Design and experiment of uav for spraying solid liquid m ixture based on finite element analysis

Ting-ting SUN，Lin-lin CHEN，Jun LI

（1 Shandong Ship Control Engineering and Intelligent System Engineering Technology Research Center，Weihai264200，China）

（2WeihaiOcrean Vocational College，Weihai264200，China）

（3 Qilu University of Technology，Jinan 250000，China）

（4Weihai Intelligent Marine Fishery Equipment Engineering Technology Research Center，Weihai264200，China）

Abstract：Aiming at solving the problem that the UAV can’t spray wettable powder and suspending agent，a kind of UAV which can spray solid-liquid mixture was designed.Based on a certain type of eight rotor UAV，themachine is composed of amedicine tank with a stirring device，a nozzle which can spray solid-liquid mixturemedicine and landing gear of the UAV.According to the type of spraying agent，a liquid medicine tank with a stirring device is designed，and then the spraying of solid-liquid mixture is designed and the hydrodynamics simulation is carried out.The UAV bracket tofix the liquid medicine tank is designed and the static and dynamic analysis is carried out.The battery is reconfigured according to the lift and endurance requirements of the UAV.The test results show that the concentration of the chemical liquid in the liquid tank is evenly distributed，and the concentration error is less than 10%；the spraying has good average coverage and average deposition parameters；the spraying operation with 5L of chemical liquid can last15.5 minutes.

Key words：UAV for plant protection，Solid liquid mixture，Nozzle design，Hydrodynamics calculation，Static and dynamic analysis

In recent years，the emergence of UAV technology injects fresh blood into the field of agricultural plant protection，and gradually becomes themainmethod of agricultural pest control.Equipped with a chemical spraying device，the UAV can solve the problems of low efficiency and difficult operation of the ground chemical spraying device［1］.UAV technology is moremature than drug spraying technology based on planting protection UAV.Therefore，the UAV-based research of drug spraying equipment is of great significance.Li Jiyu［2］et al.Studied the effect of air flow on the droplet size of UAV through the field test and wind tunnel test.The results show that the atomization performance of the nozzle in awind tunnel test is stable and the atomization grade is fine；the effect of air flow vortex on the droplet size is significant in field the test.Cai Yanlun［3］designed a contact electrostatic spray system based on F-50 type plant protection UAV.Hu Caiqi［4］et al.Designed a carbon fiber board rotor spray UAV tomeet the requirements of scientific research and spray irrigation.Han Qing［5］et al.Applied genetic algorithm to the optimization of airfoil sandwich structure UAV and achieved excellent results.The results indicate that the optimized weight is reduced by 38.2%before optimization.Liu Feng［6］et al.Optimized the whole carbon fiber wing of 10 kg class fixed wing UAV.The weight of the optimized wing is only 11.94%of the initial weight.Jiliang［7］et al.Used topology optimization to design the structure of four rotor UAV.Yu Hui［8］designed and optimized the structure of the heavy-duty four rotor UAV.At present，research direction of scholars mainly focuses on the design，the optimization design，the spray system and droplet size of UAV.However，there are few studies on plant protection UAV’s that can spray solid liquid mixed agents.

In this paper，aiming at solvingthe problems of the existing plant protection UAV spraying wettable powder and suspending agent，such as the liquidmedicine is easy to precipitate，unable to mix evenly with water，nozzle blockage and so on，based on the eight rotor plant protection UAV，we designed the liquidmedicine tank with stirring function，the spray nozzlewith spraying mixed liquid medicine and the landing gear of the UAV，and tested the operation performance of the UAV spraying solid liquid mixed liquid medicine through the inter test.

