Pulse Width Modulation Update

There’s been a lot of talk about rate control in spraying, and one key technology is pulse-width modulated spray systems (PWM). Although PWM has been commercially available for a number of years, we are seeing new products enter the market. This article explains what PWM is and how to make it work in a spray operation.

Rate Control Primer

Modern sprayers, be they self-propelled, or tractor- or truck-drawn, experience fluctuations in travel speed. Operators speed up or slow down as conditions demand. In order to maintain a constant application volume per acre, the spray liquid flow must change in direct proportion to travel speed. The sprayer achieves this with a rate controller, a device standard on most sprayers.

The rate controller uses four pieces of information to ensure a constant application rate.

  • The user enters the width of the boom and the desired water application rate.
  • The sprayer provides travel speed information (collected from a GPS signal or a radar- or wheel-based speedometer) and liquid flow rate (collected from a flow meter on the main sprayer liquid flow line).

Using a simple mathematical formula, the rate controller calculates what the required liquid flow needs to be for any given travel speed. A typical controller changes the flow by adjusting the pump pressure. The sprayer operator keeps an eye on the spray pressure to ensure it doesn’t exceed the capabilities of either the nozzle, the plumbing, or the pump or that it does not produce an undesirable droplet size or spray pattern.

Rate Control Options

There are currently about five options for rate control on the market, and all but one rely to a degree on spray pressure to manage flow rate.

  • Pressure-based rate control. The most common system, it changes spray pressure as required by the travel speed. It is limited by the flow rate capacity – both high and low – of the spray nozzles installed on the sprayer and the pressure limitations of the sprayer system.
  • Variable rate spray nozzles. Commercial systems such as the VariTarget nozzle use a plunger in a nozzle assembly that pushes down on, and deforms a flexible nozzle cap, with spray pressure. Higher pressures result in the cap’s orifice becoming larger, facilitating more flow. This system is capable of a wider range of flows than a conventional nozzle system over the same pressure range.
  • Dual boom systems. The rate controller still functions as described above, but a second boom fitted with different flow nozzles is activated when the flow rate requirements can no longer be met with a single set of nozzles. For example, if the second boom contains larger nozzles, once the boom with larger flow nozzles is activated, the spray pressure drops significantly and additional speed capacity can be realized.
  • Dual or Quadruple nozzle bodies. A similar approach to the dual boom is available from Arag (Seletron) or Hypro (Duo React). These systems utilize a single boom (no duplicate boom is required) and direct the flow through one of any two or four (Arag Seletron only) nozzles, or two nozzles simultaneously. Individual nozzle section control is also possible with this approach. Similar pressure fluctuations as with a dual boom would be experienced, requiring careful selection of nozzle flow rates to avoid large pressure jumps. The same system can also be used to manually change from one nozzle to another as conditions require.
  • Figure 1 (Capstan solenoid b) Pulse Width Modulation. PWM utilizes conventional plumbing: a single boom line and a single nozzle at each location. Liquid flow rate through each nozzle is managed via an intermittent, brief shutoff of the nozzle flow activated by an electric solenoid that replaces the spring-loaded check valve. Typical systems pulse at 10 Hz (the solenoid shuts off the nozzle 10 times per second), and the duration of the nozzle in the “on” position is called the duty cycle (DC) or pulse width. 100% DC means the nozzle is fully on, and 20% DC means the solenoid is open only 20% of the time, resulting in the nozzle flowing at approximately 20% of its capacity. The ability to control the duty cycle is referred to as pulse width modulation.

Pros and Cons

There are two chief features of a pressure-based approach that affect the spray operation.

  • Pressure affects spray quality and spray patterns. Higher pressures (the result of faster travel speeds) result in finer, more drift-prone sprays, and lower pressures may, in addition to producing a coarser spray, reduce the spray’s fan angle. The resulting narrow patterns can result in less overlap and poor pattern uniformity. When the travel speed drops below a defined point, the spray flow rate is held constant to maintain the pattern; this can result in over application.
  • Pressure is not a very effective way of changing flow rates. Increasing the travel speed by a factor of 2 requires a pressure change of four-fold, as predicted by the square-root relationship between flow rate and pressure. As a result, a system capable of pressures ranging from a low of 30 psi to a high of 90 psi (a three-fold change in pressure) results in only a 1.73-fold change in flow rate (and travel speed). 1.73 is the square root of 3.

