benefit from wireless sensors. The development of materials for hypersonic aircraft requires extreme environments like those found in spacecraft re-entry. At NASAs Ames Research Center, the Arc Jet facility uses an electric arc to accelerate heated gas from Mach 4 to Mach 12 at temperatures up to 1,727 C. The Arc Jet facility was used to develop high-temperature materials for hypersonic aircraft, such as the National Aerospace Plane (NASP).Future aircraft will fly at higher altitudes and velocities and therefore will experience more extreme environments than those encountered by todays aircraft. To address these needs, new passive wireless sensor systems will have to be developed that can operate in corresponding environments.In addition to aeronautics, other disciplines could benefit from passive wireless sensors. At the Kennedy Space Center (KSC), wireless sensor networks have been developed for monitoring cryogenic lines and for centering and aligning the space shuttle external tank. Ongoing work in the Transducers group at KSC integrates wireless communications with sensors and transducers. At the Langley Research Center, researchers have developed and tested a wireless fluid level system that worked while immersed in liquid nitrogen. This device can work in a variety of harsh environments while detecting the level of numerous fluids such as liquid nitrogen, transmission fluid, sugar, and even ground corn. While the devices just mentioned do not employ passive wireless technology, they do demonstrate the current trend toward wireless sensing for ground testing.

3.a) NASA Glenn Research CenterGlenn Research Center has six unique world-class wind tunnels with varying capabilities.The 1-by 1- Foot Supersonic Wind Tunnel specializes in conducting fundamental research in supersonic and hypersonic fluid mechanics, supersonic-vehicle-focused research, and detailed benchmark quality experiments for computational fluid dynamics. This facility is an excellent low-cost testing tool for small-scale research.The 8- by 6-Foot Supersonic Wind Tunnel is a world-class test facility that provides researchers the opportunity to explore the subsonic, transonic, and supersonic speed range. The facility tests advanced aircraft concepts and components, engines for high-speed aircraft, and launch vehicle concepts. It is NASAs only transonic Propulsion wind tunnel, operating from Mach 0.25 to 2.0 and at very low speeds from 0 to Mach 0.1. This facility is equipped for aerodynamic and propulsion scale models. The 9- by 15-Foot Low-Speed Wind Tunnel is the most utilized low-speed propulsion acoustic facility in the world specializing in evaluating aerodynamic performance and acoustic characteristics fans, nozzles, inlets, propellers, and hot gas reingestion of advanced Short Take-off Vertical Landing (STOVL) systems. It is the only national facility that can simulate take-off, approach, and landing in a continuous flow wind tunnel environment. The 10- by 10-Foot Supersonic Wind Tunnel is the largest wind tunnel at NASA Glenn specifically designed to test supersonic propulsion components such as inlets and nozzles, propulsion system integration, and full-scale jet and rocket engines. This dual-cycle wind tunnel can operate as a closed-loop (aerodynamic cycle) or open-loop system (propulsion cycle) and is equipped for large-scale aerodynamic models as well as full-scale engine and aircraft components. This facility operates at test section speeds of Mach 2.0 to 3.5 and subsonically from 0 to Mach 0.36.The Icing Research Tunnel (IRT) is one of the worlds largest refrigerated wind tunnels dedicated to the study of aircraft icing. In this facility, natural icing conditions are duplicated to test the effects of in-flight icing on actual aircraft components and models of aircraft, including helicopters. The Hypersonic Tunnel Facility (HTF) tests large-scale hypersonic air-breathing propulsion systems. The HTF is a hypersonic (Mach 5, 6, and 7) blowdown and nonvitiated (clean air) wind tunnel capable of testing large-scale propulsion systems at true enthalpy flight conditions.

Facility Benefits Provides aerodynamic and propulsion test capabilities from low subsonic through high supersonic Mach range Standardized data acquisition systems Accommodates in-house and private industry research programs Experienced staff of technicians, engineers, researchers, and operators High customer satisfaction

Commercial Applications Supersonic and hypersonic fluid mechanics Aircraft and missile developments Next-generation launch vehicles Jet and rocket engines Inlet performance and operability Propulsion system integration Engine and fan noise reduction Low-speed flight applications Advanced propulsion system components High-speed and counterrotating fans Airport noise Provides next-generation ice protection systems for military and commercial aircraft

Programs and Projects Supported High-Speed Civil Transport National Aerospace Plane (NASP) Space Shuttle Joint Strike Fighter (JSF) Aviation Society Program Integrated system test on an air-breathing rocket (ISTAR) direct connect combustion rig test.

