# Design and Test of the MEMS Coupled Piezoelectric–Electromagnetic Energy Harvester

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## Abstract

This paper researches on the design and test of the output performance of double-end clamped MEMS coupled piezoelectric–electromagnetic energy harvester. It establishes the theoretical output model of the double-end clamped rectangular beam and trapezoidal beam piezoelectric–electromagnetic energy harvester, and optimizes the structure parameters of piezoelectric and electromagnetic unit with simulation analysis. It also respectively realizes the processing of piezoelectric and electromagnetic unit by MEMS and flexible PCB technology, and completes the performance test of structure prototype through the experimental system. The result showed that the capacity of MEMS coupled piezoelectric–electromagnetic energy harvester, which taked four coil piezoelectric with integrated electromagnetic in series, was 12.23 times higher than that of piezoelectric energy harvester. Also the output voltage and power of coupled trapezoidal beam energy harvester were respectively increased 18.89% and 2.26%, compared with coupled rectangular beam energy harvester.

## Keywords

Coupled energy harvester Piezoelectric Electromagnetic MEMS Double-end clamped## List of Symbols

*t*Time

*m*_{ε}Equivalent mass

*c*_{ε}Equivalent damping

*k*Equivalent stiffness

*k*_{1},*k*_{3}Nonlinear stiffness introduced by large deformation

- θ
Coupling coefficient of piezoelectricity

- γ
Coupling coefficient of electromagnetism

*z*(*t*)Displacement function

*V*(*t*)Piezoelectric partial voltage function

*i*(*t*)Electromagnetic partial current function

*a*(*t*)Acceleration

*μ*Calibration factor of the energy harvester system model

*R*_{1}Loading resistance of the piezoelectric part

*R*_{2}Internal equivalent resistance

*R*_{3}Loading resistance of the electromagnetic part

*C*Equivalent capacitance of the piezoelectric part

*L*Equivalent inductance of the electromagnetic part

## 1 Introduction

In recent years, with the continuous development of wireless sensor network, field intelligent sensing system, environmental monitoring system, personal wearable system and other fields, the energy supply problem of these smart sensor systems has become the key to restrict its development [1, 2, 3]. With the development of vibrational energy harvest technology, it is possible to convert the vibration energy in the environment into electrical energy for the energy supply of the sensor.

A great number of research work has been done on the piezoelectric energy harvester and the electromagnetic energy harvester. It concludes that the piezoelectric energy harvester has larger output voltage, but smaller output current from a few to tens of microamperes, due to the large resistance, which is suitable for working in high loading environment; in addition electromagnetic energy harvester has larger output current, but smaller output voltage from tens to hundreds of millivolts, which is unable to meet the general requirements of rated voltage devices, and suitable for working in smaller loading environment [4]. Therefore, considering the output characteristics and performance complementarity of these two energy harvesters, the researchers put forward the combination of two energy harvest mechanisms of piezoelectric and electromagnetic in the same structure, which can output large voltage and large current simultaneously. At present, the coupled piezoelectric–electromagnetic energy harvester has become a hot spot and trend of vibration energy harvest.

Wacharasindhu designed the keyboard power of coupled piezoelectric–electromagnetic energy harvester, and PZT thin films and planar coil, based on d33 mode, are achieved by the MEMS processing [5]. When tapping the keyboard on the magnet under vibration, changes occur, the magnetic flux through the coil is also due to the magnet stress effect on PZT. Then the coil and the PZT layer will output power. According to experiment, the output power of piezoelectric and electromagnetic energy harvest unit is respectively 40.8 µW and 1.15 µW. Yang designed two kinds of coupled piezoelectric–electromagnetic energy harvester according to the location of magnet and coil, and the structure was processed by micro-machining process [6]. Under the excitation of 2.5 g, the output power of the piezoelectric and electromagnetic energy harvest units are 107 µW and 0.18 µW respectively, when the coil is directly under the magnet. When the coil is on the side of the beam, and the direction of the magnetic pole is perpendicular to the plane of the coil, the output power of the piezoelectric and electromagnetic energy harvest units are 176 µW and 0.19 µW respectively. Challa designed a small coupled piezoelectric and electromagnetic energy harvester, which adheres to the piezoelectric plate on the surface of the cantilever beam, and the magnet is fixed to the end of the beam, and a winding circle is placed in the bottom of the magnet [7]. Through the theoretical analysis of the matching relation between the mechanical damping and electrical damping, the coupled piezoelectric–electromagnetic energy harvester can increase the output power of energy harvesting structure, compared to a piezoelectric or electromagnetic energy harvester with a single a single energy harvest mechanism. According to the test results, the first-order resonant frequency of the structure is 21.6 Hz, while the output power of the piezoelectric energy harvester and electromagnetic energy harvester are 257 μW and 244 μW respectively, while the output power of the coupled is 332 μW. Robert et al. [8] also shows that the output power of the coupled energy harvester is better than that of the piezoelectric energy harvester and the electromagnetic energy harvester, and the coil in the electromagnetic energy harvest unit can also be applied to the inductance in the SSHI circuit to increase the output power of the high voltage electric energy harvest unit. Tadesse et al. [9] designed a multimodal coupled piezoelectric–electromagnetic energy harvester based on a cantilever beam mass block. The study shows that the electromagnetic energy harvest unit can output larger power in the low frequency vibration environment, while the piezoelectric harvest unit has greater output power at high frequency vibration. Therefore, the combination of the two energy harvesting mechanisms can broaden the range of the energy harvesting band. Zhang Yating and Chen Tingting designed four clamped beam mass structure of coupled micro piezoelectric–electromagnetic energy harvester based on composite type, the experimental test, in the acceleration excitation of 6 g, the peak output voltage of the piezoelectric unit is 94 mV, although the peak output voltage of electromagnetic unit is 8 mV, the output signal is quite unstable. Li has studied the electromechanical coupling characteristics, the random output response and the output characteristics of the piezoelectric–electromagnetic energy harvester under the nonlinear action. The research has shown that to increase the electromechanical coupling and nonlinear force can also increase the output power and the energy harvest bandwidth of the energy harvester [10, 11, 12, 13].