In comparison, PWM systems do not rely on pressure changes to effect new flow rates. Instead, the duty cycle of the system affects nozzle throughput. Boom spray pressure stays constant throughout the duty cycle range, and as a result, so does spray quality and spray fan pattern angle. In practice, the lowest duty cycles increase droplet size, and reduce fan angle, somewhat. These effects are minor and do not impact overall performance. The operator also has the option of adjusting the spray pressure to get a desired droplet size, even “on the fly” and the PWM system will maintain the desired application rate.

A PWM system can therefore change travel speeds by about a factor of five (from 20 to 100% DC). Duty cycles less than 20%, although possible, are not recommended.

Note that the actual measured change in flow rate achieved by a PWM system is not directly related to duty cycle. The actual nozzle flow rate is greater than that predicted by a duty cycle calculation, especially for smaller nozzles.

Commercial PWM Systems

Figure 2 (AIM Command)The original inventor of PWM for spraying, Dr. Ken Giles of the University of California at Davis, worked with Capstan Ag Systems to produce the Capstan Synchro, the first PWM system on the market. The Capstan product was later licensed to Case IH sprayers and named AIM Command. It was a factory option on Case sprayers from 1998 to 2016, manufactured by Capstan. The system featured a separate monitor, permitted PWM to range from 100 down to 15%, and featured an alternating pulse in which the every second nozzle pulses identically, and alternating nozzles work in a 180 degree offset. In other words, in a system operating at 50% DC, when any given nozzle is on, adjacent nozzles are off. This results in a “blended pulse” that minimizes the likelihood of skips.

Figure 3 (Sharpshooter Monitor)In recent years, Capstan has entered the retrofit market place and the technology has been installed on many brands of sprayers at the dealer level. The hardware is identical to Case products, with some minor differences in how the software interacts with the rate controller.

Since 2012, Case has offered an enhanced version called AIM Command Pro (Capstan calls their version Pinpoint). This system offers individual nozzle sectional control as well as turn compensation. In addition, the enhanced system offers individual nozzle diagnostics that provides operational details to the sprayer operator.

Figure 4 (Raven Hawkeye)In 2014, Raven introduced a system called the Hawkeye. Initially targeted at the retrofit market, the system uses an ISOBUS approach that works with the Viper 4 monitor. The electric solenoids are similar to those on the Capstan systems. The basic system (Hawkeye) features turn-compensation, but not individual nozzle sectional control. Section resolution is determined by the limits of the monitor, for example, 16 sections on the Raven Viper 4. Hawkeye 2.0 HD, announced December, 2015, allows for individual nozzle on-off control. Hawkeye will be available as a factory option on New Holland, Apache, and Case sprayers starting in 2017.

john-deere-exactapply_nozzle_John Deere announced their PWM version on August 29, 2016.  Called ExactApply, the system  splits the liquid flow into two streams, one through each of two solenoids. The solenoids serve to shut off the flow, and also to control liquid flow rate, each running at 15 Hz.  The body contains six nozzles on numbered feed housings, and they operate in opposite pairs.

The body is rotated manually to the preferred nozzle pair. With the longer housing in front (#4, 5, and 6) it allows for very high flows from a single nozzle only. When the shorter housing is rotated to the front, the unit will allow just the front, just the back, or both nozzle to operate.

It has three main modes.

  1. High flow (e.g. fertilizer) capacity with the long housing in front.  To the best of our knowledge, this nozzle can be pulsed at 30 Hz by using both solenoids.
  2. A/B Mode. Cab-switchable nozzles in front, back, or both for rate variation, spray quality variation or other specialty uses. Nozzle can be air-induced since in this mode the PWM would not be running. For rate control, the idea is for a smaller nozzle to be in the front (A) and to respond to travel speed changes with pressure changes (i.e., conventional pressure-based rate control). When increased speed exceeds the pressure capability of the nozzle, the unit switches to the back nozzle (B), which is slightly larger. Pressure drops immediately and faster speeds are possible. Once the the pressure reaches maximum, both nozzles switch on, making additional speed possible.
  3. PWM Mode. To the best of our knowledge, the front, or back, or both nozzles can be pulsed. The user can switch between these nozzles from the cab. When only one nozzle is pulsed from position #1, 2, or 3, the frequency is 15 Hz. When two are pulsed, the effective frequency would be 30 Hz (15 Hz each at a 180 degree offset).

