Brian H. Fisher

Orthogonal Frequency Coded SAW Sensors for Aerospace SHM Applications

National Aeronautics and Space Administration (NASA) aeronautical programs require structural health monitoring (SHM) to ensure the safety of the crew and the vehicles. Future SHM sensors need to be small, lightweight, inexpensive, and wireless. Orthogonal frequency coded (OFC) surface acoustic wave (SAW) reflectors and transducers have been recently introduced for use in communication, as well as in sensor and radio-frequency identification (RFID) tag applications (Malocha , 2004, Puccio , 2004). The OFC SAW technology approach has been investigated by NASA for possible inclusion in ground, space flight, and space exploration sensor applications. In general, SAW technology has advantages over other potentially competitive technologies, because the devices can operate in ranges from cryogenic to furnace temperature. SAW devices can also be small, rugged, passive, wireless, and radiation hard and can operate with variable frequency and bandwidth. SAW sensor embodiments can provide onboard device sensor integration or can provide integration with an external sensor that uses the SAW device for encoding the sensor information and transmission to the receiver. SAW OFC device technology can provide RFID tags and sensors with low loss, large operating temperatures, and a multiuse sensor platform. This paper will discuss the key parameters for OFC device design, which includes reflector and transducer design, coding diversity approaches, and insertion loss considerations. Examples of several OFC device sensors and RFID tags are presented to show the current state-of-the-art performance for several NASA applications. Projections for future sensor and RFID tag platform performance are discussed, along with some of the current challenges and issues of the technology.

Fastener Failure Detection Using a Surface Acoustic Wave Strain Sensor

Surface acoustic wave (SAW) strain sensors are presented for use in the detection of aircraft fastener failures. SAW sensors have the potential for the development of passive wireless systems. The SAW devices employed four orthogonal frequency coding spread spectrum reflectors in two banks on a high temperature piezoelectric substrate. Three SAW devices were attached to a cantilever panel with removable side stiffeners. Damage in the form of fastener failure was simulated by removal of bolts from the side stiffeners. During testing, three different force conditions were used to simulate static aircraft structural response under loads. The design of the sensor, the panel arrangement and the panel testing results are reported. The results show that the sensors successfully detected single fastener failure at distances up to 655 mm from the failure site under loaded conditions.

Orthogonal frequency coded SAW sensors and RFID design principles

Orthogonal frequency coded (OFC) SAW reflectors and transducers have been recently introduced for use in communication, sensor and RFID tag applications.[1,2] The OFC SAW technology approach has been funded by NASA for possible inclusion in ground, space flight and space exploration sensor applications. In general, SAW technology has advantages over possible competing technologies: passive, wireless, radiation hard, operation from cryogenic to furnace temperature ranges, small, rugged, variable frequency and bandwidth operation, encoding and commercially available. SAW sensor embodiments can provide onboard device sensor integration, or can provide integration with an external sensor that uses the SAW device for encoding the sensor information and transmission to the receiver. SAW OFC device technology can provide RFID tags and sensors with low loss, large operating temperatures and a multi-use sensor platform. This paper will discuss the key parameters for OFC device design, which include reflector and transducer design, coding diversity approaches, and insertion loss considerations. Examples of several OFC device sensors and RFID tags will be presented to show the current state-of-the-art performance for several NASA applications, as well as projections for future sensor and RFID tag platform performance.

A Passive Wireless Multi-Sensor SAW Technology Device and System Perspectives

This paper will discuss a SAW passive, wireless multi-sensor system under development by our group for the past several years. The device focus is on orthogonal frequency coded (OFC) SAW sensors, which use both frequency diversity and pulse position reflectors to encode the device ID and will be briefly contrasted to other embodiments. A synchronous correlator transceiver is used for the hardware and post processing and correlation techniques of the received signal to extract the sensor information will be presented. Critical device and system parameters addressed include encoding, operational range, SAW device parameters, post-processing, and antenna-SAW device integration. A fully developed 915 MHz OFC SAW multi-sensor system is used to show experimental results. The system is based on a software radio approach that provides great flexibility for future enhancements and diverse sensor applications. Several different sensor types using the OFC SAW platform are shown.

Study of the acoustoelectric effect for SAW sensors

Research has recently begun on the use of ultrathin films and nanoclusters as mechanisms for sensing of gases, liquids, etc., because the basic material parameters may change because of film morphology. As films of various materials are applied to the surface of SAW devices for sensors, the conductivity of the films may have a strong acoustoelectric effect, whether desired or not. The purpose of this paper is to reexamine the theory and predictions of the acoustoelectric effect for SAW interactions with thin conducting or semi-conducting films. The paper will summarize the theory and predict the effects of thin film conductivity on SAW velocity and propagation loss versus frequency and substrate material. The theory predicts regions of conductivity which result in extremely high propagation loss, and which also correspond to the mid-point between the open and short-circuit velocities. As an example of the verification and possible usefulness of the acoustoelectric effect, recent experimental results of palladium (Pd) thin films on a YZ LiNbO3 SAW delay line have shown large changes in propagation loss, depending on the Pd film thickness, exposure to hydrogen gas, or both. By proper design, a sensitive hydrogen leak detector SAW sensor can be designed.

