This paper reports a novel diaphragm-assisted open-ended hollow coaxial cable resonator (OE-HCCR) for highly sensitive pressure measurements at elevated temperatures. The coaxial line resonator is constructed using two highly reflective reflectors in a homemade hollow coaxial cable, including a metal post shorting the two concentric conductors (i.e., the inner conductor and the outer conductor) and the open end of the coaxial line. In the pressure sensor device design, a sealed metal diaphragm is mounted on the open end of the OE-HCCR and is employed as the pressure responsive element. The diaphragm’s deflection can be accurately captured by the coaxial line resonator in real-time, in response to a change in differential pressure. The sensor device’s responses to hydraulic pressures at ambient temperature and air pressures at elevated temperatures up to 1000 °C were investigated in the demonstration experiments, showing user-configurable measurement sensitivity and high-temperature survivability. The device’s pressure measurement resolution is also demonstrated as high as sub-Pascal by probing tiny hydrostatic pressure changes. A positive feedback loop is constructed as an alternative interrogator for the novel device, showing high signal quality and ease of demodulation. The pressure sensor described in this work is robust, easy-to-manufacture, user-configurable, low-cost, highly sensitive, and has great potential for industrial applications involving high-temperature and harsh environments. The presented work opens new avenues to develop a new generation of high-sensitivity and high-temperature microwave sensors based on well-known open-ended coaxial probes.

This paper presents an extrinsic Fabry–Perot interferometer-based optical fiber sensor (EFPI) for measuring three-dimensional (3D) displacements, including interfacial sliding and debonding during delamination. The idea employs three spatially arranged EFPIs as the sensing elements. In our sensor, the three EFPIs are formed by three endfaces of three optical fibers and their corresponding inclined mirrors. Two coincident roof-like metallic structures are used to support the three fibers and the three mirrors, respectively. Our sensor was calibrated and then used to monitor interfacial sliding and debonding between a long square brick of mortar and its support structure (i.e., a steel base plate) during the drying/curing process. This robust and easy-to-manufacture triaxial EFPI-based 3D displacement sensor has great potential in structural health monitoring, the construction industry, oil well monitoring, and geotechnology.

In this study, two types of smart rock were proposed to monitor bridge scour effect, including a direction-unknown (DU) smart rock and a direction-known (DK) smart rock. The DU smart rock is a concrete ball containing a cylindrical magnet, and the DK smart rock is also a concrete ball but contains a magnet whose direction is always pointing to the projected direction of the ambient magnetic field (AMF) in the horizontal plane. Based on the theory of magnetic field, the distribution of the magnetic field induced by DU and DK smart rock was derived. Algorithms were developed to localize the position of the two smart rocks under the AMF. Field tests were conducted at a bridge pier, and the intensity and direction of the AMF at the test site were measured by using a magnetometer and a home-made device, respectively. Results showed that presence of steel reinforcement and steel girders in the bridge changed the distribution of the geomagnetic field. The algorithms were able to localize the position of both the DU and the DK smart rocks with satisfied accuracy. Compared with the DU smart rock, the amount of calculation to localize DK smart rock was reduced. However, the measurement error was increased due to the instability of its direction from varying AMF caused by environmental factors such as passing vehicles on the bridge.

In this study, a smart rock, which is a magnet embedded in a concrete ball and whose direction is always pointing downwards, is proposed to monitor bridge scour depth. Based on the theory of magnetic field, the distribution of the magnet-induced magnetic field (MMF) induced by the smart rock was derived. An algorithm was developed to localize the position of the smart rock. Field tests were conducted at a bridge pier at three different times. Both the intensities of the ambient magnetic field (AMF) and the total magnetic field (TMF) were measured with a magnetometer. Results showed that the presence of steel reinforcement or steel girders in the bridge changed the distribution of the geomagnetic field. The algorithm successfully localized the position of the smart rock with an error ranging from 0.26 to 0.33 m, which satisfied the requirement for engineering applications. The effective monitoring range depends on the variation of the AMF, and the maximum monitoring depth ranged from 11.5 to 8.5 m as the standard deviation of the AMF increased from 32.3 to 80.75 nT.

For the past 50 years, open-ended coaxial lines were commonly employed for the determination of electromagnetic properties of various materials. In this article, the application of an open-ended hollow coaxial cable resonator (OE-HCCR) as a 1-D inclinometer for tilt measurements with 110 nanoradian (nrad) resolution is proposed and demonstrated. The coaxial cable resonant structure is formed between a metal post welded within the coaxial cable at the RF input end and the open end of the coaxial cable. A metal mass block, suspended in proximity to the open end in parallel is used to construct a pendulum structure, serving as the element responsive to a tilt. When the device is tilted, the distance between the mass block and the open end varies due to the displacement of the suspended pendulum. The change in the gap distance modifies the phase reflection coefficient associated with the open end, causing the resonance frequency of the coaxial cable resonator to change. By tracking the resonance frequency, the coaxial line resonator is effectively used as an inclinometer. The experimental results demonstrated that the novel device has a measurement resolution of 110 nrad over a range of 0-3 milliradians (mrad). Additionally, based on the same principle, a 2-D inclinometer based on two perpendicularly aligned OE-HCCRs, useful for simultaneous tilt measurements in two orthogonal dimensions is demonstrated. The open-ended coaxial cable resonator-based inclinometer holds several advantages, including high resolution, robustness, cost effectiveness, ease of signal interrogation, and user-configurable sensitivity and dynamic range.