Low flow metering system

A system for metering liquids at very low flows for use with agricultural sprayers is provided. The system may utilize two electronically controlled solenoid valves in series with a storage chamber in between. The flow may be a function of the PWM control of each of the valves, the phase relationship between the control signals, the storage chamber in between, the pressure difference across the metering device, and the fluid being metered.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a non-provisional application based upon U.S. provisional patent application Ser. No. 62/219,928, entitled “Low Flow Metering System,” filed Sep. 17, 2015 which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to agricultural implements, and in particular, to spray systems providing metering of fluid at very low flow rates for use with agricultural field sprayers.

BACKGROUND OF THE INVENTION

Field sprayers, as known in the art, are typically attached to, or towed, by an agricultural implement such as a tractor or other vehicle or are a dedicated self-propelled sprayer vehicle. Such sprayers generally include a fluid holding tank supported by a frame. The fluid holding tank typically stores a crop protection fluid, such as pesticides or liquid fertilizer, which often consists of a carrier fluid (such as water) mixed with a chemical at a predetermined concentration. The fluid holding tank, in turn, is fluidly coupled to a series of spray nozzles spaced apart from one another along booms extending outwardly from the frame. Accordingly, the crop protection fluid may be dispensed through the spray nozzles onto the farm field, preferably in an even distribution spray pattern, so that the fluid is applied consistently across the farm field.

In some situations, it may be necessary to meter a flow of liquid material at a very low flow rate, such as on the order of 100 milliliters per minute (mL/min) or less. This situation could arise, for example, while attempting to achieve a low concentration of chemical fluid with respect to a carrier fluid (such as water) to be sprayed. Traditional control techniques, such as reduced nozzle orifice sizes or Pulse Width Modulation (PWM) controlled valves allowing flow only at limited times, have various drawbacks. For example, when operating in agricultural environment in which the soil may be disturbed by movement of the sprayer, particles in the environment or impurities and/or lumps in the chemical fluid may cause very small orifices to clog. Also, most electronically controlled solenoid valves do not respond reliably when the duty cycle of PWM control signals are very low.

What is needed is an improved system for in which a fluid may be reliably metered at very low flow rates.

SUMMARY OF THE INVENTION

A system for metering liquids at very low flows for use with agricultural sprayers is provided. The system may utilize two electronically controlled solenoid valves in series with a storage chamber in between. The flow may be a function of the PWM control of each of the valves, the phase relationship between the control signals, the storage chamber in between, the pressure difference across the metering device, and the fluid being metered.

On a sprayer, an independent distribution system for a chemical may be installed to deliver concentrated chemical along a boom to individual spray nozzles at a pressure higher than the carrier pressure. A metering device may control the flow of concentrated chemical from that distribution system as it is injected into the carrier flow stream at the nozzle.

In one aspect, a system could include two series connected solenoid operated valves with a charge chamber in between. The charge chamber can vary in size and can be made from materials of different elasticity, or exhibiting specific expandability, such as a spring loaded diaphragm. Both valves may be PWM controlled independently, but generally at the same frequency. For low flow control, there is preferably no overlap in PWM pulses for the respective valves. The inlet valve may open to charge the chamber then close to trap pressurized fluid. The outlet valve may that open to release the charge. Smaller chambers having limited elasticity may reduce the volume of discharged fluid. Valves may also be kept relatively large to reduce clogging. For higher flow requirements, valve controls may be overlapped to cause a controlled time of continuous flow through both valves. For even higher flows, one valve can be held open while the other valve limits flow based on traditional PWM control.

Accordingly, the system may provide control of relatively low flow rates without requiring more expensive valves having fast response times. The system may also eliminate the need for very small orifices and passages that could cause clogging issues. PWM frequency may also be kept high enough to limit pulsing of the controlled flow with reliable operation.

