The research in this project led to the development of a novel sensor system for continuous health monitoring of pavement structures. The system consists of a self-powered wireless sensor capable of detecting fatigue damage in pavement structures under actual traffic and environmental load history. The developed system is based on the integration of a piezoelectric transducer with an array of ultra-low power FG computational circuits. A miniaturized sensor was developed and tested. It was shown that it is capable of continuous battery-less monitoring of strain events integrated over the occurrence duration time.
Successful development of the proposed strain sensor could dramatically transform the economics of pavement preservation/management and ultimately improve the serviceability of pavements. The developed system consists of a network of low-cost sensors distributed along the pavement. Each sensor node is self-powered and capable of continuously monitoring and storing the dynamic strain levels in the host pavement structure. The strain data are stored on-board the sensor, which consists of the self-powered sensor strip and small-scale electronics. The data from all the sensors are periodically uploaded wirelessly to a central database. The sensor can be read through standard RF transmission using a RF reader that is either manually operated or mounted on a moving vehicle. By directly outfitting service vehicles with low-cost RF transponders, the roads can be frequently monitored to detect changes in structural integrity that may not only foreshadow a future crack/distress manifestation but also allow for more accurate scheduling of preservation actions. It should be noted that only a single RF reader is needed to inspect the sensor network.
The main characteristics of the new sensor are as follows:
Self-powered, continuous, and autonomous sensing. Testing of the sensor showed that it is able to operate using only self-generated electrical energy harvested directly from the sensing signal induced by a piezoelectric transducer attached to the pavement.
Autonomous computation and non-volatile storage of sensing variables. The design uses the physics of FG transistor injection principles for computing cumulative mechanical strain patterns experienced by the pavement structure. The design details and corresponding steps are detailed in chapter 2 of this report.
Small size. The actual size of the analog electronics circuit, manufactured in a special small package, is 0.23 × 0.21 × 0.03 inches (5.84 × 5.33 × 0.76 mm). An interface board was designed and manufactured in-house. The board acts as an interface between the active piezoelectric element and the analog electronics. The board also contains all of the electronic circuitry that interfaces the sensor with the reading antenna. The board dimensions are 0.6 × 0.6 × 0.04 inches (15.24 × 15.24 × 1.01 mm). The size of the active piezoelectric element, used as sensor and source of energy, depends on the desired level of sensitivity and accuracy required at each measuring node. The amplitude of the measured strains determines the size of the transducer to be implemented. A minimum voltage of 2.3V is required at the piezo's output to activate the gate's memory system. Thus, for low-level strains, a bigger active area is required to generate the needed power. During the current research, transducers varying from 0.78 × 0.04 × 0.007 inches (19.81 × 1.01 × 0.18 mm) to 2.36 × 0.4 × 0.07 inches (59.94 × 10.16 × 1.78 mm) were used. The complete system (analog electronics, interface board, and a piezoelectric element) are integrated in a package designed and manufactured in-house. The package was designed so that it has a similar shape to existing systems, and it can be installed using existing installation procedures that are accepted by SHAs and will not constitute a major disruption to current practices.
Wireless communication. A novel RF communication module was designed in-house. The mixed mode module is specific for integration with the FG analog memories. It should be noted that commercially available RF modules cannot be associated with the used circuitry given the very low current levels in the system. The RF powering mode is dissociated from the computing and storage circuitry and is achieved by harvesting the RF signal. The salient modules of the RF module include: (1) ADC for digitizing the stored usage statistics, (2) charge-pumps for generating biases for erasing and remote initialization of the sensor, (3) encoding/decoding circuits for creating data packets with inherent clock/data recovery mechanisms, (4) state machine for command and control of the IC, and (5) RF interface circuitry operating at 13.56 MHz for harvesting energy from the reader signal and for wireless transmission of the sensor data. In addition, an external reader was designed and manufactured. The reader interfaces with an embedded antenna, which was also designed and manufactured in-house and fits the exact H-shape of the designed package. The embedded antenna is connected to the interface. Laboratory testing showed that the reading distance highly depends on the parasitic capacitances of the antennas (embedded antenna and external reader) as well as the parasitic capacitance at the sensors input. These capacitances are induced by the manufacturing process and cannot be predicted upfront. A calibration procedure and antennas resonance tuning is required.
Robustness to withstand harsh environmental conditions. As mentioned, an H-shaped packaging system was designed and manufactured in-house to encase the sensing modules. The package was designed so that it has a similar shape to existing systems, and it can be installed using existing installation procedures. Nonmetallic materials were used so that it allows for RF communication. A layered system mainly consisting of Araldite® GY-6010 epoxy, polyurethane foam, and mineral-filled urethane casting resin was used to provide the required mechanical and thermal protections for field installation. The system, entirely built in-house, was tested under a laboratory setup simulating field conditions as well as in real construction projects.
Possibility of networks deployment. As market penetration increases, sensor prices are expected to fall. The estimation for the electronics cost is based on prices of other mass produced RFID tags sensors and varies depending on vendor, system complexity, and level of demand. The cost projection for this research was developed based on literature reviews.(18) Presently, for passive tags, the cost is less than $1 per unit. For active tags, the cost ranges from $10 to $50, and these costs include the cost of batteries. The functionalities of sensor for this study are much more developed than the passive tags. Therefore, the cost should be somewhere between the cost of active and passive tags. It is envisioned that extensive sensor networks could be deployed in a given pavement structure. This allows for simple statistical compensations for external parameters (temperature variations and traffic wander).
Finally, a sensor-specific data interpretation algorithm for predicting remaining fatigue life of a pavement structure was developed using cumulative limited compressed strain data stored in the sensor memory chip. The algorithm was verified using actual laboratory fatigue test results of a notched concrete beam under constant, variable, and random loading histories.
7.1 RECOMMENDATIONS FOR FUTURE RESEARCH AND DEVELOPMENT
While the development of the new sensor constitutes a major achievement towards the future implementation of self-powered autonomous sensor networks for the continuous health monitoring of in-service pavement structures under actual traffic and environmental loadings for extended periods of time, there are still some challenges for the acceptance in to practice by SHAs. These challenges include the following:
The installation procedure in the field. The current package design described in this report and implemented in this project was selected to mimic the current installation procedures used typically in specific pavement sections for research purposes (e.g., accelerated load facilities, long-term pavement performance Specific Pavement Studies (SPS)-1 and SPS-2 instrumented test sections, and in a limited number of special pavement test sections in other countries). Such installation procedure demands require considerable care during construction (even without the use of wires, which is the case for the system developed in this project) in order to insure that the H-shaped gauges are properly bonded to the pavement surface layer (AC or PCC) and are properly aligned in both horizontal and vertical directions. One possible solution to the alignment problem is to include orientation detection devices (e.g., a miniature accelerometer). The issue of bonding quality between the gauge and the pavement surface material is not unique to the developed system; it is present in other existing strain measurement systems. Another possible alternative solution is to consider embedding the sensor in a spherical packaging with a size of the order of a coarse aggregate particle. This would entail modifications in the size and shape requirements of the piezoelectric material, the antenna, and the inclusion of orientation detection devices. The advantages of such system are the ease of installation (the pebble size sensor would be placed in the mix at the site) and the capability of measuring strains in multiple directions (the maximum strain could be calculated from the strain values in three different directions).
The continuous monitoring of pavement temperature and correlating it to corresponding loading events. A potential solution to this challenge requires additional research.
The identification of axle type (i.e., differentiating between single and multiple axle configurations). It is known that fatigue damage is different under different axle configurations even at the same peak strain level.
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