Single-bit modulator for wireless power transfer system

International Journal of Power Electronics and Drive System (IJPEDS)
Vol. 10, No. 3, September 2019, pp. 1270∼1280
ISSN: 2088-8694, DOI: 10.11591/ijpeds.v10.i3.pp1270-1280 r 1270
Single-bit modulator for wireless power transfer system
Dhafer J. Almakhles1
, Akshya Swain2
, Umashankar Subramaniam3
1,3
Renewable Energy Lab, Communications and Networks Engineering, Prince Sultan University
2
Department of Electrical and Computer Engineering, University of Auckland
Article Info
Article history:
Received Nov 19, 2018
Revised Feb 23, 2019
Accepted Mar 15, 2019
Keywords:
Wireless power transfer
(WPT)
Capacitive power transfer
(CPT)
Proportional integral (PI)
Single-bit Modulator
ABSTRACT
This paper proposes a single-bit ADC system based Proportional and Integral (PI)
controller to maintain a desired level of power transfer efficiency in Capacitive Power
Transfer (CPT) systems. In this paper, a simple single-bit ADC system i.e., Single-
Bit Modulator (SBM) is considered as an alternative to the commonly used multi-bit
ADC systems. Unique features of employing SBM are 1) its ability to convert analog
signals into single-bit signals and 2) its easy integrability in digital chips with linear
variable differential transformers (LVDTs) such as FPGAs. A SBM based PI (SBM-
PI) controller is designed to judicially interface with the single-bit output of SBM.
The proposed (SBM-PI) controller guarantees less hardware resources, latency and
regulates the output voltage to provide the desired power transfer efficiency. The be-
havior of SBM-PI controller is compared to that of a conventional multi-bit controller,
with the results of both controllers being identical. The effectiveness of the proposed
controller with SBM is further demonstrated using the experimental prototype of CPT
by implementing a SBM-PI controller using 16 MHz ATmega8 microcontroller. The
experimental results from a laboratory prototype illustrate that SBM-PI controller suc-
cessfully regulates the output voltage of CPT to control the power flow.
Copyright © 2019 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
Dhafer Almakhles,
Communications and Networks Engineering, Prince Sultan University,
P.O.Box No. 66833 Rafha Street, Riyadh 11586 Saudi Arabia.
Email: dalmakhles@psu.edu.sa
1. INTRODUCTION
Wireless power transfer (WPT) is a technique used to transmit power from a power source to a load
without using physical wires or conductors. Electrical systems using WPT are becoming increasingly popular
and have gained considerable attention in the research field for various applications including electric vehicle,
biomedical implants, cellphones, contactless battery chargers etc. [1-3]. There are two distinct techniques of
WPT, namely Inductive Power Transfer (IPT) and Capacitive Power Transfer (CPT). IPT uses the principle
of electromagnetic coupling and is suitable for both low and high power applications, whereas CPT uses the
principle of electric coupling and is suitable for lower power applications.
Among many advantages of CPT is that it has the ability to transfer power through metal barriers with
high efficiency due to its resistance to any external interference [4-9]. However, one of the issues with WPT sys-
tems and CPT systems in particular is the inability to maintain the power flow at a desired level. This is mainly
due to the uncertainties arise from load and circuit parameter variations and the operating frequency drifting of
the power supply i.e., the operating frequency of the power inverter [10-14]. Hence, proportional–integral (PI)
and proportional–integral–derivative (PID) controllers are often implemented to regulate the voltage in order
to control power flow between the transmitters and receivers of WPT systems [10, 12, 15, 16].
Journal homepage: http://iaescore.com/journals/index.php/IJPEDS

