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Control more with more control: Pulse-width modulation

In the January 2004 Illinois Pesticide Review, I wrote an article about spray-droplet size measurement and classification. That article provides important background information for the subject of this article. Please refer to that issue if you are unfamiliar with spray-droplet classification.

Pesticides labels will eventually require specific droplet sizes for applications, meaning the applicator will have to control the droplet size category created by the sprayer during applications. When selecting your nozzle size based on the flow rate (in gallons per minute) needed to get the correct gallons per acre (gpa), you will also need to select a nozzle type, size, and operating pressure to create the droplet size category required by the label. The nozzle will then have to be operated within a pressure range that maintains that droplet size category.

What happens to spray-droplet size if pressure increases? Droplet size decreases. The opposite is also true. If pressure decreases, then droplet size increases. During an application, if you increase the pressure too much, your droplet size category will decrease. Decrease the pressure too much, and your droplet size category will increase. In either situation, you will be making an application with the wrong droplet size category, thus, you will be going off label.

This brings us to the subject of spray rate controllers. Spray rate controllers maintain a constant gpa as you change speeds. If we think about the three factors that determine the gpa, which are nozzle flow rate, ground speed, and effective sprayed width (nozzle spacing), we realize that if we change the speed, the rate controller has to adjust one of the other two factors to keep the gpa constant. Because there is no way for the controller to change nozzle spacing, it has to change the nozzle flow rate. There are two ways to change the nozzle flow rate: operating pressure or orifice size. The controller has no way of switching your nozzles for you, so it adjusts the pressure.

Here is a scenario to consider, keeping in mind the calibration equations (I'm sure you have these memorized). Let's say you're making an application for which the primary speed is 8 mph with nozzles spaced 20 inches apart. The label requires a spray application rate of 10 gpa and a very coarse or coarse droplet size category. You are using TT11003 nozzles, which at a pressure of 32 pounds per square inch (psi) give the required flow rate of 0.27 gallons per minute (gpm). You are using a spray rate controller. At times during the application, the speed is increased to 10 mph. At this speed, the flow rate needs to be increased to 0.34 gpm to maintain 10 gpa. To increase the flow rate from the TT11003 nozzle, pressure must be increased to 51 psi. This pressure changes the droplet size category from coarse to medium. You are now off label because the droplet size is too small.

Is there any way to maintain a constant gpa while changing speeds without changing the droplet size category? Technology called pulse-width modulation (PWM) can provide this control.

PWM works by rapidly starting and stopping the flow of spray from the nozzle. Thus, the pulse is the rapid on-off action of 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, 10 times a second. The length of time the valve is held open during a pulse is called the duty cycle and is given as a percentage. A duty cycle of 100% means the nozzle is constantly open; a duty cycle of 50% indicates the nozzle is open only 50% (half) the time. The longer the duty cycle, the longer the nozzle is open, and the greater amount of spray that is released. Thus, nozzle flow rate can be controlled by the length of the duty cycle. Need an increase in flow rate? Increase the duty cycle, which opens the nozzle for a longer time. If a lower flow rate is needed, then the duty cycle can be decreased.

Using PWM, nozzle flow rate can be varied over an 8-to-1 range. If pressure is used to change flow rate, most nozzles can be varied only over a 2-to-1 range before the upper or lower limit of their pressure range is reached. Because the length of the duty cycle is controlling the flow rate instead of pressure, pressure can now be set independently. What can pressure control be used for? Droplet size! While the duty cycle adjusts nozzle flow rate, pressure can be adjusted to control the droplet size category created by the nozzle. If you need larger droplets, reduce pressure. The PWM duty cycle will adjust the nozzle flow to maintain the gpa required for the application.

PWM allows you to have independent control of nozzle flow rate and droplet size. You can adjust one without altering the other. An applicator can adjust nozzle flow rate to maintain a constant gpa, while changing speed without changing the droplet size category. An operator can also adjust droplet size by changing the pressure without affecting nozzle flow rate and gpa.

Certainly starting and stopping the flow of spray from a nozzle, even rapidly, would have an effect on the uniformity of the spray pattern, right? You would expect skips in the coverage during those periods when the nozzles are turned off. However, this problem does not occur. The pulses from individual nozzles are blended together to create a continuous, uniform pattern along the length of the boom that provides consistent coverage of the target. This blended pattern is accomplished in several ways. First, the pulses are alternated along the boom so that adjacent nozzles are opposite one another. When a nozzle is open, the nozzle next to it is closed. Second, nozzles with 110¡ fan angles are recommended instead of 80¡ angles, so each individual nozzle creates a wide pattern. Third, boom height is set to provide 100% nozzle overlap. Finally, it is recommended to avoid duty cycles below 33%. PWM works with most commercially available nozzles, with the exception of venturi-style nozzles. The pulsing prohibits the venturi action from working properly.

Let's reexamine our previous scenario, assuming that you recently purchased a PWM system. Now when the speed is increased to 10 mph, the gpa is held constant by increasing the nozzle flow rate to 0.34 gpm, as before. This time, however, the rate controller increases flow rate by increasing the pwm duty cycle. Pressure remains constant at 32 psi, and the droplet size category remains coarse. You are able to increase speed while maintaining the gpa and droplet size category.

Sometimes the change required during an application might be the droplet size category. For example, you might be applying a pesticide that requires a droplet size category of medium, coarse, or very coarse. Along one side of the field to be sprayed is an area containing trees that are sensitive to the pesticide being applied. For the majority of the application, the pressure can be set to provide a medium droplet size category to maximize coverage. When working close to the sensitive trees, you can reduce pressure to increase the droplet size category to very coarse. This means larger spray droplets and a decreased chance of drift occurring. Because nozzle flow rate is being independently controlled by the PMW duty cycle, the correct gpa is maintained during the pressure changes.

The Synchro spraying system is fully automatic PWM equipment that can be retrofitted to existing spray equipment. The diaphragm check valve on the nozzle body is replaced with the solenoid valve, and wires connect these valves to a flow-control unit. The flow-control unit works with conventional rate controllers to determine and set the nozzle flow rate. A separate pressure-control system with its own control unit and pressure-regulating valve is also part of the equipment. The Sharpshooter is a manually controlled PWM system that does not include pressure-control equipment. Both the Synchro and Sharpshooter systems are available from Capstan Ag Systems, Inc (

You can contact Scott Bretthauer via email at

In the January 2004 Illinois Pesticide Review, I wrote an article about spray-droplet size measurement and classification. That article provides important background information for the subject of this article. Please refer to that issue if you are unfamiliar with spray-droplet classification.

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