Research Progress on Intelligent Variable-Rate Spray Technology for Precision Agriculture

Abstract

Conventional continuous pesticide application remains prevalent in agriculture, but its limitations in addressing the spatial–temporal variability of biotic stressors have led to excessive chemical inputs and inefficiency. The emergence of precision agriculture has catalyzed significant advancements in variable-rate spray systems to optimize agrochemical deployment through real-time modulation. This technology demonstrates critical advantages in minimizing the environmental footprint while maintaining crop protection efficacy. Our systematic review analyzes three foundational variable-rate spray architectures—pressure-regulated, flow rate-regulated, and pesticide concentration-regulated mechanisms—evaluating their maturity and implementation paradigms. Pressure-regulated technology relies on the pressure–flow relationship to achieve regulation, but there is a narrow range in flow regulation, atomization stability is insufficient, and there are other defects. Flow rate-regulated technology achieves precise control through the dynamic adjustment of the nozzle orifice area or Pulse-Width Modulation duty cycles, but this technology faces mechanical wear, a nonlinear flow–duty cycle relationship, and other challenges. Pesticide concentration-regulated technology is centered on real-time mixing, which can avoid the residue of chemicals but is highly dependent on fluid characteristics and mixing efficiency. This study proposes improvement paths from the perspectives of hardware optimization, control strategy integration, and material innovation. Through the summary and analysis of this paper, we hope to provide valuable references for future research on variable-rate spray technology.

1. Introduction

Since their introduction, chemical pesticides have long been regarded as rigid technological tools for pest and weed control because of their advantages, such as high control efficiency and fast-acting speed [1,2,3,4,5]. However, inappropriate application of chemical pesticides can lead to problems such as pesticide drift and residues, which are mainly caused by the movement of droplets with the wind, the excessive height between the spray nozzle and the crop, and the excessive use of pesticides. These issues pose a threat to the stability of farmland ecosystems and are contrary to the goal of sustainable agricultural development [6,7,8]. Of particular concern is the fact that traditional pesticide application techniques commonly use fixed-dose, whole-area spraying strategies, which neither include crop growth information nor take into account the spatial heterogeneity characteristics of pests, diseases, and weeds and are the main reasons for the overuse of pesticides and the low effective utilization rate [9,10,11].

With the deepening of the concept of precision agriculture, variable-rate spray technology (VRST), which integrates Internet of Things (IoT), big data, and intelligent equipment, provides an innovative path to solve the above dilemmas [12,13]. By analyzing multi-dimensional information from the operation scene in real time (including the severity of pests and diseases, crop growth parameters, and dynamic parameters of the operation machinery, etc.), the technology constructs a multi-variable coupling decision-making model to realize precise matching between the amount of pesticide deposition and biological demand. Compared with traditional application modes, VRST technology can effectively reduce pesticide residues, improve pesticide use efficiency, and realize pesticide reduction and efficiency, which has become the core direction of global agricultural engineering research [14,15,16,17].

The information perception system of VRST mainly relies on sensor technology to obtain data, and the commonly used sensors are machine vision, LiDAR, ultrasonic sensors, etc. Current research generally adopts a multi-sensor data fusion strategy to combine sensors according to the detection object and environment; for example, machine vision can analyze vegetation coverage and pest distribution, LiDAR accurately reconstructs 3D canopy structures, and ultrasonic sensors dynamically monitor the application distance, etc. This research will not be repeated in this study [18,19,20]. According to the differences in the regulation mechanism of spray variables, VRST can be divided into three types of technology systems: pressure-regulated, flow rate-regulated, and pesticide concentration-regulated. This paper systematically analyzes the working principles and system architecture of these three types of technologies, analyzes their application scenarios and technical bottlenecks based on comparative field trial data, and ultimately puts forward optimization suggestions for smart agriculture with a view to providing theoretical references and practical directions for the iterative upgrading and large-scale diffusion of VRST.

2. Materials and Methods

This systematic review employed a structured methodology to ensure rigor and minimize bias. The process involved identifying the relevant literature, screening records based on predefined criteria, and conducting a full-text assessment to determine final inclusion. This approach prioritized transparency and reproducibility throughout the review process. The review focused on journal articles published between 1995 and 2025 that investigated the practical application of VRST in field crop or orchard settings.

2.1. Identification and Search Strategy

The Scopus database was selected for the literature search due to its comprehensive coverage of the scientific literature. A detailed search strategy was developed using a combination of keywords related to “variable-rate spray”, “variable-rate application”, “precision spraying”, and so on, for a total of 51 keywords (as shown in Figure 1). These terms were strategically combined using Boolean operators (AND/OR) and applied to the Title, Abstract, and Keyword fields within Scopus. The search was filtered to include only journal articles published between 1995 and 2025, excluding review articles, books/book chapters, letters, errata, data papers, and short surveys/notes. This initial search yielded 116 records.

2.2. Screening

The titles and abstracts of the 116 identified records were independently evaluated by the authors against three key inclusion criteria: 1. the study explicitly focused on or utilized VRST; 2. application was performed using ground equipment; 3. the study talked about spray systems. Records that did not meet all three criteria were excluded. Following this screening, 52 articles were retained for further assessment.

2.3. Reading

The full texts of the 52 screened articles were retrieved and roughly read. Articles were excluded based on the following reasons: 1. the full text was unavailable (n = 8); 2. the study did not involve practical field or orchard evaluations of VRST effectiveness, focusing instead solely on simulation, model, and algorithm development without field validation or non-VRST spraying techniques (n = 22). This full-text assessment resulted in the final inclusion of 22 articles, which formed a preparatory dataset for analysis.

2.4. Selecting and Summarizing

The complete information (title, authors, abstract, and so on) of the 22 articles that met the inclusion criteria through identification, screening, and full-text assessment were subsequently transferred to another Excel spreadsheet, and their full texts were stored in PDF format in a folder. By carefully rereading the entire text of each article, each article was categorized according to three types of VRST. The authors found that 17 of the articles were about flow-regulated VRST, while only 3 and 2 articles were about pressure-regulated VRST and pesticide concentration-regulated VRST. Therefore, the authors conducted a second search in Google Scholar using the all-field query (as shown in Figure 1). There is limited heterogeneity in the themes related to VRST, which can be summarized as follows: aspects related to VRST components, comparisons of different VRST operating parameters, and comparisons with traditional spraying methods. We discarded many articles that repeated the same technology (only changing the application objects or some spray parameters, etc.). Of course, we read all of these articles as the basis for this review but just did not summarize them in tables (there were a total of 13 articles on pressure-regulated VRST, 45 articles on flow-regulated VRST, and 24 articles on pesticide concentration-regulated VRST, with a total of 82 articles). Ultimately, from the preparatory datasets retrieved in these two searches, we selected several representative articles for each technology as the analysis objects for this review, which are summarized in the tables below (6 articles on pressure-regulated VRST, 15 articles on flow rate-regulated VRST, and 15 articles on pesticide concentration-regulated VRST).

3. Pressure-Regulated Variable-Rate Spray Technology

Pressure-regulated VRST utilizes the nonlinear relationship between pressure difference and flow rate in fluid mechanics to achieve precise control of the application flow rate by dynamically adjusting the line pressure. This technology has become the preferred solution for upgrading the variable application function of small and medium-sized plant protection machinery due to the advantages of its simple structure and low retrofit cost, and some representative studies are summarized in Table 1. Regardless of the VRST technology, the VRST system generally contains a liquid supply unit (tank, filter, agitator, valve group, pipeline, etc.), a power unit (centrifugal pump, diaphragm pump, etc.), sensors (flowmeter, pressure sensor, GPS positioning device, etc.), a control unit (PLC, microcontroller, embedded controller, etc., with control algorithms including PID, fuzzy logic, neural network, etc.), and a spray unit (spray bar, nozzle, etc.), as shown in Figure 2. The pressure regulator of a pressure-regulated variable-rate spray system mainly includes a proportional relief valve/pressure-reducing valve, ball valve, butterfly valve, etc. The technology still faces the following challenges in practical application: (1) The pressure difference in the system is proportional to the square of the flow rate, making it difficult to cover a wide range of flow rate adjustments. (2) Changes in pressure directly affect the distribution of droplet size. Droplet size is critical in plant protection, as it directly determines the target deposition efficiency, environmental safety, and pest control efficacy. When droplet size is too large, its coverage density is insufficient, penetration is poor, and the pesticide droplet is prone to rolling off the leaf surface. Conversely, when droplet size is too small, its depositing speed is slow, and the horizontal drift distance increases exponentially under the influence of wind. Additionally, high temperatures and low humidity accelerate droplet evaporation, further reducing droplet size and expanding the pollution range, posing a threat to adjacent crops and ecological safety. (3) The response speeds of traditional pressure regulators are slow, making it difficult to meet the real-time regulation requirements of high-speed operating machinery. In order to break through the above bottlenecks, future research needs to focus on the following: (1) at the hardware level, developing the pressure adaptive compensation function of the composite structure of the nozzle, with the optimization of the internal flow channel and other ways to reduce the sensitivity of the pressure change on the spray angle and droplet size; (2) at the level of the control strategy, integrating machine vision and other sources of sensor data to build a feedforward–feedback composite control model to achieve the dynamic matching of the travel speed, attitude changes, and pressure settings.

Table 1. Overview of pressure-regulated VRST research.

