**3.1 Subsystems of wireless impedance sensor nodes**

To measure the electromechanical impedances, impedance analyzers such as HP4294A are conventionally used. However, they are not quite suitable for field applications to online SHM because they are bulky (approximately 25 kg) and expensive (approximately 40,000 USD). Thus, research on the impedance based SHM technique trends toward development of self-contained sensors and wireless active sensor nodes with all required functions including actuating/sensing, data processing, damage assessments and sensor selfdiagnostics on the sensor board as well as power management with energy harvesters. Recently, Analog Devices© developed an integrated impedance converters, AD5933 (www.analog.com). It is equipped with a 12-bit analog-to-digital converter (ADC), a digitalto-analog converter (DAC) and a discrete Fourier transform (DFT) functionality. The frequency generator allows an external complex impedance with range of 100 Ω to 10MΩ to be excited with a known frequency of up to 100 kHz. AD5933 is just of a penny size, thus it provides a solution for self-contained miniaturized impedance measuring. Therefore, AD5933 has been used as a core component in developing a wireless impedance sensor node for SHM applications (Mascarenas et al. 2007, 2009, Overly et al. 2007, 2008, Taylor et al. 2009a, b, Park S. et al. 2009, Min et al. 2010). The wireless sensor node, proposed in this study, has extended the previous researches for multifunctional and environment-friendly uses in the impedance-based SHM (Mascarenas et al. 2007, Park S. et al. 2009). It was designed by adding: (1) optimal arrangement of each chip for low power consumption, (2) energy harvester equipped with solar panels, (3) peer-to-peer communication by using a RF transceiver of CC2420, which enables to construct the ubiquitous sensor network, (4) internal algorithms for operations, which are optimized by using microcontroller-dependent

Ubiquitous Piezoelectric Sensor Network

(UPSN)-Based Concrete Curing Monitoring for u-Construction 81

(a) Subsystem and enclosure

(b) Block diagram

Fig. 4. Proposed wireless impedance sensor node

instruction codes to boost the sensor node's capability and (5) miniaturized hardware system fabricated as a printed circuit board (PCB) for a high quality prototype and enclosed by waterproof plastic box for applications to real structures. The proposed wireless sensor node is composed of four functional subsystems: (1) sensing interface, (2) computational core, (3) wireless transceiver and (4) power supply. The "sensing interface" includes an interface to which a piezoelectric sensor and a temperature sensor can be connected, and an impedance chip (AD5933) for exciting a piezoelectric sensor and measuring the impedance signals. Here, NTC (Negative Temperature Coefficient) disc thermistor is equipped for temperature sensing on the structure near a piezoelectric sensor. It is a low-cost and smallsize resistance type device, and is suitable for temperature ranges from -20 ºC to +120 ºC with reference resistance of 10 kΩ at 25 ºC. The "computational core" consists of a microcontroller and a serial flash memory for computational tasks and system operations with various embedded algorithms. Through embedding technologies in microcontroller, the wireless traffic can be reduced and the survival rate of transmitted data can be increased. In this sensor node, ATmega128L is adopted because it is one of high performance and low power 8-bit microcontrollers, and has 128 kilobytes of in-system self-programmable flash program memory (www.atmel.com). The "wireless transceiver" is an integral component of the wireless system, which is composed of a RF transceiver (CC2420), a balun transformer, and an antenna to communicate with a base station (Kmote-B radio module) and/or other wireless sensor nodes and to broadcast the structural condition. CC2420 is a single chip 2.4 GHz IEEE 802.15.4 compliant RF transceiver designed for low-power and low-voltage wireless applications (www.ti.com). It provides a low-cost and highly integrated solution for robust wireless communication and extensive hardware support for packet handling, data buffering and burst transmission. These features reduce the load on the host controller and allow CC2420 to interface low-cost microcontrollers. The sensor node can be operated by one of three type "power supply" systems: 5 V AC-plug DC adapter, 3.6-7.2 V battery, or 5 V solar power system. The power can be monitored on the microcontroller using a general ADC, which transforms the analog signals acquired from batteries to the digital signals. For stable power supply to the sensor node during operations, LDO (Low-dropout regulator) is mounted for providing a fixed 3.3 V reference output to the sensor node. Solar power system for energy harvesting consists of single crystalline silicon solar cells (120 × 60 mm2) to generate the maximum power for its size, two AA Ni- MH rechargeable batteries to stand high temperature and overcharging under sunlight and to last up to 1000 charge/discharge cycles, and a step-up DC/DC solar controller to protect the appliances and the batteries with over discharge prevention circuit. Fig. 4 shows the impedance sensor node developed in this study and its block diagram, and the features are described in Table 1. The developed impedance sensor node was tested on the several operational conditions to determine the actual in-service power consumption. A multimeter was placed in line on the node's positive voltage terminal. The current draw during each condition was recorded as: 1.3 mA in idle state, 25.8 mA during measurement, 15.2 mA during calculation, and 27.3 mA during transmission, which indicates that the maximum required power is approximately 90 mW with 3.3 V.

