METHOD FOR CONTROLLING VALVES OF A SPRAYER BOOM OF AN AGRICULTURAL SPREADING MACHINE

Description

[0001] The invention relates to a method for controlling valves of a spray boom of an agricultural spreading machine according to the preamble of patent claim 1 and a system for controlling valves of a spray boom of an agricultural spreading machine according to the preamble of patent claim 11.

[0002] The line system of a spray boom of an agricultural application machine, such as a field sprayer, contains a plurality of valves that can be used to control the flow rate of spray liquid to the spray nozzles located on the boom. Such agricultural application machines can be equipped with section control or individual nozzle control, allowing the valves used to control the flow rate to be switched independently of one another. For this purpose, valve-specific voltage signals are generated and provided to the valves located on the spray boom.

[0003] US 2020 / 0101480 A1 discloses systems and methods for controlling the operation of a valve. US 5 296 702 A discloses a structure and method for distinguishing one object from another object. CN 109 479 862 A discloses a variable system for spraying pesticides and a control method therefor. US 2020 / 221682 A1 discloses systems and methods for applying a fluid, including a split spray boom and partial control valves for partial pressure control. In practice, it has been shown that the position of the respective valves on the spray boom regularly influences the valve-specific flow rate of spray liquid. In the spray boom, for example, the fluid pressure drops from the center of the boom outwards, so that with identically controlled valves, differing flow rates occur due to a pressure difference that is attributable to the position of the respective valves on the spray boom.The differing flow rates also result from a change in the valve actuation time, which depends on the fluid pressure applied to the respective valve. The higher the fluid pressure upstream of the valve, the longer the actuation time. Due to the varying lengths of the electrical cables connected to the respective valves, there are also different voltage drops during signal delivery, so that different voltage levels at the valves lead to different switching behaviors. In this context, a decreasing control voltage at the valves, for example, leads to an increase in the actuation time of the valves.

[0004] In addition, the valves on a spray boom are influenced to varying degrees by pressure waves in the pipe sections upstream of the valves, depending on their position on the spray boom.

[0005] The object underlying the invention is therefore to reduce position-related influences on the valve-specific flow rates of valves of a spray boom.

[0006] The object is achieved by the subject matter of the independent claims, in particular by a method of the type mentioned at the outset, wherein, within the scope of the method according to the invention, the valve-specific voltage signals generated are pulse-width modulated and each have a valve-specific duty cycle, wherein the valve-specific duty cycles compensate or at least reduce the influences on the valve-specific flow rate of spray liquid that are dependent on the position of the respective valves on the spray boom.

[0007] The invention takes advantage of the finding that position-dependent influences on the flow rate can be compensated or at least reduced by valve-specific adaptation of the pulse width modulation. The influences dependent on the position of the respective valves on the spray boom can, for example, relate to the fluid pressure falling outwards from the center of the boom, the signal-specific voltage drop when providing the valve-specific voltage signals, and / or pressure waves in the line sections upstream of the valves. Thus, several or all voltage signals provided to different valves can have different duty cycles, even though an identical quantity of spray liquid is to be applied via the valves. The duty cycles of individual or all voltage signals therefore differ from one another despite an identical target flow rate of spray liquid.

[0008] The valves are preferably solenoid valves and have a coil that generates a magnetic field to move a valve armature. The duty cycle corresponds to the quotient of the pulse duration and the period duration.

[0009] The valve-specific voltage signals are preferably generated and / or provided by a control system of the agricultural spreading machine. Voltage conditioning can be performed in the control system using a voltage converter. The voltage converter could then be used to increase the valve-specific voltage to compensate for the voltage drop in the electrical lines to the respective valves.