Specifically then, one aspect of the present invention provides a spray system for use with an agricultural sprayer comprising: a first distribution path for distributing a first fluid (which may be a carrier fluid such as water); a second distribution path for distributing a second fluid (which may be a full concentrate pesticide or some chemical fluid); a spray nozzle assembly providing first and second inlets for receiving the first and second fluids, respectively, a mixing chamber for mixing the first and second fluids to provide a mixed fluid, and an outlet for spraying the mixed fluid, wherein the spray nozzle assembly may be in communication with the first distribution path for receiving the first fluid at the first inlet; a fluid storage chamber; a first electronically controlled valve coupled between the second distribution path and the fluid storage chamber for controlling flow of the second fluid from the second distribution path to the fluid storage chamber; and a second electronically controlled valve coupled between the fluid storage chamber and the second inlet of the spray nozzle assembly for controlling flow of the second fluid from the fluid storage chamber to the second inlet of the spray nozzle assembly.

The fluid storage chamber may be operable to expand with pressure from the second fluid. Also, the spray system may further comprise a controller in communication with the first and second electronically controlled valves, wherein the controller may be configured to control at least one of the first and second electronically controlled valves with a Pulse Width Modulation (PWM) signal.

For a higher flow, the controller may hold one of the first and second electronically controlled valves open while the other of the first and second electronically controlled valves may be controlled with the PWM signal.

For a medium or low flow, the controller may be configured to control the first electronically controlled valves with a first PWM signal and control the second electronically controlled valves with a second PWM. The first and second PWM signals may have substantially the same frequency and differ in phase.

For the medium flow, the first PWM signal controls the first electronically controlled valve to open during a first time while the second PWM signal controls the second electronically controlled valve to open, then the first PWM signal controls the first electronically controlled valve to close during a second time while the second PWM signal controls the second electronically controlled valve to remain open, and then the first PWM signal controls the first electronically controlled valve to remain closed during a third time while the second PWM signal controls the second electronically controlled valve to close.

For the low flow, the first and second PWM signals may further differ in duty cycle. The first PWM signal may control the first electronically controlled valve to open during a first time while the second PWM signal controls the second electronically controlled valve to close, then the first PWM signal may control the first electronically controlled valve to close during a second time while the second PWM signal controls the second electronically controlled valve to remain closed, and then the first PWM signal may control the first electronically controlled valve to remain closed during a third time while the second PWM signal controls the second electronically controlled valve to open

The second distribution path will typically distribute the second fluid at a higher pressure than the first distribution path distributing the first fluid.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring generally to the drawings, and more particularly toFIG. 1, an exemplar agricultural product application system, which in the illustrated embodiment is a field spraying system10(a tractor with a three point mounted sprayer attached), is shown in accordance with the present invention. The field spraying system10may comprise a self-propelled sprayer12having an operator cab14and a primary fluid tank16supported by a chassis18. A rear end20of the chassis18may support a wing boom22(or multiple wing booms) to which one or more secondary fluid tanks, which could be provided as illustrated by reference numeral24, may be supported. The wing boom22also supports a series of spray nozzle assemblies26for spraying an area of a field. The chassis18is supported by a set of wheels28, and the wing boom22, depending on size, may be supported by a set of smaller wheels (not shown).

Primary distribution lines30are flow coupled between the primary fluid tank16and the spray nozzle assemblies26. The primary fluid tank16may typically store a earner fluid such as water. The primary distribution lines30may provide flow of the carrier fluid to the spray nozzle assemblies26directly or indirectly, such as via one or more charge pumps, accumulators, control valves, pressure relief valves, manifolds and/or supplemental distribution, lines in the path as understood in the art for effecting various flow rates, pressures and control for sprayer configurations.

