# Multi-Unit Coupled Motion Hybrid Generator Based on a Simple Pendulum Structure
## 1 Introduction  
With the continuous development and utilization of marine resources by humans, the Ocean Internet of Things (Ocean IoT) has emerged [1, 2]. As a network system connecting various marine devices and sensors, the marine IoT offers innovative solutions for marine resource exploration, environmental monitoring, and marine disaster early warning [3, 4, 5]. Up to now, energy harvesting systems based on artificial intelligence (AI), and adaptive hybrid devices have gradually become research hotspots. In this field, energy harvesting devices, as the hardware foundation empowered by AI, play a crucial role [6, 7, 8]. However, the complexity and vastness of the marine environment pose significant challenges for traditional energy supply technologies in the marine IoT [9, 10]. Additionally, the irregular amplitude and low frequency of wave energy limit the performance of traditional energy harvesting devices in converting blue energy [11, 12, 13]. Therefore, there is a pressing need to develop new devices capable of efficiently harvesting wave energy. In recent years, the triboelectric nanogenerator (TENG) has been introduced for harvesting mechanical energy from the environment [14, 15, 16, 17]. TENGs collect energy through the slight deformation of materials under external forces and are characterized by their small size, light weight, and strong scalability, making them highly suitable for wave energy harvesting [18, 19, 20, 21, 22, 23]. Currently, energy harvesting devices based on swinging structures have garnered widespread attention due to their ability to operate under low-frequency excitation and produce multi-frequency outputs [24, 25, 26]. For example, [PERSON] et al. developed a hybrid wave energy harvester (HW-NG) based on a pendulum structure, integrating TENGs and electromagnetic generators (EMGs) [27]. However, a large portion of theinternal space in this structure remains underutilized. Similarly, [PERSON] et al. designed and fabricated a durable rolling-based swinging structure triboelectric nanogenerator (RS-TENG) for low-frequency water wave energy harvesting [28]. When triggered by water waves, it can generate an output power of 4.27 mW, with a corresponding power density of 1.16 W/m\({}^{3}\). Nevertheless, there is still a significant amount of unused space within this structure, leaving room for improvement in space utilization. Addressing the issue of low space utilization in traditional swinging structures, this paper proposes a multi-unit coupled motion hybrid generator (MCM-HG) based on a simple pendulum structure. The device integrates eight power generation units, including triboelectric nanogenerators (TENGs), electromagnetic generators (EMGs), and piezoelectric nanogenerators (PENGs). The structural design, which integrates multiple power generation units, significantly enhances the utilization of internal space compared to traditional single-unit structures. Moreover, it achieves coupled motion between TENG units and EMG units, resulting in high voltage and large current outputs. The coupled motion between TENG units and PENG units enables multiple outputs from a single excitation. First, this paper systematically investigates the effects of the number of turns in the PCB coil and the range of magnetic flux cutting distance on the electrical output performance of the EMG unit; the mass of the simple pendulum and the support structure angle on the electrical output performance of the TENG unit; and the movement distance of the pulley clamp on the electrical output performance of the PENG unit. Next, the output performance of each power generation unit of the MCM-HG on a six-degree-of-freedom platform is studied. At a frequency of 0.5 Hz and a swing angle of 15\({}^{\circ}\), the MCM-HG achieves an output power of 22.07 mW, with a corresponding power density of 7.36 W/m\({}^{3}\), which is 5.35 times higher than that of traditional swinging structures. Finally, the electrical output characteristics of TENGs and EMGs connected in parallel with the same phase are demonstrated.