Automatic spraying with electronic controllers

The automation of operational processes such as spraying is increasingly present in Brazilian crops. However, it is necessary to be very attentive to electronic devices in order to obtain the desired phytosanitary control.

Automation can be defined as a set of operational processes that are controlled and executed through mechanical or electronic devices, being useful in controlling repetitive procedures. Boom sprayers, especially self-propelled ones, are increasingly making use of application volume automation, popularly also known as application rate. To function, onboard electronics are required, which have sensors, actuators and the electronic spray controller. As sensors and actuators are normally not as visible to operators, they end up forgetting their existence when adjusting or overhauling the machine, which can lead to the application of erroneous doses of phytosanitary products or interfere with the deposition of drops and, consequently, in control.

Spray controllers underwent major development in the 1990s. Initially, this equipment was used in large land-based sprayers, justified by the dilution of its price in the total cost of the sprayer. However, more recently, its use has been extended to trailed sprayers and even in equipment set up for specific applications, such as sugarcane cultivation. The controllers were initially developed only to manage the application volume, but began to integrate other functions such as satellite guidance, application management information and even droplet size control.

Electronic controllers

The minimum expected from electronic spray controllers is that they are capable of automatically adjusting the flow rate at the nozzles by varying the sprayer speed, keeping the application volume per unit area constant.

These controllers receive information from flow meters and/or circuit pressure (through the pressure transducer, popularly known as pressure switch), calculating the total instantaneous flow in the bar (L/min). The controllers also receive information from radars or pulse sensors installed on the sprayer's wheels, determining the machine's instantaneous speed. Speed ​​and flow information are integrated into the calculation of the instantaneous application volume.

Despite offering accuracy in correcting the application volume depending on speed variations, the controllers make this adjustment by changing the pressure in the hydraulic circuit, using an electric motor installed in the pressure regulating valve in the main line. Thus, increasing the sprayer speed causes an increase in flow rate (in order to maintain a constant application volume per unit area), due to the increase in pressure and, as a consequence, a decrease in droplet size. Opposite behavior occurs when decelerating. The consequent decrease in droplet size at a given moment of application may compromise the deposition of the solution due to drift. Increasing droplet size can compromise deposition in the lower stratum of the culture.

Therefore, the operator must be careful not to vary the speed too much. A worrying scenario is when the application is carried out crossing terraces, an increasingly common practice as it increases the operational field capacity of the sprayer (“return”). This problem becomes worse when a low application volume is used (such as 70 L/ha or less). As a practical example, if a sprayer is calibrated to operate at 15km/h with a pressure of 45 PSI (Pound Per Square Inche or lb/in², equivalent to 3,1 BAR), accelerating to 20 km/h will increase the pressure to 80 PSI, while reducing to 10 km/h will reduce it to 20 PSI. This magnitude of pressure variation may be enough to compromise the success of the application, as they cause significant changes in the size of the drops generated by hydraulic nozzles (most used throughout Brazil).

Therefore, it is important that the tip selected allows small variations in pressure without excessively affecting the size of the droplets. Unfortunately, the producer invests little in tips and, often, the cause of inefficient control is due to inadequate use of the tips. The relationship between pressure and droplet size is found in the technical tables of the tips and must be made available by the manufacturers. Extended-use fan tips are the most used in land-based boom sprayers, as they are cheaper, however, they have a greater variation in droplet size. It is recommended to use more specific tips depending on the purpose of the applications, such as an air induction tip if the application allows thick to very thick drops, or a tip with pre-orifice technology, which tend to not be as susceptible to pressure variations in relation to the size of drops formed.

Flow sensor

Most sprayers with on-board electronics on the market have turbine-type flowmeters, as they are cheaper and have good accuracy when in good condition. Some commercial equipment uses this electromagnetic type sensor. Turbine-type flowmeters measure the instantaneous flow by counting the revolutions of a small propeller inside their body, comparing it with a previous calibration carried out with a known volume. Electromagnetic flowmeters work in a similar way to the turbine type, but without moving parts inside the sensor, resulting in less maintenance and a longer useful life. With less frequent use, another sensor used to infer the instantaneous flow of the system is the pressure transducer (pressure switch). Special care with this sensor is related to its maximum working pressure. Exposure to pressure greater than that accepted irreversibly damages it, consequently affecting the volume of application.

A periodic check of the moving parts of turbine-type flowmeters is recommended (during crop inspection, for example), as there is natural wear on the propeller due to the time of service and the abrasiveness of the phytosanitary products sprayed, especially those sold in vehicles. solid, such as wettable powders. Therefore, flowmeters have a defined useful life and must be replaced according to the manufacturer's recommendations. Worn or defective flowmeters lead to calculation errors on the part of the controller, causing incorrect application volumes to be sprayed, compromising target control. Nor should one type of sensor be replaced by another without correctly reconfiguring the electronic controller.

Another precaution to consider when using a turbine-type sensor is related to the application of mixtures of phytosanitary products in the mixture. Some mixtures present incompatibility, and even if small to the naked eye, they can modify the physical characteristics of the mixture and adhere to the flowmeter propeller, altering the flow reading in relation to the calibrated constant. Therefore, if the mixture is adhering to the tank or piping, it is most likely also adhering to the sensor propeller, requiring more frequent cleaning.

Speed ​​sensor

The most common sensors for calculating speed are pulse sensors installed on the sprayer wheels, but there are also radars and the use of GPS receivers. Pulse sensors are inexpensive, virtually maintenance-free and work well, especially when there are many holes or magnets in the sprayer wheels. Each hole or magnet results in a pulse sent to the electronic controller, so the more pulses, the more accurate the calculated speed will be. However, an important point to consider is that pulse sensors are normally installed on only one wheelset, resulting in erroneous speed calculations when the equipment operates on long curves, as the internal wheelsets have a lower rotation (angular speed). Another important problem to consider in operation is the slipping of the wheelset where the pulse sensors are installed. Hence the need to calibrate the speed sensor in the field where spraying will occur, preferably under the same soil moisture conditions and compaction level. These characteristics do not become a concern when installing radars on equipment. However, radar readings may be influenced by excessive variations in the vegetation cover of the area being worked on.

Some electronic controllers use the signal from a GPS antenna to obtain speed. This technique is interesting, as the system does not need calibration, just as it offers the real speed on the field, that is, wheel slippage does not interfere with the speed reading. They generally use a GPS navigation antenna, which does not have signal problems. The only caution is with its use at extremely low speeds, such as 1 or 2 km/h, as GPS errors can greatly influence the speed calculation, however, self-propelled vehicles operate at speeds much higher than these.

Future trend

The spray tip must work at a pressure regime within the range where droplet size is maintained, but this does not always occur in practice. Thus, one of the alternatives for varying the flow rate without affecting the size of the droplets is by using PWM valves (in English “Pulse-Width Modulation”), which modulate the time between opening and closing the nozzle. This technology allows control of the flow rate and operating pressure of each tip during application, keeping the application volume constant (or varying, if the objective is to apply at variable VRT rates), without significantly affecting the droplet size. The PWM works by electronically controlling the opening and closing of the flow at each end, causing the flow to start and quickly stop. This control is carried out by the presence of a solenoid valve in the nozzle that can open and close up to 10 times per second. The length of time the valve remains open and closed in each cycle determines the application volume. Thus, the operator can vary the travel speed, such as when crossing terraces, without changing the application volume and droplet size.

Fábio HR Baio, UFMS; Ulisses R. Antuniassi, FCA/UNESP

Article published in issue 158 of Cultivar Máquinas.