Threshold voltage control to improve energy utilization efficiency of a power management circuit for energy harvesting applications

This is the author accepted manuscript. The final version is available from MDPI via the DOI in this record.Eurosensors 2018 Conference, 19-12 September 2018, Graz, AustriaThis work presents a design approach that improves power management circuit (PMC) for energy harvesting applications so that more of the harvested energy can be utilized by the wireless sensor nodes (WSNs) to perform useful tasks. The proposed method is widely applicable to different circuits by setting an appropriate threshold voltage at the energy flow control interface of the circuit. Experimental results show that with a threshold voltage difference of around 20 mV, the energy output from the PMC can differ by more than 5%. This difference is significant over a long period of time as more tasks can be performed by the WSN with the extra energy.This work has been partly supported by the Engineering and Physical Sciences Research Council, U.K., through the project En-ComE under Grant EP/K020331/1 and Innovate UK through the project Multi-source power management to enable autonomous micro energy harvesting systems

Strain Energy Harvesting Powered Wireless Sensor Node for Aircraft Structural Health Monitoring

This is the final version of the article. Available from Elsevier via the DOI in this record.Proceedings of the 30th anniversary Eurosensors Conference – Eurosensors 2016, 4-7. Sepember 2016, Budapest, HungaryThis paper presents a wireless sensor node (WSN) powered by a strain energy harvester (SEH) through an adaptive power management module (PMM) for aircraft structural health monitoring (SHM). The energy distribution in the system, the efficiencies of the whole systems, and the WSN powering capability of the SEH under different strain loadings were studied to understand the developed system performance for practical applications of an autonomous WSN. Experimental results show that the SEH is able to produce up to 3.34 mW under strain loading of 600 μɛ at 10 Hz. The WSN can be powered up through the adaptive PMM at efficiency from 70 to 80% under different test conditions.The authors gratefully acknowledge financial support from EPSRC in the UK through funding of the research into EPSRC via the project entitled “En-ComE” (EP/K020331/1)

Energy Harvesting Powered Wireless Sensor Nodes With Energy Efficient Network Joining Strategies

This is the author accepted manuscript. The final version is available from IEEE via the DOI in this recordThis paper presents strategies for batteryless energy harvesting powered wireless sensor nodes based on IEEE 802.15.4e standard to join the network successfully with minimal attempts, which minimizes energy wastage. This includes using a well-sized capacitor and different duty cycles for the network joining. Experimental results showed a wireless sensor node that uses a 100 mF energy storage capacitor can usually join the network in one attempt but multiple attempts may be needed if it uses smaller capacitances especially when the harvested power is low. With a duty-cycled network joining, the time required to form a network is shorter, which reduces the overall energy usage of the nodes in joining the network. An energy harvesting powered wireless sensor network (WSN) was successfully formed in one attempt by using the proposed methods.Engineering and Physical Sciences Research Council (EPSRC

Energy autonomous wireless sensing system enabled by energy generated during human walking

This is the final version of the article. Available from the publisher via the DOI in this record.PowerMEMS 2016,December 6 – 9, 2016. The 16th International Conference on Micro and Nanotechnology for Power Generation and Energy Conversion Applications, Paris, FranceRecently, there has been a huge amount of work devoted to wearable energy harvesting (WEH) in a bid to establish energy autonomous wireless sensing systems for a range of health monitoring applications. However, limited work has been performed to implement and test such systems in real-world settings. This paper reports the development and real-world characterisation of a magnetically plucked wearable knee-joint energy harvester (Mag-WKEH) powered wireless sensing system, which integrates our latest research progresses in WEH, power conditioning and wireless sensing to achieve high energy efficiency. Experimental results demonstrate that with walking speeds of 3~7 km/h, the Mag-WKEH generates average power of 1.9~4.5 mW with unnoticeable impact on the wearer and is able to power the wireless sensor node (WSN) with three sensors to work at duty cycles of 6.6%~13%. In each active period of 2 s, the WSN is able to measure and transmit 482 readings to the base stationThe authors gratefully acknowledge the financial support from the EPSRC through project EP/K017950/2

Adaptive Maximum Power Point Finding Using Direct VOC/2 Tracking Method with Microwatt Power Consumption for Energy Harvesting

This is the author accepted manuscript. The final version is available from IEEE via the DOI in this record.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.Funding Agency: Engineering and Physical Sciences Research Counci

