Tag: controller

  • Rate Controllers and a Pulse Width Modulation Update

    Rate Controllers and a 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), Hypro (Duo React), and a host of European manufacturers. 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.
    • 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 or 15 Hz (the solenoid shuts off the nozzle 10 or 15 times per second), but some pulse at 50 and even 100 Hz. 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, particularly because the time spent at low duty cycles tends to be low. 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 and also for higher pulse frequencies.

    Commercial PWM Systems

    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.

    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.

    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 virtual sections on the Raven Viper 4. Hawkeye 2.0 HD, announced December, 2015, allows for individual nozzle on-off control. Hawkeye is available as a factory option on New Holland (called IntelliSpray), Apache, Rogator, Horsch Leeb, and Case (AIM Command HD) sprayers.

    John Deere announced their PWM version in 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.  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. When pulsing is turned off, AB AutoSelect mode is available which automatically switches nozzles based on flow needs. 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 limit of the nozzle (set by user), 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 the user-defined maximum, both nozzles switch on, making additional speed possible.
    3. Pulsing Mode. 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 quasi 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 offers higher maximum flow rates through its solenoid (up to 50 US gpa at 15 mph).

    We’ve provided an in-depth overview of ExactApply here.

    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.

    PWM systems are also available on Agrifac (StrictSprayPlus), WEEDit Quadro, and BBLeap. These systems operate at higher frequencies, up to 50 Hz for WEEDit and up to 100 Hz for Agrifac and BBLeap, as conditions require.

    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, John Deere ExactApply, Agrifac StrictSprayPlus, TeeJet DynaFlex, and WEEDit Quadro 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 60 to 70% to offer the maximum flexibility. Second, the diameter of the object being turned around should not be much smaller than 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 most PWM manufacturers. 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 10 psi or nearly 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 five nozzles for PWM. In order, from finest to coarsest:

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

    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. The UR is a dicamba-specific nozzle to meet US label requirements for drift protection.

    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, as seen below.

    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 11012 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. Recently, TeeJet has tested and approved several air-induced nozzles for PWM, among these are the AITTJ60, the TTI, and the TTI TwinJet.

    Hypro’s Guardian and 3D nozzles are well suited for PWM, but are somewhat finer than current air-induced standards. The new ULDM nozzle is Ultra Coarse, and approved for PWM despite being air-induced. Hypro also manufacture the LDM nozzle exclusively for John Deere for ExactApply, producing a spray quality similar to the well-established ULD. Most other manufacturers, including Lechler, Hardi, 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.

    Agrotop (Greenleaf in the US) offer two unique PWM nozzles. The BPDF is an asymmetric twin configuration featuring two modified AirMix tips which have eliminated air-induction. The much coarser SoftDrop nozzle is intended for the dicamba market.

    Arag has introduced PWM nozzles that produce Coarse or Very Coarse spray qualities, called CFLD-C or CFLD-VC nozzles. These are the a good spray quality for general use. These models are not yet offered in flow rates above 06, however.

    Nozzle Selection Process

    The sizing of nozzles requires a small amount of calculations from traditional spray calibration tables. Follow these steps, or look at this dedicated post:

    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 normal field 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.

  • Rate Controllers on Air-Assist Sprayers

    Rate Controllers on Air-Assist Sprayers

    There are many advantages to using rate controllers, but their primary role is to maintain a constant application rate. All sprayers change speed on hills, at row-ends, or in response to surface conditions. Since flow from an uncontrolled sprayer is constant, the application rate varies significantly (up to 40% in hilly conditions). Rate controllers compensate for changing speed by adjusting flow.

    Hilly operations create highly variable application rates. Changes in travel speed can translate to 40% variability in rate applied. Rate controllers adjust flow to compensate.

    Pesticide is not saved directly (since increased uphill rates already cancel out reduced downhill rates), but consider the pesticide label. Labels that list a range of rates are contingent on pest pressure and crop size, but also compensate for poor coverage from low-performing equipment. When coverage uniformity is improved, experience has shown that operators can safely spray at minimal rates.

