PHOTOS: MICK ROBERTS
SPRAY TECHNOLOGY ▶▶▶
Test proves that advanced spray control can boost accuracy
BY MICK ROBERTS I
ndependent, on-farm research carried out in the UK reveals that Pulse Width Modula- tion (PWM) not only works well, but is ca- pable of improving application accuracy in a wide range of situations. But it does have its limitations. PWM allows the pressure in the sprayline to remain constant while maintain- ing the correct flow rate by turning the nozzles on and off very quickly. So, if forward speed in- creases, the nozzles stay open longer (‘wide’ pulse) but, if it slows, they stay shut for a longer period (‘narrow’ pulse). The trials, which involved Tom Robinson, an in- dependent spray application specialist at Agri- fac along with James Thomas and the New Farm Technologies Team at Syngenta, were carried out at Thurlow Estates, which farms in the counties of Essex, Suffolk and Cambridge- shire. Farms Director Andrew Crossely invested in PWM to benefit from the higher application accuracy, its individual nozzle control and turn compensation technology. But he was also keen to ensure that the farm was making the most of its potential. “My first step was to look at existing research and trials in the US,” says Tom Robinson. “I was quite surprised to find only five scientific studies. So we had to virtually start from the beginning and between us devise our own testing procedures. Our trial appears to be the first research into the performance of PWM in the field worldwide. It is still, howev- er, not much more than an experiment to find out what the machine is capable of doing for the farmer,” he adds.
Four main questions Uniformity – is it as uniform as a conventional sprayer under standard conditions and operat- ing parameters e.g. 100 litres/ha at 12 km/hr? Turn compensation – how big a curve can it cope with without compromising the applica- tion compared with driving in a straight line? Variable rate – can it cope with variable rate
60 ▶ FUTURE FARMING | 20 November 2020
Research shows that Pulse Width Modulation (PWM) works well and is also capable of improving accuracy. To use it for variable rate application, it needs to be improved futher for rates lower than 50% of the standard application.
applications and how much variation can be achieved? Operation – what can you do to avoid reduc- ing efficacy when setting up PWM. The tests were carried out using water with Helios tracer dye and Activator 90 to ensure that the liquid behaved properly. This was sprayed through the 11005 fan jets at 2.5 bar and a 75% duty cycle applying a rate of 136 li- tres/ha at 12 km/hr. Deposits were collected on strips of ten 50 mm discs laid in the direction of travel – placed lengthways directly under a nozzle and 25 cm away – in between two nozzles. This reveals the uniformity of the deposits at 50 mm spacing over 0.5m of travel. Each disc was washed in solvent and the amount of deposit collected measured using a Fluorimeter. This provided a direct com- parison of the deposits on each separate disc. Strips of water-sensitive paper fixed to 0.5 m battens were placed alongside the discs to provide a visual indication of the coverage. It’s
important to note these do not show the amount of deposition but simply where the spray has landed.
Forward speed compensation How well did it work? It worked well – with forward speed compensation receiving a five star rating in the trials. The measured deposi- tion at speeds of 16km/hr, 12 km/hr, 8 km/hr and 4 km/hr was the same: within +/- 10%.
Turn compensation The trials tested the effectiveness of turn com- pensation in a 55 m radius at 12 km/hr and a tighter 22.5 m radius at 6 km/hr, comparing the depositions with no PWM (100%) and with PWM operating at a 75% duty cycle. How well did it work? It worked well – with variable speed turn compensation receiving a 4.5 star rating in the trials. At the tighter 22.5 m radius, at 6 km/ hr, there was a pronounced difference in the depositions between when PWM was used and
Solenoids in PWM systems turn nozzles on and off at speeds up to 100 times/sec. The ‘duty cycle’ de- notes the length of time the solenoid is open and the nozzle is spraying.
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