Reference	Year	System Components and Working Principle	Control and Spray Performance
[21]	2003	It consists of a liquid tank, pump, GPS positioning device, SCS750 controller, flow meter, speed sensor, butterfly valve, and flow regulator valve. It receives real-time location information through GPS to match the preset application map, and the controller adjusts the butterfly valve opening based on the feedback from the flow meter and the speed sensor to change the nozzle pressure to realize flow control.	The average application error is ±2.25%, and the pressure adjustment response time is about 1.5 s. Combined with the GPS signal delay (0.5 s) and the preset 3 s “look-ahead time” algorithm, the total delay is about 2.35 s, which can trigger changes in the application of chemicals 0.65 s in advance, basically meeting the demand for accurate application of chemicals. However, the preset fixed look-ahead time algorithm cannot fully compensate for delay changes under dynamic field conditions and cannot eliminate delay differences between nozzles due to differences in piping lengths.
[22]	2004	It consists of a liquid tank, pump, proportional relief valve, and several parallel spraying units; each spraying unit contains a proportional pressure-reducing valve, solenoid valve, pressure-type variable-rate spray nozzle, pressure sensor, and flow sensor. The proportional relief valve stabilizes the pressure of the main line, the proportional pressure-reducing valve regulates the pressure of each unit independently, the hydraulic spray nozzle changes the flow rate according to the input pressure.	MATLAB 6.5 simulation shows that its step response time is 0.2 s, frequency width is 4.5 Hz, input current and output flow rate are linear, and it can dynamically adjust the application volume according to the target characteristics and the speed of the unit and, at the same time, keep the droplet size and distribution quality stable so as to realize the precise application of medicine. The disadvantage is that the inherent frequency of the proportional pressure-reducing valve limits the ability of the high-frequency dynamic response, and the actual flow rate adjustment range is constrained by the nozzle pressure–flow characteristics.
[23]	2005	It consists of a liquid tank, centrifugal pump, RavenSCS440 controller, on-board radar, GPS positioning unit, flow meter, and ball valve. Fifteen sets of three-position nozzle bodies are mounted on the spray bar, utilizing TurboTeeJet 11002, 11003, and 11004 series nozzles. The system regulates the main medicament pipeline flow through a flow meter and quick-closing ball valve.	The system was able to achieve weed management in 7.6 m × 7.6 m grid cells by adjusting pressure and speed to vary the application rate per unit area (140–336 L/ha), with herbicide rates of 21–98% of conventional rates. However, the following limitations existed: delayed nozzle switching resulted in uneven application in the 1.5–2.8 m transition zone, fast closing valves were prone to instantaneous application spikes of 450 L/ha, and the controller deadband setting (3% deviation tolerance) left small application rate changes unresponsive.
[24]	2012	It consists of two switching solenoid valves, a proportional solenoid valve, liquid tank, double membrane pump, pressure sensor, and ATmega64 embedded controller. The system dynamically regulates the pressure through a nonlinear PI controller.	The average tracking error is less than 0.3 bar over the 2–14 bar operating range, and the controller has better resistance to pump pressure pulsation noise than a fixed-parameter PI controller (4.4 Hz low-pass filtering is required). However, the polynomial fitting of the controller parameters is only applicable to the 4–14 bar range, and the low-pressure region needs to be initialized manually; the pressure sensor noise affects control accuracy.
[25]	2020	It consists of a liquid storage container, motor pump set, overflow regulator, liquid pipeline, flow meter, and pressure gauge. Flow control is realized by adjusting the system pressure.	The flow rate adjustment range was narrow (about 2:1), and when the pressure was increased to increase the flow rate, the spray angle increased at a rate of 1.08°/%, resulting in the expansion of droplet spatial coverage; the droplet sizes (DV0.1, DV0.5, and DV0.9) decreased dramatically with increasing pressure at a rate of −1.023, −1.562, and −3.609 μm/%, respectively, resulting in a sharp decrease in droplet spectra toward small particle sizes and an increased risk of drift.
[26]	2021	It consists of a reservoir, a diaphragm pump, a 12-foot spray bar with six solenoid valve nozzle SVNs spaced 22 inches apart, and an Atmega2650-based controller. The system achieves pressure control by PWM (Pulse-Width Modulation) regulation of the diaphragm pump voltage using two control strategies, PID (Proportional Integral Derivative) and cascade feedback, and combining feedforward compensation with a polynomial model of the number of SVNs (Solenoid Valve Nozzles) activated and pressure perturbation.	The cascade control has an overshoot rate of 7.93% (19.61% for PID) at a target pressure of 16 psi, with a pressure regulation time of about 2 s, ensuring a constant flow rate (1.27–4.08 L/min) with a stable spray angle (43–120°) from a single nozzle and a reduction in pesticide use of 75%. However, the diaphragm pump’s maximum pressure of 26 psi limits the system’s flow scalability, and there is still brief pressure fluctuation when the nozzle is quickly switched (about 0.5 s to recover).

Table 1. Overview of pressure-regulated VRST research.

Reference	Year	System Components and Working Principle	Control and Spray Performance
[21]	2003	It consists of a liquid tank, pump, GPS positioning device, SCS750 controller, flow meter, speed sensor, butterfly valve, and flow regulator valve. It receives real-time location information through GPS to match the preset application map, and the controller adjusts the butterfly valve opening based on the feedback from the flow meter and the speed sensor to change the nozzle pressure to realize flow control.	The average application error is ±2.25%, and the pressure adjustment response time is about 1.5 s. Combined with the GPS signal delay (0.5 s) and the preset 3 s “look-ahead time” algorithm, the total delay is about 2.35 s, which can trigger changes in the application of chemicals 0.65 s in advance, basically meeting the demand for accurate application of chemicals. However, the preset fixed look-ahead time algorithm cannot fully compensate for delay changes under dynamic field conditions and cannot eliminate delay differences between nozzles due to differences in piping lengths.
[22]	2004	It consists of a liquid tank, pump, proportional relief valve, and several parallel spraying units; each spraying unit contains a proportional pressure-reducing valve, solenoid valve, pressure-type variable-rate spray nozzle, pressure sensor, and flow sensor. The proportional relief valve stabilizes the pressure of the main line, the proportional pressure-reducing valve regulates the pressure of each unit independently, the hydraulic spray nozzle changes the flow rate according to the input pressure.	MATLAB 6.5 simulation shows that its step response time is 0.2 s, frequency width is 4.5 Hz, input current and output flow rate are linear, and it can dynamically adjust the application volume according to the target characteristics and the speed of the unit and, at the same time, keep the droplet size and distribution quality stable so as to realize the precise application of medicine. The disadvantage is that the inherent frequency of the proportional pressure-reducing valve limits the ability of the high-frequency dynamic response, and the actual flow rate adjustment range is constrained by the nozzle pressure–flow characteristics.
[23]	2005	It consists of a liquid tank, centrifugal pump, RavenSCS440 controller, on-board radar, GPS positioning unit, flow meter, and ball valve. Fifteen sets of three-position nozzle bodies are mounted on the spray bar, utilizing TurboTeeJet 11002, 11003, and 11004 series nozzles. The system regulates the main medicament pipeline flow through a flow meter and quick-closing ball valve.	The system was able to achieve weed management in 7.6 m × 7.6 m grid cells by adjusting pressure and speed to vary the application rate per unit area (140–336 L/ha), with herbicide rates of 21–98% of conventional rates. However, the following limitations existed: delayed nozzle switching resulted in uneven application in the 1.5–2.8 m transition zone, fast closing valves were prone to instantaneous application spikes of 450 L/ha, and the controller deadband setting (3% deviation tolerance) left small application rate changes unresponsive.
[24]	2012	It consists of two switching solenoid valves, a proportional solenoid valve, liquid tank, double membrane pump, pressure sensor, and ATmega64 embedded controller. The system dynamically regulates the pressure through a nonlinear PI controller.	The average tracking error is less than 0.3 bar over the 2–14 bar operating range, and the controller has better resistance to pump pressure pulsation noise than a fixed-parameter PI controller (4.4 Hz low-pass filtering is required). However, the polynomial fitting of the controller parameters is only applicable to the 4–14 bar range, and the low-pressure region needs to be initialized manually; the pressure sensor noise affects control accuracy.
[25]	2020	It consists of a liquid storage container, motor pump set, overflow regulator, liquid pipeline, flow meter, and pressure gauge. Flow control is realized by adjusting the system pressure.	The flow rate adjustment range was narrow (about 2:1), and when the pressure was increased to increase the flow rate, the spray angle increased at a rate of 1.08°/%, resulting in the expansion of droplet spatial coverage; the droplet sizes (DV0.1, DV0.5, and DV0.9) decreased dramatically with increasing pressure at a rate of −1.023, −1.562, and −3.609 μm/%, respectively, resulting in a sharp decrease in droplet spectra toward small particle sizes and an increased risk of drift.
[26]	2021	It consists of a reservoir, a diaphragm pump, a 12-foot spray bar with six solenoid valve nozzle SVNs spaced 22 inches apart, and an Atmega2650-based controller. The system achieves pressure control by PWM (Pulse-Width Modulation) regulation of the diaphragm pump voltage using two control strategies, PID (Proportional Integral Derivative) and cascade feedback, and combining feedforward compensation with a polynomial model of the number of SVNs (Solenoid Valve Nozzles) activated and pressure perturbation.	The cascade control has an overshoot rate of 7.93% (19.61% for PID) at a target pressure of 16 psi, with a pressure regulation time of about 2 s, ensuring a constant flow rate (1.27–4.08 L/min) with a stable spray angle (43–120°) from a single nozzle and a reduction in pesticide use of 75%. However, the diaphragm pump’s maximum pressure of 26 psi limits the system’s flow scalability, and there is still brief pressure fluctuation when the nozzle is quickly switched (about 0.5 s to recover).

4. Flow Rate-Regulated Variable-Rate Spray Technology

Flow rate-regulated VRST is currently based on variable nozzle and PWM technology in two ways. It is based on pressure-regulated variable-rate spray systems, one of which uses variable nozzles for spraying, and the other adds a PWM signal generator and actuators to turn the spray on/off.

4.1. Variable Nozzle

VRST based on variable nozzles achieves precise control of the flow rate by dynamically adjusting the spray orifice morphology or area. The advantages of this technology include a high flow rate adjustment ratio, a fast response, and controllability of droplet parameters (droplet size, distribution, and spray angle). Some representative studies are summarized in Table 2, and the structure of several variable nozzles is shown in Figure 3. Currently, variable nozzles are mainly classified into three types: mechanical (e.g., conical sleeve plunger nozzles, which are regulated by pressure and a spring balance), smart material (e.g., magnetorheological fluid nozzles, which utilize a magnetic field to change the viscosity of the fluid), and multistage modules (e.g., triple-layer atomizing disks that are regulated hierarchically by layered on/off functions). The limitations of this technology include the following: (1) the long-term wear and tear of mechanical parts (e.g., plunger, lever) cause a decline in adjustment accuracy; (2) magnetorheological fluid and other materials have limited ranges in flow rate adjustment and risk residue buildup; (3) complex structures (such as a multi-layer atomizing disks) increase the difficulty of maintenance. Future directions of improvement should focus on (1) developing stainless steel or ceramic composites to enhance the wear resistance of moving parts; (2) optimizing the nozzle runner design to alleviate the problem of spray angle attenuation at low flow rates; and (3) developing new smart materials for wide-ranging flow adjustment and lightweight nozzle structure designs.

Table 2. Overview of variable nozzle-based VRST research.

