圆形PWM变量喷灌机喷灌特性的仿真研究

Simulation Research on Irrigation Features of PWM Center Pivot Sprinkler Irrigation System

变量喷灌(Variable Rate Irrigation,VRI)是精准农业的一个重要分支,目前的发展方向是单喷头变量喷灌(Individual Sprinklers VRI,IS-VRI),即每个喷头均采用脉宽调制(Pulse Width Modulation, PWM)变量技术控制水量。为此,通过分析面向IS-VRI应用的圆形PWM变量喷灌机在喷灌过程中水量累积的过程后,数值仿真研究了PWM脉冲占空比、PWM脉冲周期、喷灌机行进角速度及喷头离围转中心距离4个参数对喷灌均匀性及喷灌水深误差的影响规律。采用Nelson R3000喷头的圆形PWM变量喷灌机数值仿真结果表明:喷灌水深与喷灌机行进角速度和喷头离围转中心距离成反比,与PWM脉冲占空比成正比,与PWM周期存在较弱的负相关关系;PWM周期和喷灌机行进角速度越小,喷灌均匀性越好,当PWM周期取10s或行进角速度取1(°)/min时,径向和周向克里斯琴森均匀系数均可达到75%和80%以上;喷灌水深误差主要出现在喷灌机中间段的径向水深误差上,约为8%,其余误差均在5%以下,水深误差可以通过调整PWM脉冲占空比补偿消除。

Variable Rate Irrigation(VRI)is an important branch of precision agriculture.Its developing direction will be Individual Sprinklers VRI(IS-VRI),which means that the flow rate of each sprinkler will be controlled with Pulse Width Modulation(PWM)technology.In this article,the process of water accumulation in sprinkler irrigation process of PWM center pivot sprinkler irrigation system based on IS-VRI was analyzed firstly.Then simulation experiments were conducted to find out effects of PWM duty cycle,PWM period,angular velocity of sprinkler irrigation system and distance of sprinkler from center pivot on application uniformity and error of application depth.Simulation results of PWM center pivot sprinkler irrigation system with Nelson R3000 sprinkler showed that application depth is inversely proportional to angular velocity of sprinkler irrigation system and distance of sprinkler from center pivot and proportional to PWM duty cycle.It also has a weak negative correlation with the PWM period.Generally the smaller the PWM period and the angular velocity are,the better the application uniformity is.And when PWM period is 10 s or the angular velocity is 1°/min,the radial and circumferential Christensen uniformity coefficients can reach more than 75%and 80%,respectively.Besides,the error of application depth is mainly the radial error and occurs in the middle section of sprinkler irrigation system,which is about 8%.And errors of other parts are all below 5%.The error of application depth can be eliminated by adjusting the duty cycle of PWM valve.