Fig: 3.a.1) Tunnel Details

3.b ) Wind Tunnels

Wind tunnels are large tubes with air moving inside. The tunnels are used to copy the actions of an object in flight. Researchers use wind tunnels to learn more about how an aircraft will fly. NASA uses wind tunnels to test scale models of aircraft and spacecraft. Some wind tunnels are big enough to hold full-size versions of vehicles. The wind tunnel moves air around an object, making it seem like the object is really flying.Most of the time, powerful fans move air through the tube. The object to be tested is fastened in the tunnel so that it will not move. The object can be a small model of a vehicle. It can be just a piece of a vehicle. It can be a full-size aircraft or spacecraft. It can even be a common object like a tennis ball. The air moving around the still object shows what would happen if the object were moving through the air. How the air moves can be studied in different ways. Smoke or dye can be placed in the air and can be seen as it moves. Threads can be attached to the object to show how the air is moving. Special instruments are often used to measure the force of the air on the object.NASA has more wind tunnels than any other group. The agency uses the wind tunnels in a lot of ways. One of the main ways NASA uses wind tunnels is to learn more about airplanes and how things move through the air. One of NASA's jobs is to improve air transportation. Wind tunnels help NASA test ideas for ways to make aircraft better and safer. Engineers can test new materials or shapes for airplane parts. Then, before flying a new airplane, NASA will test it in a wind tunnel to make sure it will fly as it should.NASA also works with others that need to use wind tunnels. That way, companies that are building new airplanes can test how the planes will fly. By letting these companies use the wind tunnels, NASA helps to make air travel safer.NASA also uses wind tunnels to test spacecraft and rockets. These vehicles are made to operate in space. Space has no atmosphere. Spacecraft and rockets have to travel through the atmosphere to get to space. Vehicles that take humans into space also must come back through the atmosphere to Earth.Wind tunnels are important in making the new Ares rockets and Orion spacecraft. Ares and Orion are new vehicles that will take astronauts into space. NASA engineers tested ideas for the design of Ares in wind tunnels. They needed to see how well Ares would fly. Engineers tested Orion models.

Long after the first design work is finished, NASA can still use wind tunnels. Wind tunnel tests have helped NASA change the space shuttle to make it safer. Wind tunnels will keep helping make all spacecraft and rockets better.

Wind tunnels can even help engineers design spacecraft to work on other worlds. Mars has a thin atmosphere. It is important to know what the Martian atmosphere will do to vehicles that are landing there. Spacecraft designs and parachutes are tested in wind tunnels set up to be like the Martian atmosphere.NASA has many different types of wind tunnels. They are located at NASA centers all around the country. The wind tunnels come in a lot of sizes. Some are only a few inches square, and some are large enough to test a full-size airplane. Some wind tunnels test aircraft at very slow speeds. But some wind tunnels are made to test at hypersonic speeds. That is more than 4,000 miles per hour!Air is blown or sucked through a duct equipped with a viewing port and instrumentation wheremodelsor geometrical shapes are mounted for study. Typically the air is moved through the tunnel using a series of fans. For very large wind tunnels several meters in diameter, a single large fan is not practical, and so instead an array of multiple fans are used in parallel to provide sufficient airflow. Due to the sheer volume and speed of air movement required, the fans may be powered by stationary turbofanengines rather than electric motors.The airflow created by the fans that is entering the tunnel is itself highly turbulent due to the fan blade motion (when the fan is blowingair into the test section when it issuckingair out of the test section downstream, the fan-blade turbulence is not a factor), and so is not directly useful for accurate measurements. The air moving through the tunnel needs to be relatively turbulence-free andlaminar. To correct this problem, closely spaced vertical and horizontal air vanes are used to smooth out the turbulent airflow before reaching the subject of the testing.Due to the effects ofviscosity, the cross-section of a wind tunnel is typically circular rather than square, because there will be greater flow constriction in the corners of a square tunnel that can make the flow turbulent. A circular tunnel provides a smoother flow.The inside facing of the tunnel is typically as smooth as possible, to reduce surface drag and turbulence that could impact the accuracy of the testing. Even smooth walls induce some drag into the airflow, and so the object being tested is usually kept near the center of the tunnel, with an empty buffer zone between the object and the tunnel walls. There are correction factors to relate wind tunnel test results to open-air results.The lighting is usually embedded into the circular walls of the tunnel and shines in through windows. If the light were mounted on the inside surface of the tunnel in a conventional manner, the light bulb would generate turbulence as the air blows around it. Similarly, observation is usually done through transparent portholes into the tunnel. Rather than simply being flat discs, these lighting and observation windows may be curved to match the cross-section of the tunnel and further reduce turbulence around the window.Various techniques are used to study the actual airflow around the geometry and compare it with theoretical results, which must also take into account theReynolds numberandMach numberfor the regime of operation.

Figure 3.b.1 Wing Tunnel

Fig: 3.b.2 Block Wind Tunnel

Measurements3.b.1) Pressure measurementsPressure across the surfaces of the model can be measured if the model includes pressure taps. This can be useful for pressure-dominated phenomena, but this only accounts for normal forces on the body.

3.b.2) Force and moment measurements

Figure 3.b.3) Measurements

A typicallift coefficientversusangle of attackcurve.With the model mounted on aforce balance, one can measure lift, drag, lateral forces, yaw, roll, and pitching moments over a range ofangle of attack. This allows one to produce common curves such aslift coefficientversus angle of attack (shown).Note that the force balance itself creates drag and potential turbulence that will affect the model and introduce errors into the measurements. The supporting structures are therefore typically smoothly shaped to minimize turbulence.

Qualitative methods Smoke TuftsTufts are applied to a model and remain attached during testing. Tufts can be used to gauge air flow patterns and flow separation.