1 Structure and w orking p rincip le of the whole m achine

1.1 Overall structure and main technical parameters

In order to achieve the purpose of spraying solidliquid mixture in the plant protection UAV，the tank with a stirring device，nozzle and landing gear for spraying solid-liquid mixture were designedbased on the eight rotor UAV.The UAV has the advantages of uniform spraying of mixed liquid medicine and no clogging of nozzle.Fig.1 is the structural diagram of the UAV.The other side of the pharmaceutical injection port is the front direction of the UAV’s flight.The nozzle is fixed under the left and right four rotors.The landing gear is composed of aluminum tubes of different specifications and fixed on the lower part of the UAV’s controlmodule bracket.The liquid medicine box is fixed on the landing gear of the liquid medicine box fixing buckle.The main technical parameters are shown in Table 1.

Fig.1 Tank structure diagram of UAV spraying so Iid Iiquid m ixture

Tab Ie1 UAV m ain technica Iparam eters

1.2Working principle

Due to the inherent characteristics of wettable powder and suspending agent，it is easy to cause uneven mixing of liquid medicine without stirring for a long time after mixing water，and the nozzle will be blocked during spraying.In this case，the spraying operator stirs the liquid several times in the spraying process，and solve the problem of nozzle blockage，which greatly reduces the efficiency of spraying liquid medicine.In order to solve these problems，the liquid medicine tank with stirring function and fan-shaped nozzle for spraying wettable powder and suspending agentwere designed.The working principle of the liquid medicine with the mixing function is：the motor drives the shaft with the mixing blade at the bottom end to mix the liquid medicine regularly through the deceleration device，which can solve the uniformity of the liquidmedicine entering the liquidmedicine pipe.The working principle of fan-shaped nozzle for spraying wettable powder and suspending agent is to allow solid micro particles to pass through without blocking.Before spraying solid-liquid mixture，inject the mixture through the injection port and tighten the cover，open themixing device switch in the tank，startmixing，start the UAV tofly over the target crop，turn on the air pump to make the air pressure in the tank higher than the atmospheric pressure，so as to spray.

2 Design ofmain com ponents and determ ination of param eters

2.1 Design of liquid medicine tank

The tank with mixing function is the key part of the UAV for spraying solid-liquid mixture，which has the function of storing the liquid medicine.It sets under the UAV and fixed on the landing gear.As shown in Fig.2，it is a liquid medicine tank with stirring function.The liquidmedicine tank withmixing function is mainly composed of liquid medicine tank，air pump，mixingmotor，mixing rod coupling，mixing rod bracket，liquid medicine outlet elbow，mixing rod，mixing fan blade，mixing rod bearing and liquid medicine filter screen with a certain height of bottom drum.In Fig.2，the design of the bottom of the liquid medicine tank with a certain height is tomake the liquid medicine fully stirred and flow into the liquid medicine pipe as much as possible，and also to provide space for the installation of the mixing device；the mixing motor and air pump are installed in the concave position on the top of the liquid medicine tank，and the mixing beltmotor is located on the central axis，which is conducive to the installation of the liquid medicine tank and the power supply of the UAV Fix the lithium battery；themixing fan blade is the key component of the liquid medicine tank with the mixing function，which is located at the lower part of the mixing rod.Themixing fan blade is connected with the coupling through the mixing rod and then connected with the mixingmotor.Themixing fan blade rotates under the driving of the mixing motor to mix the liquid medicine，so that the wettable powder and the suspending agent are evenlymixed with thewater；themixing rod bearing is fixed on the mixing rod bracket，starting and stopping themixing rod Push positioning function；the filter screen is located at the bottom of the liquid medicine tank and fixed under the mixing support tofilter the liquid medicine；the air pump makes the air pressure in the liquid medicine tank higher than the external air pressure，and the liquid medicine flows into the liquid medicine pipe through the liquid medicine outlet elbow under the air pressure.

Fig.2 Structure diagram of UAV spraying so Iid Iiquid m ixture

2.2 Nozzle designs and simulation analysis

The typical pesticide sprays such aswettable powder and suspending agent，will settle in a short time after being prepared，which will cause the effective components in the solution to notbe evenly distributed in the solution.Simultaneously，because the active ingredient is slightly soluble in water，there will be some small particles thatwill block when they enter the air through the traditional nozzle，which will affect the spraying operation of themedicinal solution.

