# A Novel Ropes-Driven Wideband Piezoelectric Vibration Energy Harvester
## 1 Introduction  
Energy harvesting from ambient vibration--such as industrial machines, human activity, vehicles, structures, and environment sources--offers a clean, regenerative means for powering small-scale systems [1; 2]. The vibration energy can be transformed into electrical power through three main mechanisms including piezoelectric, electromagnetic, and electrostatic. Since piezoelectric material can effectively convert mechanical vibration into electrical energy using a very simple structure, the piezoelectric energy-harvesting technique is highlighted as a self-powered source for wireless sensor network systems [3]. Typical piezoelectric vibration energy harvesters (VEHs) are composed of a mass-spring-damper system with a transducer based on linear mechanical principles [4]. Such devices give an appreciable response amplitude only if the dominant ambient vibration frequency is close to the resonant frequency of the harvester [5], and the output drops dramatically when ambient vibration frequency is slightly different from the resonant frequency. Hence, it is impractical for the VEH, which is designed based on one resonant frequency mode with narrow bandwidth, to harvest energy from a variable environment.  
In order to overcome this gain bandwidth dilemma, researchers have paid attention to different approaches on broadening the operation bandwidth of vibration energy harvesters, which can be simply classified into the nonlinear harvesting technique and the multimodal harvesting technique [6; 7]. In the case of the nonlinear harvesting technique, usually extra magnetic force or mechanical force is purposely introduced as a nonlinear restoring force to control the magnitude and propertyof the nonlinearity in the system, thus enhancing transduction of the energy harvester under broadband excitations can be achieved [8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18], while the bandwidth is still limited. In the case of the multimodal harvesting technique, it is mainly based on the structure of beam arrays, which have different frequencies to match the wideband excitation's components, can be classified into unconnected mechanical structure systems [19; 20; 21; 22; 23]--connected mechanical structure systems using springs [24; 25]--and multi-resonance systems [26; 27; 28; 29; 30; 31]. Compared with a conventional single piezoelectric beam EH, much higher bandwidth of the EH can be realized by the multimodal harvesting technique. Moreover, wider or even unlimited bandwidth can theoretically be achieved with a greater number of beams added; however, output power will dramatically decrease with the number of beams, making it impractical for real applications, and this phenomenon is obviously worse for VEHs of a small size.  
In this paper, we propose a novel wideband piezoelectric vibration energy harvester (PVEH), in which a high-frequency generating beam (HFGB) is driven by an array of low-frequency driving beams (LFDBs) using ropes. We take advantages of frequency upconversion and multimodal harvesting technique mechanisms, which not only can produce unchanged output power with the increasing number of beams, but also has the potential to achieve unlimitedly wideband energy harvesting. Meanwhile, the characteristics of each LFDB driving HFGB is affected by multiple parameters, thus, for practical applications, the output and bandwidth can be optimized by changing these parameters, such as acceleration, rope margin, and stiffness of LFDBs. This wideband operation of the device offers a promising approach to work effectively in very complicated environments.