    Experience has also demonstrated that when coverage uniformity is improved, pack-out benefits follow. Even a modest improvement represents a quick return on investment. Equally important, a more consistent application reduces the risk of higher residue levels on the uphill and improves crop protection on the downhill.

    Now, if you are wondering if a rate controller is right for your operation, or if you should just stop reading now, consult this handy decision support matrix:

    This decision support matrix will help you decide if a rate controller is right for your operation. Spoiler alert: It probably is.

    Rate controller categories

    The following table categorizes controllers based on how they control flow. The categories are successively more expensive and complicated, but there’s commensurate value. For example, while not specified here, high-end rate controllers offer value-added features such as as-applied mapping (a powerful management tool).

    DescriptionProsCons
    Good:
    Monitors and adjusts pressure. Uses math to assume flow.    		-Fewest moving parts.
    -Simple interface.
    -Lowest cost.    		System monitors pressure, but does not register flow. For example, if nozzle flow is restricted, back pressure increases. The controller will compensate to correct pressure, implicitly reducing flow, but the operator is not alerted to the actual problem.
    Better:
    Monitors and adjusts flow, not pressure.    		Alerts operator to changes in flow. Operator usually sets the percent error threshold a little high to ignore transient changes.System will not register pressure deviations. At threshold speed, pressure may drop too low. This can cause inconsistent check valve operation and spray pattern collapse. With tall booms, the top nozzles may close completely.
    Best:
    Monitors flow and pressure and adjusts flow.    		-Best likelihood of a consistent application.
    -Alarms or automatic compensation of flow and pressure (user sets hard stops).
    -Provides a low tank level warning.
    -Stores preset calibrations to quickly switch between blocks.    		-Highest cost.
    -Steepest learning curve.
    -More “wire-wiggling”.
    -Operators often choose to over-apply at low speeds as a tradeoff for uniform output and consistent atomizer performance.

    Rate controller adoption and components

    As we write this, less than 10% of air-assist sprayers have rate controllers. In the dark old days of the 1980’s, air-assist operators were ill-advised to install high flow, low pressure field sprayer controllers. That history of mismatched components and subsequent bad experiences continues to hinder widespread adoption.

    Today’s components, however, are specific to air-assist sprayers and have made installations easier and more successful. Do your homework and speak with the manufacturer (not necessarily the local dealer) to ensure the controller, and all its components, meet your needs. Let’s describe the components so you’re prepared to have the conversation:

    • Console
    • Flow meter(s)
    • Flow control valve (including electric boom shut-offs)
    • Speed sensor
    • Wire harness
    Examples of rate controller components.

    Console

    The console is the interface. The user enters criteria about the sprayer, the planting, and calibration data and receives information about sprayer performance. Select a console designed for air-assist sprayers and not field sprayers. Controllers intended for horizontal booms perceive swath in two dimensions, but air-assist controllers account for multiple vertical booms or boom sections in the swath (see the following figure).

    Field sprayer rate controllers used in vertical crops must be “tricked” when programming swath. Leading air-assist rate controllers can assign flow to zones on a single vertical section (left) and adjust swath (sometimes called width) for multiple booms (right).

    Flow meter

    With rate controllers, flow is detected by one or more flow meters positioned pre-manifold. The relief valve becomes more of a safety device, defining the high pressure limit and bypassing flow if required. Most rate controllers use a flowmeter with no ability to monitor pressure. While still effective, adding a pressure sensor ensures nozzles are operating in the desired pressure range.

    Turbine or paddle meters are inexpensive and acceptably accurate. They require periodic cleaning because some chemistry can accumulate and interfere with their moving parts. Filtration helps to minimize this issue. Magnetic or ultrasonic meters have no moving parts, higher resolution, wider metering ranges and aren’t affected by the viscosity of the spraying solution or entrained foam. However, they are considerably more expensive than mechanical meters.

    Flow control valve

    Unlike boom control valves that are open or closed, flow control valves are capable of a range of adjustments. Valve actuation is controlled by 12 volt servomotors. The level of precision depends on the style of valve.

