Design and Research of Efficient Adaptive Spray Plant Protection UAV

Chao ZHU, Hongxin ZHANG, Wanlin SUN, Miao WANG

1. Physics Department, Changji College, Changji 831100, China; 2. Xinjiang Agricultural Vocational Technical College, Changji 831100, China

Abstract When the plant protection UAV is spraying on plants, the operator usually performs visual inspection on the vertical distance between the UAV and the plant to adjust the spray height to complete the application. Visual inspection by human eyes is easy to cause the error of spray deposition and deposition density. In order to improve the full utilization of drugs and the spraying efficiency of plant protection UAV, an efficient adaptive spray plant protection UAV was designed, and the vertical distance from the UAV to the plant was collected by ultrasonic wave. Meantime, the plant density was detected in real time, and the flight attitude of plant protection UAV and application rate of spray nozzle were automatically adjusted. The experimental results showed that the designed highly efficient adaptive spray plant protection UAV had fast response speed, precise spray location and less drug loss.

Key words Plant protection UAV; Ultrasonic wave; Adaptive adjustment; High efficiency; Less drug loss

1 Introduction

Modern crop plant protection is featured by precision, refinement, high efficiency and less drug loss[1]. In conventional plant protection operations, the unmanned aerial vehicle (UAV) pesticide application task is completed by visual inspection of the operator. Due to the limited visual range of the operator, it is not possible to adjust the flight application action dynamically according to the height and density of plants in real time[2]. In the process of drug application, precise application of drugs according to plant growth height can effectively improve drug deposition rate and reduce droplet drift. Precise application of drugs according to plant growth density can reduce drug loss and lower the damage of drugs to water and soil[3-4]. Ultrasonic measuring distance technology of UAV can be used to collect the vertical distance and plant density, complete the dynamic perception of the operating environment in real time, and adjust the parameters of the UAV (spray height, spray time) adaptively, so as to achieve accurate and precise spraying of the operating environment.

2 Precise spray control system

2.1 Hardware framework of plant protection UAVRich hardware configuration can complete the efficient coupling between flight attitude (flight speed, flight height) and nozzle opening by systematically processing the real-time operational environment data collected, and delivering precise control instructions according to the adaptive control system is the key to complete an efficient spraying operation of UAV. The UAV used STM32F407 microprocessor of STMicroelectronics Cortex-M4 kernel as the MCU of centralized controller, which not only had high processing capacity and short instruction processing cycle, but also added abundant peripheral interfaces, and adopted more advanced battery management mode, so that the processor had lower power consumption.

The peripheral interface of microcontroller was developed again to realize the acquisition and real-time control of UAV’s own state. Meantime, six axis acceleration transducer for attitude adjustment, high precision angle measurement transducer, air pressure transducer, electronic compass, GPS, electronic speed regulator and wireless communication module,etc., as well as supply component for application operation, ultrasonic distance measuring module, differential pressure liquid level measuring system[5], spray nozzle, and electronic speed regulator control module were equipped. The hardware framework of efficient plant protection UAV spray control system is shown in Fig.1.

2.2 Principle of fixed height sprayUltrasonic measuring distance technology does not harm the detected object, and is not interfered by dust, electromagnetic wave and light wave, with the measurement accuracy reaching centimeter level[6]. The important factors affecting the spraying efficiency of plant protection UAV in the process of pesticide application are composed of target plant height, transport characteristics of droplet flow field, degree of atomization and penetrating power of droplets. When the UAV flies at a high altitude, droplets are susceptible to the influence of natural wind, wind direction and temperature, which leads to drift of droplets and uneven distribution of droplets; when the UAV flies at a low altitude, droplets are mostly concentrated in the middle and lower part of the plant[7], leading to the problems such as low spraying efficiency and high drug pollution.

The plant protection UAV flies at different altitudes and speeds for different crops. In order to maximize the efficiency of pesticide application to crops, the vertical flight height of UAV from plants and its flight speed should be initialized according to the field operation environment and agricultural expert knowledge before pesticide application, and the normal operation of all work modules should be checked.

