Development of Intelligent Drone Battery Charging System Based on Wireless Power Transmission Using Hill Climbing Algorithm

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Article Development of Intelligent Drone Battery Charging System Based on Wireless Power Transmission Using Hill Climbing Algorithm

Ali Rohan 1,* , Mohammed Rabah 1 , Muhammad Talha 1 and Sung-Ho Kim 2

1 Department of Electrical, Electronics and Information Engineering, Kunsan National University, Gunsan-Si 573-360, Korea; [email protected] (M.R.); [email protected] (M.T.) 2 Department of Control and Robotics Engineering, Kunsan National University, Gunsan-Si 573-360, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-10-2857-6080

Received: 13 September 2018; Accepted: 5 November 2018; Published: 7 November 2018

Abstract: In this work, an advanced drone battery charging system is developed. The system is composed of a drone charging station with multiple power transmitters and a receiver to charge the battery of a drone. A resonance inductive coupling-based wireless power transmission technique is used. With limits of wireless power transmission in inductive coupling, it is necessary that the coupling between a transmitter and receiver be strong for efficient power transmission; however, for a drone, it is normally hard to land it properly on a charging station or a charging device to get maximum coupling for efficient wireless power transmission. Normally, some physical sensors such as ultrasonic sensors and infrared sensors are used to align the transmitter and receiver for proper coupling and wireless power transmission; however, in this system, a novel method based on the hill climbing algorithm is proposed to control the coupling between the transmitter and a receiver without using any physical sensor. The feasibility of the proposed algorithm was checked using MATLAB. A practical test bench was developed for the system and several experiments were conducted under different scenarios. The system is fully automatic and gives 98.8% accuracy (achieved under different test scenarios) for mitigating the poor landing effect. Also, the efficiency η of 85% is achieved for wireless power transmission. The test results show that the proposed drone battery charging system is efficient enough to mitigate the coupling effect caused by the poor landing of the drone, with the possibility to land freely on the charging station without the worry of power transmission loss.

Keywords: wireless power transfer; unmanned aerial vehicle; automatic charging station; drone station; hill climbing

1. Introduction

1.1. Introduction and Motivation The quadcopter, also known as a quadrotor, is a type of unmanned aerial vehicle (UAV), lifted and propelled by four rotors [2,3]. Quadcopters use two pairs of identical ﬁxed pitched propellers: two clockwise and two counter-clockwise. The quadcopter has high maneuverability, as it can hover, take off, cruise, and land in narrow areas. It also has a simpler control mechanism compared to other UAVs [4] and is equipped with different components such as an inertial measurement unit (IMU), a global positioning system (GPS), an electronic speed control (ESC), a standard radio control (RC), a radio-frequency module (RF) used to transmit live videos to a personal computer (PC), and a ﬂight controller [5,6].

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The quadcopter is used for applications such as surveillance, search and rescue, and object detection [7–9]. As mentioned earlier, a quadcopter with two pairs of fixed pitched propellers has a very short operation time because it has to generate lift force all the time to move around, which requires high electrical power. Batteries are used as an electrical power source in quadcopters; however, because of the high electrical power requirement, the normal operation time of a quadcopter is just 20 to 30 min [10]. This limits the quadcopter flight range and operation time drastically, and accordingly, the quadcopter might not be able to fulfill the purpose of its use in a specific application. Generally, to continuously operate quadcopters, the batteries are changed or recharged after the normal operation time. These batteries can be recharged using wired power transmission which requires some physical connection or via wireless power transmission which does not require any physical connection. Even though wired power transmission is more efficient than wireless power transmission, the wireless power transmission technique is currently utilized for its lower maintenance and increased safety (due to no physical connection of wires) for the delivery of power [11,12]. In order to charge the battery using wireless power transmission, the quadcopter is equipped with electromagnetic coils. These coils can be single or multiple depending upon the size and design of the charging system. Generally, a wireless power transmission system is composed of a transmitting and a receiving side. Both the transmitting and receiving sides are equipped with coils to transfer the power from the source to load. The battery with the receiving coil is installed on the quadcopter and the transmitting side is normally a ground station composed of a transmitting coil. For efficient power transmission, it is necessary that the quadcopter land on the ground station in such a way that the receiving and transmitting coils are aligned properly; however, due to the poor landing effect of quadcopters, there is always a chance of misalignment. This misalignment causes power loss and affects the efficiency of the charging system. To eliminate this misalignment issue, there is a need to develop a system which can easily mitigate the poor landing effect. For that, a charging system is proposed in this work which can cope with such issues and increase the power transmission efficiency.

1.2. Related Works Previously, there were different researches on wireless power transmission. In References [13,14], continuous inductive coupling in radio-frequency identification (RFID) tags were implemented. In References [15,16], a wireless power technique based on an inductive coupling mechanism was used for medical implants. Few wireless power systems for robotic applications were developed using a large array of smaller coils driven by microelectromechanical systems (MEMS) switches and organic field effect transistors to selectively transmit wireless power [17]. Some researchers developed efficient wireless power transmission systems by designing a switched-mode direct current/alternating current (DC/AC) inverter based on Class E topology [17–21]. Recently, there were many researches on power control in wireless power transmission. In References [22–25], the authors proposed a microcontroller-based power control method. In Reference [26], the authors developed a power control scheme using analog feedback circuits. Some researchers used the closed-loop power control technique using a commercial off-the-shelf (COTS) chipset [27–29]. For wireless charging of drones, some authors proposed laser beam systems which deliver the power directly to the drone [30]. Solar energy to support a drone’s long flight time was proposed in Reference [31]. Charging docking stations to recharge the drone battery were proposed in Reference [32]. Some authors applied smart contact arrays [33], whereas some proposed a drone charging station [34]. In Reference [35], the authors presented an approach based on optimal designing of the transmitting and receiving coil with the goal of becoming less sensitive to the misalignment of coils. The system was composed of a charging station with multiple arrays of primary or transmitting coils with a specifically designed secondary or receiving coil. The receiving coil was designed to perfectly fit in the landing skid of the drone. The authors proposed a transmitting coil overlapping scheme to entirely cover the charging area on the charging station. By calculating the impedance of the multiple Appl. Syst. Innov. 2018, 1, 44 3 of 19 transmitting coils and choosing the transmitting coil with maximum impedance, power transmission between the transmitting and receiving coil was achieved. In Reference [36], the authors presented a target detection technique based on image processing. After landing, the center of the coil was aligned with the transmitting coil using a specific color detection and image-processing scheme. The image-processing algorithm worked by taking the images using a drone camera and converted red/green.blue (RGB) color space to hue/saturation/value (HSV) color space. After applying some filters, the red color was detected and considered as a target. In References [37], the authors presented a positioning system using a binary distance laser sensor and ultrasonic sensors. The system was composed of a charging station comprising a single transmitting coil and a drone equipped with a single receiving coil. The system worked by detecting the position of the receiving coil and aligning the transmitting coil with it for wireless power transmission. The positioning system took almost 5 s to detect and align with the receiving coil. Using sensors, the system complexity increased; it required specific places and areas for installation on the drone and the charging station.