[0010] In a preferred embodiment of the method according to the invention, when generating the valve-specific voltage signals, the fluid pressure applied to the respective valves is taken into account to compensate for the pressure-dependent influences on the valve-specific flow rate of spray liquid. The valve-specific duty cycles are therefore pressure-dependent. When generating the valve-specific voltage signals, the control system can, for example, use a correction function or a correction table that describes the relationship between the fluid pressure applied to the respective valves and a suitable adjustment of the duty cycle. The pressure-dependent influences on the valve-specific flow rate of spray liquid can be attributed, for example, to the fact that the fluid pressure applied to the respective valves decreases from the boom center outwards.For example, fluid pressure influences the flow rate of the spray liquid through open valves. Due to the different flow rates, a valve located in the center of the boom will have a larger volume of spray liquid flowing through it than a valve located on the outside of the spray boom, even if both valves have an identical opening time. This influence can be compensated for or at least reduced by voltage signals with a valve-specific duty cycle. The fluid pressure applied to the valves can also influence their switching behavior, so that valves located in the center of the boom open more slowly than valves located on the outside of the spray boom due to the higher fluid pressure.This influence can also be compensated or reduced by providing voltage signals with valve-specific duty cycles. A drop in fluid pressure at a valve leads to faster opening behavior, so the valve remains open longer. Furthermore, a drop in fluid pressure upstream of the nozzle also leads to a decrease in flow rate. These pressure-related influences can be compensated with valve-specific duty cycles.

[0011] In another embodiment of the method according to the invention, the fluid pressures present at the respective valves are determined. Based on the fluid pressures present at the respective valves, the control system can determine, in particular calculate, the pressure-dependent influences on the valve-specific flow rate of spray liquid. For this purpose, the control system can, for example, use a correction function or a correction table that describes the relationship between pressure, volume flow, and the opening and closing times of the valves. Based on these determinations, valve-specific voltage signals with valve-specific duty cycles can then be generated and provided, which compensate for or at least reduce the pressure-dependent influences on the valve-specific flow rate of spray liquid.The fluid pressures applied to the respective valves are preferably determined via one or more pressure measurements in the spray boom. Consequently, one or more pressure measuring devices can be arranged on the spray boom. Alternatively or additionally, the fluid pressures applied to the respective valves can be determined by the control system by modeling the pressure behavior in the spray boom via the volume flow. The volume flow is either measured via one or more flow measuring devices or calculated based on the existing machine settings. The pressure conditions in the spray boom can also be modeled via the line geometry of the spray boom.

[0012] In another preferred embodiment of the method according to the invention, the fluid pressures applied to the respective valves are determined via at least one pressure measurement and a pressure calculation based on the pressure measurement and taking into account the pressure change in the spray boom. A pressure drop model can be used in the pressure calculation. The pressure drop model can, for example, also take into account the line geometry of the spray boom. The fluid pressure is measured, for example, at a central point in the spray boom, and the pressure drop is calculated based on the line cross-section and / or the fluid output along the boom. In this way, a valve-specific fluid pressure for several valves of the spray boom can be determined based on a pressure measurement. The influences of the fluid pressure differences at the valves are compensated or at least reduced by the valve-specific duty cycle.

[0013] In another advantageous development of the method according to the invention, when generating the valve-specific voltage signals, the signal-specific voltage drop caused by the provision of the valve-specific voltage signals is taken into account to compensate for the voltage-dependent influences on the valve-specific flow rate of spray liquid. The valve-specific duty cycles are therefore voltage-drop dependent. When generating the valve-specific voltage signals, the control system can, for example, use a correction function or a correction table that describes the relationship between voltage drop and a suitable adjustment of the duty cycle. The voltages applied to the respective valves during active energization decrease from the center of the boom toward the outside.The voltages decreasing from the center of the boom outwards result, for example, from the different lengths of the electrical cables used to provide the signals. The different lengths of the electrical cables result in different electrical resistances. Typically, the voltage signals for valves located on the outside of the spray boom are provided via longer electrical cables than the voltage signals for valves located in the center of the spray boom. Due to the different electrical resistances of the electrical cables, different voltage levels arise at the valves of the spray boom. The signal-specific voltage drop or the different voltage levels at the valves can be calculated, for example, based on a voltage drop model.Alternatively or additionally, the signal-specific voltage drop or the different voltage levels at the valves can be measured, for example using voltage sensing. The voltages applied to the respective valves when energized influence the switching behavior of the valves. Valves with comparatively high PWM pulses switch faster than valves with lower PWM pulses because the valve armature moves faster. The shorter actuation time extends the time the valve is open, allowing a larger amount of fluid to flow through the valve and be dispensed. If the armature moves slowly due to lower voltage, the actuation time is longer and thus the period in which the valve is open is shorter.Because the voltage level of the PWM pulses decreases with increasing cable length, the valves' on and off times vary. This effect is compensated or at least reduced by valve-specific duty cycles. When the voltage at the valves varies, the valve-specific duty cycles ensure that the valves switch on and off at the same speed despite the voltage variation. For this purpose, an average on-time for all valves can be used, for example. The average on-time can be determined cyclically.

