Supplying battery-free power to wireless sensor systems (WSSs) mounted on rotating shafts remains a major challenge due to limited installation space, low rotational speed, and the requirement for long-term autonomous operation. This paper presents a compact dual-rotor energy harvester (EH) based on multilayer printed circuit board (PCB) sheets, designed for powering WSSs installed on ship propulsion shafts. Stacked multilayer PCB coils forming a three-dimensional structure are arranged on both the inner and outer rotors to enhance magnetic flux linkage and power density. The experimental results show that the EH generates power levels up to 959 mW at a shaft speed of 300 rpm. The output power improved nonlinearly with increasing rotational speed, demonstrating its suitability for real-time monitoring applications. The proposed EH offers a promising solution for powering WSS in autonomous driving technologies, with the potential for further optimization and integration into various mobility systems.

This paper presents the real-ship implementation and validation of an energy harvesting-based self-powered wireless sensor system (SP-WSS) for propulsion shaft monitoring. The proposed system integrates a compact electromagnetic energy harvester (EH), based on stacked flexible printed circuit board (F-PCB) coils, with a wireless sensor system (WSS) mounted directly on a ship propulsion shaft. The EH converts low-speed rotational energy into electrical power to continuously operate sensing, processing, and wireless communication units without external power sources. The SP-WSS was installed on a training ship and evaluated under realistic propulsion shaft operating conditions over a continuous testing period of 7.2 hours under realistic propulsion shaft speeds of 0-120 rpm. Experimental results demonstrate stable power generation, reliable wireless data transmission, and uninterrupted system operation during ship operation. The measured output voltage ranged from 0 to 4.42 V, with an average voltage of 2.69 V, while the corresponding output power reached up to 1137.71 mW with an average value of 487.87 mW. These results confirm the feasibility and robustness of the proposed SP-WSS for practical marine propulsion shaft monitoring and smart ship condition monitoring.

This letter investigates the relationship between window penetration loss (WinPL) and building entry loss (BEL) over the 3-40 GHz band using twelve traditional and thermally-efficient window types. WinPL was derived from measurement, simulation, and analytical modeling, showing consistent frequency-dependent trends. In traditional buildings, WinPL showed negligible correlation with BEL (r = 0.06), whereas a weak but significant correlation (r = 0.20, p < 0.001) was found in thermally-efficient buildings. The results suggest BEL models should consider window characteristics for modern buildings to improve prediction accuracy in high-frequency indoor scenarios.

Self-powered multi-parameter wireless sensing enables autonomous condition monitoring of rotating marine machinery, where wired power delivery and frequent maintenance are impractical. This paper presents a self-powered wireless sensor system (SP-WSS) that integrates a compact electromagnetic energy harvester (EH) with sensors for shaft speed, torsional strain/vibration, temperature, and power monitoring. The system was installed on a 300 mm training-ship propulsion shaft and evaluated for 7.2 h under real operating conditions. The harvester delivered an average power of 487.87 mW, exceeding the system demand of 374 mW by 30.4%, and maintained wireless data acquisition during the investigated period. The measured torsional responses captured operational shaft behavior and provided fatigue-relevant loading histories. These results confirm the feasibility of the proposed SP-WSS as a practical sensing platform for prognostics and health management (PHM) applications in marine propulsion systems.

This letter presents a novel outdoor-to-indoor (O2I) propagation loss model that incorporates the impact of building-related clutter to enhance indoor power prediction in complex environments. The key contribution is the introduction of the compound power (CP) parameter, which captures the combined effects of diffraction, reflection, and penetration in non-line-of-sight (NLoS) scenarios. Analytically derived and experimentally validated through a unique dual-method extraction process, the model demonstrates that clutter, such as large nearby structures, significantly influences building entry loss (BEL) between line-of-sight (LoS) and NLoS paths. This comprehensive approach improves the accuracy and generalizability of indoor power predictions by accounting for the surrounding clutter.

In this work, we propose a novel outdoor-to-Indoor (O2I) propagation loss model centered on the novel compound power (CP) parameter, which encapsulates the combined effects of diffraction, reflection, and penetration, specifically in non-line-of-sight (NLoS) environments. The model is rigorously developed through analytical derivations and experimentally validated using a unique measurement setup, proving that the inclusion of clutter, such as nearby large structures, has a substantial impact on building entry loss (BEL). By accurately accounting for both the building and its surrounding environment, the model significantly improves indoor power predictions, offering a comprehensive approach to O2I propagation in real-world, cluttered environments.

