Chapter 12

Variable-Rate Technology in Precision Agriculture

Variable-Rate Liquid Chemical Technology

Variable-rate liquid applicators are precision agriculture tools that adjust the application rate of liquid fertilizers or pesticides based on specific field conditions or crop needs. They offer more efficient and targeted applications compared to constant-rate applicators, potentially reducing input costs and environmental impact. Variable-rate liquid applicators or field sprayers are designed to provide an adjustable product output rate (volume per unit time). However, application rate (volume per unit area) is affected by both product output rate and applicator travel speed. So, to maintain a preset application rate, travel speed is monitored, and the output rate is adjusted to compensate for changes in ground speed. Different types of control systems are on the market that are adaptable to precision applications, including automatic section control systems and pulse width modulation systems

Automatic Section Control System

The flow of product to the boom is shut ON and OFF using a boom shut-off valve. These valves allow the operator to shut the sprayer ON and OFF. The two-way boom valve is simply an ON or OFF boom valve that either allows (on-state) or does not allow (off-state) product flow to the boom section. When the two-way boom valve is shut off, the product volume intended for that boom section is not delivered. A sprayer setup with two-way boom shut-off valves uses flow feedback to adjust the flow Most agricultural sprayers have booms split into what are termed “boom-sections,” allowing for independent control. Automatic section control rate controllers track the location of each section with respect to field boundaries, keep-out zones, and previously applied areas as the tractor moves through a field.

Flow-Based Control Systems

The flow-based control system has a flow meter, a ground speed sensor, and a controllable valve (servo valve) with an electronic controller to apply the desired rate of the tank mix (Figure 12.6). A microprocessor in the console uses information regarding sprayer width and prescribed gallons per acre to calculate the appropriate flow rate (gallons per minute) for the current ground speed. The servo valve is then opened or closed until the flow-meter measurement matches the calculated flow rate.

Chemical Direct-Injection Systems

An alternative approach to flow-based control system uses direct injection of the chemical into a stream of water. These systems utilize the controller and a chemical pump to manage the rate of chemical injection rather than the flow rate of a tank mix. The flow rate of the carrier (water) is usually constant, and the injection rate is varied to accommodate changes in ground speed or changes in prescribed rate.

Modulated Spraying-Nozzle Control Systems

A modulated spraying-nozzle control (MSNC) or pulse width modulation system utilizes solenoid valves mounted at each individual nozzle to provide automatic proportional flow compensation based on the speed of the sprayer and desired application rate (Figures 12.7 and 12.8). The solenoid valve is an electromagnet that opens and closes the nozzle flow using a plunger held in the closed position by a spring. The MSNC system maintains the constant desired nozzle application pressure irrespective of the number of nozzles in the ON or OFF states. This is a tremendous advantage over flow-based systems, where pressure is affected during speed transitions and boom-section actuations.

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