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Dave's Concise Pulse Width Modulation Guide

 

 

"The seemingly confusing topic of Pulse Width Modulating systems put simply!"

 
 

 Although popular in the US for around 20 years, in Australia we have only fairly recently been seeing growth in the importation and use of Pulse Width Modulated (PWM) Self Propelled boom sprayers, primarily the Patriot and more recently Agrifac and JD’s ExactApply.

Their system of operation and the way in which they achieve a constant rate per hectare, is very different to conventional sprayers.

Instead of a constant flow, the nozzles are “pulsing” on and off in many cycles per second controlled by solenoids on each individual nozzle.

It is important to grasp the difference between the FREQUENCY of the pulses, measured in Hertz (cycles per second), and the DUTY CYCLE, which is the percentage of time the nozzle is on during each cycle/pulse.

Rather than the controller reducing or increasing the pressure to maintain a constant application rate, they increase or decrease the “duty cycle”. The frequency remains fixed at 10Hz or 15Hz or whatever the case may be. 

To explain this simply, a nozzle operating at a 30% Duty Cycle is open for 30% of the time, 50% Duty Cycle, 50% of the time and so on up to 100% where the nozzle is ON for the full length of the cycle/pulse.

This pulsing technique creates a number of advantages, the principal ones amongst them being that the operating pressure for the nozzle does not change, remaining constant as speed increases or decreases. The advantage here is that you can select a nozzle for a specific Spray Quality for chosen pressure and that is what you will have all day!

Secondly, with a smart computer in charge of the nozzle solenoids individually, PWM generally allows for turn compensation altering the Duty Cycle independently on each boom wing.

Thirdly, PWM allows for a very broad range of operating speeds. This is a great feature if you have a lot of go slow country or have to hurry home for lunch!

 

Optimum Duty Cycle

According to Tom Wolf at sprayers101.com, when writing about JD’ ExactApply, the optimum Duty Cycle recommended is 60% to 80% (average 70%) and he lists a number of reasons why including; 

  • Speed flexibility,
  • Drift reduction through lower spray pressures, which is important as Air Inducted nozzles are generally not suitable for PWM.
  • Having sufficient rate/room at both boom ends for turn compensation. As one end speeds up and the other end slows down.
The approach we have taken to nozzle selection here at NOZZLES.ONLINE for PWM becomes quite clear if you take a look at our chart below, which is “optimised” for 20 km/h. That is the Duty Cycle is approximately 70% at 20 km/h
 
This allows for a speed range from 10 km/h to 25 km/h while still staying clear of the absolute extremes of 100% and minimum (probably factory set) of 25%.
 
Our chart makes it is very easy to select your desired nozzle by Spacing (25cm or 50cm), pressure and Spray Quality. We will be adding a new chart soon optimised for 25 km/h
 
 
Summary

Factors involved in the process are;

  • YOU set rate/ha and pressure.
  • System has fixed Hertz (cycles per second).
  • Spray computer varies the Duty Cycle (%age of time a nozzle is on per cycle) to maintain your rate/ha.
 
 
More Information
 
That about covers the basic principles so here are some other references for you;

Specific information about our Low Drift Ceramic nozzles for PWM is here; 

https://www.fantasticnozzles.com.au/fantastic-nozzles/ldc/

 

Downloadable charts for PWM nozzles and everything else, are available here; 

https://www.dropbox.com/sh/tubmuue16i3qxv6/AACNvHvEYa9tDvNSsHwHn_Tpa?dl=0

 

AND, if you wish to discuss the suitability of of our Low Drift Ceramic nozzles for your PWM sprayer, I will be happy to hear from you anytime.

 

Regards,

Dave Young

08 6102 5886

  

Reference

 

The general principles as well as the practical specifics are explained very well in a series of articles by Tom Wolf at http://sprayers101.com/?s=pwm

 

 

 

 Click here for our PDF Chart Library

PWM 20.jpegPWM 25.jpeg

 

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