It is slightly larger than the required power of 60 mW by Overly et al. (2008), which may be caused by additionally equipped NTC thermistor for temperature sensing, three LEDs for informing the node status by twinkling with different colors, and other operation subsystems. The required power may be reduced further with proper use of sleep modes in Overly et al. (2008).

instruction codes to boost the sensor node's capability and (5) miniaturized hardware system fabricated as a printed circuit board (PCB) for a high quality prototype and enclosed by waterproof plastic box for applications to real structures. The proposed wireless sensor node is composed of four functional subsystems: (1) sensing interface, (2) computational core, (3) wireless transceiver and (4) power supply. The "sensing interface" includes an interface to which a piezoelectric sensor and a temperature sensor can be connected, and an impedance chip (AD5933) for exciting a piezoelectric sensor and measuring the impedance signals. Here, NTC (Negative Temperature Coefficient) disc thermistor is equipped for temperature sensing on the structure near a piezoelectric sensor. It is a low-cost and smallsize resistance type device, and is suitable for temperature ranges from -20 ºC to +120 ºC with reference resistance of 10 kΩ at 25 ºC. The "computational core" consists of a microcontroller and a serial flash memory for computational tasks and system operations with various embedded algorithms. Through embedding technologies in microcontroller, the wireless traffic can be reduced and the survival rate of transmitted data can be increased. In this sensor node, ATmega128L is adopted because it is one of high performance and low power 8-bit microcontrollers, and has 128 kilobytes of in-system self-programmable flash program memory (www.atmel.com). The "wireless transceiver" is an integral component of the wireless system, which is composed of a RF transceiver (CC2420), a balun transformer, and an antenna to communicate with a base station (Kmote-B radio module) and/or other wireless sensor nodes and to broadcast the structural condition. CC2420 is a single chip 2.4 GHz IEEE 802.15.4 compliant RF transceiver designed for low-power and low-voltage wireless applications (www.ti.com). It provides a low-cost and highly integrated solution for robust wireless communication and extensive hardware support for packet handling, data buffering and burst transmission. These features reduce the load on the host controller and allow CC2420 to interface low-cost microcontrollers. The sensor node can be operated by one of three type "power supply" systems: 5 V AC-plug DC adapter, 3.6-7.2 V battery, or 5 V solar power system. The power can be monitored on the microcontroller using a general ADC, which transforms the analog signals acquired from batteries to the digital signals. For stable power supply to the sensor node during operations, LDO (Low-dropout regulator) is mounted for providing a fixed 3.3 V reference output to the sensor node. Solar power system for energy harvesting consists of single crystalline silicon solar cells (120 × 60 mm2) to generate the maximum power for its size, two AA Ni- MH rechargeable batteries to stand high temperature and overcharging under sunlight and to last up to 1000 charge/discharge cycles, and a step-up DC/DC solar controller to protect the appliances and the batteries with over discharge prevention circuit. Fig. 4 shows the impedance sensor node developed in this study and its block diagram, and the features are described in Table 1. The developed impedance sensor node was tested on the several operational conditions to determine the actual in-service power consumption. A multimeter was placed in line on the node's positive voltage terminal. The current draw during each condition was recorded as: 1.3 mA in idle state, 25.8 mA during measurement, 15.2 mA during calculation, and 27.3 mA during transmission, which indicates that the maximum required power is approximately 90 mW

It is slightly larger than the required power of 60 mW by Overly et al. (2008), which may be caused by additionally equipped NTC thermistor for temperature sensing, three LEDs for informing the node status by twinkling with different colors, and other operation subsystems. The required power may be reduced further with proper use of sleep modes in

with 3.3 V.