[0014] In another preferred embodiment of the method according to the invention, the signal-specific voltage drops caused by the provision of the valve-specific voltage signals are determined by one or more voltage measurements. The voltages present at the respective valves during active energization can, for example, be measured directly at a control unit of the control system. The valve-specific duty cycle is adjusted depending on the voltage present at the respective valves during active energization.

[0015] Furthermore, a method according to the invention is preferred in which the generated valve-specific voltage signals compensate for or at least reduce the influence of pressure waves in the line sections of the spray boom connected to the respective valves on the valve-specific flow rate of spray fluid. The pressure waves can be overpressure regions or underpressure regions, which move at a propagation speed through the fluid line and thus pass through various line sections. The valve-specific voltage signals are therefore pressure wave-dependent. Pulse width modulation creates pressure waves in the fluid lines of the spray boom. The pressure waves in the fluid lines of the spray boom influence the valve-specific flow rate during the opening phase of the respective valves.This effect is further amplified by the overlapping pressure waves in the rod, further increasing the risk of misapplication. These pressure waves can be avoided or at least reduced by synchronized valve control. To achieve this, the valves are controlled in a coordinated or offset manner so that the pressure waves do not overlap and / or existing pressure waves are eliminated or at least weakened by newly generated pressure waves.

[0016] Furthermore, a method according to the invention is preferred in which, when generating the valve-specific voltage signals, pressure wave propagation, in particular the propagation speed of pressure waves, in the spray boom is taken into account to avoid or reduce pressure wave superposition. Overlays of pressure waves in the lines of the spray boom can occur via the flow velocity and the frequency of the pulse width modulation. Pressure waves in the fluid lines arise, for example, when a valve is opened and closed. Pressure wave superposition occurs when the flow duration of the spray fluid between valves connected in series, i.e., arranged one behind the other, corresponds to the time between the opening processes of the valves connected in series.To prevent or reduce pressure wave overlaps, valves arranged side by side and connected in series must be coordinated or offset from each other in such a way that pressure wave propagation does not lead to pressure wave overlap. This prevents overpressure or underpressure areas from accumulating.

[0017] In an alternative embodiment of the method according to the invention, the flow velocity of the spray liquid in the spray boom and the length of the fluid line between consecutive valves arranged in series are taken into account when generating the valve-specific voltage signals to avoid or reduce pressure wave overlaps. The valve-specific voltage signals are therefore dependent on the flow velocity of the spray liquid in the spray boom. Thus, for example, the switching-on and switching-off times of adjacent valves can overlap. Preferably, the length of the fluid line between consecutive valves arranged in series is also taken into account when generating the pressure wave compensation signal. For example, a pressure wave trough moving toward a valve can be used to partially or completely compensate for a pressure wave generated by the following valve.

[0018] In a further development of the method according to the invention, pressure pulses are introduced into the lines of the spray boom using a pressure accumulator to compensate for or reduce pressure waves in the spray boom. For the targeted introduction of pressure pulses, the pressure waves and their propagation in the spray boom lines must be known. If the pressure waves introduced into the spray boom lines by the valve circuits are known, suitable counter-pulses can be generated via the pressure accumulator, resulting in partial or complete pulse compensation within the spray boom lines.

