A secured wireless charging scheme with data transmission

This research task proposes a magnetic resonant coupling wireless power transfer(MRC-WPT) scheme for electric vehicles, which realizes secure data communication between the wireless charging station and the electric vehicle using binary phase shift-keying (BPSK) data transmission, thereby waving any radio frequency(RF) communication links. The secondary side is designed to accept power, while it transmits data back to the primary side simultaneously. The BPSK design ensures data transmission robustness to against disturbance interference. The data are encrypted by the user's biometric features for information security. The magnetic resonance performs better than that of the traditional magnetic induction in the power transmission efficiency against coil misalignments. With the Internet of Thing(IOT) architecture, the encrypted data can be stored and retrieved via the cloud server.


Introduction
Existing power grids are almost everywhere, how to build up an easy-access charging environment for electric vehicles is becoming a crucial issue. However, most wireless power transmission (WPT) technologies are dedicated to develop alternative possibilities to provide portable devices or pick-up units that receive power wirelessly. WPT has been extensively used in various fields, and versatile wireless charging schemes have recently been proposed according to specific product requirements. WPT technology can be applied universally to various application fields such as modern industry, electric vehicles [1,2], aerospace [3], consumer electronics [4] and implantable medical devices [5,6].
The WPT is an innovative technology that transmits electric energy via time-varying electromagnetic field. The benefits of WPT have attracted much attention recently and have become a crucial part for consumer electronics and daily applications such as smartphone wireless charging [4] and the medical implantation system [6]. The WPT systems for electric vehicles with large power transfer have gradually received much attention to the vehicle battery manufacturers. However, most wireless charging systems only transfer power, while data communication is realized separately [7][8][9][10][11]. With regard to data transmission, the modern electric vehicle wireless charging systems still rely on extra communication links for the purpose. This leads to higher costs in hardware with extra power rating of the system.
In the published literature, researchers have proposed various wireless electric vehicle charging schemes [12]. In [13], the authors used phase synchronization method by tracking the maximal output current to fulfil power transfer. The simultaneous wireless power and data transfer was recently considered in [14][15][16]. In our pioneered research task [14], simultaneous wireless power/data transfer for electric vehicle charging with amplitude modulation and frequency shift keying (FSK) was attempted. In the simplified architecture proposed in [15], power and data are transmitted to the same direction at the same time. That meant, one cannot monitor charging status and engine information of the vehicle. In [16], data transmission protocol of the system used two different frequencies to realize data transmission based on FSK. However, frequency of the power carrier did not comply to the SAE standard. Among the research tasks depicted above none of them has addressed the issue of information security during communication.
Power line communication (PLC) is a communication technology that sends data over the existing wires [17]. The idea of simultaneous power and data transfer in the WPT system is a bit similar to PLC but with a completely different structure.
Vehicle charging using WPT is unmatured at the moment; there are still weaknesses to be resolved such as low power transferring efficiency and alignment of power transmission pads [18][19][20]. Compared with electric vehicles using wired charging, the wireless charging methods do have potentials from the viewpoint of driver convenience. However, most wireless charging technology that currently appeared on the market provides the function of power transfer only, no data feedback from the vehicle while charging. Unless there are repeaters or gateways, communication of the underground parking lots is not always available. Signals such as FM and 4G network signals would be shielded by basement structure, making it difficult for vehicles to communicate with the outside world.
For the WPT with communication, there are still critical issues worthy of further study such as optimization of power transfer efficiency, power transfer range and data transfer security and throughput. The major objective of this research task is to develop a WPT system that electric power transfer between two isolated units should be able to allow slight misalignment of power transmission pads and the secondary side can simultaneously feedback data to the primary side with security. The wireless power transfer operating frequency used is set to 83 kHz to comply with the frequency standard (81.39-90 kHz) proposed by the SAE J2954. In addition, by combining the wireless charging system and the Internet of Things (IoT), the vehicle (charging) status can be monitored and recorded via mobile networks.
Three major contributions specify the features of the proposed design: (1) Simultaneous WPT power/communication transfer The wireless charging system charges the vehicle battery via a magnetic resonant coupler. Simultaneously, weak power signals carry data at the vehicle side can be sent back to the charging station via the binary phase shift-keying (BPSK) technique and currentvoltage phase difference detection to avoid influence by the received amplitude variation. The process of communication does not rely on external modulator to superimpose information. (2) Biometric-based data encryption Vehicle information is encrypted for information security.

(3) IoT application
The vehicle status can be monitored on the web page via the grid side connecting the server. It allows users to monitor vehicle charging status (and more) via mobile networks. It should be emphasized that IoT components are being used for communications via Ethernet at the grid side. When there is in the absence of radio frequency (RF) links (such as the underground parking lots), the vehicle is still able to communicate with the cloud server-a fundamental function for the smart grid.

