Download Self-contained Self-powered Wireless Sensing Node for AC

PIERS Proceedings, Stockholm, Sweden, Aug. 12–15, 2013
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Self-contained Self-powered Wireless Sensing Node for AC Power
Supply Cords Monitoring
D. Lu, Y. Wen, P. Li, S. Pan, and Z. Zhang
Research Center of Sensors and Instruments, Department of Optoelectronic Engineering
Chongqing University, Chongqing 400044, China
Abstract— This paper presents a self-contained self-powered wireless sensing node for AC
power supply cords monitoring. It consists of an energy harvester, a current sensor, a temperature
sensor and a wireless module. The underlying component of the node is a magnetic split-core,
which readily embraces the monitored cord in operation. There are two sets of coils wound around
the core, one is sensing the current and another is scavenging the magnetic energy originated
from the carried-on AC current. The output of the energy harvesting coil is connected with
the power management circuit to produce adequate DC supply for the wireless sensing node.
The outputs of the current and temperature sensors are connected to the module for wireless
transmission. The node is demonstrated to monitor AC power supply cord with carrying current
from 1 A to 100 A of electric appliances exclusive of externally physical connections. With the
proposed wireless sensing nodes, a wireless sensor network can achieve distributed monitoring of
the AC power supply cords.
1. INTRODUCTION
Energy harvesting is thought to be a promising solution for sustainably powering wireless sensors.
As a matter of fact, in general ambience, it is quite difficult to sustainably scavenge adequate energy
from ambient sources for power supply. However there are guaranteed electric/magnetic fields along
power cords carrying AC currents. Based on this fact, quantities of researches have been done on
the self-powered sensor systems, which scavenge the electromagnetic energy originated from the
carried-on AC current [1].
Harvesting the energy aroused from AC current carrying power lines have been designed for
the electric power monitoring systems of power transmission lines [2] and substations [3], in which
cases, the line voltage reaches kV-level. There are also self-powered sensors applied in the power
supply cords, whose voltage is typically 110 V ∼ 380 V [4, 5]. In the case of wireless sensing of power
cord condition, a battery usually has to be contained in the wireless node [6, 7]. Here, for wireless
monitoring low-voltage AC power supply cords of electric appliances, a self-contained self-powered
wireless sensing node is proposed.
The self-contained self-powered wireless sensing node wirelessly transmits the outputs of the
current and temperature sensors and scavenges the magnetic energy surrounding the monitored cord
for node’s operation simultaneously, achieving the wireless monitoring AC power cords exclusive of
externally physical connection to a DC power supply. Actual tests have been carried out to verify
the validity of the self-contained self-powered wireless sensing node. Furthermore, a wireless sensor
network employing the proposed sensing nodes is also constructed for distributed power supply
cords monitoring.
2. NODE DESIGN
The architecture of the self-contained self-powered wireless sensing node is shown in Figure 1.
There are three parts included, namely an energy harvester, sensors and a wireless module. The
energy harvester is composed of an energy harvesting coil and a power management circuit. Sensors
consist of a current sensor and a temperature sensor. For easy installation and maintenance, the
core is designed as a split-core.
Both the energy harvesting coil and current sensing coil are on the basis of the same principle
— electromagnetic induction. The coils couple to the alternating magnetic field produced by an
AC current-carrying cord passing through the core, as a result, an alternating electromotive force
which is proportional to the AC current is induced across the coils. Two sets of coils both wrap
around the magnetic core, consequently, energy harvesting and current sensing can be achieved in
the same magnetic path.
However the energy scavenged by harvesting coil exists in AC form, and the instantaneous
output power is not always enough for the operating of the node, a power management circuit is
Progress In Electromagnetics Research Symposium Proceedings, Stockholm, Sweden, Aug. 12-15, 2013 1749
Figure 1: Architecture of the self-contained self-powered wireless sensing node.
designed to accumulate and store energy and provide a high-power DC output to drive the sensing
node. The scavenged energy is stored in a storage capacitor/rechargeable battery of the power
management circuit. When discharging, the capacitor/battery acts as the power supply for the
sensing node with the voltage dropping from U1 to U2 . The energy stored in the capacitor/battery
available for the sensing node is
¢
1 ¡
E = C U12 − U22
(1)
2
where C is the value of the storage capacitor/rechargeable battery.
