WIRELESS POWER TRANSFER FOR INDUSTRIAL APPLICATIONS
THROUGH STRONGLY COUPLED MAGNETIC RESONANCES
Jan Pannier1, Dries Hendrickx1, Frederik Petré2, Tiene Nobels3
1
Master student Electromechanical Engineering, GROUP T – Leuven Engineering College, Vesaliusstraat 13, B-3000 Leuven, Belgium
2
Flanders’ Mechatronics Technology Centre (FMTC), Celestijnenlaan 300D, B-3001 Heverlee, Belgium, Frederik.Petre@fmtc.be
3
Unit Energy, GROUP T – Leuven Engineering College, Vesaliusstraat 13, B-3000 Leuven, Belgium Tiene.Nobels@groupt.be
Abstract: Because of the numerous advantages related to the use of wireless sensors, the feasibility of extending an existing
wireless power setup, initially designed for static power transfer, to a wireless power setup with a rotary receiver has been
investigated. The main difference between both setups is the emergence of intermittent behavior due to movement of the
receiver. Besides that, existing parts such as the tank and rectifier circuits, were optimized in order to increase efficiency and
improve integrability. Most of the conducted experiments were therefore efficiency measurements for a variety of realistic
situations. Also, an analysis of the major losses was performed. Throughout all static experiments the new coil design proves
to be better than the original one when it comes down to transfer efficiency. Furthermore, the new design is far more suited for
integration on a PCB. Finally, the operation of the wireless power system was successfully demonstrated for a rotating shaft
application with speeds up to 120 rpm.
Keywords: Magnetic Resonance, RF Coil Design, RF Rectification, Wireless Power
INTRODUCTION
Wireless sensors offer tremendous advantages in the
manufacturing value chain, with value-added industrial
applications as diverse as wireless condition monitoring,
wireless control and wireless safety. Next to wireless data
communication, realizing an efficient wireless power
solution is one of the main technical challenges to realize
self-powered wireless sensors that do not require battery
maintenance. Recently, inductive wireless power technology
has gained significant momentum for consumer applications
as evidenced by the rising number of high-tech startup
companies in this area (WiTricity, Fulton Innovation,
Wisepower, etc.) and the interest shown by consumer giants
such as Intel and Sony [1,2,3,4,5]. In this perspective, the
Wireless Power Consortium is setting the international
standard for interoperable wireless battery charging of all
kinds of mobile devices such as digital cameras and smart
phones [6].
Already back in 2007, FMTC demonstrated the technical
feasibility of inductive wireless power transfer based on
strongly coupled magnetic resonant circuits at 27 MHz [7].
This setup was designed for a specific industrial application
involving a translation movement of the receiver coil and,
although very different from our new application, served as
a starting point for our project. This setup will be referred to
as the original setup throughout this text.
This work focuses on the specific industrial application case
of wireless power transfer to a wireless sensor node attached
to a rotating shaft. This case, which is representative to
many rotating machinery applications, is not only very
suitable to demonstrate the use of wireless power transfer
but also introduces an extra difficulty, namely intermittent
behavior. Tackling these problems together with the
improvement of the overall efficiency and the integrability
of the wireless power receiver were the main goals of this
project. To save time and effort, the other components of the
previous wireless power transmitter, such as the oscillator
and power amplifier, were maximally reused because the
primary focus is on the wireless power receiver (mounted on
the rotating shaft).
Since the original setup already uses magnetic resonance,
increasing the overall power efficiency comes down to
optimization and re-design of the tank circuits and the
bridge rectifier in particular. Because of their major role in
optimizing efficiency and improving integrability, the
biggest challenge resides in the design of the coils.
This work provides the reader with information about the
architecture, the design and the evaluation of the new
wireless power system. Finally, our conclusions are
summarized and some potential improvements for future
research are proposed.
SYSTEM ARCHITECTURE
Decomposition of the wireless power setup at top level
yields four major subsystems as shown in Figure 1. The red
arrows represent the interfaces between the subsystems. The
industrial application provides the necessary context to
demonstrate the correct operation of the wireless power
setup. The main component of the industrial application
context is the rotating shaft. The other subsystems require a
more extensive step by step decomposition.
Figure 1: Top Level Architecture.
