Each year in U.S., federal, state, and local governments spend billions of dollars to maintain, upgrade, repair, and renovate various civil structures such as buildings, bridges, dams, tunnels, pipelines and offshore platforms. The factors that affect the integrity of these large and complex civil infrastructures cannot be perfectly predicted. As a result, the degradation process of the structures must be monitored in some form. Embedded sensors have the most desired capability for structural health monitoring (SHM). These sensors are used for measurement of various structural and environmental parameters such as strain, force, crack, deflection, vibration, and corrosion under normal and extreme conditions for in situ assessment of the health of the structures.
Monitoring of structure integrity is technically challenging because these engineered structures are inherently large in dimension and geometrically complex. The general requirements on the monitoring technology include high resolution, large dynamic range, low cost, excellent reliability, and remote operation at a long working distance. However, current sensing technologies still have difficulty meeting these requirements. Therefore, there is a continuing need for developing new sensor technologies to address the challenges and ensure the safe operation of the nation's critical infrastructures.
The development of large strain sensors has recently attracted worldwide attentions. A significant challenge remains in achieving a large dynamic range while maintaining high resolution. Conventional strain sensors represented by electro-resistive strain gauges have a satisfactory resolution but a limited dynamic range of less than 1.5%. For strains higher than 2%, extensometers, linear variable differential transformers, and grating based mark tracking technique are commonly used. They can typically measure a strain of up to 5% with low resolution of 0.45%. A common issue associated with these large strain sensors is that these sensors are difficult to embed into a building structure due to the large size of the sensor transducer. Other issues include electrical wiring/connection, poor stability, and large temperature cross-sensitivity.
In the past two decades, fiber optic sensors have found many successful applications in SHM due to their unique advantages such as compactness, high resolution, immunity to electromagnetic interference, remote operation and multiplexing capability. In general, fiber sensors have relatively small dynamic ranges due to the limited deformability of silica glass. Various strain transfer mechanisms have been investigated to extend the dynamic range of the sensor devices. For example, through a specially-designed sensor package, a high strain resolution of 10μ∈ within a large dynamic range (12,000μ∈) has been demonstrated using an extrinsic Fabry-Perot interferometer (EFPI). However, when embedded into a structure, the signal transmission line, i.e., the optical fiber, can easily break when it is subjected to a large strain (about 10 m∈ or 1%) and/or a shear force, causing serious challenges for sensor installation and operation. As such, fiber optic sensors have restricted applications in heavy duty or large strain measurement.
For more robust transmission cables, plastic optical fibers have been explored for sensor development by taking the advantage of their inelastic nature. Plastic fiber sensors with a strain measurement range of up to 15.8% have been reported. However, current plastic fibers have poor optical transmission and wave-guiding properties, which makes it difficult to fabricate high performance sensors from plastic fibers. The strong thermo-optic coupling effect and a large thermal expansion coefficient have also resulted in a large temperature-strain cross sensitivity of the sensors.
A coaxial cable and an optical fiber are the two basic forms of wave-guiding structures that have been widely used in telecommunications for transmitting signals over a long distance. These two types of cables share the common fundamental physics governed by the same electromagnetic (EM) theory, except that the frequencies of the EM waves supported by them are quite different. In comparison with optical fibers, coaxial cables can survive a large longitudinal strain and are relatively resistant to lateral force and bending.
Prior research has already started to explore various coaxial cable devices mimicking their optical fiber counterparts. Inspired by the well-known optical fiber Mach-Zehnder interferometer (MZI), the coaxial cable based MZI implementations have been demonstrated and the superluminal and negative group velocities in the radio frequency (RF) regime were observed experimentally. RF band-gap structures have also been explored to mimic the photonic crystal (PC) device that has found many interesting applications such as wavelength specific filters, reflectors, waveguides, light trappers, and super-lenses. In one case, alternating 50Ω and 75Ω coaxial cable segments were connected in a row to create periodic impedance variations along the cable length to form the so called coaxial PC. Experimental evidence such as band-gaps, sub- or super-luminal velocities and defect modes were observed and investigated in the RF regime.
Recently, a coaxial cable sensor was demonstrated and investigated for health monitoring of concrete structures. The sensor was fabricated with tightly wrapped tin-plated steel spiral covered with solder. The cracks that developed within concrete structures could cause out of contact of the steel spirals. This topology change results in an impedance discontinuity that can be measured using time-domain reflectometry (TDR). The reported sensor has low resolution and works only for detection of extremely large loads, such as cracks, in the concrete. In addition, the method is prone to noise and random reflections generated during cable deployment and it does not have a deterministic relation between the signal and the applied load. Nevertheless, the work successfully proved the general feasibility of using coaxial cable sensor for SHM.
Additionally, in U.S. Pat. No. 7,421,910 to Chen et. al, another type of coaxial cable sensor is disclosed based on the electric time domain reflectometry (ETDR) and electric time domain transmission (ETDT) technologies. The sensor cable disclosed therein includes an inner conductor, a dielectric jacket, and an outer conductor that is configured to passively deform responsively to strain in an associated structure. The deformation translates strain into a measurable change in a reflection coefficient associated with the outer conductor. However, the deformation of the outer conductor or the dielectric jacket induced by the strain is limited, and the discontinuity only contributes limited changes to the local reflection coefficients. As a result, the ETDR or ETDT signal is relatively weak, which hinders the resolution as well as the least strain change of the strain sensing.
Therefore, there is a need to provide a new and improved coaxial Bragg grating cable with high resolution and capability of multiplex and remote operation.