Alfred J. Bahr

Design and performance of wireless sensors for structural health monitoring

Wireless sensors can be realized by integrating a sensor with a passive commercial radio-frequency identification (RFID) chip. When activated, the chip responds with a digitally encoded signal that not only identifies the sensor but also contains information about the sensor state. Two devices have been developed to date: a temperature-threshold indicator and a chloride-threshold indicator. This paper discusses basic concepts, design issues, and preliminary performance.

On the use of active coupling networks with electrically small receiving antennas

An analysis of the noise performance of an active receiving antenna system consisting of an antenna, active coupling network, and high-gain preamplifier is described. It is shown how this noise performance can be described by circular loci in the output-impedance plane of the coupling network. These loci are not restricted to the right-half plane. A coupling network having an output impedance in the left-half plane was built using a negative-impedance converter (NIC), and experimental results obtained by using this circuit in conjunction with a short monopole are presented.

Statistical Detection Model for Eddy-Current Systems

In comparing different eddy-current systems (or probes), it is convenient to base the comparison on the magnitude of the signal produced by a certain flaw size and type. However, as in any detection problem, the real effectiveness of such a system in detecting a given flaw is determined by the statistical variations in the flaw signal and the characteristics of the noise added by the system. Hence, the objective of this task has been to develop a statistical model for determining the probability of detecting a given flaw using an eddy-current system. Such a model would be useful not only for comparing different systems, but also for optimizing the detection process. This optimization would consist of using the model to select the probe and operating frequency(ies) that maximize the signal-to-noise ratio (SNR).

Wireless subsurface microsensors for health monitoring of thermal protection systems on hypersonic vehicles

Health diagnostics is an area where major improvements have been identified for potential implementation into the design of new reusable launch vehicles in order to reduce life cycle costs, to increase safety margins, and to improve mission reliability. NASA Ames is leading the effort to develop inspection and health management technologies for thermal protection systems. This paper summarizes a joint project between NASA Ames and SRI International to develop SensorTags, radio-frequency identification devices coupled with event-recording sensors, that can be embedded in the thermal protection system to monitor temperature or other quantities of interest. Two prototype SensorTag designs containing thermal fuses to indicate a temperature overlimit are presented and discussed.

Radar scattering from intermittently contacting metal targets

Radar returns from moving multielement metal targets often exhibit an unexpected modulation that has both random (or noise-like) and semicoherent components. One possible mechanism for producing this effect is the modification of the current distribution on the target that results when electrical contacts between target elements are altered intermittently by the forces associated with target motion. We propose that this mechanism can be modeled in terms of an impedance-loaded scatterer, and we present preliminary experimental results obtained using resonant dipole scatterers.

Experimental Techniques in Microwave NDE

The objectives of this paper are to provide a historical perspective of microwave NDE and to review the associated experimental techniques by discussing them in the context of specific applications. The application examples are drawn from the published literature and the author’s own experience. The applications discussed include measurements of layer thicknesses and material parameters, detection and quantification of internal flaws inside dielectric objects, detection and quantification of surface flaws on conductors, and imaging of flaws inside material media. The discussion will seek to clarify the potential advantages and disadvantages of various microwave NDE techniques.

Using electromagnetic scattering to estimate the depth of a rectangular slot

A theory for predicting the electromagnetic scattering from a rectangular slot is developed and is used to evaluate the possibility of determining the slot depth from scattering measurements. Experimental results obtained at 100 GHz are presented and compared with theory. Practical limitations of the technique are discussed.

The grating array: A new surface acoustic wave transducer

A novel surface acoustic wave transducer, called the grating array, is described. Experimental results obtained at 9.2 MHz on PZT and 290 MHz on LiNbO 3 are presented which demonstrate the capability of this transducer to efficiently generate and detect Rayleigh waves on piezoelectric materials.

Backscattering from a cylindrical dipole centrally loaded by a time-varying impedance

Amplitude and spectral data obtained from backscattering measurements made on a mechanically switched bisected dipole are presented. The observed behavior is explained in terms of a dipole loaded at its center with a time-varying capacitance.

A Novel Continuously Variable Delay Line

A novel scheme for preducing continuously variable time delay is proposed. The scheme makes use of two dispersive delay lines and spectral inversion to achieve dispersionless variable delay. Experimental results obtained at 1 to 2 GHz are presented.