MRC-WPT system
We propose a bi-functional magnetic resonant coupling wireless power transfer (MRC-WPT) scheme. The method for transmission is to convert the DC power source at the primary (grid) side to a high-frequency magnetic field and transmits power wirelessly to the secondary side through the magnetic coupling resonator. The secondary (vehicle) side induces power from the high-frequency magnetic field and converts voltage back to DC voltage through a full bridge rectifier for vehicle battery charging (Fig. 1). The MRC-WPT system performs better than the magnetic inductive type in terms of power transmission while there are inductive coil misalignments. The magnetic resonance coupling wireless charging system realizes the wireless transmission of electric energy by magnetically coupling the resonant circuit at the transmitting side (primary side) and the resonant circuit at the receiving side (secondary side), thereby realizing power transmission. Inductive coils on both sides connect in either series or parallel connection with a capacitor. Thus, the magnetic resonant field may occur on both sides at the resonant frequency.
It is well known that the coupling coefficient reflects the amount of magnetic flux in the coupling coil to induce voltage at the secondary side. It is closely related to the mutual inductance between both coils and the primary and secondary self-inductances. When the primary inductance and capacitance are tuned at the resonant frequency, a strongly enhanced magnetic field will be established in the transmitter coil. The magnetic field grows significantly due to resonance.

LC compensation topologies
The air gap between two inductive coils affects inductance leakage and coupling coefficient. Therefore, a compensation Fig. 2 The SS compensation configuration circuit benefits in minimizing the volt-ampere (VA) rating of the power supply at resonance. By adjusting current of the power supply circuit, the secondary-side voltage efficiency can be improved. To improve the power transfer efficiency, the MRC-WPT system needs to operate as close as possible to the resonant frequency of the compensation networks. When both sides are tuned near to the resonant frequency ω, it can be expressed as It is crucial to reduce the VA rating by making the primaryside resonant, which is the way to reduce power conversion loss. The compensation capacitor at the primary side is tuned appropriately so that the input voltage and current are in phase under the coupling and loading conditions-known as zerophase-angle (ZPA). By tuning the compensation network at the primary side, it can maintain a small amount of reactive power at the primary side to reach zero-voltage switching (ZVS) or zero-current switching (ZCS) conditions and hence to realize soft-switching for the inverter.
Moreover, when the coupling coefficient is low, the power factor decreases causing energy conversion loss, because voltage and current are not in phase. To have an appropriate power factor, the resonant operation is required. The reactance of the circuit can be divided into two types: inductive reactance and capacitive reactance. When the circuit operated at the resonant frequency ω and current lags voltage, it acts as an inductive reactance X L = ωL. On the other hand, when current leads voltage, the circuit is considered as a capacitive reactance X C = 1 ωC . The fundamental topology arrangements include seriesseries compensation (S-S), series parallel compensation (S-P), parallel-series compensation (P-S) and parallel-parallel compensation (P-P) topologies. The SS compensation topology in Fig. 2 is more suitable for EV wireless charging than others. The major advantage is that the capacitance values of the transmitting end and the receiving end are unrelated with the load and mutual inductances. Therefore, the In Fig. 3, i i denotes the input power current, i P denotes the primary coil current, C P denotes the primary compensation network capacitor, i S denotes the secondary-side current, R 1 is primary coil resistance, R 2 is the secondary coil resistance, i L denotes the load current, C S denotes the secondary compensation network capacitor, and R L represents the load at the secondary side.
To tune the primary-side capacitance, first, the load quality factor at the secondary side is defined. The ratio of inductance to the resistance of the secondary coil is adopted for the purpose [21,22]. The secondary side's load quality factor Q s for the series-compensated topology is given by For the parallel-compensated topology, it becomes According to the quality factor at the secondary side, the primary side's capacitance C p of the compensation of S-S topology has a constant value of capacitance regardless of the coupling and load conditions, which is For the S-S compensation topology and circuit adopted here, the secondary impedance Z S is given by The reflected impedance Z r from the secondary side to the primary side is The total impedance Z P for the primary side, determined by combining the primary and secondary circuits, can be described as The phase angle of the total impedance Z P at the primary side is known to be the relationship between the active power W over the apparent power S: Once the change of phase angle ϕ between the voltage and current phasors is obtained, the power factor is specified. The phasors can be lagging, leading or in-phase. Under the premise of insignificantly affecting the power factor, the phase change is introduced to modulate data which record vehicle battery status at the vehicle side. The data are then returned to the charging station for the vehicle battery status monitoring and energy management. Compared with our previous approach based on amplitude modulation [14], data modulation based on the phase change would be more robust to against voltage variations at the primary side.

