Development of mobile electrical vehicle for EMI applications

: This paper presents the relevancy, design, and test results of a mobile electrical vehicle for the applications of testing and providing electromagnetic interferences in Wi-Fi signal in the band 2.4–2.5 GHz. The mobile device includes power electronics supplying a magnetron device for the interference. Propulsion of the mobile device is implemented by using a system of stepper motors. The mobile device is controlled with the help of the microcontroller ATmega 16. For the interfering part, it is necessary to design the power electronics to supply the magnetron from the battery. The power electronics interface between a battery and magnetron must be able to transfer output battery voltage 12 VDC to the variable output 4000 VAC and power around 1 kW. The magnetron is used as a source of electromagnetic interference high-frequency acting on targets, which operates in the band of Wi-Fi signal. In addition, the system must be able to control the direction and power of EMI interferences. The investigations of the power-efficiency of power inverter versus power density will be discussed and optimised here.


Introduction of mobile electrical vehicles
The mobile vehicle for EMI applications, which we are designed, can drive forward, backward, turn left, and turn right and change the velocity of the device. The propulsion of mobile device is implemented by four stepper motors. It also has a fifth stepper motor, which is placed on top of the mobile device and used to rotate the antenna of the interfering part with a magnetron. The rotation of the antenna is regulated in sector 0-180° due to antennas part. The interfering mobile device is presented in Fig. 1.
The device consists of six parts. The first part is 'powerbattery', which provides power for all parts of the mobile device. For this mobile device, this part will be the power supply by using batteries Li-Po (TP5000-7SPF70). The second part is 'propulsion of the device' containing five stepper motors. Here, we use four hybrid bipolar stepper motors SX34-2970 [1] for driving the mobile device and one hybrid bipolar stepper motor LDO-57STH76-2804 [2] for driving the rotation of the antenna. The third part is 'drivers of stepper motors' that provides the rated currents to the motor's windings in the specific sequence. The fourth part is 'block control', which controls the overall functionality of the interfering mobile device, for example, movement of the mobile device, activation interfering part, rotation of the antenna. This part is implemented by using microprocessor ATmega 16. For this application, remote control is selected after deliberation. Remote control is made by using a system of the subminiature transmitter MTX2 and the universal receiver MRX1 from company Flajzar [3]. The fifth part is 'limit rotation of the antenna' that will be implemented with two inductive proximity sensors LJ12A3-4-Z/BY. This part allows setting the antenna to initial position 90° and limit rotation of the antenna in the 0-180°s pace. The sixth part is 'interfering block' that includes an interfering circuit with the magnetron. The main elements of this part are the inverter 12 VDC to 4000 VAC, 1 kW power and the antenna for directing electromagnetic energy from interfering part.
The first thing, we have to discuss, is the reason why stepper motor was used for this application. In the practice, there are some kinds of electronic motors. The device can be implemented by using a system of the servo motors or stepper motors etc. Torque, speed, cost consideration, and control algorithm play an important role in selecting the best motor for this application.
The interfering mobile device will be used as a preparation interference of electronic equipment in the band of wireless signal and used as a component for measurement of electromagnetic compatibility of electronic equipment. The propulsion of the device must have enough torque to move in the field, flexibility, e.g. move forward, backward, turn right, turn left, and change velocity. The antenna of interfering part rotates in the limited space (0-180°) and must know his precise initial position (90°). Therefore, the used motor for the rotation of the antenna must accurately tune his location.
For precise setting location, servo motor needs a system of an absolute position encoder to track the position of the motor's shaft [4]. For example, a system of optical absolute encoder, a light source, and photo detector array reads the optical pattern that results from the disc's position at any one time. The grey code is often used. This code can be read by a controlling microprocessor to determine the angle of the shaft. This is one of the most common technologies. In the practice, the results can be wrong because optical encoders are very sensitive to dust [5]. Driving a stepper motor to a precise position is much simpler than driving a servo motor. The motor's position can then be commanded to move and hold at one of these steps without any position sensor for feedback (an open-loop controller), as long as the motor is carefully sized to the application in respect to torque and speed [6].
The stepper motor has full torque in the rest position (when currents are applied to the coils) and greater torque at low speed than servo motor of the same size and speed. A stepper motor divides a full rotation into a number of equal steps. Stepper motors have a high reliability because of this, that the engine does not contain any of the contact brushes [6]. According to these advantages, the stepper motor is selected for application of the propulsion of the mobile device.

