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Scott Bretthauer Scott Bretthauer
Extension Specialist in Application Equipment
Department of Agricultural and Biological Engineering
Pesticide Safety Education Program
(217) 333-9418; sbrettha@illinois.edu

Control More With More Control: Pulse Width Modulation

Control of spray droplet size is critical for making an effective application and minimizing the risk of drift. The droplet size recommended for an application is often given as a droplet size classification, such as medium or coarse. The applicator needs to create and maintain that droplet size during an application. The type and size of the nozzle as well as the operating pressure determine both the droplet size spectrum and the nozzle flow rate. An increase in pressure results in a decrease in droplet size and an increase in flow rate. A decrease in pressure results in an increase in droplet size and a decrease in flow rate.

Spray rate controllers maintain a constant gallons per acre (GPA) rate during an application by using pressure to change nozzle flow rate in response to changes in speed. A decrease in speed requires a reduced flow rate, which is achieved by reducing the pressure. An increase in speed is compensated for by increasing pressure. By changing the pressure, however, the controller is also changing the droplet size. If droplet size is increased, coverage and pest control can be reduced. If droplet size is decreased, there is a greater potential for drift.

Dual nozzle body
Dual nozzle body with an AIM Command
solenoid valve on the outer (left) nozzle.
This valve is wired to a controller which
sets the duty cycle.

Technology called pulse width modulation (PWM) allows a spray controller to maintain a constant GPA during speed changes without altering the droplet size. PWM works by rapidly starting and stopping the flow of spray from the nozzle. A computer controlled solenoid valve located at the inlet of the nozzle controls the flow. The valve can open and close very rapidly, cycling 10 times every second. The length of time the valve is held open creates the pulse of spray and is called the duty cycle. A duty cycle of 100 percent means the nozzle is constantly open; a duty cycle of 50 percent means the nozzle is open only half of the time. A longer duty cycle means the nozzle is open for a longer period of time, releasing a greater amount of spray. The nozzle flow rate can be controlled by altering the length of the duty cycle. Using PWM, nozzle flow rate can be varied over an 8 to 1 range. Pressure based rate controllers can only vary flow rate over a 2 to 1 range before the pressure limits of the nozzle are exceeded.

Because the length of the duty cycle is controlling the flow rate, pressure can be used to control droplet size. Increase pressure for smaller droplets, or decrease pressure for larger droplets. This gives the applicator independent control of nozzle flow rate and droplet size. The length of the duty cycle controls nozzle flow rate to maintain a constant GPA during speed changes, and pressure remains unchanged so droplet size remains constant. The GPA can also be changed easily, making PWM suitable for variable rate applications. Alternatively, the droplet size can be adjusted by changing the pressure without altering the GPA because the PWM duty cycle is controlling the flow rate.

Spray pattern uniformity is maintained by alternating the pulses along the length of the boom. When one nozzle is closed, those adjacent to it are open. Also, nozzles with 110-degree fan angles mounted to provide 100 percent overlap are recommended. This arrangement blends the pulses from individual nozzles into a continuous spray pattern along the length of the boom that provides uniform coverage.

PWM technology is available commercially as the Synchro spraying system from Capstan Ag systems, Inc. The Synchro system can be retrofitted to existing spray equipment. The solenoid valve replaces the diaphragm check valve on the nozzle body. PWM is also available as the AIM Command spray system on Case IH sprayers.

Department of Crop Sciences
College of Agriculture, Consumer and Environmental Sciences
University of Illinois Extension
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