Reference	Year	Variable Nozzle Components and Working Principle	Control and Spray Performance
[27,28]	2001, 2002	It consists of a conical nozzle body sleeve, split-end metering plunger, diaphragm actuator, and pressure control port. Its core is the longitudinal displacement adjustment of the split-end plunger within the conical sleeve, which dynamically adjusts the flow rate by changing the nozzle opening through the balance between the control pressure and the supply pressure.	The nozzle flow rate adjustment range is 0.227–3.028 L/min, and the adjustment ratio reaches 13.3:1, which can obtain a DV0.5 of 141–522 μm while maintaining spray distribution uniformity. However, the spray angle decreases significantly at low flow rates (the angle of a 90° nozzle decreases to 65° at a flow rate of 0.227 L/min), which affects the coverage width. Also, the plunger position is sensitive to pressure balance, and the spring constants and diaphragm design may cause adjustment complexity.
[29]	2005	It consists of a variable area pre-hole, a variable area spray hole, a flexible nozzle, a metering assembly (with wedge-shaped groove structure), a diaphragm, a spring, and a nozzle body. The system makes use of the dynamic balance between liquid pressure and spring force to adjust the position of the metering component and synchronously change the area of the pre-hole and spray hole.	The pre-orifice controls flow (0.15–0.80 GPM at 15–50/65 PSI pressure) and the spray orifices maintain a constant spray angle (110°) and optimized droplet size (325–425 μm for systemic pesticides VMD, 200–240 μm for contact pesticides) by lever-adjusting the V-cut angle, with a response time of <0.25 s and a coefficient of variation for uniformity of spray distribution of 8 to 13%. However, the mechanical adjustment structure is at risk of wear and tear under high-frequency use, and the ease of operation is limited by the need to change nozzles for different pesticide types.
[30]	2007	It consists of a spray tip with a variable orifice, a metering device (with adjusting lever and wedge groove), a diaphragm, a spring, and a nozzle body, which dynamically adjusts the area of the spray orifice to realize flow control through pressure changes.	The system achieves flow regulation ratios of 12:1 and 10:1 for the black and clear nozzles, respectively, over a pressure range of 10–110 psi. Field tests showed that the nozzle droplet size fluctuations were small (VMD 498–621 μm for the black nozzle and 465–599 μm for the clear nozzle) under the conditions of a 4–12 mph variable speed and a 4–12 GPA variable application rate. However, at pressures above 40 psi, the droplet size is out of the standard range, and the system relies on precise pressure control.
[31]	2019	It consists of a water inlet, magnetorheological fluid cavity, coil winding axis, elastic membrane, cyclone cavity, and outer shell, etc. Its core function is to control the viscosity and distribution pattern of magnetorheological fluid through the magnetic field generated by the coil and then adjust the inlet cross-sectional area of the nozzle to realize flow control.	By adjusting the magnetic field strength with 0–28 V, the system can achieve a 25% change in flow rate at a magnetorheological fluid injection volume of 1.5 mL with a fast and reversible response. Under the constant pressure of 0.3 MPa, the flow rate decreases by 28.57% when the magnetorheological fluid injection volume is increased from 0 to 2.5 mL, while it decreases by only 14.29% under 24 V, which shows that the magnetic field can effectively inhibit flow rate attenuation. However, the system has limitations, such as a limited flow rate adjustment range (maximum 25%), magnetic field saturation effect (the flow rate stabilizes after the voltage exceeds 24 V), and the large volume and weight of the nozzle (50 mm diameter of the casing), and the magnetorheological fluid may produce residuals that affect stability in long-term use.
[32]	2023	Composed of three layers of independent atomization disks, each layer is equipped with a liquid supply pipe, solenoid valve, and peristaltic pump, using STM32F103-embedded controller output adjustable PWM wave control peristaltic pump flow combined with the combination of an on/off solenoid valve to achieve 4 levels of flow regulation (1 level of only upper level open, 2 levels of the upper layer + middle, 3 levels of the upper layer + the bottom, 4 levels of full open).	By adjusting the motor voltage to control the rotational speed of the atomizing disk (5540–8395 r/min), the spray width was stabilized at 1.98 m, the amount of droplet deposition increased from 1183.2 mL to 2696.5 mL with the flow rate level, and the droplet particle size continued to decrease with the increase in rotational speed (187.6–113.2 μm), and the distribution of the deposition was in accordance with the normal distribution. However, the study only verified the performance of the clear water medium and did not address the effect of actual agent viscosity; the three-layer independent structure increased mechanical complexity, and there may be sealing problems in long-term use.

Table 2. Overview of variable nozzle-based VRST research.

Reference	Year	Variable Nozzle Components and Working Principle	Control and Spray Performance
[27,28]	2001, 2002	It consists of a conical nozzle body sleeve, split-end metering plunger, diaphragm actuator, and pressure control port. Its core is the longitudinal displacement adjustment of the split-end plunger within the conical sleeve, which dynamically adjusts the flow rate by changing the nozzle opening through the balance between the control pressure and the supply pressure.	The nozzle flow rate adjustment range is 0.227–3.028 L/min, and the adjustment ratio reaches 13.3:1, which can obtain a DV0.5 of 141–522 μm while maintaining spray distribution uniformity. However, the spray angle decreases significantly at low flow rates (the angle of a 90° nozzle decreases to 65° at a flow rate of 0.227 L/min), which affects the coverage width. Also, the plunger position is sensitive to pressure balance, and the spring constants and diaphragm design may cause adjustment complexity.
[29]	2005	It consists of a variable area pre-hole, a variable area spray hole, a flexible nozzle, a metering assembly (with wedge-shaped groove structure), a diaphragm, a spring, and a nozzle body. The system makes use of the dynamic balance between liquid pressure and spring force to adjust the position of the metering component and synchronously change the area of the pre-hole and spray hole.	The pre-orifice controls flow (0.15–0.80 GPM at 15–50/65 PSI pressure) and the spray orifices maintain a constant spray angle (110°) and optimized droplet size (325–425 μm for systemic pesticides VMD, 200–240 μm for contact pesticides) by lever-adjusting the V-cut angle, with a response time of <0.25 s and a coefficient of variation for uniformity of spray distribution of 8 to 13%. However, the mechanical adjustment structure is at risk of wear and tear under high-frequency use, and the ease of operation is limited by the need to change nozzles for different pesticide types.
[30]	2007	It consists of a spray tip with a variable orifice, a metering device (with adjusting lever and wedge groove), a diaphragm, a spring, and a nozzle body, which dynamically adjusts the area of the spray orifice to realize flow control through pressure changes.	The system achieves flow regulation ratios of 12:1 and 10:1 for the black and clear nozzles, respectively, over a pressure range of 10–110 psi. Field tests showed that the nozzle droplet size fluctuations were small (VMD 498–621 μm for the black nozzle and 465–599 μm for the clear nozzle) under the conditions of a 4–12 mph variable speed and a 4–12 GPA variable application rate. However, at pressures above 40 psi, the droplet size is out of the standard range, and the system relies on precise pressure control.
[31]	2019	It consists of a water inlet, magnetorheological fluid cavity, coil winding axis, elastic membrane, cyclone cavity, and outer shell, etc. Its core function is to control the viscosity and distribution pattern of magnetorheological fluid through the magnetic field generated by the coil and then adjust the inlet cross-sectional area of the nozzle to realize flow control.	By adjusting the magnetic field strength with 0–28 V, the system can achieve a 25% change in flow rate at a magnetorheological fluid injection volume of 1.5 mL with a fast and reversible response. Under the constant pressure of 0.3 MPa, the flow rate decreases by 28.57% when the magnetorheological fluid injection volume is increased from 0 to 2.5 mL, while it decreases by only 14.29% under 24 V, which shows that the magnetic field can effectively inhibit flow rate attenuation. However, the system has limitations, such as a limited flow rate adjustment range (maximum 25%), magnetic field saturation effect (the flow rate stabilizes after the voltage exceeds 24 V), and the large volume and weight of the nozzle (50 mm diameter of the casing), and the magnetorheological fluid may produce residuals that affect stability in long-term use.
[32]	2023	Composed of three layers of independent atomization disks, each layer is equipped with a liquid supply pipe, solenoid valve, and peristaltic pump, using STM32F103-embedded controller output adjustable PWM wave control peristaltic pump flow combined with the combination of an on/off solenoid valve to achieve 4 levels of flow regulation (1 level of only upper level open, 2 levels of the upper layer + middle, 3 levels of the upper layer + the bottom, 4 levels of full open).	By adjusting the motor voltage to control the rotational speed of the atomizing disk (5540–8395 r/min), the spray width was stabilized at 1.98 m, the amount of droplet deposition increased from 1183.2 mL to 2696.5 mL with the flow rate level, and the droplet particle size continued to decrease with the increase in rotational speed (187.6–113.2 μm), and the distribution of the deposition was in accordance with the normal distribution. However, the study only verified the performance of the clear water medium and did not address the effect of actual agent viscosity; the three-layer independent structure increased mechanical complexity, and there may be sealing problems in long-term use.

4.2. Pulse-Width Modulation

PWM technology was initially used in industrial production and was subsequently applied to flow regulation in areas such as pesticide spraying and irrigation [33,34]. PWM-based VRST adjusts the duty cycle and frequency of actuators to achieve precise control of the flow rate. The advantages of this technology include the following: a wide range of flow rates; a fast response speed; compatibility with existing types of nozzles (fan, hollow cone, etc.); and low impact on droplet size. With these advantages, PWM technology has become the most widely used VRST today. Some representative studies are summarized in

Table 3. Overview of PWM-Based VRST research.