1.3. Contribution of the Paper and Road Map To solve the problem of misalignment of coils caused by the drone’s imperfect landing, a battery charging system based on wireless power transmission was developed in this work. The system is composed of a ground charging station and a receiving coil with the load. The charging station is equipped with multiple transmitting coils, whereas, on the receiver side, just one receiving coil is used. Multiple transmitting coils are placed on a movable bed which can move in four directions (positive X-direction, negative X-direction, positive Y-direction, and negative Y-direction). A control technique based on the hill climbing algorithm was implemented to control the alignment between the transmitting and receiving coil. The system was developed in a way allowing the drone to land freely on the charging station without the worry of alignment between the receiving and transmitting coil. The drone can land freely on charging station and the coils will adjust automatically in proper alignment to start the power transmission. Previously, the misalignment issues were solved using different methods, such as the design of a charging station with overlapped coils [35], using a target detection technique based on image processing [36], and using a positioning system based on physical sensors [37]. The proposed system is based on a novel method which uses the hill climbing algorithm on backscattered voltage signals of the transmitting coils. It improves the accuracy of the system and eliminates the ﬂaws found in previous techniques such as the image-processing technique, where there are chances of missing the target. Also, it gives the freedom for the drone to land freely on a charging station and makes the system more adoptable by avoiding the use of physical sensors. The conﬁguration of the proposed system is discussed in detail in Section2.

2. Conﬁguration of the Proposed Wireless Battery Charging System for a Quadcopter Figure1 shows the proposed wireless battery charging system for a quadcopter. The system is composed of the following three parts:

1. Wireless power transmitter, comprising transmitting coil array which can move in four directions. 2. Wireless power receiver, a receiving coil, and battery charger. 3. Control unit which can measure the terminal voltage of each transmitting coil, and align the centroid of the transmitting and receiving coil using the hill climbing algorithm. Appl. Syst. Innov. 2018, 1, 44 4 of 19 Appl. Syst. Innov. 2018, 2, x FOR PEER REVIEW 4 of 19

FigureFigure 1. 1. BlockBlock diagram diagram of of the the proposed proposed wireless wireless battery battery charging charging system for a quadcopter.

The proposed wireless battery charging system consists of multiple transmitting coils which The proposed wireless battery charging system consists of multiple transmitting coils which can can be moved in four directions (positive X-direction, negative X-direction, positive Y-direction, and be moved in four directions (positive X-direction, negative X-direction, positive Y-direction, and negative Y-direction). The use of multiple coils can potentially allow the system to efficiently adapt negative Y-direction). The use of multiple coils can potentially allow the system to efficiently adapt to magnetic field propagation conditions, similar to the way multiple antennas are used to adapt to to magnetic field propagation conditions, similar to the way multiple antennas are used to adapt to channel conditions in wireless communication systems [38]. Also, multiple transmitting coils decrease channel conditions in wireless communication systems [38]. Also, multiple transmitting coils the time for coil alignment, providing a chance for the design of big charging stations for large drones, decrease the time for coil alignment, providing a chance for the design of big charging stations for where coil size and design are limited due to power transmission characteristics. In this proposed large drones, where coil size and design are limited due to power transmission characteristics. In this system, if a drone with a receiving coil lands at any position on the multiple transmitting coils, the proposed system, if a drone with a receiving coil lands at any position on the multiple transmitting voltages across the transmitting coils decrease depending on the coupling of each transmitting coil coils, the voltages across the transmitting coils decrease depending on the coupling of each with the receiver coil, and the controller knows where the drone lands by observing the change in transmitting coil with the receiver coil, and the controller knows where the drone lands by observing terminal voltages of the multiple coils. When the power is transferred from the transmitting coil to the the change in terminal voltages of the multiple coils. When the power is transferred from the transmittingreceiving coil, coil the to terminal the receiving voltage coil, of thethe transmittingterminal voltage coil tendsof the totransmitting decrease due coil to tends the phenomenon to decrease duecalled to the signal phenomenon backscattering. called By signal observing backscattering the backscattered. By observing signal the from backscattered each transmitting signal from coil, each the transmittingcontroller automatically coil, the controller knows which automatically transmitting kn coilows is which nearest transmitting to the receiving coil coil. is Ifnearest the controller to the receivingpinpoints coil. the nearestIf the controller transmitting pinpoints coil, the the multiple nearest transmitting transmitting coil coil, array the multiple is moved transmitting to automatically coil arrayalign theis moved centroid to ofautomatically the detected transmittingalign the centroid coil with of the that detected of the receiving transmitting coil using coil thewith hill that climbing of the receivingalgorithm. coil The using practical the hill system climbing used algorithm. to implement The practical the proposed system system used comprisesto implement a receiver the proposed circuit systemfor battery comprises recharging a receiver and a circuit power for transmission battery recharging station. and a power transmission station. 2.1. Transmitter and Receiver Circuit Design and Description 2.1. Transmitter and Receiver Circuit Design and Description Figure2 shows a simple wireless power transmission circuit. The circuit comprises a transmitter Figure 2 shows a simple wireless power transmission circuit. The circuit comprises a transmitter circuit, a receiving circuit, and the control part (voltage divider and analog-to-digital converter (ADC) circuit, a receiving circuit, and the control part (voltage divider and analog-to-digital converter (ADC) of the microcontroller). The power amplifier drives the transmitting coil Lt. When the receiving coil of the microcontroller). The power amplifier drives the transmitting coil 𝐿. When the receiving coil is brought near to the transmitting coil, the voltage at receiving coil Lr is induced and it is processed is brought near to the transmitting coil, the voltage at receiving coil 𝐿 is induced and it is processed by a bridge rectifier to convert AC to DC voltage. To achieve the optimum performance, values of by a bridge rectifier to convert AC to DC voltage. To achieve the optimum performance, values of Ct, Lo, Co, and Cr are calculated using Equations (1) to (4). 𝐶,𝐿,𝐶, and 𝐶 are calculated using Equations (1) to (4).

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FigureFigure 2.2. Basic circuit diagramdiagram forfor wirelesswireless powerpower transmission.transmission.

