The ins and outs of Pulse Width Modulation (PWM) and why to prefer WEED-IT

Over the past 10 years, interest in Pulse Width Modulation (PWM) technology for agricultural sprayers has increased significantly. Its benefits - regulation of the application rate independently of pressure and the ability to apply crop protection products evenly in corners - have greatly contributed to this.

WEED-IT Precision Spraying

Yet, there’s also some pitfalls involved and there’s a common (mis)understanding that higher frequencies are better than lower frequencies. In practice, it is not the frequency itself, but the way manufacturers are using the right frequency in combination with the duty cycle to reach the best spray result. Therefore, WEED-IT’s PWM expert wrote a white paper on the do’s and don’ts of the use of PWM technology on sprayers.

Download our PWM whitepaper!

Duty cycles and frequencies - PWM

PWM is used to vary the ON and OFF time of nozzles within a cycle time. Within this cycle, comparable to a circle or clock, the time that a nozzle is ON or OFF can be varied. The percentage of time that the nozzle is ON, is called the duty cycle (DC). There’s a second important factor involved and that is the time it takes to make one cycle. The first PWM systems on the market used a fixed cycle time of 100 ms and thus able to run 10 cycles per second, a 10 Hz frequency. Today, available PWM systems use frequencies between 10 and 50 Hz in practice while 20 Hz is the most commonly used frequency today. Higher frequencies then 50 Hz can be made, but have no advantages in practice as it requires (much) more power and does not affect the spray quality any further.

(Nearly) always the correct application rates

Over the last 10 years, interest in PWM technology for agricultural sprayers has increased for several reasons. One of these is the ability to regulate the application rate independently of the pressure. With a constant pressure, the droplet size can be kept constant, while the PWM regulates the flow of each individual nozzle. Another advantage of PWM is the ability to apply crop protection products evenly in corners as PWM can adjust the flow for each individual nozzle to match the desired application rate depending on the ground speed of each individual nozzle. Sometimes, the speed is that high, that the end of the boom is applying too less. In this case, the WEED-IT system triggers a ‘Too Fast’ warning, asking the driver to slow down.

Not just the higher the better

Frequency is an often-used argument, stating that a higher frequency is better. In practice, it is not the frequency itself, but the way the manufacturer is using the right frequency in combination with the duty cycle to reach the best spray result. Manufacturers of more advanced PWM systems like Rometron WEED-IT use a variable frequency depending on the driving speed. This way, they are minimising the skip risks and increasing the spray distribution and effectiveness of the spray job.

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