Secondary distribution lines, which could be provided as illustrated by reference numeral32, may be flow coupled between one or more of the secondary fluid tanks24and the spray nozzle assemblies26. The secondary fluid tanks24may typically store a chemical fluid, such as a liquid fertilizer, pesticide, herbicide, or the like. The secondary distribution lines32may provide flow of the chemical fluid to the spray nozzle assemblies26directly or indirectly, such as via one or more charge pumps, accumulators, control valves, pressure relief valves, headers, manifolds and/or supplemental distribution lines in the path as understood in the art for effecting various flow rates, pressures and control for sprayer configurations. Accordingly, the carrier fluid and the chemical fluid may be stored in different tanks and subsequently mixed at each of the spray nozzle assemblies26thereby providing improved distribution in the field. The secondary fluid tanks24are typically smaller than the primary fluid tank16.

Referring now toFIG. 2, in a spray system, a pictorial view of an exemplar spray nozzle assembly26is provided in accordance with the present invention. The spray nozzle assembly26may generally include a nozzle body40, coupled in turn to a mixing body42, coupled in turn to a control valve44. In one aspect, the nozzle body40may be thread coupled to the mixing body42, and the mixing body42may be thread coupled to the control valve44, although other temporary or permanent coupling techniques known in the art could be used, such as pressure fittings and/or adhesive agents.

The nozzle body40includes a nozzle outlet46(exposing an orifice) for spraying a mixed fluid which will typically consist of a carrier fluid (such as water) mixed with a chemical fluid at some concentration. The nozzle body40may also include a nozzle body inlet48for receiving the carrier fluid. The carrier fluid may come from the primary fluid tank16via the primary distribution lines30.

The mixing body42may include a mixing body inlet50for receiving the chemical fluid (such as a liquid fertilizer, pesticide, herbicide, or the like). The chemical fluid may come from either of the secondary fluid tanks24via the secondary distribution lines32. Within the mixing body42, a flow control mechanism (shown inFIG. 3) may provide a mixing chamber for mixing the carrier fluid with the chemical fluid in the nozzle to provide the mixed fluid.

The control valve44operates to stop the mixed fluid from flowing to the nozzle outlet46, or to allow the mixed fluid to flow to the nozzle outlet46for spraying. The control valve44could be a passive check valve, as shown inFIG. 2, in which the mixed fluid is mechanically stopped from flowing if there is insufficient pressure applied by the mixed fluid against a valve mechanism, or the mixed fluid is allowed to flow if there is a build-up of sufficient pressure of the mixed fluid against the valve mechanism. Alternatively, the control valve44could be an actively controlled solenoid valve, as shown inFIG. 3by reference numeral74, in which the mixed fluid is stopped from flowing or allowed to flow depending on a control signal provided to a solenoid which actuates a valve. Accordingly, the control valve44may serve to prevent undesirable leaking of the mixed fluid. Also, the control valve44may be operator or computer controlled in the field.

Still referring toFIG. 2, a light source52and a light sensitive receiver54may each be connected to the spray nozzle assembly26. The light source52and the light sensitive receiver54may be contained in separate housings, and each of the housings may fit in opposing openings of the mixing body42with fluid tight seals. The light source52may be any circuit, element or device for emitting light in the mixing body, and may preferably be a Light. Emitting Diode (LED). First and second light source signals56and58, respectively, may interface with other control systems or circuitry of the field spraying system10and may allow for turning on or off the light source52, biasing, and/or controlling the intensity, brightness and/or wavelength of light produced by the light source52.

The light sensitive receiver54may be any circuit, element or device for receiving light in the mixing body and generating an electrical signal indicating an amount of light received by the light sensitive receiver54. The light sensitive receiver54may preferably be a photodiode. In particular, the light sensitive receiver54may receive light from the light source52(passing through the mixed fluid) within the mixing body42. First and second light sensitive receiver signals60and62, respectively, may interface with other control systems or circuitry of the field spraying system10and may allow for sending an electrical signal indicating the amount of light received by the light sensitive receiver54, biasing, and/or controlling the wavelength of light to which the light sensitive receiver54may be sensitive.