The nozzle is not only the device to realize the atomization of liquid medicine，but also the key component of UAV for plant protection.The nozzle with excellent performance has not only excellent droplet size but also excellent application amount［9］.The most commonly used nozzle in plant protection UAV is spherical fan mist nozzle，which has the characteristics of large spray angle，large flow range and small atomization pressure requirement.In this design，spherical sector nozzle is used，as shown in Fig.3 is the three-dimensional structure of a spherical sector nozzle.Its atomization principle is：when a certain pressure of the liquid medicine flows through the nozzle orifice，the fan shaped nozzle cuts the wedge surface to squeeze through the pressure liquid flow，so that the liquid changes into a plane liquid film，and continues to thinner under the action of external pressure，and then becomes a fine thread，and finally becomes a droplet.The droplet becomes smaller and finer after being rubbed with the air，and falls on the plant to complete the spray operation.

Fig.3 3D structure of sphericaIsector nozzIe

As a standard part，the spherical fan-shaped nozzle can be purchased from the market，but at the beginning of its development，itwas not used to spray wettable powder and suspending agent，so the existing fan-shaped nozzle was redesigned.Before designing spherical sector nozzles，it is necessary to know the structural parameters affecting the spray volume，so as tofacilitate the subsequent determination of nozzle parameters.Structural parameters of spherical sector nozzle are shown in Fig.4.

Fig.4 Structura Iparameters of spherica Isector nozzIe

Parameter in the figure：αis groove angle（°）；his hole edge distance（mm）；His groove depth（mm）；Ris spherical radius（mm），D=2R，Dis spherical diameter；eis eccentricity（mm）.

After knowing the structure of the nozzle，it is necessary to calculate the spray quantity of the nozzle.Through the formula（1）（2），the spray volumeQcan be obtained.

Where：uis liquid flow coefficient；Fis the outlet area of f is sector nozzle（mm2）is related to spherical diameterD，groove angleαand eccentricitye.Gis acceleration of gravity（N/kg）；Pis liquid pressure（kPa）；γis Liquid density（kg/m3）.

The flow coefficient is calculated by equation（2）：

Where：∑uis the sum of the contraction coefficient of the nozzle section，determined by the nozzle structural parameters.

Finally，the structural parameters of the spherical sector nozzle are determined as follows：eccentricitye=0.92，spherical diameterd=4.6mm，groove angle α=30°.Finally，the structural parameters of the spherical fan nozzle were determined as：eccentricitye=0.92，spherical diameterD=4.6 mm，and groove angleα=30°.Different from its finite elementmodel，when the fluid model of the nozzle is set up，not only the fluid area inside the nozzle but also a part of the external area is needed in order to observe the spray field of the nozzle.In order to achieve the simulation results of observing the outside of the nozzle，a pentahedron was established based on the cutting angle of the nozzle.As shown in Fig.5，the finite elementmodel is established.The flat bottom surface of the cylinder is used as the inlet，its side，the spherical surface and a part of the plane as thewall，and the rest of the pentahedron as the outlet.There are 31 657 nodes and 159 013meshes in the finite elementmodel.

Fig.5 Finite e Iem entm ode Iof spherica Isec tor nozzIe

It is a typical Eulermultiphase flow model that the solid-liquid mixture flows through the nozzle.In fluent software，turbulence model，Euler multiphase flow，medium，boundary condition，iterative algorithm and relaxation factor，initialization and iterative calculation settings are carried out successively.In order to describe the results of flow field calculation conveniently，a plane named a plane is established by cutting the groove angle and perpendicular to the cylinder plane and the ground.The flow field analysis results in the plane a are shown in Fig.6-9.