Figure 2 shows a more detailed decomposition of the
wireless power transmitter. A mechanical stand, mounted on
the table, allows the transmitter (or primary) tank circuit to
be placed at the correct height. The transmitter tank circuit is
fed by a power amplifier which amplifies the signal from the
signal generator.
Figure 5: Architecture of Performance Registration.
Figure 2: Architecture of Wireless Power Transmitter.
The wireless power receiver is decomposed as shown in
Figure 3. The mechanical clamp acts as a solid support for
all rotating parts. The receiver (or secondary) tank circuit
captures the power signal (magnetic flux) from the air. Next,
the power signal is conditioned and finally delivered to the
load (purely resistive).
Figure 3: Architecture of Wireless Power Receiver.
An even further analysis of the conditioning circuitry results
in the structure shown in Figure 4. The rectifier, consisting
of a rectifier bridge and a smoothing capacitor, serves as an
AC to DC converter and also smoothes the ripple on the
voltage. Next, an energy buffer is inserted to deal with the
intermittent behavior. Based on the buffer voltage the
On/Off circuit decides whether the switching regulators
should be active or not. In this way the correct amount of
voltage is always guaranteed.
Figure 4: Architecture of Conditioning Circuitry.
Figure 5 represents the decomposition of the performance
registration, the last top level subsystem. The voltage
measurements are performed using voltage dividers to make
sure none of the analog inputs of the data logger are
damaged. Next, the data logger transmits a wireless data
signal which is then captured by the base station. The base
station is connected to the PC through USB. Once the data is
available on the PC, Labview is used to alter it to its desired
form.
DESIGN OF TANK AND CONDITIONING CIRCUITS
A. Tank Circuits
One of the main goals is to research how to optimize the
tank circuits for maximum transfer efficiency while keeping
them as small as possible. Eventually integration on a
Printed Circuit Board (PCB) should be possible. Since the
capacitors are commercial off the shelf (COTS)
components, the main difficulty for this part of the setup lies
within the coil design. The original setup used small
rectangular coils. This coil concept was optimized for one
specific application, so next to this one, experiments with
three new coil concepts were conducted. All concepts are
shown in Figure 6, and will from now on be referred to as
rectangular, solenoid, planar circular and planar square. The
red transparent surface shows at which side the transmitter
coil will couple with the receiver coil.
Figure 6: Rectangular (a), Solenoid (b), Planar Circular (c)
and Planar Square (d) Coil Concepts.
Figure 7: Rectangular (a), Solenoid (b), Planar Circular (c)
and Planar Square (d) Coil Prototypes.
Discrete coils with insulated copper wire (shown in Figure
7) were made based on calculations.
The inductance of each coil is approximated using
expressions for DC inductance based on the Harold A.
Wheeler approximations [8].
No calculations and
measurements for the rectangular concept are made, because
this concept is only used as a baseline for comparison. The
theoretical inductance of the solenoid is calculated as shown
in Equation (1).
capacitance of 2.71 pF and 2.51 pF respectively.
(1)
Here L is the inductance, r the coil radius, N the number of
turns, and l the length of the coil. All dimensions are in
inches. The prototype has a radius of 0.0125 m (0.49 inch), a
length of 0.011 m (0.43 inch) and 6 windings resulting in an
inductance of 0.995 μH.
For the calculation of the planar coils’ inductance, Equation
(2) is used.
(2)
Here r is the inner radius, N is the number of turns and w is
the wire diameter. All dimensions are in inches. Since no
formula is available for a planar square coil, the equation for
a planar circular coil is used for both concepts. The
prototypes have an inner radius of 0.0085 m (0.33 inch), a
wire diameter of 0.0017 m (0.066 inch) and 6 windings,
resulting in an inductance of 1.181 μH.
The inductance is also determined experimentally. When a
known capacitor (47 nF) is added in parallel with the coil,
the resonant frequency needs to be measured. Subsequently,
the inductance value is derived with Equation (3).
(3)
The circuit containing the solenoid resonates at 746 kHz.
This results in an inductance of 0.969 μH, proving the
accuracy of the proposed formula.
The circuits containing the planar circular and planar square
coil resonate at 678 kHz and 654 kHz respectively. This
results in an inductance value of 1.173 μH for the planar
circular coil and 1.261 μH for the planar square coil. The
equation proves to be accurate for the planar circular coil.
As expected, the planar square coil has a higher inductance
value than calculated, due to its somewhat larger cross
section.
