Intensive research on energy harvesting powered wireless sensor nodes (WSNs) has been driven by the needs of reducing the power consumption by the WSNs and increasing the power generated by energy harvesters. The mismatch between the energy generated by the harvesters and the energy demanded by the WSNs is always a bottleneck as the ambient environmental energy is limited and time varying. This paper introduces a combined energy-aware interface with an energy-aware program to deal with the mismatch through managing the energy flow from the energy storage capacitor to the WSNs. These two energy-aware approaches were implemented in a custom developed vibration energy harvesting powered WSN. The experimental results show that, with the 3.2-mW power generated by a piezoelectric energy harvester under an emulated aircraft wing strain loading of 600 με at 10 Hz, the combined energy-aware approaches enable the WSN to have a significantly reduced sleep current from 28.3 μA of a commercial WSN to 0.95 μA and enable the WSN operations for a long active time of about 1.15 s in every 7.79 s to sample and transmit a large number of data (388 B), rather than a few ten milliseconds and a few bytes, as demanded by vibration measurement. When the approach was not used, the same amount of energy harvested was not able to power the WSN to start, not mentioning to enabling the WSN operation, which highlighted the importance and the value of the energy-aware approaches in enabling energy harvesting powered WSN operation successfully.

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record.The continuous progress made in wearable energy harvesting technology is delivering sophisticated devices with increasing power output, which are possible to provide sustainable energy supply for body sensors to achieve an energy-autonomous wireless sensing system. This paper reports the development and characterisation of a wearable energy harvesting powered wireless sensing system with system-level strategies to address the challenges in energy harvesting, power conditioning, wireless sensing and their integration into a system. The system comprises four parts: (1) a magnetically plucked wearable knee-joint energy harvester (Mag-WKEH) to scavenge energy from knee-joint motion during human walking, (2) a power management module (PMM) with maximum power point tracking (MPPT), (3) an energy-aware interface (EAI) for dealing with mismatch between energy generated and energy demanded, and (4) an energy-aware wireless sensor node (WSN) for data sensing and transmitting. Experiments were performed with a human subject wearing the system walking on a treadmill at different speeds. The experimental results showed that as the walking speed increased from 3 to 7 km/h, the power output of the Mag-WKEH increased from 1.9±0.12 to 4.5±0.35 mW, and the generated power was able to power the WSN to work at a duty cycle from 6.6±0.36% to 13±0.5% with an active time of 2.0±0.1s. In each active time, the WSN was able to sample 482 readings with an interval of 10 ms from the sensors and transmit all data to a base station at a distance of 4 m.The authors gratefully acknowledge financial support from the Engineering and Physical Sciences Research Council (EPSRC) in the UK through funding of the research into ‘smart multifunctional architecture & technology for energy aware wireless sensors’ (EP/K017950/2)

Maximum power transfer occurs in many energy harvesters at their half open-circuit voltage (VOC/2). A novel implementation method of maximum power point finding based on the VOC/2 method is presented by exploiting the capacitor charging voltage across a capacitor connected in parallel with the energy harvester. The presented technique has a specifically designed high-pass filter which has a peak output voltage that corresponds to the VOC/2 of the energy harvester. The control circuit filters and differentiates the voltage across the smoothing capacitor to directly determine the timing of reaching the VOC/2 of the energy harvester without having to find the VOC first, and is fully implemented using discrete analog components without the need of a microcontroller, leading to low power consumption of the method. In this paper, the control circuit is used in conjunction with a full wave diode bridge rectifier and a dc-dc converter to harvest energy from a piezoelectric energy harvester (PEH) as the studied case. The PEH was subjected to various strain levels at low frequencies from 2 to 10 Hz. Experimental results show that the implemented circuit is adaptive to various vibration amplitudes and frequencies and has a maximum power point finding efficiency of up to 98.28% with power consumption as low as 5.16 μW.

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.

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.