HARVESTING ENERGY FROM PLANETARY GEAR USING PIEZOELECTRIC MATERIAL

of thesis presented to the Senate of Universiti Putra Malaysia in fulfilment of the requirement for the degree of Master of Science In this study, a rotational piezoelectric (PZT) energy harvester has been fabricated and tested. The prototype can enhance output power by frequency up-conversion and provide the desired output power range from a fixed input rotational speed by increasing the interchangeable planet cover numbers which is the novelty of this work. The prototype ability to harvest energy has been evaluated with four experiments, which determine the effect of rotational speed, interchangeable planet cover numbers, the distance between PZTs, and PZT number. Increasing rotational speed shows that it can increase output power. However, increasing planet cover numbers can increase the output power without the need to increase speed or any excitation element. With the usage of one, two, and four planet cover numbers, the prototype is able to harvest output power of 0.414 mW, 0.672 mW, and 1.566 mW, respectively, at 50 kΩ with 1500 rpm, and 6.25 Hz bending frequency of the PZT. Moreover, when three cantilevers are used with 35 kΩ loads, the output power is 6.007 mW, and the power density of piezoelectric material is 9.59 mW/cm3. It was concluded that the model could work for frequency up-conversion and provide the desired output power range from a fixed input rotational speed and may result in a longer lifetime of the PZT.

