Meiling Zhu

This article presents a self-configurable and self-poweredactive rectifier that operates from 0.25–20 V for energy harvesting applications. The proposed circuit self-startups from a low voltage using a charge pump and amplifies the voltage with a voltage doubler topology to provide succeeding circuits such as boost converters with a higher voltage. When the voltage of the energy harvester reaches a high threshold, the circuit switches its topology to a full-wave rectifier that does not amplify the voltage. The start-up circuit can limit its voltage intake to prevent boosting the high voltage, which may damage the whole circuit. Comparators with a maximum operating voltage of 5.5 V used in the implementation of the rectifier are protected by a diode and resistor based circuit. A piezoelectric energy harvester (PEH) that has a wide open-circuit voltage of 0.4–15 V under the acceleration of 0.04–0.3 g was used to test the circuit. The experiment results showed the rectifier can startup from 0.25 V and switch its topology according to the PEH voltage. The voltage and power conversion efficiencies are over 90% in most cases.

Magnetic field energy harvesters (MFEHs) from current-carrying structures/conductors are usually modelled as decoupled electromagnetic and electrical systems. The current-carrying structures may affect the performance of MFEH through the generation of the eddy current and the alteration of the magnetic reluctance. Moreover, the load circuit affects the current generated in the coil and therefore the flux density and eddy current generated. The effects of the current-carrying structure and the load circuit cannot be fully described by the decoupled models. This work develops a finite element model (FEM) that fully couples the electromagnetic and electrical systems by simulating both the magnetic field and eddy current distribution of an MFEH connected to an electrical circuit. The FEM first simulates the coil inductance and resistance of a magnetic field energy harvester (MFEH) placed close to a current-carrying structure exemplified by a rail track. The FEM then simulates the outputs of the MFEH connected to an electrical circuit consisting of a compensating capacitor and optimal load resistor determined by the first step. An MFEH was fabricated and tested under a section of current-carrying rail track. Both experiment and simulation show an increase of both coil resistance and inductance when the MFEH is placed close to the rail track. The good agreement between experimental and simulation results validates that the FEM can predict the full-matrix performances of the MFEH, including the coil parameters, power output and magnetic flux density under the influence of the current-carrying structure and the load circuit. Simulation results reveal that in addition to the permeability of the magnetic core, the electrical conductivity and magnetic permeability of the current-carrying structure considerably affect the performance of the MFEH, which cannot be predicted by decoupled models.

This paper presents methods that enable batteryless energy harvesting powered Time Synchronized Channel Hopping (TSCH) wireless sensor nodes to join a network with less energy wastage. Network joining of wireless sensor nodes is a very power hungry yet inevitable process to form a working wireless sensor network (WSN). Since the energy level from energy harvesting is scarce, low energy methods are essential. A duty-cycled network joining process in combination with an appropriate capacitor size is proposed here as they are among the factors that can be easily controlled without extra energy. When a node joins the network in a duty-cycled manner, other nodes may join the network during the gap time, which reduces energy wastage of the nodes in waiting. With an appropriate capacitor size, the capacitor can be charged up within a reasonable time and power up the node for a sufficiently long time, which increases the probability to complete the network joining process of the node. With the combination of a join duty cycle of 50% with a 100 mF capacitor, a WSN was successfully formed by two energy harvesting powered wireless sensor nodes in one network joining attempt.

This paper presents an adaptive self-configurable rectifier to increase the range of vibration excitation that a piezoelectric energy harvesting power management circuit (PMC) can operate with. The proposed circuit configures itself as a voltage doubler (VD) to increase the low voltage of piezoelectric energy harvesters to meet the minimum operating voltage of the PMC. This allows energy harvesting from low voltage range that is not viable using a full-wave bridge (FB) rectifier. When the piezoelectric voltage increases and causes the voltage amplified by the VD to approach the maximum operating voltage of the PMC, the proposed circuit reconfigures itself to FB rectifier that does not amplify the voltage. This allows operation at a higher voltage range that is restricted by the VD topology. Two different reference voltages each for the VD topology and the FB rectifier topology with low-pass filtering are used to ensure correct switching of the rectifier topology. The proposed control circuit can cold start from a piezoelectric voltage of 1.2V in the VD topology to amplify the voltage to allow an existing PMC to operate, while consuming 240-410nW of power. With the two-reference-voltages control method, 20% more energy can be transferred through the PMC than the one without it.

This paper presents an adaptive power management circuit which maximizes the energy transfer from the energy harvester to wireless sensor nodes in real world applications. Low power consumption techniques were adopted in the power management circuit to maximize the delivery of the harvested energy to the load instead of being consumed by the circuit. The presented circuit incorporates an analogue control circuit (ACC) for maximum power transfer from the energy harvester to the storage capacitor and an energy-aware interface (EAI) for controlling the energy flow from the storage capacitor to the load. To evaluate the performance of the presented circuit, piezoelectric energy harvesting was used as a studied case. The piezoelectric energy harvester (PEH) was mechanically excited at different strain loadings and frequencies. The experimental results show the circuit can self-start and powered directly by the PEH since the EAI and ACC have low power consumption in the range of micorwatts. The circuit is adaptive to energy harvesters of varying output and various electrical loads, with a peak efficiency of 76.18 % in transferring the harvested energy from the PEH to the storage capacitor. More than 96% of the energy released from the storage capacitor is effectively transferred to the electrical load.