Shock and Vibration

Volume 2015, Article ID 345191, 9 pages

http://dx.doi.org/10.1155/2015/345191

## The Effect of the Parameters of a Vibration-Based Impact Mode Piezoelectric Power Generator

^{1}Division of Electronics and Informatics, School of Science and Technology, Faculty of Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515, Japan^{2}Research and Development Department, Mitsuba Corporation, 1-2681 Hirosawa-cho, Kiryu, Gunma 376-8555, Japan

Received 12 February 2015; Accepted 11 May 2015

Academic Editor: Nuno M. Maia

Copyright © 2015 Amat A. Basari et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

This study reports the effects of the parameters of a vibration-based impact mode piezoelectric power generator. First, an evaluation of the effects of the impact parameters, the mass, and the impact velocity is presented. It is found that the output voltage of the piezoelectric device in impact mode is directly proportional to the velocity, whereas the output power is equal to a quadratic function of the same variable. For the same impact momentum, the effect of the velocity in generating a higher peak output is dominant compared with the mass. Second, the vibration-based impact mode piezoelectric power generator is discussed. The experimental results show that a wider operating frequency bandwidth of the output power can be achieved with the preloading configuration. However, regarding magnitude, due to the high velocity of impact, the configuration with a gap between the tip and the piezoelectric device produces a higher output.

#### 1. Introduction

Research advancements in mechanical vibration energy harvesting have been widely reported for decades. Energy from these harvesting systems is expected to be used to power low-power devices, such as LEDs, tire pressure monitoring systems, and many others. The objectives of the study are to propose a new design and to evaluate the factors that affect the output power generation. In general, mechanical vibration is converted to electrical energy using three types of devices: piezoelectric, electrostatic, and electromagnetic devices. Both* *analytical [1] and experimental analyses [2–5] have shown that there are various factors that affect the performance of such devices. The evaluation of each factor is very subjective that strongly depends on the devices, the environment, and the type of the vibration.

In the case of vibration energy harvesting using piezoelectric devices, for linear vibration motion, the basic operation of power generation can be divided into two modes: bending mode and impact mode. In bending mode for power generation using a piezoelectric cantilever beam, one end of the device is attached to the vibration sources and the other end freely vibrates with the sources of the vibration. To improve the output power of the piezoelectric power generator in bending mode, the shape of the device is critical, as it has been shown that devices of certain shapes are more effective than others [6, 7].

Another important factor for the optimum output is impedance matching [8, 9]. However, impedance matching is dependent to the resonant frequency of the structures, which means that for low resonant frequency structures, a large matching impedance is required. These are among the important factors that have been considered in the design of vibration-based bending mode piezoelectric power generators.

In the case of vibration-based impact mode power generation, piezoelectric devices do not deform due the vibration. The deformation is due to the impact. As reported in [10], a structure with a freely moving steel ball repeatedly hits the piezoelectric wall to generate electricity. At the beginning of the design process, a weight drop experiment is conducted. The output power from this experiment is found to be relatively higher compared with the output power of the designated device. One suggestion regarding optimizing the output power is that the steel ball must be large and heavy. Another design of an impact mode piezoelectric power generator is reported by [11]. An impact mode power generator consisting of a vibrating beam with a piezoelectric device on top and two additional piezoelectric cantilever beams placed at each side of the vibrating beam is proposed. The vibrating beam has an extended rectangular tip on which a mass is fixed. When the beam vibrates, it hits both piezoelectric beams and, due to the impact, electricity is generated. The implementation target of the device is to harvest low frequency vibrations, such as human motion-related movements. The optimization procedure is based on the impedance matching technique. Another analysis and discussion on the combination of a bending and impact mode power generator is reported in [12]. It is reported that, in terms of voltage, the bending mode piezoelectric specimen generates higher values than that of the impact mode piezoelectric specimen, although how the output can be optimized for the designated device is not discussed.

Other studies have reported the effect of the dimensions of piezoelectric ceramics [13] and the type of vibrations [14] on impact mode piezoelectric power generation. However, the effects of mechanical impact parameters on impact mode piezoelectric ceramic power generation are less discussed by researchers. Therefore, this study presents an analytical and experimental study on how to optimize the output power of impact mode piezoelectric power generators by analyzing two parameters that closely effect the output power: the velocity of impact and mass. To identify the relationship of the output power with these two parameters, a weight drop experiment was conducted. Variations in the weight of the objects and the height at which the objects were dropped, which is related to the impact velocity, were considered. The findings were then used to analyze the performance of a forced vibration-based impact mode piezoelectric power generator in harvesting vibration energy.

#### 2. Power Generation of the Piezoelectric Device in Impact Mode

##### 2.1. Piezoelectric in Impact Mode: Effects of the Impact Variables

Piezoelectric devices generate electricity through deformation of its structure. The cause of deformation can be vibration or direct impact on the structure. The characteristics of impact mode power generation of piezoelectric devices for various impact parameters can be studied by performing weight drop experiments. Theoretically, when an object with a weight of is dropped from a predetermined height, its impact force is given by the following:where is the kinetic energy, is the penetration distance, and is the velocity upon impact. Theoretically, the maximum output energy per cycle of piezoelectric devices that operate in 33-mode is given by (2). From the equation, the amount of electrical energy that can be generated by a piezoelectric device is directly proportional to the square of the impact force:In this equation, , , and are the width, length, and thickness of the piezoelectric device, respectively, and are the piezoelectric charge (strain) and voltage (stress) constants, respectively, and is the force that acts on the device. Substituting (1) into (2) yields (3), which reveals that the output energy is directly proportional to and and inversely proportional to the penetration distance . In drop-weight experiments, the two important impact variables are the weight and the velocity of the impact object. Assuming the object is dropped in a free fall and its height is , the velocity of impact can be calculated using the equation , where is the acceleration due to gravity.

##### 2.2. Experiment and Discussion

The weight drop experimental setup is shown in Figure 1. The specifications of the piezoelectric device are listed in Table 1. The steel balls used in this experiment have diameters of 9.52 mm and 12.7 mm and weights of 4 g and 8 g, respectively, and the material of the steel balls is carbon. Two types of supporting bases of the piezoelectric device are used: a flat base and a base with a hole. Two types of supporting bases are used to evaluate the output power with respect to the change in the stiffness. The diameter of the hole of the base is 30 mm.