Nowadays, most wireless sensor nodes and portable electronic devices are powered by traditional chemical batteries. To overcome the drawbacks of batteries such as limited operating life, expensive maintenance/replacement cost and chemical pollution, energy harvesters collecting electrical energy from a variety of ambient sources (solar energy, thermal energy, wind energy, and vibration energy) have received a great deal of attention in recent years [1
]. Various forms of miniaturized energy harvesters have been developed to power wireless sensor nodes and small electronic devices. For the relatively low output powers of the energy harvesters and the special requirements of the electrical loads on the electrical sources, a power management circuit (PMC) is essential to efficiently store and release the collected energy to the loads [8
]. Moreover, optimal data scheduling, admission control, anti-collision, and sensitive and efficient energy harvesting for backscatter sensor networks have been studied [11
]. The energy detection technology for Decision Fusion in multiple-input multiple-output (MIMO) Wireless Sensor Networks, the decision fusion algorithms, and distributed detection have been discussed [14
Different types of energy harvesters require various PMCs. For example, the standard energy-harvesting (SEH) circuit, synchronous charge extraction (SCE) circuit, parallel synchronized switch harvesting on inductor (P-SSHI) circuit, or series synchronized switch harvesting on inductor (S-SSHI) circuit have been proposed to manage the piezoelectric energy harvesters (PEHs) with the characteristics of high output voltage and low current [18
]. Electromagnetic energy harvesters (EMEHs) possess the characteristics of low output voltage and high output current. The voltage with hundreds of millivolts is generally lower than the forward conduction voltage of the rectifier diode or the working voltage of most wireless senor nodes. This complicates the PMC by an indispensable voltage-boosting element. A synchronous magnetic flux extraction (SMFE) circuit uses coil inductance to construct a boost direct current-direct current (DC–DC) converter circuit. However, the switch control of the SMFE circuit is not yet self-powered and requires an external power supply [22
]. In addition, the voltage multiplying circuit adopts a series of diodes and capacitors to boost the voltage. However, the multiplying circuit is complex and causes high power consumption, which decreases the charging speed [24
Three types of the models of electromagnetic energy harvesting systems have been developed. First, a linear electromagnetic energy harvester is connected to a linear interface circuit, which is a pure resistive electrical load [25
]. Second, a nonlinear electromagnetic energy harvester is connected to a linear circuit [26
]. Third, a linear electromagnetic energy harvester is connected to a nonlinear circuit containing non-resistive components [27
]. However, the model of a nonlinear electromagnetic energy harvester connected to a nonlinear circuit has not been investigated, which will be discussed in this paper.
Our group has developed an impact-based nonlinear EMEH with arrays of magnet and coil [26
]. The most prominent characteristic of this device is the high output voltage, with the voltage of 16 V across an 11 kΩ resistor under the base excitation of 0.3 g at 8.5 Hz. The resistive impedance-matching method is usually adopted to match the internal resistance of the harvester, neglecting the effects of non-resistive components [28
]. There are many ways to produce the control signals for the impedance matching. Lefeuvre et al. [28
] used an active crystal oscillator to produce a control signal to match the impedance of PEHs. Shen et al. [29
] used the active crystal oscillator to produce a control signal for the impedance matching of EMEHs. However, the operating voltage of the active crystal ranges from 1.6 V to 5.5 V, which puts forward additional requirement on the output voltage of the harvester. Kong et al. [30
] and Guo et al. [31
] used a square-wave-generating circuit to produce a control signal. The control circuit is powered by a storage element; as a result, once the storage energy is exhausted, the system cannot work normally again. Chen et al. [32
] adopted a square-wave-generating circuit to produce a control signal and used a capacitor after rectifiers to power the control circuit. A voltage stabilizer after the capacitor was used to stabilize the control signal, which would affect impedance matching and cause energy dissipation.
This paper proposes a nonlinear interface circuit model for the nonlinear EMEHs with high output voltage and designs an autonomous PMC to power a wireless sensor node. The PMC applies the average input resistance of the buck–boost converter to match the load resistance of the nonlinear interface circuit, instead of roughly matching the internal resistance of the harvester. The circuit mainly contains two open-loop branches. The first branch is the main branch powered by most of the coils of the EMEH to store the harvested energy, and the second branch is the control branch that is individually powered by the rest of the coils. The signal generated by the second branch controls the operation of the main branch to store the harvested energy. Furthermore, the experimental results demonstrate that, compared with the SEH circuit, the proposed PMC transfers more energy from the harvester to the load and improves the charging efficiency. The joint design of EMEH and PMC improves the whole performance of the system. In addition, a wireless sensor node powered by the nonlinear electromagnetic energy harvesting system is developed and tested.
The remainder of this paper is organized as follows. The structure of the EMEH and the nonlinear model are introduced in Section 2
. After giving an SEH circuit as a comparison, Section 3
presents the proposed power management circuit. Section 4
describes the constitution and operation process of the self-powered wireless sensor node. In Section 5
, the experimental results are presented and discussed. Finally, conclusions are drawn in Section 6
This paper proposed a PMC for an impact-based electromagnetic energy harvester to accumulate energy and power a wireless sensor node without any external power source. The model of the nonlinear electromagnetic energy harvester with a nonlinear interface circuit was investigated and simulated. The PMC applied a buck–boost DC–DC converter to match the load resistance of the nonlinear interface circuit, instead of roughly matching the internal resistance of the harvester. To take use of the coil array of EMEH, the PMC contained two open-loop branches, and a fixed duty cycle operation of the switching converter might provide resistance matching under low frequency excitations. The PMC operated well over a wide input voltage range. The experimental results demonstrated that, compared with the SEH circuit, the proposed PMC transferred more energy from the harvester to the load and improved the charging efficiency. The power transfer efficiency of 41.7% was obtained under the base excitation of 0.3 g at 8 Hz.
For simplicity, the rolling friction was neglected in the simulation in this paper. This effect should be further analyzed in the future. More works need to be conducted on the optimization method of the electrical parameters. In addition, to improve the adaptability of the harvesting system to different base excitations, a reconfigurable adaptive structure should be considered, which may automatically adjust the coil numbers of the two branches of the PMC and transfer excess energy from the control branch to the storage capacitor of the main branch.