Reference	Year	System Components and Working Principle	Control and Spray Performance
[41]	1996	Consisting of a solenoid valve (mounted upstream of the nozzle), an electronic interface unit (generates a PWM signal with a phase difference of 180° in order to reduce the deposition gap), a closed-loop pressure control loop (regulates droplet size), and a GPS positioning module, it can be integrated into existing spray controllers to enable retrofitting.	The flow rate can be adjusted up to 10:1 at fixed pressure (30:1 in combination with pressure adjustment) with a dynamic response speed of more than 5 Hz. The coefficient of variation for lateral deposition uniformity for PWM intermittent sprays ranges from 15 to 23%. Major limitations include intermittent spraying results in poorer deposition uniformity compared with continuous spraying systems, and it relies on nozzle overlap compensation; high system complexity involving the integration of multiple modules (pressure control, PWM signal generation, GPS interface) and retrofitting to existing equipment.
[42]	2009	It consists of a proportional adjustment solenoid valve, 24 kHz high-frequency PWM drive circuit, liquid supply system, and typical nozzles (fan, hollow cone, solid cone). The core function of the system is to control solenoid valve opening to realize the continuous adjustment of the flow rate by adjusting the duty cycle (100–40%) through a DRV101 chip.	At 0.3 MPa pressure, the fan, hollow cone, and solid cone nozzle have a flow regulation ratio of 7.14:1, 3.57:1, and 3.70:1, respectively; the spray angle and flow rate decrease linearly (sensitivity 0.83–0.58 degrees/%), with a maximum drop of the droplet median diameter only 9.9%. At lower flow rates, the droplets are finer (facilitating deposition), and the distribution is concentrated directly below the nozzle. However, at low flow rates (duty cycle < 40%), the spray pattern is severely distorted, leading to measurement failures. The hollow and solid cone nozzles have a limited range of adjustment, and a significant reduction in edge coverage may lead to non-uniform application.
[43]	2012	It consists of a solenoid valve, a driving circuit, and a state–space modeling algorithm. The system regulates the solenoid valve coil current to control spool displacement through high-frequency PWM (10 kHz) to accurately regulate the valve pressure drop and nozzle inlet pressure to realize the dynamic control of the spray droplet size spectrum; at the same time, the low-frequency PWM (10 Hz) regulates the time-averaged flow rate by cyclically opening/closing the valve (100 ms cycle) to form the two-tier hierarchical control architecture. The drive circuit integrates a microcontroller, current feedback module, and pressure sensor, combined with a control algorithm to dynamically calculate the coil charging time and duty cycle.	After triggering the low-frequency PWM signal, the pressure turn-on and turn-off time averages about 8 ms, and the outlet pressure reaches a steady state after about 15 ms. The average flow rate is highly linear with respect to the low-frequency duty cycle (R2 > 0.99) and possesses robustness to voltage fluctuations (12–13.8 V) and inlet pressure variations (4.14–6.21 bar). However, the system’s steady-state pressure control suffers from deviations (maximum error of 32% between measured pressure and target value), which requires improvement of the algorithm’s accuracy; the mechanical balance of the valve spool is affected by the coupling of the spring preload and the fluid force, which leads to a nonlinear response.
[44]	2014	It consists of a pressure supply and stabilization unit (including medicine box, plunger pump, accumulator), flow regulation unit (high-speed solenoid valve and pressure sensor), signal acquisition unit (oscilloscope and sampling resistor), and PWM control unit (STC80C52 microcontroller and driving circuit), and the instantaneous flow rate is deduced through the Kalman filter algorithm by processing the data from the pressure sensor.	A high-precision model of PWM duty cycle and spray flow rate (coefficient of determination R2 > 0.995) was established. The system can realize ±6% flow rate control error in the pressure range of 0.2–0.4 MPa, which can effectively reduce the amount of liquid used and have less influence on the droplet size, but its limitations are that the small flow rate control accuracy is significantly affected by pressure fluctuations (error of −5.4%), and the lower limit of the duty cycle adjustment of the PWM signal is higher (≥4% is needed to trigger the solenoid valve action).
[45]	2018	It consists of 40 XR8004 fan nozzles equipped with individual PWM solenoid valves, a microcontroller unit, and a pressure regulator. The nozzles are connected to eight sets of five-port pneumatic manifolds via polyethylene tubing, and the solenoid valves are driven by a 10 Hz synchronization signal to achieve precise control of the duty cycle (10–100%).	Flow control linearity was good at constant pressure (242 kPa) (slope increased 51–103% with increasing number of nozzles). The system reduces pesticide use by more than 50% and deposits better than a constant spray, but there are significant limitations: at unregulated pressure, an increase in the number of nozzles (1→40) or duty cycle (10%→100%) results in a sudden drop in line pressure from 345 kPa to 104 kPa, causing a flow deviation of up to 27.6%; the PWM control shows flow nonlinearities at both low (<20%) and high (>90%) duty cycles (<20%) and at high (>90%) pressure. During PWM control at low duty cycles (<20%) and high duty cycles (>90%), there is a nonlinear fluctuation in flow rate, and the prediction error of a single nozzle at a 30% duty cycle is 207.9%; the system relies on artificial pressure compensation, and there is a lack of closed-loop feedback control, and the difference between the measured flow rate and the theoretical value under extreme conditions (40 nozzles + 90% duty cycle) is 27.6%.
[46]	2021	Equipped with the DynaJet® Flex7140 PWM control system and XR8002VS fan nozzle, flow control is achieved by adjusting the duty cycle (30–100%) and forward speed (4–8 km/h) while maintaining a constant pressure (0.4 MPa) to maintain droplet spectrum stability. The system utilizes a 180 mm spaced array of solenoid valves on the vertical spray bar for precise flow regulation, combined with an axial fan directional airflow to enhance droplet canopy penetration.	The system was effective in reducing the spray coverage coefficient of variation (CV% < 25%) at duty cycles ≥50%, and the spray uniformity index (Iu ≈ 0.8) was better than that of the laboratory simulations under field conditions, with optimal deposition densities achieved at application rates of 200–250 L/ha (insecticides) and 300–370 L/ha (fungicides). However, the system suffers from the following limitations: frequent opening and closing of the solenoid valve at low duty cycles (30%) resulted in significant inhomogeneity of spray coverage in both vertical and traveling directions (CV% up to 35%); the effect of PWM on the spray angle (a 40% reduction in duty cycles may result in a 49.8% reduction in spray angle) may affect deposition uniformity in complex canopy environments.
[47]	2022	It consists of an electromagnetic suction mechanism, two-way water valve, spray body, standard fan nozzle, and other core components. The electromagnetic suction mechanism, through a coil-energized electromagnetic force-driven spool, achieves high-frequency reciprocating motion (frequency 0–40 Hz). Combined with the opening and closing of the two-way water valve, it controls the water flow channel and uses PWM signals to regulate the duty cycle of a single cycle of water channel opening (duty cycle of 7–96%).	Under the conditions of 20 Hz frequency and 0.3 MPa pressure, the system has an average relative error of 3.1% and a maximum error of 13.1% in the spray time of a single cycle in the range of 20–96%. Its limitations are mainly reflected in the low duty cycle interval (7–19%), control stability is poor (average error of 7.5%, the maximum error of 55.1%), and the system pressure exceeds a 0.5 MPa spool reset force, which is insufficient to lead to delayed closure, while the selection of the PWM frequency needs to be dynamically adjusted according to the operating speed (critical frequency of 1–26 Hz corresponds to the speed of 1–10 km/h), and spray uniformity is speed-dependent.
[48]	2023	The system consists of a host computer (HMI serial screen), spray controller (with PIC18F258 microcontroller as the core), electronic switch, and solenoid electronically controlled nozzle body, and data communication is realized through CAN bus. The system receives the parameters of pesticide application through the host computer, and the spray controller calculates the corresponding PWM duty cycle and outputs multiple independent signals, which control the nozzle solenoid valve on/off time through the MOS electronic switch to realize flow adjustment of a single nozzle.	Under a pressure of 0.3–0.5 MPa and frequency of 10 Hz, the flow rate of the nozzle is highly linear with the duty cycle (R2 > 0.95), the maximum response time is 0.179 s, the on/off time RMS error is ≤2.54 ms, and the relative error is ≤6.8%, which provides accurate control of the application of pesticides. However, there is a linear deviation phenomenon that slows down the flow growth rate after the duty cycle exceeds 80%, and the relative error is higher (up to 6.8%) in low duty cycle intervals (30–50%).
[49]	2024	It consists of a liquid tank, diaphragm pump, pressure stabilizing tank, PWM controller, solenoid valve, and pressure gauge, etc. The delayed opening and closing of the solenoid valve is controlled by an STM32F103 microcontroller, which realizes the asynchronous working mode of multiple nozzles.	Asynchronous opening and closing can significantly reduce the pipeline pressure pulse, it has a 50% duty cycle pressure peak reduction of up to 14%, the coefficient of variation of pressure fluctuates with the increase and decrease in PWM frequency, the nozzle flow rate and duty cycle have a good linear relationship (R2 ≥ 0.90), the ST03 nozzle flow rate has an adjustable range of 0.28–1.22 L/min, asynchronous PWM spray deposition distribution has a frequency of 20 Hz, and the coefficient of variation (CV) of uniformity is less than 15%. However, the solenoid valve responds abnormally at low duty cycles (20%) under high-frequency conditions (e.g., 20 Hz), resulting in pressure peaks that cannot be effectively controlled. Asynchronous mode reduces pressure fluctuations, but pressure pulsation still affects the accuracy of flow regulation (maximum error 12–15%).

. Variable-rate spray systems based on PWM technology mainly use three kinds of actuators: an independent nozzle solenoid valve, a multi-channel integrated pneumatic manifold, and a high-speed proportional solenoid valve. The limitations of this technology are as follows: (1) at low PWM duty cycles and low frequencies, the inhomogeneity of spray coverage increases and the flow rate is nonlinear with respect to the duty cycle [35,36,37,38]; (2) pressure fluctuates in the pipeline due to changes in the number of activated nozzles or frequent opening and closing [39,40]; (3) high-frequency PWM (>20 Hz) solenoid valves are frequently opened and closed, increasing mechanical wear such as spool reset spring fatigue. Future improvement directions should focus on (1) developing adaptive pressure compensation nozzles to reduce the impact of pipeline pressure fluctuations on the flow rate; (2) optimizing the mechanical structure and magnetic circuit parameters of the solenoid valve to improve the responsive speed and reliability; and (3) enhancing the robustness of the control algorithm (fusion fuzzy or BP PID control) to improve the flow rate and pressure control performance.

5. Pesticide Concentration-Regulated Variable-Rate Spray Technology

Pesticide concentration-regulated VRST is used to control the flow rate of water so that it remains unchanged during application while changing the flow rate of the pesticide to realize a variable-rate spray. This technology can be mainly divided into pesticide direct injection systems and jet mixing devices. Direct injection systems need to configure a special metering pump and online mixing chamber (such as a buffer tank with a static mixer) for each nozzle section or nozzle to realize the complete decoupling of the pesticide path and the carrier path. The jet mixing system is based on the venturi principle, with a single jet pump throat enabling the negative pressure adsorption of pesticides in the mixing chamber to complete the concentration adjustment by independently adjusting the flow ratio between the working fluid and the drawn fluid. The essential difference from the traditional pressure/flow regulation system (relying on the global parameter adjustment of the main loop pressure valve or flow valve) is that both of these systems accurately separate the pesticide and carrier regulation paths through independent flow channel architecture, thus realizing the true concentration variable while maintaining isolation from carrier pressure fluctuations.

5.1. Direct Injection

The direct injection type injects the pesticide directly into the nozzle or spray boom through a metering pump or doser. The advantages of this technology include the following: it avoids pesticide pre-mixing residues and reduces the waste of chemicals; it is adaptable to a variety of pesticide characteristics; pressure or flow fluctuations have little effect on pesticide concentration. Some representative studies are summarized in Table 4, and a typical direct injection system is shown in Figure 4. The limitations of this technology are as follows: (1) the pressure and flow rate of the carrier can significantly affect the response time and accuracy [50,51,52]; (2) high-viscosity pesticides are prone to cause metering pump flow attenuation and piping residue; (3) there is a delay in the concentration transfer and pressure fluctuations resulting in concentration differences between the nozzles. Future improvements should focus on (1) developing adaptive mixing chambers that adjust the length of the mixing path according to the flow rate automatically; (2) developing corrosion-resistant and wear-resistant metering pumps to improve the accuracy of the measurement of high-viscosity agents; (3) optimizing the buffer tank flow channel or introducing self-cleaning designs to reduce pesticide residue.