Energy-aware approaches for energy harvesting powered wireless sensor nodes

This is the author accepted manuscript. The final version is available from IEEE via the DOI in this record.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 the 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 (EAI) 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 (PEH) 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 bytes), 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

Power Management Circuit for Wireless Sensor Nodes Powered by Energy Harvesting: On the Synergy of Harvester and Load

This is the author accepted manuscript. The final version is available from IEEE via the DOI in this record.Data availability: All data are provided in full in the results section of this paper.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.Engineering and Physical Sciences Research Council (EPSRC

Strain Energy Harvesting Powered Wireless Sensor System Using Adaptive and Energy-Aware Interface for Enhanced Performance

This is the author accepted manuscript. The final version is available from IEEE via the DOI in this record.This paper presents a wireless sensor system (WSS) powered by a strain energy harvester (SEH) through the introduction of an adaptive and energy-aware interface for enhanced performance under variable vibration conditions. The interface is realized by an adaptive power management module (PMM) for maximum power transfer under different loading conditions and an energy-aware interface (EAI) which manages the energy flow from the storage capacitor to the WSS for dealing with the mismatch between energy demanded and energy harvested. The focus is to realize high harvested power and high efficiency of the system under variable vibration conditions, and an aircraft wing structure is taken as a study scenario. The SEH powered WSS was tested under different peak-to-peak strain loadings from 300 to 600 µε and vibrational frequencies from 2 to 10 Hz to verify the system performance on energy generation and distribution, system efficiency, and capability of powering a custom-developed WSS. Comparative studies of using different circuit configurations with and without the interface were also performed to verify the advantages of the introduced interface. Experimental results showed that under the applied loading of 600 µε at 10 Hz, the SEH generates 0.5 mW of power without the interface while having around 670 % increase to 3.38 mW with the interface, which highlights the value of the interface. The implemented system has an overall efficiency of 70 to 80 %, a long active time of more than 1 s, and duty cycle of up to 11.85 % for vibration measurement under all the tested conditions.This work was supported in part by the Engineering and Physical Sciences Research Council, U.K., through the project En-ComE-Energy Harvesting Powered Wireless Monitoring Systems Based on Integrated Smart Composite Structures and Energy-Aware Architecture under Grant EP/K020331/1. All data are provided in full in the results section of this paper

Magnetic field energy harvesting from current-carrying structures: electromagnetic-circuit coupled model, validation and application

This is the final version. Available on open access from IEEE via the DOI in this record. 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.Engineering and Physical Sciences Research Council (EPSRC)Royal Societ

A high-power, robust piezoelectric energy harvester for wireless sensor networks in railway applications

This is the final version. Available on open access from Elsevier via the DOI in this recordData availability: Data will be made available on request.Piezoelectric energy harvesting techniques are increasingly seen as promising power sources for wireless sensor networks that monitor railway infrastructure. However, the piezoelectric generators currently available for railway applications suffer from low power output, as well as inadequate durability and robustness. To tackle these issues, this study introduces a novel, high-power, sturdy piezo stack energy harvester's design, optimization, and testing for powering wireless sensor networks in rail systems. The aim is to improve both the power output and the durability and robustness of the device. The proposed harvester's high-power generation is facilitated by a frequency up-conversion mechanism, mechanical transformer design and optimization, and the application of the piezo stack's compression mode (d33 mode). The frequency up-conversion mechanism allows the harvester to function at low-frequency track vibrations with high power. The mechanical transformer significantly magnifies the force exerted on the piezo stack. The compression mode boots the energy conversion efficiency due to its higher coupling factor. To enhance durability and robustness, innovative approaches are employed. The mechanical transformer is optimized for maximum energy transmission efficiency without exceeding the material's fatigue limit. Moreover, the piezo stack is designed to operate under pre-compression, preventing tensile stress and taking advantage of the piezoelectric ceramics' remarkable compressive strength. Plate springs are also integrated into the mechanical transformer to maintain motion along the vibration direction. Experimental results from prototype testing provide strong evidence for the high-power output of the proposed harvester and its ability to power a wireless sensor. A maximum power of 511 mW and an average power of 24.5 mW are achieved at a harmonic excitation with 21 Hz and 0.7 RMS (Root Mean Square) g, while a maximum power of 568 mW and an average power of 7.3 mW are generated under a measured railway track vibration signal.Engineering and Physical Sciences Research Council (EPSRC)University of Exete