Compensation topology
Data communication for the MRC-WPT adopts a serial protocol that uses phase deviation between inductive voltage and current. The binary data 1 and 0 are transmitted through the resonant magnetic field. The waveforms exhibit different amplitudes and phases to represent specific signs. The higher the magnetic resonant frequency of the MRC-WPT is the faster the data transmission speed.
The phase modulated signal S p can be described as [23]: where A c is the amplitude of the modulated wave, the constant K p represents the phase sensitivity of the modulator, m(t) is the message signal, and f c is the modulation frequency.
The technique of data transmission in this research is named phase shift keying or BPSK shown as in Fig. 3. To discriminate the binary data "1" and "0", the equations are defined here as S 0 (t) = A c cos(2π f c t + π), to modulate "0 The length of the data packet is 122 bits (Fig. 4) involving a start bit, encrypted vehicle data, secret key, vehicle battery level and a stop bit. The bit period is nearly as same as the inverse of the operational frequency, i.e., the resonant frequency f. The baud rate is simply the number of signal changes per second, i.e., Baudrate = bps log 2 N (11) where N represents the number of distinct messages that could be sent and the data transmission speed is measured in bits per second (bps). To speed up data transmission, this research adopts the BPSK scheme with each symbol occupying 6 bits. The symbol encoder is commonly used in the data transmission protocol. It consists of 37 upper case alphabet characters, 10 numbers of characters, and space. Ifx is the bit number, the maximum symbol possibility will be 2 x .
The modulator calculates the operating resonant frequency to incorporate data in right timing. The modulation takes 6 period cycles as a reference to transmit a single bit of the data packet. The method of data transmission allows one to increase robustness of communication quality and optimize power transmission. The total bits in modulation can be defined by total bits = data packet · N where ΔN is frequency cycle, i.e., the number of occurrences of a repeating event per unit of time. The bit allocation for complete transmission is shown in Fig. 5. Figure 6 illustrates how the  Data are decoded by measuring the primary-side LC tank voltage and current. The converter allows AC voltage and current signals converted into pulse width signals to the microcontroller. The signals interrupt the timer of the microcontroller to calculate the pulse-width and frequency of the MRC-WPT system. See Fig. 7, by calculating the duty cycle D one can map it into the phase difference between voltage and current in each cycle. To decode data from the modulated signal of the MRC-WPT system, the MCU 3 measures voltage and current of the LC tank circuit. The MCU timer proceeds to calculate the phase angle between voltage and current of the LC tank at the primary side. See Fig. 7 for illustration of waveform of the primary LC compensation network.

IoT model and data security
The IoT system here is to conduct data decryption and provide web hosting services as shown in Fig. 8. The hardware includes a single-board computer as the server and Ethernets to a Wi-Fi module settled at the grid side and links to the cloud server. Figure 9 illustrates the wireless power transfer and data feedback structure. The simplified circuit diagram involves a primary side, a secondary side and a cloud server. See Fig. 10 for the experimental configuration. To comply with the SAE standard, the primary side generates 83 kHz magnetic field to transmit power and the secondary side receives transmitted power from the alternating magnetic field to the load. Data feedback is performed simultaneously at the secondary side,

Architecture
The hardware for experimental verification in Fig. 10 involves three microcontrollers: two in the primary side and one in the secondary side. For the primary side, the MCU1 controls the full-bridge inverter with 4 PWM signals, while the MCU2 demodulates the data from the demodulator. The data transmission from the secondary side is manipulated by the MCU3. The synchronization circuit provides the operating frequency to the MCU3 for data transmission. The microcomputer serves as the server of the IoT system. It restores encrypted data and provides web hosting services.
To modulate data sent from the secondary side, it needs a modulator to increase the capacitance at the secondary-side circuit, thereby alternating the resonant frequency shift. The green block in Fig. 11 shows the modulator schematic which is connected in parallel with an LC tank at the secondary-side.
The modulator consists of a N-MOSFET and an optocoupler to isolate the MCU3 from the high-voltage circuit, while Fig. 12 The IoT scenario the it converts user's input data for transmission. When there are data to be transmitted, the MCU3 controls the secondary capacitance by switching the MOS switch which is realized by a frequency synchronization circuit to alternate resonant frequency. It turns on the MOS switch to drag down resonant frequency by placing C m in parallel with the resonant capacitor C s which leads to the effect of current leading voltage phase when modulating "0". It otherwise turns off the MOS switch when modulating "1". Figure 12 illustrates the network and I/O port connection for the IoT. The decoded data at the primary side are transmitted to the server side via port 4100 through a Wi-Fi module. The web-based dashboard displays the decrypted data on port 3000. The client and server are connected to the wide-area network (WAN).

