Electromagnetic interference shielding in unmanned aerial vehicle against lightning strike

Electromagnetic interference (EMI) due to lightning strike in Unmanned Aerial Vehicle (UAV) became a critical issue, since it could cause a severe damaged to electronic equipment which was installed in UAV. This paper proposed a method to reduce EMI by creating shield over the body of UAV with aluminum foil with thickness 0.15mm. A standard lightning impulse-voltage generated by impulse generator was delivered to the high voltage (HV) electrode at a particular distance from UAV. Indirect effect of lightning strike was investigated by measuring induced voltage inside the compartment of UAV fuselage during flashover between the HV electrode and UAV. Flashover between the HV electrode and UAV simulated the lightning strike. The result showed that the highest voltage only 1V at UAV compartment when 150kV impulse voltage supplied to the electrode with striking area on the wing. The measured voltage was far below 100V as the insulation level of low voltage equipment inside UAV. Although the direct effect of lightning has created a burning hole on the surface aluminum foil with thickness 0.15mm, the effect was overcome by increasing the thickness to 0.3mm. The application of aluminum foil over the surface of UAV repelled the electromagnetic interference which acted as a Faraday cage. The shielding method was successfully reduced the effect of EMI.


Introduction
In general, Unmanned Aerial Vehicle (UAV) structure made of kevlar, fiberglass, carbon fiber, balsa wood, and elapor foam [1].Those materials are dielectric which are not conductive at all, except carbon fiber which is less conductive than metal.Dielectric materials are vulnerable to lightning strikes damage.Lightning strike to aircraft has two effects: direct and indirect.Direct effect is mainly related to physical damage of the structure at the striking area [2][3][4].Indirect effects occur when electromagnetic field induce transient voltages in the compartment of UAV fuselage [2,3].
The Lightning Strike Protection (LSP) serves as a conductor path over the skin of aircraft to ground without damage and it gives shield against electromagnetic interference [5].Due to low electrical conductivity of dielectric materials, some materials and methods have been studied to improved the conductivity [2,[6][7][8][9][10][11][12].Several works proposed the use of metallic coating such as: aluminum, silver, pristine graphene or carbon based composited for LSP [6][7][8][9][10], or hybrid coating consisted of silver nano particles dispersed in conducting polymer [10].Other works proposed paper made of carbon nanofiber and nickel nanostrand [2].A conducting polymer which has a conductivity similar to metal such as (Polyaniline) was suggested by [11] as a filler of dielectric matrix.Previous researchers also used copper wire mesh or pristine as an envelop of carbon fiber composite [12].Among all, expanded copper foil (ECF) showed as the best material used for LSP [10].However, copper foil is heavier than aluminum.Moreover, copper is more expensive than aluminum.
Many papers have discussed the LSP solution in the aircraft but the method did not effectively overcome electromagnetic interference in UAV.The authors carried out lightning strike tested using a lightning current waveform to investigate the performance of LSP [2,5,[8][9][10]12].While few paper proposed the LSP solution in UAV.Other works developed a numerical simulation and experimental validation method to assess  ISSN: 1693-6930 TELKOMNIKA Vol.17, No.2, April 2019: 915~919 916 electromagnetic compatibility of composite based UAV by using a DC injection test to measure induced current [13,14].However, the test can not be used to study electromagnetic interference due to lightning strike.
Therefore, this research was to study the electromagnetic interference shielding against lightning strike.This paper proposed a method to reduce electromagnetic interference by covering body of UAV with aluminum foil.Both direct and indirect effects of lightning strikes were investigated by testing UAV under high voltage test.This research used a standard lightning impulse voltage supplied by impulse generator.The lightning impulse voltage was used instead of lightning current because this research mainly investigates the induced voltage cause by electromagetic field during lightning strike.