Because of the high operating frequency, the parasitic
capacitance cannot be neglected [9]. The parasitic
capacitance is hard to calculate and is therefore determined
experimentally. Since the inductance value is known, the
parasitic capacitance can be calculated with Equation (4) if
the resonant frequency is known.
B. Rectifier
Rectification at 27 MHz introduces specific technical
challenges that are not encountered at lower frequencies. For
this application, the most obvious choice is a full-bridge
rectifier, because a single diode rectifier would cause more
efficiency loss and an active rectifier would be very hard to
implement at this frequency.
The voltage drop across the diodes as well as the diodes’
reverse recovery time, both related to their physical device
nature, turn out to be the biggest loss factors. The nonlinearity introduced by the bridge rectifier does not have a
significant influence on the system’s power efficiency, due
to the filtering behavior of the tank circuits which act as
band pass filters.
The voltage drop across a diode is a function of the current
running through it. Also, when a diode switches from
conducting to non-conducting state or vice versa the current
will still flow in the wrong direction through the diode for
some time. This is called the reverse recovery time [10]. The
theoretical efficiency of the bridge rectifier is then
calculated according to Equation (5).
(5)
Where η is the rectification efficiency, Vin is the input
voltage, Vdiode is the voltage drop across a single diode, T is
the period of the rectified signal and trr is the reverse
recovery time.
Although the bridge rectifier produces an output of fixed
polarity, a smoothing capacitor is added to smooth the
amplitude of the continuously varying output.
Based on Equation (5) a few suitable types were chosen.
Next, the one with the highest efficiency was determined
experimentally.
C. Energy Buffer
The energy buffer in the system has to deliver energy to the
load when the transmitter and receiver are too far apart for
efficient power transfer. An electric double-layer capacitor,
also known as a supercap, is chosen. The main advantages
of such a supercap over an electrochemical battery are:
longer lifetime, very high rates of charge and discharge,
extremely low internal resistance and high power density.
The only significant disadvantages over normal batteries for
this application are higher self-discharge, lower specific
energy and a less stable output voltage [11].
(4)
When the solenoid is excited without any added capacitance,
it resonates at 94.15 MHz., this results in a parasitic
capacitance of 2.95 pF. Under the same conditions, the
planar circular coil resonates at 99.72 MHz and the planar
square coil resonates at 89.44 MHz, resulting in a parasitic
D. Voltage Regulation Circuitry
The voltage regulation circuitry needs to ensure that a stable
3.3 V and 5 V is delivered to the loads. Clock controlled
switching regulators are used. These only work properly if
the input voltage is sufficiently high. Therefore, a system is
needed that only enables the conditioning IC’s when the
input voltage (this is the voltage across the energy buffer)
reaches a certain high level and disables them whenever the
input voltage drops beneath a certain low level. The
thresholds are set to 6.5 V and 12 V for the low level and
high level respectively. The On/Off circuit designed for the
original setup was reused, only the high voltage level was
adjusted. The schematic for the voltage conditioning circuit
is shown in Figure 8.
Figure 11: Efficiency as a Function of Distance between
Transmitter and Receiver Surface.
Figure 8: Voltage Regulation Circuit Diagram.
PERFORMANCE EVALUATION
First, the results of the static efficiency tests for the different
coil concepts are discussed. Influence of distance, angular
displacement, surroundings and transferred power is tested.
The setups for these tests are represented using the coupling
surfaces defined in Figure 6. Second, an efficiency test on
various bridge rectifiers is performed. Finally, the effect of
intermittent behavior is examined. Figure 9 shows the static
test setup where influence of distance is measured. To carry
out other types of experiments the static test setup is each
time adapted in a proper way.
From the results shown in Figure 11 two conclusions can be
drawn. First, each coil concept has an optimal distance due
to critical coupling. For the rectangular coils and the
solenoids the optimal distance is situated below 5 mm. The
optimal distance for the planar coils is situated around 20
mm, below this distance efficiency decreases due to
overcoupling. Second, once beyond the point of critical
coupling efficiency decreases with increasing distance.
The second test examines the influence of angular
displacement between the transmitter and receiver surface as
shown in Figure 12.
Figure 12: Setup for Angular Displacement Test.
Figure 13: Efficiency as a Function of Angular
Displacement of the Receiver Surface.