Block control
This part is very important for application of the mobile device. The 'block control' controls overall functionality of the mobile device. The main element of this part is the microcontroller ATmega 16. Its' construct provides enough input/output ports for control of all function of the mobile device.
The 'block control' is energised from the part 'power-battery' that includes three Li-Po batteries. One of them is used to energise the 'block control', rotation of the antenna and inverter DC/AC of the interfering part. Each one of other two batteries will be used for stepper motors on each side of the mobile device (left and right side). Fig. 2.
The 'block control' is integrated with receiver micromodule MRX1-SMA and then the 'block control' will process received signals from transmitter micromodule MTX2. By using a suitable algorithm, these signals command other parts of the mobile device. Transmitter and receiver work in the range of the frequency of 868.35 MHz with FSK modulation (range up to several hundred meters). Micromodules themselves have a protocol for the communication, hence for application in this article, we do not need to make any protocol of the communication [3].
Stepper motors on each side (left, right) of the mobile device will be controlled simultaneously. The front left and back left stepper motor will always be controlled by the same signal, and similarly for the right side. The stepper motor of the antenna is controlled separately. According to control theory of stepper motor, to control a stepper motor, it is necessary to have a driver circuit. The driver circuit plays a role like a current amplifier control signals from a microprocessor and make stepper motor move. Construction and principle of the driver circuit depend on the principle control of a stepper motor [7,8]. In this application, for each stepper motor, we need one driver circuit. The name of driver circuit is DIV268N-HY-5A that is two-phase hybrid stepper driver and suitable for both of used stepper motors.

Control algorithm of the mobile device
The mobile device is controlled by two modes that are mode '1' and mode '2'. Mode '1' is used to control the movement of the mobile device, e.g. driving forward, backward, and change speed. In this application, the mobile device has three levels of the speed. Turning left or right is only possible by slowing down all wheels on one side of the mobile device. Mode '2' is used to control block interfering, e.g. on/off jammer, rotation of the antenna, setting an initial position of the antenna.
The total algorithm to control mobile device has been given, so when the mobile device is in motion, the block noise and rotation of antenna will not be worked. When block noise and rotation of antenna work, the propulsion of mobile device is at rest. The reason for this solution is dependent on the electromagnetic compatibility of the mobile device. The interfering part with magnetron works with a high level of electromagnetic energy and can impact negatively on driver circuits when they work together. So, in terms of electromagnetic compatibility, the control algorithm was divided separately into two modes. The algorithm is implemented in C language through Atmel Studio 7.0 environment. Fig. 3.

Block of the interfering part with magnetron
The magnetron is a special kind of vacuum tube and is used as a source of electromagnetic interference high-frequency and highefficiency acting on targets, which operate in the band of wireless signal [9,10], for example acting on electronic devices on board of the UAV or acting on communication of the UAV that uses wireless signal for transmit control signals, by the way we can disable fly of UAV in demarcated space. To supply magnetron, it is necessary to design an inverter 12 V/4000 V and 50-60 Hz. Fig. 4 shows a block diagram of the interfering part.
The interfering part is energised from the part 'Power-battery'. The main element of the interfering part is DC/AC inverter 12 V/ 230 V, 50 Hz. In this article, we will be also showing the design and measurement of the antenna in the terms of power density of the interfering part.

