Chih-Kung Lee

Self-powered piezoelectric energy harvesting device using velocity control synchronized switching technique

With the rapid development of low power consumption electronics, wireless sensor networks (WSN) are highly investigated and used to improve our life quality. Using piezoelectric materials to transfer the mechanical energy into electrical energy for batteries of WSN in order to extend the life time is the focus in many researches in the recent years. It is important and efficient to improve the energy harvesting by designing an optimal interface between piezoelectric device and the load. In this paper, a self-powered piezoelectric energy harvesting device is proposed based on the velocity control synchronized switching technique (V-SSHI). Comparing to diode full bridge rectifier standard technique, the synchronized switching harvesting on inductor (SSHI) technique can highly improve harvesting efficiency. However, in real applications when the energy harvesting device is associated with WSN, the SSHI technique needs to be implemented and requires being self-powered. The conventional technique to implement the self-powered SSHI is to use bipolar transistor as voltage peak detector. In this paper, a new self-powered device is proposed, using velocity control to switch the MOSFET more accurately than in the conventional technique. The concept of design and theoretical analysis is presented in detail. Experimental results are examined and show better performance.

Self-powered semi-passive piezoelectric structural damping based on zero-velocity crossing detection

In recent years, semi-passive vibration damping using non-linear synchronized switching methods has been intensively investigated and discussed. In this paper, a self-powered synchronized switch damping on inductor (SSDI) technique based on zero-velocity crossing detection is proposed and investigated. The control signal used to drive the switches is obtained by sensing velocity. A totally self-powered damping system powered by harvested energy using the SSDI technique with velocity sensing and without external power is established. Compared with the conventional technique based on voltage peak detector, this technique does not generate lag in detection of switching time. The theoretical model, the experimental evaluation and the drawback of the self-powered zero-velocity crossing detection switching technique are discussed in this study. The system performance is also compared with the externally powered system.

Targeted origin placement for the autonomous gain-phase tailoring of piezoelectric sensors

Targeted origin placement can be shown to compliment mathematical tools such as the image method, window functions and two-sided Laplace transformation for designing a whole new class of piezoelectric sensors that can be used to tailor the gain and phase portions of the sensor transfer functions independently. It can be demonstrated that by properly applying the boundary conditions and the method of imaging, the targeted origin of this new class of sensor design can be traversed anywhere along the surface of the testing structure. In addition, the active sensor concept that integrates classical control theory with piezoelectric sensors can be adapted to further advance sensor technology. The design concept, theoretical derivations, numerical calculations and experimental verifications of implementing this class of new sensor design to influence flexible structure control and point sensor design methodologies are detailed.

Power enhancement of piezoelectric transformers by adding heat transfer equipment

It is known that piezoelectric transformers have several inherent advantages compared with conventional electromagnetic transformers. However, the maximum power capacity of piezoelectric transformers is not as large as electromagnetic transformers in practice, especially in the case of high output current. The theoretical power density of piezoelectric transformers calculated by stress boundary can reach 330 W/cm3, but no piezoelectric transformer has ever reached such a high power density in practice. The power density of piezoelectric transformers is limited to 33 W/cm3 in practical applications. The underlying reason is that the maximum passing current of the piezoelectric material (mechanical current) is limited by the temperature rise caused by heat generation. To increase this current and the power capacity, we proposed to add a thermal pad to the piezoelectric transformer to dissipate heat. The experimental results showed that the proposed techniques can increase by 3 times the output current of the piezoelectric transformer. A theoretical-phenomenological model which explains the relationship between vibration velocity and generated heat is also established to verify the experimental results.

Electrical and mechanical field interactions of piezoelectric systems: foundation of smart structures-based piezoelectric sensors and actuators, and free-fall sensors*

Based on an idea to fully integrate the advantages of both distributed and point sensors, we adopted mathematical tools such as method of image, linear superposition, and window functions to develop a fundamental approach towards creating no-phase delay filters for finite sensor structures. Through an expanded design freedom for piezoelectric sensors and actuators as a result of adding in situ, we can obtain a no-phase delay signal filtering capability by properly designing an effective surface electrode onto the piezoelectric layer. An inertia-based free-fall sensor that can measure the start of free-fall motion in addition to creating a piezoelectric sensor and actuator pair that possesses a spatial dependent transfer function and demonstrates the wide range applicability of the new concepts are disclosed. The fundamental perspective of pursuing signal processing through an electromechanical interaction of wave modes and of piezoelectric materials is examined in detail. It can be shown that the fundamental design criteria of electromechanical-coupled systems are highly related to the temporal and spatial characteristics of the mechanical structure and its electrical interaction between interface circuits. After pursuing the physical characteristics of these systems, the possibilities of extending the no-phase delay signal processing capabilities to other physical fields are discussed as well.

