Greg E. Bridges

Embeddable wireless strain sensor based on resonant rf cavities

In this article we describe a type of sensor to monitor strain. The strain sensor is a passive device that can be embedded or attached to a structure and then remotely interrogated though a wireless interface. Such a system has the advantage of requiring no permanent physical connection, either electrical or optical, to an interrogation system. The sensor is a conducting coaxial electromagnetic cavity that is embedded in or bonded to the structure in which strain is to be measured. The cavity will exhibit resonance for electrical wavelengths two times the cavity length. Changes in the structure’s dimensions will be reflected in changes in the dimensions of the cavity, and will result in a shift of the resonant frequency of the cavity. The sensor incorporates an antenna so that the resonant frequency of the cavity can be determined by remote interrogation. The acquired resonant frequency is then used to calculate the strain in the structure. The sensor presented in this article operates at a frequency of a...

Filter-Antenna Module Using Substrate Integrated Waveguide Cavities

A design procedure for filter-antenna modules based on substrate integrated waveguide cavities is presented in this letter. The filter-antenna module is modeled as an asynchronously tuned coupled-resonator circuit in which the last resonator also contains the radiating element. The design of the filter-antenna module is based on the classical process applied to obtain filters through the use of coupled resonators. The designed filter-antenna module is manufactured and measured, obtaining the following: a central frequency (fo) of 1.94 GHz, a -3-dB fractional band- width (FBW) of 5.57%, a gain (G) of 4.87 dBi, a front-to-back ratio (FTBR) of 25.60 dB, and a co-to-cross-polarization ratio of 22.86 dB in the direction of maximum radiation. The integration of the filter and the antenna into just one module leads to a reduction of size and weight in the RF front-end, while the implementation by means of the substrate integrated waveguide technique makes the integration with planar circuits easier.

RF Cavity Passive Wireless Sensors With Time-Domain Gating-Based Interrogation for SHM of Civil Structures

Many existing sensing technologies for application to the monitoring of civil structures have a serious deficiency in that they require some type of wired physical connection to the outside world. This causes significant problems in the installation and long-term use of these sensors. This paper describes a new type of passive wireless sensor that is based on resonant RF cavities, where the resonant frequency is modulated by a measurand. In the case of a strain sensor, the electrical length of the cavity directly modulates its resonant frequency. A probe inside the cavity couples RF signals from the cavity to an externally attached antenna. The sensor can then be interrogated remotely using microwave pulse-echo techniques. Such a system has the advantage of requiring no permanent physical connection between the sensor and the data acquisition system. In this type of sensor, the RF interrogation signal is transmitted to the sensor and then reradiated back to the interrogator from the sensor resulting in a signal strength that decreases with the forth power of distance. This places an upper limit on the distance over which the sensor can be interrogated. Theoretical estimates show that these sensors can be interrogated with sufficient SNR at distances exceeding 10 m for radiated powers of less than 1 mW. We present results for a strain sensor and a displacement sensor that can be interrogated at a distance of 8 m with a strain resolution of less than 10 ppm and displacement resolution of 0.01 mm, respectively.

“Zeptofarad” (10−21 F) resolution capacitance sensor for scanning capacitance microscopy

We describe a sensor for use in a scanning capacitance microscope (SCM) that is capable of “zeptofarad” (10−21 F) capacitance measurement resolution in a 1 Hz bandwidth with a peak-to-peak sense voltage on the probe tip of no more than 300 mV. This sensitivity is based on experimental data and simulation results that are in excellent agreement. The complete sensor incorporates an oscillator (phase locked to a 10 MHz crystal oscillator), a coupled transmission line resonator, an amplifier, and a peak detector. The resonator is fabricated from copper-clad, low-loss dielectric material and its size is such that it is easily incorporated with a scanning probe microscope. The sensor’s use in the SCM enables capacitance resolution that has not previously been possible while retaining the instrumental advantages of imaging at low sense voltages. The performance of this sensor is discussed and compared to alternative scanning capacitance microscopy methodologies.

Asymptotic Limits of Negative Group Delay in Active Resonator-Based Distributed Circuits

In this paper the asymptotic limits of negative group delay (NGD) phenomena in multi-stage RLC resonator-based circuits are discussed. A NGD-bandwidth-product limit is derived as a function of the number of stages and the out-of-band gain, which is independent of the circuit topology and can include active gain compensation. The limit is verified experimentally at microwave frequencies using a gain-compensated NGD circuit employing a parallel RLC resonator in the feedback path of a high-frequency op-amp. It is shown that, in the asymptotic limit, the NGD-bandwidth-product is proportional to the square root of the number of stages, and also to the square root of the logarithm of the out-of-band gain. The relation between the time-domain transient amplitude and the out-of-band gain is analyzed for finite-duration modulated signals, indicating an exponential increase in transient amplitudes with the square of NGD. Analysis shows that any attempt to increase the NGD of a finite-duration modulating waveform, by cascading more stages, is thwarted by the transients.

A Wireless Passive Sensor for Temperature Compensated Remote pH Monitoring

Temperature must be accounted for in order to provide accurate measurements in electrode-based pH sensors. We present an integrated wireless passive sensor for remote pH monitoring employing temperature compensation. The sensor is a resonant circuit consisting of a planar spiral inductor connected in parallel to a temperature-dependent resistor (thermistor) and a voltage-dependent capacitor (varactor). A pH combination electrode consisting of an iridium/iridium oxide sensing electrode and a silver/silver chloride reference electrode, is connected in parallel with the varactor. A potential difference change across the electrodes due to pH variation of the solution changes the voltage-dependent capacitance and shifts the resonant frequency, while temperature of the solution affects the resistance and changes the quality factor of the sensor. An interrogator coil is inductively coupled to the sensor inductor and remotely tracks the resonant frequency and quality factor of the sensor. The sensor is calibrated for temperature over a range of 25

The changing dielectric properties of CHO cells can be used to determine early apoptotic events in a bioprocess

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Differential electronic detector to monitor apoptosis using dielectrophoresis-induced translation of flowing cells (dielectrophoresis cytometry)

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Scanned electrostatic force microscope for noninvasive high frequency potential measurement

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A wireless embedded passive sensor for monitoring the corrosion potential of reinforcing steel

and pH over a 1.5–12 dynamic range. By employing temperature compensation, a measurement accuracy of less than 0.1 pH is achieved and the response time of the sensor is demonstrated to be less than 1 s. The sensor overcomes the pH measurement error due to the temperature dependence of electrode-based passive pH sensors and has applications in remote pH monitoring where temperature varies over a wide range.