    • Butterfly valves: Simple, inexpensive, and typically for pressures <10bar (150psi). Some have minor leak-by when closed. Control is less precise as the valve opens because the orifice gets geometrically larger. This gives a narrow metering range.
    • Calibrated ball valves: Versions available for all pressures. May be simple flow through balls with similar metering limits to a butterfly. A better ball design is also available that offers a linear flow rate through the entire adjustment range, offering more stable rate control over the entire flow range. Several manufacturers offer these. All ball valves offer zero flow when closed.
    Left- A butterfly valve. Right- A ball valve. Notice how a small change in the opening angle translates into a large change in the orifice size; this is difficult to control manually. Servomotors not pictured.

    Compared to field sprayers, air-assist sprayers travel slower and use lower flow rates. It is a mistake to employ valves intended for high-flow, high-speed sprayers.

    • Speed: Valves are rated by connection size (½”, ¾”, etc.) and opening time (e.g. 1-14 seconds are common). Many rate controllers can be programmed to optimize adjustments for the speed and size of the valve.
    • Precision: As control valves open over their 90° range, the ability to control flow is less precise. Slower valves give less precision, but greater stability.
    • Size: Valve size should accommodate maximum flow and no more. If the valve is too large, it can only meter flow over the first few degrees of opening. For example, let’s say a valve capable of 200 L/min (50 gpm) and rated 1 second is used. Your sprayer meters 0-20 L/min (0-5 gpm). This means the whole metering range happens in the first tenth of a second. Even lightning-fast consoles will give unstable readings (aka hunting) as the computer overshoots the target in an effort to comply.

    Control valves are “service parts”. Seals, moving parts and abrasive liquids mean they will require regular care and eventual replacement. It’s a wise precaution to make them accessible and easily removable. We suggest installing them with quick-connects (see top-right of the previous collage of rate controller components above) to make field-maintenance fast and easy.

    Speed sensor

    Speed can be based on GPS, engine tachometer readings, radar, or wheel rotations. Newer rate controllers may even take the speed directly from the tractor’s data feed. Price, reliability and crop conditions are all factors you should consider in the choice.

    • GPS: Easiest to deploy, very accurate (especially RTK-GPS) and reasonably priced. However, overhead canopy can block satellite signals. Some controllers compensate for the GPS losses with sophisticated internal kinematic devices that measure the inertia of the sprayer and calculate speed when the GPS is not reliable.
    • Wheel rotation speed sensors:  An entry-level sensor, it’s typically a reed switch or Hall effect sensor that detects either the lug nuts or magnets installed on the rotating wheel. More magnets improve accuracy. Its exposure makes it prone to physical damage, and readings change with tire radius (which changes as the tank empties, on soft ground and with temperature). This is why wheel sensors are calibrated in the alley, with the tank half full and both tires at the same pressure.
    • Radar speed sensors: Employing the Doppler effect to measure speed, radar is the most accurate sensor. They are unaffected by terrain, slope or tank volume. They can be mounted anywhere in sight of the ground. They are, however, the most expensive and are typically not repairable if they fail.
    • Tachometer speed sensors: Largely obsolete, they measure the tractor’s tachometer speed and convert it to travel speed. Difficult to install and prone to the same inaccuracy as wheel sensors.
    • Interface sensors: Relatively new, some rate controllers interface with tractor electronics to receive speed data. ISOBUS, the standard interface language that agricultural electronics are increasingly adopting, makes this data exchange more common.

    Wire harness

    It may seem we’re drilling deep to mention wires, but standards are changing. Many controllers employ traditional analog wiring, but they are being made obsolete by the newer ISOBUS option.

    • Traditional Analog: Simple wires with automotive or custom plugs designed to match components. Relatively inexpensive and sometimes field repairable, analog wiring carries signal voltage (and power) to and from the controller to drive valves and receive analog sensor data. Communication is one-way: Sensor to controller, controller to valves.
    • Modern ISOBUS: Bus systems are more like a computer network, where digital signals travel back and forth between the controller and each component. Components that require power are wired directly to a battery. This results in a greatly simplified harness. The controller’s single ISOBUS wire “daisy chains” all components to relay commands and receive status, which makes system monitoring and diagnosis easier and more effective.