This paper proposes an improved outdoor-to-indoor (O2I) path loss model using the compound power (CP) parameter to capture excess attenuation in cluttered non-line-of-sight (NLoS) environments. CP values are extracted via two consistent methods using multi-frequency measurement data. The CP-based model is applied within the ITU-R framework, showing enhanced prediction accuracy without increasing model complexity.

A self-powered wireless sensor system (SP-WSS) integrating a high-power energy harvester (EH) has been proposed for real-time monitoring of the propulsion shaft. The main contribution of this article is to demonstrate that the status of the propulsion shaft can be monitored in real time by integrating a high-power EH that generates power solely from the rotational force of the propulsion shaft. The proposed EH features a simple structure consisting of 12 coils and six magnets, facilitating easy design and manufacturing. The output power of the EH increases in proportion to the number of coils and rotation speed, simplifying the design of the power required for the WSS. The fabricated EH achieved power outputs of 1.5 and 5.1 W at shaft speeds of 200 and 300 r/min, respectively. The complete SP-WSS, integrating the EH, four sensors, a control module, and a radio module, was designed and installed on the propulsion shaft. The SP-WSS accurately measured the power production of the EH, power consumption of the WSS, shaft vibration, temperature changes around the shaft, and shaft rotation speed in real time, successfully transmitting the data wirelessly to a main management system.

This paper presents a self-powered wireless sensor system (WSS) integrated with a simple, compact, high-power energy harvester (EH) for real-time monitoring of rotating shafts in ships. The EH utilizes multiple flexible PCB coils and magnets, achieving a power output of 1.71 W. A comprehensive WSS was designed with four sensors specifically for the rotary shaft. The system's performance was validated on a small-scale test bench with a 200 mm shaft diameter, successfully demonstrating real-time monitoring of the propulsion shaft with the additional function of a charging battery.

This work introduces a compact and high-power energy harvester (EH) designed for use on ship propulsion shafts to power wireless sensor systems (WSS). The proposed EH employs stacked flexible printed circuit board (F-PCB) coils to generate power from low-speed propulsion shafts. The rotor, measuring 3 mm in thickness, consists of six F-PCB sheets and integrates 12 coil bundles with a total coil turn count of 1104. The stator is assembled within a fixture suitable for installation in bearing housings. Voltage and power characteristics of the EH were evaluated over a rotational range of 20 to 100 rpm. Notably, at 100 rpm, the average voltage measured was 6.9 V, with a corresponding power output of 2,778.5 mW. This performance signifies a significant advancement over previous EH designs. The proposed EH features a simple structure, facilitating power design and prediction, and is adaptable to various rotational shaft applications.

This study investigates the effects of heatinsulating glass on window penetration loss (WinPL) and building entry loss (BEL) in the 3-40 GHz range. Simulated and measured results for 12 types of glass windows, including single, double-, and triple-glazed structures with metallic coatings, were analyzed. Thermally-efficient windows (TEW) with metallic coatings showed an average WinPL about 20 dB higher than traditional windows (TW). Both window types exhibited periodic oscillations in WinPL, and TEW demonstrating shorter periods and slight irregularities. TEW showed lower WinPL in narrow bands around 3, 5, 10, and 20 GHz. The study highlights the significant influence of WinPL on BEL, particularly in thermally-efficient buildings, and suggests the need for updated ITU-R BEL models to account for modern construction materials.

In this Letter, a compact-size and high-power energy harvester (EH) based on multilayer flexible printed circuit board (F-PCB) sheets is presented for wireless sensor system (WSS) applications on propulsion shafts. A 2 mm thin rotor is designed using two F-PCB sheets that integrate 12 coil bundles with a total number of coil turns of 368. A stator is designed in a fixture that can be installed in the bearing housing. Voltage and power of the fabricated EH were measured and analyzed at a rotational speed of 10–100 rpm. At 80–100 rpm, the average voltage and output increased linearly with increasing speed. In particular, at 100 rpm, the average voltage and power were 5.43 V and 1714 mW, respectively. This performance represents a significant improvement compared to previously published EHs. The proposed EH features the straightforward structure, facilitating easier power design and prediction, and it is adaptable to various rotational shaft applications.

In this work, measured results of window penetration loss (WinPL) and their impact on BEL were analyzed in the 3 similar to 40 GHz band for 13 types of glass windows. Traditional double-glazed windows exhibit odd harmonic periodic oscillation characteristics due to properties of the multilayer dielectric. The WinPL of thermally -efficient windows is on average 20 dB higher than that of the traditional windows due to the metal coating layer and does not exhibit regular oscillation characteristics. In thermally-efficient buildings, WinPL contributes significantly to the BEL, while it does not significantly affect the BEL in the traditional buildings. In order to improve the BEL models in the future, it is necessary to develop appropriate and relevant parameters for traditional and thermally-efficient buildings based on these analysis results.