Fluorescent mini-tufts attached to a wing in the Kirsten Wind Tunnel showing air flow direction and separation. Angle of attack ~ 12 degrees, speed ~120 Mph.Figure 3.b.4

4) AIRCRAFT PROPULSION APPLICATIONSActive wireless sensor systems have been developed for monitoring the health of aircraft engines for commercial, military, and NASA aircraft, but all of these systems require batteries. NASA would prefer that future sensors to be passive. The Army, Air Force, and NASA require high- temperature propulsion sensors that can operate in environments of up to 1,538 C around the engine and inside the gas path. Both wiring and batteries become an issue in these applications; therefore, high-temperature-resistant, passive wireless sensors, such as the passive engine-bearing sensor, are needed.NASAs Glenn Research Center is researching new propulsion technologies in their Advanced Subsonic Combustion Rig (ASCR). The ASCR simulated combustor inlet can test conditions of pressures up to 6,300 kPa and temperatures up to 1,871 C. Environments with temperature this high could benefit from wireless sensors. Commercial airlines use turbofan engines for routine flights. This schematic comes from NASAs Glenn Research Center and is available online. These engines run with a combustion station temperature of ~1,116 C. High-temperature wireless sensors that can operate for long periods are required to optimize the engine parameters for better fuel efficiency.High-temperature materials are currently being researched for wireless sensor applications. Aluminium nitride is being investigated for the high-temperature (800 C) operation of temperature-compensating sensors. Gallium phosphate (GaPO) has been used as a substrate in the development of a temperature sensor. The sensor is wireless, operates at 433 MHz, and withstands temperatures of 600 C for 192 hours. Sensors made with exotic materials such as Langasite, langatite, and langatate have been characterized from -100 C to 900 C. These sensors require metal conductors. Thin films, however, do not behave the same as bulk materials. Therefore, research is ongoing on thin film metal characterization for passive wireless sensors. These new materials may enable wireless passive sensors to operate at temperatures higher than 1,000 C. In addition to sensory materials research, radio frequency (RF) transponders are being developed for passive wireless sensor use on turbine blades [14]. These devices have been characterized for operation up to 1,100 C.

5) AIRCRAFT STRUCTURAL APPLICATIONSNASA envisions the addition of structural health monitoring (SHM) sensors to existing aircraft; however, installing wiring for the sensors adds cost and weight to the aircraft. In addition, wires are prone to damage such as nicks, breaks, and degradation due to wear, excessive heating, and arcing. Wiring problems have led to major aircraft accidents and delays of space vehicle launches. In contrast, wireless systems present a desirable option for retrofitting sensors onto existing aircraft for structural health monitoring. High speeds mean high temperatures from skin friction heating. Sensors for hypersonic (greater than Mach 5) aircraft may experience aerodynamic heating above 1,000 C. Hypersonic aircraft based on NASAs HyperX X-43 design will fly at Mach 10 and therefore require sensors that can withstand temperatures up to 1,282 C. Thus, hypersonic vehicles will need high-temperature wireless sensors like those needed for propulsion applications.The X-51 Waverider is another vehicle that required high temperature sensors. Ground tests of the X-51 engine were conducted in the 8-ft high-temperature tunnel at NASAs Langley Research Center. The X-51A Waverider set a hypersonic flight record when it flew at Mach 5 for 200 seconds, beating the X-43 record of 12 seconds. On May 1, 2013, the X-51A broke another world record when it flew for six minutes. During this final flight, the aircraft achieved Mach 5.1. The nose of the prototype X-51 was expected to reach 1,480 C during flight due to skin friction heating.Hypersonic aircraft often contain cryogenic fluids in composite overwrapped pressure vessels (COPV) . Hydrogen, liquid oxygen (LOX), kerosene, and other fuels are often kept at cryogenic temperatures. In addition, COPVs are also used to store gaseous helium , xenon , and 90% hydrogen peroxide for propulsion applications.The tanks are constructed from metal liners that are wrapped in composite fibers (usually carbon or Kevlar), which are then cured in an epoxy matrix to keep them from bursting at high pressures. The operating temperature of a typical COPV is around -195 C to -184 C, and the maximum operating pressure is close to 31.1 Mpa. Evaluation of the structural health of the COPV tanks is necessary due to the thin liners being used, the high pressures, and the risk of impacts causing lower burst pressures. For applications where sensors need to be installed within a COPV, the sensors must be able to withstand both high pressure and cryogenic temperatures. Some passive wireless sensing technologies, such as SAW devices, have already demonstrated operation in cryogenic liquid environments. RFID sensors are also being investigated for operation down to -196 C. Wireless sensors utilizing energy harvesting in the form of solar power have also been proposed for monitoring the structural health of COPV tanks.

6) CHEMICAL SENSINGMany of the aerospace research vehicles, such as the Space Shuttle, HyperX, and Helios, all contained hydrogen tanks. These vehicles could have benefited from NASAs high- temperature chemical sensors that can detect hydrogen. A surface acoustic wave (SAW) hydrogen sensor based on a Langasite substrate that utilizes palladium as the sensing medium has been shown to operate at 250 C. Since SAW technology can be used for hydrogen sensing, all that is needed is the addition of passive wireless capability. SAW devices can be used to detect other chemicals as well. The integrity of wires on board aeronautical vehicles could be determined by monitoring the effluents given off by the wires insulation. The effluents are generated during aging, over-currents, arcing, and high-temperature conditions. SAW chemical sensors could be used to detect effluents and give an indication of wire integrity. SAW technology is very promising for passive wireless operation but is not the only technology being investigated. Prime Photonics has developed a passive wireless temperature sensor that can operate at 1649 C based on RFID technology. Other technologies include resonant circuits for chemical sensing that can operate at 675 C. A high-temperature (800 C) wireless temperature sensor has been developed for aircraft engine applications. This sensor is the first step in developing a high-temperature wireless CO2 sensor. Passive wireless sensors, including chemical sensors, have been proposed for NASAs Development Flight Instrumentation (DFI) Technology Roadmap. The roadmap calls for passive wireless sensors as part of low power electronics, which is one of six enabling technologies. Single-walled carbon nanotubes (SWNTs) have been used to develop a chemical sensor with high sensitivity to nitrogen dioxide, acetone, benzene, nitrotoluene, chlorine, and ammonia in the concentration range of ppm to ppb. In an effort to develop a portable wireless airborne ammonia, chlorine gas, and methane sensing system, researchers at NASAs Ames Research Center have developed a chemical sensor that plugs into an iPhone. The device utilizes 16 high-speed nano sensors that sniff the air and communicate wirelessly using the cell phone. Researchers at NASAs Glenn Space Flight Center are investigating small wireless chemical sensors for space and aeronautical applications.

