S. Percy and C. Knight, "Wireless power transfer, sensor positioning and power monitoring," Active and Passive Smart Structures and
Integrated Systems, Mehrdad N. Ghasemi-Nejhad, Editor, Proc. SPIE 7977, 7977-4, (2011).
Copyright 2011 Society of Photo Optical Instrumentation Engineers. One print or electronic copy may be made for personal use only.
Systematic electronic or print reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes,
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Wireless Power Transfer, Sensor Positioning
and Power Monitoring
a
S. Percy a and C. Knight a
Commonwealth Scientific and Industrial Research Organisation (CSIRO)
PO Box 330, Newcastle, NSW 2300, Australia
ABSTRACT
A system has been designed that will allow a network of sensor nodes to request power from a base node and receive it
wirelessly. The system consists of a central transmitting node which can be powered from an indefinite power source or
from a reliable source of energy harvesting such as solar. This energy is converted into UHF radio waves and transmitted
to individual stationary or mobile nodes making up the remainder of the network. When a sensor node detects that its
onboard power supply is at a critical level it will request a top up from the base station. The base station will scan
through 360° for the sensor node and once located begins charging. The charging station will remain in this position until
the sensor batteries are fully charged. At this point the base station will seek out another sensor node if required, or go
into a standby mode. If a mobile node is moved out of the charging position or interference of the beam occurs this is
indicated to the charging station and the transmitting node will scan again until another node is relocated. Results
indicate that charging can be obtained within a radius of up to 1.5 meters or greater for a higher transmission power. The
sensor positioning and power monitoring aspects of the system could be retained for a laser based system, which would
increase the transmission range. The system has the advantage that if sufficient solar energy can be captured during the
day, charging of the sensor nodes can be maintained over night allowing the battery size of each sensor node to be
reduced significantly.
Keywords: Energy harvesting, energy scavenging, power harvesting, RF transmission, wireless sensor, sensor network,
UHF, sensor tracking
1. INTRODUCTION
Frequently sensor networks are being situated in locations where there is no substantial ambient power source to allow
for energy harvesting. Examples of this include mine shafts, under dense tree foliage and inside enclosed structures.
Commonly to address this problem sensor nodes are equipped with large capacity batteries that can supply power for the
lifetime of the sensor node, however this can become impractical where size, weight and ‘indefinite’ operation are
important within the sensor network. To adopt this situation power needs to be supplied to the location of the node. This
has lead to research into the use of ultra high frequency (UHF) radio waves to transfer power to the location where it is
required.
The most common application of radio frequency transmission is within communication where microwatts or less of
radio frequency power are required to receive and decode a signal. For many years UHF power has also been used by
radio frequency identification (RFID) systems for the passive or semi-passive identification and tracking of objects, and
these systems are able to operate on tens of microwatts of received radio frequency power [1]. RFID systems operate by
radiating energy from a reader; normally a high power source with a directional transmitting antenna, this power is then
received by an omni-directional receiving antenna. A portion of this power is rectified and used to energise a small
microchip contained within the RFID tag. The remainder of this power is used to backscatter a signal for reception by the
reader indicating a small piece of data stored on the chip [2].
The described system utilises a similar idea developed within RFID design contained in [1] and [2], with an expansion to
track the positions and power requirements of nodes within a sensor network. The experimental system works by
integrating the power management and power supply of the sensor network into the sensor network itself, allowing the
normal transmissions within the network to indicate if the node’s stored energy has become critically low and request
Active and Passive Smart Structures and Integrated Systems 2011, edited by Mehrdad N. Ghasemi-Nejhad,
Proc. of SPIE Vol. 7977, 797705 · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.880367
Proc. of SPIE Vol. 7977 797705-1
power from the base transmitter when required. The base station then makes a decision whether power will be supplied
to this node and indicates this to the leaf node.
Additional proposed improvements to the system include replacing the UHF based transmission with an alternative
directive wireless power source, such as a laser-based transmitter. With this system the process of communicating a
node’s power state, requests for power transmissions, and the location of the node, all remain the same as the described
system. The source of power would be changed to a laser-based transmitter, with a corresponding photovoltaic-based
receiver. This would allow a significant improvement to the range of the system.