2.3 Ultrasonic ranging circuitIn order to realize the real-time monitoring of the distance between UAV and leaf surface, an ultrasonic ranging structure was designed according to crop leaf orientation and the specific requirements of spray operation. The

Fig.1 Hardware framework of plant protection UAV

structure was installed directly below the spray nozzle, mainly composed of ultrasonic transmitting circuit (ultrasonic driving circuit, ultrasonic probe) and ultrasonic receiving circuit (signal acquisition, signal conditioning). When flying to crops with lower (higher) height, the plant protection UAV adaptively adjusted (descend or ascend) according to the pre-set vertical height, to maximize the spraying efficiency.

Ultrasonic signal transmitting circuit was mainly composed of ultrasonic driving circuit and ultrasonic probe (Fig.2). SP3232 was used as an ultrasonic transducer chip in the driving circuit, which input a specific driving voltage into the probe to obtain a specific frequency of ultrasonic signals. The operating voltage was 3.0-5.5 V, and it had a low power consumption charged mode. The C2- and C2+ of pins were used for voltage reversal, and C1+ and C1- for energy storage of charge pump.

Fig.2 Ultrasonic transmitting circuit

In ultrasonic generation, STM32 high resolution timer sent complementary adjustable duty cycle PWM signals to T2IN and T1IN pins of SP3232, and generated +6V and -6V voltages through the voltage reversal function of the chip. Ultrasonic transducer was driven by T2OUT and T1OUT interfaces to send ultrasound at a specific frequency.

When ultrasonic wave propagates in medium, the amplitude of echo signals reflected by obstacles varies with distance. In order to get the accurate echo signal with less noise, it is necessary to amplify the echo signal. The system used NE5532 operational amplifier to reprocess the echo signal. The chip had the characteristics of low noise, multi-op amplifier and high performance. Since the amplitude and distance measurement of echo signal attenuated exponentially with the increase of distance, a time gain compensation circuit was used in the test to gain compensation for attenuated echo. The ultrasonic receiving circuit is shown in Fig.3.

Fig.3 Ultrasonic receiving circuit

2.4 Precise spray principleThe growth density of crops is affected by soil acidity and alkalinity, watering suitability and other factors, resulting in differences in growth density of crops[8]. Spray should not be applied equally in plant areas with uniform growth density and less density. Meantime, nozzle diameter and fluid pressure have different effects on the flow rate of medicine liquid, spray particle size and deposition distribution of droplets[3-4]. High efficiency UAV adopts electric centrifugal nozzle, which can realize uniform spray on the front and back side of plant leaves and stem in the application process.

In the process of plant protection, when the UAV flied to the area with low plant density, ultrasonic wave determined whether there were plants and plant growth density under the spray nozzle according to the transit time. Meantime, the detection data were transmitted to the microprocessor, and the electronic speed regulator was activated by special communication protocol, so as to realize the speed control of the diaphragm pump and the electric centrifugal nozzle used for drug application in the medicine tank, and then regulate the drug flow, spray range and droplet radius according to the crop density (Fig.4). There was a certain mathematical relationship between the flight speed of UAV and the required volume of spray[9],i.e.Q=AWU/166.67.

Where,Qis the calculated flow rate (L/min);Ais the spraying width of spray nozzle (m);Wis the application volume of target area (L/hm2);Uis the flight speed of UAV (m/s).

Fig.4 Control principle diagram of precise spraying control system

3 Control principle of UAV position

The plant protection UAV designed in this paper was a quad-rotor aircraft with a cross structure model (Fig.5). There were four DC motors at the end of the cross structure of UAV.

Fig.5 Structural model of quad-rotor aircraft

The speed was controlled by controlling the current size of each motor, and the lift force and resistance generated varied with motor speeds, so that the flight attitude of the aircraft could be changed by controlling the motor speed. A typical principle of attitude adjustment is shown in Fig.6.

