PIERS Proceedings, Stockholm, Sweden, Aug. 12–15, 2013
1744
An Energy Management Circuit Based on Up-conversion to Power
Wireless Sensor Nodes
S. Q. Pan, P. Li, Y. M. Wen, Z. Q. Zhang, D. Lu, and D. F. Sun
Research Center of Sensors and Instruments, Department of Optoelectronic Engineering
Chongqing University, Chongqing 400044, China
Abstract— This paper presents an up-conversion energy management circuit for harvesting
electrical energy induced from the current-carrying electric cord and driving wireless sensor nodes.
Since the low frequency signal induced from the electric cord is up-converted into a new one
focused on a higher frequency by the management circuit the energy harvesting power of the
management circuit can be improved. More energy can be accumulated in the storage capacitor
and the charging time can also be improved. It is demonstrated that the maximum energy
harvesting efficiency of the management circuit can reach to 91.67%. Under 1 A current through
the electric cord circumstances, the energy stored in the storage capacitor can be released to drive
a wireless sensor node, whose power consumptions are 18 mW in acquiring data mode (199 ms)
and 54 mW in transmitting data mode (1 ms) at a communication distance of 20 m, respectively.
1. INTRODUCTION
Nowadays, as indispensable facilities, electric cords need to be monitored for safe and reliable
operation. Since electric cords are generally installed in unapproachable environments, it is hard
and inconvenient to monitor the electric cords by traditional approaches. Wireless sensor nodes
(WSNs) possess many characteristics, such as low power consumption, wireless connection, and
handy installation [1]. Hence, it is not complicated to monitor the electric cords by WSNs. The
power supplies are significant to the operation lifespan of WSNs. The conventional power supplies
are batteries, but batteries have many disadvantages including limited energy and bulky size [2].
Harvesting ambient energy as the power supplies of WSNs is gradually becoming an excellent
alternative solution to batteries [3].
Electromagnetic energy distributes around the current-carrying electric cord and can be scavenged as the energy source of WSNs by the energy harvester. An energy management circuit is
necessary to store and govern the energy from transducer. Traditional energy management circuit
is composed of an ac-dc rectifier, a storage capacitor, and a dc-dc converter [4]. An input voltage
of 120 mV can be converted to an output voltage of 1.2 V with a maximum efficiency of about 30%
by the proposed circuit [5]. Different kinds of interface circuits based on synchronized switch harvesting on inductor (SSHI) technique are developed to improve energy harvesting efficiency [6, 7].
In these SSHI circuits more power consumptions can be caused by the auxiliary devices including
voltage detection sensor and microcontroller. However, due to weak outputs of transducers and
poor impedance matching of the aforementioned management circuits, it is difficult to accumulate
adequate energy and drive WSNs under weak current through the electric cord circumstances. A
high efficiency energy management circuit is necessary to harvest enough power from the currentcarrying electric cords for WSNs.
This paper describes an energy management circuit based on up-conversion for scavenging electrical energy induced from the electric cord. The accumulated energy by the proposed management
circuit can drive the WSN with the power consumption of 54 mW under 1 A current through the
electric cord circumstances.
2. ENERGY MANAGEMENT CIRCUIT
An electromagnetic transducer is proposed to convert electromagnetic energy distributing around
the current-carrying electric cord into electrical energy, as shown in Figure 1. The output power
and voltage of the harvesting coil (L2 in Figure 1) change as a function of load resistance under
1 A current through the electric cord circumstances, as shown in Figure 2. The output voltage
increases with load resistance and the maximum output power of 0.72 mW can be obtained under
a resistance of 8 kΩ. For a large storage capacitor, the output power is weak due to capacitive load.
Hence, a high efficiency energy management circuit must be developed to achieve good impedance
matching and accumulate more energy.
Progress In Electromagnetics Research Symposium Proceedings, Stockholm, Sweden, Aug. 12-15, 2013 1745
Figure 1: Electromagnetic transducer.
Figure 2: Output power and voltage as a function
of load resistance.