[0019] The object underlying the invention is further achieved by a system of the type mentioned at the outset, wherein the system according to the invention has a control system which is configured to generate pulse-width-modulated voltage signals, each having a valve-specific duty cycle, so that the influences on the valve-specific flow rate of spray liquid, which are dependent on the position of the respective valves on the spray boom, are compensated or at least reduced. The system according to the invention is preferably configured to control valves of a spray boom of an agricultural spreading machine according to the method according to one of the embodiments described above. With regard to the advantages and modifications of the system according to the invention, reference is therefore first made to the advantages and modifications of the method according to the invention.

[0020] In a preferred embodiment of the system according to the invention, the control system is configured to take into account the fluid pressures present at the respective valves when generating the valve-specific voltage signals in order to compensate for the pressure-dependent influences on the valve-specific flow rate of spray liquid. Alternatively or additionally, the control system is configured to take into account the signal-specific voltage drop caused by the provision of the valve-specific voltage signals when generating the valve-specific voltage signals in order to compensate for the voltage-dependent influences on the valve-specific flow rate of spray liquid.

[0021] Furthermore, a system according to the invention is advantageous in which the control system is configured to take into account, when generating the valve-specific voltage signals, the pressure waves occurring in the line sections of the spray boom connected to the respective valves in order to compensate for the pressure wave-dependent influences on the valve-specific flow rate of spray liquid.

[0022] Preferred embodiments of the invention are explained and described in more detail below with reference to the accompanying drawings. Fig. 1 shows a system according to the invention including the fluid pressures applied to the valves and the resulting valve-specific duty cycles of the generated voltage signals; Fig. 2 shows a system according to the invention including the level of the supply voltages applied to the valves and the resulting valve-specific duty cycles of the generated voltage signals; Fig. 3 shows the temporal developments of the voltage applied to a valve and the current flowing through the valve as well as the temporal pressure curves exhibiting pressure waves upstream and downstream of the valve; Fig. 4 shows temporal pressure curves at three different valves without pressure wave compensation; and Fig. 5 shows temporal pressure curves at the Fig. 4 underlying valves with pressure wave compensation.

[0023] The Fig. 1 shows a system 10 for controlling valves 14a-14j of a spray boom 12 of an agricultural spreading machine. The agricultural spreading machine can be, for example, a field sprayer. A spreading nozzle 16a-16j is arranged on each of the valves 14a-14j, with the valves 14a-14j and the spreading nozzles 16a-16j each forming a valve-nozzle unit.

[0024] The system 10 further comprises a control system 20, which is connected to the valves 14a-14j in a signal-conducting manner via the electrical lines 18a-18j. The control system 20 generates valve-specific voltage signals U a , U e for controlling the valves 14a-14j, wherein the valve-specific voltage signals U a , U e are provided to the valves 14a-14j via the lines 18a-18j. The valve-specific voltage signals U a , U e shown are provided to the valves 14a, 14e, wherein the valve-specific voltage signals provided to the other valves 14b-14d, 14f-14j are not shown.

[0025] The valve-specific voltage signals U a , U e generated by the control system 20 are pulse-width modulated and each have a valve-specific duty cycle D a -D j . The valve-specific duty cycles D a -D j compensate for or reduce the influences on the valve-specific flow rate of spray liquid that depend on the position of the respective valves 14a-14j on the spray boom 12.

[0026] The bar chart shown below the spray boom 12 shows the fluid pressure P a -P j applied to the respective valves 14a-14j. The bar chart shows that the fluid pressure P a -P j applied to the respective valves 14a-14j decreases from the boom center outwards. Since the fluid pressure P influences the flow velocity of the spray fluid through the valves 14a-14j, the pressure-dependent influence on the valve-specific flow rate of spray fluid must be taken into account when controlling the valves 14a-14j.