Figure 9: Static Test Setup.
The first test examines the influence of the distance between
the transmitter and receiver surface as shown in Figure 10.
Figure 10: Setup for Distance Test.
The results shown in Figure 13 lead to two conclusions.
First, the efficiency decreases more or less linearly with
increasing angular displacement. Second, the planar coil
concepts are clearly less affected by angular displacement.
The third test is used to compare the various diode types
based on their efficiency for different levels of input power.
buffer’s voltage and the power dissipated by the loads.
Figure 14: Power Efficiency of Various Diode Types at 27
MHz.
Figure 14 shows that the BAT43 diode performs best at
power levels below 200 mW. For power levels above 200
mW, the 1N5282 diode performs best.
Figure 15 shows the test setup used for the fourth and fifth
test. In the fourth test, the dynamic test setup is used with
the shaft in standstill. Figure 16 shows the conditioned
voltages, the energy buffer’s voltage and the power
dissipated by the loads.
Receiver
Transmitter
Figure 17: Results for End-To-End Dynamic
Measurements.
The maximum constant conditioned power the setup is able
to deliver to the loads during rotation of the shaft is 60 mW
when 2000 mW of input power 1 is used. A change in
rotation speed does not influence the transferred power, only
the coupling angle is of importance. The coupling angle is
that part of an entire rotation when power transfer is
possible. For this case, the coupling angle is 43°, because 60
mW of output power is obtained during rotation and 500
mW during standstill.
It should be noted that the rotating parts are not shielded,
causing the wireless power link to interfere with the
measurement signal.
CONCLUSIONS AND FUTURE WORK
Figure 15: Dynamic Test Setup.
Based on the different static tests, we conclude that the
overall performance of the planar coil concepts is better than
that of the other concepts. The planar coils, especially the
square ones, are also very well suited for integration on a
Printed Circuit Board (PCB). With the planar square
concept, a transfer efficiency of about 80% can be reached.
Passive rectification, although the most obvious option for
this application, is still subject to large energy losses. The
maximum rectification efficiency for this application is
about 70%. Improving the rectification efficiency is directly
related to the availability of better diodes optimized for
operation at 27 MHz.
Figure 16: Results for End-To-End Static Measurements.
When the transmitter is powered on, the buffer voltage
increases until the high voltage of the On/Off circuit is
reached. When this happens, the conditioning circuit is
enabled and the loads are powered. The buffer voltage
decreases until it reaches a stable point, which depends on
the amount of transmitted power. To constantly provide the
loads with 500 mW, a power of 2000 mW has to be
transmitted. This results in a static end-to-end efficiency of
25%.
The fifth test examines the intermittent behavior at 120
rpm. Figure 17 shows the conditioned voltages, the energy
For this application, a supercap offers the best solution to
buffer differences between received and consumed power,
thanks to low losses and very good dynamic properties.
Switching regulators are used to condition the buffered DC
signal. Although the current solution works well, efficiency
is only about 50%, so a thorough redesign of this circuit can
result in a considerable increase in efficiency.
A transfer efficiency of 80%, a rectification efficiency of
70%, and a voltage regulation efficiency of 50% results in a
total achievable efficiency of 28%. This is in agreement
1
An input power of 2000 mW is measured at optimal coupling. The
input power fluctuates during rotation due to a varying coupling factor.
with the measured end-to-end efficiency of 25%.
The dynamic test proves the feasibility of powering a load
on a rotating shaft with only one transmitter and one
receiver. Compared to static power transfer, transferred
power is reduced by a factor 10 during rotation.
However, a lot of improvements can still be made. These
improvements go beyond the scope of this work, but are
listed as topics for future research:
1.
2.
3.
4.
5.
6.
7.
Reconsideration of the frequency choice, taking
every part of the system into account;
Research into new wire material for improved coil
designs;
Integration of transmitter and receiver coils on a
PCB;
Optimization of the efficiency of the bridge
rectifier;
Redesign of the voltage regulation circuitry;
Addition of transmitters or receivers to increase the
coupling angle;
Shielding of all electronics to reduce interference
from wireless power link.
ACKNOWLEDGEMENTS
We want to thank Ludo Somers for his help with all kinds of
practical issues. Also, we want to thank René Boonen as he
was able to clarify some theoretical mysteries. Finally, many
thanks to Sam Buls for developing our Labview program.
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