In order for wireless communication, a module is required to wirelessly transmit the outputs of
the current and temperature sensors. A wireless sensor network with the proposed sensing nodes
is essential for multi-point monitoring of AC power supply cords.
To reduce the power consumption of the sensing node for long-time AC power cords monitoring,
the node is set to work and sleep alternately. The work duration includes the activity cycle and
the transmission interval. During the activity cycle, the node wakes up and acquires current and
temperature information. In one period, the energy consumed by the sensing node is
En = Psleep Tsleep + Pactive Tactive + Ptrans Ttrans
(2)
where Psleep , Pactive , Ptrans are the power consumption of the node in the sleep, active and transmission stage separately and Tsleep , Tactive , Ttrans are the corresponding time of the three stages
respectively.
3.
EXPERIMENTS
The fabricated self-contained self-powered wireless sensing node is shown in Figure 2. Because of
the compact size and self-contained device architecture, the sensing node can be readily installed to
encircle a power cord without interruption of the power supply from the monitored cord. Nordic’s
nRF24LE1 SOC solution which is comprised of a high performance microcontroller core, 6–12 bit
ADC and a transceiver is employed for wireless transmission.
In the experiment, the sleep time of the sensing node is set to be 2 minutes. An activity cycle
and a transmission process requires 201 ms and 500 µs respectively. As shown in Figure 3, the
Figure 2: Experimental setup of the self-contained self-powered wireless sensing node.
PIERS Proceedings, Stockholm, Sweden, Aug. 12–15, 2013
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power consumed by the sensing node in one period is
En = 6.4 µW × 120 s + 18 mW × 201 ms + 54 mW × 0.5 ms ≈ 4.42 mJ
(3)
A storage capacitor is used for energy storage element in the test. During the work duration
of the sensing node, the voltage of the capacitor drops from 3.43 V to 3.39 V, which is shown in
Figure 4, the energy provided by the storage capacitor is
E=
¡
¢
1
× 0.1 × 3.432 − 3.392 = 13.64 mJ
2
(4)
The normal efficiency of a regulated power supply circuit is more than 70%. Consequently, the
energy supplied to the sensing node is at least 13.64 mJ ∗ 70% = 9.548 mJ. It’s obvious that the
energy provided by the power management circuit can meet the power consumption requirement
of the sensing node.
Figure 3: Power consumption of the sensing node.
Figure 4: Discharge process of the storage capacitor.
The data loss ratio of the point-to-point communication test is performed in the indoor condition,
whose result is shown in Figure 5. According to the figure, the data loss rate of the point-to-point
communication is lower than 0.25‰ with communication distance within 10 meters. The data loss
rate in the open field is considerably lower than the one in the indoor condition.
The wireless sensor network is constructed in a star topology distributively with the proposed
self-contained self-powered sensing nodes, which is shown in Figure 6.
Figure 5: Data loss rate as a function of distance.
Figure 6: Schema of the wireless sensor network.
4. CONCLUSION
In this paper, the self-contained self-powered wireless sensing node for AC power supply cords
monitoring is presented. The energy induced from the magnetic field is transferred to the wireless
module by using the power management circuit. It’s verified that the self-contained self-powered
Progress In Electromagnetics Research Symposium Proceedings, Stockholm, Sweden, Aug. 12-15, 2013 1751
wireless sensing node can operate while the carried-on current is from 1 A to 100 A with a wireless
distance of 10 meters and the sensing data is wirelessly transmitted at a data loss rate of point-topoint communication less than 0.25‰. The wireless sensor network is constructed in a star topology
for distributed monitoring with the proposed wireless sensing nodes.
The application fields of the self-contained self-powered wireless sensing node not only can be
applied to monitoring of power supply cords, but also can be easily extended to realize intelligent
community or intelligent buildings etc., and the concept of “self-contained self-powered wireless
sensor” can be applied to almost all “M2M” industries.
ACKNOWLEDGMENT
This work is supported by the National Natural Science Foundation of China (No. 6107104) and
National Education Ministry Doctor Foundation of China (No. 20100191110009).
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