7) SHOCK & IMPACT TESTINGNASAs Glenn Research Center has three specialty guns for ballistic impact testing. The various rigs in this facility are used to study the dynamics of high-speed projectiles and the impact damage they cause. These tests help to develop new advanced materials and structures that are lighter, stronger, and more impact-resistant. Failure and deformation characteristics of both structures and materials are evaluated under ballistic impact loading conditions. These guns are used in aeronautics research for assessing the impact resistance of aged composite fan containment materials and structures and for the development of lightweight and multifunctional fan containment structures. In addition, this laboratory performs the development and validation of impact damage models. For this application, wireless sensors that can detect the damage caused by impacts are sought.

One of the landmarks at NASAs Langley Research Center is the Lunar Landing Research Facility. The 200-ft, 61-m high tower was built to help train the original Apollo astronauts how to land on the moon. Currently, the tower is used for research on landing and impacts for both aircraft and spacecraft such as the Orion space capsule. On Aug. 28, 2013, a Navy CH-46 Sea Knight helicopter fuselage was dropped from 13.7 m to simulate a crash. Rugged wireless sensing that can withstand the shock loads experienced from aircraft crashes would be useful for this application.

8) IONIZING RADIATIONHigh-altitude sensing applications require sensors that exhibit tolerance to ionizing radiation. Tests conducted on SAW devices have demonstrated an inherent radiation tolerance of up to 10 Mrad. The constant size reduction of commercial electronics has led to less radiation tolerance and therefore to more soft errors in standard commercial electronics. In 1993, static RAM memories were inadvertently found to have neutron-induced bit errors or single-event upsets (SEU) when flown in aircraft at 10 km. In addition, in 1993, a series of tests on static RAMs demonstrated that they have SEUs due to radiation at 8.8 km and at 19.8 km. The author used the data from these flights to predict that error correcting coding (ECC) will be necessary for avionic systems. The author was correct in his prediction: not only are ECCs used in avionics now but they are also available for memory used in consumer computers. The need for radiation-tolerant avionics has not diminished, as the autopilot memory in a modern commercial airliner has been found to have one upset every 200 hours. The amount of radiation received varies by altitude, longitude, latitude, and the natural solar cycles of the sun. For example, four dosage cases. The radiation dosage versus altitude is plotted for the solar minimum (10/86) and the solar maximum (7/89) for 90 degrees west longitude and for both 35 and 70 north latitude. Radiation dosages rise with altitude, making spacecraft and high-altitude aircraft more susceptible to radiation effects.

Micro-electro-mechanical systems (MEMS) are also inherently radiation tolerant and may be used to develop sensors for extreme environments. Radiation-tolerant electronics are very expensive compared to commercial electronics. Therefore, inherently radiation-tolerant technologies are better candidates for sensors in high-altitude and space applications than devices made from conventional electronics.

9) PASSIVE WIRELESS SENSORSOne of the main challenges for wireless sensors is power. Often, batteries cannot be used due to inaccessible locations or exposure to large temperature extremes. Energy- harvesting systems that rely on batteries for energy storage are eliminated for the same reasons. Thus, passive wireless sensing systems that do not include batteries should be developed. Several passive wireless technologies may be adapted to meet the needs of the extreme environment community. MEMS, RFID, SAW, and backscatter techniques have all shown potential for passive wireless and extreme environment operation.

