Abstract: High frequency solenoid valve has the advantage of fast response, and it can effectively improve the deposition uniformity especially for high-speed operation.In order to study the variable spraying characteristics of high frequency solenoid valve, a pulse width modulation (PWM) variable rate spraying system based on high frequency solenoid valve was developed.The minimum pulse width of the solenoid valve used in the research was 1.3 ms.The PWM variable rate spraying system was made up of power supply, drive, single chip microcomputer, solenoid valve, serial communication module, computer, and so on.Frequency and duty cycle of solenoid valve could be regulated by the PWM control system in real time.This article applied the ST standard fan nozzle and IDK anti-drift nozzle which are commonly used in the agriculture to carry out experiment, which focused on how frequency and duty cycle of solenoid valve influenced flow rate, droplet diameter, atomization process and longitudinal deposition uniformity.In the flow rate test, 12~37 Hz and 20%~100% were chosen as the working frequency and duty cycle and the working pressure was 0.2~0.5 MPa.The water weight was measured by precision balance in 30 s to get flow rate(L/min), and each test was repeated 5 times.It was observed that the flow rate had a linear relation with the duty cycle in the fixed working pressure and the R2 of regression line was greater than 0.9 at 12~37 Hz.Linear interval was 0.8 at 12 Hz and 0.56 at 37 Hz, and it was concluded that the linear interval decreased with the increasing of frequency.For the nozzle of ST120-04 under the working condition of 0.4 MPa and 30 Hz, the flow rate increased by 6 times at duty cycle of 100% compared to 20%, while the flow rate increased by 10 times in the pressure of 0.5 MPa.Therefor it was essential to collect proper frequency and spraying pressure to meet working requirement.In the article, 2 measurement modes of Spraytec particle sizer were chosen in the test of droplet diameter.The continuous measurement mode could measure the droplet diameter continuously for 10 s.Droplets could be measured discontinuously at the measure frequency of 100 Hz which meant that droplets were measured for 100 times in one second by the Spraytec particle sizer.The frequency and duty cycle were 15~35 Hz and 20%~100% and the working pressure was 0.3 MPa.Nozzle was fixed at 50 cm above the Spraytec particle sizer and volume medium diameter(VMD) was used to describe the droplet diameter in the article.The results showed that in the continuous measurement mode, the droplet diameter decreased from 20% to 100% respectively for the nozzle of ST110-02, ST110-03 and ST120-04.Moreover, for ST110-03 and ST120-04, the droplet diameter increased from 20% to 40%, because these 2 nozzles needed more flow rate to form the atomization zone yet the flow rate was too small from 20% to 40%.In the discontinuous measurement mode, the droplet diameter changed periodically and the average VMD of 3 nozzles was consistent with the test result of continuous measurement mode.In order to further research the atomization characteristics of PWM variable spraying system based on the high frequency solenoid valve, high-speed camera was used to observe the atomization zone at different frequency and duty cycle.The frame and shutter of high-speed camera were set in 3 000 FPS and 1/4000 s respectively.It was observed that the atomization zone would break up because of nozzle's instantaneous open-close.The duty cycles at the fracture of atomization zone were 26%~43% and 35%~60% for the ST110-03 and ST120-04 nozzle from 15 to 35 Hz.Deposition test bench based on the traveling boom was developed in the Center of Chemical Application Technology, China Agricultural University.Petri dishes (35 mm) were used to collect droplets.The collectors were set in 5 parallel columns and the interval of adjacent 2 columns was 12 cm.Each column contained 16 Petri dishes and the 3rd column was placed under the traveling direction of nozzle.Tartrazine(3‰) was chosen as the tracer material and spray boom speed was 2 m/s.In the deposition test, 15 and 30 Hz, and 40%, 70% and 100% were chosen as the working frequency and duty cycle.The results showed that frequency and duty cycle would influence deposition uniformity.For the IDK120-04 nozzle at duty cycle of 40%, the CV (coefficient of variation) at 30 Hz decreased by 6.07% compared with 15 Hz.From 40% to 100%, CV decreased by 11.75% and 18.31% respectively for IDK120-02 and IDK120-04 at 15 Hz while decreased by 8.72% and 12.24% respectively at 30 Hz.Also, the deposition uniformity of ST standard nozzle was better than the IDK anti-drift nozzle.These results show that the high frequency solenoid valve can be used in the PWM variable rate spraying system and it has advantage in longitudinal deposition uniformity.