    Conclusion

    Rate controllers are a worthy consideration for your existing or future air-assist sprayer. Assess your needs and work with a knowledgeable dealer or manufacturer that can assemble and install a system appropriate for your operation.

  • Exploding Sprayer Myths (ep.1): Rate Controllers

    Exploding Sprayer Myths (ep.1): Rate Controllers

    This is the first of a series of short, educational and irreverent videos made with Real Agriculture to bring a little levity to sprayer education. Let’s face it – ironically, nozzles can be pretty dry.

    This first video discusses what a rate controller can be expected to do, and what it cannot do. Plus, we got to blow up a sprayer in the intro… so there’s that.

  • Rate Controllers and Spray Pressure

    Rate Controllers and Spray Pressure

    Automatic rate controllers are standard equipment on almost all new sprayers. They ensure consistent application volumes, but they don’t do all the thinking for you.  We explore how to make them work properly.

    A rate controller needs to know the boom width (entered by the user), the total spray liquid flow rate (from a flow meter), and the sprayer speed (gps, radar).  It controls the spray liquid pressure by opening or closing a bypass valve. More pressure equals more flow to the boom.

    The rate controller allows the applicator to enter a desired application volume and the controller sets the spray pressure that gives the necessary flow for the application volume and sprayer travel speed being used. In practice, this means that higher travel speeds result in higher spray pressure, and vice versa.

    But it’s not that simple. Rate controllers aren’t smart enough to know how pressure affects nozzle performance. Some nozzles don’t work well at low pressures. Others do a poor job at high pressures. Some sprayer pumps may even have a problem generating some of the higher pressures a rate controller calls for. What does that mean for the available travel speed range that’s possible with any given nozzle? To answer that question, we first have to have a closer look at how pressure affects nozzle performance.

    Spray Pressure and Nozzle Performance

    Nozzle performance depends on a number of factors. Of these, the most critical is spray pressure. Pressure affects the flow rate of the nozzle, the spray pattern (fan angle) and the spray quality (droplet size range). The last two of these affect coverage, overlap, and spray drift, so it’s important to get them right. Each nozzle model has a unique spray pressure range and unique spray qualities within that range, so one must obtain information that is specific to the nozzles on the spray boom from the nozzle manufacturer.

    ASABE spray quality for the TeeJet AIXR nozzle.

     Catalogues Contain Important Information

    Nozzle manufacturer catalogues identify the pressure range over which the nozzle should be operated. At low pressures, engineers look for a uniform pattern that meets the advertised fan angle. The upper pressure limits are kept low enough to prevent the formation of excessively fine sprays. Manufacturers now publish tables containing “Spray Quality”, a broad categorization of droplet size, for their various nozzles and spray pressures in their product line. Common spray qualities for agricultural nozzles are Fine (orange), Medium (yellow), Coarse (blue), Very Coarse (green), and Extremely Coarse (white). An example table from a catalogue is shown in Figure 1. Note that for any given nozzle flow rate (left column), the spray quality changes with spray pressure. For example, the TT110025 nozzle can produce a Very Coarse or a Fine spray, depending on the pressure. Also note that for any given pressure, higher flow rate nozzles produce coarser sprays. At 40 psi, the TT nozzle can produce a Medium, Coarse, or Very Coarse spray, depending on its nominal flow. Both of these relationships depend on the nozzle model and manufacturer.

    Speed-Pressure-Spray Quality Relationship

    As we increase spray pressure, flow rate increases with a square-root relationship.

    Speed-Pressure resize
    The square root relationship between travel speed (or flow rate) and spray pressure for hydraulic nozzles

    This means that in order to double the flow rate, we need to increase spray pressure by a factor of four. Figure 2 shows three different flow rate tips, each applying 10 US gpa at a range of travel speeds. Assume the operator uses a AIXR11004 to apply 10 US gpa at 12 mph. The nozzle would operate at about 40 psi, producing an Extremely Coarse spray quality. If the sprayer slows down to 7 mph to initiate a turn, spray pressure will drop to 15 psi, producing an Ultra Coarse spray. The spray pattern would likely become noticeably narrower, and poor pest control performance is likely in this situation due to the coarseness of the spray.