Table 4. Overview of direct injection VRST research.

Reference	Year	System Components and Working Principle	Control and Spray Performance
[53]	2007	It consists of two parts: the injection system contains pesticide delivery devices (gear pumps, diaphragm pumps, and pneumatic tanks) and injection equipment (proportional valves and PWM control valves), which control the amount of pesticide injected in real time; the transportation and mixing system uses optimized mixing chambers (including KMS, SMX, and Quadro static mixers) to achieve rapid and uniform mixing.	The system reduces the injection time to less than 80 ms with a novel control strategy (initial high-flow pulse injection to shorten the response time), combined with a CFD-optimized compact mixing chamber (diameter ratio L/D = 16) that reduces the transport mixing time to 52 ms, for a total response time of about 150 ms, to meet real-time application requirements (<0.33 s). The system achieves mixing homogeneity with a 5% concentration coefficient of variation (CoV), significantly reducing pesticide usage and environmental risks. However, the following limitations exist: mixing efficiency is significantly affected by the carrier flow rate and flow regime (laminar/turbulent), and long mixing paths are still required at low flow rates; and there is limited adaptability to highly viscous pesticides.
[54]	2007	It consists of a carrier pump, a chemical metering pump, a proportional adjustment valve, a mixer, a nozzle, and a sensor (conductivity sensor/transmittance sensor), which mix the herbicide with the carrier water at the end of the line using direct injection technology. The system realizes variable control by two injection methods: spray rod section injection injects the agent into the spray rod main pipe, and nozzle direct injection sets a metering orifice plate in front of the nozzle for local distribution. The system realizes closed-loop flow control by means of a proportional valve with Kv-value characteristics and a differential pressure flow meter.	The response time of the spray bar injection system is 2.75–8.84 s, while the lag time for direct nozzle injection can be optimized to less than 0.3 s. A flow range of 0.002–0.4 L/min can be covered but with flow calibration errors of up to 18% for high-viscosity agents (219 mPa·s). It can save 59% of herbicide dosage, but there is a mixing uniformity problem, and the coefficient of variation of concentration between nozzles is as high as 16.3–39%, which is mainly affected by the residual liquid in the pipeline (2.5 L residual liquid in the 12.7 mm pipe diameter and 300 mm pipe length), the viscosity of the agent affecting the flow rate stability (differential pressure fluctuation of ±0.2 bar due to 60 mPa·s solution), and the response delay leading to the offset of the application position (30 s delay corresponds to 66 m deviation), among other limitations.
[55]	2008	It consists of a mixing chamber, static mixer (e.g., KMS type), light transmission sensor, and pesticide direct injection device. Its core innovation is to adopt the principle of the iodine–sodium thiosulphate decolorization reaction combined with light transmission sensor to monitor the mixing uniformity at the nozzle outlet in real time.	Control performance is dependent on the geometric configuration of the static mixer (e.g., 12 L/D mixer length) and optimization of the injection position (90° is optimal), which achieves a mixing time of 40.2 ms at a flow rate of 20 mL/s and a mixing homogeneity of 99.67% (with a standard deviation of 0.25%), which meets the criterion of a coefficient of variation of 5%, significantly improves pesticide uniformity of distribution, and reduces environmental residues. However, its limitations are reflected in the highly sensitive mixer configuration: low-concentration (<0.3%) or high-viscosity (600 mPa·s) pesticides need the mixer to be extended to 16 L/D to maintain the effect; injection position shift will lead to a 1/3 reduction in mixing efficiency.
[56]	2011	It consists of a hydraulic spray bar (series or parallel layout), diaphragm pump (conveying carrier liquid), and peristaltic pump (chemical agent injection) driven by a DC power supply, PID feedback controller, pressure sensor, and fluorescence concentration sensor, etc. It realizes precise control of concentrations by real-time adjustment of the ratio of the carrier flow rate to the amount of chemical agent injected.	The system uses a tandem spray bar (6 mm diameter) with 97% application uniformity and a concentration response delay of 1.5 s, while the parallel spray bar (4 mm diameter) reduces the delay to 2 s by supplying fluids through separate nozzles but with a 7–10% increase in pressure loss. The constant current-carrying strategy has a delay of 2.5 s for sudden changes in velocity (0.6→1.2 m/s), whereas the variable total flow strategy reduces the delay to 2.3 s by pressure regulation (1–3 bar). Limitations are that the operating speed variations need to be limited to 0.5 m/s2 for stability, and the parallel layout reduces delay but increases system complexity and cost.
[57]	2014	It consists of a chemical storage tank, peristaltic metering pump (equipped with motor drive, speed sensor), tractor driving speed sensor, GPS positioning module, electronic control unit (ECU), venturi mixing device, and other core components. The system operates through ECU-integrated processing, GPS geographic information, real-time driving speeds, spray pressure data, and dynamic adjustment of the metering pump speed to achieve precise application control.	The working pressure has a significant effect on the response time, with average response times of 25.8 s, 22.8 s, and 17.9 s at 3, 4, and 5 bar, respectively. The system is capable of proportioning the pesticide concentration on demand and compensating for the response time by means of an “advance trigger” algorithm. Its limitations include the following: the system does not integrate an automatic pressure adjustment module, so the spray pressure needs to be preset manually; the long response time (especially in low-pressure conditions) limits the accuracy of the dynamic adjustment; the physical distance between the mixing unit and the spray bar directly affects the response characteristics of the system, which may result in differences in the concentration of multiple nozzles.
[58]	2014	It consists of a ceramic piston metering pump, centrifugal water pump, diaphragm spray pump, embedded computer control unit, 56 L water tank, 5 L liquid tank, 20 L pre-mixing tank, buffer tanks and other core components. The microprocessor-controlled metering pump injects pesticide liquid (accuracy of ±0.05 mL) into the mixing chamber together with 10 L of water injected by the centrifugal pump. The hollow cone nozzle provides mandatory mixing in the pre-mixing tank, and the pesticide is ultimately transferred to the buffer tank for dynamic output from the spray pump.	The embedded computer has a touch screen, which achieves human–computer interaction, the real-time adjustment of the liquid ratio (0.1–1.0%), and a metering pump in the 50–400 rpm speed range. With viscosities of 0.9–31.3 mPa·s, the liquid flow control error is less than 0.5%, and the coefficient of variation of the mixing uniformity is less than 6.1%. The system effectively solves the problems of lag effect (response time reduced to less than 30 s), inaccurate measurement of low flow rate (minimum flow rate of 5 mL/min), and uneven concentration between nozzles that exist in traditional online mixing systems, and it reduces the waste of pesticides by more than 60%. The main limitation is that the buffer tank and pre-mixing tank each cycle up to 20 L of residual liquid, although compared with a traditional large-capacity tank, this is a significant reduction, but there is still a small amount of waste.
[59]	2019	The core components include a precision metering pump, water pump, static mixer, pre-mixing tank, and buffer tank. The system accurately controls the flow of the pesticide liquid through the metering pump, and the water delivered by the pump is initially mixed in the static mixer before entering the pre-mixing tank and is then supplied to the variable flow nozzle through the secondary mixing of the buffer tank. The Arduino control unit automatically triggers the mixing cycle based on the signal from the level sensor, and the metering pump rotational speed and running time are adjusted.	The system can achieve liquid metering accuracy with an error of <5% in the viscosity range of 0.9–32.0 mPa·s. The relative error in concentration of the mixture is up to 7.6%, with a coefficient of variation of ≤4.5%, which satisfies a coefficient of variation of <5%. The main limitations are the 2.3% attenuation of the metering pump flow when dealing with high-viscosity pesticides (32 mPa·s) and the relatively large error (5.8%) in the 2% high-concentration ratio.
[60]	2019	It consists of a water solvent-regulating pipeline and pesticide injection pipeline: the water solvent pipeline realizes the main flow rate closed-loop regulation by controlling the bypass throttling through the electric regulating valve, and the pesticide injection pipeline adopts an electromagnetic metering pump combined with a dynamic matrix control algorithm to realize precise flow rate control and is equipped with a SK static mixer to enhance mixing homogeneity. The system adopts an STM32F4 controller to realize double closed-loop control. In automatic mode, the flow instruction is generated by a prescription map or GPS positioning, and manual mode supports manual parameter input.	The system has a mixing ratio range of 10:1–270:1, a steady-state response time of about 3 s for aqueous solvent flow, a steady-state error of no more than 5% for dynamic matrix control of pesticide flow, a spatial coefficient of variation of the mixture of less than 5%, and good nozzle spray volume uniformity. The main limitations are that the system still suffers from a concentration transfer delay of about 0.8 s, and the strong dependence of the static mixer on homogeneity may increase maintenance costs.
[61]	2024	The system consists of pesticide pumps, pressure storage buffer tanks, accumulators, electric regulating valves, pumps, static mixers, photoelectric detection units, and other core components. The real-time adjustment of electric valve openings achieves precise control of pesticide flow, combined with a non-contact photoelectric detection module (based on the principle of solution transmittance) for the monitoring of the mixing ratio.	With a maximum standard deviation of 0.00145 over the mixing ratio range of 1:40 to 1:200, an average coefficient of variation of less than 3.41%, and a response time of up to 3.5 s, the system is adaptable to simulated fluids of different viscosities (1.01–34.76 mPa·s). However, the system still has technical limitations: the micro-flow sensor is susceptible to corrosion, leading to measurement errors; high-viscosity liquid (75% glycerol content) may lead to fluctuations in mixing homogeneity (maximum detection error of 5.5%) and an increase in response latency; there are fluctuations in flow rates of 8% during the charging and discharging switching process; and it needs to rely on the addition of conductive salts to realize the calibration of the concentration.

Table 4. Overview of direct injection VRST research.