Cryogenic Liquid Sensing Using SAW Devices

Sensing at cryogenic temperatures is required for many critical applications, but is extremely difficult. There are a wealth of problems for cryogenic sensor devices, which include the extreme cold, which makes many sensors inoperable due to freeze-out of conduction carriers, mechanical stress and strain which impacts reliability, undesirable device heat generation in the vessel, and a host of others. In addition, consideration of sensor wiring and vessel penetration is required. In principle, acoustic devices can successfully operate to extremely low temperatures without any serious performance degradation. In particular, SAW devices operate as sensors, and certain embodiments are passive, wireless, and coded for multi-sensor applications. However, few results are reported on SAW devices for cryogenic applications. Research has been performed on the use of SAW devices for operation as liquid level sensors. The initial application is for a level sensor to be used by NASA in its cryogenic liquid fuel tanks, for both ground and space vessels. The work has investigated the use of both quartz and lithium niobate devices as liquid level sensors in liquid nitrogen. In principle, the devices can operate as simple switches, with the devices turned off due to liquid SAW damping when submerged. One primary concern in using SAW devices is the survivability of the devices when shocked by rapid, large, thermal changes in the submersion and withdrawal from the liquid. Another concern is the ability of a packaged device to be reliable. Experiments were performed on bare die, commercially available packaged devices, and on UCF fabricated devices. Samples underwent a host of rapid thermal cycling by liquid submersion and data was taken under varying test conditions. Survivability of devices and the operational parameters were measured throughout the year long study. The results found that the samples are remarkably robust. All packaged devices showed no failures in the entire study. This paper will present the experimental data and results from the study on SAW quartz and lithium niobate device operational performance parameters. A review of the various test conditions and device results will be shown. The results of this study conclude that under proper conditions, SAW devices can be used as liquid sensors at cryogenic temperatures.

Wireless SAW Sensor Temperature Extraction Precision

Surface acoustic wave (SAW) devices are suitable for use in harsh environments because of their ruggedness under radiation and extreme temperatures and ability to be configured as passive wireless sensors. This orthogonal frequency coded (OFC) sensor system consists of a transceiver that wirelessly interrogates the passive SAW sensor, which can be used in several types of environments. A modified radar equation is presented that can be used to estimate the signal power from the returned sensor signal as a function of practical parameters. The precision performance of the OFC passive wireless sensor, not previously investigated, is shown to be a function of signal-to-noise ratio as expected from an analog sensor; extracted temperature precision better than 0.001 °C is reported. Signal integration is an effective means to increase the precision and/or range of the wireless temperature extraction scheme when deciding if the OFC passive wireless sensing scheme will fit a particular application.

A study on the aging of ultra-thin Palladium films on SAW hydrogen gas sensors

Traditionally, low-powered, room temperature sensing of gaseous hydrogen (H2) is difficult. With renewed interest in H2 as a source of energy, there is a need for reliable, energy-efficient sensors. A potential solution can be found in using surface acoustic wave (SAW) devices, which have been implemented as passive, wireless RFID tag-sensors. Thus, in concept, it is advantageous to develop a SAW device with H2 sensing capabilities. Prior experiments have successfully demonstrated a passive SAW-based H2 gas sensor by placing an ultra-thin Palladium (Pd) film (<50Å) in the propagation path [1–3]. These sensors have an instantaneous response and a significant fractional change in SAW propagation loss; however, the lifetime of these sensors are still unknown. Hence, the objective of this study was to examine the influence of aging of ultra-thin Pd films on the usable life of passive SAW H2 gas sensors.

Surface Acoustic Wave Pulsed-Correlator Transceiver for Aerospace Applications

This paper will present current efforts on wireless passive surface acoustic wave (SAW) sensor transceiver development for aerospace applications. Our groups SAW sensor work has been sponsored by NASA for the past ten years in efforts to develop wireless sensing for ground base and space exploration in extreme environments. The devices are radiation hard and work from cryogenic to several hundred degrees centigrade with standard device processing. The focus of this paper will be on a synchronous, pulsed correlator transceiver for interrogation of SAW delay-line sensors. The 915-MHz system has a bandwidth of 15 MHz and uses a pulsed noise-generated signal of 1-\(\mu \) s duration. The design principles, system analysis, and receiver measurements are given. The prediction of signal-to-noise ratio versus range is shown for given system parameters. The system development leading to the first testing of SAW liquid level sensors at NASA KSC will be discussed.

In situ observation and measurement of the SAW thin-film acoustoelectric effect

The thin-film acoustoelectric effect in SAW devices describes the interaction of electrical energy between a SAW in a piezoelectric medium and a thin film in the waves propagation path. The real-time observation of the thin-film acoustoelectric interaction is useful in the design and characterization of SAW sensors (i.e., temperature, humidity, viscosity, voltage, current, Hall effects, etc.). An in situ test fixture was designed to be mechanically, thermally, and electrically stable. Data acquisition software and an electron beam evaporation system were configured for real-time thin-film characterization during film growth. Data have been observed for more than 20 SAW devices and over a wide range of frequencies (i.e., 62 MHz to 1 GHz). The results suggest that the use of the in situ procedure yielded good agreement between theoretical predictions and the measured data, which demonstrates a method for the characterization of a SAW H2-gas sensor in real-time.