2.33 (a) The harvester design with its excitation force using magnetics (Wu et al., 2018); (b) The design of rotational harvester using gear (Janphuang et al., 2011) 35 2.34 (a) Diagram of the harvester with a shape of octagon gear and piezoelectric cantilever and the gear in plain view (Wei and Duan, 2015); (b) Initial model variables (Park et al., 2012)  Converting mechanical energy such as kinetic energy, vibration or distortion energy into electrical energy is known as energy harvesting. The fast development of wireless sensor networks (WSNs) and the solution of storage power better efficacy will, ultimately, increase the devices that use self-power in an automotive application, monitoring of the environment, and health-care (Yang et al., 2018). However, the limitation of the power source of WSNs is one of the significant problems in this technology. Further, there are issues such as volume, weight, and short lifetime of batteries, which is much shorter than the WSN life, and batteries must be changed frequently. Consequently, researchers must find an alternative power source by focusing more attention on energy-harvesting technology (Kherbeet et al., 2015;Wu et al., 2014).
An energy harvester for powering WSNs and microdevices is a feasible approach in our environment, due to its low power consumption, small size, and special working environment. Piezoelectric energy harvesting is one of the novel approaches that has been developed to harvest power for these devices using different types of mechanical sources (Elahi et al., 2018). Depending on the application, different types of harvesting energies, as well as the handiness of the mechanical power sources, are available most of the time. Mechanical energies such as airflow, vibration, pressing, rotational machine, and human motion are variable frequencies and amplitude energies, which can be called random energies that are available everywhere (Kumar and Kim, 2012).
Rotational mechanical energy is one of the important power sources for piezoelectric energy harvesting. Different types of rotational power sources have been used in the past decades for piezoelectric energy harvestings, such as rotational machines ( The principle of rotational piezoelectric energy harvesting operation is based on the PZT plucking for excitation, which results in PZT bending, vibration, or pressing, and thus voltage is generated. Researchers have used different excitation elements in rotational piezoelectric energy harvestings, such as mass (Mohamad Hanif et al., 2018;Zhu et al., 2017), magnetic (Çelik et al., 2017;Karami et al., 2013;Pozzi, 2018) (Gu andLivermore, 2010a, 2012), gravitational force (Febbo et al., 2017;Hsu et al., 2014;Farbod Khameneifar et al., 2013), and gear teeth force (Park et al., 2012;Wei and Duan, 2015) or a compilation of these elements ( Yeo, et al., 2018;Yunshun Zhang et al., 2018, 2016. They have also applied these elements to widen the broadband range (Bai et al., 2015;Fu and Yeatman, 2017;Rezaei-Hosseinabadi et al., 2015) or for frequency up-conversion (Fu and Yeatman, 2019;Pillatsch et al., 2014;Yeo, et al., 2018) and rotational frequency, which is considered to be low in some cases compared to piezoelectric resonant frequency. While these different methods for rotational piezoelectric energy harvesting offer many advantages, they have numerous challenges. These include harvesting the desired output power ranges from a fixed input rotational speed, finding new methods in frequency up-conversion with better results, less force on PZT and hence a longer lifetime, and avoiding using slip ring or any other extra device for power transfer. Using a new source power type, design or method that can achieve these challenges is still desirable.
Most of the previously published papers on rotational piezoelectric energy harvesting area focused on frequency up conversion; however, frequency up conversion methods continues to be enhanced. Hence, rotational power sources, especially for wind, human motion, some rotary machines, and some vehicle tires, are generally considered as low frequencies, compared to PZT frequency. In some cases, the wiring for output power transfer is still an issue because the PZT is rotating with the system and thus a slip ring or any other wireless transfer is needed, which makes the device more complicated (Guan and Liao, 2016;Larkin and Tadesse, 2013;Li et al., 2011;Ramírez et al., 2017;Resali and Salleh, 2017b;Wu et al., 2014). Also, the heavy and rapid repetitive bending or striking on the PZT with the direct excitation contact can be considered an issue because it will reduce its lifetime; however, this can be avoided in different ways (Çelik et al., 2017).
Using a gear as a rotational mechanical input for piezoelectric energy harvesting excitation has been considered a good and new method. Only a few works have been accomplished using the gear as input in rotational piezoelectric energy harvesting. Juil Park et al. (Park et al., 2012) proposed a design where the cantilever free tip was excited by any rotary motion of mechanical devices. Using a mouse gear as the rotary mechanical device, the frequency can be changed from 0 to 800 Hz according to the changes in rotational speed and gear teeth numbers. Further, the output voltage and power generated from the gear model frequency up-converted kinetic energy harvesting device have reached a level of interest for practical applications and could be easily increased using multiple cantilevers within a single gear ( Janphuang et al., 2015). Energy harvesting is produced from an impact by using a piezoelectric Micro-Electro-Mechanical Systems (MEMS) scavenger. Useful electrical power is generated by the impact of the rotating gear on the MEMS piezoelectric transducer (PJanphuang et al., 2011). Moreover, Janphuang et al. 2011( Janphuang et al., 2013 conducted an experiment where the results revealed that free vibrations of the harvester after plucking contributed significantly to the power efficiency. The efficiency and output energy can be greatly improved by adding a proof mass to the harvester. Yang et al. (Yang et al., 2019) proposed a gullwing-structural piezoelectric energy harvester that consists of two typical non-linear buckled-bridges to scavenge low-frequency rotational energy based on a gear mechanism induced oscillation. This design is promising for self-powered sensors, especially at changeable and low frequency, such as tire pressure monitoring. However, as far as the authors are concerned, no research has been done using the planetary gear. Also, the PZT lifetime, wiring, and getting the desired output power from a fixed input rotational speed have not been clearly studied.
The novelty of this work is to design, fabricate, and test a compact frequency upconversion energy harvester that can harvest sustainable energy in broad range frequencies. This work is inspired from previous studies (that use normal gear), but with an enhanced gear design (planetary gear), which has less effect on the PZT and its primary system. The planetary gear design can harvest energy using a piezoelectric cantilever. The advantage of this study design over the previous designs that use a standard gear ( Janphuang et al., 2011;Pattanaphong Janphuang et al., 2015Park et al., 2012) is that it uses interchangeable planet covers so that it will have the ability to provide the desired output power range from a fixed input rotational speed, thus, by increasing the planet cover numbers without the need to increase the input speed, use of multiple PZT cantilevers, or increase the PZT pressing force. Moreover, it could be applied in any rotational machine such as a steam turbine with a pump for powering WSNs. Also, the PZT will be gently bent by the pressing of planet teeth gear, due to the way the planet gear moves in the planetary gear system, and this may result in a longer life-time for PZT. The wiring will not be an issue; there is no need for a slip ring or any other wireless device to transfer output power because the PZT will be fixed and will not rotate with the system.
Many applications can be suggested to apply this piezoelectric energy harvester prototype due to its properties, such as when it is connected to any rotational source that runs with variable speeds such as steam turbine or wind rotational source; moreover, it could be used in human motion energy harvesting with a more compact design.

Problem statement
Energy harvesting is the most effective way to respond to the energy shortage and to produce sustainable power source from the surrounding environment. In particular, rotational piezoelectric energy harvesting, which uses a direct energy conversion from any rotational mechanical input energy to the electrical energy, has been considered as an important type of harvesting (Elahi et al., 2018).. However, it is a challenge to harvest the energy at the desired output power range from a fixed input rotational speed. Frequency up conversion is still an issue for rotational power source, a lot of research has been done on frequency up conversion (Fu and Yeatman, 2019;Williams, et al., 2018) . However, new method are still desirable especially, most of the rotational power sources and their applications are considered as low frequency compared to high resonant frequency of piezoelectric (Febbo et al., 2017).
Only few works have been done on piezoelectric energy harvesting using gear, and there are lot of rooms to be filled with these kinds of design and enhance it (Janphuang et al., 2011;Janphuang et al., 2014;Park et al., 2012) . further, it has been found that no research has been done on piezoelectric energy harvesting using planetary gear. One of the issues in using gear for piezoelectric energy harvesting is that the gear teeth may damage the piezoelectric, due to the hard bending of the teeth, so, finding new contactless method or using less friction will enhance the output power (Janphuang et al., 2015).
However, it is the usage of planetary gear may increase the lifetime of piezoelectric cantilever, as the way the planet gear moving in the planetary gear system and thus, it bend the piezoelectric cantilever smoothly without damaging it. In addition, the wiring to transfer output power in rotational energy harvesting has also been considered as an issue because most of the time, the source of power rotates within the system and thus an extra device such as slip ring or Bluetooth will be needed for output power transfer (Guan and Liao, 2016;Lallart, 2017;Ramírez et al., 2017;Resali and Salleh, 2017b).