Fig.6 Principle of attitude adjustment of quad-rotor aircraft

When the four motors kept the same rated speed, the aircraft would keep hovering action; when the speeds of four motors were increasing (decreasing) simultaneously, the aircraft would complete the ascending (descending) movement; when the speed of motor 2 decreased and that of motor 4 increased, the aircraft completed left-leaning (right-leaning) movement; when motors 2 and 4 accelerated and motors 1 and 3 decelerated, the aircraft completed clockwise (anticlockwise) rotation. The position motion of flight attitude could be realized by the combination of increasing and decreasing the speed of four motors. The specific position control steps and analysis of quad-rotor UAV can be modeled and derived according to literature[10-13], which will not be described here.

4 Efficient adaptive spray control system

The variable application control system based on ARM framework and application flow control method based on pulse width modulation technology can compare the height measured by ultrasound with the pre-set application height in real time. The comparison results showed that acting on each rotor motor by high precision fuzzy reasoning system realized dynamical adjustment of application height of UAV following the optimal application distance; according to the information of crop growth density, the optimal configuration of parameters of spray nozzle could achieve the maximum spray efficiency and precise quantification of drugs; the whole adaptive spray process realized the automatic matching of application flow and application amplitude with flight height and speed. The control flow chart of efficient adaptive spray control system is shown in Fig.7.

5 Results and analysis of spray test

The ultrasonic distance measuring system was tested in the laboratory environment, and the parameters of each vertical distance were measured respectively. The total error of the designed ultrasonic distance measuring module was within 4%, which reached the standard. The test results are shown in Table 1.

Table 1 The results of ultrasonic distance measurement

Fig.7 Control flow chart of efficient adaptive spray control system

The test was carried out in team 5 of the horticulture farm in Changji Prefecture. The average relative humidity of the environment was 53%, the average temperature was 27 ℃, and the wind speed was less than 1.4 m/s (Fig.8).

Five groups of cotton columns with different densities were selected in the test field, and the plant protection UAV could load 15 L pesticide. By comparing the volume of drugs applied in the operation of efficient spraying system with that of conventional spraying operation, the overall saving volume of the efficient spraying system reached 18.81%, and the average saving volume was 16.63% (Table 2).

Fig.8 Spray test picture of plant protection UAV

Table 2 Comparison of volume between efficient spray and conventional spray

A column of cotton plants with uneven growth density was selected in the test field, and droplet deposition on unit leaf surface was detected by droplet deposition detection method introduced in literature (Table 3)[14]. The statistical results of 20 crops selected in this column showed that the efficient spray control system could spray and deposit uniformly under uneven plant growth density, and the deviation could be controlled within a small range.

Table 3 Statistics of droplet deposition

6 Conclusions

(i) Aiming at uneven application caused by visual application of drugs in the process of plant protection, the efficient spray plant protection UAV based on ARM framework is equipped with abundant information collection module of peripheral operating environment, which is convenient for in-depth study on the influence of spray efficiency on generated airflow and environmental wind speed and direction in the process of spray. The highly efficient spray control system developed at the present stage can realize the adaptive adjustment of UAV attitude according to the height of the plant, and at the same time, the pulse width modulation technology is used to control the spray volume and amplitude automatically, so as to realize the adaptive matching of flight state and spraying volume according to the operational environment.

(ii) Multiple groups of tests were conducted in team 5 of the horticulture farm in Changji Prefecture with an area of 6.8 hm2. The results showed that the spray plant protection UAV designed could accurately detect the distance to the plant during flight, and the error could be controlled within 4%. According to the plant density, the spraying flow was controlled adaptively and reliably, and the spraying volume was significantly reduced. Compared with the conventional operation, the spraying amount was reduced by 16.63% on average, and the deposition of droplets was more uni-

form in the spraying operation. During the test process, the UAV system ran stably and reliably, had fast adaptive response and high robustness.