[0027] The pressure-dependent influence on the valve-specific flow rate of spray liquid is compensated or at least reduced by the fact that the valve-specific voltage signals U a , U e generated by the control system 20 have a valve-specific duty cycle D a -D j .

[0028] The bar chart relating to the duty cycle D shows that the duty cycle D a of the voltage signal U a generated for valve 14a is greater than the duty cycle D e of the voltage signal U e generated for valve 14e. The fluid pressure P falling from the center of the rod outwards is thus compensated for by the fact that the outer valves remain open longer than the valves located in the center of the rod, using a valve-specific duty cycle.

[0029] The duty cycle D corresponds to the quotient of pulse duration τ and period duration T. The period duration T is the same for all voltage signals U a , U e generated by the control system 20. The differing duty cycles D a , D e result from different pulse durations τ a , τ e .

[0030] To implement the valve control, the control system 20 must know the fluid pressures P a -P j present at the respective valves 14a-14j. Within the scope of the control method, the fluid pressures P a -P j present at the respective valves 14a-14j are determined, for example, via a pressure measurement and a pressure calculation based on the pressure measurement and taking into account the pressure change in the spray boom 12. During the calculation, the control system 20 can, for example, use a pressure drop model so that the actual fluid pressure P only needs to be measured at a central point in the spray boom 12 and the pressure drop is then calculated based on the line cross-section and / or the liquid output along the spray boom 12. In this way, it is possible to compensate for or at least reduce the influences of the fluid pressure differences at the valves 14a-14j by means of valve-specific duty cycles D a -D j.

[0031] The Fig. 2 also shows a system 10 for controlling valves 14a-14j of a spray boom 12. The control system 20 generates valve-specific voltage signals U a , U e for controlling the valves 14a-14j. The electrical lines 18a-18j, via which the control system 20 is connected to the valves 14a-14j, have different line lengths, so that different signal-specific voltage drops result when providing the voltage signals U a , U e due to different electrical resistances.

[0032] The bar chart shown below system 10 shows that the supply voltage level UV , i.e. the voltage level of the PWM pulses arriving at the valves 14a-14j, decreases from the rod center outwards due to the different lengths of the electrical lines 18a-18j. The voltages UV,a -UV,j applied to the respective valves 14a-14j when energized decrease from the rod center outwards. The voltages UV,a -UV,j applied to the respective valves 14a-14j when energized influence the switching behavior of the valves 14a-14j. Due to the falling voltages U v,a -U v,j , the switch-on and switch-off times of the valves 14a-14j vary from one another, so this must be compensated for with valve-specific duty cycles D a -D j .To compensate for or at least reduce the voltage-dependent influences on the valve-specific flow rate of spray fluid, the control system 20 generates valve-specific voltage signals U a , U e , which take into account the signal-specific voltage drop during signal provision. The voltage signal U a , which is provided to the external valve 14a, thus has a larger duty cycle D a than the voltage signal U e , which is provided to the valve 14e.

[0033] In order for the control system 20 to be able to take the signal-specific voltage drop into account to compensate for the voltage-dependent influences on the valve-specific flow rate of spray liquid, the control system 20 must be aware of the signal-specific voltage drop. Within the scope of the control method, the signal-specific voltage drops caused by the provision of the valve-specific voltage signals U a , U e are determined through one or more voltage measurements. The valve-specific duty cycle D a -D j is adjusted depending on the voltages U v,a -U v,j applied to the respective valves 14a-14j when the current is actively applied.

[0034] The Fig. 3 shows the time course of a voltage U a applied to a valve 14a, the time course of a current I a flowing through a valve 14a, and the time course of pressure P a , P a,h upstream and downstream of the valve 14a. In the diagrams, the voltage U, the current I, and the pressure P are plotted over time t. The valve 14a, not shown, is a solenoid valve.