Passive wireless RFID chips have been developed that can use RF energy to power the electronic circuitry that comprises the wireless sensor node. A backscatter temperature sensor that can operate to at least 300 C has been developed. The device uses resonators to modulate the frequency of the backscattered radar cross section to make measurements. SAW devices have been proposed as passive wireless devices that can operate in high- temperature environments. A wireless temperature sensor on Langasite (LGS) has been demonstrated to operate at up to 850 C. The company Evironetix has capitalized on this technology and now has a SAW LGS product that can measure temperatures up to 910 C. A wireless oxygen sensor that can operate at up to 650 C has been developed on a LGS/ZnO substrate. SAW pressure sensors on LGS that operate at up to 500 C have also been developed. Many other SAW high-temperature sensors are under development.Vibration, ionizing radiation, RF issues, and certifications are just a few of the issues that must be addressed when developing new sensor systems. There are many challenges in developing wireless sensors for extreme environments. Pressure variations from vacuum to high pressures should not be an issue for solid-state devices such as MEMS, ICs, or SAW devices. For most cases, corona discharge and arcing at low pressures should not pose a problem when the voltages are low. However, an issue may arise when devices are miniaturized and the spacing between charged components is reduced, allowing corona discharge and arcing to occur at lower voltages. Devices must be designed with Paschens law in mind to avoid arcing and corona discharge when the pressure drops. Vibration is an issue for all aircraft. Component failures from the high levels of shock and vibration during operation are not uncommon. Monitoring of dynamic loading during flight is important for SHM and fatigue life analysis. Structural health monitoring using conventional sensors (such as strain gauges) has been difficult during flight due to the dynamics of aircraft loading and the random vibrations that are generated. Strain is often measured on research aircraft like those found at NASAs Dryden Research Center. Typical small aircraft can experience vibrational noise in the range of 60 peak to peak, while the aircraft experiences up to 1,000 loads during flight. These strain measurements are similar in magnitude to those taken on other research aircraft [47, 48]. Aircraft such as the P3 experience 0.6 g of vibrational noise during flight [49]. The data in Fig. 7 are from three strain sensors mounted on the wing leading edge during take-off. The raw sensor data were filtered with a two-point moving average. The moving average was subtracted from the original data, leaving only the vibrational noise. The structural noise, which is mostly due to vibrations of the aircraft, is -60 to +85 . Although SAW sensors can filter out this noise from strain measurements, many other types of sensors, such as conventional strain gauges, cannot. Another concern for avionics is exposure to corrosive environments. The Kennedy Space Center operates accelerated corrosive chambers that use salt fog and acids to investigate corrosive environments and their effects on aerospace vehicles. They also operate the outdoor KSC Beachside Corrosion Laboratory, where material degradation and harsh corrosive environments are studied. The Glenn Research Center operates an extreme environment chamber that measures 0.9 m x 1.2 m. The chamber temperature can reach 538 C, with a pressure of 10.1 MPa and an atmosphere of corrosive chemicals such as hydrogen fluoride and hydrogen chloride. The chamber is used primarily for simulating the conditions found on the planet Venus. Although this seems to be a space application instead of an aeronautical application, it has been suggested that NASA fly an aircraft in the atmosphere of Venus. RF communication issues pose a significant challenge to the successful implementation of SAW systems. Modulation methods must be chosen to allow large numbers of devices to communicate without interference. The bandwidth must be utilized carefully to enable high data rates while adhering to FCC limitations. Encoding schemes must be developed to allow for efficient operation in noisy environments. The devices must be small, which means higher frequencies and therefore smaller antenna sizes. Higher frequencies enable higher data rates; however, the range will begin to decrease when the frequency reaches 10 GHz. Higher- frequency devices may also mean that the fabrication feature sizes must shrink, which may lead to manufacturing issues. In addition, the frequency of operation must follow FCC guidelines. A 60 GHz SHM system is being developed for aircraft applications. The system exploits an unlicensed band in the communication spectrum for 5764 GHz. The devices use Ultra Wide Band Impulse Radio (UWB-IR) technology and very low power. The wireless communicating nodes are passive and fabricated on a flexible Kapton substrate that can be attached to curved surfaces within the aircraft. Many of NASAs test environments are closed metallic structures with metal test articles. Direct wireless links may not be possible in enclosed metallic spaces of aircraft, so multi-hop or mesh network architectures may be required. Electromagnetic interference poses a problem for all wireless systems. All wireless electronics must be designed to pass tests for both electromagnetic compatibility and interference. The certification of wireless sensor networks for flight is another issue that must be addressed. This includes the allocation of frequencies for wireless sensing on aircraft, along with the determination of RF power levels and FAA acceptance for aircraft use. There is a concern that wireless devices within the cabin may interfere with aircraft antennas located outside the cabin. Personal electronic devices and cell phones have been tested to determine the effects that wireless devices may have on aircraft avionics. Active RFID tags have been tested for compliance with RTCA/DO160 aircraft emission limits. The tags exceeded the thresholds, but further investigations are needed before the effects of the interference can be understood. The interference from passive tags was not considered a concern, however, because the study focused on RFID tags contained in cargo pallets without an interrogator. Unfortunately, passive SAW sensors would require an active interrogator within the aircraft structure. Because SAW sensors will share the same frequency bands with RFID tags and some of the same RF modulation techniques, they may exceed the emission thresholds as well. Before any wireless sensor system can be certified for use on aircraft, it will have to be tested.

8.a) Surface Acoustic WavesSAW filters offer high performance at frequencies up to 5 GHz range. They are small and suitable to low-cost mass manufacturing; Therefore, they are widely used in mobile communications applications. SAW RFID tag is an emerging technology that holds promises for long-range identification in various applications, with reading distance of several meters and completely passive tag devices.We have developed several low-loss SAW filter designs that are based on longitudinally coupled resonator structures. The designs aim at minimizing the effect of the principal loss mechanisms due to the filter structure, such as resistive losses and bulk-wave radiation losses. Operation of the filters is optimized using software developed here and based on the coupling-of-modes (COM) model. Novel SAW tag topologies are studied and developed using Finite Element Method (FEM) software.

Figure 9.a.1: Schematic picture of a simple SAW device. The acoustic wave propagating on a piezoelectric substrate is generated and received using interdigital transducers (IDTs).

