## 1 Introduction
### Development of Sensors with MEMS Technologies  
It is expected that various sensing devices would be installed in infrastructure and a lot of services utilizing big data would be created and socially provided by the Internet [13; 14; 15]. In a \"Trillion sensors society\", which is aimed for by the TSensors (Trillion Sensors) project [16; 17], it is clearly indicated that our society and life are greatly changed in all fields, such as medical health care, agriculture, environment, and infrastructure, and the whole society is connected to the network by many sensor terminals.  
In infrastructure monitoring, developments in sensor device and IoT technology are indispensable. At the same time, it is considered that a self-powered (battery-free) device is necessary to build the sensor network and deliver reliable and long-term performance for civil infrastructures in their service life with environmental conditions. Energy harvesters are categorized into three types depending on the operational principles: electromagnetic transduction [18; 19], electrostatic transduction [18; 20], and piezoelectric transduction [18; 21]. Several studies have been conducted on characterizing the environment- and bridge structural type-dependent vibrations and on quantifying the powers harvested from those vibrations using comb-shaped silicon electrets (see Figure 1) [22].  
In the electret electrostatic microelectromechanical systems (MEMS) energy harvester, as shown in Figure 2, the electret is a vibrating mass supported by a spring and forms a variable capacitance when it is facing a fixed electrode through an air gap. It is possible to output the current with power generation based on the frequency response synchronized with the movement of the electrode (oscillation). The structure can be designed to efficiently obtain the generation amount only in a certain frequency band [22]. As a small wireless device of coin size for the purpose of frequency detection of environmental vibration, it can be easily mounted without any space and wiring problems.  
A vibration-based micro energy harvester (MEH) device is proposed to monitor the health of RC concrete slabs and pier scours in this study. MEH devices are introduced with microelectromechanical systems (MEMS) technology. They harvest micro energy from its environmental energy source such as light, wind, heat, and electromagnetic field vibration and convert it into electric energy. If they can be integrated into (or embedded in) infrastructure monitoring sensors, they can supply the sensors' power and, to a certain extent, make the sensors self-sustaining, needing no cable connection to the electricity power grid.  
In terms of power-generating capacity, light-, vibration-, and wind-sourced MEH devices perform comparatively better than the other choices, such as heat- and electromagnetic-sourced MEH. Among the three types of MEH (light-, vibration-, and wind-based), a vibration-based MEH device is the only one that can function in lightless, windless, dark, or night conditions and inside the box girders. Therefore, a vibration-based MEH is proposed for bridge structural health monitoring. The vibration-based MEH device used herein (see Figure 3) was identical to those proposed in previous studies [22], which is an inexpensive coin-sized device that can generate over 100 \(\mu\)W RMS (root mean square) of output electric power under 0.03 G\({}_{\text{RMS}}\) accelerations.