Int J Pow Elec & Dri Syst ISSN: 2088-8694 r 1271
Conventional digital controllers manipulate discrete multi-bit signals in order to provide control to
a particular system. A significant disadvantage of the conventional controller is the method in which the
desired analog information is processed, sampled and converted into the discrete-time domain. Multi-bit analog
to Digital Converters (ADC’s) increase the complexity of hardware implementation and latency in order to
accurately convey analog signals digitally. The hardware resources of the conventional multi-bit converters
i.e., wires, input/output ports, etc., increase with resolution in order to maintain the high accuracy required in
many control systems [17-21]. Multi-bit ADC’s encompass additional limitations; such as the limitation of
the finite word length of particular processors, setting further boundaries to the maximum available resolution
and hardware resources. Furthermore, some multi-bit systems require a DAC which increases the required
hardware even further.
As an effort to mitigate the demerits of large number of wires, complexity of implementations and
maintenance, one technique known as Sigma-Bit Modulation (SBM), has become quite popular for achieving
concise data transmissions and signal processing [22]. This technique uses SBM to convert the analog signal
or multi-bit digital signal to single bit signal, which is known as single-bit signal, providing several advantages
over the conventional control scheme and signal processing technique. First and the most obvious advantage
of SBM is that the signal is represented by single bit, which simplifies transmission route to one wire per
signal. Second, employing SBM reduces the hardware resources, complexity of implementation and remove
the latency caused by the decimation filters in multi-bit ADC. Similar to discrete controllers with multi-bit
inputs, discrete controllers with single-bit inputs manipulate discrete single-bit signals as opposed to multi-
bit signals in order to provide control to a particular system. Single-bit controllers have the advantage of
having the ability to use multiplexers as gains as opposed to the conventional multiplication gains. Thus, the
processors with single-bit inputs are built simpler than conventional Nyquist rate processor due to the less usage
of logic gates [18, 23, 24, 25, 26, 30]. The aforementioned features of single-bit control techniques make SBM
is suitable many applications [30, 31]. In particular, it becomes suitable to replace multi-bit ADC in power
electronic systems controlled by high operating frequency; CPT is a prime example [28].
This paper proposes a SBM-PI controller to regulate the output voltage a CPT system. The effec-
tiveness of the proposed control system will be examined by the regulating ability of a SBM-PI controller and
comparing the performance against a conventional multi-bit controller executing the same task. The robustness
of the controller will also be examined to fully identify the feasibility of a SBM based controllers on a WPT
system.
The layout of this paper is as follows; Section 2 contains an overview of the CPT system to be con-
trolled in this paper. Section 3 introduces the control problem, information about the single-bit signal, its
generation, SBM and SBM-PI controller. Section 4 briefly covers the simulation and experimental results of
CPT system controlled by SBM-PI controller. Finally, sections 5 and 6 will cover future recommendations and
conclusions respectively.
2. CAPACITIVE POWER TRANSFER
This section encompasses the WPT system chosen to be controlled, i.e., a CPT. Briefly discussing the
system, how it works, how it was designed and the response of the system.
2.1. Overview
Capacitive power transfer (CPT) systems take advantage of the principle of electric coupling in or-
der to successfully maintain WPT. CPT system advantages include high voltage based efficiency, the ability
to transfer between metal and low electromagnetic radiation. However, they are also associated with the dis-
advantages of being difficult to operate over large distances and having poor electric coupling safety which
can be overcome with adequate insulation. The most critical disadvantage is the inability for a CPT system to
naturally regulate their output voltage to a high degree of accuracy. Hence, a controller is implemented in order
for accurate regulation of system output voltage. For a more in-depth discussion on CPT refer to [29].
For this paper, a simplistic CPT topology was chosen to allow for ease of analysis and is shown in
Figure 1. As can be seen in the figure, the structure of the utilized CPT system includes a half-bridge inverter,
an inductor L in series with a capacitive coupling interface consisting of two pairs of coupling plates, and a
full-bridge rectifier followed by a DC load.
Single-bit modulator for... (Dhafer J. Almakhles)

