Pro Operator

New – PWM holds potential to improve accuracy – Autumn ’18 edition

Pulse width modulation in agriculture

In the autumn ’18 issue of Pro Operator NRoSO members can find out how Pulse Width Modulation in agriculture works, as well as how to select, set-up and use the technology.

In the video below, you can clearly see the effect PWM has on spray application.

With its ability to turn nozzles on and off up to 100 times a second, Pulse Width Modulation (PWM) will maintain not only the right application rate, but also the correct droplet size, regardless of changes in forward speed.

At the same time, if equipped with individual nozzle control, and many systems are, it will provide ‘turn compensation’. This will reduce or even eliminate the over- and under-application caused by the inside of the boom hardly moving, while the outer end whips around when cornering.

These poor applications on corners result in inferior control. Worst still, it is now becoming quite obvious that years of sub-lethal treatment contributes to herbicide resistance in these areas, exacerbating the weed control problem.

pulse width modulation close up on a spray boom

How PWM works

In normal situations the output of a nozzle is fixed and determined by the orifice size and the pressure. If the sprayer speeds up or slows down the only way to change the flow is to alter the pressure to maintain the set application rate.

Automatic rate controllers will change the pressure to match speed changes. But the difficulty with standard nozzles is to double the flowrate you need to increase the pressure by a factor of four. This will inevitably alter the size of the droplet, coarsening them as you slow down or making them finer as you speed up – increasing the risk of drift.

Pulse width modulation in agriculture shown at night on spray boom

Pulse frequency

PWM overcomes this problem by essentially maintaining the application rate by using solenoids to turn the nozzles on and off in milliseconds. The actual switching speed – or pulse frequency – is rated in hertz (Hz) – if the nozzles can be pulsed at up to 100 times/sec this is shown as 100Hz.

pulse width modulation diagram

Duty Cycle

Another important parameter is the ‘Duty Cycle’ – this is the ‘pulse width’ part of the name and denotes the length of time the solenoid is open and the nozzle is spraying. This can range from 10% – open for just a tenth of the time – up to 100%, or fully open. Experts recommend setting systems to operate at between 60% to 80% DC. To do this operators need to select a nozzle for optimum droplet size for the target and essentially add about 20% to 40% more flow compared with the normal size. This forces the system into running most of the time at 70% DC, which allows room to move in either direction.

Forward speed changes

At 70% DC the system can operate 30% faster – for example when speed increases to say 16km/hr taking it up to 100%. Or, when slowing down to about 4km/hr, it is able to drop to 20%. This will determine the working speed range over which the pressure remains constant and consequently the droplet size will stay the same.

pulse width modulation nozzle selction

Nozzle selection

For any nozzle, the pressure determines the droplet size, and the benefit with PWM is that’s held constant. The key is to select a nozzle for optimum droplet size for the target and essentially that will work within the duty cycle parameters.

It’s important to note that, due to the temporary break in spray pressure during the off-cycle, PWM doesn’t work well with air-inclusion nozzles.

pulse width modulation showing turn compensation

Turn compensation

With turn compensation sensors monitor the differences in speed the boom is travelling when making a turn. These detect the tips will being whipping around the outside of a corner, while the inner section hardly moves – or even goes backwards.

Equipped with individual nozzle control the PWM system changes the pulsing of the each of the nozzles along the boom to ensure they deliver the correct flowrate for the actual speed in that position.

Look out for more information…

In the coming weeks Pro Operator will be compiling in-depth reports on all the systems that are currently available on the market – those supplied by the manufacturers or for retro-fitting. This will enable to operators to consider the various duty cycles, control systems and nozzle set-ups to help choose the system that best suits their individual situation.

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