Through the above investigation and analysis, the best design criteria of the coupled energy harvester in the literature are rarely mentioned. A lot more work should be done to design the coupled piezoelectric–electromagnetic energy harvest structure with excellent performance. Considering the vibration frequency in the work environment is usually distributed in a certain frequency range, if the harvester has narrow bandwidth, it can only work in the smaller frequency range, and reduce the energy harvesting efficiency. Therefore, we need to broaden the transducer bandwidth, in order to cover the vibration frequency in the entire working environment, improve the energy harvesting efficiency, and increase the output power.

The paper has designed the MEMS coupled piezoelectric–electromagnetic energy harvester. The structure of piezoelectric part has double-end clamped rectangular four beam structure and double-end clamped trapezoidal beam, in which double-end clamped rectangular four beam structure is used to increase the series output voltage, and double-end clamped trapezoidal beam is used to optimize the output power. The electromagnetic part consists of a micro planar coil and a micro high performance permanent magnet. In the study, we optimize and make the MEMS micro processing the structure of energy harvest, and finally tests the output performance of the micro coupled piezoelectric–electromagnetic energy harvester prototype.

## 2 Theoretical Model of MEMS Double-End Clamped Beam Structure

## 3 Design of MEMS Double-End Clamped Beam Structure

### 3.1 Design of Rectangular Beam Structure Energy Harvester

By means of simulation software ANSYS, we get the first resonance frequency with the modal analysis, and through the harmonic response analysis, obtain corresponding voltage data structure. On this basis, output power is calculated according to the structure parameters. This part will optimize the design by changing the beam width, beam length and PZT material length, so as to get the optimal structure. In addition, a comparative analysis is made on the rectangular beam and trapezoidal beam of MEMS double-end clamped beam.

By using ANSYS software, the optimal beam length, beam width and PZT width are selected and the simulation was carried out under the conditions of other parameters unchanged.

### 3.2 Comparison and Analysis of MEMS Double-End Clamped Rectangular Beam and Trapezoid Beam

From the comparison result, the trapezoid beam has greater displacement and more uniform distribution of stress and strain than the rectangular beam structure. The center line (x direction) of the bottom surface of a beam is used as the research object to observe the strain distribution.

The analysis shows that the output voltage of the trapezoid beam is 5 times than that of the rectangular beam in the same size, and the power is about 10 times than that of the rectangular beam in the same size.

## 4 Processing of the Piezoelectric Energy Harvester with Double-End Clamped Beam

_{2}on in the silicon, the beam is fabricated by the etching process. Then, PZT thin film is prepared by sol–gel method on the beam, where Ti and Pt are used as the top and down electrode. In the fabrication, it needs to ensure the integrity of PZT layer and avoid the interconnection of top electrode and bottom electrode. Besides, it needs to check there are no cracks on the surface of PZT layer. These aspects always result in the failure or lower power output for MEMS piezoelectric energy harvester. The structure after processing is shown in Fig. 15.

## 5 Performance Test of MEMS Coupled Piezoelectric and Electromagnetic Energy Harvester

^{2}in order to facilitate the comparison and analysis of the piezoelectric structure test.