The electromagnetic transducers are installed in the 220 V distribution box, as shown in Figure 3(a). The energy management circuit is composed of a matching circuit, an up-conversion
circuit, a rectifier, a storage capacitor, and a regulator, as shown in Figure 3(b). The proposed
matching circuit can achieve good impedance matching by converting a low frequency signal into
a high frequency signal.
(a)
(b)
Figure 3: (a) Transducers installed in distribution
box. (b) Schematic diagram of management circuit.
Figure 4: Equivalent management circuit.
The equivalent management circuit is shown in Figure 4. The current-carrying electric cord can
be equivalent to a serial circuit with a current source and an inductor with only one turn of coil.
S is an ultra-low power consumption bidirectional switch, which is opened or closed by a trigger
signal generated by a rectangular wave generating circuit (1 µA). C2 is the matching capacitor to
produce damped oscillating response with inductor L2 . The signal of ω0 induced from electric cord
can be up-converted into a signal focused on new frequencies (ω0 , ω1 , and ω2 ). The voltage across
capacitor C2 can be expressed as
½
Ae−α1 t sin (ω1 t + γ1 ) + C sin ω0 t. S is on
vc2 (t) =
(1)
Be−α2 t sin (ω2 t + γ2 ) + D sin ω0 t. S is off
It is evident that vc2 (t) contains a damped oscillating signal of ω1 (or ω2 ) and a sinusoidal signal
of ω0 when S is on (or off). When S is controlled by a trigger signal with a frequency of 114 Hz
and a duty cycle of 50%, the output voltage across capacitor C2 is shown in Figure 5. The 50 Hz
ac signal is up-converted into a new one mainly focused on 4.72 kHz by the management circuit.
PIERS Proceedings, Stockholm, Sweden, Aug. 12–15, 2013
1746
Figure 5: Voltage across capacitor C2 .
Figure 6: Charging power and voltage of storage
capacitor.
3. EXPERIMENT
Under 1 A current through the electric cord circumstances, the charging power and voltage of
storage capacitor (0.1 F) are shown in Figure 6. The storage capacitor can be charged to 3.3 V at
a charging time of 30 minutes. The maximum charging power of 0.66 mW can be obtained when
the voltage across storage capacitor is 1.1 V. Hence, the maximum energy harvesting efficiency of
the management circuit is
η = (0.66 mW/0.72 mW) × 100% = 91.67%
(2)
The WSN is composed of a wireless System-on-Chip (nRF24LE1) and sensor units (DS600).
The power consumptions of receiving data (199 ms) and transmitting data (1 ms) are 18 mW and
54 mW, respectively. The necessary energy of WSN in an operation period is
En = 54 mW × 0.001 s + 18 mW × 0.199 s = 3.64 mJ
(3)
The experimental results show that the voltage across storage capacitor drops from 3.16 V to 3.11 V
after transmitting data. Thus, the energy provided by the management circuit in a discharging
period is
h
i
Eo = (3.16 V)2 − (3.11 V)2 × 0.1 F/2 = 15.68 mJ
(4)
From Equations (3) and (4), Eo > En . Hence, the scavenged energy from the electric cord by the
management circuit can drive the WSN under 1 A current through the electric cord circumstances.
4. CONCLUSION
In this paper, an electromagnetic energy management circuit based on up-conversion is proposed
to harvest energy from electric cord and drive WSNs. The management circuit can achieve good
impedance matching. Under 1 A current through the electric cord circumstances, the maximum
energy harvesting efficiency is 91.67% and the storage capacitor can be charged to 3.3 V within
30 minutes. The weak energy from electric cord can be accumulated to drive the WSN with an
output power of 54 mW at a distance of 20 m. The proposed approach based on up-conversion
technique can also be applied in other energy harvesting circuits to harvest weak energy from
transducers.
ACKNOWLEDGMENT
This work is financially supported by the National Natural Science Foundation of China (Grant
Nos. 50830202 and 61071042) and the National High Technology Research and Development Program of China (863 Program) (No. 2012AA040602).
Progress In Electromagnetics Research Symposium Proceedings, Stockholm, Sweden, Aug. 12-15, 2013 1747
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