This paper describes a low cost, passive wireless dielectric constant and conductivity sensor using a microfabricated inductor with interdigitated capacitors (IDC). A self-resonant-structure (SRS) is ...designed by incorporating IDC electrodes in the inter-winding space of the inductor. The distributed capacitance and conductance of the sensor is affected by dielectric constant (ε) and conductivity (σ) of its environment or material under test (MUT). The ε and σ can be used to provide information about the surrounding environment. This serves as an impedance transducer changing the resonant frequency and phase dip of the SRS. The SRS is interrogated using a non-contact inductively coupled reader coil. The change in resonance frequency and phase dip of the SRS is used to detect material properties of the environment/MUT. The relationship between sensor layout and coupling factor between sensor and reader is investigated. Optimizations of the coupling factor based on this relationship are discussed. IDC design trade-offs between the sensor’s sensitivity and coupling factor are also investigated. The sensor’s response to variety of liquid MUTs with a wide range of dielectric constant and conductivity is presented.
We develop a model to quantify the signal strength in passive inductively coupled resonant sensor (PICRS) systems. We also explore the relationship between detectability and signal strength, which is ...critical for signal strength-sensitive applications. Either the phase or magnitude of the input impedance of a coupled reader can be used as a measurand; here, we focus on phase, but the model can be easily adapted to other measurands. The model employs dimensionless parameters, i.e., the quality factor (Q) of the PICRS and coupling factor (k) between the reader and PICRS. We experimentally verify the model using readers and sensors of different Q and k factors. The model incorporates the effect of the reader coil, which is essential for co-selection of readers and sensors. This model can find broader application as a tool for the design and detection of nonisochronous weakly coupled resonant sensors.
We present a miniature passive wireless resonant sensor and its use in a novel platform for chemical-memory-based threshold sensing. The sensors are microfabricated with polyimide passivation and ...utilize various customizable polymer sensor layers allowing for capacitive transduction. Sensors of ≈2-mm 3 active volume and 8-mm working distance are demonstrated, with experimental data for sensing using multiple polymers in multiple solvents. The novel design with configurable sensor layers enables a versatile platform for chemical and environmental monitoring applications.
A surface micromachined Fabry-Perot cavity used as a pressure sensor has been fabricated using standard IC technology. Dielectric film stacks consisting of layers of silicon dioxide and silicon ...nitride were used as mirrors. Polysilicon was used as a sacrificial layer that was then removed to form an air gap cavity. The Fabry-Perot sensor was optically interrogated using a multimode optical fiber. The measured response of the sensor agrees well with theoretical simulation, which takes into account the averaging effect caused by the shape of the deflected mirror in the cavity.
A passive sensor platform was developed to detect the onset of corrosion in concrete bridge decks. The term “passive” is used to describe the sensors because they do not include onboard processing ...capabilities or sources of power. The sensors are powered and interrogated in a wireless manner by measuring the impedance through an external reader coil, which is magnetically coupled to resonant circuits within the sensor. The sensors are designed to be embedded in a concrete bridge during construction and interrogated periodically over its service life. Reinforced concrete slabs were subjected to long-term exposure tests to demonstrate the reliability of the prototype sensors. The middle region of each slab was exposed to alternating moisture cycles with saltwater. Sensors in this region indicated that threshold amounts of corrosion occurred, whereas sensors located in regions that were not exposed to saltwater indicated the absence of corrosion. At the conclusion of the tests, the sensor readings were confirmed by removing the concrete cover and examining the condition of the embedded reinforcement. One flaw in the initial design of the sensors, however, was that the transducing element (a corroding steel wire) passed through the epoxy potting that protected the circuit components. This configuration provided a pathway for contaminants to infiltrate the sensors, causing corrosion. To improve the durability of the passive sensors, a new configuration is being developed in which the corroding element is physically isolated from the resonant circuits within the sensor.
Prototype sensors have been developed to detect the onset of corrosion in steel reinforced concrete using non-invasive techniques. These sensors are designed to be extremely simple and low cost. The ...sensors are embedded in the concrete and are powered and interrogated through the use of inductively coupled magnetic fields. New analysis and data processing techniques show that the sensor can provide useful information about the current extent of corrosion
The Federal Highway Administration requires that highway bridges in the US be inspected at least every 2 years. A research investigation at the University of Texas at Austin aims to develop a ...wireless data acquisition system that can be used to monitor the response of steel bridge systems in an economic manner and thereby enhance this traditional inspection process. Wireless systems feature many advantages, such as minimizing the amount of wire required and reducing installation costs. However, there are also a number of challenges associated with implementing wireless systems and the research team has previously experienced communication difficulties when using wireless data acquisition systems to monitor steel highway bridges. Radio frequency (RF) studies were conducted to determine if wireless networks using IEEE 802.15.4 radios provide reliable communication in steel bridge environments. Measurements were also taken to determine the impact on the network performances using four types of antennas. While improvements in network performance were observed using high-gain antennas in an open field, the performance improvements were inconsistent when used on steel bridges and therefore their use is not recommended.
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