Reference	Year	System Components and Working Principle	Control and Spray Performance
[53]	2007	It consists of two parts: the injection system contains pesticide delivery devices (gear pumps, diaphragm pumps, and pneumatic tanks) and injection equipment (proportional valves and PWM control valves), which control the amount of pesticide injected in real time; the transportation and mixing system uses optimized mixing chambers (including KMS, SMX, and Quadro static mixers) to achieve rapid and uniform mixing.	The system reduces the injection time to less than 80 ms with a novel control strategy (initial high-flow pulse injection to shorten the response time), combined with a CFD-optimized compact mixing chamber (diameter ratio L/D = 16) that reduces the transport mixing time to 52 ms, for a total response time of about 150 ms, to meet real-time application requirements (<0.33 s). The system achieves mixing homogeneity with a 5% concentration coefficient of variation (CoV), significantly reducing pesticide usage and environmental risks. However, the following limitations exist: mixing efficiency is significantly affected by the carrier flow rate and flow regime (laminar/turbulent), and long mixing paths are still required at low flow rates; and there is limited adaptability to highly viscous pesticides.
[54]	2007	It consists of a carrier pump, a chemical metering pump, a proportional adjustment valve, a mixer, a nozzle, and a sensor (conductivity sensor/transmittance sensor), which mix the herbicide with the carrier water at the end of the line using direct injection technology. The system realizes variable control by two injection methods: spray rod section injection injects the agent into the spray rod main pipe, and nozzle direct injection sets a metering orifice plate in front of the nozzle for local distribution. The system realizes closed-loop flow control by means of a proportional valve with Kv-value characteristics and a differential pressure flow meter.	The response time of the spray bar injection system is 2.75–8.84 s, while the lag time for direct nozzle injection can be optimized to less than 0.3 s. A flow range of 0.002–0.4 L/min can be covered but with flow calibration errors of up to 18% for high-viscosity agents (219 mPa·s). It can save 59% of herbicide dosage, but there is a mixing uniformity problem, and the coefficient of variation of concentration between nozzles is as high as 16.3–39%, which is mainly affected by the residual liquid in the pipeline (2.5 L residual liquid in the 12.7 mm pipe diameter and 300 mm pipe length), the viscosity of the agent affecting the flow rate stability (differential pressure fluctuation of ±0.2 bar due to 60 mPa·s solution), and the response delay leading to the offset of the application position (30 s delay corresponds to 66 m deviation), among other limitations.
[55]	2008	It consists of a mixing chamber, static mixer (e.g., KMS type), light transmission sensor, and pesticide direct injection device. Its core innovation is to adopt the principle of the iodine–sodium thiosulphate decolorization reaction combined with light transmission sensor to monitor the mixing uniformity at the nozzle outlet in real time.	Control performance is dependent on the geometric configuration of the static mixer (e.g., 12 L/D mixer length) and optimization of the injection position (90° is optimal), which achieves a mixing time of 40.2 ms at a flow rate of 20 mL/s and a mixing homogeneity of 99.67% (with a standard deviation of 0.25%), which meets the criterion of a coefficient of variation of 5%, significantly improves pesticide uniformity of distribution, and reduces environmental residues. However, its limitations are reflected in the highly sensitive mixer configuration: low-concentration (<0.3%) or high-viscosity (600 mPa·s) pesticides need the mixer to be extended to 16 L/D to maintain the effect; injection position shift will lead to a 1/3 reduction in mixing efficiency.
[56]	2011	It consists of a hydraulic spray bar (series or parallel layout), diaphragm pump (conveying carrier liquid), and peristaltic pump (chemical agent injection) driven by a DC power supply, PID feedback controller, pressure sensor, and fluorescence concentration sensor, etc. It realizes precise control of concentrations by real-time adjustment of the ratio of the carrier flow rate to the amount of chemical agent injected.	The system uses a tandem spray bar (6 mm diameter) with 97% application uniformity and a concentration response delay of 1.5 s, while the parallel spray bar (4 mm diameter) reduces the delay to 2 s by supplying fluids through separate nozzles but with a 7–10% increase in pressure loss. The constant current-carrying strategy has a delay of 2.5 s for sudden changes in velocity (0.6→1.2 m/s), whereas the variable total flow strategy reduces the delay to 2.3 s by pressure regulation (1–3 bar). Limitations are that the operating speed variations need to be limited to 0.5 m/s2 for stability, and the parallel layout reduces delay but increases system complexity and cost.
[57]	2014	It consists of a chemical storage tank, peristaltic metering pump (equipped with motor drive, speed sensor), tractor driving speed sensor, GPS positioning module, electronic control unit (ECU), venturi mixing device, and other core components. The system operates through ECU-integrated processing, GPS geographic information, real-time driving speeds, spray pressure data, and dynamic adjustment of the metering pump speed to achieve precise application control.	The working pressure has a significant effect on the response time, with average response times of 25.8 s, 22.8 s, and 17.9 s at 3, 4, and 5 bar, respectively. The system is capable of proportioning the pesticide concentration on demand and compensating for the response time by means of an “advance trigger” algorithm. Its limitations include the following: the system does not integrate an automatic pressure adjustment module, so the spray pressure needs to be preset manually; the long response time (especially in low-pressure conditions) limits the accuracy of the dynamic adjustment; the physical distance between the mixing unit and the spray bar directly affects the response characteristics of the system, which may result in differences in the concentration of multiple nozzles.
[58]	2014	It consists of a ceramic piston metering pump, centrifugal water pump, diaphragm spray pump, embedded computer control unit, 56 L water tank, 5 L liquid tank, 20 L pre-mixing tank, buffer tanks and other core components. The microprocessor-controlled metering pump injects pesticide liquid (accuracy of ±0.05 mL) into the mixing chamber together with 10 L of water injected by the centrifugal pump. The hollow cone nozzle provides mandatory mixing in the pre-mixing tank, and the pesticide is ultimately transferred to the buffer tank for dynamic output from the spray pump.	The embedded computer has a touch screen, which achieves human–computer interaction, the real-time adjustment of the liquid ratio (0.1–1.0%), and a metering pump in the 50–400 rpm speed range. With viscosities of 0.9–31.3 mPa·s, the liquid flow control error is less than 0.5%, and the coefficient of variation of the mixing uniformity is less than 6.1%. The system effectively solves the problems of lag effect (response time reduced to less than 30 s), inaccurate measurement of low flow rate (minimum flow rate of 5 mL/min), and uneven concentration between nozzles that exist in traditional online mixing systems, and it reduces the waste of pesticides by more than 60%. The main limitation is that the buffer tank and pre-mixing tank each cycle up to 20 L of residual liquid, although compared with a traditional large-capacity tank, this is a significant reduction, but there is still a small amount of waste.
[59]	2019	The core components include a precision metering pump, water pump, static mixer, pre-mixing tank, and buffer tank. The system accurately controls the flow of the pesticide liquid through the metering pump, and the water delivered by the pump is initially mixed in the static mixer before entering the pre-mixing tank and is then supplied to the variable flow nozzle through the secondary mixing of the buffer tank. The Arduino control unit automatically triggers the mixing cycle based on the signal from the level sensor, and the metering pump rotational speed and running time are adjusted.	The system can achieve liquid metering accuracy with an error of <5% in the viscosity range of 0.9–32.0 mPa·s. The relative error in concentration of the mixture is up to 7.6%, with a coefficient of variation of ≤4.5%, which satisfies a coefficient of variation of <5%. The main limitations are the 2.3% attenuation of the metering pump flow when dealing with high-viscosity pesticides (32 mPa·s) and the relatively large error (5.8%) in the 2% high-concentration ratio.
[60]	2019	It consists of a water solvent-regulating pipeline and pesticide injection pipeline: the water solvent pipeline realizes the main flow rate closed-loop regulation by controlling the bypass throttling through the electric regulating valve, and the pesticide injection pipeline adopts an electromagnetic metering pump combined with a dynamic matrix control algorithm to realize precise flow rate control and is equipped with a SK static mixer to enhance mixing homogeneity. The system adopts an STM32F4 controller to realize double closed-loop control. In automatic mode, the flow instruction is generated by a prescription map or GPS positioning, and manual mode supports manual parameter input.	The system has a mixing ratio range of 10:1–270:1, a steady-state response time of about 3 s for aqueous solvent flow, a steady-state error of no more than 5% for dynamic matrix control of pesticide flow, a spatial coefficient of variation of the mixture of less than 5%, and good nozzle spray volume uniformity. The main limitations are that the system still suffers from a concentration transfer delay of about 0.8 s, and the strong dependence of the static mixer on homogeneity may increase maintenance costs.
[61]	2024	The system consists of pesticide pumps, pressure storage buffer tanks, accumulators, electric regulating valves, pumps, static mixers, photoelectric detection units, and other core components. The real-time adjustment of electric valve openings achieves precise control of pesticide flow, combined with a non-contact photoelectric detection module (based on the principle of solution transmittance) for the monitoring of the mixing ratio.	With a maximum standard deviation of 0.00145 over the mixing ratio range of 1:40 to 1:200, an average coefficient of variation of less than 3.41%, and a response time of up to 3.5 s, the system is adaptable to simulated fluids of different viscosities (1.01–34.76 mPa·s). However, the system still has technical limitations: the micro-flow sensor is susceptible to corrosion, leading to measurement errors; high-viscosity liquid (75% glycerol content) may lead to fluctuations in mixing homogeneity (maximum detection error of 5.5%) and an increase in response latency; there are fluctuations in flow rates of 8% during the charging and discharging switching process; and it needs to rely on the addition of conductive salts to realize the calibration of the concentration.

5.2. Jet Mixing

Jet mixing devices utilize water flowing through a venturi tube or other jet device to generate negative pressure, then draw the pesticide in and mix it well with the water before spraying. Compared to direct injection, the advantages of this injection type are in the speed of response and system simplification. The core component of the jet mixing system is the jet mixing device, whose structural parameters (such as aperture distribution, shrinkage angle, guide vane layout, etc.) directly affect mixing homogeneity. Some representative studies are summarized in Table 5, and several jet mixing apparatuses are shown in Figure 5. The limitations of this technology are mainly reflected in the following: (1) the mixing effect is sensitive to fluid viscosity, with high-viscosity pesticides easily leading to jet energy attenuation and local pesticide residues; (2) the jet device can easily cause mixing instability under different working conditions, which may lead to concentrations that are too high or too low, especially under the conditions of load fluctuation or nozzle parameter mismatch [62,63,64]; (3) the pesticide mixing efficiency and uniformity of jet mixing devices have been significantly improved by optimizing design parameters such as the nozzle diameter, mixer structure, and fluid flow path. However, most of these studies are based on static laboratory tests, and there is insufficient flexibility in structural adjustments for practical applications (e.g., fixed nozzle diameters, non-dynamically adjustable guide vanes). Future directions of improvement should focus on (1) developing adaptive jet mixing chambers, adjusting mechanical structures to adapt to different viscosity chemicals; (2) optimizing jet mixing and spray system pressure control strategies to reduce the risk of reflux; (3) promoting the design of a modular jet unit to support the rapid replacement of the nozzle, guide vane, and other core components in order to match the needs of different operations.

Table 5. Overview of jet mixing VRST research.

