Ki Hwan Baek

Design of a New Piezoelectric Energy Harvester Based on Secondary Impact

Two models for an energy harvester imparting rotational energy to piezoelectric materials are presented, in order to compare the effects of applying identical amounts of energy to a cantilever beam by changing the total displacement per unit time, and applying a secondary impact. For a piezoelectric energy harvester given a high total impulse per unit time with low total displacement per unit time, higher power outputs were generated at lower resistive loads. Conversely, for a harvester given high total displacement per unit time with low total impulse per unit time, power output was higher at high resistive loads. At matched impedance, the secondary-impact-type piezoelectric energy harvester generated higher power output than the hitting-type piezoelectric energy harvester did at low resistive load. Optimized response of secondary-impact-type piezoelectric energy harvester was obtained at a frequency of 60 Hz with a low resistive load of 1 kΩ. The generated output power was measured as 124 mW, which corresponds to power density of 140 mW/cm3 for the entire cantilever beam, and a power density of 342 mW/cm3 for only the piezoelectric material volume (including sliver paste volume). For a harvester without a secondary impulse at low resistive loads (1 kΩ), the optimizing frequency was between 20 and 30 Hz, with an output power of 22 mW, which corresponds to a 25 mW/cm3 power density for entire cantilever beam and power density of 60 mW/cm3 for only the piezoelectric material volume(including sliver paste volume).

Restoration and Reinforcement Method for Damaged Piezoelectric Materials

This study investigated the effects of strain and crack formation on the performance of damaged piezoelectric materials as well as methods to restore damaged electrodes and reinforce the piezoelectric material. Specifically, this study investigated whether Ag paste (thickness: 0.1 mm) could restore electrical contact in damaged electrodes, and whether a coating of UV curable resin could provide reinforcement against crack formation. Experiments were conducted with a piezoelectric material on a steel cantilever substrate. The substrate was subjected to impact at various distances from the free end of the piezoelectric material to vary the applied strain. It was found that the output voltage increased with the strain (distance of impact from free end) until crack formation, which led to a large decrease in output. However, Ag paste could successfully restore electrical contact. Furthermore, before crack formation, coating with UV curable resin increased the maximum strain, and therefore, maximum electrical output, as well as cycle life.

Design of Piezoelectric Energy Harvesting System by Magnetic Force–Controlled Resonance Frequency

We designed a piezoelectric energy harvesting system that can be controlled the resonance frequency to the frequency of external energy. A permanent magnet (10 mm × 10 mm × 5 mm) was affixed to the free end of cantilever, and a permanent magnet was affixed to each of the four faces of a rotor at 90° angles. The effect of the dimension of the permanent magnets (20 mm × 20 mm × 10 mm, 30 mm × 20 mm × 10 mm, and 40 mm × 20 mm × 10 mm) and the effect of the pole array (NNNN, SSSS, NSNS, and NNSS) were experimentally tested. The optimum conditions were selected by testing varied distances between the magnets at varied rpm. The experiments demonstrated that the maximum output voltage was generated for the largest magnet and the minimum distance. The most effective way to control the resonance frequency was to modify the pole arrays of magnets affixed to the rotor. Furthermore, the optimum conditions were determined at each distance by changing the pole array and rpm. Simulation software supports the experimental results.

Study of Charging Efficiency of a Piezoelectric Energy Harvesting System Using Rectifier and Array Configuration

This paper suggests the design in rectifier and array configuration for an impact-based piezoelectric energy harvesting (PEH) system, to investigate the charging efficiency. In the design of the rectifier, it is suggested that rectifying the signal from each piezoelectric module separately generates more electrical energy and charges a capacitor faster than using a single rectifier for all modules connected in series. In the structural design, the array was designed into two conditions: impacted simultaneously and impacted sequentially with a phase difference (30°, 60°, 90°, 120°, 150° and 180°). We show that when impacted sequentially with a phase difference, this allows faster charging rates of energy harvesting. Additionally, distribution of the torque required to transform the piezoelectric modules was found to be another advantage from this condition. These results could be used in the design of more-efficiently-charging impact-based PEH system.

Stress Distribution Design of Additional Substrate for Piezoelectricity

This study was conducted to analyze a relationship between deformation degree curvature and output voltage of piezoelectric system, as well as to interpret the calculated and experimented data. Basically, the experiment was conducted by using tip masses of 10, 20, and 30 g to two different thicknesses of piezoelectric materials (0.15 mm and 0.2 mm). With this approach, the result showed that the lowest average Ix value had the highest voltage, 26.12 V m/s2 when the vibration acceleration was 4 m/s2. On the other hand, the lowest standard deviation of curvature ratio, ρ, had highest voltage, 58.98 V when the vibration acceleration was of 40 m/s2. Overall, this study demonstrates that the deformation degree and curvature ratio are related and proves they can be controlled.

