The goal of this paper is to review current methods of energy harvesting, while focusing on piezoelectric energy harvesting. The piezoelectric energy harvesting technique is based on the materials’ property of generating an electric field when a mechanical force is applied. This phenomenon is known as the direct piezoelectric effect. Piezoelectric transducers can be of different shapes and materials, making them suitable for a multitude of applications. To optimize the use of piezoelectric devices in applications, a model is needed to observe the behavior in the time and frequency domain. In addition to different aspects of piezoelectric modeling, this paper also presents several circuits used to maximize the energy harvested.
Multilayer Ceramic Capacitors (MLCC) have a major role in modern electronic devices due to their small price and size, large range of capacitance, small ESL and ESR, and good frequency response. Unfortunately, the main dielectric material used for MLCCs, Barium Titanate, makes the capacitors vibrate due to the piezoelectric and electrostrictive effects. This vibration is transferred to the PCB, making it resonate in the audible range of 20 Hz–20 kHz, and in this way the singing capacitors phenomenon occurs. This phenomenon is usually measured with a microphone, to measure the sound pressure level, or with a Laser Doppler Vibrometer (LDV), to measure the vibration. Besides this, other methods are mentioned in the literature, for example, the optical fiber and the active excitation method. There are several solutions to attenuate or even eliminate the acoustic noise caused by MLCC. Specially designed capacitors for low acoustic levels and different layout geometries are only two options found in the literature. To prevent the singing capacitor phenomenon, different simulations can be performed, the harmonic analysis being the most popular technique. This paper is an up-to-date review of the acoustic noise caused by MLCCs in electronic devices, containing measurements methodologies, solutions, and simulation methods.
The goal of this paper is to review up to date energy harvesting techniques, while focusing on energy harvesting with piezoelectric materials. A classification of various energy harvesting sources is provided in order to properly locate piezoelectricity. Piezoelectric energy harvesting uses the special material property that exists in many single crystalline materials: the direct piezoelectric effect. Those materials are generating electric potential when mechanical stress is applied. There are two types of mechanical stress suitable for piezoelectric energy harvesting: hitting and vibrating. The hitting method involves the direct transfer of energy to piezoelectric modules, so it generates more power than the vibrating method. This kind of energy harvesting is used to drive low energy consuming devices and is suitable for applications where replacement of battery or maintenance is unpractical, like sensors in the human body, for powering portable devices or it can be used for improvement of a smart building concept. If the piezoelectric transducers are placed in the floor of a crowded area or in shoes, it can theoretically generate 4.9 J/Step; therefore, this energy can be used to replace the chargeable batteries. This review is useful for a proper positioning of this type in the IoT broad context and mainly as an alternate energy source for wearables.
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