DC/AC inverter voltage 12 V/230 V, 50 Hz
In practice, there are many types of the DC/AC inverter. In this article, it will be shown one of our experiments that is a design of a push-pull forward inverter. This inverter is very convenient for high power application [11][12][13]. In Fig. 5, it is shown that the diagram of the push-pull forward inverter.
The control circuit includes high-and low-side driver IR2101 that is used to amplify voltage of the control signals from the part 'block control'. There are two control signals from control circuit that are designed for driving two MOSFETs (Q1, Q2) transistors. Here, two control signals are two PWM pulses with the same frequency 50 Hz and reversed polarity. The ratio of the PWM control signals will be varied in the range from 30 to 70%. The output of the transformer is loaded with a light bulb (40 W, 240 V). The results of voltage on the secondary winding of the transformer are shown in Table 1.   The output voltage has maximum value 182 V with ratio of 50% and has maximum efficiency that is 70.6%. Fig. 6 shows the waveforms of the output voltage (the green line) and one output of the control circuit (the yellow line).
The value of maximum efficiency is suitable to theory about this inverter [14] but the output voltage did not achieve value 230 V. The reason of this problem is losses in the transformer that is designed only for application of low electronic performance. So, for this application, this transformer is not suitable. This first experiment is used only to test and verify characteristics of a pushpull forward inverter in terms of power efficiency and will be not used to apply magnetron of the interfering part. In the next work, we will design a transformer that has suitable parameters to apply magnetron.

Measurement of the characteristics of the magnetron
In this part will be shown measurement of the input and output of the magnetron. We set a system like Fig. 4 but did not use the part 'Power-battery' and 'inverter 12 V/230 V'. We used supply network 230 VAC for supplying high-voltage transformer. Fig. 7.
The high-voltage transformer used is transformer MOT (Microwave Oven Transformer) that has two secondary windings. The former has 3700 V with the current around of hundred mA, and the latter has 3.15-3.3 V with a current of about 10 A.
MOT began to generate a characteristic sound after applying. The voltage has a shape like a sinusoidal waveform with voltage −6.35 kV. The voltage was measured two more times during 2 s. In the next second, the signal was roughly begun to form in rectangular shape with voltage −6.35 kV. In the final third second, signal already has a rectangular shape with a slight overshoot in the rising and falling edges and voltage is −3.7375 kV. Fig. 8.
In the next step, we will verify the frequency of magnetron by using spectrum analyser. The result is shown in Fig. 9.
The result shows working frequency of interfering part with magnetron that is in the range Wi-Fi signal with centre value 2.463 GHz. The magnetron is a special kind of vacuum tube with a high level of power can influence on the system of electronic devices that also work in Wi-Fi signal.