1 2 R𝑅0L𝐿r ±± 2 ω𝜔𝑀M C𝐶r == 2 ;; (1) (1) R ω2L2 𝑅0𝜔 𝐿r − ω𝜔1 C𝐶0 == ;; (2) ( − 2) + ( + − ( )) (2) ω𝜔𝐿Lt(11−𝐾K ) +𝑅R0(𝑄+1−secQ 1 sec (𝜑))ϕ (22𝜔ω−1) C𝐶t == ;; (3) 𝜋π2 (3) 11++ 4 R𝑅 4 −1 L0 = ω QR0. (4) 𝐿 =𝜔 𝑄𝑅. (4) InIn thethe aboveabove equations, M𝑀 is is thethe mutual inductance, K𝐾 is is thethe couplingcoupling factor, Q𝑄 is is thethe qualityquality ◦ ◦ factorfactor betweenbetween thethe transmitting and receiving coils, coils, and and 𝜑ϕ is is thethe phasephase angleangle rangingranging from 4040° toto 70° 70 . TheThe derivationderivation ofof thethe equationsequations forfor aa Class-EClass-E amplifieramplifier cancan bebe foundfound inin ReferenceReference [[39].39]. TransmittingTransmitting coilscoils and the receiving receiving coil coil are are made made of of copper copper wire wire with with 15 15and and 40 turns, 40 turns, respectively respectively (Tables (Tables 1 and1 and2). According2). According to tothe the power power transmission transmission range range ca capabilities,pabilities, wireless wireless powe powerr transmissiontransmission cancan bebe categorizedcategorized intointo threethree typestypes [[40,41].40,41]. FirstFirst isis thethe inductiveinductive powerpower transmissiontransmission (IPT)(IPT) andand capacitivecapacitive powerpower transmissiontransmission (CPT),(CPT), usedused forfor short-rangeshort-range distances;distances; secondsecond isis thethe resonantresonant inductiveinductive couplingcoupling powerpower transmission,transmission, widely widely used used for medium-rangefor medium-range distances; distances; and third and is thethird laser is beamthe laser or microwave beam or powermicrowave transmission, power transmission, used for long-range used for distances.long-range Indistances. order to In achieve order to high achieve efficiency, high efficiency, inductive couplinginductive requirescoupling very requires close very coupling close coupling between betw theeen transmitting the transmitting and receiving and receiving coil. coil. Whereas, Whereas, in resonantin resonant inductive inductive coupling, coupling, efficient efficient power power transmission transmission can be can achieved be achieved with some with distance some betweendistance thebetween transmitting the transmitting and receiving and receiving coil via the coil use via of the resonant use of circuits.resonant Also, circuits. resonant Also, inductiveresonant inductive coupling hascoupling better has tolerance better thantolerance inductive than coupling.inductive Therefore,coupling. theTherefore, resonant the inductive resonant coupling inductive is consideredcoupling is anconsidered effective an technique effective for technique coping withfor coping the coil with misalignment the coil misalignment issues, and issues, for drone and batteryfor drone charging battery systems.charging Therefore,systems. Therefore, in this work, in athis resonance work, a inductive resonance coupling-based inductive coupling-based wireless power wireless transmission power techniquetransmission was technique used for was charging used for the charging drone battery. the drone The battery. detailed The block detailed diagram block of diagram the circuit of forthe wirelesscircuit for power wireless transmission power transmission is shown in is Figure shown3. in At Figure the transmitter 3. At the transmitter side, Class-E side, power Class-E amplifiers power areamplifiers used to are generate used amplifiedto generate AC amplified voltages AC with voltages a resonance with frequency a resonance of 240frequency kHz for of each 240 coil. kHz for each coil.

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Figure 3. BlockBlock diagram diagram of of the the circuit circuit fo forr wireless wireless power power transmission transmission with multiple multiple transmitting coils and a receiving coil. Table 1. Excitation circuit component values. Table 1. Excitation circuit component values. Component Value Component Value Number of Turns 15 Number of Turns 15 Lt 14.5 µH 𝐿 14.5 µH Lc 1 mH 𝐿 1 mH Lo 9.9 µH 𝐿 9.9 µH Ct 15 nF 𝐶 15 nF Co 28 nF 𝐶 28 nF R1 100 Ω 𝑅 100 Ω R2 30 Ω 𝑅 30 Ω

Table 2. Receiver circuit component values.

Table Component2. Receiver circuit component Value values. Number of Turns 40 Component Value NumberLr of Turns 733.39 40 µH Cr 680 pf 𝐿 733.39 µH CL 0.05 µF 𝐶 680 pf RLoad 2 kΩ 𝐶 0.05 µF 𝑅 2 kΩ These amplifiers amplifiers are driven by a high-frequency clockclock signal. The clock signal is provided to the gate driver of each amplifier amplifier of the respective transmittingtransmitting coil simultaneously. A microcontroller is used for generating a high-frequency clock signal andand it keeps measuring the output voltages of each transmitting coil using an ADC, and performs envelo envelopp detection of the measured voltages to identify which excitation coil is the closest to the receiving coilcoil ofof thethe drone.drone. At thethe receivingreceiving side, side, the the receiving receiving coil coil is installedis installed on theon dronethe drone and itand is connected it is connected to a full to bridgea full bridgerectifier, rectifier, which which is used is to used convert to convert the AC the to AC DC. to DC. Finally, Finally, a battery a battery charger charger (DC (DC to DCto DC converter) converter) is isused used for for battery battery charging. charging. The The system system works works by by detecting detecting thethe changechange inin thethe voltagevoltage of any of the envelope-detected voltage voltage signals signals of of the transmitting coils, which (changes in voltage) refer to the presence of the receiving coil onon thethe chargingcharging station.station. After detecting the receiving coil, a control algorithm is activated to align the coilscoils and start wireless power transmission.transmission.

2.2. Four-Way Directional XY Table A four-way directional XY table was constructed to control the position of the transmitting coil array, as shown in Figure4 4.. TheThe XY table is controlled by two stepper motors that are driven by two stepper motor drivers. Motor 1 is responsible for moving the transmitting coil array in the Y-direction, while motor 2 is responsible for moving it in the X-direction. When the quadcopter approaches the

Appl. Syst. Innov. 2018, 1, 44 7 of 19 Appl. Syst. Innov. 2018, 2, x FOR PEER REVIEW 7 of 19 whilecharging motor station, 2 is responsiblethe transmitting for moving coil, already it in the excitedX-direction. having When a constant the quadcopter voltage, experiences approaches a thechange charging in voltage. station, This the change transmitting in voltage coil, refers already to the excited presence having of the a constantreceiving voltage, coil and experiencesload (battery). a changeBy measuring in voltage. the This voltage change change in voltage from referseach transmitting to the presence coil, of thethe receivingcharging coilstation and is load moved (battery). in a Bydirection measuring where the one voltage of the changetransmitting from coils each is transmitting aligned to the coil, receiving the charging coil. The station controller is moved sends in the a directionrequired wherepulse width one of modulation the transmitting (PWM) coils signals is aligned to the tostepper the receiving motor driver coil. Theto move controller the transmitting sends the requiredcoil array pulse to the width proper modulation position to (PWM) initiate signals the wireless to the stepper power motortransmission driver toand move battery the transmittingcharging. coil array to the proper position to initiate the wireless power transmission and battery charging.