The system also features individual nozzle shutoff, turn compensation, programmable rates by nozzle, nozzle plug detection, and LED lighting. It is expected to be competitively priced compared to the other systems, and offers higher maximum flow rates through its solenoid (up to 50 US gpa at 15 mph).

Units are expected to be tested on sprayers in 2017, and become available as a factory option or a retrofit in 2018. We will update this space as more information becomes available.

 

Figure 5 (DynaJet-b)TeeJet has a system called the DynaJet Flex 7120 that uses either a monitor or Android tablet to display pressure, duty cycle, and droplet size. DynaJet is available to OEMs and to the aftermarket. The unique aspect of the TeeJet system is the ability to dynamically select different droplet sizes, and the system will maintain that droplet size across a wide range of speeds or application rates. The TeeJet system is compatible with any flow-based rate control system, and does not require a TeeJet spray control. The product is available from TeeJet dealers.

System Capabilities

Spray Quality: Since PWM systems can alter flow rate without affecting spray pressure, the user can select a spray pressure that meets their spray quality goals and expect this spray quality to remain constant throughout the field, regardless of travel speed.

Spray Drift Control: Although PWM does not by itself have any unique capabilities to reduce spray drift, it does make spray drift management easier. For example, the most accessible tool for reducing spray drift is to increase droplet size by reducing spray pressure. In a conventional system, the reduction of spray pressure can only be achieved with a reduction in travel speed because the lower spray pressure also reduces the overall flow rate. With PWM, the loss of flow with a reduction in spray pressure can be compensated by an increase in DC. As a result, lower pressures do not require a reduction in travel speed provided there is sufficient DC capacity in the system. Also, PWM systems use larger orifice nozzles, which naturally produce larger droplets.

Rate Control: A PWM system can be used for variable rate application. The spray volume, as determined by duty cycle, can vary as desired within its operational envelope without a change in travel speed.

Turn Compensation: AIM Command Pro, Capstan Pinpoint, Raven Hawkeye, and John Deere ExactApply feature turn compensation capabilities. During a turn, the outside boom moves faster than the inside boom, resulting in under- and over-application. A turn-compensated system can deliver additional flow to the outside, reducing the flow towards the boom end on the inside of the turn. In practice, there are limits to this feature. For example, the system’s average DC needs to be about 70% to offer the maximum flexibility. Second, the diameter of the object being turned around must not exceed the width of the boom, or else the inside boom moves too slowly in relation to the outside boom. The system’s lag must also be minimal to avoid a counteracting effect during turn initiation and completion.

Sectional Control: In a PWM system, sectional configuration is determined by wiring and software, not plumbing. All section valves remain open during operation, and sectional shutoff is effected directly at the nozzle solenoid. Individual nozzle sectional control is offered by the AIM Command Pro and Capstan Pinpoint, as well as Hawkeye 2.0 HD and John Deere ExactApply. This feature may provide product savings when field margins are not straight, or fields feature obstacles resulting in significant overlaps.

Shutoff response: The traditional nozzle check valve is designed to prevent nozzle dripping on boom shutoff. However, due to the presence of air pockets in most booms as well as the elastic nature of rubber hoses leading to the boom, the shutoff is delayed until the boom pressure reaches about 1 bar. This can take up to 10 seconds, resulting in unintended overspray and other safety concerns. In a PWM system, the solenoid shuts off the flow to the nozzle instantly, and conversely, turn it on instantly as well. The boom remains fully pressurized while the nozzles are shut off, allowing the spray patterns to be fully developed upon flow resumption.