In this paper, we introduce a novel energy harvester (EH) using rolling magnets, which can supply power to a wireless sensor system (WSS) for monitoring the status of rotating ship shafts. The EH consists of twelve coils and seven magnets, which generate energy from the shaft's motion and supply it to the WSS for monitoring the shaft system's status. We installed the EH on a 20 cm diameter shaft and obtained a transferred voltage of 0.84 V and an output power of 47.2 mW with 2000 turns of each coil, despite a rotation speed of 25 rpm and a 3 mm air gap. The proposed EH shows great potential for monitoring rotating shaft on ships.

In this paper, we present a wireless sensor system (WSS) that integrated an inductive wireless power transfer (I-WPT) module for battery-free real-time monitoring of the status of rotating shaft in ships. Firstly, an optimized I-WPT module for seamless power supply was implemented using multiple Tx and Rx coils, and its power capability of 1.75 W with an efficiency of 75% was achieved. Secondly, as a result of the high-power transfer performance of the implemented I-WPT module, an entire WSS that integrated four sensors was designed on the rotary shaft. Finally, the designed WSS was installed on a small-scale test bench system with a shaft diameter of 200 mm; it was demonstrated that the status of a propulsion shaft could be monitored in real time without a battery.

In this paper, we present a self-powered wireless sensor system (WSS) integrated with an energy harvester (EH) to enable battery-free real-time monitoring of rotating shafts in ships. The EH implementation involved the use of multiple flexible coils and magnets, resulting in a power capability of 0.5 W. Subsequently, a comprehensive WSS was designed, incorporating four sensors, specifically tailored for the rotary shaft. To validate the system’s performance, the designed WSS was successfully deployed on a small-scale test bench system with a 200 mm shaft diameter, demonstrating real-time monitoring of the propulsion shaft status without the need for a battery.

We present a capacitive wireless power transfer (C-WPT) system using rotating capacitors for wireless sensor system (WSS) on propulsion shaft. In order to supply stable power to the WSS consisting of four sensors, a controller, and a radio module, we designed the rotating capacitor connected in parallel with multiple plates that minimizes the change in capacitance of the power coupling capacitor of the C-WPT system. A class-E converter and transformers topology are utilized to drive the C-WPT system for WSS. The fabricated C-WPT system transmitted stable power even when the rotational speed of the shaft was changed from 100 to 300 revolution per minute (rpm), and achieved power of 20.48 W and transmission efficiency of 64.29%.

In this work, we present an energy harvester (EH) for a battery-free wireless sensor system (WSS) on a marine propulsion shaft. To produce power for driving the WSS on a low-speed propulsion shaft, an EH with multiple coils and magnets is proposed. Using eight coils with 3,000 winding turns and six 1.43 T magnets, the EH produces a power of 1.5 W at 200 rpm. The proposed EH demonstrates that it is possible to implement a battery-free WSS for real-time monitoring of a marine propulsion shaft. As far as we know, the proposed EH shows the highest power compared to other systems.

An induction wireless power transmission (I-WPT) system using multiple coils has been proposed to drive a wireless sensor system (WSS) that monitors the status of the propulsion shaft. While maintaining a constant transfer voltage, the minimum number of coils and the minimum air gap between coils were extracted through several experiments. The optimized air gap between transmitter (Tx) and receiver (Rx) coils was 3 mm and the number of the Rx and Tx coils was four and six, respectively. The optimized I-WPT system was installed on the shaft with a diameter of 20 cm and obtained the stable transferred voltage characteristics and output power of 1.75 W with above 75% efficiency in spite of different rotation speed of the shaft.

We investigated the impact of window penetration loss (WinPL) on the frequency dependence of building entry loss (BEL) from 3.5 to 24 GHz. The WinPL characteristics of an ideal double-glazed glass and an actual double-glazed window were simulated and measured on-site, respectively, and both results showed almost the same oscillatory characteristics with respect to the frequency changes that occurred due to the impedance oscillation of the double glass-like multilayer dielectrics. Two BEL measurement scenarios were examined to analyze the frequency dependence of BEL in a traditional office building with double-glazed windows identical to those analyzed in the on-site WinPL measurements. The experiments included a complex propagation route (the first scenario) from the facade of the building to the corridors through windows and offices and a simple propagation route (the second scenario) through only windows lateral to the building. The two main findings are (i) BEL showed strong frequency-dependent behavior regardless of the propagation route and (ii) the WinPL characteristics of the outer double-glazed window were the main contributors to the frequency dependence of BEL.