Overly et al. (2008).

(b) Block diagram

Fig. 4. Proposed wireless impedance sensor node

Ubiquitous Piezoelectric Sensor Network

or processed data from the designated sensors.

from a lowered battery voltage of more than 2 V.

which was measured every one hour.

**3.3 Self-powered wireless system incorporated with solar cells** 

produced by collecting sunlight and converting it into electricity.

(UPSN)-Based Concrete Curing Monitoring for u-Construction 83

measurement block, the embedded analysis engine optionally performs the analysis for structural damage detection and sensor self-diagnosis. Two algorithms are embedded on the microcontroller for the structural status monitoring: the RMSD metric and the temperature compensated CC metric calculated by EFS method. Sensor self-diagnosis is simply carried out calculating the slope of the imaginary part of admittance. Here, the baseline impedance is stored at the serial flash memory. Depending on input arguments, the users can get raw

Power scavenging enables "place-and-forget" wireless sensor node. Considering that the necessary cost and efforts for battery maintenance and replacement may over-shadow the merits of the wireless SHM system, the ability to scavenge energy from the environment is a quite important and it permits deploying self-powered sensor nodes onto inaccessible locations. Thus, many researchers have shown interest in power scavenging and the related technologies have steeply grown. Especially, the solar power is most often used, which is

This is done by using solar panels, which are large flat panels made up of many individual solar cells. In this study, a solar power system for operating a wireless sensor node is designed with single crystalline silicon solar cells (120 × 60 mm2), two AA Ni-MH rechargeable batteries (1.2 V × 2ea), and a step-up DC/DC solar controller, considering onetime measurement per day. A step-up DC/DC solar controller offers 4.8 V reference output

This solar power system provides maximum 750 mW, which may be enough to operate the developed sensor node of 90 mW. If the larger power is needed for more frequent measurements per day, the recharging capacity of the solar power system may be increased by using higher-efficient and bigger size solar panels and higher-voltage batteries. To validate the ability of the solar power system, a simple experiment has been carried out on an aluminum plate as shown in Fig. 6. A macro-fiber composite (MFC) patch of 47 × 25 × 0.267 mm3 (2814P1 Type; Smart Material©) was surface-bonded to the aluminum specimen of 50 × 1,000 × 4 mm3. The MFC is a relatively new type of PZT transducer that exhibit superior ruggedness and conformability compared to traditional piezoceramic wafers. At the beginning, the batteries were fully recharged by an electric battery charger. Then, the experiment started at 00:00 am on 6 September, 2009. Raw impedance signals and the processed structural damage detection results were wirelessly transmitted to a base station at every 10:00 am for five days. The weather condition was changed in five days as follows: sunny (19.6-31.1 ºC; cloud 0.8), mostly cloudy (20.9-27.9 ºC; cloud 7.6), partly cloudy (21.0- 29.8 ºC; cloud 5.3), partly cloudy (17.9- 28.6 ºC; cloud 4.3), and partly cloudy (14.5-28.5 ºC; cloud 6.8). Fig. 7 shows the voltage level in two AA rechargeable batteries during five days,

Although the voltage steeply declined during the measurement of impedances and on-board

It may indicate that it is able to operate the sensor node several times per day. The recharged voltage remained on stable condition under sun light, but it decreased at 0.005 V/hour at night. When cloudy, the solar cells could not be recharged due to the lack of sun light, but it shortly returned to stable condition as the sun rose. From the above results, it may be concluded that the solar power system is able to provide a solution for maintenance-

calculation of damage index, it was almost fully recovered in one hour under sun light.


Table 1. Features of the proposed wireless impedance sensor node

#### **3.2 Data control and on-board data analysis**

TinyOS is the most typical open-source operating system designed for wireless embedded sensor networks. It features a component-based architecture which enables rapid innovation and implementation while minimizing code size as required by the severe memory constraints inherent in sensor networks.

