PRESSURE DYNAMICS OF A SPRAY NOZZLE UNDER PULSE-WIDTH MODULATION CONTROL

Nozzle pressure uniformity is one of the most critical indicators of sprayer performance. Pulse-width modulation (PWM) sprayer control systems adjust average flow by rapidly cycling a solenoid valve on and off. The changes in pressure supplied to the nozzle body result in higher-order pressure dynamics that lead to less-than-ideal pressure uniformity at the nozzle tip. This research focused on measuring actual nozzle pressure dynamics and modeling nozzle pressure as a function of upstream pressure before the solenoid valve and the PWM control signal. The experiments were conducted in a laboratory test environment using a single spray nozzle to collect the nozzle pressure data under seven nozzle orifice sizes, five system pressures, and three PWM duty cycles at a 10 Hz operating frequency. The system identification toolbox in MATLAB was used to obtain transfer function models that represent the nozzle pressure dynamics under different treatment combinations. The results showed that the damping ratio (ζ) depends on orifice size, whereas the natural frequency (ω_n) depends on orifice size and nominal system pressure. The wide range of ζ values under various nozzle orifice sizes resulted in high variation in maximum overshoot (M_p) above the target pressure. Settling times (t_s) were longer for the smallest and largest nozzle orifice sizes due to low ω_n. Results also showed that nozzle orifice size and nominal pressure substantially impacted rise time (t_r), which had a direct inverse relationship. Further statistical analysis showed that orifice size and nominal system pressure had a significant effect on mean ζ and ω_n—largely at smaller orifice sizes and lower nominal system pressures. Therefore, it is important to select the optimal input parameters that yield adequately fast and damped system pressure dynamics to ensure an accurate application rate with the desired droplet size distribution.