In sending the electrical signal indicating the amount of light received, one of the first and second light sensitive receiver signals60and62, respectively, could be used to provide an analog voltage having a magnitude in proportion to the amount of light received by the light sensitive receiver54, while the other of the first and second light sensitive receiver signals60and62, respectively, could provide a reference level. In an alternative aspect, digital circuitry could be employed in the light sensitive receiver54so that the first and/or second light sensitive receiver signals60and/or62, respectively, provide a digital representation of the magnitude of light received.

Referring now toFIG. 3, an exploded pictorial view of an alternative spray nozzle assembly76having the mixing body42ofFIG. 2, but with an alternative nozzle body70and an alternative control valve74, is provided in accordance with the present invention. In this aspect, the nozzle body70may include first and second nozzle body inlets78aand78b, respectively, for receiving the carrier fluid instead of a single nozzle inlet. Accordingly, the multiple inlets (the first and second nozzle body inlets78aand78b, respectively) may allow for alternative implementations of the spray nozzle assembly76in the field spraying system10, such as ganging a plurality of spray nozzle assemblies76together. The nozzle body70may be coupled to the mixing body42, for example, via nozzle body threading79.

Also in this aspect, the control valve74is an actively controlled solenoid valve. Accordingly, mixed fluid is stopped from flowing or allowed to flow depending on a control signal provided, via wiring/interconnect75, to a solenoid which controls the valve. The wiring/interconnect75may interface with other control systems or circuitry of the field spraying system10for control of spraying applications in the field. The control valve74may be coupled to the mixing body42, for example, via mixing body threading53. It will be appreciated that with this configuration, if desired, the mixing body42could be removed, and the control valve74coupled directly to the nozzle body70, via nozzle body threading79, to revert to a de-featured implementation.

Also in this aspect, the mixing body42ofFIG. 2is used. The mixing body42may include a mixing body inlet50(or alternatively first and second mixing body inlets) for receiving the chemical fluid.

Within the mixing body42, a flow control mechanism90may be provided for directing fluid flow within the spray nozzle assembly76. With additional reference to4A, fluid flow is depicted by way of arrows reference characters. In particular, arrows with the reference character “A” denote flow of the carrier fluid; arrows with the reference character “B” denote flow of the chemical fluid; and arrows with the reference character “C” denote flow of the mixed fluid.

In operation, the carrier fluid A is received via the first and second nozzle body inlets78aand78b, respectively, of the nozzle body70. The carrier fluid A is directed through a first interior opening92(which may be a plurality of openings) in the flow control mechanism90, leading to a mixing chamber94. The mixing chamber94may be defined by a cavity formed by exterior walls of the flow control mechanism90and interior walls of the mixing body42.

The chemical fluid B is received via the mixing body inlet50of the mixing body42. The chemical fluid B is directed to the mixing chamber94, thereby mixing in the nozzle to form the mixed fluid C. The mixed fluid C, in turn, is directed through a second interior opening96(which may be a plurality of openings) in the flow control mechanism90, leading to the control valve74.

Upon sufficient pressure of the mixed fluid C, such as with a check valve, or upon actuation of the control valve74, such as with the solenoid valve, the mixed fluid C will then flow through the control valve74and exit via a control valve outlet98. The control valve outlet98is fluidly coupled with an interior channel100of the flow control mechanism90and may be fluid sealed with a sealing member99. The mixed fluid C may then, in turn, travel through the interior channel100to an orifice102proximal to the nozzle outlet72of the nozzle body70for spraying.

Still referring toFIGS. 3 and 4A, the mixing body includes first and second openings104aand104b, respectively, for accommodating the light source52and the light sensitive receiver54with fluid tight seals. In one aspect, the first openings104acould receive the light source52, and the second opening104bcould receive the light sensitive receiver54, and the first and second openings104aand104bcould be opposing such that a fluid inspection region for transmitting light through the mixed fluid is formed in between. Transmission of light from the light source52to the light sensitive receiver54, through the fluid inspection region, may allow determining a concentration of the chemical fluid in the mixed fluid by determining how much light is received by the light sensitive receiver54(and how much light is inhibited by the mixed fluid).