Fig.6 Liquid phase ve Iocity distribu tion of section A

Fig.7 So Iid phase ve Iocity distribution of section A

It can be seen from Fig.6 and Fig.7 that the solidliquid two-phase flow velocity inside the nozzle tends to increase，and reaches the maximum value at the nozzle slot，becoming the inflection point of the change of the solid-liquid two-phase velocity；the solid-liquid two-phase flow velocity outside the nozzle tends to decrease and spread outwards，and the shape of the fog field is fan-shaped.It can be seen from Fig.8 and Fig.9 that the volume distribution of solidliquid two phases in the flow field outside the nozzle is fan-shaped.From the above analysis，it can be concluded that this design can provide favorable conditions for the atomization and tearing of solid-liquid mixture，and it also proves that the structural parameters of spherical fan-shaped nozzle are correct in the design process.

Fig.8 Finite e Iem entm ode Iof spherica Isec tor nozzIe

Fig.9 Finite e Iem entm ode Iof spherica Isec tor nozzIe

2.3 Design and analysis of landing gear

The landing gear is the key part of the plant protection UAV，which is used to connect the UAV and themedicine tank，and also to protect the medicine tank.During the flight of the UAV，it is used to carry the weight of themedicine tank.During the landing，it is used to support the UAV and provide the landing conditions.Due to the redesign of themedicine tank of the UAV，the original landing gear does not have the corresponding function，so it needs to be designed.The landing gear designed in this paper is a double inverted U-shaped structure，connected by four beams in themiddle，and connected by two circular tubes with a length of 500 mm at the bottom.The joint is fixed by TEE corner and bolt，thematerial is aluminum alloy，and the pipe thickness is 1.5 mm.Thethree-dimensional dimensions of the landing gear are：500 mm in length，375 mm in width，and 425 mm in height.

Fig.10 CIoud chart of Ianding gear disp Iacem ent distribution

Due to the complex force on the landing gear，in order to verify whether itmeets the requirements of UAV operation，the static analysis of the landing gear under full load static state is usually selected［10］.Apply a corresponding force according to the landing gear’s load and perform a static analysis.The analy-sis results are shown in Fig.10 and 11.The maximum displacement is located at the midpoint of the top end of the inverted U-shaped beam of the landing gear，which is only 0.02 mm.The connection between the inverted U-shaped beam of the landing gear and the fixed beam of the liquid tank is 2.669 MPa，which is far less than the allowable value of thematerial（σmax=370 MPa）.

Fig.11 CIoud chart of Ianding gear stress distribution

In order to ensure that the landing gear can still maintain structural stability in the process of UAV operation，the freemodal analysis of the landing gear is carried out.Generally，the first four natural frequencies greater than 1Hz in the free modal analysis results are selected for analysis［11］.Table 2 shows the results of the first four frequencies and themaximum deformation of the landing gear of the UAV.Because of the symmetry of the eight rotor plant protection UAV，the propeller is completely balanced and has no excitation source，but itwill acton the air in the flight process，the propeller will flutter and generate high-frequency vibration，generally above 1 100 Hz［12］.Through comparison，it is found that the first four natural frequencies of UAV are far lower than the vibration source frequencies，so the landing gear will not resonate during the operation of UAV.From the static and dynamic analysis results of the landing gear，it can be concluded that the design of the landing gear is reasonable.

2.4 Lift calculations and endurance calculation

Generally，when calculating the lift of plant protection UAV，the first consideration is the total weight of UAV.The total weight of UAV is not only affected by its own quality，but also related to the ambient temperature and atmospheric pressure［13-14］.The totalweightW of UAV can be obtained by formula（3）.

Where：Pais atmospheric pressure（MPA）；PWis partial pressure ofwater vapor（MPA）；WBisweight of UAV body（including battery）（n）；Nmotoris number ofmotors；Wais cantilever weight of rotor（n）；Wlis UAV load（n）；Tais ambient temperature（k）.