Research Method
The UAV Skywalker X8 with specification listed in Table 1 was used in this research.It made of elapor foam wrapped with fiberglass using lycal resin to strengthen the structure.Hence, the UAV was totally made of dielectric material.The EMI shielding made from aluminum foil with 0.15mm thickness.Figure 1 shows the UAV has covered with aluminum foil.The UAV which has EMI shielding was undergone high voltage (HV) testing.The aim of testing was to measure induced voltage during flashover between high voltage electrode and UAV.When flashover occurred an electromagnetic field was created in vicinity of striking point.Figure 2 shows an impulse generator used to generate lightning impulse voltage.A standard lightning impulse voltage 1.2/50s was delivered to high voltage electrode as shown in Figure 3.

Testing Electric Field in Vicinity Area of UAV during Flashover
The idea of conducting the test was to measure the induced voltage inside the compartment of fuselage without EMI shielding.However, the conducted test is harmful to the electrostatic field meter.A modification has been made by placing EFM at 4cm below the fuselage.A 50kV impulse voltage was delivered to HV electrode.The measured result was used to calculate the induced voltage at 1cm distance between HV electrode and UAV.

Testing Electric Field in Compartment of UAV Fuselage
The induced voltage was an average value from ten times experiment results as shown in Table 2.The electric field obtained by dividing induced voltage with 2.54, as calibrating factor to get electric field in unit of V/cm.Table 2 shows the highest induced voltage was only 1V inside UAV compartment.It was measured at striking point on wing with impulse voltage 150kV.The electronic equipment placed inside the compartment, such as: APS, GPS, telemetry and camera, has impulse breakdown voltage below hundreds volts.An electronic device with IC bipolar technique has 100V as a lowest breakdown voltage limit [15].Therefore, the highest measured voltage of 1V did not damaged to the equipment.Figure 5(a) shows burning holes found at surface of aluminum foil during measurement.In order to the prevent physical damaged the thickness of the foil was increased to 0.3mm.Increasing the thickness is to increase flash point of material.The lowest temperature at which aluminum foil will ignite becomes higher.

Testing Electric Field in Vicinity Area of UAV during Flashover
The induced voltage and electric field measured at 4cm below UAV are 456V and 179.53V/m, respectively.Theoretically the electric field is inversely proportional to the distance.The distance between compartments to the striking points on the exterior surface of UAV was assumed around 1cm.Hence, the calculated result of induced voltage and electric field should be four times higher than those measured at distance of 4cm.The induced voltage and electric field were 718V/m and 1824V, respectively.
The calculated induced voltage inside compartment of UAV was 1824V.The measured voltage inside compartment of UAV without electromagnetic interference shielding presume comparable with the calculated ones.The voltage could be harmful to the electronic equipment inside the compartment since it is higher than 100V as the maximum insulation voltage of electronic equipment.This result proved that the electromagnetic shielding successfully reduced the effect of electromagnetic interference due to lightning strike.

Conclusion
This paper proposed an electromagnetic interference shielding by covering body of UAV with aluminum foil thickness 0.15mm.The measured induced voltage and electric field in the compartment with electromagnetic interference shielding was far below the insulation voltage of the equipment.The pyhsical damage on the surface of aluminum foil can be avoided by increasing the thickness to 0.3mm.The EMI shileding are mandatory to prevent direct and indirect effect of lightning strike in UAV.

Figure 5 (Figure 5 .
Figure 5(a) shows burning holes found at surface of aluminum foil during measurement.In order to the prevent physical damaged the thickness of the foil was increased to 0.3mm.Increasing the thickness is to increase flash point of material.The lowest temperature at which aluminum foil will ignite becomes higher.Figure5(b) displays no burning hole on the surface of 0.3mm aluminum foil.

Table 1 .
Figure 1.Application of aluminum foil on Skywalker X8 UAV to shield electromagnetic interference

Table 2 .
Measured Result of Induced Voltage And Electric Field when Flashover Occur between HV Electrode and UAV

Table 2
shows the measured induced voltage with electromagnetic Electromagnetic interference shielding in unmanned aerial... (Diah Permata) 919 interference shielding was only 1V at UAV compartment when the impulse voltage in 150kV.