Figure 4. Four-way directional XY table. Figure 4. Four-way directional XY table. 2.3. Control Part 2.3. Control Part The control part of the proposed system performs specific tasks such as generating the clock signal for drivingThe control transmitting part of coils, the proposed monitoring system each coil’sperforms terminal specific voltage, tasks andsuch controlling as generating the four-waythe clock XYsignaltable for to driving align the transmitting centroids of coils, the monitoring transmitting ea andch coil’s receiving terminal coils. voltage, and controlling the four- wayGenerally, XY table to it align is not the easy centroids to land aof drone the transmitting at a specific and point receiving on the drone coils. station precisely. In most cases,Generally, the centroids it is of not the easy transmitting to land a drone and receiving at a specific coil point are misaligned, on the drone as shownstation inprecisely. Figure5 .In most cases,When the centroids the misalignment of the transmitting between and the rece coilsiving happens, coil are themisaligned, efficiency as shown of the in wireless Figure power5. transmissionWhen the deteriorates. misalignment In order between to solve the this coils problem, happens, an intelligent the efficiency automatic of alignmentthe wireless algorithm power basedtransmission on the hill deteriorates. climbing algorithm In order is to proposed solve th andis implemented.problem, an intelligent automatic alignment algorithmThe automatic based on alignment the hill climbing algorithm algorithm was implemented is proposed inside and a implemented. microcontroller. The microcontroller keepsThe measuring automatic the terminalalignment voltages algorithm of each transmittingwas implemented coil simultaneously, inside a microcontroller. and finds out which The transmittingmicrocontroller coil keeps has the measuring lowest voltage. the terminal Generally, voltages when of each the receivingtransmitting coil coil of thesimultaneously, drone is near and to afinds certain out transmitting which transmitting coil, the coil corresponding has the lowest transmitting voltage. Generally, coil’s terminal when voltage the receiving tends to coil decrease. of the Therefore,drone is near the microcontrollerto a certain transmitting can detect thecoil, transmitting the corresponding coil with transmitting a voltage difference coil’s terminal and, from voltage this momenttends toon, decrease. the microcontroller Therefore, the tries microcontroller to align the centroid can detect of the the transmitting transmitting coil coil with with the receivinga voltage coildifference by moving and, thefrom four-way this moment directional on, theXY microcontroltable. ler tries to align the centroid of the transmitting coil with the receiving coil by moving the four-way directional XY table.

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FigureFigure 5.5. Misalignment of coils forfor wirelesswireless powerpower transmission.transmission. Figure 5. Misalignment of coils for wireless power transmission. 2.3.1.2.3.1. Signal BackscatteringBackscattering 2.3.1. SignalSignal backscatteringBackscatteringbackscattering is aa phenomenonphenomenon basicallybasically usedused inin wirelesswireless communicationcommunication forfor RFIDRFID (radio-frequency identification). The transmitting side voltage tends to decrease by increasing the (radio-frequencySignal backscattering identification). is a phenomenon The transmitting basica sidelly usedvoltage in tendswireless to decreasecommunication by increasing for RFID the coupling between the transmitting and receiving coils. When the drone lands on the charging (radio-frequencycoupling between identification). the transmitting The and transmitting receiving coils. side Whenvoltage the tends drone to lands decrease on the by charging increasing station, the station, the XY table starts the aligning process and, during the time of movement, the voltage couplingthe XY tablebetween starts the thetransmitting aligning andprocess receiving and, coils.during When the thetime drone of movement, lands on the the charging voltage station, at the at the transmitting side decreases rapidly. This change in voltage produces a natural backscattered thetransmitting XY table sidestarts decreases the aligning rapidly. process This changeand, duri in voltageng the timeproduces of movement, a natural backscattered the voltage atsignal. the signal. This backscattered signal at a very high sampling frequency of 84 MHz is continuously read transmittingThis backscattered side decreases signal at rapidly. a very highThis samplingchange in frequencyvoltage produces of 84 MHz a natural is continuously backscattered read signal. by the by the ADC of the microcontroller. Figure6 shows the sampling process of the voltage signal during ThisADC backscattered of the microcontroller. signal at a Figurevery high 6 shows sampling the sa frequencympling process of 84 MHz of the is voltagecontinuously signal read during by theone one period of time for aligned (transmitting coil voltage at load) and misaligned (transmitting coil ADCperiod of of the time microcontroller. for aligned (transmitting Figure 6 shows coil voltage the sampling at load) process and misaligned of the voltage (transmitting signal during coil voltage one voltage at no load) coils. Inside of the microcontroller, an envelope detection algorithm is used to periodat no load) of time coils. for alignedInside of (transmitting the microcontroller, coil voltage an envelope at load) and detection misaligned algorithm (transmitting is used tocoil detect voltage the detect the envelope of the backscattered signal. After the detection of an envelope, the peak value of atenvelope no load) of coils. the backscattered Inside of the microcontroller,signal. After the andetecti envelopeon of andetection envelope, algorithm the peak is value used ofto thedetect voltage the the voltage signal is selected and this peak value is observed continuously to provide the information envelopesignal is ofselected the backscattered and this peak signal. value After is theobserv detectied continuouslyon of an envelope, to provide the peak the value information of the voltage about about aligned or misaligned coils. In the case of aligned coils, the peak value will be very low (almost signalaligned is orselected misaligned and thiscoils. peak In the value case isof observaligneded coils, continuously the peak value to provide will be the very information low (almost about 25 V; 25 V; Figure6) and, in the case of misalignment, the peak value will be high (almost 70 V; Figure6). alignedFigure 6)or and,misaligned in the casecoils. of In misalignment, the case of aligned the peak coils, value the peak will valuebe high will (almost be very 70 low V; Figure(almost 6). 25 The V; The hill climbing algorithm processes this voltage information data and finds the optimum solution by Figurehill climbing 6) and, algorithmin the case processes of misalignment, this voltage the informationpeak value will data be and high finds (almost the optimum 70 V; Figure solution 6). The by moving the XY table in a specific direction and aligning the transmitting and receiving coils. hillmoving climbing the XY algorithm table in processesa specific directionthis voltage and information aligning the data transmitting and finds and the receivingoptimum coils. solution by moving the XY table in a specific direction and aligning the transmitting and receiving coils.

FigureFigure 6.6. Sampling ofof voltagevoltage signalsignal duringduring oneone timetime period.period. Figure 6. Sampling of voltage signal during one time period.