1272 r ISSN: 2088-8694
o
V
dc
V
NOT
PWM
ref
V
L 1
C
2
C
1
D
2
D
3
D
4
D
R
3
C


Figure 1. CPT system design
2.2. Design principles
In order to ensure the CPT is operating appropriately a few basic design principles must be followed.
The first being that ‘shoot-through’ of the switches does not occur. Shoot-through occurs when the two switches
are both turned on (closed) which effectively shorts the supply to ground. For simulations, in order to ensure
this procedure does not occur the switches are controlled by a single PWM signal along with a simple NOT
conductor. As for the hardware design, a half-bridge driver with a single input ensured that the switches never
turned on simultaneously. This driver responded to a single PWM input signal and produced the appropriate
signals which incorporated dead time to enable switches to successfully turn completely off.
The second design principle is to design for LC resonance within the system. Resonance optimizes
efficiency by matching the capacitor and inductor voltage magnitudes allowing for maximum power to the
resistive load. The impedance of inductor and the two capacitors are opposite and directly related to frequency
XL = jωL , XC =
1
jωC/2
where XL and XC are the inductor and capacitor impedance respectively, ω = 2πf where f denotes the
operating frequency of the half-bridge inverter, L is the inductance and C is the capacitance of each capacitor.
Generally, the inductor value L is chosen dependent on the system frequency f and the capacitance C of the
electrically coupled metal plates. Due to the infinite reactance of capacitors at zero frequency, the two switches
operating out of phase are required to act as a DC to AC inverter enabling current flow through the electrically
coupled metal plates. The AC/DC rectifier then converts the signal back into DC for the load to consume.
3. CONTROL SYSTEMS DESIGN
This section encompasses the controller aspect of this paper, primarily designed to remove the error
of the steady state value for applications that require precision in the power transferred. The section will cover
the type of controller design and how the conventional controller is manipulated for single-bit based controller.
3.1. Open loop control for CPT system
The control scheme for the CPT systems is described as follow. A reference voltage Vref is used to
determine the duty cycle of the PWM switches ranging from 0V to 5V, representing 0% to 50% duty cycle,
respectively. The output voltage Vo is directly proportional to the duty cycle of the PWM signals controlling
the DC to AC inverter. The maximum output voltage can be obtained when Vref = 2.5V (or 50% duty cycle)
and described as
Vo = Vdc/2γ
where γ > 1 denotes uncertainties due to variation of operating frequency f, system parameters, distance
between conductive plates, load changing in the CPT system, etc. The presence of the nonlinear components
i.e., diodes and the uncertainties factors denoted by γ in CPT often introduces high steady state error i.e.,
e = Vref − Vo (1)
Int J Pow Elec & Dri Syst, Vol. 10, No. 3, September 2019 : 1270 – 1280

Int J Pow Elec & Dri Syst ISSN: 2088-8694 r 1273
where CPT systems fail to accurately regulate voltage. Achieving exact or variable outputs for particular
applications is difficult as the exact duty cycle required needs to be known. This behavior is undesired and
attenuating steady state error is one of the main focuses for the upcoming controller where a PI is proposed in
order to reduce the steady state error e ≈ 0 along with the potential rise time.
3.2. Single-Bit Modulator and Single-bit based controller
This section encompasses the basics of the single-bit signal, briefly discussing its implementation and
generation through the chosen modulation along with some of its advantages.
In terms of the single-bit based controller for a CPT, a single-bit signal is a method of representing an
analog signal in the discrete time domain by using only one-bit of data per sample time. This signal consists of
only two possible values; ‘high − low’ or ‘on − off’; which interchanges between the two possible values at
a relatively high switching frequency. The average value of the single-bit signal is equivalent to the average or
root mean square (RMS) of the analog signal being replicated. Both the frequency and duty cycle are able to
vary in order for the single-bit signal to accurately resemble the analog signal. Demonstrated in Figure 2 is the
single-bit signal representation of a sine wave, with the sampling frequency reduced for clarity.
Figure 2. Single-bit representation of sine wave
3.3. Single-Bit Modulator
SBMs are type of single-bit ADCs where the input is an analog signal and the output is the corre-
sponding digital single-bit signal. Basic SBMs are extremely simplistic, comprising of only a few components
in comparison to multi-bit ADC. There are multiple topologies for producing a single-bit signal from an analog
signal. Three possible techniques of SBM are delta (4-M), delta-sigma (Σ4-M) or hybrid Σ4-M. Cascad-
ing these techniques produces multi-order topologies of single-bit converters which enables an increase in the
resolution of the digital signal. Figure 3 shows a simple SBM, which is used in this paper.
e
0
k