Other improvements include the type of energy storage (capacitor, lithium or nickel battery chemistry, etc.) which
determine the upper and lower limits of discharge, and an ability of a transmitting node to ‘demand’ charge, rather than
‘request’ it if the remaining capacity falls below a further, lower level. The results of the recharging system shows the
effect of a node requesting charge when a different node is already being charged, and the ‘handshake’ routine utilised to
deny charge and allow that node to return to a low-power sleep mode.
2. EXPERIMENTAL DETAILS AND METHODS
This section will explain each of the elements of the experimental system that constitute the sensor network. These
include the charging base station and the power receiving leaf nodes. Discussion will include how the system power
supply is integrated into the Fleck3B™ wireless sensor network platform to perform the power management and supply
to the complete sensor network. For the experiment 433MHz was selected as the operating frequency for the power
transmission arrangement.
2.1 The Leaf Node Harvester Design
The elements that make up the leaf nodes of the sensor network are outlined in Figure 1 below, these include the energy
harvesting circuitry, energy storage device and Fleck3B™ wireless sensor, and these elements will be described
individually. The antenna which is responsible for receiving the power from the transmitting base station was selected to
have an omni-directional field pattern. This was chosen so that the vertical rotation of the antenna was not critical for the
systems operation, a monopole whip antenna was used.
Energy Harvesting Circuitry
Antenna
433MHz
Matching
Network
Rectifier/
Voltage
multiplier
Power
Management
Energy
Storage
Fleck3B™
Wireless
Sensor
Power
Received
Indicator
true/false
Figure 1 - Leaf node elements.
To convert the received UHF power to a usable DC voltage a highly sensitive voltage rectifier circuit was required; the
layout selected for this was a Dickson voltage multiplier [3]. This was chosen since it exhibits a good efficiency,
manageable impedance change and a linear circuit layout which aids in the design and operation of the radio frequency
circuit. In order to maximise the rectifier circuit’s efficiency the change in circuit input impedance as a result of diode
Proc. of SPIE Vol. 7977 797705-2
capacitance variation at different input power levels had to be managed; the change in capacitance is discussed in [4]. If
this was not done impedance mismatch between the antenna and rectifying circuit would occur, resulting in inefficient
AC to DC conversion. The method to deal with this impedance change, predominantly used within RFID design in [1]
and [2], is to select a fixed matching network that will provide a good power transfer between the antenna and rectifier
circuit for the minimum received power required for the circuit to operate. The advantage of this is that the circuit will
still operate at higher power levels from which the matching network is optimised. This work had to be extended, as the
energy storage device will appear to the output terminals of the rectifier as a load that is non-linear and increasing in
voltage as energy is stored, causing the circuit input impedance to change as the energy is stored. To deal with this the
matching circuit also had to be designed for a storage voltage equal to the normal voltage of the storage device being
used. For the NiMH battery used this was 3.6V. The result of this was an excellent power transfer at one input power and
battery voltage and a good impedance match at other combinations of the two.
The power management circuitry was designed to operate on the rectified received power, to disconnect the storage
device from the rectifier to avoid over charging and indicate to the sensor if radio frequency power is being received.
Components were selected to minimise power consumption of this circuit. A small amount of hysteresis was added to
avoid oscillation at the switching point. Two storage devices have been selected for examination within this experiment,
outlined in Table 1.
Table 1 - Leaf node and storage.
Node
number
Storage type
Usable Storage
capacity*
Nominal
voltage
Request
Charging
Voltage
Cease
Charging
Voltage
Node 1
2 Farad
Supercapacitor
9J at 4.25V
3.8V
4.05V
4.25V
Node 2
3-Cell NiMH
battery 7mAh
~90 J
3.6V
3.97V
4.1V
* an estimation of storage energy between the upper limit and lowest operating voltage of the Fleck3B™.
The entire leaf node fits inside a small box and is shown in Figure 2. This shows the two supercapacitors attached to the
side of the box, the large power receiving antenna, and smaller Fleck3B™ transmitting antenna. The harvesting circuit is
on the lighter coloured board, and the Fleck3B™ node sits underneath.
Figure 2 - Supercapacitor sensor node with Fleck3B™ sensor.