Objective of Research
The primary objective of this study is to design a rotational energy harvester model by using an interchangeable planetary gear cover that can harvest different ranges of output power from piezoelectric without changing the excitation pressure or rotational speed, and this can be done with the following secondary objectives.

1.
To fabricate and test compact energy harvester using planetary gear.

2.
To evaluate the performance of the prototype for frequency up conversion and test it with four experiment which is; effect of rotational speed, effect of planet cover numbers, effect of distances between PZT, and effect of PZT number, and prove its ability to provide the desired output power range from a fixed input rotational speed by increasing the planet covers numbers.

Hypothesis
Using planetary gear as energy harvester; the output power can be increased by increasing; rotational speed, choose the best angular distance between PZTs, and increasing PZTs numbers. However, increasing planet cover numbers can increase the output power without need to increase rotational speed or any other elements.

Contributions
It is clearly noted that no research has been done using planetary gear for piezoelectric energy harvesting. The ability to give different ranges of output power without increasing the frequency (rotational speed) or pressing on piezoelectric by simply plugging in more planet covers is the significance of this work. Moreover, the device was worked as a frequency up conversion, when more planet covers are used and in addition to that, the bending of piezoelectric cantilever is done smoothly because of the way that the planets gear move in the planetary gear. Consequently, this may increase the lifetime of piezoelectric.

1.
Rotational speed range 300 rpm (5 Hz) to 1500 rpm (25 Hz), has been chosen according to the suggested application which is human motion applications to wind turbine generator sensor. The frequency of gait motion is normally about 0.5-3 Hz which corresponds to a walking speed of 1.3-7.8 km/h, however human motion maximum frequency such as for walking is less than 10 Hz (Choi et al., 2017). The digital sensors rotating part that use to measure the rotational speed is mounted on wind turbine generator shaft with a typical speed range from 750 to 1500 rpm (Van Engelen and Kanev, 2010).

2.
The sufficient output power is 100 µW which is enough for powering sensors. However, the Optimum power output that supposed to reach by this prototype is more than 1 mW (Adu-Manu et al., 2018;Grady and Jeff, 2019;M‫׳‬boungui et al., 2015)(Grady and Jeff, 2019).

3.
The PZT cantilever is 5H material, d31 parallel connection and a sealed type named (S234-H5FR-1803XB) from Mide technology company with a dimension of 71×10.4×0.84 mm. These type as stated by the manufacturer is sealed by using reinforcement laminated between the two piezoelectric layers that make it a rugged PZT (Mide Technology, 2019a).

4.
The planetary gear type is chosen with 100 mm diameter to cope with the selected PZT dimension. The planet gear maximum numbers that can be fit in this size is four planets (Kohara Gear Industry Co., 2015) .

5.
3-dimension finite element method simulation using COMSOL Multiphysics program has been done to predict the performance of the harvester. According to the target applications and the size of the planetary gear, the simulation has been done to test these range of frequencies and planet cover numbers and with the using of 50% of the maximum allowable displacement of the PZT. The FE check whether the harvester with these elements were able to harvest the sufficient output power.

Thesis outline
First chapter of this study gives a background on energy harvesting, its piezoelectric rotational movement, problem statement, objective of the study and its outline. The literature review in the second chapter contains factors that affect output power energy, review, comparison of previous studies from different aspects with mechanical input which has been divided it into four parts; rotational from wind, vehicle tires, human motion and any rotary machine that include gear. Chapter three describes the methodology which includes planetary gear system, planetary gear components, fabrication of the prototype, experimental setup and procedures for four experiments which are the effect of rotational speed, effect of planets cover number, effect of distances between PZT, and increasing PZT number, effect on primary system, and modelling and simulation. Chapter four provides the experimental results and the discussions. Finally, chapter five draws the conclusion with the recommendations of future work.