Tab Ie 2 The frequencies and m axim um deform ation

The lift force of the UAV in this design is the result of eightmotors driving the propeller at the same time.In order tofacilitate the motor selection，the maximum pull force provided by a single motor needs to be calculated.The maximum tension provided by a singlemotor can be obtained from equation（4）.

Where：Lmaxismaximum tension required for a single motor（n）；αis safety factor，generally 0.5.

Although the flight state of the UAV is changeable and the working state of themotor is constantly changing，the UAV keepsmovingmostof the time，so the battery capacity can be approximately solved by equation（5）.

Where：kis safety factor；IAis current of singlemotor in UAV working state（A）；IPair pump current（A）；Isismixingmotor current（A）；tis flight time（h）.

3 Experim ent

3.1 Comparative test results and analysis

The main purpose of the test is to test the mixing function of the liquid medicine tank.Test conditions：indoor with temperature of 24℃and humidity of 40%.Testmethod：first，open the mixing device of the liquid medicine tank，inject the configured liquid medicine of specific concentration（5%）into the liquid medicine tank，turn on the power switch of the air pump，use the same time interval of the beaker to connect the liquid medicine from the liquid medicine pipe（the amount of liquid medicine is random），cal-culate the solution concentration after heating and evaporation，and make a comparison with the previously configured concentration；on the premise of not opening themixing device Next set of tests.Materials for the test：prepared liquid medicine，several beakers，alcohol lamp and electronic balance.Table 3 shows the test results with agitator and table 4 shows the test resultswithout an agitator.

Tab Ie 3 Concentration vaIue w ith m ixing device

Tab Ie 4 Concentration vaIue w ithout agitator

From the test results in Table 3，it can be seen that the difference between themaximum concentration and theminimum concentration is only 0.9%in the five groups of tests conducted randomly，the concentration distribution of the liquid medicine is uniform，and the error between the concentration of the liquid medicine in the liquidmedicine tube and it configured less than 10%.From the test results in Table 4，it can be seen that in the absence of a stirring device，the difference between the concentrations of the liquid medicine is up to 3.8%，themaximum error between the concentration in the liquid medicine pipe and the concentration of the configured liquid medicine is up to 50%，and it shows a decreasing trend with the increase of time，which is caused by the slightly soluble in water of the wettable powder and the suspending agent and the settlement is easy to occur.It can be seen from the above comparative test that themixing device has excellentworking performance.

3.2 field test results and analysis

The field test ismainly used to test the comprehensive performance of the UAV which can spray solidliquid mixture.Test conditions：outdoorwith breeze of 0.1 m/s and temperature of 9℃.Testmethod：the test field was simulated by placing water sensitive paper.Six water sensitive paperswere arranged in a row with an interval of10m and repeated three times.The UAV was located at2 m above the water sensitive paper and maintained a speed of 3 m/s for cruise spraying.Materials required：water sensitive paper，drying dish，notebook computerwith droplet coverage analysis software DepositScan.The test results are shown in Table 5.The test results are the average values of three repeated tests.

Tab Ie 5 Sp ray test resu Its

From the test results in Table 5，it can be seen that the UAV spraying solid-liquid mixture has better average coverage and average deposition parameters.The test is also verified that the UAV can carry 5 L of liquid medicine for stable spraying operation，with endurance of 15.5 min，and the mixing device works stably.

4 Conclusion

A drone capable of spraying solid-liquid mixed liquids is designed.It consists of an eight-rotor drone，a liquid tank with stirring function，a nozzle that can spray solid-liquid mixed liquids，and a landing gear.The characteristics of the medicinal solution can also be sprayed with conventionalmedicinal solutions.

（1）The results show that the tank with agitation function has excellent performance，the concentration distribution in the tank is uniform，and the concentration error is less than 10%；

（2）A nozzle was designed to spray solid-liquid mixture，and the simulation of solid-liquid two-phase flow was carried out；

（3）The landing gear with fixed liquid medicine tank is designed，and the static and dynamic analysis is done；

（4）The results of field experiments show that the spraying has better average coverage and average deposition parameters.