2.2 Transmitting base station
The set up of the transmitting base station was fairly simple; the elements are outlined in Figure 3. It consists of a
433MHz oscillator, radio frequency amplifier to boost the power, and a directional Yagi-Uda antenna. The transmitting
antenna is switched on and off by the base station when required to conserve power. The Fleck3B™ wireless sensor
outputs a pulse width modulated (PWM) signal to control the azimuth position of the transmitting antenna as required.
Proc. of SPIE Vol. 7977 797705-3
433MHz
Oscillator
3dBm
433MHz
RF
Amplifier
18.1dBm
433MHz
9dBi
Yagi
Antenna
On/Off
Fleck3B™
Wireless
Sensor
PWM
Azimuth
Position
Motor
Figure 3 – Base node elements.
2.3 Operation of the Sensor Network
The charging base station is responsible for supplying power to the leaf nodes in the network upon request. The flow
diagrams in Figure 4 and Figure 5 outline the operation of this part of the system. The network is made up of a number
of leaf nodes located within range of the base node. The base node is the central receiving point for a local network of
sensor nodes. The base node receives the data transmissions from each of the leaf nodes within the network and performs
a variety of tasks dependant on the type of message. There are five types of message in this network. These transmissions
are colour coded in Figure 4 and Figure 5, and defined as:
A) A normal transmission from the leaf node – no battery charging is required and no charging is being supplied,
or battery charging is required and it is being received. This packet is sent periodically (15 minutes) and is
stored in a permanent data storage location. The base node may be charging another node, and if so this
continues. The contents of the packet are:
a. sensor data (for this example that is temperature from the onboard temperature sensor),
b. a battery voltage reading, and
c. an incrementing counter that indicates how many continuous samples have been taken by this node.
B) A cease charging request – the leaf node is receiving power from the base node transmitter, but the battery
voltage is greater than the required maximum. This packet is sent at the standard transmission rate (15 minutes),
contains the same information as a type A transmission, and is also stored in the permanent data storage. Once
received the base node will turn off the RF transmission and await the next incoming transmission.
C) An initial charge request – the leaf node requires charge, but is not receiving it. Once received, the base node
will determine if it can supply the charge (that is, it is not currently charging another node). If it can’t, it denies
the request and continues charging the current node. The denial is received at the leaf node and the leaf node
returns to deep sleep. If the base node can supply the request, it slaves the antenna to the last known azimuth of
the node and turns on the antenna. A series of handshake transmissions (type D and type E) then occur between
the base and leaf node to determine if the antenna is correctly oriented towards the required node.
D) A power handshake query (leaf) – the leaf node responds to a power handshake query from the base node (type
E, as described below). If the leaf node indicates it is receiving power the base node antenna stops searching,
stores the position of the antenna against this leaf node and waits for the next sample from the network. If the
leaf node indicates it is not receiving sufficient power the base node increments the antenna position and repeats
the handshake query (type E).
E) A power handshake query (base) – causes the leaf node to respond with information about how much power is
being received (a type D message). If the leaf node is receiving sufficient power, the leaf node will go into sleep
mode (while still receiving power) and the base node will wait for the next data transmission from any node. If
it is not receiving enough power it will indicate this and the base node will increment the antenna position and
resend a type E message. Type E is the only packet type sent from the base node.
Proc. of SPIE Vol. 7977 797705-4
Figure 4 – Flow diagram of leaf node programme
Figure 5 – Flow diagram of the base node programme.
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2.4 System investigation
The received power of the leaf node is affected by three main constraints – the path loss, the antenna gains and the power
of the transmitter. The items that are being investigated here are the advantages of increasing the gain of the base
transmitting antenna, and how this can be used to provide power to a complete sensor network. The first investigation
into the power transfer system was to simply demonstrate that using a directive transmitting antenna in the near field can
increase the received power significantly. This was done by replacing the directive antenna described above with an
omni directional monopole antenna and measure the power received at 1.5 meters separation with another monopole
antenna. The monopole antenna was then replaced with the 9dBi Yagi-Uda antenna and again the received power was
measured at 1.5 meters separation, and the two powers compared. The results are discussed below. A further
examination of the transmitting and receiving system was conducted to plot the system path loss with distance. This was
done by transmitting power from the yagi antenna and receiving it at different distances to determine dB path loss. The
results are shown in Figure 8 Figure 6.