Data encryption and decryption
The Hill cipher cryptography is adopted to encrypt and decrypt information for data transfer security that is a polygraphic substitution cipher based on linear algebra [24]. The core of the Hill cipher is a matrix manipulation and inversion for encryption and decryption, respectively. The matrix used for encryption is the cipher key. It is chosen randomly from the set of invertiblen × n matrices. Here, modulo 37 was used. The encryption is represented by the following expression: where C denotes the ciphertext, the non-singular K denotes the secret key for encryption, and P is text of user input information. Figure 13 shows the signal waveforms of the MRC-WPT system worked at transmitting 400 W output power. Channels 1 to 8 show the inverter S1 PWM signal, the voltage waveform (V P ) and the current waveform (i P ) of the compensation circuit at the primary side, the voltage waveform (V S ) and the current waveform (i S ) of the compensation circuit at the secondary side, and the modulated signal.

Communication quality
To verify data transmission stability, five data packets are randomly selected for verification while transmitting power at the primary side. Each data packet consists of 18 bits including a start bit, 8 bits of the data packet, 8 bits of the checksum, and a stop bit. The communication quality is defined as Comm quality = Correct received bits Total transmitted bits × 100% (13) where total bits are the length of data packet transmitted from the secondary side, correct received bits are the bit number received and decoded correctly by the decoder circuit at the  Figure 16 shows the converted waveforms of the primary side's LC tank and the modulated signal in which the first channel to the third channel show, respectively, the voltage waveform, the current waveform and the modulated data signal. The compensated capacitance C m for modulation of "0" at the secondary side is 0.07 µF. When the secondary side modulates data, the LC tank's capacitance of the secondary side changes accordingly, and the primary side reacts to the change in less than 16 us (Fig. 14). When the MRC-WPT operates at the resonant frequency while transmitting power, the current phase (orange line) leads voltage (green line) by 25 deg, which was used to modulate the digitized "0", as displayed in Fig. 15. See the horizontal width between the dash lines. Figure 16 displays the modulated data from the secondary side which shows the current phase lags voltage by 27 deg. It was used to modulate the digitized "1".

Communication performance
The results of data communication quality are recorded and displayed in Fig. 17. The proposed system is shown to perform signal demodulation correctly when it transmits data from the secondary side to the primary side with a lower power input (15-30 W) and higher power input (120-200 W). Since phase difference is used to modulate data rather than the amplitude modulation (AM), its data communication quality is more robust to against input voltage variations.
The power transmission efficiency for the 100 W rated power is shown in Fig. 18. By changing the coil lateral offset, the transmission efficiency of the proposed system still maintains at above 80%. When the power transmission efficiency of the MRC-WPT system keeps at above 80%, the communication quality remains perfectly at 100%. The vehicle information modulated at the secondary side contains 732 bits. Data modulation used is BPSK. Figure 19 shows the time period taken for the whole modulation.

Data transfer security
The simultaneous power and data transfer with biometric encryption also conducted with the HMI and application. To simulate the application, first, the user inputs name and license plate via the HMI as shown in Fig. 20.
The HMI's graphical user interface (GUI) allows users to enter vehicle information and extract fingerprint features for access control. In Fig. 20, the 2 blanks are for the driver's name and license plate which are considered as sensitive user data in (12). By clicking the register button on the GUI, it extracts fingerprint features and converted in the matrix K in (12) which serves as the secret key for encryption and data compression. The user registration GUI also provides a QR code allowing users to launch web-based dashboards using mobile devices. When completing the fingerprint scan for data encryption, the encrypted data are converted into the binary form. The MCU3 modulates data and adds to the secondary compensation circuit through a modulator. The decoder decodes the encrypted information at the primary side shown as in Fig. 21.  Scanning the QR code on the HMI directs the mobile device to the web URL, which allows the user to check vehicle information at any place. The encrypted information is also uploaded to the cloud server via the Ethernet for data decryption. The vehicle information is shown on the webpage via a mobile phone (Fig. 22).

Conclusions
A new wireless power transferring scheme for vehicles with built-in data feedback from the vehicle side to the charging station is proposed which reduces manufacturing cost by waiving the need of extra communication links. The MRC-WPT scheme equipped with the BPSK data transmission mechanism ensures performance robustness of power transmission and secure data communication. The IoT application allows users to monitor vehicle charging status (and more) via the cloud server through the proposed scheme. The encrypted vehicle information is adopted to ensure information security. Real-world experiments have been conducted to verify functionalities and performance.
The application scenario of this attempt might be at the underground parking lot or certain parking spaces where RF communications are unable to access by vehicles. This function serves as a part to form a microgrid by linking vehicles to the grid side provided that bidirectional power transfer is ultimately fulfilled.