Energizing low power
WSN using Microstrip Antennas at GSM-900 band
Abstract-
The global demand for "green" Technology is driving a new generation of low-power
Wireless sensor networks. Wireless sensor networks (WSN) are being developed
for use in remote sensor-based systems, for both industrial and control
applications. This
article presents an RF energy system that can harvest energy from the ambient
surroundings at the downlink radio frequency range(935.2MHz – 959.8MHz) of
GSM-900 band. The harvesting system provides an alternative source of energy for
energizing low sensor networks. The system design consists of three modules:
two E-shaped patch antennas, a pi-matching network and a 7-stage voltage
doubler circuit. All the three modules can be fabricated on a single printed
circuit board. Previous harvesting systems constitute a single antenna, so the
output power is considerably low. In this system, two Microstrip antennas are
used to receive the RF signal and therefore the combined power of the two
antennas is much higher than the previous system. The harvester nearly produces
a voltage of 6V for a received signal of -27dBm. This voltage is enough to
power a wireless sensor system.
1.Antenna:
1.Antenna:
The antenna applied
here was an E-shaped single patch from the conventional wide band microstrip
antennas. The antenna is designed and optimized to capture the energy from the
ambient at a downlink radio frequency range of GSM-900 band. In order to expand
its bandwidth, two parallel slots are incorporated into this patch. The pi
matching network is designed and optimized to provide an impedance matching for
the antenna. The feed line is appropriately positioned on the upper leg of the
E-shaped patch antenna as shown in Figure 1. The slot length, width, and
position are important parameters in controlling the achievable bandwidth.
Traditionally, the property of the patch antenna is suitable for narrow
bandwidth applications. The challenge here is to make the patch antenna for the
wideband energy harvesting environment. The antenna design required to look
into the permittivity or dielectric constant of the substrate, width, length of
the patch antenna and the ground plane. The permittivity of the substrate plays
a major role in the overall performance of the antenna.
2. Matching Network:
The matching network
in the geometry was designed to provide a good impedance match for complex load
(RF-DC convertor) impedance 63-j117 Ω to source (antenna) impedances 377
Ω to transform maximum power from the source to the load. The output of
the pi matching network is directly connected to the input of RF-DC converter
circuit.
3. RF-DC Conversion Module:
The
An energy conversion module is a voltage doubler circuit used to convert the
harvested energy from ambient radio frequency into DC voltage. The design
contains stages of the Villard voltage doubler circuit. A 7-stage Schottky
diode voltage doubler circuit is designed, modeled, simulated, fabricated and
tested. Multisim is used for the modeling and simulation. Simulation and
measurement were carried out for various input power levels at the specified
frequency band. The voltage
multiplier circuit in this design uses zero bias Schottky diode HSMS-2850 from
Agilent.
Each independent
stage with its dedicated voltage doubler circuit can be seen as a single
battery with open circuit output voltage V0, internal
resistance R0.The number of stages in
the system has the greatest e®ect on the output voltage. The capacitors, both
in the stages and at the final stage of the circuit, affect the speed of the
transient response and the stability of the output signal. The number of stages
is essentially directly proportional to the amount of voltage obtained at the
output of the system.
The simulation and practical implementation was done with fixed RF at 945 MHz ± 100 MHz, which is close to the down link center radio
frequency
(947.5 MHz) of GSM-900 band.
The voltage obtained at the final node (VDC 7) of the
doubler
circuit
was recorded for various input
power levels from −40 dBm to +5 dBm with power level interval
(spacing) of
5 dBm.
The input impedance
63-j117 Ω of the
voltage doubler is obtained using
the network analyzer. This 63-
j117 Ω was tested from 900
MHz to 1000 MHz. as the
antenna was designed
for the down link radio frequency range
of GSM-900 band.
4. Step-up Circuit:
A Booster converter
(Step-up converter) is a DC-DC power converter with its output voltage greater
than its input voltage. It
is a class of
switched-mode power supply (SMPS) containing at least two semiconductor switches (a diode and a transistor)
and at least one energy storage element, a capacitor, inductor,
or the two in combination. It is applied between the
Energy conversion system and Load to successfully accomplish the load
requirement.
Operating Principle:
(a) When the switch is closed, current flows through the
inductor in clockwise direction and the inductor stores the energy. The
polarity of the left side of the inductor is positive.
(b) When the switch
is opened, current will be reduced as the impedance is higher. Therefore,
change or reduction in current will be opposed by the inductor. Thus the
polarity will be reversed (means left side of inductor will be negative now).
As a result two sources will be in series causing a higher voltage to charge
the capacitor through the diode D.
·
The
step-up circuit is used depending on Voltage requirement.
5.
Load:
A wireless sensor network (WSN) consists
of spatially distributed autonomous sensors
to monitor physical or environmental conditions such as temperature,
sound, pressure etc. and pass the data to a corresponding control center.
A WSN is
built of ‘nodes’- each having a transceiver, an electronic circuit for
interfacing with the energy source usually a battery or from an energy
harvester. Each node can be equipped with a power of less than 0.5- 2 amp-hour
and 1-3V.
An example
of such sensor node is a STLM20 temperature sensor. All the nodes in the
network are connected to a master node through which the data is communicated
and controlled bidirectionally.
Conclusion:
A
novel 900MHz RF energy harvesting system for powering low power sensors has
been analyzed and discussed.
Firstly, two 377Ω E-shaped patch antenna with
compacted size were implemented.
Subsequently the pi matching network
located in between the antennas and the RF-DC conversion module is designed to
simulate and
implemented to
provide a good match from the load to the source.
The energy conversion module that
comprises of 7-stage voltage doubler circuit with zero bias Schottky diodes was
successfully implemented and found to be efficient in converting the RF signals
captured by the antenna to the required DC output voltage for powering the WSN.
Finally, a booster converter which
can be applied based on the voltage requirement is discussed.
