Session: 08-02 Alternative Energy Conversion Techniques

Paper Number: 114615

114615 - Mems Electrostatic Energy Harvesters With Dual Frequency Up-Conversion Techniques 

With the rise of smart home applications, there is an increased need for power harvesters at the microscale to support their sensors and associated circuitry. One way of powering these types of devices is through harvesting the ambient vibrational energy which is omnipresent. There is a plethora of vibrational energy harvesters, including electromagnetic, piezoelectric and electrostatic vibrational energy harvesters (E-VEHS). E-VEHS are variable capacitors fabricated using microfabrication technologies. This makes them attractive due to their potential for on-chip integration with sensors and integrated circuits. However, current electrostatic energy harvesting MEMS devices show limited bandwidth and power output. Thus, there is a need for an improved design able to produce usable energy output.

The objective of this research is to investigate a novel way to increase the response bandwidth and power output of E-VEH based on frequency up-conversion techniques. These devices have a spring-supported shuttle mass (large area silicon layer) with finger like electrodes on its edges. The variable capacitor is formed between the finger electrodes attached to the mobile mass and their pairs anchored to the substrate. The frequency up-conversion refers to a higher frequency response of the device when actuated by a lower frequency excitation. In this work, frequency up-conversion is triggered by two approaches implemented simultaneously: 1) collision between electrodes, designed to be trapezoidal to prevent pull-in; and 2) collision of a springless mass, namely a microsize ball free to move and placed within a cavity of the shuttle mass.

The MEMS device was fabricated using SOIMUMPS fabrication by MEMSCAP. They were then mounted and wire bonded to a custom built PCB board for testing. To understand the device response to vibrations, the PCB was installed on a shaker table and connected to a DC bias voltage supply and a series resistor. The voltage output across the resistor was recorded as a function of frequency and amplitude of the applied vibration, as well as applied DC bias voltage. Tests include frequency sweep and ringing testing. Ringing testing is performed to understand the oscillation triggered after impact, which can be harvested to produce power beyond that produced in the absence of ringing.  Ringing with and without a microball was measured to see the effect of a microball on frequency up-conversion. The ringing output of the device tested with the microballs shows an increase in the output voltage peaks. To understand the impact of different microball sizes and materials, a frequency sweep was performed while monitoring the voltage output. Tests were performed with 0.5mm Tungsten, 0.8mm Zirconia, and 0.8mm Silicon Nitride microballs, respectively. Frequency sweeps performed with all microballs showed an increase in device bandwidth up to twice the bandwidth of the device without a microball. Of all microball diameters and materials tested, the Zirconia 0.8mm ball performed the best in both voltage RMS output and increased bandwidth. These results show a promising approach to further increase the power output of electrostatic harvesters.

Presenting Author: Hannah Arnow Rensselaer Polytechnic Institute

Presenting Author Biography: Hannah Arnow is a PhD student studying Mechanical Engineering at Rensselaer Polytechnic Institute focusing on energy harvesting systems. Specifically, her research explores ways to improve energy conversion in emerging systems, such as electrostatic vibrational energy harvesters and luminescent solar concentrators.