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2.3.2.Hill climbing Hill Climbing is an Algorithm optimization technique that is used to find an optimum solution to a computational problem. It starts off with a solution that is normally very poor compared to the optimal solutionHill climbing and then is aniteratively optimization improves technique from there. that It is does used this to by find generating an optimum other solutionsolutions to a whichcomputational are better than problem. the current It starts solution. off with It repeats a solution the process that is normallyuntil it finds very the poor optimal compared solution to the whereoptimal it can solutionno longer and find then any iteratively improvements. improves from there. It does this by generating other solutions whichGenerally, are better drones than tend the to currentland at solution.any place Itupon repeats the thedrone’s process wireless until battery it finds charging the optimal station. solution Thatwhere means it canthe nocentroid longer of find the any receiving improvements. coil on the drone may not be aligned with any of the transmittingGenerally, coils. In drones this case, tend totransmitting land at any coils place are upon moved the drone’s using the wireless XY table battery in chargingan arbitrary station. directionThat to means get closer the centroid to the receiving of the receiving coil. By measuring coil on the terminal drone voltages may not of be each aligned transmitting with any coil, of the the controllertransmitting can coils. detect In thiswhether case, the transmitting receiving coils coil aregets moved closer usingto one the ofXY thetable transmitting in an arbitrary coils. directionThis kindto of getarbitrary closer movement to the receiving of the XY coil. table By continues measuring until terminal it detects voltages the decrease of each in transmittingterminal voltages coil, the of thecontroller transmitting can detect coils, whether as shown the in receiving Figure 7. coil When gets closerone transmitting to one of the coil, transmitting which corresponds coils. Thiskind to of transmittingarbitrary coil movement 2 in Figure of the 7,XY istable chosen, continues the hi untilll climbing it detects algorithm the decrease is activated in terminal to voltagesmove the of the transmittingtransmitting coil coils, to the as shownproper inposition Figure7 .where When the one transmittingvoltage measured coil, which from corresponds that transmitting to transmitting coil reachescoil its 2 in minimum. Figure7, is chosen, the hill climbing algorithm is activated to move the transmitting coil to the properFigure position8 shows wherethe overall the voltage flowchart measured of the fromproposed that transmittingmethod. Figure coil 9 reaches shows itsthe minimum. flowchart of the hill climbingFigure8 algorithmshows the used overall in this flowchart system. of Prev theiously, proposed authors method. tried Figure to solve9 shows a different the flowchart kinds of of controlthe problems hill climbing using algorithm the hill usedclimbing in this algorithm system. [42,43]. Previously, In this authors work, tried the hill to solve climbing a different algorithm kinds of startscontrol by sending problems the required using the PWM hill climbing value to algorithmthe stepper [42 motor,43]. In driver this work, to move the the hill XY climbing table. At algorithm the start,starts when by there sending is no the change required in the PWM voltage, value i.e., to the the drone stepper is not motor on the driver charging to move station, the XY theretable. is no At the movementstart, when and the there voltage is no changeof the transmitting in the voltage, coil i.e., is constant. the drone When is not the on drone the charging lands on station, the charging there is no station,movement there is anda change the voltage in the of voltage the transmitting value, and coil the is voltage constant. decreases When the in dronethe presence lands on of thea drone charging withstation, a receiving there coil. is a changeThe hill in climbing the voltage algorithm value, andis activated the voltage and decreases one transmitting in the presence coil with of athe drone with a receiving coil. The hill climbing algorithm is activated and one transmitting coil with the maximum change in the voltage value is selected. The current terminal voltage 𝑉 of the selected coil maximum change in the voltage value is selected. The current terminal voltage Vn of the selected is measured and compared with the previous measured voltage 𝑉. The minimum value is stored coil is measured and compared with the previous measured voltage Vn−1. The minimum value is as 𝑉. After that, the controller sends the required PWM to move the XY table to the position of the minimumstored as V value.min. After This that, kind the of controllerprocess keeps sends re thepeating required until PWM it reaches to move the the minimumXY table voltage. to the position In Figureof the9, 𝑛=1 minimum to 4 is value. the number This kind of transmitting of process keeps coils, repeatingand 𝑘 is the until number it reaches of the the sample. minimum voltage. In Figure9, n = 1 to 4 is the number of transmitting coils, and k is the number of the sample.

Figure 7. (a) Receiving coil’s initial position. (b) Receiving coil gets close to a transmitting coil after Figurearbitrary 7. (a) Receiving movement. coil’s initial position. (b) Receiving coil gets close to a transmitting coil after arbitrary movement.

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Figure 8. Overall flowchart of the proposed method. Figure 8. Overall ﬂowchart of the proposed method. Figure 8. Overall flowchart of the proposed method.

Figure 9. Flowchart of the hill climbing algorithm.

Figure 9. Flowchart of the hill climbing algorithm. Figure 9. Flowchart of the hill climbing algorithm.

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3. Explanation of the Test Bench In order to verify the feasibility of the proposed algorithm, we made a test bench of a wireless power transmission and battery charging station fo forr the drone. The developed test bench is shown in Figure Figure 10.10 .The The test test bench bench was was composed composed of four of four transmitting transmitting coils coils which which are mounted are mounted on the on four- the wayfour-way XY table,XY table, one receiving one receiving coil connected coil connected to an to electrical an electrical load, load, and anda controller a controller which which performs performs the measurementthe measurement and andhill hillclimbing climbing algorithm. algorithm.

Figure 10. PracticalPractical test bench for drone battery charging station.

3.1. Battery Charging Station The battery charging station was built using an XY table.table. The The XYXY tabletable is is used to move the transmitting coil array to the position where the cent centroidroid of the transmitting coil is aligned with the centroid of the receiving coil using the hill climbing algorithm. The dimensions of the XY table were 394394mm mm × ×414 414mm mm× ×93.6 93.6mm mm(l (𝑙× w × h𝑤). The ×ℎ ). mainThe main frame frame of the XYof tablethe XY was table made was of aluminum made of aluminumand the rectangular and the rectangular plate, where plate, the coilswhere are the placed, coils wasare placed, made of was a plastic made sheetof a plastic with a sheet thickness with ofa thickness2.5 mm. Four of 2.5 transmitting mm. Four transmitting coils were placed coils onwere top placed of the on rectangular top of the plate rectangular within theplateXY withintable andthe XYthey table were and controlled they were by controlled two stepper by motors two stepper for positioning. motors for positioning. In thisthis work, work, four four excitation excitation circuits circuits for the for four the transmitting four transmitting coils were coils developed. were developed. Each transmitting Each transmittingcoil was connected coil was to connected the excitation to the circuit. excitation A gate circuit. voltage A ofgate 15 voltage V and a of supply 15 V and voltage a supply of 12 voltage V were ofapplied 12 V were to the applied IRF510 to metal-oxide-semiconductor the IRF510 metal-oxide-semiconductor ﬁeld-effect transistor field-effect (MOSFET), transistor and (MOSFET), it was driven and itby was a low-power driven by 240 a low-power kHz clock signal, 240 kHz generated clock signal, by the generated microcontroller. by the A microcontroller. voltage divider circuitA voltage was divideralso built circuit to decrease was also the built output to voltagedecrease into the a output voltage voltage that was into readable a voltage by the that controller. was readable Based by on the controller.proposed method Based andon the the equationsproposed presentedmethod and in Reference the equations [40], the presented optimum in values Reference of the electrical[40], the optimumcomponents values for theof the excitation electrical circuits components were calculated. for the excitation Table1 circuitsshows thewere component calculated. values Table 1 used shows in the excitationcomponent circuit, values and used Figure in 11the shows excitation a closer circ viewuit, and of the Figure excitation 11 shows circuits a andcloser controller. view of the excitation circuits and controller.

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Figure 11. CloserCloser view of the excitati excitationon circuits and controller.