Reference	Year	System Components and Working Principle	Control and Spray Performance
[65]	2010	It consists of a cylindrical fluid storage tank, circulation pump, jet mixer, replaceable nozzles (5 types of active area 11.4–53.2%), and a flow meter, which realizes fluid mixing control by adjusting the active area of the nozzles, the position of the jet, and the flow rate.	The nozzle, with 20% spray area, has the largest liquid retention rate and the best mixing effect at jet position T/1.8. The system forms a complete circulating flow path in low-viscosity liquid (water), with high mixing efficiency and low energy consumption, but for high-viscosity fluids (CMC/guar concentration > 0.3%), the injection speed needs to be greatly increased in order to maintain effective mixing, and the system has the limitations of not integrating real-time monitoring or feedback control of the concentration, relying on the static experiments of the optimization parameters of the nozzle and drastically increasing the energy consumption of high-viscosity working conditions.
[66]	2014	It consists of a jet mixing device, power liquid pump, water tank, liquid tank, spraying system (including spraying rod and 16 nozzles with 0.5 m spacing), and measuring elements (pressure gauge, flow meter, etc.). The core structure of the jet mixing device includes nozzles (adjustable diameter of 2–4 mm), a suction port, a suction chamber, and mixing and diffusion tubes, and the switching of the working state (suction, reflux, or cavitation) is realized by changing the diameter of the nozzles and the load of the system.	In the no-load bench test, the 2 mm nozzle causes bubbles in the inhalation chamber due to cavitation, which affects mixing efficiency, while in the actual spray bar spray system, the 2 mm nozzle can realize a stable suction state, and the 3 mm and above nozzle triggers a reflux state, which avoids the cavitation phenomenon. However, its limitations are as follows: 1. existing research focuses on the flow-field simulation of the jet mixing device itself, and the overall synergistic control of the spray system is insufficient; 2. the system relies on manually adjusting the nozzle diameter and other structural parameters, it lacks real-time dynamic regulation, and it is difficult to achieve accurate concentration feedback control; 3. the cavitation phenomenon will still destroy the uniformity of mixing under specific conditions, and the reflux state may result in the circulation of the liquid medicine, with efficiency decline.
[67]	2014	It consists of a jet mixing device (the core component, including jet nozzle, mixing tube, diffusion tube, and other structures), a spray bar sprayer, a liquid pump, a water tank, a liquid tank, and a flow/pressure measuring device, in which the structural parameters of the jet mixing device (such as the diameter of the jet nozzle outlet d, the diameter of the mixing tube D, and the area ratio m) can realize the online control of the concentration of the pesticide solution by adjusting the fluid dynamics characteristics. The system regulates the mixing performance by changing the structural parameters of the jet device and spray pressure (0.26–0.36 MPa).	When the diameter of the jet nozzle outlet is small (e.g., d = 2.00 mm, area ratio m = 4.00) and the resistance of the spraying system is low (e.g., F110-015 spray nozzle), the system can be stably in the suction state, and the spatial coefficient of variation (CV) of the mixing concentration is less than 5%. However, there are significant limitations of the system: first, the working state is highly dependent on the matching of the parameters of the jet device and the resistance of the spraying system, which may lead to reflux problems when the diameter of the jet nozzle is too large or the area ratio exceeds a reasonable range; second, the experimental conditions are based on the simulated liquid (cochineal) and the laboratory environment, and the differences in the physicochemical properties of the pesticide, dynamic pressure fluctuations, and complex piping conditions in the actual field applications may affect the stability of the pesticide mixing.
[68]	2016	It consists of liquid supply system (including water tank, centrifugal pump, and electric motor), pesticide supply system (including liquid tank, metering pump, and flow control module), and rotating jet mixer. The mixer adopts the structure of a spiral bending contraction tube, spinning device, and diffusion tube with a guide vane to realize the online mixing of water and fat-soluble pesticides through pressure difference, and its core innovation lies in the enhancement of mixing uniformity through fluid spinning.	Through the closed-loop adjustment of the flow rate of the electric metering pump (microcontroller control of the motor frequency to adjust the pump speed), the precise control of the pesticide flow rate can be realized (error < 0.001 kg/s). Numerical simulations and tests show that the system can achieve a uniformity index of 0.9989 for the volume fraction of pesticide in the outlet cross-section under 1 MPa working pressure, but there are the following limitations: the traditional structure of the mixing of fat-soluble pesticides exists in the phenomenon of agglomerated suspension, with a low degree of mixing uniformity; the improved rotary structure improves the mixing effect but increases flow resistance and energy loss (pressure loss of about 0.2 MPa), precision structures such as guide leaves are easily affected by changes in pesticide viscosity, and structural parameters (e.g., guide leaf height, pitch, etc.) need to be adjusted for different pesticide characteristics.
[69]	2016	It consists of a rotary jet mixer, pressure drive system, and mixing effect monitoring system. Among them, the core structure of the mixer includes a spiral contraction tube (contraction degree 0.095, pitch 128 mm, contraction angle 16°), a tangential diverter (adjustable tangential angle β), a guide vane diffuser tube (guide vane height 0–6.5 mm gradient, wrapping angle 15 °), and a relay rotator (including three 6.5 mm high, 15 ° wrapping angle guide vane), using spiral flow generation and maintenance technology to achieve the mixing of the pesticide solution. The diaphragm pump provides water pressure (0.23 MPa) and pesticide pressure (0.22 MPa), with flow meters and manometers to achieve dual-channel precision control.	The volume fraction uniformity index at the nozzle reaches 0.9995, and the maximum mixing ratio is 99.4425%, which significantly improves the dispersion uniformity of fat-soluble pesticides and effectively solves the problem of pesticide suspension in the traditional jet mixer. However, the system still has limitations: first, the structural parameters (such as diverter tangential angle, guide vane layout) need to be dynamically adjusted according to the viscosity and concentration of pesticides, which increases the complexity of the operation; second, it relies on the pesticide pump to maintain the pressure (the outlet directly connected to the spray nozzle requires 0.22 MPa pesticide pump pressure), and energy consumption and equipment costs are high.
[70]	2019	The core structure consists of a jet mixer (e.g., ordinary jet mixer A and improved sandwich orifice tube mixer B), a pesticide supply device (e.g., volumetric metering pump or pulse-width modulated solenoid valve), and an online mixing and detection system (with high-speed camera and fluorescent tracer technology). The system optimizes the control performance by adjusting the carrier flow rate (800–2000 mL/min) and the mixing ratio (4:100–10:100), wherein the sandwich orifice tube mixer B attenuates the injection pulsation through the multi-orifice plate structure, which significantly improves the dynamic concentration consistency.	The average dynamic concentration inconsistency index (CV value) under the test conditions was reduced from 0.039 to 0.011 for mixer A and met the stringent criterion of CV < 0.020 at an injection frequency of >5.10 Hz or a mixing ratio of >5:100. Mixer B achieved an instantaneous homogeneity index (D-value) of about 13.00 for homogeneous mixing at high carrier flow rates (>1400 mL/min) and mixing ratios (>9:100), but its limitation is that the homogeneity of the common jet mixer A decreases due to a single-point injection at high mixing ratios (up to a maximum of D-value of 21.13), and the dynamic homogeneity of mixer B at low flow rates or low mixing ratios still has fluctuations (e.g., CV increased to 0.018 at Qc = 2000 mL/min), and transient uniformity is not sufficiently optimized (D-value vs. CV correlation was only 0.684).

Table 5. Overview of jet mixing VRST research.

