Pulse Width Modulation

Pressure-based rate control has become the norm on agricultural crop sprayers around the world, however there are some inherent drawbacks with this method of control. By fluctuating the spray pressure to maintain the rate as forward speed changes, droplet size and therefore efficacy and drift characteristics are effected. As a result nozzle choice is a compromise, often with multiple sets required to achieve the operators targets.

Pulse Width Modulation (PWM) offers solutions to these issues and whilst increasing spray efficacy at the same time. On a PWM system each nozzle body is fitted with an electric solenoid that pulses the spray on and off multiple times a second, this can be anywhere between 10-40Hz (pulses per second). Rate changes are achieved by varying the duration the nozzle is in the “on” position or “duty cycle”. This duty cycle is changed in relation to changes in forward speed, allowing instant rate control at each spray nozzle, with the pressure being fixed at all times to give a consistent droplet size across the whole field.

Features such as turn compensation, individual nozzle control and the ability to change the droplet size from the drivers seat, offer Chafer customers the opportunity to maximise application efficacy like never before.

PWM: What does it offer growers?

Tailor the Droplet to the Target

Different targets, weather conditions and environmental considerations all effect the droplet size required. Pulse Width Modulation allows the operator to not only specify the droplet size from the cab, but also to maintain it over the whole field.

Turn Compensation as Standard

The Raven Hawkeye system uses in built gyroscopes to sense boom movement during spraying. Using this information the application rate is varied proportionally along the boom to prevent over application on the inside of turns and under application on the outside.

Perfect Rate Control on Every Nozzle

To ensure the target rate is achieved at all forward speeds every PWM nozzle along the boom has the ability to vary the flow rate in less than a fiftieth of a second, ensuring consistent application rates are achieved even during rapid speed changes made at the headland.

Individual Nozzle Control

Available as an unlock on the Raven Hawkeye PWM system, individual nozzle control allows every nozzle to operate as its own spray section. By minimising under and overlaps savings of up to 5% can be made on yearly chemical and liquid fertiliser bills.

No Additional Terminals

Chafer utilises the ISObus compatible Raven Hawkeye system, allowing connectivity through any ISObus terminal. This allows your business to continue to use your preferred control system from the like of Topcon, TeeJet, John Deere etc, with no additional screens required.

The Facts: How does PWM work?

Frequency & Duty Cycles

The Raven Hawkeye system uses a blended pulsing setup on each nozzle to vary the application rate. Simply put the nozzle is turned on and off multiple times every second, with a frequency high enough to prevent an misses as the machine moves down the field. As standard this frequency is set at 10Hz, simply meaning each nozzle turns on and off ten times per second.

The application rate can then be adjusted by adjusting the “on time” or “duty cycle” during each of these cycles. Hawkeye can operate between 25 and 100% duty cycles, with spray nozzles specified to operate at a 70% duty cycle during normal conditions, allowing increases and decreases as speed changes or turns are made. While all these changes are being made, two things remain consistent: pressure and droplet size.

25% Duty Cycle75% Duty Cycle

Phased Valves to Avoid Misses

To further reduce the possibility of an application “miss” Hawkeye groups the spray nozzles and switches them in phases, so while any nozzle is in the off position, the one next to it will be on. By alternating this phasing the system is making use of the double overlap pattern which occurs when the nozzles are at their correct spraying height. The images show how the nozzles are phased and also a plan view of how this application appears when viewed from above.

Proportional Turn Compensation

Along with lighting fast rate changes and consistent droplet sizing one of the largest benefits of adopting a PWM system is turn compensation. The Raven Hawkeye system uses integrated gyroscopes inside the main control ECU located on the boom centre carriage. When turning the sprayer movements are measured and using the machines geometry a revised application rate can be calculated for each spray nozzle. As each nozzle has a duty cycle range of 25 to 100%, rate changes can be made for each individual nozzle whilst turning to prevent over/under applications. On tighter turns nozzles on the inside of the boom can be switched off where no forward movement is occurring, especially useful for chemicals that can cause damage if application doses are exceeded. As gyroscopes are used rather than a GPS signal to provide turn sensing, Hawkeye can also adapt the application rate should any yaw occur in the boom.

The Facts: How does PWM work?

A pulsing solenoid creates short durations of low pressure inside the nozzle body, and this can result in poor performance of some air induction nozzles. As a result, the PWM manufacturers have recommended that pre-orifice nozzles be used instead.

It’s worth noting that some PWM users have used air induction nozzles successfully, with variance between different models noted. Some air-induced nozzles blow spray liquid out the air-induction intakes once a pulse is applied, so this must be checked at a range of duty cycles and rates.

Due to the unique way the PWM system works, users will be specifying a slightly larger nozzle than they currently use to achieve the 70% duty cycle. The best place to start is let us know what nozzle you currently use and at what pressure, that way we can specify an equivalent to gives the correct droplet sizing.

A PWM system can be used with liquid fertiliser, either by setting the duty cycle to 100%, or by running a higher duty cycle. Low duty cycles can create cavitation so these are best avoided.

Chafer are currently looking into Streambar compatibility to provide guidelines for the setup on the Raven Hawkeye System.

Some manufacturers of Pulse Width Modulation systems advertise that a higher frequency is preferable. The Hawkeye system generally operates anywhere between 10-40Hz, with the two phase pulsing creating a quasi 20-80Hz range, but in general we advise the system be run at the standard 10Hz. This maximises service intervals and as long as the nozzle is sized correctly removes the changes of misses being created.

The other issue with a higher frequency is a reducing in the effective duty cycle range. If a nozzle body is operating at 100Hz, most of the time is taken up by physically moving the poppet from its open to closed position and back. This reduces the time available to actually vary the rate, resulting in poorer performance during turn compensation and a reduced speed range. A bigger number isn’t always better!

Like all spray systems Raven Hawkeye likes a properly filtered liquid, so keep an eye on filters and ensure they are properly sized and clean. The only wearing part on the system is the stainless steel poppet in the NCV, with the rubber seal on the end wearing over time. Users may experience dripping from nozzles when this part is beginning to wear, but users have extended service intervals well beyond 2500hrs before having to swap any poppets.

For more information on the Raven Hawkeye PWM system please get in touch.

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