3.2. Receiving Coil and Electric Load When the drone lands on the battery charging station and the XY tabletable startsstarts movingmoving thethe transmitting coil coil array array to to the the proper proper position position using using the the hill hill climbing climbing algorithm, algorithm, the thetransmitting transmitting coil coilstarts starts to couple to couple inductively inductively with the with receiving the receiving coil. This coil. inductive This inductive coupling coupling between between the coils the induces coils inducesvoltage in voltage the receiving in the receiving coil. The coil.receiving The receiving coil is connected coil is connected to the receiving to the receivingcircuit as circuitshown asin shownFigure in Figure3. The induced voltage at Lr is processed by a full-wave bridge rectiﬁer to convert AC to DC. 3. The induced voltage at 𝐿𝐿 is processed by a full-wave bridge rectifier to convert AC to DC. In this Insystem, this system, rectification rectiﬁcation is achieved is achieved via four via insulated- four insulated-gategate bipolar transistors bipolar transistors (IGBTs), (IGBTs),designed designed to work to work on high-frequency AC input signals. The output voltage is obtained across the load resistor on high-frequency AC input signals. The output voltage is obtained across the load resistor 𝑅. TableRLoad. 2 Table shows2 shows the component the component values values used in used the in rece theiving receiving circuit, circuit, and Figure and Figure 12 shows 12 shows a closer a closer view viewof the of receiving the receiving circuit circuit and receiving and receiving coil. coil.

Figure 12. Closer view of the receiving circuit and receiving coil. 3.3. Controller 3.3. Controller In order to perform the control tasks, an STM32f4 discovery kit featuring a 32-bit ARM Cortex-M4 In order to perform the control tasks, an STM32f4 discovery kit featuring a 32-bit ARM Cortex- was used. It can generate up to a 168-MHz clock signal. It also supports ADC with 19 channels and M4 was used. It can generate up to a 168-MHz clock signal. It also supports ADC with 19 channels 12-bit resolution. The controller was used to generate 240-kHz clock signals for the four excitation and 12-bit resolution. The controller was used to generate 240-kHz clock signals for the four excitation circuits and it read the terminal voltage of transmitting coils simultaneously. circuits and it read the terminal voltage of transmitting coils simultaneously. When the drone lands on the battery charging station, the controller sends different PWM values When the drone lands on the battery charging station, the controller sends different PWM values to the two stepper motor drivers to move the XY table in random directions. At the same time, the to the two stepper motor drivers to move the XY table in random directions. At the same time, the controller keeps measuring the output voltage of the fourfour transmitting coils. Once there is a drop in the voltage across one of the transmitting coils, the controller triggers the hill climbing algorithm and sends the required PWM to the stepper motor drivers until it measures the minimum voltage value

Appl. Syst. Innov. 2018, 1, 44 13 of 19

theAppl. voltage Syst. Innov. across 2018, one2, x FOR of the PEER transmitting REVIEW coils, the controller triggers the hill climbing algorithm 13 of and 19 sends the required PWM to the stepper motor drivers until it measures the minimum voltage value where both the centroids of the transmittingtransmitting and receivingreceiving coils are aligned; then, the hill climbing algorithm stops working, and the wireless power transm transmissionission begins until the the drone drone is is fully fully charged. charged.

4. Experiments and Results Several experiments are conducted with a test be benchnch in order to verify the feasibility of the proposed scheme. The The system system was was tested under different test scenarios depending on the drone landing position on the charging station. The receiving coil attached with load was placed at different different positions onon thethe chargingcharging station station and and the the response response of of the the system system was was observed. observed. In oneIn one test test scenario, scenario, the systemthe system was was tested tested for the for misalignment the misalignment of x = 100of xmm = 100 and mmy = and 50 mm, y = whereas50 mm, inwhereas othertest in other scenarios, test thescenarios, misalignment the misalignment was x = 75 was mm, x y= =75 30 mm, mm y and= 30x mm= 50 and mm, x =y 50= 10mm, mm. y = Regardless 10 mm. Regardless of the position of the ofposition the receiving of the receiving coil, the charging coil, the stationcharging was statio ablen to was perfectly able to align perfectly the centroid align the of thecentroid transmitting of the andtransmitting receiving and coil receiving with an accuracycoil with ofan 98.8%. accuracy Figure of 98.8%. 13 shows Figure one 13 of shows the test one scenarios. of the test Initially, scenarios. we assumedInitially, we that assumed the receiving that the coil receiving and four coil transmitting and four transmitting coils were positioned coils were aspositioned shown in as Figure shown 13 ina. AtFigure the start,13a. At the theXY start,table the was XY moved table was randomly moved until randomly the receiving until the coil receiving got closer coil to got one closer of the to fourone transmittingof the four transmitting coils. During coils. this process, During the this microcontroller process, the microcontroller kept measuring thekept four measuring terminal the voltages four simultaneously.terminal voltages By simultaneously. detecting the voltage By detecting drop between the voltage all four drop transmitting between all coils, four the transmitting microcontroller coils, activatedthe microcontroller the hill climbing activated algorithm the hill andclimbing the XY algorithmtable was and moved the XY to aligntable thewas nearest moved transmitting to align the coilnearest with transmitting a receiving coil coil. with The distancea receiving and coil. position The distance of the nearest and position transmitting of the nearest coil were transmitting judged on thecoil basiswere ofjudged the voltage on the drop.basis of The the closer voltage the drop. transmitting The closer and the receiving transmitting coil, and the largerreceiving the coil, voltage the droplarger would the voltage be due drop to power would transmission be due to power between transmission the two coils. between As shown the intwo Figure coils. 13 Asb,c, shown after the in randomFigures 13b,c, movement after the of therandomXY table, movement transmitting of the XY coil table, 1 was transmitting found as the coil nearest 1 was found coil to as the the receiving nearest coil.coil to Then, the receiving the hill climbing coil. Then, algorithm the hill was climbing activated algorithm by the microcontrollerwas activated by and the the microcontrollerXY table moved and to alignthe XY transmitting table moved coil to 1align with transmitting the receiving coil coil. 1 with Figure the 14 receiving shows the coil. recorded Figure voltage14 shows waveforms the recorded for thevoltage scenario waveforms shown infor Figure the scenario 13. It can shown be seen in clearly Figure that 13. theIt can voltage be seen of transmitting clearly that coil the 1 voltage decreased, of redtransmitting in the case coil of 1 Figuredecreased, 13a, red green in inthe the case case of Figu of Figurere 13a, 13 greenb, and in blue the case in in of the Figure case of13b, Figure and blue 13c. Figurein in the 15 showscase of the Figure voltage 13c. of Figure other transmitting 15 shows the coils voltage under of load other (wireless transmitting power transmission)coils under load and no(wireless load (normal) power transmission) condition. and no load (normal) condition.

Figure 13. ((aa)) Initial Initial position position of of the the coils. coils. (b ()b XY) XY tabletable movement movement toward toward nearest nearest transmitting transmitting coil. coil. (c) (Transmittingc) Transmitting and and receiving receiving coil coil aligned aligned for for wireless wireless power power transmission. transmission.

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Figure 14. 14. TransmittingTransmittingTransmitting coilcoil coil 11 voltagevoltage 1 voltage underunder under wirelesswireless wireless powerpower power transmission.transmission. transmission. (For(For casecase (For scenarioscenario case inin scenario FigureFigure 13).in Figure 13).

Figure 15. VoltageVoltage under no no load (normal)(normal) and loadload (wireless(w(wirelessireless power transmission) for for transmitting transmitting coilcoil 2. 2.

