Electromechanical Analysis of a New Integral Arrayed Cymbal Harvester

www.ijep.org International Journal of Energy and Power (IJEP) Volume 4, 2015 doi: 10.14355/ijep.2015.04.010

Electromechanical Analysis of a New Integral Arrayed Cymbal Harvester SUN Chun‐hua*1, DU Jian‐hong2, WANG Hong‐bing3, SHANG Guang‐qing4 Department of Mechanic and Electronic Engineering, Suzhou Vocational University, 215104, China *1

chh_sunny@163.com; 2djh@jssvc.edu.cn; 3whb@jssvc.edu.cn; 4shanggq@jssvc.edu.cn

Abstract To harvest energy from ambient waste vibration energy effectively on larger area, a new integral arrayed cymbal harvester is proposed. The harvester is composed of two pieces of steel endcaps and several disks of PZT lying between the endcaps. The two endcaps are stamped into several cymbalʹs cavities which are arrayed evenly. Electromechanical analysis is done with the finite element method, including natural frequency, open output voltage and stress distribution. Results show that the integral arrayed cymbal harvester possesses lower natural frequency and the first three order frequencies lay the range of 800‐3000Hz. The excited open output voltage decreases with the thickness of PZT disks under the same pressure. And the voltage increases with the pressure. Stress analysis reveals that most of the applied stress is expended in deforming the cavity along the radial direction and the stress factor along the thickness direction is significantly small. The proposed harvester is similar to the classic cymbal transducer. It reduces the natural frequency and is available for capturing lower frequency vibration effectively on large area.

Keywords Piezoelectric Energy Harvesting; Cymbal Harvester; Integral Arrayed; Finite Element Analysis

Introduction The cymbal harvester, which was developed by R.E.Newnham in 1992[1], has been widely applied in hydro‐ acoustic, electro‐acoustic, ultrasonic, pressure sensors, actuator and so on[2‐5]. Several recent researches have focused on investigating piezoelectric power harvesting with the cymbal transducer from ambient waste vibration energy. The relations between sizes of cymbal and performances were discussed [6‐8]. Further, to enlarge the energy harvesting efficiency and to broaden the applicable scope, some studies took the scheme by arraying the cymbal harvesters along the longitudinal or horizontal direction, and electrically connecting in series and parallel[9‐10]. However, due to the limitations of the piezoelectric ceramic manufacturing technique, the size of PZT disk is smaller. That leads to small size of the existing cymbal harvester and makes it to be restricted for capturing energy on a large area with one piece of harvester.

The objective of the present paper is to develop a new kind of the integral arrayed cymbal harvester. The finite element method(FEM) is adopted for analyzing electromechanical properties of the harvester, including natural frequency, open voltage and stress distribution.

The Structure of New Integral Arrayed Cymbal Harvester As we all know, cymbal harvester has larger displacement, larger generative forces, and lower cost[1]. However, due to the limitation of PZT ceramic processing technology, its size is not larger. So the harvested electricity is limited by using a single cymbal. A new harvester based on cymbal’s working principle needs to be explored.

Figure 1 shows a new kind of the integral arrayed cymbal harvester. The harvester is composed of two pieces of metal endcaps and several disks of PZT lying between them. The two endcaps are punched into several cavities like cymbal which are arrayed evenly. A series of PZT ceramic disks are bonded on the two endcaps using the binder, which are laid with the same polar direction. Epoxy resin is filled between two endcaps except PZT disks.

The electrical connection is made by soldering the conductive wires at the capʹs edge of the harvester.

The arrayed distribution lists as following: 1 is on the center; 6 are on the second circle at 60 degree spacing; 12 are one the third at 30 degree. And the spacing distance of p between adjacent cavities is set along the horizontal axis.

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Thus, this integral arrayed harvester is similar to classic cymbals in electrical parallel. In addition to the performance like a single cymbal transducer, this new transducer is available for harvesting vibration energy on large area and can be more easily manufactured.

FIG.1 THE INTEGRAL ARRAYED CYMBAL HARVESTER

FEM Model and Material Parameters A detailed FEM analysis is performed on the integral arrayed cymbal harvester using ANSYS software to analyze electromechanical performance of the proposed harvester.

PZT‐5A is chosen as piezoelectric ceramic material because of its higher (d∙g) value. Here, d is piezoelectric field constant and g is piezoelectric voltage constant. The PZT‐5A material properties are listed in Table 1.

TABLE 1 PZT‐5A MATERIAL PROPERTIES

Parameters Density（kg/m3）

Value 7500

Poissonʹs ratio

0.32

Relative permittivity  33 r T

（10‐9F/m）

4.46    2.301    4.46

Piezoelectric voltage constant matrix [d]（pC/N）

0 0 0 12.5 0  0   0 0 0 0 12.5 0   3.1 13.5  3.1 0 0 0

Flexible piezoelectric constant matrix [s] （GPa）

158.3 69.1 67.5 0 0 0     69.1 158.3 67.5 0 0 0   67.5 67.5 149.9 0 0 0    0 0 45 0 0   0  0 0 0 0 45 0    0 0 0 0 45  0

Aluminum is used as the metal endcap, whose density, Poisson’s ratio and elastic modulus are 2790 kg/m3, 0.34 and 72.5GPa, respectively. Epoxy is taken as the binder and its material’s parameters lists as following: density of 0.98×103kg/m3, Poisson’s ratio of 0.36 and elastic modulus of 2.5GPa. Dimensional details of the proposed harvester list as following: thickness of the metal endcap is 0.3mm, diameters of PZT disk and the harvester are 10mm and 100mm, respectively. The interval of PZT disks is p=22mm.

