Piezoelectric energy harvesting interface circuits for energy autonomous sensors
Koottungal Paramba, Arish Shareef
Date of Issue2017
Interdisciplinary Graduate School
Energy Research Institute @ NTU
A*STAR Institute of Microelectronics
The Internet of Things (IoT) era has unlocked a new dimension for interconnected devices. Numerous sensors are being deployed around us for applications such as environmental, structural health and human health monitoring. These sensors are sometimes expected to work in a deploy-and-forget mode, in which accessing them for replacing their power source might not be a feasible option. Though energy harvesting has been in existence for a long time, it is only in the recent past that they have been increasingly applied for powering miniature sensor nodes, mainly due to the ultra-low power nature of the contemporary sensors. Harvesting energy from ambient sources such as light, heat, motion and radio waves is an attractive alternative to perishable batteries. Though energy from light has the highest power density, the availability of the same cannot be guaranteed for all environments. Harvesting energy from motion, especially vibration, has garnered much research interest due to their ubiquitous nature. Vibrational energy is often harvested using three methods viz. Electromagnetic, Electrostatic and Piezoelectric, the latter being the focus of this thesis. Harvesting vibrational energy using Piezoelectric Transducers (PET) is highly desirable owing to their higher power density amongst the three methods. However, in space-constrained designs, it becomes imperative to harvest the last bit of energy left in the tiny transducer. MEMS based transducers subjected to ambient vibrations typically produce low voltages, which are not efficiently harvested using conventional diode based rectifier circuits. Moreover, the voltage output of a PET is highly susceptible to the rapidly varying frequency of ambient vibrations, causing the voltage to drop during off-resonant conditions. Impedance matching is essential to maximize the power output from any energy source and performing the same in low power environments is a challenge due to the power overhead involved. Non-linear processing techniques have been proposed in the past, which aim to enhance the power output from a PET. The work in thesis is based on Synchronous Electric Charge Extraction (SECE), which is one of the many non-linear techniques available. This thesis introduces three rectifier-less topologies based on the SECE technique. The rectifier-less feature in all the three topologies is implemented using a bi-directional switching converter. The first topology is capable of harvesting energy from a single PET with a measured peak efficiency of 73 % in the input voltage range of 0.5 V – 1.75 V. The second topology achieves a peak efficiency of 80 % and targets the same voltage range, but combines multiple PET sources in an inductor-sharing configuration. The combination of multiple PETs operating at different resonant frequencies widens the operating frequency range of the Piezoelectric Energy Harvesting (PEH) system. The third topology has a slightly wider input voltage range of 1.1 – 3.5 V and achieves efficiencies up to 79 % in simulations. Two complete PEH systems have been developed using the proposed rectifier-less, SECE topologies. The first PEH system is for a single PET, whereas the second is for multiple PETs. The efficacy of the multiple input PEH system is verified using three commercially available piezoelectric cantilevers. The PEH systems are implemented on a PCB using discrete components. Performance data from the PCB prototype demonstrate a peak power-conversion efficiency of 80 % in agreement with simulations. The PEH systems are self-powered, battery-less, and self-starting. The start-up circuit kick-starts the system from a zero-energy state with an input voltage of 650 mV. During the harvesting process, the control circuits consume an intrinsic power of 7 µW at 2.7 V. In the absence of vibrations, the system draws a standby power of only 2.6 µW at 2.7 V. In comparison to other state-of-the-art PEH systems, the proposed system achieves comparable or better power conversion efficiencies by using a small sized inductor.
DRNTU::Engineering::Electrical and electronic engineering::Power electronics