Surface acoustic wave sensorsare a class ofmicro electro mechanical systems(MEMS) which rely on the modulation of surface acoustic waves to sense a physical phenomenon. The sensor transduces an input electrical signal into a mechanical wave which, unlike an electrical signal, can be easily influenced by physical phenomena. The device then transduces this wave back into an electrical signal. Changes in amplitude, phase, frequency, or time-delay between the input and output electrical signals can be used to measure the presence of the desired phenomenon.Device Layout

Fig: 9.a.2) Surface Acoustic Wave Sensor Interdigitated Transducer DiagramThe basic surface acoustic wave device consists of a piezoelectric substrate, an input interdigitated transducer (IDT) on one side of the surface of the substrate, and a second, output interdigitated transducer on the other side of the substrate. The space between the IDTs, across which the surface acoustic wave will propagate, is known as the delay-line. This region is called the delay line because the signal, which is a mechanical wave at this point, moves much slower than its electromagnetic form, thus causing an appreciable delay.Device OperationSurface acoustic wave technology takes advantage of thepiezoelectric effectin its operation. Most modern surface acoustic wave sensors use an inputinterdigitated transducer(IDT) to convert an electrical signal into an acoustic wave.The sinusoidal electrical input signal creates alternating polarity between the fingers of the interdigitated transducer. Between two adjacent sets of fingers, polarity of the fingers will be switched (e.g. + - +). As a result, the direction of the electric field between two fingers will alternate between adjacent sets of fingers. This creates alternating regions of tensile and compressive strain between fingers of the electrode by the piezoelectric effect, producing a mechanical wave at the surface known as asurface acoustic wave. As fingers on the same side of the device will be at the same level of compression or tension, the space between them---known as the pitch---is the wavelength of the mechanical wave. We can express the synchronous frequencyf0of the device with phase velocityvpand pitchpas:

The synchronous frequency is the natural frequency as which mechanical waves should propagate. Ideally, the input electric signal should be at the synchronous frequency to minimize insertion loss.As the mechanical wave will propagate in both directions from the input IDT, half of the energy of the waveform will propagate across the delay line in the direction of the output IDT. In some devices, a mechanical absorber or reflector is added between the IDTs and the edges of the substrate to prevent interference patterns or reduceinsertion lossesrespectively.The acoustic wave travels across the surface of the device substrate to the other interdigitated transducer, converting the wave back into an electric signal by the piezoelectric effect. Any changes that were made to the mechanical wave will be reflected in the output electric signal. As the characteristics of the surface acoustic wave can be modified by changes in the surface properties of the device substrate, sensors can be designed to quantify any phenomenon which alters these properties. Typically, this is accomplished by the addition of mass to the surface or changing the length of the substrate and the spacing between the fingers.The structure of the basic surface acoustic wave sensor allows for the phenomena of pressure, strain, torque, temperature, and mass to be sensed. The mechanisms for this are discussed below:Pressure, Strain, Torque, TemperatureThe phenomena of pressure, strain, torque, temperature, and mass can be sensed by the basic device, consisting of two IDTs separated by some distance on the surface of a piezoelectric substrate. These phenomena can all cause a change in length along the surface of the device. A change in length will affect both the spacing between the interdigitated electrodes---altering the pitch---and the spacing between IDTs---altering the delay. This can be sensed as a phase-shift, frequency-shift, or time-delay in the output electrical signal.When a diaphragm is placed between the environment at a variable pressure and a reference cavity at a fixed pressure, the diaphragm will bend in response to a pressure differential. As the diaphragm bends, the distance along the surface in compression will increase. A surface acoustic wave pressure sensor simply replaces the diaphragm with a piezoelectric substrate patterned with interdigitated electrodes. Strain and torque work in a similar manner, as application to the sensor will cause a deformation of the piezoelectric substrate. A surface acoustic wave temperature sensor can be fashioned from a piezoelectric substrate with a relatively high coefficient of thermal expansion in the direction of the length of the device.

MassThe accumulation of mass on the surface of an acoustic wave sensor will affect the surface acoustic wave as it travels across the delay line. The velocityvof a wave traveling through a solid is proportional to the square root of product of the Youngs modulusEand the densityof the material.

Therefore, the wave velocity will decrease with added mass. This change can be measured by a change in time-delay or phase-shift between input and output signals. Signal attenuation could be measured as well, as the coupling with the additional surface mass will reduce the wave energy. In the case of mass-sensing, as the change in the signal will always be due to an increase in mass from a reference signal of zero additional mass, signal attenuation can be effectively used.The inherent functionality of a surface acoustic wave sensor can be extended by the deposition of a thin film of material across the delay line which is sensitive to the physical phenomena of interest. If a physical phenomenon causes a change in length or mass in the deposited thin film, the surface acoustic wave will be affected by the mechanisms mentioned above. Some extended functionality examples are listed below:Chemical VapoursChemical vapour sensors use the application of a thin film polymer across the delay line which selectively absorbs the gas or gases of interest. An array of such sensors with different polymeric coatings can be used to sense a large range of gases on a single sensor with resolution down to parts per trillion, allowing for the creation of a sensitive "lab on a chip."Biological MatterA biologically-active layer can be placed between the interdigitated electrodes which contains immobilized antibodies. If the corresponding antigen is present in a sample, the antigen will bind to the antibodies, causing a mass-loading on the device. These sensors can be used to detect bacteria and viruses in samples, as well as to quantify the presence of certain mRNA and proteins.HumiditySurface acoustic wave humidity sensors require a thermoelectric cooler in addition to a surface acoustic wave device. The thermoelectric cooler is placed below the surface acoustic wave device. Both are housed in a cavity with an inlet and outlet for gases. By cooling the device, water vapor will tend to condense on the surface of the device, causing a mass-loading.Ultraviolet RadiationSurface acoustic wave devices can be made sensitive to optical wavelengths through the phenomena known as acoustic charge transport (ACT), which involves the interaction between a surface acoustic wave and photo generated charge carriers from a photo conducting layer. Ultraviolet radiation sensors employ the use of a thin film layer of zinc oxide across the delay line. When exposed to ultraviolet radiation, zinc oxide generates charge carriers which interact with the electric fields produced in the piezoelectric substrate by the traveling surface acoustic wave.[1]This interaction decreases the velocity and the amplitude of the signal.Magnetic FieldsFerromagnetic materials, such as iron, nickel, and cobalt, exhibit a characteristic called magnetostriction, where the Young's modulus of the material is dependent on magnetic field strength. If a constant stress is maintained on such a material, the strain will change with a changing Young's modulus. If such a material is deposited in the delay line of a surface acoustic wave sensor, a change in length of the deposited film will stress the underlying substrate. This stress will result in a strain on the surface of the substrate, affecting the phase velocity, phase-shift, and time-delay of the signal.ViscositySurface acoustic wave devices can be used to measure changes in viscosity of a liquid placed upon it. As the liquid becomes more viscous the resonant frequency of the device will change in correspondence. A network analyser is needed to view the resonant frequency.