The system was was designed designed for for the the transmission transmission power power of of P𝑃tx ==6060 W. Figure Figure 1616 showsshows the the output output The system was designed for the transmission power of 𝑃 =60 W. Figure 16 shows the output power at the load resistance R𝑅L under under differentdifferent testtest scenarios.scenarios. The misalignment distance between power at the load resistance 𝑅 under different test scenarios. The misalignment distance between thethe coils coils was was changed changed and and the the load load power power 𝑃P L responseresponse was was observed. observed. The The maximum maximum load load power power 𝑃 the coils was changed and the load power 𝑃 response was observed. The maximum load power 𝑃 η wasPL was measured measured to be to 52 be W, 52 which W, which gives gives an efficiency an efficiency η of of 85%.85%.of 85%. ItIt cancan It bebe can observedobserved be observed inin FigureFigure in Figure 1616 that,that, 16 asthat, the as distance the distance of misalignment of misalignment between between the transmit the transmittingtingting andand receivingreceiving and receiving coilscoils increased,increased, coils increased, thethe timetime the takentaken time bytaken the bycharging the charging station station (XY table)table) (XY tototable) properlyproperly to properly alignalign toto align thethe nearesneares to thett nearest possiblepossible possible coilcoil alsoalso coil increased.increased. also increased. InIn thethe caseIn the of casea misalignment of a misalignment of x == 100100 of mm,xmm,= 100 y ==mm, 5050 mm,mm,y = itit 50 tooktook mm, almostalmost it took 1.851.85 almost ss toto properlyproperly 1.85 s to align properlyalign thethe coilscoils align andand the transfertransfercoils and thethe transfer powerpower the withwith power designeddesigned with efficiency.efficiency. designed efficiency. ForFor misalignmentsmisalignments For misalignments ofof x == 7575 mm,mm, of x y= == 75 3030 mm, mmmm yandand= 30 x == mm 5050 mm,and xy = == 501010 mmmm,mm,, ythethe= 10timetime mm toto, alignalign the time thethe coils tocoils align waswas the almostalmost coils 1.581.58 was ss almost andand 1.51.5 1.58 s,s, respectively.respectively. s and 1.5 s, In respectively.In aa timetime rangingranging In a betweentime ranging 1.5 s and between 1.9 s, 1.5the s misalignment and 1.9 s, the caused misalignment by the imperfect caused by drone the imperfect landing was drone eliminated landing wasand thetheeliminated powerpower transmissiontransmission and the power waswas transmission increasedincreased toto was thethe increased maximummaximum to power.power. the maximum power. InIn order order to to validate validate the the hill hill climbing climbing algorithm, algorithm, the the data data obtained obtained from from the the practical practical test test bench bench was used in MATLAB and simulations were were carried out out to to assure assure the the feasibility feasibility of of the the algorithm. algorithm. Figure 17 shows the trajectory of the XY positionposition for for one one transmitting transmitting coil coil in in a a three-dimensional space.space. It It can can be be seen seen that, that, at at the the st start,art, there there is is no no voltage voltage drop drop and and the XYXY tabletable isis movedmoved toto aa randomrandom position. After After two two random random movements, movements, the the positi positionon with with the the minimum possible possible voltage value is selectedselected and, and, from from that position, position, the the hill hill climbing climbing algorithm is activated and it keeps finding finding the best possible positions to align the transmitting and receiving coils until the minimum level is achieved.

Appl. Syst. Innov. 2018, 1, 44 15 of 19 Appl. Syst. Innov. 2018, 2, x FOR PEER REVIEW 15 of 19 Appl. Syst. Innov. 2018, 2, x FOR PEER REVIEW 15 of 19 possibleOnce the positions minimum to level align is the achieved, transmitting the transmitting and receiving and coils receiving until the coils minimum are aligned level properly is achieved. and Once the minimum level is achieved, the transmitting and receiving coils are aligned properly and Oncewireless the power minimum transmission level is achieved, starts with the full transmitting efficiency. and receiving coils are aligned properly and wirelesswireless powerpower transmissiontransmission startsstarts withwith fullfull efﬁciency.efficiency.

Figure 16. Load power response for different misalignment cases. Figure 16.16. Load power response forfor differentdifferent misalignmentmisalignment cases.cases. The results obtained from the test bench shows that the proposed system is quite efficient in TheThe resultsresults obtainedobtained fromfrom thethe testtest benchbench showsshows thatthat thethe proposedproposed systemsystem isis quitequite efﬁcientefficient inin resolving the misalignment issue. The power transmission efficiency of 85% is reasonable for resolvingresolving thethe misalignment misalignment issue. issue. The The power power transmission transmission efﬁciency efficiency of 85% of is reasonable85% is reasonable for resonant for resonant inductive-based wireless power transmission, while also reducing the need for the inductive-basedresonant inductive-based wireless power wireless transmission, power transm whileission, also reducing while also the needreducing for the the implementation need for the implementation of complex tracking and landing algorithms for the drone. The time to align the ofimplementation complex tracking of complex and landing tracking algorithms and landing for the algorithms drone. The for timethe drone. to align The the time centroid to align is also the centroid is also minimized due to the use of the hill climbing algorithm. The drone is free to land minimizedcentroid is also due minimized to the use ofdue the to hill theclimbing use of the algorithm. hill climbing The algorithm. drone is free The to drone land is anywhere free to land on anywhere on charging station and, within a time of just 2 s, the centroid of the coils are aligned charginganywhere station on charging and, within station a timeand, ofwithin just 2 a s, time the centroid of just 2 of s, the the coils centroid are aligned of the properlycoils are andaligned the properly and the power transmission loss is mitigated. powerproperly transmission and the power loss istransmission mitigated. loss is mitigated.

FigureFigure 17.17. TrajectoriesTrajectories ofof hillhill climbingclimbing algorithm.algorithm. Figure 17. Trajectories of hill climbing algorithm. ComparativeComparative StudyStudy Comparative Study AA comparativecomparative studystudy ofof thethe proposedproposed andand previouslypreviously presentedpresented solutionssolutions forfor dronedrone imperfectimperfect A comparative study of the proposed and previously presented solutions for drone imperfect landinglanding (misalignment (misalignment issues) issues) is presentedis presented in this in section.this section. The system The system is compared is compared with three with solutions three landing (misalignment issues) is presented in this section. The system is compared with three presentedsolutions presented in Reference in [Reference37–39]. [37–39]. solutions presented in Reference [37–39]. InIn ReferenceReference [[37],37], thethe authorsauthors presentedpresented anan approachapproach basedbased onon thethe optimaloptimal designdesign ofof thethe In Reference [37], the authors presented an approach based on the optimal design of the transmittingtransmitting andand receivingreceiving coilscoils withwith thethe goalgoal ofof becomingbecoming lessless sensitivesensitive toto thethe misalignmentmisalignment ofof coilscoils.. transmitting and receiving coils with the goal of becoming less sensitive to the misalignment of coils. TheThe systemsystem waswas composedcomposed ofof aa chargingcharging stationstation withwith multiplemultiple arraysarrays ofof primaryprimary oror transmittingtransmitting coilscoils The system was composed of a charging station with multiple arrays of primary or transmitting coils withwith aa speciﬁcallyspecifically designeddesigned secondarysecondary oror receivingreceiving coil.coil. TheThe receivingreceiving coilcoil waswas designeddesigned toto perfectlyperfectly with a specifically designed secondary or receiving coil. The receiving coil was designed to perfectly ﬁtfit inin thethe landinglanding skidskid ofof thethe drone.drone. TheThe authorsauthors proposedproposed aa transmittingtransmitting coilcoil overlappingoverlapping schemescheme toto fit in the landing skid of the drone. The authors proposed a transmitting coil overlapping scheme to entirelyentirely covercover thethe chargingcharging areaarea onon thethe chargingcharging station.station. ByBy calculatingcalculating thethe impedanceimpedance ofof thethe multiplemultiple entirely cover the charging area on the charging station. By calculating the impedance of the multiple transmittingtransmitting coilscoils andand choosingchoosing thethe transmittingtransmitting coilcoil withwith maximummaximum impedance,impedance, powerpower transmissiontransmission transmitting coils and choosing the transmitting coil with maximum impedance, power transmission betweenbetween thethe transmittingtransmitting andand receivingreceiving coilscoils waswas achieved.achieved. between the transmitting and receiving coils was achieved. This system works well for a specific type of drone with some specific dimensions; however, if This system works well for a specific type of drone with some specific dimensions; however, if the size and dimension of the drone (physical size) changes, a whole new design of the charging the size and dimension of the drone (physical size) changes, a whole new design of the charging