Design and testing of the antenna
In this part will be shown design and testing of antenna system of the interfering part. The antenna system will be used for wireless signal 2.45 GHz.
An antenna with a high gain is often necessary to keep the signal level as well as to have constant connectivity. One of the simplest and probably the most widely used antennas is the horn. Horn antennas are widely used as a feed element for large radio astronomy, satellite tracking, and wireless communication [15]. Horn antenna creates the largest group of primary emitters used in the radars and has good impedance characteristics over the frequency band, the possibility to define the shape of emission characteristics and their structural simplicity. One of the most widely used horn is rectangular pyramidal horn antenna. Fig. 10.
The antennas will be placed on the mobile device, for this reason, the design of the antenna must be presented from the perspective of size and power density at a given distance. In this application, we will use concept optimum gain-horn that is preferred as it results in the shortest axial length for a specified gain [16].
All calculation and modulation to optimise the antenna will be implemented by using MatLab. The size of the antenna will be calculated for semi-half-power beamwidth, which is narrow as  The gain of pyramidal horn antenna is calculated from 10 to 25 dBi. For each value of gain, the program in MatLab will calculate beamwidth in E-plane and H-plane and calculate power density at given distance 30 m. To optimising the antenna in terms of size and power density at a given distance, the program in MatLab will compare the values of power density, half-power beamwidth, and dimensions of the antenna. In the next step, the program will choose two variants resulting in the highest possible power density, the narrowest possible half-power bandwidth and the shortest possible dimensions of the horn antenna.
Two selected optimum horns are gain G = 17, 18 dB for the frequency f = 2.45 GHz [17]. The power density will be calculated for 30 m distance. The results of the calculation are shown in Table 2.
For verifying the functionality of the program in MatLab, the pyramidal horn antenna will be simulated by using antenna design tool Antenna Magus. Fig. 11 shows results by using two programs, MatLab (a, b) and Magnus software (c). Magnus software is professional tool for antenna-design and helps us to control our approach of modelling by MatLab.
The results of modulation from the use of the program in MatLab are satisfied with those from the use of Antenna Magus Software in terms of half-power beamwidth (−3 dB) and gain (see Table 2 and Fig. 11).
In the practice part, the measurement of radiation pattern was performed at the anechoic chamber. The result of measurement of the antenna with G = 17 is shown in Fig. 12.
The red lines are the radiation patterns of E-plane and the green lines show the radiation patterns of H-plane. The data of measurement were saved in file.txt. By using the program in MatLab, the beamwidth of the radiation pattern on E-and H-planes were determined and shown in Table 3.
For G = 17, the value of E-plane beamwidth is smaller than that from the calculation about 1.04°. Moreover, the value of H-plane   beamwidth is higher than that from the calculation about 0.14°. For G = 18, the results of measurement differ more from the calculation and modulation. The value of the E-plane beamwidth is smaller than that from the calculation about 1.48° and for H-plane about 1.54°. From these results, we can verify the proper functioning of the program in MatLab. The program is convenient for the design and modulation of horn antenna. The next measurement is about power density of the horn antenna. The selected transmitter power was 850 W from the magnetron. The set-up of the measurement is shown in Fig. 13.
The distance was 30 m from the horn antenna. The measurement was implemented by using of the device broadband field meter Narda NBM-90. The results show the value of 3.25 W/m 2 for G = 17 dBi and the value of 4.2 W/m 2 for G = 18 dBi. The results from measurement are very close to values from calculation. The difference between measurement and calculation can be a result of a measurement error or inaccurate building of the antenna.

Conclusion
This article presents a design and implementation of the test version of the mobile electrical vehicle for EMI applications. The mobile device was implemented by using the system of stepper motors and controlled with the help of microprocessor ATmega 16. The mobile device can move forward, backward, turn left, turn right, and change speed. In the next mode, mobile device drives the rotation of the antenna, on/off interfering part.
The paper showed one experiment of the power electronic that is a design of a push-pull forward inverter 12 VDC/230 VAC, 50 Hz. This designed inverter used transformer that had low electronic performance and was not suitable for applying magnetron. In the next work, for applying magnetron, it is necessary to design a new transformer with desired parameters. The measurements of highvoltage transformer and magnetron also were shown in this article. The experimental results showed working frequency of the magnetron that is in the band of Wi-Fi signal. The magnetron with high-electronic energy can interrupt electronic device in Wi-Fi band.
The last part of this paper is about design and testing system antenna of the interfering part. The pyramidal horn antenna was selected for this application. The antenna system was designed by using a concept of optimum -gain horn. All calculation, modulation of the antenna was implemented by using the program, which was created in Matlab. The program allows us to optimise parameters of the antenna system. The experimental result showed proper functioning of the program for practical design.
In the next work, we will continue to solve problem with the interfering part with the magnetron. The output voltage for magnetron device must be variable around 4000 VAC to control power output of the magnetron. In addition, the problem that must be solved is based on the balancing the power density and weight of power supply of the magnetron. The results of the optimisation of size, weights of power converter due to need of mobile vehicle to achieve low weight to improve dynamic behaviour of vehicle, consequently high-frequency switching are required.

Acknowledgments
The work presented in this article has been supported by the Czech Republic Ministry of Defence -University of Defence development program 'Pulse High Voltage Converters for Mobile Interference Device with Magnetron' (K-207).