Pulse-width modulation (PWM) for rate control in spraying has been available in North America for about 20 years, primarily through Capstan Ag Systems (PinPoint) and more recently, Raven (Hawkeye). Case pioneered its widespread use through AIM Command, first with in partnership with Capstan, and more recently, with Raven. We’ve provided an overview of these systems in earlier editions of Farming For Tomorrow and on Sprayers101.com.
ExactApply is an application system capable of PWM, introduced by John Deere in August 2017, with its first customer field season in 2018. ExactApply offers several unique features that differentiate it from the existing systems. Following is a brief description of its major components and capabilities.
Nozzle Body Design:
- The body contains a turret with six numbered nozzle locations, all pointed down, and two solenoids, one on either side of the body. Three nozzle locations are on short feeds (locations 1, 2 and 3), whereas the remainder are on long feeds (4, 5 and 6). The front locations and left solenoid is called “A,” whereas the right solenoid and rear location is “B.”
<FIGURE 1 Caption: ExactApply body. Note the long (#4) and short (#3) nozzle feeds on turret. >
- Nozzles are paired so that A or B or both are capable of spraying at a time, depending on the selected mode. Pairs are 1 and 4, 2 and 5, and 3 and 6. The operator manually rotates the desired nozzle pair into position.
- When a short feed is placed at the front of the body, the system is in separated mode. In this mode, the left solenoid controls the front nozzle and the right solenoid controls the rear nozzle. Either or both can be used in pulsing (PWM) or conventional mode, selected through the monitor.
- When a long feed is placed at the front, the body is in combined mode. Now, all flow from the right and left solenoid can only exit the front nozzle. Very high flows are achievable in combined mode, making it suitable for liquid fertilizer application. It may not have other practical applications in Western Canada.
- In pulsing mode, each solenoid pulses at 15 Hz, meaning it completes 15 open-and-close cycles per second. The A and B solenoid timing is offset by 180 degrees, so that the B nozzle is in the middle of its on cycle when the A nozzle is in the middle of its off cycle. In combined mode, this means the system operates at 30 Hz.
<FIGURE 2 Caption: Plunger inside solenoid interrupts liquid flow 15 times per second.>
- The proportion of each cycle that the solenoids are open is known as the duty cycle (DC). At 100 per cent DC, the valves are always open. At 50 per cent DC, the valves are open 50 per cent of the time. The minimum DC allowed by the system in default is 25 per cent. This can be lowered to a smaller value within the monitor.
- DC is closely related to the flow rate of the nozzle. There are two ways of looking at this. A 08-sized tip operating at 40 psi will have a flow rate of 0.8 U.S. gpm at 100 per cent DC, about 0.4 U.S. gpm at 50 per cent DC, and close to 0.2 U.S. gpm at 25 per cent DC. This feature is primarily useful when sprayer speed is changed, requiring new flow rates without a change in spray pressure.
- Pulsing mode is not available for nozzles sized smaller than 02, or for air-induced tips.
- Pulsing can be disabled to allow the use of air-induced or other tip technologies that may not function well when pulsed. This is called auto mode.
- Auto mode can be used to achieve three unique flow rates. “A” alone, “B” alone, or “A” and “B.” When properly staggered, a travel speed range similar to pulsing mode can be achieved, although pressure rise will occur within each nozzle as travel speeds increase.
- In auto mode, the user selects a tip for position A, and an incrementally larger tip for position B. The monitor requires that the user inputs minimum and maximum pressures for A, B, and A and B. Travels speeds corresponding to these tip and pressure choices are calculated, and the monitor warns the user when speeds don’t overlap. The user either changes minimum and maximum spray pressures, or selects a different sized tip to eliminate the gap.
- Auto mode is useful when a certain level of drift protection is required that can’t be achieved in pulsing mode due to the inability to use air-induced tips.
Pulsing Mode Nozzle Selection
At this time, John Deere’s nozzles suited to the ExactApply’s pulsing mode are the LD, the LDX and the 3D. All three are similar in spray quality, obtaining medium to coarse sprays at 20 to 40 psi. These tips would need to be operated at those lower pressure for satisfactory drift management similar to that obtained with air-induced tips.
Proper sizing for PWM requires that tips be sized for about 20 to 40 per cent extra capacity. In other words, at expected average travel speeds, the pulsing duty cycle should be approximately 60 to 80 per cent. The following chart has a highlighted column at 70 per cent duty cycle for that reason. Assuming an ExactApply operator expects to apply five gpa and travel at 15 mph on average, possible nozzle options are:
03 at 60 psi
04 at 30 psi
05 at 10 psi
<FIGURE 3 Caption: Calibration chart for PWM systems. Nozzles are sized at about 70 per cent duty cycle.>
The decision will likely be either of the first two options, as the third has a spray pressure that is clearly too low. The decision would depend on the spray quality obtained for each of the remaining two options.
Why 70 Per Cent Duty Cycle?
There are four main reasons a nozzle should be sized to run at approximately 70 per cent DC. The first is to provide speed flexibility. An operator may need to speed up somewhat, but usually not more than 30 per cent. On the other hand, slowing down is much more common to accommodate challenging terrain, and a factor of two to three is possible (from 70 per cent DC to 25 per cent DC).
Secondly, drift reduction through lower spray pressure usually requires less speed due to the associated lower flow rate. With some DC room to spare, the loss of flow can be corrected without requiring a speed change.
Thirdly, spot spraying at a slightly higher rate is possible, again through DC alone. Finally, turn compensation, during which the outside boom travels faster than the tractor unit and the inside boom slower, requires this additional capacity.
Auto Mode Nozzle Selection
Auto mode allows for three flow rates to be used in succession: A, then B, then AB. The key to success is to use small size increments between A and B, and to use tips that have a wide pressure range. In the example below, the A location was a 02 tip and the B was a 03, for a total of 05. Pressure was not allowed to drop below 30 psi to retain good patterns. Pressure at switch over to the next largest flow rate therefore needed to be 90 psi to make the moves possible without pressure gaps resulting in over-application. As a result, the spray quality can be expected to fluctuate three times as the sprayer accelerates through A, B and AB in succession. Nozzle selection should seek to emphasize the middle of the pressure range of either B or AB to avoid unnecessary fluctuations.
<Figure 4 Caption: The system moves through three separate pressure curves as the sprayer accelerates.>
ExactApply joins Capstan PinPoint, Raven Hawkeye and TeeJet DynaJet with PWM capable systems. Auto mode is a version of nozzle switching first introduced into the market as Arag Seletron and Hypro Duo React. It appears to be a full-featured system that is fully integrated into the new John Deere 4600 display but is also available on the older 2630. It’s expected that systems originating in Europe, notably by Rometron (WEED-it) and Agrifac (StrictSprayPlus), will enter the North American market in the near future.