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

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 produced by collecting sunlight and converting it into electricity.

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 from a lowered battery voltage of more than 2 V.

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, which was measured every one hour.

Although the voltage steeply declined during the measurement of impedances and on-board calculation of damage index, it was almost fully recovered in one hour under sun light.

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-

Ubiquitous Piezoelectric Sensor Network

2

wireless systems.

**Sunny 16** °**C / 27** °**C**

**4. Experimental verification** 

**4.1 Experimental setup and test procedure** 

data acquisition software (MATLAP), as shown in Fig. 9, 10.

Measurement

**Sunny 15** °**C / 27** °**C**

Fig. 7. Voltage monitoring of a wireless SHM system with solar cells

2.2

2.4

2.6

Recharged Voltage by Solar Cells (V)

2.8

3

3.2

(UPSN)-Based Concrete Curing Monitoring for u-Construction 85

09/20 09/21 09/22 09/23 1.8

Date (MM/DD)

In order to verify the feasibility of the proposed electromechanical impedance technique for online monitoring of the strength developed during the curing process of the concrete structures, a series of experimental studies have been carried out using both wired and

Two types of concrete cylinders with design strength of 60MPa and 100MPa were prepared to measure the impedance signals during the curing process of concrete, as shown in Fig. 8. The cylinders were developed by isothermal air curing. PZT sensors, 20 mm × 20 mm × 0.508 mm in size, were attached to the concrete cylinders. The PZT sensors were installed on the cylinders in the first 24 hours after casting. Since concrete is a non-conducting material, a conducting copper paste was applied to the specimen before bonding the PZT sensor to the host structure. The PZT patches were bonded to the top center of the cylinder surface, as shown in Fig. 8. The experimental setup for the wired impedance measurement system consisted of cylinders with the PZT sensors, a self-sensing circuit board and a DAQ system (PXI 1042Q, National Instruments Inc.). The DAQ system consisted of an Arbitrary Waveform Generator (AWG), a Digitizer (DIG), embedded controller and data acquisition software (LabVIEW). The wireless system was comprised of the cylinders with the PZT sensors, a wireless sensor node, a RF receiver (KETI), and a laptop computer equipped with

**Partly Cloudy 16** °**C / 24** °**C**

Lower Operation Level When Using Solar Panel

**Partly Sunny 15** °**C / 25** °**C**

free wireless sensor nodes in spite of sensitive reaction to the environment, which would be complemented by development of the more efficient energy scavenging technologies.

Fig. 5. Overall command/data flow of embedded software

Fig. 6. Sensor node with a solar panel

free wireless sensor nodes in spite of sensitive reaction to the environment, which would be complemented by development of the more efficient energy scavenging technologies.

Fig. 5. Overall command/data flow of embedded software

Fig. 6. Sensor node with a solar panel

Fig. 7. Voltage monitoring of a wireless SHM system with solar cells