ˆ

ê
Q

1
Q

Low-Pass
Filter
clk
s
Figure 3. Single-bit modulator topology
This modulator comprises of a comparator, first order RC low-pass filter and a gain/multiplexer. Using
a simple feedback network, this system attempts to pull the error between the comparators inputs to 0, resulting
in a single-bit signal representing that of the input analog signal. The output of the modulator is either 1 or 0
due to δ function, which is given by
δk =
1
2
(1 + sgn (s)) =
(
1 ∀ s ≥ 0
0 ∀ s < 0
(2)

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since sgn (s) =
(
+1 ∀ s ≥ 0
−1 ∀ s < 0
for all t ∈ [kTs, (k + 1)Ts] where 0 < Ts 1 denotes the sampling period.
The input of the comparator is given by s = e − ê where e is defined in (1) and ê denotes the average value
(i.e., slow components) of the discontinuous function sgn (s) which is obtained by filtering it through first order
low-pass filter
ê := Qsgn (s) − τ ˙
ê (3)
where τ is the time constant and Q is known as quantization boundary [26].
3.4. Single-bit based controller
The single-bit based controller can be classified to conventional PI controller and hybrid PI controller
[26]. Implementing conventional single-bit PI controller is limited to which the proper Q values used in SBMs
of the proportional and the integral have to be carefully analyzed and selected in order to minimize the quanti-
zation noise. The SBM-PI controller which is used to control CPT system, is defined as
û = Qkpsgn (s) + kiQ
t
ˆ
0
sgn (s) dτ (4)
where û is the control signal, Q is the quantization boundary value of controller input SBM, kp is proportional
gain and ki is integral gain.
The requirement for the single-bit based controller is to have an input-output interface in single-
bit format, which means the multi-bits arithmetic elements in the digital controller are replaced by single-bit
function blocks. These blocks integrating SBM in arithmetic elements, such as adder, subtractor, and integrator,
have been used and tested in different papers [26, 27].
In this paper, SBM based PI is implemented on microcontroller. Thus, it is of importance to convert the
continuous-time system into its quasi-continuous approximation using the well-known Euler’s discretization
method. The discrete-time transformation of (4), which is shown in Figure 4, is given by ûk = ûp
k + ûi
k where
ûp
k =
(
KUp = +Qkp ∀ δ̂k = 1
KLp = −Qkp ∀ δ̂k = 0
and
ûi
k =
(
KUiρ−1
∀ δ̂k = 1
KLiρ−1
∀ δ̂k = 0
where KUi = QTski, KLi = −QTski, Ts denotes the sampling frequency and ρ = z − 1. The bold line is the
multi-bit signal and the unbold line is the single-bits signal. These signal processing elements are detailed in
[26].
Up
K
Lp
K
k
ˆ
Ui
K
Li
K
1

 
i
k
û k
û
p
k
û
Figure 4. Hybrid single-bit PI
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Int J Pow Elec Dri Syst ISSN: 2088-8694 r 1275
4. SIMULATION AND EXPERIMENTAL RESULTS
Contained within this section are the findings of this paper. It demonstrates the performance and
ability of the SBM-PI on a CPT system. Both simulation results and hardware results are presented.
4.1. Hardware prototype and specifications of CPT system
Multiple hardware prototypes were produced in the overall design process, with the final single-bit
based control CPT system shown in Figure 5. The system parameters of CPT system, which is described by
the equivalent circuit in Figure 1, are given in Table 1.
Figure 5. Prototype of the CPT system.
Table 1. Parameters of experimental prototype
Symbol Value
Vdc 12 V
L 27.6 µH
C1 470 nF
C2 470 nF
C3 470 µF
R 47 Ω
Controller ATmega8 (max clk = 16MHz)
PWM’s sampling frequency fs = 62.5 kHz (dead time of 0.5 µs)
half-bridge driver IR2183
MOSFETs S1 and S2 IRF3205
Diodes D1 − D4 1N4001
4.2. Simulation results
Simulations were conducted in Matlab’s simulink with the PLECS extension. Figure 6 (a) and (b)
show both the simulated results as well as the hardware results for the open loop CPT system, with 3V input
resulting in a 4V output. The system response has a steady-state error of approximately 1V with an input of
3V ’s. By examining the issues pertained within the open-loop response of the CPT system the type of controller
required to successfully achieve the desired response can be deduced. Although the system contained severe
steady state error there was no oscillations residing in the transient of the open-loop response.
In order to gauge the success of the single-bit controller it was first produced in a simulated envi-
ronment and compared to the performance of a simulated conventional multi-bit controller operating on the
exact same system. Simulations provided sufficient evidence that single-bit based controllers provided accu-
rate voltage regulating abilities. Figure 7 (a) shows the open loop system response to a 2V and 4V reference
while Figure 7 (b) shows the controlled responses of the CPT to the same reference voltage, i.e., 2V and 4V
reference using single-bit based PI controller with kp = 255 and ki = 55. As can be seen in Figure 7 (b), the
controller was able to successfully eliminate the steady state value error present in the open loop responses in
both cases for 2V and 4V input signals. Additionally, there was a decrease in both rise time and settling time
for both scenarios. Furthermore, both the conventional and single-bit controllers responded exactly the same
in simulations, this suggests that the single-bit controller performs at an equal level to that of the conventional
controller.