Referring now toFIG. 5, a pictorial view of a metering system120having two electronically controlled valves in series with a storage chamber in between for metering a fluid at a low flow rate is provided in accordance with the present invention. First and second electronically controlled valves122and124, respectively, are provided, which may each be actively controlled solenoid valves operating in a manner similar to the solenoid control valve74described above with respect toFIG. 3. Accordingly, the first and second electronically controlled valves122and124, respectively, may each be controlled to stop fluid or allow fluid to flow depending on a control signal provided via first and second wirings126and128, respectively. The first and second wirings126and128, respectively, may interface with other control systems or circuitry of the field spraying system10. Accordingly, the first and second electronically controlled valves122and124, respectively, may receive first and second Pulse Width Modulation (PWM) signals, respectively, for there modulated control from the control systems or circuitry of the field spraying system10.

The first electronically controlled valve122may have a first electronically controlled valve inlet130for receiving fluid from a fluid supply. In a preferred aspect, the first electronically controlled valve inlet130may be coupled to a distribution path drawing the chemical fluid from the secondary fluid tank24via the secondary distribution line32.

The first electronically controlled valve122may also have a first electronically controlled valve outlet132for providing the received fluid to a fluid storage chamber134when open. The fluid storage chamber134may be operable to expand with pressure from the fluid, and the fluid in the fluid storage chamber134may also undergo an amount of compression. The fluid storage chamber134, in turn, may be coupled to a second electronically controlled valve inlet136of the second electronically controlled valve124. The second electronically controlled valve inlet136may also have a second electronically controlled valve outlet138for providing, the fluid further downstream, such as to the mixing body inlet50of the spray nozzle assembly76.

Referring briefly toFIG. 6A, the fluid storage chamber134may operably expand from a first volume140to a second volume142, the material of which, accordingly exhibiting elasticity. In this arrangement, the fluid storage chamber134may operably expand from the first volume140to the second volume142, for example, when the second electronically controlled valve124is closed and the first electronically controlled valve122is open, thereby causing a pressure of the fluid in the fluid storage chamber134to build against interior walls of the fluid storage chamber134to increase the fluid storage chamber134volume. In addition, the fluid held in the second volume142may also undergo an amount of compression due to the first electronically controlled valve122continuing to push fluid into the fluid storage chamber134.

Referring briefly toFIG. 6B, in an alternative arrangement, a fluid storage chamber148may instead consist of first volume150which may operably expand to a second volume152via a diaphragm154and a spring156. In this spring loaded diaphragm arrangement, the fluid storage chamber148may operably expand from the first volume150to the second volume152, for example, when the second electronically controlled valve124is closed and the first electronically controlled valve122is open, thereby causing a pressure of the fluid in the fluid storage chamber148to build against the diaphragm154to thereby compress the spring156to increase the fluid storage chamber148volume. In addition, the fluid held in the second volume152may also undergo an amount of compression due to the first electronically controlled valve122continuing to push fluid into the fluid storage chamber148.

Referring now toFIG. 7, a schematic view of a control system160having the metering system120ofFIG. 5and the spray nozzle assembly76ofFIG. 3is provided by way of example in accordance with the present invention. A first distribution path162is provided for distributing a first fluid, which may be the carrier fluid stored in the primary fluid tank16. The first distribution path162may receive the carrier fluid via the primary distribution line30, and may include a third electronically controlled valve164(identified as “V3”), which may be a solenoid valve operating in a manner similar to the solenoid control valves described above with respect toFIGS. 3 and 5, for metering the carrier fluid to the spray nozzle assembly76(and to the mixing chamber94).