The contract pressure is accepted to act on the cavities of the harvester along the z direction, as shown in Fig.1.

The unit of SOLID45 is used to mesh the end caps and the binder, and SOLID5 is used for PZT disks. The units are meshed by using HyperMesh. Through transferring the units into ANSYS, the modal and statics analysis are then done after applying the boundary conditions. By taking into account of axial symmetry of the proposed harvester,

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one quarter FEM model of the integral arrayed harvester is constructed, as shown in Fig.2.

FIG.2 ONE QUARTER MODEL OF THE HARVESTER

Result and Discussion The Natural Frequency The natural frequency of the integral arrayed harvester is firstly done through modal analysis with ANSYS software. The model with PZT thickness of 1mm is adopted. The experiment is also done. The results are listed in Table 2. It can be seen from Table 2 that the finite element analysis(FEA) result is contrast with the experimental one very well and the difference is small.

TABLE 2 NATURAL FREQUENCIES

Order

FEA result(Hz)

Experimental result(Hz)

Error(%)

1 2 3

807.45 1404.5 2998.1

787.188 1395.46 2891.101

2.51 0.64 3.57

Note that the harvester possesses the first three orders of resonance frequencies adjustable between 800 and 3000Hz. So the proposed harvester is capable of harvesting low frequency vibration in 800‐3000Hz range.

Therefore, the finite element method is an effective tool to perform an electromechanical analysis of the integral arrayed cymbal harvester. The proposed finite element model is available.

Figure 3 shows modal graphs of the first three order.

(a) f=807.45Hz

(b) f=1404.5Hz

(c) f=2998.1Hz

FIG.3 THE MODAL GRAPHS OF THE FIRST THREE ORDERS

Open Circuit Output Voltage The open circuit output voltage is then analyzed. Figure 4 shows the output voltages from the cymbal harvesters with various thicknesses of PZT under the pressure of 0.7MPa. The other sizes of the harvester are the same as those of the above modal analysis. It is obvious that the output voltage decreases with the thickness of PZT ceramic disk. This trend can be explained as follows: with increasing of the PZT thickness and the rigidity, the deformation of the PZT disk drastically decreases under the same force from the metal endcap, and the excited voltage reduces as well.

The conditions under which various pressures act on the proposed harvester of PZT thickness of 1mm are also analyzed. The result is shown in Fig.5. Note from Figure 5 that the open output voltage increases with the pressure.

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This is due to increasing of the pressure and the deformation of the endcap along the tangential direction, leading to a higher electric charge generation on the PZT disk.

FIG.4 OUTPUT VOLTAGE WITH VARIOUS PZT THICKNESS FIG.5 OUTPUT VOLTAGE WITH PRESSURE

Stress Distribution Figure 6 shows the stress distribution of metal end cap when PZT thickness is 1mm under 0.7MPa pressure . As it is evident from this calculation, stress is concentrated on the cavities, and very low static stress is located on most region of metal endcap, as shown in Fig.6(a). Note again that most of the input mechanical energy is converted to the PZT deformation along the radial direction(XY), as shown in Fig.6(b), while the magnitude of the XY tensile stress is significantly higher than ZZ stress, as shown in Fig.6(c). Along the cavity diameter, the magnitude of the XY stress is up to the order of 8 while the ZZ stress is 7. This indicates that the local cavity in the metal endcap also serves as a cymbal transducer, which is a mechanical transformer for transferring and amplifying a portion of the incident axial stress in the radial stress.

(a) VON MISES STRESS DISTRIBUTION (b) XY STRESS DISTRIBUTION (c) ZZ STRESS DISTRIBUTION FIG.6 STRESS DISTRIBUTION ON METAL ENDCAP

The above phenomenon also reminds us that structure optimization of the metal endcap can be done further.

Conclusion This study reports a new kind of the integral arrayed cymbal harvester. The proposed harvester has two pieces of metal endcaps, on which there lie many cavities evenly. This structure is similar to many cymbal transducers in electrical parallel. FEM analysis shows that the proposed harvester possesses the first three orders of low resonance frequencies in a range of 800‐3000Hz. The excited open output voltage decreases with the thickness of PZT disks under the same pressure. The voltage is about 75V when the PZT thickness is 1mm under pressure of 0.7MPa. Meanwhile, the output voltage increases with the pressure and the voltage is up to 107V when the pressure is 1MPa. Stress concentration is taken on the cavities and most regions of metal endcap have low static stress. Most of the applied stress is expended in deforming along the radial direction and the stress factor along the thickness direction is significantly small. Based on the results of this study it can be conjectured that the proposed harvester can harvest vibration energy on larger area and provide an alternative harvester on lower frequency conditions.

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ACKNOWLEDGMENT

This research was supported by the National Natural Science Foundation of China (No.51175359) and the 4th “333 Engineering” Research Funding Project of Jiangsu Province (BRA2014086).

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