Blades play a critical role in the wind turbine system. Therefore, their structural health monitoring is very important. Blades are damaged by sudden changes in wind load, cracks due to collision of foreign objects, and disasters, such as lightning strikes, hail, and typhoons. Moreover, blades are expensive to maintain. Defects or damages to wind turbine blades reduce the life span and power generation efficiency of the wind turbine and increase safety risks and maintenance costs. Therefore, it is very important to detect blade damage to prevent problems in the wind turbine. Ultrasonic inspection is suitable for blades made of composite materials. Piezoelectric ceramic, which is a typical piezoelectric element, has relatively high sensitivity compared to other sensors. However, it suffers from brittle fractures and thus difficult to apply to curved structures. To overcome the limitations of piezoelectric ceramics, a piezoelectric flexible line sensor that can be applied to curved surfaces was manufactured using the dice-and-fill method for a [Pb(Li_0.25Nb_0.75)]_0.06 [Pb(Mg_0.33Nb_0.67)]_0.06 [Pb(Zr_0.50Ti_0.50)]_0.88O₃ with 0.7 wt% MnO₂ (PZTNMML) ceramic disc. Instead of a typical ultrasonic inspection method with limited surface contact, a laser capable of producing ultrasonic excitation of ultrasonic waves over a large area from a long distance was used. The possibility of detecting a defect on the wind turbine blade using a piezoelectric flexible line sensor and laser ultrasound was confirmed in this study.

The demand for efficient water use and automatic systems has been increasing due to the frequent drought damage to crops as a result of climate change, the shortage of water resources in rural areas, and the aging of farmers. The existing automatic irrigation systems reduce the amount of labor required for irrigation and maintain soil moisture. However, the irrigation threshold criteria are user-determined as opposed to being automated according to input objectives such as improving crop productivity and saving water. In this study, an algorithm that could automatically determine suitable soil moisture according to a database and an automatic irrigation system with intermittent irrigation for efficient water use were developed. An experiment was then conducted on the productivity of crops for protected cultivation according to the application of the system. As the frequency domain reflectometry (FDR) sensor used in this system measured the volumetric water content of the soil, the soil moisture tension corresponding with the set value was converted into the volumetric water content using a regression equation. The process of intermittent irrigation was defined by using the moisture movement modeling of Hydrus 2D to reduce water loss on the soil surface and allow moisture to penetrate the soil unobstructed. An experimental field of a tomato farm was divided into empirical manual and controlled automatic irrigation plots. A total of 97.3% of the soil moisture values in the −33 kPa-controlled automatic irrigation plot and 96.6% of the soil moisture values in the −25 kPa-controlled automatic irrigation plot were within each set range during the first cropping season. During the second cropping season, a total of 94.8% of the soil moisture values in the −33 kPa-controlled automatic irrigation plot was within the set range. Compared with the empirical manual irrigation plot, the water productivity in the first cropping season was 113.9% in the −33 kPa-controlled automatic irrigation plot and 106.3% in the −25 kPa-controlled automatic irrigation plot. In the second cropping season, the water productivity was 117.3% in the −33 kPa-controlled automatic irrigation plot. Therefore, an automatic irrigation system applied with intermittent irrigation could be critical to increasing agricultural production and improving water-use efficiency.

We studied the detection and visualization of defects in a test object using a laser ultrasonic guided wave. The scan area is irradiated by a laser generated from a Nd:YAG 532 nm Q-switched laser generator through a galvanometer scanner. The laser irradiation causes the surface temperature to suddenly rise and then become temporarily adiabatic. The locally heated region reaches thermal equilibrium with the surroundings. In other words, heat energy propagates inside the object in the form of elastic energy through adiabatic expansion. This thermoelastic wave is typically acquired by a piezoelectric sensor, which is sensitive in the ultrasonic domain. A single piezoelectric sensor has limited coverage in the scan area, while multi-channel piezoelectric sensors require many sensors, large-scale wiring, and many channeling devices for use and installation. In addition, the sensors may not acquire signals due to their installed locations, and the efficiency may be reduced because of the overlap between the sensing areas of multiple sensors. For these reasons, the concept of a piezoelectric line sensor is adopted in this study for the first time. To verify the feasibility of the line sensor, I- and L-shaped sensors were attached to a steel structure, and the ultrasound signal from laser excitation was obtained. If the steel structure has defects on the back, the ultrasonic propagation image will be distorted in the defect area. Thus, we can detect the defects easily from the visualization image. Three defects were simulated for the test. The results show that the piezoelectric line sensor can detect defects more precisely and accurately compared to a single piezoelectric sensor.