Roberto T. Leon

Passive wireless antenna sensor for strain and crack sensing—electromagnetic modeling, simulation, and testing

This research investigates a passive wireless antenna sensor designed for strain and crack sensing. When the antenna experiences deformation, the antenna shape changes, causing a shift in the electromagnetic resonance frequency of the antenna. A radio frequency identification (RFID) chip is adopted for antenna signal modulation, so that a wireless reader can easily distinguish the backscattered sensor signal from unwanted environmental reflections. The RFID chip captures its operating power from an interrogation electromagnetic wave emitted by the reader, which allows the antenna sensor to be passive (battery-free). This paper first reports the latest simulation results on radiation patterns, surface current density, and electromagnetic field distribution. The simulation results are followed with experimental results on the strain and crack sensing performance of the antenna sensor. Tensile tests show that the wireless antenna sensor can detect small strain changes lower than 20???, and can perform well at large strains higher than 10?000???. With a high-gain reader antenna, the wireless interrogation distance can be increased up to 2.1?m. Furthermore, an array of antenna sensors is capable of measuring the strain distribution in close proximity. During emulated crack and fatigue crack tests, the antenna sensor is able to detect the growth of a small crack.

Thermal effects on a passive wireless antenna sensor for strain and crack sensing

For application in structural health monitoring, a folded patch antenna has been previously designed as a wireless sensor that monitors strain and crack in metallic structures. Resonance frequency of the RFID patch antenna is closely related with its dimension. To measure stress concentration in a base structure, the sensor is bonded to the structure like a traditional strain gage. When the antenna sensor is under strain/deformation together with the base structure, the antenna resonance frequency varies accordingly. The strain-related resonance frequency variation is wirelessly interrogated and recorded by a reader, and can be used to derive strain/deformation. Material properties of the antenna components can have significant effects on sensor performance. This paper investigates thermal effects through both numerical simulation and temperature chamber testing. When temperature fluctuates, previous sensor design (with a glass microfiber-reinforced PTFE substrate) shows relatively large variation in resonance frequency. To improve sensor performance, a new ceramic-filled PTFE substrate material is chosen for re-designing the antenna sensor. Temperature chamber experiments are also conducted to the sensor with new substrate material, and compared with previous design.

Design and simulation of a slotted patch antenna sensor for wireless strain sensing

In this work, a slotted patch antenna is employed as a wireless sensor for monitoring structural strain and fatigue crack. Using antenna miniaturization techniques to increase the current path length, the footprint of the slotted patch antenna can be reduced to one quarter of a previously presented folded patch antenna. Electromagnetic simulations show that the antenna resonance frequency varies when the antenna is under strain. The resonance frequency variation can be wirelessly interrogated and recorded by a radiofrequency identification (RFID) reader, and can be used to derive strain/deformation. The slotted patch antenna sensor is entirely passive (battery-free), by exploiting an inexpensive offthe- shelf RFID chip that receives power from the wireless interrogation by the reader.

Large Scale Shake Table Test of a Port Container Crane under Strong Motion Excitation

Earthquakes pose a significant threat to seaports in the United States and around the World. Container cranes, used to load and unload goods, represent one of the most vulnerable components of ports. Cranes are critical to the economy and post disaster response of a region. In order to better understand how cranes behave under earthquake loading, shake table experiments are conducted on a 1:10 scale model of a typical container crane found on the west coast of the USA. During previous earthquakes, three failure modes have been observed: derailment, local buckling of the legs and collapse. To investigate the various failure modes, a 1:10 scale model is constructed and tested on the six-degree-of-freedom shake table at the University at Buffalo. For ease of testing, the model is simplified by modifying the boom structure, while still preserving the most influential vibration modes. This paper presents important results from the study.

Passive Frequency Doubling Antenna Sensor for Wireless Strain Sensing

This paper presents the design, simulation, and preliminary measurement of a passive (battery-free) frequency doubling antenna sensor for strain sensing. Illuminated by a wireless reader, the sensor consists of three components, i.e. a receiving antenna with resonance frequency f0, a transmitting antenna with resonance frequency 2f0, and a matching network between the receiving and transmitting antennas. A Schottky diode is integrated in the matching network. Exploiting nonlinear circuit behavior of the diode, the matching network is able to generate output signal at doubled frequency of the reader interrogation signal. The output signal is then backscattered to the reader through the sensor-side transmitting antenna. Because the backscattered signal has a doubled frequency, it is easily distinguished by the reader from environmental reflections of original interrogation signal. When one of the sensor-side antennas, say receiving antenna, is bonded to a structure that experiences strain/deformation, resonance frequency of the antenna shifts accordingly. Through wireless interrogation, this resonance frequency shift can be measured by the reader and used to derive strain in the structure. Since operation power of the diode is harvested from the reader interrogation signal, no other power source is needed by the sensor. This means the frequency doubling antenna sensor is wireless and passive. Based on simulation results, strain sensitivity of this novel frequency doubling antenna sensor is around −3.84 kHz/μe.Copyright

Seismic Behavior of Steel-Concrete Composite Frame Structures and Design Practice in the United States

Steel-concrete composite frames have been shown to be a sensible option for use as the primary seismic force resisting system of building structures. However, there remains little data to justify the structural system performance factors (i.e., R, Cd, and Ωo) that characterize the overstrength and ductility of these systems, which are a vital component of successful design in the United States. Based on a suite of new finite element formulations, this work investigates the behavior of composite moment and braced frames under seismic loading and develops rational system performance factors. A set of archetype frames, selected to be representative of the range of frames seen in practice, were designed according to current design specifications. Nonlinear static pushover and transient dynamic analyses were performed on the frames to generate the statistical data on the seismic response from which the performance factors are quantified. The results from this investigation enable a better understanding of the variability in collapse performance of composite frame systems and will facilitate more effective designs of these systems.

Compressive strain measurement using RFID patch antenna sensors

In this research, two radiofrequency identification (RFID) antenna sensor designs are tested for compressive strain measurement. The first design is a passive (battery-free) folded patch antenna sensor with a planar dimension of 61mm × 69mm. The second design is a slotted patch antenna sensor, whose dimension is reduced to 48mm × 44mm by introducing slots on antenna conducting layer to detour surface current path. A three-point bending setup is fabricated to apply compression on a tapered aluminum specimen mounted with an antenna sensor. Mechanics-electromagnetics coupled simulation shows that the antenna resonance frequency shifts when each antenna sensor is under compressive strain. Extensive compression tests are conducted to verify the strain sensing performance of the two sensors. Experimental results confirm that the resonance frequency of each antenna sensor increases in an approximately linear relationship with respect to compressive strain. The compressive strain sensing performance of the two RFID antenna sensors, including strain sensitivity and determination coefficient, is evaluated based on the experimental data.

Behavior of Composite CFT Beam-Columns Based on Nonlinear Fiber Element Analysis

The results obtained from nonlinear fiber element analyses for concrete filled tubes (CFT) are discussed. The studies were aimed at assessing primarily the overall behavior and stability effects on these structural elements as a prelude to a large full-scale testing program. The study focuses on ultimate strength analyses for CFT composite columns with different stress-strain models for both concrete and steel. Fiber analyses using OpenSees are used to assess the impact on the ultimate strength based on the assumed stress-strain material curves, member slenderness, initial imperfections, and both material and geometric nonlinearities. Fiber analysis results are also compared with those obtained from AISC (2005). Fiber-based results show a compatible correlation with the expected element behavior, which is also captured in the current AISC (2005) Specifications.