After this the two nodes outlined in Table 1 were added to the system 80 degrees apart and 1m from the driven element
of the transmitting antenna. These were pre-charged above the ‘request charging’ voltage at the beginning of the test.
The system was switched on and the storage device voltages logged via dSPACE™. Figure 6 shows the supercapacitor
circuit being charged.
Figure 6 - System charging supercapacitor node.
3. RESULTS
3.1 Matching network
The measured impedance at 433MHz, without the impedance matching components in the circuit, was predominantly
capacitive at 0.861-37.1j ohms for the supercapacitor node and 0.97-39.4j ohms for the NiMH node. Due to tolerance
and availability of components this network remained identical for the different normalised voltages, the difference being
the adjustment of the series trimmer capacitor. The network schematic as shown in Figure 7. While connected to a
network analyser this capacitor was tuned to improve the impedance match for the different storage normal voltages. An
input reflection coefficient, s11 of -13dB was achieved for each circuit; this equates to a power ratio of 0.0501.
Proc. of SPIE Vol. 7977 797705-6
Figure 7 - Impedance Matching Network.
3.2 Range and power transfer.
The measured power from the base transmitter used in the experiment was +18.1dBm (64.56mW), while the range
associated with the system is dependent on transmission power and gain of the antennas. This was demonstrated by using
a low gain monopole transmitting antenna at a range of 1.5 meters. At this distance the received power was -5.22dBm
(0.3mW). By replacing this with a 9dBi antenna a power of 3.80dBm (2.4mW) was received at 1.5 meters from the
transmitter (distances were measured from the driven elements of the antennas). This indicates the advantage of using a
directional transmitting antenna. It is also noted that the system is operating within the near field and in an indoor
laboratory situation where reflections from objects in the room can affect the antenna field. 8 below shows the dB path
loss measured within the laboratory. Using this path loss in a link budget calculation demonstrates that by increasing the
transmission power to 30dBm (1 Watt), transmission range should increase to ~3 meters where 7.9dBm (6.16mW) could
be received. However, issues regarding human tissue exposure and relevant radiation regulations and standards would
need to be investigated.
Figure 8 - dB path loss.
3.3 System in operation
The system during operation is shown in Figure 9. This demonstrates four situations occurring during operation, shown
by the numbered arrows. The periodic voltage spikes are caused by current draw due to the data transmissions.
Point (1) is the situation where the supercapacitor voltage has fallen below the set level of 4.05V and the NiMH voltage
remains above the minimum level of 3.97V. This triggers the leaf node to request charge. As the charging system is idle
the request is accepted and a handshake routine to determine location of the leaf node is begun. This occurs for a short
Proc. of SPIE Vol. 7977 797705-7
period of time (indicated by a thick black line at point (1)), until the node is located; the time this takes defines the
energy used for this task. Once located the supercapacitor battery voltage begins to increase, peaking at point (3).
Point (2) indicates the situation where the NiMH node storage voltage has dropped below the set level of 3.97V; this
triggers the NiMH node to request charging. As the base is currently charging the supercapacitor node a ‘charge request
denied’ message is sent back to the leaf node, putting it back to sleep. There is a slight increase in power consumption
due to the node going into receive mode for a short period of time.
Point (3) shows a node charge peak, and begins decreasing when the supercapacitor voltage has reached the defined level
of 4.25V. This triggers the base to switch off the transmitting antenna until the next data packet from the NiMH node is
received indicating that power is required. The base and NiMH node then go into a handshake routine to determine
location of the NiMH node, until the node is found. Again, the added power consumption for this routine shows as a
thick dashed red line. After this, charging of that node begins.
Node Storage Voltges Showing System Operation
4.4
(3)
4.3
4.2
Storage Voltage (V)
4.23V to
4.05V
(4)
4.1
4
4.1V to
3.97V
3.9
3.8
3.7
(2)
3.6
NIMH Node
3.5
3.4
Supercapacitor Node
(1)
0
10
20
30
40
50
60
70
80
Time (minutes)
Figure 9 – System operation supplying 2 sensor nodes.
The final situation which is not indicated in Figure 9 is when both nodes are above the set levels. This would occur when
a high power is received and charge time is short. In this situation both node voltages would be decreasing until are
charge was required.