A controller170may be configured, among other things, to control the third electronically controlled valve164, such as via a carrier fluid metering PWM signal166. The controller170may be a microprocessor, a microcontroller or other programmable logic element as known the art.

A second distribution path172is provided for distributing a second fluid, which may be the chemical fluid stored in the secondary fluid tank24. The second distribution path172may receive the chemical fluid via the secondary distribution line32. The second distribution path172preferably distributes the chemical fluid at a higher pressure than the first distribution path162distributing the carrier fluid.

The second distribution path172may lead to the metering system120, which may include the first electronically controlled valve122(identified as “V1”), coupled in turn to the fluid storage chamber134, coupled in turn to the second electronically controlled valve124(identified as “V2”). The controller170may be configured to control the first and second electronically controlled valves122and124, respectively, via first and second PWM signals180and182, respectively.

The metering system120, in turn, meters the chemical fluid to the spray nozzle assembly76(and to the mixing chamber94). Within the spray nozzle assembly76, from the mixing chamber94, the mixed chemical and carrier fluids (i.e., mixed fluid) may then pass through a fluid inspection region in which the light source52transmits light through the mixed fluid to the light sensitive receiver54to produce feedback to the controller170for indicating the concentration of the chemical fluid in the mixed fluid. Depending on the feedback, the controller170may adjust the metering system120, such as via the first electronically controlled valve122, the second electronically controlled valve124, or both, to achieve a target concentration. Moreover, the controller170may continuously receive feedback and adjust the metering system120as part of a closed loop control system, including for example, by implementing a Proportional-Integral-Derivative (ND) controller or the like.

Although only one metering system120and spray nozzle assembly76are shown inFIG. 7by way of example, it will be appreciated that the control system160may include numerous metering systems120and spray nozzle assemblies76of the field spraying system10.

Low Flow

Referring now toFIG. 8A, a graph for controlling the metering system120, and the first and second electronically controlled valves122and124, respectively, for achieving a low flow rate is provided in accordance with the present invention. Accordingly, the controller170may provide the first and second PWM signals180and182, respectively, at substantially the same frequency. However, the phase and the duty cycles may differ.

During a first time identified as “1,” the first PWM signal1180may control the first electronically controlled valve122(or V1) to open while the second PWM signal182controls the second electronically controlled valve124(or V2) to close. As a result, fluid enters the fluid storage chamber134causing an expansion of the fluid storage chamber134and compression of the fluid. Then, during a second time identified as “II,” the first PWM signal180may control V1to close while the second PWM signal182controls V2to remain closed. As a result, the fluid held under pressure in the fluid storage chamber134is held in delay for a predetermined amount of time. Then, during a third time identified as “III,” the first PWM signal180may control V1to remain closed while the second PWM signal182controls V2to open. As a result, the fluid held under pressure is released from the fluid storage chamber134toward the spray nozzle assembly76(and to the mixing chamber94). Finally, during a fourth time identified as “IV” the first PWM signal180may control VI to remain closed while the second PWM signal182controls V2to also remain closed for another predetermined amount of time. The aforementioned cycle, including I, II, III and IV, may repeat.

Accordingly, in this configuration, the second PWM signal182may be entirely out of phase with the first PWM signal180such that the second PWM signal182is active when the first PWM signal180is inactive. Moreover, the duty cycle of the second PWM signal182may be less than the duty cycle of the first PWM signal180(which may be 50%), such that the fluid held under pressure in the fluid storage chamber134may be held in delay for the predetermined amount of time during the second time (II). In some aspects, the active times of the second PWM signal182may be advantageously centered during the inactive times of the first PWM signal180.

Medium Flow

Referring now toFIG. 8B, a graph for controlling the metering system120, and the first and second electronically controlled valves122and124, respectively, for achieving a medium flow rate is provided in accordance with the present invention. Accordingly, the controller170may provide the first and second PWM signals180and182, respectively, at substantially the same frequency and duty cycle. However, the phase between the first and second PWM signals180and182, respectively, may differ.