Appl. Syst. Innov. 2018, 1, 44 16 of 19

This system works well for a specific type of drone with some specific dimensions; however, if the size and dimension of the drone (physical size) changes, a whole new design of the charging station is required, and the transmitting coils will need to be readjusted again with some proper overlapping to cover the charging area properly. On the other hand, our proposed system has non-overlapped multiple transmitting coils which can be moved in four directions. Furthermore, the proposed system utilizes the feature of backscattering which generally happens in wireless communication in order to identify the most closely coupled receiving coil among them. Even though the closely coupled receiving coil is detected, a further fine alignment of the centroids of the transmitting coil and the receiving coil is required to obtain efficient wireless power transmission by moving multiple transmitting coils. The proposed system is autonomous and totally free from the physical dimensions of the drone; it just needs to detect a receiving coil. This coil could be placed at any part of the drone where it can come in contact with any of the multiple transmitting coils placed on the charging station. The drone can land at any part of the charging station and, within just 1 to 2 s (min to max, depending on the distance of misalignment), the coils would be properly aligned with a strong coupling factor. In Reference [33], the impedance of the transmitting coils was monitored, which requires some current and voltage sensors to provide the measured value at every instance of charging operation. It increases the complexity of the charging station and requires more electronic components. On the other hand, in our case, the voltages of the transmitting coils are enough to ensure proper alignment between coils. Regarding power transfer capabilities and efficiency, in Reference [37], the authors presented results based on the efficiency of wireless power transmission. The results showed that the efficiency of the system changed and decreased when there was misalignment between the coils. The efficiency met the minimum level requirement of 75% in all cases; however, upon misalignment, there was a reduction in efficiency from 88% (normal; no misalignment) to 83% (100-mm misalignment) and 78% (200-mm misalignment). In our system, the efficiency will always be the same (normal; no misalignment) because there is no misalignment and power transmission is done after ensuring this. In Reference [38], the authors presented a target detection technique based on image processing. After landing, the center of the coil was aligned with the transmitting coil using some specific color detection and image-processing scheme. The image-processing algorithm worked by taking the images using a drone camera and converted RGB color space to HSV color space. After applying some filters, the red color was detected and considered as a target. There is always a chance of a failure in such a scheme contingent upon the outer environment and weather conditions; research work is still being pursued to improve such image-processing techniques. In our work, the drone is independent of such outer environmental uncertainties and the system works autonomously with the use of the hill climbing algorithm. In Reference [39], the authors presented a positioning system using a binary distance laser sensor and ultrasonic sensors. The system was composed of a charging station comprising a single transmitting coil and a drone equipped with a single receiving coil. The system worked by detecting the position of the receiving coil and aligning the transmitting coil with it for wireless power transmission. The positioning system took almost 5 s to detect and align with the receiving coil. Using sensors, the system complexity increases; it requires some specific areas for installation of the drone and the charging station. There is the possibility of errors in installing these sensors properly at the proper position so that the alignment between coils is always perfect. In our system, there is no physical sensor that needs to be installed on the drone or charging station. The time it takes to get the best possible solution is 1 to 2 s (min to max, depending on the distance of misalignment). The algorithm used in our system is much faster and much more robust, giving accurate results. Moreover, in Reference [35], there is just a single transmitting coil used to transfer the power. In our case, the charging station is composed of multiple transmitting coils because using multiple transmitting coils gives the possibility of designing large charging stations for big drones. If the drone size is big, it will require more space for landing on the charging station. Using multiple transmitting coils decreases the time required Appl. Syst. Innov. 2018, 1, 44 17 of 19 to align the coils. Generally, the drone lands at any point on the charging station. In Reference [35], it was made sure that the drone lands in the range of the sensors to initiate the alignment process; however, it would be almost impossible to initiate this process of alignment if the drone landed on the charging station and was out of range of the sensors. For this, in our charging station, multiple arrays of transmitting coils were installed to allow the drone to land freely anywhere on the charging station without the possibility of getting out of range.

5. Conclusions In this work, an efficient wireless power transmission system for drone battery charging was developed. A charging station with multiple transmission coils was used to transfer power to the receiving side (drone) to charge the battery. This study aimed to solve the problem caused by uneven drone landing on a charging station which leads to inefficient wireless power transmission due to the poor alignment between the transmitting and receiving coils. To solve this problem, a control mechanism based on the hill climbing algorithm was proposed. The control mechanism was used to mitigate the uneven landing effect of the drone on a charging station by carefully moving the charging station to a point where wireless power transmission was maximum. A practical test bench was developed to test the system feasibility. The power transmission efficiency was 85% and the test accuracy of the system was 98.8%. It can be observed from the results that the proposed system performed well compared to previously used techniques, without any need of a physical sensor such as a position sensor. Also, the use of the hill climbing algorithm gives the system a much faster response compared to any image-based target detection scheme. It also eliminates the possibility of getting affected by environmental conditions. The drone simply needs to land on the charging station, and, within 2 s, wireless power transmission starts at its maximum designed efficiency without any misalignment.

Future Work We are working on the real-time implementation of the system. For that, there are several things which need to be improved and addressed carefully. These include the battery charging issues such as charging time and the capacity of the battery. Additionally, the wireless power transmission system will be improved in the future. The efﬁciency of the system can be improved by looking into some design parameters. The distance between the transmitting and receiving coils will be improved and medium-range transmission will be introduced into the system.

Author Contributions: Conceptualization, A.R.; Data curation, M.R.; Formal analysis, M.R.; Investigation, A.R., M.T. and S.-H.K.; Project administration, A.R.; Resources, M.T.; Software, A.R., M.T.; Supervision, S.-H.K.; Validation, A.R. and S.-H.K.; Visualization, S.-H.K.; Writing–review & editing, A.R. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂicts of interest.

References

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