### 5.1 Four Beam Structure with Double-End Clamped Beam and Piezoelectricity Test

#### 5.1.1 Vibration Response Test

#### 5.1.2 Piezoelectric Performance Test

Optimal structure data table

Parameter | Beam width | Beam length | PZT length |
---|---|---|---|

Numerical (μm) | 1150 | 4000 | 1250 |

Performance comparison table of optimal structure data and single item optimal structure data comparison table

Structure | Optimal beam length structure | Optimal beam width structure | Optimal PZT structure | Comprehensive optimal structure |
---|---|---|---|---|

Voltage (V) | 0.1220 | 0.1814 | 0.1705 | 0.2433 |

Voltage increase ratio (based on the optimum beam length structure) | 0.00% | 48.69% | 39.75% | 99.43% |

Power (W) | 5.610e−7 | 6.711e−7 | 6.999e−7 | 7.701e−7 |

Power increase ratio (based on the optimum beam length structure) | 0.00% | 19.63% | 24.76% | 37.27% |

### 5.2 Test of Coupled Piezoelectric–Electromagnetic Energy Harvester with Rectangular Beams

Designed parameters of coupled piezoelectric-electromagnetic energy harvester

Type | Number of coils | Piezoelectric chip number | Distance between coils and permanent magnets (mm) |
---|---|---|---|

Coupled 1 | 6 layer × 1 | 1 | 1 |

Coupled 2 | 6 layer × 2 | 1 | 1 |

Coupled 3 | 6 layer × 4 | 1 | 1 |

Coupled 4 | 6 layer × 4 | 2 | 1 |

Output performance of coupled piezoelectric-electromagnetic energy harvester

Type | Piezoelectricity | Electromagnetism | Composite power (nW) | Performance | Bandwidth (Hz) | ||
---|---|---|---|---|---|---|---|

Voltage (mV) | Power (nW) | Voltage (mV) | Power (nW) | ||||

Single piezoelectricity | 81.98 | 10.50 × 4 | – | – | 42.00 | – | 10 |

Coupled 1 | 69.18 | 7.61 × 4 | 3.52 | 74.43 | 104.87 | ↑1.50 times | 15 |

Coupled 2 | 64.14 | 6.43 × 4 | 9.30 | 270.28 | 296.00 | ↑6.05 times | 17 |

Coupled 3 | 109.81 | 18.84 × 4 | 17.21 | 473.06 | 548.42 | ↑12.06 times | 38 |

Coupled 4 | 157.26 | 19.32 × 4 | 17.52 | 478.52 | 555.80 | ↑12.23 times | 41 |

### 5.3 Output Performance of Coupled Piezoelectric–Electromagnetic Energy Harvester with Trapezoid Beam

Comparison of structural parameters between coupled piezoelectric and electromagnetic rectangular beams and trapezoid beams

Type | Structure | Coil | Coil and detection quality distance (mm) | Test quality size (mm) | Piezoelectric connection |
---|---|---|---|---|---|

Coupled 5 | Rectangular beam | 6 layer × 4 | 0.1 | 3.5 × 3.5 × 0.8 | Single beam series |

Coupled 6 | Trapezoidal beam | 6 layer × 4 | 0.1 | 3.5 × 3.5 × 0.8 | Single beam series |

Comparison of the output performance of coupled piezoelectric and electromagnetic rectangular beams and trapezoid beams

Type | Piezoelectricity | Electromagnetism | Composite power (nW) | Piezoelectric/composite performance | ||
---|---|---|---|---|---|---|

Voltage (mV) | Power (nW) | Voltage (mV) | Power (nW) | |||

Coupled 5 | 123.55 | 53.00 × 4 | 19.35 | 570.04 | 782.04 | – |

Coupled 6 | 142 | 63.01 × 4 | 18.55 | 547.66 | 799.70 | ↑18.89%/↑2.26% |

## 6 Summary

This paper studies the optimization design, processing technology and structural performance test of the coupled MEMS piezoelectric–electromagnetic energy harvester. The piezoelectric structure is optimized designed by ANSYS software. The MEMS process design and implementation of the optimized structure is carried out, and the comparative analysis of the prototype is done. The analysis results show that after introducing the large deformation nonlinearity, the energy harvesting band of MEMS structure has been greatly broadened, and the use of planar multilayer microcoils makes the characteristics of electromagnetic capture energy more significant. In addition, the analysis also shows that compared with the single piezoelectric energy harvesting mode, the nonlinear effect of the coupled energy harvester is enhanced, the capture frequency band is broadened, and the output performance is improved significantly. MEMS coupled piezoelectric–electromagnetic energy harvester with the electromagnetic coil and four piezoelectric in series is increased 12.23 times in power; in addition, the output voltage and couple power of the coupled energy harvester with piezoelectric beam structure is increased 18.89% times and 2.26% times.

## Notes

### Acknowledgements

This work supported by the Natural Science Foundation of Shandong Province, China (ZR201709220253).

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