Nozzle Options

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

Case sprayers are equipped with a nozzle body manufactured by Wilger Industries that fits the ComboJet nozzle caps. This company offers four nozzles for PWM. In order, from finest to coarsest:

  • ComboJet ER
  • ComboJet SR
  • ComboJet MR
  • ComboJet DR

In practice, the ER and DR are rarely used in PWM systems. The MR is typically suited for lower water volume rates (3 US gpa to 6 US gpa achieved with the MR11003 or MR11004), whereas higher volumes (6 US gpa to 15 US gpa) are typically delivered using the SR tip (SR11005, SR11006, or SR11008, depending on average travel speed). In some cases, the ER nozzle is use when flow rates require 11010 or 110125 sizes. Spray pressures are typically 40 psi at the nozzle.

Wilger’s nozzle body can be purchased as retro fits for other sprayers. The company also offers adaptors that allow Wilger nozzles to be used on TeeJet-style bodies and vice versa.

PWM systems operating on TeeJet style bodies are well served by TeeJet Technologies’ Turbo TeeJet nozzles. These wide-angled tips are available in sizes up to 11008 and generate suitable spray qualities at pressures ranging from 15 to 60 psi. Many operators use the Turbo TwinJet, another good option, which is available in a large selection of flow rates to 11010.

Hypro has a pre-orifice tip similar to the Turbo TeeJet, called the Hypro Guardian. Most other manufacturers, including Lechler, Hardi, Arag, and others, have traditional pre-orifice flat fan nozzles that may also work. It is important to always select 110 degree fans to ensure that 100% overlap is achieved to maintain the concept of blended pulse. Limitations are in the maximum flow rates available in a specific model, many nozzles are not available in sizes larger than 05 or 06.

Some PWM users have used air-induced nozzles successfully, but this approach is not currently supported by PWM manufacturers. Some air-induced nozzles blow spray liquid out the air-induction intakes one a pulse is applied, so caution is advised.

Nozzle Selection Process

The sizing of nozzles requires a small amount of calculations from traditional spray calibration tables. Follow these steps:

  1. Target a duty cycle of 70% ± 10% on average during operation. This permits the best travel speed flexibility. Say you want to apply 10 US gpa at 15 mph. In a conventional system, the 05 nozzle size could meet this flow at 40 psi. For PWM, the 06 size would operate at about 83% duty cycle (0.5/0.6 = 0.83). Assuming a minimum DC of 20%, a minimum travel speed of about 3.6 mph is possible. The 08 size would operate at 63% DC (0.5/0.8=0.63), allowing a minimum speed at 20% DC of 4.8 mph. Either option can work as long as the operator recognizes the travel speed limitations of both. Remember that the actual flow rate change is not directly related to duty cycle. Expect to see higher flows than calculated, especially for the smaller flow rate nozzles.
  2. Calculate the travel speed range. The travel speed range of any nozzle selection and water volume can easily be calculated. The maximum travel speed is limited by the capacity of the selected nozzle and pressure at 100% DC. The minimum travel speed can be assumed to be 20% of that value, at which the system would be operating at 20% DC. Assuming a user selected the 08 nozzle size at 40 psi in the above example, the maximum travel speed can be read from a traditional calibration chart, 24 mph. The minimum would be one fifth of that, 4.8 mph. Capstan has charts that show the theoretical travel speed range (assuming a direct relationship between DC and flow). For Raven and TeeJet, the charts were developed using actual flow measurements. These reveal that actual flows are greater than predicted, especially for smaller nozzles.
  3. Consider the pressure drop across the solenoid. The pressure drop depends on the total flow through the solenoid, it varies from 3 to 5 psi for 04 flow rates to 5 to 13 psi for 08 flow rates for the Case, Capstan, and Raven products. If targeting 40 psi spray pressure, set the pressure to 40 psi plus these values. Traditional spray charts do not account for pressure drop across the PWM solenoids. When using a Capstan Ag system, always refer to the tip charts from Capstan Ag Systems, Inc. at capstanag.com, (or here) and when using a TeeJet system, refer to the charts at www.teejet.com. Both show the pressure drop at various flow rates. Raven’s tip chart is in their operator’s manual.

