טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentHar-Nes Idan
SubjectBandwidth expansion for piezoelectric energy harvesting
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Haim Abramovich
Full Thesis textFull thesis text - English Version


Abstract

The present research thesis is related to the conversion of mechanical vibrational energy to electrical energy using piezoelectric patches. The trend of monitoring the “health” of structures using various sensors requires supplying energy for the sensors, and one of the advanced ways is self-power using transducers which convert mechanical energy, mainly in the form of vibration into electrical energy. Most of those devices, called harvesters, make use of a cantilever type beam with piezoelectric layers bonded on it. A single bonded piezoelectric layer is called unimorph, while two layers sandwiching the carrying substrate beam is named bimorph. The harvester is attached to a vibrating platform causing it to vibrate and leading to the conversion of the vibrational energy into electrical energy in the form of electrical charge which using an electrical circuit composed diode bridge will charge a storage device , like capacitor or supercapacitor.  

The use of a single cantilever harvester as a harmonic oscillator to harvest vibrational energy is not effective due to its inherent narrow frequency bandwidth stemmed from the need to adjust the natural frequencies of the harvester to the platform excitation frequencies. Therefore, the present research focuses applying an advanced system with three bimorphs, lowering their natural frequencies by tip end masses and interconnected by springs, thus enlarging the system’s bandwidth. An analytical model was developed for the three bimorphs interconnected by two springs, with three end masses. The model is capable of predicting the output voltage from each bimorph and then the output power on a given outside resistor as a function of the material properties, the geometric dimensions of the vibrating beams, the end-masses and the spring constants. The analytical model was then compared with data in the literature yielding a good correlation. To further increase the reliability of the model, a test set-up was designed and manufactured which included three bimorphs with three end-masses connected by two springs. The system was excited using a shaker, and the output voltage was measured for each bimorph for various configurations. Then the analytical model was updated based on the test results by introducing two factors, the quality and the stiffness factors, and the predictions of the calibrated analytical model were compared with the experimental results yielding a good correlation.

The calibrated analytical model was used to perform a comprehensive parametric investigation for two and three bimorphs systems, in which the influences of various parameters like, spring constant, mass value, thickness, width and length of the bimorph and the substrate beam, on the output generated power were investigated.  

The main conclusion from this parametric investigation was that by correctly choosing the geometric sizes of the cantilevers, the adequate tip end masses and the ratio between constants of the springs, the frequency bandwidth is expanded yielding higher harvested power.

Typical harvested power of the present designed system can reach up to 20 [mW] at the first natural frequency and up to 5 [mW] for the second natural frequency.