8.b) Radio Frequency Identification Energy HarvestingRF energyis currently broadcasted from billions of radio transmitters around the world, including mobile telephones, handheld radios, mobile base stations, and television/ radio broadcast stations. The ability toharvest RF energy, from ambient or dedicated sources, enableswireless chargingof low-power devices and has resulting benefits to product design, usability, and reliability. Battery-based systems can be trickled charged to eliminate battery replacement or extend the operating life of systems using disposable batteries. Battery-free devices can be designed to operate upon demand or when sufficient charge is accumulated. In both cases, these devices can be free of connectors, cables, and battery access panels, and have freedom of placement and mobility during charging and usage.

Energy SourcesFig: 9.b.1) RFID Energy Harvesting

The obvious appeal ofharvesting ambient RF energyis that it is essentially free energy. The number of radio transmitters, especially for mobile base stations and handsets, continues to increase. ABI Research and iSupply estimate the number of mobile phone subscriptions has recently surpassed 5 billion, and the ITU estimates there are over 1 billion subscriptions for mobile broadband. Mobile phones represent a large source of transmitters from which to harvest RF energy, and will potentially enable users to provide power-on-demand for a variety of close range sensing applications. Also, consider the number of WiFi routers and wireless end devices such as laptops. In some urban environments, it is possible to literally detect hundreds of WiFi access points from a single location. At short range, such as within the same room, it is possible to harvest a tiny amount of energy from a typical WiFi router transmitting at a power level of 50 to 100 mW. For longer-range operation, larger antennas with higher gain are needed for practical harvesting ofRF energyfrom mobile base stations and broadcast radio towers. In 2005, Powercast demonstrated ambient RF energy harvesting at 1.5 miles (~2.4 km) from a small, 5-kW AM radio station.RF energy can be broadcasted in unlicensed bands such as 868MHz, 915MHz, 2.4GHz, and 5.8GHz when more power or more predictable energy is needed than what is available from ambient sources. At 915MHz, government regulations limit the output power of radios using unlicensed frequency bands to 4W effective isotropic radiated power (EIRP), as in the case of radio-frequency- identification (RFID) interrogators. As a comparison, earlier generations of mobile phones based on analog technology had maximum transmission power of 3.6W, and Powercasts TX91501 transmitter that sends power and data is 3W.RF Harvesting ReceiversRF energy harvesting devices, such asPowercasts Powerharvester receivers, convert RF energy into DC power. These components are easily added to circuit board designs and work with standard or custom 50-ohm antennas. With current RF sensitivity of theP2110 Powerharvesterreceiver at -11dBm, powering devices or charging batteries at distances of 40-45 feet from a 3W transmitter is easily achieved and can be verified with Powercastsdevelopment kits. Improving the RF sensitivity allows for RF-to-DC power conversion at greater distances from an RF energy source. However, as the range increases the available power and rate of charge decreases.An important performance aspect of an RF energy harvester is the ability to maintain RF-to-DC conversion efficiency over a wide range of operating conditions, including variations of input power and output load resistance. For example,Powercasts RF energy-harvesting components do not require additional energy-consuming circuitry for maximum power point tracking (MPPT) as is required with other energy-harvesting technologies. Powercasts components maintain high RF-to-DC conversion efficiency over a wide operating range that enables scalability across applications and devices. RF energy-harvesting circuits that can accommodate multi-band or wideband frequency ranges, and automatic frequency tuning, will further increase the power output, potentially expand mobility options, and simplify installation.

Typical ApplicationsRF energy can be used to charge or operate a wide range of low-power devices. At close range to a low-power transmitter, this energy can be used to trickle charge a number of devices including GPS or RLTS tracking tags, wearable medical sensors, and consumer electronics such as e-book readers and headsets. At longer range the power can be used for battery-based or battery-free remote sensors for HVAC control and building automation, structural monitoring, and industrial control. Depending on the power requirements and system operation, power can be sent continuously, on a scheduled basis, or on-demand. In large-scale sensors deployments significant labor cost avoidance is possible by eliminating the future maintenance efforts to replace batteries.

