A wireless, battery-free sensor for strain and crack monitoring
Nearly one third of US bridges are made of steel and aluminum iron, and almost one quarter of those are rated as structurally deficient or functionally obsolete.1 For aging bridges, fatigue-induced fractures and cracks are among the most common concerns for inspectors and owners.2 Traditional approaches to monitoring strains and cracks require running lengthy cables in the large structure, covering only very limited areas, or involving human-operated equipment that is not convenient for in situ continuous application.
Among the many new structural health monitoring technologies, wireless sensing helps to significantly reduce instrumentation time and system cost.3 A typical wireless sensing device contains three functional modules: a sensing interface (e.g., converting an analog sensor signal to digital data), a computing core (data storage and processing), and a wireless transceiver (digital communication with peers or a wireless gateway server). To obtain different types of measurements, various sensors can be connected with the wireless sensing device. For example, strain measurement is obtained by interfacing the device with a metal foil strain gauge. In addition, current wireless sensing devices usually operate on an external power source such as batteries.
Our research adopts a different approach by directly exploiting the electromagnetic properties of a radio frequency identification (RFID) antenna for strain sensing. The novel sensor design relies on the combination of interdisciplinary expertise in electromagnetics and structural mechanics because of the multi-physics nature of the sensor operation. Instead of using wireless technologies to transmit digitized data, the strain-dependent behavior of the electromagnetic waves in the antenna is used as the sensing mechanism. When a small piece of electromagnetic antenna (usually with a 2D shape) is under strain or deformation, its electromagnetic resonance frequency may change, and that change can be interrogated by a wireless reader and used as the strain indicator.
Other researchers have explored similar ideas. Some of those studies focus on inductively coupled sensors to measure the strain change, but they have a limited measurement distance of around 10cm.4, 5 Instead, we developed an RFID-based folded patch antenna as a passive wireless strain and crack sensor.6 It operates without battery power and can be interrogated wirelessly by a portable reader. The sensor can be used to monitor stress concentration and crack growth in various civil, mechanical, and aerospace structures, including metallic and nonmetallic ones. This technology provides a relatively inexpensive and simple solution for monitoring stress concentration and crack growth.
We used a low-cost RFID chip to make the sensor passive and to reduce design and manufacturing cost. The chip obtains all of its operating power from interrogation of the RF signal emitted by a wireless reader, eliminating the need for a battery or cabled power supply. The folded patch antenna sensor contains a dielectric substrate, top and bottom copper cladding, and an RFID chip (see Figure 1). Through a line of vias connecting top and bottom coppers, antenna folding is achieved and antenna dimension is reduced. The folded patch antenna is further optimized for strain and crack sensing by impedance matching.
The folded patch antenna has a resonance frequency that depends on the antenna's physical length. The two quantities are inversely proportional. After being mounted on a structural surface, the antenna sensor is put under stress and deformation along with the structure, causing the sensor's length to change and its resonance frequency to shift. An associated wireless measurement technique was developed for strain and crack sensing using the interrogation power threshold, which refers to the output power level from the reader, and is just enough to activate the RFID chip and sensor response. Once wirelessly interrogated by a reader, the antenna resonance frequency is used to derive the strain or crack growth of the underlying structure.
To verify sensor performance, we first conducted tensile tests in both small and large strain ranges. The sensor was able to capture small deformations at around 20με strain change, where 1με (microstrain) means 1×10−6 or 1ppm (parts per million) change in dimension.7 A large strain experiment was also conducted. The experimental results showed that the sensor can measure strain up to 10,000με, which is more than enough for most applications. Furthermore, we conducted crack sensing experiments showing that the proposed sensor can detect submillimeter crack generation.8 In addition, using a high-gain reader antenna, a long interrogation distance over six feet was recently demonstrated.
Due to the very low cost of battery-free wireless sensors, sensor instrumentation at high nodal density can be achieved at a reasonable expense. The proposed novel antenna sensors have immense application opportunities, since stress concentration and fracture are among the most common concerns for many engineering structures. Wireless and battery-free, the antenna sensors will provide unprecedented convenience in operation, ease of fabrication, and low cost. We are now working to reduce the sensor dimensions and to further optimize its sensing accuracy and range.
This material is based on work supported by the Federal Highway Administration under agreement DTFH61-10-H-00004. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the view of the Federal Highway Administration.
Yang Wang, assistant professor of civil and environmental engineering, earned a PhD degree in civil engineering from Stanford University and recently received a 2012 National Science Foundation Early Faculty Career Development Award to support his research on smart structures.