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0.02 0.16
0.14
0.1
0.08
0.06
0.04 0.12 0.20
0.18
Time (sec)
0.00
0.02 0.16
0.14
0.1
0.08
0.06
0.04 0.12 0.20
0.18
Time (sec)
0.00
1
2
4
3
0
Voltage
(V)
1
2
4
3
0
Voltage
(V)
(a)
(b)
Figure 6. : Open loop CPT systems responce (a) Simulated resulted with 3V input, and (b) Hardware
response with 3V input.
0.5
Time (sec)
0.00
1
2
4
3
0
Voltage
(V)
(a)
5
1
2
4
3
0
Voltage
(V)
5
2.5
2.0
1.5
1.0 3.0
0.5
Time (sec)
0.00
(b)
2.5
2.0
1.5
1.0 3.0
Figure 7. Simulation results for open and closed loop CPT systems responce (a) Simulated resulted for
open-loop CPT systems with 2V and 4V inputs, and (b) Simulated resulted for controlled CPT systems with
2V and 4V inputs and PI controller.
4.3. Experimental results
Due to the success of the simulations on the CPT system, both a conventional and a single-bit con-
troller were designed and implemented using hardware to prove the viability of the single-bit control methods
even further. The responses obtained from the hardware resembled the simulations closely. They maintained

similar overshoot, rise time and settling time along with obtaining the same response in terms of steady state
voltage regulation. Demonstrated in Figure 8 is the controlled hardware response for a 2V , 3V and 4V refer-
ence voltage. Finally, observing the robustness of the controllers was undertaken. A well performing controller
is able to adequately regulate the response of a system even when system parameters are to venture away from
their optimal states. Examples of parameters that could vary are the values of original component values, the
DC supply, the operating frequency and the deterioration of components. The feature of a CPT system that is
the most susceptible to change is the equivalent capacitance of the electrically coupled metal plates due to the
potential air gap variances. The controller is able to successfully sustain the steady state voltage regulation with
both a large increase and decrease in equivalent capacitance values. This suggests that the system is adequately
robust.
1
0
0.5
Time (sec)
0.00 2.5
2.0
1.5
1.0 3.0 3.5 4.0
2
4
3
Voltage
(V)
Figure 8. Controlled hardware response of the CPT system.
5. FUTURE RECOMMENDATIONS
One of the largest flaws in this paper was the accuracy of the single-bit signal. The technique used as
a single-bit modulator was one of the simplest methods which reduced the overall accuracy slightly. However,
the most significant issue was the ATmega8 microcontroller chosen. The ATmega8’s limited clock speed was
a severe hindrance. For future work in this field I would recommend using a processor with a larger clock
speed. This improvement will result in a larger sampling frequency for the single-bit modulator which results
in an increase of the analog to digital conversion resolution. Additionally, a multi-order delta-sigma modulator
could be implemented in order to further increase the accuracy. The second flaw in this project is also related
to the chosen microcontroller. The ATmega8’s PWM feature, which is used to control the switches in the CPT
system, was limited to 62.5 kHz at 8-bits of accuracy which is much too low. Practical CPT Systems generally
operate at higher frequencies in order to operate successfully with the low capacitance of electrically coupled
metal plates. This issue would be rectified by using a faster processor and would allow for the controller to be
implemented on a more practical system. Finally, implementing the single-bit controller in a networked control
system. Networked control systems are becoming increasingly popular and single-bit is theorized to be a more
viable option due to the benefits of transmitting single-bit as opposed to multi-bit packages.
6. CONCLUSION
In conclusion, a proposed method to manipulating a single-bit controller for a CPT system has been
presented. This work included the design of both a conventional multi-bit controller and single-bit controller.
The single-bit controller’s performance was compared to that of the conventional multi-bit controller and the
results demonstrated that both controllers were able to successfully regulate the response of the specified CPT
system. This paper has proven the concept of using a single-bit controller on a CPT system and has provided
headway for further research on the topic.
ACKNOWLEDGEMENT
The authors would like to thank both Nathan Pyle and Hossein Mehrabi for the help during the prepa-
ration of this work.