10) CHALLENGES AND ISSUES OF WIRELESS SENSINGVibration, ionizing radiation, RF issues, and certifications are just a few of the issues that must be addressed when developing new sensor systems. There are many challenges in developing wireless sensors for extreme environments. Pressure variations from vacuum to high pressures should not be an issue for solid-state devices such as MEMS, ICs, or SAW devices. For most cases, corona discharge and arcing at low pressures should not pose a problem when the voltages are low. However, an issue may arise when devices are miniaturized and the spacing between charged components is reduced, allowing corona discharge and arcing to occur at lower voltages. Devices must be designed with Paschens law in mind to avoid arcing and corona discharge when the pressure drops. Vibration is an issue for all aircraft. Component failures from the high levels of shock and vibration during operation are not uncommon. Monitoring of dynamic loading during flight is important for SHM and fatigue life analysis. Structural health monitoring using conventional sensors (such as strain gauges) has been difficult during flight due to the dynamics of aircraft loading and the random vibrations that are generated. Strain is often measured on research aircraft like those found at NASAs Dryden Research Centerypical small aircraft can experience vibrational noise in the range of 60 peak to peak, while the aircraft experiences up to 1,000 loads during flight. These strain measurements are similar in magnitude to those taken on other research aircraft. Aircraft such as the P3 experience 0.6 g of vibrational noise during flight. The raw sensor data were filtered with a two-point moving average. The moving average was subtracted from the original data, leaving only the vibrational noise. The structural noise, which is mostly due to vibrations of the aircraft, is -60 to +85 . Although SAW sensors can filter out this noise from strain measurements, many other types of sensors, such as conventional strain gauges, cannot. Another concern for avionics is exposure to corrosive environments. The Kennedy Space Center operates accelerated corrosive chambers that use salt fog and acids to investigate corrosive environments and their effects on aerospace vehicles. They also operate the outdoor KSC Beachside Corrosion Laboratory, where material degradation and harsh corrosive environments are studied. The Glenn Research Center operates an extreme environment chamber that measures 0.9 m x 1.2 m. The chamber temperature can reach 538 C, with a pressure of 10.1 MPa and an atmosphere of corrosive chemicals such as hydrogen fluoride and hydrogen chloride. The chamber is used primarily for simulating the conditions found on the planet Venus. Although this seems to be a space application instead of an aeronautical application, it has been suggested that NASA fly an aircraft in the atmosphere of Venus. RF communication issues pose a significant challenge to the successful implementation of SAW systems. Modulation methods must be chosen to allow large numbers of devices to communicate without interference. The bandwidth must be utilized carefully to enable high data rates while adhering to FCC limitations. Encoding schemes must be developed to allow for efficient operation in noisy environments. The devices must be small, which means higher frequencies and therefore smaller antenna sizes. Higher frequencies enable higher data rates; however, the range will begin to decrease when the frequency reaches 10 GHz. Higher- frequency devices may also mean that the fabrication feature sizes must shrink, which may lead to manufacturing issues. In addition, the frequency of operation must follow FCC guidelines. A 60 GHz SHM system is being developed for aircraft applications. The system exploits an unlicensed band in the communication spectrum for 5764 GHz. The devices use Ultra Wide Band Impulse Radio (UWB-IR) technology and very low power. The wireless communicating nodes are passive and fabricated on a flexible Kapton substrate that can be attached to curved surfaces within the aircraft. Many of NASAs test environments are closed metallic structures with metal test articles. Direct wireless links may not be possible in enclosed metallic spaces of aircraft, so multi-hop or mesh network architectures may be required. Electromagnetic interference poses a problem for all wireless systems. All wireless electronics must be designed to pass tests for both electromagnetic compatibility and interference. The certification of wireless sensor networks for flight is another issue that must be addressed. This includes the allocation of frequencies for wireless sensing on aircraft, along with the determination of RF power levels and FAA acceptance for aircraft use. There is a concern that wireless devices within the cabin may interfere with aircraft antennas located outside the cabin. Personal electronic devices and cell phones have been tested to determine the effects that wireless devices may have on aircraft avionics. Active RFID tags have been tested for compliance with RTCA/DO160 aircraft emission limits. The tags exceeded the thresholds, but further investigations are needed before the effects of the interference can be understood. The interference from passive tags was not considered a concern, however because the study focused on RFID tags contained in cargo pallets without an interrogator. Unfortunately, passive SAW sensors would require an active interrogator within the aircraft structure. Because SAW sensors will share the same frequency bands with RFID tags and some of the same RF modulation techniques, they may exceed the emission thresholds as well. Before any wireless sensor system can be certified for use on aircraft, it will have to be tested.11) CONCLUSIONFrom the ground up to the edge of space, NASA has applications that require passive wireless sensor technologies in extreme aeronautical environments. NASA sensor systems (including acceleration, temperature, pressure, strain, shape, chemical, acoustic emission, ultrasonics, imaging, eddy current, thermography, and terahertz waves) could benefit from becoming passive and wireless. However, each of these applications has its own requirements and issues. Extreme environments offer many challenges that must be addressed, such as temperature, pressure, vibration, ionizing radiation, and certifications. However, despite the issues and challenges, new technologies such as MEMS, SAW, backscatter, and RFID are leading to the development of robust passive wireless sensor systems that may one day operate in extreme environments.The need for passive wireless sensors has been presented, but NASA does not possess all of the necessary resources to develop them. Thus, NASA encourages partnerships, including those with universities and industry, to aid in the development of passive wireless sensors for the extreme environments found in aeronautical applications.

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