GopalaPillai et al. tested an electrical flow control system for site-specific herbicide applications
and its steady state performance [6]. Solenoid valves were used and controlled by a PWM signal in the
system. The chemical flow rate was varied by changing the duty cycle of the PWM signal from 10%
to 100%. However, this was an open-loop system. The precision and stabilization of chemical flow
rate were not discussed. Han et al. also tested a DIS with solenoids controlled by a PWM signal in an
open-loop control system [7]. They found the flow rate control error ranged from 15% to +12%.
Effective flow rate control requires fast and accurate metering of chemicals for the direct injection
sprayer, which could be accomplished by using a positive driven pump or a closed–loop control
system with the help of flowmeters [8]. Frost et al. developed a metering system in which a metered
flow of water was used to control the flow rate of chemical [9]. That was a kind of central direct
injection system (CDIS), where chemical was injected into the system downstream from the main
carrier tank and prior to branching of the distribution hoses carrying the solution to different boom
sections. The advantage of that system is that the chemical was metered by a closed-loop contoller
so that it could provide a wide range of chemical dose rates precisely, while the disadvantage was the
long lag time from a change in the chemical flow rate to the corresponding changes in its concentration
at the spraying points [10].
To solve the long lag time problem, Vondricka and Schulze Lammers proposed a direct nozzle
injection system (DNIS) concept, in which the point where the chemical was injected into the system
was changed to be at the nozzles [11]. This improvement shortened the chemical flow path and
hence reduced the lag time significantly. Subsequently, their team focused on many aspects of this
DNIS. Vondricka addressed on the problem of mixture homogeneity and Doerpmund assessed the
cleanability of this DNIS [12–14]. However, there is still a need to investigate the performance for
DNIS metering and chemical flow rate control, which is the premise for realizing site-specific and
variable-rate application of chemicals.
The objective of this article was to test the injection uniformity of RRVs within a boom section
of the DNIS, to present a closed-loop control method to meter and stabilize the chemical application
rate with the help of a thermodynamic flowmeter, and to evaluate the performance of the control
system and the DNIS by an EC sensor.