Design of vibration exciter by using permanent magnets for application to piezoelectric energy harvesting

We designed a piezoelectric energy harvesting system that can shift the resonant frequency to match the fixed external energy frequency. A permanent magnet (10 × 10 × 5 mm3) was attached to the free end of a cantilever beam, and a permanent magnet was attached to each of the four faces of a rotor at 90° angles. The effect of the size of the permanent magnets (40 × 20 × 10 mm3, 30 × 20 × 10 mm3, and 20 × 20 × 10 mm3) and the effect of the pole array distribution (NNNN, SSSS, NSNS, and NNSS) were experimentally investigated. The optimum conditions were determined by testing various distances between the magnets at different rpms. The experiments showed that the maximum output power was generated for the minimum distance and largest magnet. The most effective approach to adjust the resonance frequency was to change the pole arrays of the magnets attached to the rotor. Furthermore, the optimum conditions were determined at each distance by changing the pole array and rpm. Software simulations support the experimental results.

Study on reinforcement and repair of cracked piezoelectric materials

This study investigated the effects of strain and crack formation on the performance of piezoelectric materials as well as methods to repair damaged electrodes and reinforce the piezoelectric material. Specifically, this study investigated whether Ag paste (thickness: 0.1 mm) could restore electrical contact in damaged electrodes, and whether a coating of UV curable resin could provide reinforcement against crack formation. Experiments were conducted with a piezoelectric material on a steel cantilever substrate. The substrate was subjected to impact at various distances from the free end of the piezoelectric material to vary the applied strain. It was found that the output voltage increased with the strain (distance of impact from free end) until crack formation, which led to a large decrease in output. However, Ag paste could successfully restore electrical contact. Furthermore, before crack formation, coating with UV curable resin increased the maximum strain, and therefore, maximum electrical output, as well as cycle life.

Rectifier and structural design for efficient energy harvesting system from impact-based piezoelectric array

This paper describes an efficient method for piezoelectric energy harvesting (PEH) system from impact forces using an array of piezoelectric modules. Specifically, the rectifier circuit and array configuration are considered. It is shown that rectifying the signal from each piezoelectric module separately generates more energy and charges a capacitor faster than using a single rectifier for all modules connected in series. In addition, the array can be designed such that the modules are impacted simultaneously or sequentially with a phase difference. We show that the latter allows faster energy harvesting. These results could be used in the design of efficient impact-based PEH system.

Design and optimization of secondary shock type piezoelectric system

Two models for energy harvesting system imparting rotational energy to piezoelectric materials are presented in order to compare the effects of applying identical amounts of energy to a cantilever beam by strain-changing and shock application. For a piezoelectric system given a high impulse with low displacement, higher power outputs were generated at lower resistive loads. Conversely, for a system with high displacement at a low impulse, power output was higher at high resistive loads. At matched impedance, the secondary shock system generated higher power output than the hitting system did at low resistive load. Optimized response of secondary shock system was obtained at a frequency of 60 Hz with a low resistive load of 1 k Ω. The generated output power was measured 124 mW, which corresponds to power density of 140 mW/cm3 for entire cantilever beam and power density of 342 mW/cm3 for only piezoelectric material volume. For a system without a secondary impulse at low resistive loads (1 k Ω), the optimizing frequency was between 20 and 30 Hz, with an output power of 22 mW, which corresponds to a 25 mW/cm3 power density for entire cantilever beam and power density of 60 mW/ cm3 for only piezoelectric material volume.

Design of supplemental plate for piezoelectric system to distribute impact force

The relationship between deformation degree and curvature to the output voltage of piezoelectric system was analyzed. The calculated data and experimental data were compared. The experiment was held by using tip mass of 10 g, 20 g, and 30 g for the various thickness of piezoelectric material as 0.15 mm and 0.2 mm for various type design of supplemental plate. As results, at the vibration acceleration of 4 m/s2, the lowest average Ix value was resulted the highest voltage of 26.12 V. However, when the vibration acceleration of 40 m/s2, the lowest standard deviation of curvature ratio, ρ, and resulted the highest voltage of 58.98 V. This study proves that the deformation degree and curvature ration can be controlled.