Reference	Year	System Components and Working Principle	Control and Spray Performance
[65]	2010	It consists of a cylindrical fluid storage tank, circulation pump, jet mixer, replaceable nozzles (5 types of active area 11.4–53.2%), and a flow meter, which realizes fluid mixing control by adjusting the active area of the nozzles, the position of the jet, and the flow rate.	The nozzle, with 20% spray area, has the largest liquid retention rate and the best mixing effect at jet position T/1.8. The system forms a complete circulating flow path in low-viscosity liquid (water), with high mixing efficiency and low energy consumption, but for high-viscosity fluids (CMC/guar concentration > 0.3%), the injection speed needs to be greatly increased in order to maintain effective mixing, and the system has the limitations of not integrating real-time monitoring or feedback control of the concentration, relying on the static experiments of the optimization parameters of the nozzle and drastically increasing the energy consumption of high-viscosity working conditions.
[66]	2014	It consists of a jet mixing device, power liquid pump, water tank, liquid tank, spraying system (including spraying rod and 16 nozzles with 0.5 m spacing), and measuring elements (pressure gauge, flow meter, etc.). The core structure of the jet mixing device includes nozzles (adjustable diameter of 2–4 mm), a suction port, a suction chamber, and mixing and diffusion tubes, and the switching of the working state (suction, reflux, or cavitation) is realized by changing the diameter of the nozzles and the load of the system.	In the no-load bench test, the 2 mm nozzle causes bubbles in the inhalation chamber due to cavitation, which affects mixing efficiency, while in the actual spray bar spray system, the 2 mm nozzle can realize a stable suction state, and the 3 mm and above nozzle triggers a reflux state, which avoids the cavitation phenomenon. However, its limitations are as follows: 1. existing research focuses on the flow-field simulation of the jet mixing device itself, and the overall synergistic control of the spray system is insufficient; 2. the system relies on manually adjusting the nozzle diameter and other structural parameters, it lacks real-time dynamic regulation, and it is difficult to achieve accurate concentration feedback control; 3. the cavitation phenomenon will still destroy the uniformity of mixing under specific conditions, and the reflux state may result in the circulation of the liquid medicine, with efficiency decline.
[67]	2014	It consists of a jet mixing device (the core component, including jet nozzle, mixing tube, diffusion tube, and other structures), a spray bar sprayer, a liquid pump, a water tank, a liquid tank, and a flow/pressure measuring device, in which the structural parameters of the jet mixing device (such as the diameter of the jet nozzle outlet d, the diameter of the mixing tube D, and the area ratio m) can realize the online control of the concentration of the pesticide solution by adjusting the fluid dynamics characteristics. The system regulates the mixing performance by changing the structural parameters of the jet device and spray pressure (0.26–0.36 MPa).	When the diameter of the jet nozzle outlet is small (e.g., d = 2.00 mm, area ratio m = 4.00) and the resistance of the spraying system is low (e.g., F110-015 spray nozzle), the system can be stably in the suction state, and the spatial coefficient of variation (CV) of the mixing concentration is less than 5%. However, there are significant limitations of the system: first, the working state is highly dependent on the matching of the parameters of the jet device and the resistance of the spraying system, which may lead to reflux problems when the diameter of the jet nozzle is too large or the area ratio exceeds a reasonable range; second, the experimental conditions are based on the simulated liquid (cochineal) and the laboratory environment, and the differences in the physicochemical properties of the pesticide, dynamic pressure fluctuations, and complex piping conditions in the actual field applications may affect the stability of the pesticide mixing.
[68]	2016	It consists of liquid supply system (including water tank, centrifugal pump, and electric motor), pesticide supply system (including liquid tank, metering pump, and flow control module), and rotating jet mixer. The mixer adopts the structure of a spiral bending contraction tube, spinning device, and diffusion tube with a guide vane to realize the online mixing of water and fat-soluble pesticides through pressure difference, and its core innovation lies in the enhancement of mixing uniformity through fluid spinning.	Through the closed-loop adjustment of the flow rate of the electric metering pump (microcontroller control of the motor frequency to adjust the pump speed), the precise control of the pesticide flow rate can be realized (error < 0.001 kg/s). Numerical simulations and tests show that the system can achieve a uniformity index of 0.9989 for the volume fraction of pesticide in the outlet cross-section under 1 MPa working pressure, but there are the following limitations: the traditional structure of the mixing of fat-soluble pesticides exists in the phenomenon of agglomerated suspension, with a low degree of mixing uniformity; the improved rotary structure improves the mixing effect but increases flow resistance and energy loss (pressure loss of about 0.2 MPa), precision structures such as guide leaves are easily affected by changes in pesticide viscosity, and structural parameters (e.g., guide leaf height, pitch, etc.) need to be adjusted for different pesticide characteristics.
[69]	2016	It consists of a rotary jet mixer, pressure drive system, and mixing effect monitoring system. Among them, the core structure of the mixer includes a spiral contraction tube (contraction degree 0.095, pitch 128 mm, contraction angle 16°), a tangential diverter (adjustable tangential angle β), a guide vane diffuser tube (guide vane height 0–6.5 mm gradient, wrapping angle 15 °), and a relay rotator (including three 6.5 mm high, 15 ° wrapping angle guide vane), using spiral flow generation and maintenance technology to achieve the mixing of the pesticide solution. The diaphragm pump provides water pressure (0.23 MPa) and pesticide pressure (0.22 MPa), with flow meters and manometers to achieve dual-channel precision control.	The volume fraction uniformity index at the nozzle reaches 0.9995, and the maximum mixing ratio is 99.4425%, which significantly improves the dispersion uniformity of fat-soluble pesticides and effectively solves the problem of pesticide suspension in the traditional jet mixer. However, the system still has limitations: first, the structural parameters (such as diverter tangential angle, guide vane layout) need to be dynamically adjusted according to the viscosity and concentration of pesticides, which increases the complexity of the operation; second, it relies on the pesticide pump to maintain the pressure (the outlet directly connected to the spray nozzle requires 0.22 MPa pesticide pump pressure), and energy consumption and equipment costs are high.
[70]	2019	The core structure consists of a jet mixer (e.g., ordinary jet mixer A and improved sandwich orifice tube mixer B), a pesticide supply device (e.g., volumetric metering pump or pulse-width modulated solenoid valve), and an online mixing and detection system (with high-speed camera and fluorescent tracer technology). The system optimizes the control performance by adjusting the carrier flow rate (800–2000 mL/min) and the mixing ratio (4:100–10:100), wherein the sandwich orifice tube mixer B attenuates the injection pulsation through the multi-orifice plate structure, which significantly improves the dynamic concentration consistency.	The average dynamic concentration inconsistency index (CV value) under the test conditions was reduced from 0.039 to 0.011 for mixer A and met the stringent criterion of CV < 0.020 at an injection frequency of >5.10 Hz or a mixing ratio of >5:100. Mixer B achieved an instantaneous homogeneity index (D-value) of about 13.00 for homogeneous mixing at high carrier flow rates (>1400 mL/min) and mixing ratios (>9:100), but its limitation is that the homogeneity of the common jet mixer A decreases due to a single-point injection at high mixing ratios (up to a maximum of D-value of 21.13), and the dynamic homogeneity of mixer B at low flow rates or low mixing ratios still has fluctuations (e.g., CV increased to 0.018 at Qc = 2000 mL/min), and transient uniformity is not sufficiently optimized (D-value vs. CV correlation was only 0.684).

6. Discussion

VRST has evolved as a cornerstone of precision chemical management, yet its implementation pathways diverge significantly across technical architectures and operational contexts. Our analysis reveals that no single approach universally dominates; rather, each technology aligns with distinct agricultural scenarios based on inherent trade-offs between precision, robustness, and cost.

Pressure-regulated systems excel in retrofit simplicity for legacy ground equipment (e.g., tractor-mounted sprayers), making them pragmatic for large-scale field crops where moderate flow adjustment (±20–50%) suffices [71,72]. However, their fundamental dependency on pressure–flow quadratic relationships inherently limits the dynamic range and atomization stability. We contend that this technology is nearing maturity for homogeneous field applications but remains inadequate for orchards with complex canopy structures, where rapid droplet size shifts exacerbate drift risks during vertical spraying [73,74]. The persistent challenge lies in decoupling pressure from droplet spectra—a hardware-level constraint demanding nozzle redesigns beyond conventional operation parameter optimizations [75,76,77].

Flow rate-regulated systems, particularly PWM-based architectures, offer superior granularity in flow control with smaller changes in droplet size. This responsiveness is particularly suitable for scenarios that require a rapid, precise adjustment of spray volume and high consistency in atomization quality. Nevertheless, our assessment identifies a critical gap between laboratory validations and field durability. Solenoid valve wear under high-frequency cycling and nonlinear flow–duty cycle relationships at extreme ends degrade performance in prolonged operations. While smart materials (e.g., magnetorheological fluids) show promise in wear reduction, their scalability is hampered by material fatigue and residual contamination—issues magnified in abrasive pesticide formulations. Despite these durability challenges, PWM-based flow regulation currently stands as the most mature and widely adopted VRST in practical ground applications, owing to its proven effectiveness, relative implementation simplicity, and compatibility with existing nozzle types [78,79,80,81,82].

Pesticide concentration-regulated systems represent the most environmentally conscious approach by minimizing carrier waste and enabling on-demand formulation switching. Direct injection suits continuous large-scale operations requiring sequential multi-chemical applications (e.g., herbicide/pesticide/fungicide cycles), whereas jet mixing excels in dynamic precision-demanding scenarios necessitating rapid responses to real-time variations. However, systemic delays in mixing homogeneity and viscosity sensitivity remain unresolved. High-viscosity adjuvants (e.g., oil-based emulsions) challenge metering pump accuracy and venturi efficiency, risking underdosing in critical zones. The technology’s future hinges on adaptive mixing chambers that dynamically adjust fluid path geometry based on rheology—a direction validated by emerging research on mechanically tunable flow channels (e.g., shape-memory polymer actuators altering mixing path length) and real-time viscosity compensation algorithms, but it is still in the early stages of research and not yet applied in pesticide concentration-regulated systems [83,84,85,86].

This review intentionally focuses on ground-based machinery (boom sprayers, air-assisted sprayers) for field crops and orchards. This is because Unmanned Aerial Spray Systems (UASSs) exhibit fundamentally distinct technical paradigms. (1) UASSs prioritize low volume and small droplet-sized spray, employing centrifugal or rotary atomizers incompatible with ground-based pressure/flow architectures [87,88]. Moreover, their rotor-induced airflow alters droplet dispersion physics, necessitating unique deposition and drift models [89,90]. (2) UASSs generally use flow rate-regulated (mainly PWM control) or, occasionally, rotational speed-regulated VRST and rarely use pressure-regulated VRST due to restrictions on droplet size (rotational speed-regulated VRST is also subject to this restriction). These methods minimize weight/complexity compared to ground systems, as real-time concentration mixing is impractical due to payload constraints and rapid response requirements during dynamic flight maneuvers [91,92,93,94]. (3) In recent years, researchers have published review articles on UASSs in the field of precision application technology [95,96,97,98,99]. Repeating these studies would dilute our analysis of innovations in VRST for ground equipment.

Finally, VRST has been commercialized and widely adopted in developed countries such as in Europe and the United States, but it has yet to be implemented in most developing countries. We still need to enhance the acceptance and adoption rate of VRST, and the key lies in addressing several practical “pain points”: (1) The technology must be able to operate stably over the long term under harsh conditions, such as dust, bumps, and temperature and humidity changes in real fields; complexity is no match for reliability. (2) Users require clear evidence of benefits (cost savings, reduced over-spraying or missed spraying, improved pest control effectiveness, etc.), and investments in technological upgrades must yield reasonable returns. (3) The system should be as simple as possible to operate and maintain, ensuring that frontline agricultural technicians can use it, afford it, and are willing to use it. We should understand that ensuring the stable and reliable implementation of VRST is more practical than pursuing theoretical advancements in high-tech precision.

7. Prospects

The key to the future development of VRST lies in the deep integration of multiple technologies and systematic intelligent upgrades. By addressing current technical bottlenecks, such as nonlinear flow drift caused by nozzle wear, the uneven mixing of high-viscosity pesticides, and reduced application accuracy under environmental interference, we could focus on advancing in three areas. At the hardware level, we could develop ceramic composite wear-resistant nozzles, adaptive compensation channels, and efficient self-cleaning mixing chambers, significantly enhancing the durability of core components (e.g., controlling key nozzle wear rates to extremely low levels) and adaptability to complex pesticides (achieving high-precision mixing ratios across a wide viscosity range, with mixing uniformity CV values targeting below 15%). At the control strategy level, we could deepen environmental sensing (integrating LiDAR, multispectral, and other sensor data) and intelligent decision-making, establishing a “feedforward–feedback” composite control model based on real-time canopy information (volume, density etc.) and machine dynamics (speed, attitude etc.) to create a “feedforward–feedback” composite control model, applying advanced algorithms (such as fuzzy PID combined with neural networks) to rapidly compensate for disturbances, ensuring dynamic precise matching of application parameters (target response delay below 200 ms, concentration control error within ±5%). At the system integration level, efforts could be focused on establishing a coordinated control mechanism for pressure–flow rate and pesticide concentration, supported by edge computing for end-to-end digital management (enabling millisecond-level decision-making), combined with sky-ground collaborative application technology (drone prescription maps and intelligent variable-rate ground equipment integration), driving the system from single-parameter regulation toward multi-objective dynamic optimization, ultimately achieving a significant reduction in application volume and enhanced ecological benefits. Given the current challenges faced by UASs, such as wind field sensitivity and payload limitations, recent developments would prioritize enhancing the intelligence and reliability of ground equipment, with cross-platform technologies integrated in phases over the medium to long term to ultimately establish an autonomous optimization spray application network.