[0035] The temporal voltage curve U a shows that the voltage signal is pulse width modulated. During an opening phase t Ö , a voltage pulse causes a current to be injected into the valve 14a, whereby a valve armature is moved from a closed position to an open position by the current injection during the opening phase t Ö . After the valve armature has reached the open position, a current is injected during a holding phase t H which is sufficient to hold the valve armature in the open position. To close the valve 14a, the voltage is first briefly inverted and then the voltage supply is interrupted during a closing phase t S . During the closing phase t G , the voltage signal is at zero level, so that no current is injected into the valve 14a. To open the valve 14a again, the closing phase t G is followed by the opening phase t Ö and the holding phase t H .

[0036] The pressure curve diagram shows that the fluid pressure P a applied to valve 14a is constantly at a high level and exhibits a plurality of pressure waves PW. The fluid pressure P a,h downstream of valve 14a increases during the opening phase t Ö and decreases again during the closing phase t S .

[0037] Pulse width modulation therefore creates pressure waves PW in the lines of the spray boom 12. The pressure waves PW influence the valve-specific flow rate during the opening phase t Ö and the holding phase t H . This effect is amplified by the fact that the pressure waves PW can overlap in the spray boom 12, thus further increasing the misapplication.

[0038] The Fig. 4 shows three wave-like pressure curves P a -P c at three valves 14a-14c over time t when no pressure wave compensation occurs.

[0039] Within the scope of the method, the generated valve-specific voltage signals are generated by the control system 20 taking into account the influences of pressure waves PW in the line sections of the spray boom 12 connected to the respective valves 14a-14j on the valve-specific flow rate, so that the valves 14a-14j are controlled in a coordinated or offset manner, whereby the pressure waves PW do not overlap and existing pressure waves PW are eliminated or at least weakened by newly generated pressure waves PW.

[0040] The effect of this pressure wave compensation is in the Fig. 5The deflection or amplitude of the pressure waves within the spray boom is significantly reduced by pressure wave compensation. When generating the valve-specific voltage signals U a , U e , the pressure wave propagation, namely the propagation velocity of the pressure waves PW in the spray boom 12, is taken into account to avoid or reduce pressure wave overlaps. Overlapping pressure waves PW can be avoided or at least reduced by appropriately controlling the valves 14a-14j via the flow velocity of the spray fluid in the spray boom 12 and the PWM frequency.When generating the valve-specific voltage signals U a , U e to avoid or reduce pressure wave superpositions, the control system takes into account the flow velocity of the spray liquid in the spray boom 12 and the line length of the fluid line between successive and series-arranged valves 14a-14j when generating the valve-specific voltage signals U a , U e. List of reference symbols

[0041] 10System 12Spray boom 14a-14jValves 16a-16jDispensing nozzles 18a-18jLines 20Control system D, D a -D j Duty cycles ICurrent I a Current PW Pressure waves PFluid pressure P a -P j Fluid pressures P a,h Fluid pressure tTime t Ö Opening phase t H Holding phase t S Closing phase t G Closed phase τ, τ a , τ e Pulse durations TPeriod duration USolation U a , U e Voltage signals UV Supply voltage level UV,a -UV,j Supply voltage levels

Claims

1. Method for controlling valves (14a-14j) of a spray boom (12) of an agricultural spreading machine, comprising the steps of: - generating valve-specific voltage signals (Ua, Ue) for controlling a plurality of valves (14a-14j) arranged on the spray boom (12); and - providing the valve-specific voltage signals (Ua, Ue) to the respective valves (14a-14j), characterized in that the generated valve-specific voltage signals (Ua, Ue) are pulse width modulated and each has a valve-specific duty cycle (Da-Dj), wherein the duty cycle corresponds to the quotient of pulse duration and period duration, wherein the valve-specific duty cycles (Da-Dj) compensate for, or at least reduce, the influences on the valve-specific flow rate of spray liquid which depend on the position of the respective valves (14a-14j) on the spray boom (12), wherein the duty cycles of each individual or all voltage signals differ from one another, despite identical target flow rates of spray liquid.