Miniature piezoelectric actuators: design concept, fabrication and performance evaluation

Magnifying mechanisms, which include bimorph configurations, waveguides, resonance, mechanical levers etc, that can convert a large-force, small-displacement piezoelectric material to become a smaller-force, large-displacement actuator are examined in detail. Sputtering processes for zinc oxide and lead-zirconate-titanate films, which are important for fabricating miniature laminated piezoelectric actuators, are presented. A newly developed laser Doppler interferometer that is especially designed for evaluating the performance of miniature piezoelectric actuators is also described. The design concept, optimization process, fabrication techniques and evaluation approach for such things as a piezoelectric impact hammer, a resonant dual-dimensional piezoelectric scanner, a Langevin type ultrasonic motor and a high-precision piezoelectric positioning stage are also discussed.

Design considerations of Piezoelectric transformers with voltage-mode rectifiers for DC/DC converter application

Piezoelectric transformers (PT) are widely adopted in recent year due to many advantages, including safety, no EMI problem, low housing profile, and high power density, etc. The characteristics of the PT are well-known when the load impedance is a pure resistor. When PTs are used in AC/DC or DC/DC converters, there are always a rectifier circuit block connected after the PT. The rectifier circuit block sometimes can be equivalent to a resistor, but not always true. Thus, this paper starts from the model of the voltage-mode rectifier directly. Then, three design constraints are discussed in detail, including the minimum input voltage, maximum mechanical current and the constraint of the PT configuration. Different from the typical optimal loading condition of the PT, the concept of the maximum mechanical current leads to a new optimal efficiency condition that is suitable for voltage regulation. Further, the ability of the regulation and the efficiency are found to be a trade-off. This paper proposes a simple criterion to obtain the good regulation and good efficiency at the same time. Finally, the completed design procedure is discussed by a given specification. The PT derived from the proposed design procedure can guarantee both good efficiency and enough range for voltage regulation.

Tailoring the performance of flexible loudspeakers by varying cell actuator formation

The AVID™ and the ESPI measurement systems were established to obtain the first resonance frequency and the corresponding vibration mode shape of an electret cell actuator. The FEA model was validated by experiment results. The size, form and configuration of an electret cell actuator were confirmed to be the essential factors to be taken into consideration to obtain a good design for good results.

Study on free-form electret actuators and its applications

A large-area curtain loudspeaker was fabricated in this paper. The objective of creating a large-area projection screen which possessed functions for excellent visual image scattering and sound radiation was successfully created. Our novel sound generating techniques were shown to be suitable for futuristic audio systems which may include implementation in smart living spaces, theaters, showrooms, conference rooms, etc. Our experimental results show that the sound volume, frequency bandwidth and sound quality generated from our free-form electret-based loudspeakers are excellent for music enjoyment considering its new novel shape and size.

Power improvement of piezoelectric transformer based DC/DC converter

In this paper a novel strategy to increase the power of piezoelectric transformer (PT) by using a cooling system is presented. A thermal pad is directly attached to a PT to dissipate the heat and enhance the power capacity. In a piezoelectric ceramic the heat increase quickly when the vibration velocity become too large. Therefore there is a limit in vibration velocity magnitude. To explain the relationship between the vibration velocity, the output current and the output power we propose a theoretical-phenomenological model based on nonlinear equivalent circuit. It will be shown that the vibration velocity and temperature influence the characteristics of the piezoelectric transformer significantly. A large vibration velocity may lead that the internally thermo-physical feedback loop of the PT enters into an unstable state. In this paper, a PT with the cooling system was implemented in a DC/DC converter. A zero voltage switching (ZVS) half-bridge is used to drive the PT and a full-wave rectifier is used to obtain the DC load voltage. As a result, the maximum power of the PT based DC/DC converter can be increased from 4.41 W to 10.2 W at specific temperature. The study comprises of a theoretical part and experimental proof-of-concept demonstration of the proposed system.