The gradient of the charge/discharge curves in Figure 9, can be related to the energy storage capacity of the battery and
supercapacitors used. The slightly greater discharge gradient of the supercapacitor indicates less energy capacity and also
indicates a level of self discharge or the supercapacitor. The magnitude of the lowest voltage limit is selected to reflect
the energy stored in the device. If the storage device has low capacity (for example a capacitor) the minimum voltage
should be higher than for a device with a larger energy capacity. If the low-point of the bandwidth is too low the leaf
node will reset as the power consumption during radio transmission causes a large drawdown in available voltage.
Conversely, if the energy device has a larger capacity (for example a battery), a higher minimum voltage can be used. It
should also be noted that at 4.1V the NiMH node battery should be close to 100% capacity. By reducing this it could be
possible to increase the cycle life time of the batteries used.
Proc. of SPIE Vol. 7977 797705-8
The nature of the discharge curve needs to be considered when setting the bandwidth and voltage minimums for the leaf
node trigger points. Critically, if the storage has a large capacity the discharge time will be longer, but the recharge time
will also increase.
For indefinite operation of the system the energy stored between the maximum voltage and the minimum voltage of the
storage device must be such that it will be able to supply the sensor for a time greater than it takes to charge the other
nodes in the network. So if a higher power is transmitted or range is reduced then the time to charge is reduced. Hence
either the storage voltage range can be reduced or the energy storage capacity can be reduced in order to reduce the time
required for charging.
An important aspect of the system is that if a node moves location between charges it will be rediscovered and charging
remains possible. Also if a node moves out of the beam while charging or insufficient power is being received, this can
be indicated to the base within the normal data transmissions, and adjustment of the antenna position can occur, or a new
node can be charged. Extensions to the system charge scheduling could also be made. If, for example, one node has a
large storage capacity, requiring a longer time to charge, and another has a small energy capacity, requiring a short time
to charge, priority can be given to the lower capacity node when the energy storage level of that node becomes critical.
This would improve the reliability of the nodes in the network and reduce the chance of a node dropping below its
minimum operating voltage.
The ideas developed here are not isolated to a radio frequency power transmission system; this could also be applied to
any directional wireless power source such as laser. In this situation the range of the system could be increased
markedly, although assessment of the danger of beam interference, and the reaction of the network, needs to be
considered.
4. CONCLUSION
A system has been designed that allows a complete short range network of sensor nodes to be remotely supplied with
power. The system consists of a base node with access to a good power supply to transmit power to leaf nodes that have
poorer access to an ambient power supply. The system outlined in this work is based around UHF transmission of power.
Results indicate that the use of a directive transmitting antenna in the near field can be increase the transmission range of
the system and also be used as the means of tracking the position of the sensor nodes. For further increase in
transmission range a higher power transmitter or an even higher gain antenna can be used.
Although RFID systems have used UHF transmission (at extremely short ranges) for powering ID tags, this system
utilises the transmitted power in an active nodal network. Each leaf node is used to sense some ambient condition, such
as temperature or wind speed, and also its own internal energy state. Once this energy state falls below a predetermined
level, information is appended to the normal transmission to request a top up charge. The preset point can be varied,
depending on the capacity and discharge characteristics of the energy supply. In this experiment a relatively small energy
supply in the form of a capacitor, is compared with a NiMH battery of larger capacity. It is noted that the selection of the
operating voltage range of the storage type and capacity has a large effect on the ability for indefinite operation of the
system. Another proposed way of improving the system is to force a charge request if power drops to a second critical
level. The continued top up, whilst avoiding over charging of the battery, can allow battery cycle life to be improved.
The hand shake protocol used by the sensor network was successful in locating the position of the sensor nodes. This
ability to search for a leaf node allows the use of mobile nodes in this style of network. Additional proposed
improvements to the system include replacing the UHF based transmission with alternative power transmission, such as
a laser based transmitter. With this system the process of communicating a node’s power state, and requests for power
transmissions, and the location of the node remains the same as the described system. The source of power is changed to
a laser based transmitter, with corresponding photovoltaic based receiver. This would allow an improvement to the range
of the system.
Proc. of SPIE Vol. 7977 797705-9
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