Development of micro built-in calibration pressure sensors with piezoelectric energy harvesters

It is a compelling concept that energy harvesting devices will increasingly enhance or even replace batteries for lower-power applications as low-power circuit design techniques and transducer energy efficiencies improve. Piezoelectric materials are promising candidates to make such devices, owing to their natural electromechanical coupling. A piezoelectric energy harvester usually consists of three major components: (1) a piezoelectric harvesting structure for generating electrical energies from ambient energy source, such as ambient acoustics/vibrations, (2) an energy storage device, typically an electrochemical battery, and (3) a modulating circuit that converts the generated AC current into a DC current for charging the battery efficiently.; We model piezoelectric energy harvesters as elecrro-mechanically coupled systems. The analysis indicates that the performance of the harvesters, measured by the power densities can be optimized by altering the mechanical structure configurations, modifying the structure geometries, or optimizing the electric circuits. The numerical results show that: (1) Being a harvesting structure, the piezoelectric bimorph with a concentrated mass attached at one end vibrating in the flexural mode is more efficient than a piezoelectric plate oscillating in the thickness-stretch mode due to a relatively low resonance frequency and high average strain of the bimorph for a given force input; (2) The power density increases dramatically if the structure geometry is chosen by satisfying that the first natural frequency of the harvesting structure matches that of the ambient acoustic vibration; (3) Introducing a switching inductor in parallel with the piezoelectric bimorph called synchronized switch harvesting inductor (SSHI) will significantly increase the power density. The efficiency of change depends on the external excitation lever. (4) An optimized electrical circuit using a step-down dc-dc converter can also increase the power density a lot.; Later, we design a wireless micro pressure sensor of Built-in calibration, with the commercially available MEMS fabrication technology for mass production and low unit cost. The design will enable in-situ calibration over a wireless network and with power being provided by a piezoelectric energy-harvesting device. The built-in automated-calibration and self-power can remove the manual procedures in the traditional calibration, and thus reduce or eliminate the maintenance cost.