    Relationship between travel speed and spray pressure for three nozzles applying 10 US gpa

    It would have been better to use the AIXR11003 nozzle.  At 12 mph, this nozzle would have operated at about 70 psi, producing a Coarse spray.  Slowing down to 7 mph would drop the pressure to 25 psi, producing an Extremely Coarse spray.  If the pesticide being used is sensitive to spray quality, then perhaps such slow speeds should be avoided in order to maintain a higher pressure and finer spray.

    The lesson from this exercise is three-fold: (a) size the nozzle to operate at a higher pressure at your target speed to avoid dropping the pressure too low when you slow down, (b) avoid going as slow as 7 mph to prevent the pressure from dropping too low (c) compromise by setting a minimum spray pressure on the rate controller, in which case you’d over-apply product somewhat when their speed dropped too low.

    Spray Pattern Overlap

    Flat fan nozzle patterns need the correct overlap in order to achieve a uniform spray pattern under the boom. Research has shown that the amount of overlap for low-drift nozzles needs to be at least 100% to achieve optimum nozzle performance. In other words, the edge of a fan should reach into the centre of the adjacent fan (Figure 3), with each fan covering twice the nozzle spacing at target height. This amount of overlap assures that not only the spray volume is uniformly distributed, but that the droplet density is equally uniform. Less overlap may result in fewer droplets depositing in the overlap region, resulting in poor coverage and reduced pesticide performance.

    Nozzle Pattern Overlap
    100% overlap means that all areas under the boom receive spray from two adjacent nozzles.

    Adjust the boom height so that at the lowest expected spray pressure (slowest planned travel speed), the nozzles still achieve 100% overlap. There is no disadvantage with greater than 100% overlap, but higher booms will lead to greater drift. When a choice exists, choose 110º fan angle nozzles. Most air-induced nozzles are produced at one (usually wide) fan angle only, but actual angles often differ from those advertised. It is important to visually check the overlap before spraying.

    Recommendations

    What does this mean in practice? Spray operators need to know the right spray quality for the job, and should consult with the pesticide product manufacturer. They also need to use nozzle manufacturers’ charts to identify the spray quality their nozzle will likely produce at their expected application volume and travel speed. If it’s a poor match, a different nozzle may need to be found. Here are some rules of thumb:

    1. Choose a nozzle that produces a Coarse spray over most of the operating pressures you expect to use. Although Very Coarse sprays can work in most situations, avoid them when using lower water volumes, controlling grassy weeds, or using contact modes of action.
    2. Minimize spray drift by avoiding nozzles or pressures that produce Medium or Fine spray qualities.
    3. Make your pressure gauge your speedometer. First, choose a pressure that is in the middle of the nozzle’s recommended operating range. If the range is 15 to 90 psi, select 50 psi. If it’s 40 to 100 psi, select 70 psi. This allows you slow down or speed up somewhat without breaching the nozzle’s capabilities.
    4. Identify the travel speeds that are possible without creating spray qualities that could compromise your application goals.
    5. Visually inspect the spray pattern at the pressure extremes you expect to spray at. At the lowest pressure, your nozzle should still produce 100% overlap (the edge of the spray fan should come to the middle of the next nozzle at target height). If it doesn’t, choose a wider fan angle nozzle, increase spray pressure or elevate the boom.
    6. Make sure your pump can produce the higher spray pressures you expect to need. Pressure limitations are greatest at high flow rates (fast travel speeds applying large water volumes).
    7. Be prepared to compromise. It’s rarely possible to travel at the exact speed, obtain the perfect pressure, and apply the desired water volume that’s been worked out in the office or using manufacturer’s charts. If in doubt, choose slower speeds or higher water volumes to make things work out.

    Nozzle manufacturers are getting much better at producing information that helps applicators produce good spraying outcomes. Learning how to use this information is the first step.