Common Questions

  1. Will the pulsing of the spray create skips in control? This is very rarely the case, usually only when a mistake in nozzle selection has been made. Skips are more likely with a combination of low duty cycles, fast travel speeds, low booms, narrow fan angles, and extremely coarse sprays. At high speeds, the system is usually operating at a high duty cycle unless a nozzle size which is far too large has been selected. At boom heights above 20 inches and Medium to Very Coarse sprays, there is enough blending of the spray cloud from the nozzle to the target to remove any skips in coverage. We can see skips on the outer edge of a boom during a sharp turn, when duty cycle is taken from the tractor unit speed (slow during a turn) and the outer edge of the boom is travelling at two to three times that speed. A conventional system would see similar under-application under these conditions.
  2. Does the droplet size really stay constant throughout the Duty Cycle range? At low duty cycles, we have seen a slight increase in the droplet size, and also a slight decrease in the fan angle. This could be because the longer off-phase reduces the internal pressure in the nozzle body, resulting in an effectively lower pressure. These changes are not significant in their magnitude. It remains important to avoid the lowest duty cycles (travel speeds) for prolonged periods.
  3. Can I do all my spraying with one nozzle? A PWM system offers the advantage of maintaining consistent pressure over a wide range of travel speeds for any given water volume. When moving to a new water volume that is more than 25 to 30% different, a different flow nozzle is recommended. Keeping the same nozzle for two volumes can technically work, but at the cost of limiting the travel speed range for one or both volumes. A typical PWM user has three nozzles, one each for low, intermediate, and high water volume needs assuming similar travel speed ranges. Some choose to use the same nozzle for intermediate and high water volumes, on the assumption that the high volumes is in maturing canopies and travel speeds will be reduced as a result.
  4. Does PWM reduce drift? PWM does not reduce drift in any special way. Drift is related to droplet size, which is controlled by nozzle choice and operating pressure. A conventional system will use low-drift nozzles that maintain reasonably low drift sprays over their pressure range. But at high speeds, high pressures will be used and that can increase drift potential. In a PWM system, high speed does not increase pressure, offering a more consistent amount of drift. However, even at the same pressure, higher speeds increase drift potential because more drift-prone droplets are pulled from the spray plume. Some users of PWM may drive faster than they should simply because they avoid the pressure spike. Fast travel speeds remain a poor practice from a drift perspective.
  5. Is the system prone to breakdowns? PWM has been on the market for about 15 years with Case and Capstan and has proven to be robust. The solenoids themselves have a good wear life, but do require replacement from time to time. Inside the solenoids, a poppet seal can wear over time, requiring fairly inexpensive and easy replacement. As with all electronics, regular inspection of the wiring harness to ensure no abrasion or pinching is required.

Additional Help

One of the more useful websites for PWM users is the Tip Wizard by Wilger (www.wilger.net). It is geared towards selecting the right nozzle for Case AIM Command, which uses the proprietary Wilger nozzle bodies and caps. The website helps users to select nozzles that match their volume, speed, and droplet size requirements.

Once a user understands the basic principles of the system, any conventional calibration chart can be used to identify the needed size nozzle. A user simply needs to choose a nozzle that is about 30 to 40% larger in flow rate, to allow the system to run at approximately 70% DC on average.

Wilger also produces a smartphone app, Tip Wizard, that offers much of the same features as their website for selecting tips.

Capstan (www.capstanag.com) produces a useful calibration table that identifies the pressure drop for various nozzles and pressures, as well as travel speed ranges for these nozzles when applying a range of water volumes.

Raven Industries offers information on the Hawkeye on their website (ravenprecision.com). The company offers useful videos on their youtube channel that illustrate installation procedures.

TeeJet Technologies has DynaJet information on their website (www.teejet.com). The site contains have product information, installation and operator manuals, application rate charts, and drop size information.

Troubleshooting and Maintenance

The main hazard for the PWM solenoids is contaminants. Granules can become lodged on the poppet seal surface, reducing the metering accuracy. Regular inspection of screens, and occasional removal and disassembly of the solenoids to expose the poppet is recommended.