Spray application manual
31 January 2025
Module 16: Overview of the spraying systems available
16.2 Overview of pulse-width modulation
Published 24 January 2025 | Last updated 20 January 2025
Pulse width modulation (PWM) systems can provide constant pressure in the spray line, which maintains a constant spray quality.
The flow rate (L/ha) is electronically controlled by PWM, where the nozzle rapidly turns on and off (pulses) many times each second. The length of each pulse is varied by the controller to match the travel speed; longer pulses = more flow, shorter pulses = less flow.
Pulse Width Modulation how it works
10 January 2025How does pulse width modulation works?. Another video from GRDC's Spray Application GROWNOTES™ series.
This video appears in modules 16 and 17.
Bill Gordon: There are a number of spraying systems now available that growers can choose to get their product onto the target. This particular system here is a pulse width modulation (PWM) system. Like all other spray systems, it relies on hydraulic nozzles to deliver the spray as liquid droplets to the target.
This system achieves that by using a solenoid. Inside the solenoid, it opens and closes approximately ten times every second. The key feature of a PWM system is that the pressure in the line remains constant, ensuring the droplet size from the nozzle doesn’t change.
What changes is the number of pulses or how long the system stays on versus off during each cycle. This is referred to as the duty cycle. For example, this system operates at 10 hertz, meaning it opens and closes 10 times per second. The duration it stays open versus closed during each cycle is what adjusts the spray output. This precise control helps maintain consistent application and efficiency.
The pulse-width modulation nozzle body
The Case AIM Command®, Capstan SharpShooter®, Raven Hawkeye® and TeeJet DynaJet® PWM systems operate at 10 hertz (or 10 cycles of open and closed per second). Other systems can operate at higher frequency (number of cycles per second). The John Deere ExactApply system can operate a single nozzle at 15 hertz, or two nozzles operating simultaneously at 30 hertz. Agrifac systems operate at 100 hertz.
The percentage of the time that the nozzle is open (relative to closed) is known as the duty cycle. It is important that the duty cycle is high enough to ensure good coverage.
With a 10-hertz system it is good practice to operate the system above 70 per cent duty cycle at your average speed, and try not to let the system drop below 50 per cent when slowing down if using coarser droplets. Operating at closer to 100 per cent at your average spraying speed will provide the greatest practical speed range.
Strengths of PWM
Pressure in the spray line remains constant, so droplet size does not change during spraying (unless the operator chooses to change the pressure to manipulate droplet size).
When operating the system at close to 100 per cent duty cycle at the normal spraying speed, the effective speed range is at least half that of your spraying speed (e.g. if spraying at 24 km/h you can generally slow to around 12 km/h without sacrificing coverage too much).
Control of the flow rate and on/off occurs at the nozzle, which introduces the ability to have auto-section control that can operate individual nozzles.
The ability to adjust pulse width allows for innovations such as turn compensation and increased flow on individual nozzles adjacent to the wheel tracks.
Possible limitations of PWM
When a PWM system pulses at 10 hertz, it is usually not possible to run air-induction nozzles effectively on the line with the pulsing nozzle bodies. However, many systems are also fitted with a standard spray line where other hydraulic nozzles (such as air-induction) may be mounted, although with some set-ups it may take quite some time to change between the two lines.
If the orifice size of the nozzles fitted to a PWM system is too large for the application volume and spraying speed, the duty cycle may be too low. With nozzles producing a coarse spray quality, operating at a low duty cycle (e.g. at low spraying speeds) may result in patchy coverage. As a rule of thumb it is best to select nozzle sizes that are not more than one-third larger in orifice size than you would normally choose for the same application volume and spraying speed on a standard boom.
Find out more
For more information on how Pulse Width Modulation works, go to Module 18: Single line and multi- step systems.