2. Method according to claim 1, characterized in that when generating the valve-specific voltage signals (Ua, Ue), the fluid pressure (Pa-Pj) applied to the respective valves (14a-14j) is taken into account in order to compensate for the pressure-dependent influences on the valve-specific flow rate of spray liquid.

3. Method according to claim 2, characterized by the step of: - determining the fluid pressures (Pa-Pj) applied to the respective valves (14a-14j), preferably via one or more pressure measurements in the spray boom (12).

4. Method according to claim 3, characterized in that the fluid pressures (Pa-Pj) applied to the respective valves (14a-14j) are determined via at least one pressure measurement and a pressure calculation which is based on the pressure measurement and takes into account the pressure change in the spray boom (12).

5. Method according to any of the preceding claims, characterized in that, when generating the valve-specific voltage signals (Ua, Ue), the signal-specific voltage drop, which is caused by providing the valve-specific voltage signals (Ua, Ue), is taken into account in order to compensate for the voltage-dependent influences on the valve-specific flow rate of spray liquid.

6. Method according to claim 5, characterized by the step of: - determining, via one or more voltage measurements, the signal-specific voltage drops caused by the valve-specific voltage signals (Ua, Ue).

7. Method according to any of the preceding claims, characterized in that the generated valve-specific voltage signals (Ua, Ue) compensate for, or at least reduce, influences of pressure waves (PW) on the valve-specific flow rate of spray liquid in the line sections of the spray boom (12) connected to the respective valves (14a-14j).

8. Method according to claim 7, characterized in that, when generating the valve-specific voltage signals (Ua, Ue), a pressure wave propagation, in particular the propagation velocity of pressure waves (PW), is taken into account in order to avoid or reduce pressure wave superposition in the spray boom (12).

9. Method according to claim 8, characterized in that when generating the valve-specific voltage signals (Ua, Ue), the flow velocity of the spray liquid in the spray boom (12) and the length of the fluid line between successive valves (14a-14j) arranged in series are taken into account in order to avoid or reduce pressure wave superpositions.

10. Method according to any of claims 7 to 9, characterized by the step of: - introducing pressure pulses into the lines of the spray boom (12) using a pressure accumulator in order to compensate for or reduce pressure waves (PW) in the spray boom (12).

11. System (10) for controlling valves (14a-14j) of a spray boom (12) of an agricultural spreading machine, in particular by means of a method according to any of the preceding claims, comprising - a plurality of valves (14a-14j) arranged on a spray boom (12); and - a control system (20) which is designed to generate valve-specific voltage signals (Ua, Ue) for controlling the valves (14a-14j) and to provide the valves (14a-14j) with the valve-specific voltage signals (Ua, Ue); characterized in that the control system (20) is designed to generate voltage signals (Ua, Ue) which are both pulse-width modulated and have a valve-specific duty cycle (Da-Dj), so that the influences on the valve-specific flow rate of spray liquid, which are dependent on the position of the respective valves (14a-14j) on the spray boom (12), are compensated for, or at least reduced, wherein the duty cycle corresponds to the quotient of pulse duration and period duration and wherein the duty cycles of individual or all voltage signals differ from one another, despite the identical target flow rate of spray liquid.

12. System (10) according to claim 11, characterized in that the control system (20) is designed, when generating the valve-specific voltage signals (Ua, Ue) - to take into account the fluid pressures (Pa-Pj) applied to the respective valves (14a-14j) in order to compensate for the pressure-dependent influences on the valve-specific flow rate of spray liquid, and / or - to take into account the signal-specific voltage drop caused by providing the valve-specific voltage signals (Ua, Ue) in order to compensate for the voltage-dependent influences on the valve-specific flow rate of spray liquid.

13. System (10) according to either claim 11 or claim 12, characterized in that the control system (20) is designed, when generating the valve-specific voltage signals (Ua, Ue), to take into account the pressure waves (PW) occurring in the line sections of the spray boom (12) connected to the respective valves (14a-14j) in order to compensate for the pressure wave-dependent influences on the valve-specific flow rate of spray liquid.