The proposed sensor node is based on TinyOS for system operation. On the other hand, the server is controlled by users through MATLAB® software, which is a high-level language and interactive environment to perform computationally intensive tasks faster than traditional programming languages such as C, C++, or FORTRAN, and includes a number of mathematical functions including Fourier analysis, filtering, signal processing and serial communications. Moreover, it provides GUI (graphical user interface) development environment, from which the user can easily change the control variables and monitor the wirelessly transmitted raw and/or processed data, temperature and node status such as battery condition. The serial communication is established between a server and a base station using two service daemons, which are cross-complied using Cygwin. These daemons provide a Linux-like environment for Windows, and enable to communicate between MATLAB® (Windows) and base station/sensor node (TinyOS).

For continuous and autonomous SHM using wireless sensor nodes, it is strongly required to construct the embedded data analysis system. More power-efficient wireless SHMs could be achieved, if the measured impedance is analyzed on microcontroller of the sensor node and only the analyzed results Table 1 Features of the proposed wireless impedance sensor node could be wirelessly sent to a base station. Especially, this fact is crucial for self-powered wireless sensor nodes incorporating several kinds of energy harvesters. In the proposed sensor node, multifunctional algorithms are implemented for temperature/power measurement, impedance measurement and analysis engine for both structural damage detection and sensor self-diagnosis, as shown in Fig. 5.

The impedance measurement block consists of the TWI library, AD5933 control library and the default sweep function (512 points) library. Using raw data from the impedance

Output Frequency Range 1 ~ 100 kHz Output Frequency Resolution > 1 Hz

Outdoor Transmission Range 150 m

Table 1. Features of the proposed wireless impedance sensor node

MATLAB® (Windows) and base station/sensor node (TinyOS).

detection and sensor self-diagnosis, as shown in Fig. 5.

Power Supply Options

**3.2 Data control and on-board data analysis** 

constraints inherent in sensor networks.

Impedance Range 1 kΩ ~ 1 MΩ Temperature Range -40 ~ 125 ºC Temperature Resolution > 0.03 ºC

On-Board Processing Yes (MCU : ATMega128L)

Operating Frequency 2.4 GHz IEEE 802.15.4 / Zigbee RF Transceiver

Feature 150 x 100 x 70 (mm) ; 310 (g)

TinyOS is the most typical open-source operating system designed for wireless embedded sensor networks. It features a component-based architecture which enables rapid innovation and implementation while minimizing code size as required by the severe memory

The proposed sensor node is based on TinyOS for system operation. On the other hand, the server is controlled by users through MATLAB® software, which is a high-level language and interactive environment to perform computationally intensive tasks faster than traditional programming languages such as C, C++, or FORTRAN, and includes a number of mathematical functions including Fourier analysis, filtering, signal processing and serial communications. Moreover, it provides GUI (graphical user interface) development environment, from which the user can easily change the control variables and monitor the wirelessly transmitted raw and/or processed data, temperature and node status such as battery condition. The serial communication is established between a server and a base station using two service daemons, which are cross-complied using Cygwin. These daemons provide a Linux-like environment for Windows, and enable to communicate between

For continuous and autonomous SHM using wireless sensor nodes, it is strongly required to construct the embedded data analysis system. More power-efficient wireless SHMs could be achieved, if the measured impedance is analyzed on microcontroller of the sensor node and only the analyzed results Table 1 Features of the proposed wireless impedance sensor node could be wirelessly sent to a base station. Especially, this fact is crucial for self-powered wireless sensor nodes incorporating several kinds of energy harvesters. In the proposed sensor node, multifunctional algorithms are implemented for temperature/power measurement, impedance measurement and analysis engine for both structural damage

The impedance measurement block consists of the TWI library, AD5933 control library and the default sweep function (512 points) library. Using raw data from the impedance

• 5V AC-plug DC Adapter • Commercial batteries (3.6-7.2V)

Solar Panels (3V)

• 2AA Ni-MH rechargeable battery with

measurement block, the embedded analysis engine optionally performs the analysis for structural damage detection and sensor self-diagnosis. Two algorithms are embedded on the microcontroller for the structural status monitoring: the RMSD metric and the temperature compensated CC metric calculated by EFS method. Sensor self-diagnosis is simply carried out calculating the slope of the imaginary part of admittance. Here, the baseline impedance is stored at the serial flash memory. Depending on input arguments, the users can get raw or processed data from the designated sensors.
