Research Of Spray Deposition Uniformity For Dynamic PWM Variable Spray System

Chemical control is an effective way to control plant diseases,insect pests and weeds.In the meantime,excessive pesticide residues could cause huge environmental pollution and human injury.In recent years,as an environmentally friendly spray method,variable spray has becoming more and more important.However,it's difficult to control the spray deposition uniformity because of the intermittent characteristic of PWM(Pulse Width Modulation)variable spray system.And the spray deposition uniformity is an important measure index for the spray quality.Only improve the spray deposition uniformity,can we save pesticides and improve the effective utilization of pesticides under the premise of high pesticide effect.So studying the spray deposition uniformity of PWM variable spray system,especially in the dynamic condition,is significant.The paper focus on the impacts of spray operating parameters on spray deposition uniformity from three evaluative aspects of whole area,forward moving and boom spray bar direction,which could compensate for the one-sided conclusions obtained by recent studies of static condition and boom spray bar direction.Designing a method of spray deposition uniformity measurement based on dynamic experiment platform of PWM variable spray:using carmine solution as spray reagent,putting matrix tray with holes on the conveyor to collect droplets,calculating the spray deposition concentrations of each hole by the method of concentration-absorbance measurement,then the spray deposition uniformity could be judged.It indicates that the new method could measure the spray deposition concentrations of trace droplets and visual assessment could be conducted after picturing the concentration distribution 3D map of spray deposition.Based on the dynamic experiment platform of PWM variable spray,the impacts of spray pressure,frequency and duty cycle of PWM signal on spray deposition uniformity for single nozzle were tested from three evaluative aspects of whole area,forward moving and spray bar direction.The research results suggest that the frequency of PWM signal has bigger impact on spray deposition uniformity of forward moving direction than spray bar direction and there is no need to set too large value of frequency unless required to guarantee the continuity of spray.What's more,before the duty cycle of PWM signal reaches a certain value that the flow rate of spray becomes stable,the duty cycle only influences the spray deposition uniformity of forward moving direction significantly,and when the duty cycle is bigger than such certain value,it can influence both the forward moving and spray bar direction.The research also indicates that the spray pressure has opposite effect on spray deposition uniformity of forward moving and spray bar direction,because the droplets gather at the two sides of spray bar direction and middle area of forward moving direction.BBD(Box-Behnken Design)method of RSM(Response Surface Methodology)with three variables including frequency,duty cycle of PWM control signal and forward moving velocity of spray was employed to study the effects of these three factors on dynamic spray distribution uniformity.CV(Coefficient of Variation)values of spray deposition distribution in the direction of forward moving and spray bar was selected as response values.And two mathematical models about the response values were built.Meanwhile,A mathematical model for dynamic spray distribution uniformity in the direction of whole area was also built using CV values of spray deposition distribution in the direction of forward moving and spray as variables.The results suggest that the mathematical model of spray deposition distribution uniformity in the direction of forward moving shows satisfactory reliability,and the influencing degree of every factors is PWM frequency>PWM duty cycle>forward moving velocity>the square of PWM frequency>interaction between PWM duty cycle and frequency.It also indicates that spray deposition distribution uniformity in the direction of spray bar has no significant relation with each factor.And the mathematical model of dynamic spray distribution uniformity in the direction of whole area has high reliability after 12 runs verification experiments with small errors between predictive and actual values.The concentration distribution maps of spray deposition in the direction of forward moving and spray bar were fit by method of curve fitting with trigonometric function and normal distribution function.What's more,the relationship between function parameters and spray operating parameters including frequency,duty cycle of PWM control signal and forward moving velocity of spray was discussed and calculation models for every function parameters were built.Based on MATLAB,the concentration distribution simulation maps of single nozzle and multiple nozzles in the whole area were simulated by the fog shape curve equations in the direction of forward moving and spray bar.In addition,based on GUI of MATLAB,concentration distribution simulation of multiple nozzles for PWM dynamic variable spray was developed.It indicates that each fog shape could be fitted effectively and spray operating parameters have different degrees of impact on the fitting function parameters.The study shows that the optimum spray scheme is 0.2MPa spray pressure,53%PWM duty cycle,6.6Hz PWM frequency and 0.39m/s forward moving velocity,which could obtain uniform distribution of spray deposition.And under the condition of such parameters,the CVs in the direction of forward moving,spray bar direction and whole area are 1.17%,34.08%and 29.79%,respectively.