During a first time identified as “I,” the first PWM signal180may control V1to open while the second PWM signal182controls V2to open. As a result, fluid enters the fluid storage chamber134and continues through V2toward the spray nozzle assembly76without stopping. Then, during a second time identified as “II,” the first PWM signal180may control V1to remain open while the second PWM signal182controls V2to close. As a result, fluid enters the fluid storage chamber134and stops, causing an expansion of the fluid storage chamber134and compression of the fluid. Then, during a third time identified as “III,” the first PWM signal180may control V1to close while the second PWM signal182controls V2to remain closed. As a result, the fluid held under pressure in the fluid storage chamber134may be held in delay for a predetermined amount of time. Finally, during a fourth time identified as “IV,” the first PWM signal180may control V1to remain closed while the second PWM signal182controls V2to open. As a result, the fluid held under pressure is released from the fluid storage chamber134toward the spray nozzle assembly76(and to the mixing chamber94). The aforementioned cycle, including I, II, III and IV, may repeat.

Accordingly, in this configuration, the second PWM signal182may be partially out of phase with the first PWM signal180such that the second PWM signal182is active: (1) for a time when the first PWM signal180is active; and (2) for a time when the first PWM signal180is inactive. The duty cycle of the second PWM signal182may be equal to the duty cycle of the first PWM signal180(which may be 50%). However, the duty cycle of either PWM signal could be increased with respect to the other to increase flow, or could be decreased with respect to the other to decrease flow.

High Flow

Referring now toFIG. 8C, a graph for controlling the metering system120, and the first and second electronically controlled valves122and124, respectively, for achieving a high flow rate is provided in accordance with the present invention. In this configuration, the controller170may provide the first PWM signal180at a frequency and duty cycle, and provide the second PWM signal182at a constant level to control V2to remain open. As a result, fluid enters the fluid storage chamber134and continues toward the spray nozzle assembly76at a rate of the frequency of the first PWM signal180without expansions of the fluid storage chamber134.

Alternatively, the controller170could provide the second PWM signal182at a frequency and duty cycle, and provide the first PWM signal180at a constant level to control VI to remain open. As a result, fluid enters the fluid storage chamber134and continues toward the spray nozzle assembly76at a rate of the frequency of the second PWM signal182with periodic expansions of the fluid storage chamber134.

The frequency of the first PWM signal180(or the second PWM signal182) could be increased to increase flow or could be decreased to decrease flow.

Referring now toFIG. 9, a graph comparing exemplar frequencies of the first and second electronically controlled valves122and124, respectively, on a horizontal axis (Cartesian “x-axis”) with respect to flow rates on a vertical axis (Cartesian “y-axis”) is provided in accordance with the present invention. The first and second electronically controlled valves122and124, respectively, may be operated at the same frequency via the first and second PWM signals180and182, respectively. As the frequency of operation of the first and second electronically controlled valves122and124increases, the flow rate of the fluid correspondingly increases. At a lower frequency, such as about 10 Hz, the flow rate may be less than 60 milliliters per minute. However, at a higher frequency, such as about 25 Hz, the flow rate may be greater than 100 milliliters per minute.

Referring now toFIG. 10, a pictorial view of an alternative spraying system is provided in accordance with the present invention. A field spraying system210may be comprised of a self-propelled sprayer212having primary and secondary fluid tanks216and217, respectively, that are supported by a chassis218in a known manner. As also known in the art, a rear end220of the chassis218may supports a pair of wing booms222,224to which a series of the spray nozzle assemblies (not shown) may be coupled. The chassis218may be supported by a set of tires228, and the wing booms may be supported by smaller wheels230. Primary and secondary distribution lines232and233, respectively, may be flow coupled to the primary and secondary fluid tanks216and217, respectively, in order to provide field spraying capability similar to the field spraying system10described above with respect toFIG. 1.