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BIOGRAPHIES OF AUTHORS
Dhafer Almakhles received B.E. degree in Electrical Engineering from King Fahd University of
Petroleum and Minerals, Dhahran, Saudi Arabia in 2006 and completed his Masters degree (Hons.)
and PhD from The University of Auckland, New Zealand in 2011 and 2016, respectively. Since
2016, he has been with Prince Sultan University - Saudi Arabia, where he is currently an assistant
professor with the department of Communications and Networks Engineering and the Director of
Science and Technology Unit. He has authored more than twenty published papers in the area of
control systems. He served as a reviewer for many journals including IEEE Transactions on: Fuzzy
Systems, Control of Network Systems, Industrial Electronics, Control Systems Technology as well
as IEEE Control Systems Letters and International Journal of Control. His research interests include
the hardware implementation of control theory, signal processing, networked control systems and
sliding mode.
Akshya Kumar received the B.Sc. degree in electrical engineering and the M.Sc. degree in elec-
tronic systems and communication from the Sambalpur University, Sambalpur, India, in 1985 and
1988, respectively, and the Ph.D. degree from the Department of Automatic Control and Sys-
tems Engineering, University of Sheffield, Sheffield, U.K., in 1997.,From 1994 to 1996, he was
a Commonwealth Scholar in the United Kingdom. Since September 2002, he has been with the
Department of Electrical and Computer Engineering, The University of Auckland, Auckland, New
Zealand. His present research interests include nonlinear system identification and control, biomed-
ical signal processing, sensor networks, and control applications to power system and wireless
power transfer systems.,Dr. Swain is an Associate Editor of the IEEE Sensors Journal. He is a
Member of the Editorial Board of the International Journal of Automation and Control and the
International Journal of Sensors, Wireless Communications and Control.

1280 r ISSN: 2088-8694
Umashankar Subramaniam is working as Associate Professor in Renewable Energy Lab, Col-
lege of Engineering, Prince Sultan University, Saudi Arabia has 15+ years of teaching, research
and industrial R and D experience. Previously, he worked as Associate Professor and Head, VIT
Vellore as well as Senior R and D and Senior Application Engineer in the field of power elec-
tronics, renewable Energy and electrical drives. He is a Senior member-IEEE, and Member of
IACSIT, IDES and ISTE. He has taken charge as Vice Chair - IEEE Madras Section and Chair -
IEEE Student Activities from 2018. He was executive member (2014-16) and Vice Chair of IEEE
MAS Young Professional from 2017 by IEEE Madras Section. He has published more than 250+
research papers in national and international journals and conferences. He has also authored/co-
authored/contributed 12 books/chapters and 12 technical articles on power electronics applications
in renewable energy and allied areas. He is an Editor of Heliyon, an Elsevier journal. He received
Danfoss Innovator Award-Mentor during 2014-15 and 2017-18, Research Award from VIT Univer-
sity during 2013-18. Also he received the INAE Summer Research Fellowship for the year 2014.
Under his guidance 24 P.G students and more than 25 U.G Students completed the senior design
project work. Also 6 PhD scholars completed Doctoral thesis as Research Associate. He is also in-
volved in collaborative research projects with various international and national level organizations
and research institutions.

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