Agriculture Reference
In-Depth Information
FIGURE 10.5
A commercially available PWM nozzle (Capstan Ag. Systems).
A larger duty cycle results in smaller average nozzle pressure, whereas a smaller
duty cycle results in higher nozzle pressure. This ability to adjust the flow rate
through the use of the solenoid duty cycle creates an additional degree of freedom
for both flow rate and droplet size to be individually controlled. Flow rate and thus
application rate can be adjusted through varying the duty cycle, whereas both flow
rate and droplet size can be selected through adjusting the operating pressure.
PWM nozzles (Figure 10.5) are especially effective in VRAs (Section 10.4.1)
where the desired application rate can be varied quite often during an application
(Schueller, 2009). Although it is desirable to adjust application rate, it is also desir-
able to maintain a consistent droplet size. This valve is pulsed on and off at a rapid
rate, and by varying the duty cycle of the square wave input, the flow rate of the
nozzle can be varied continuously without the droplet size variations associated with
pressure-based flow control (Giles et al., 1996). A PWM controller can change its
flow rate very quickly while maintaining a constant droplet size.
In addition, optimization of nozzle characteristics including variable orifice
nozzles has been a focus in recent years to reduce the drift while achieving higher
coverage and efficacy in the field. In variable orifice nozzles, orifice area is adjust-
able based on liquid or air pressure that will adjust the flow based on pressure while
keeping the droplet size uniform (Bui, 2005, 2006; Tian, 2007). Some researchers
have also investigated multijet nozzles to deliver air and chemical through nozzles
(Zhu et al., 2006).
10.3.4 S
ECTION
C
ONTROL
Dividing the length of a sprayer unit into multiple sections and automatically control-
ling those individual sections can enable more efficient spraying. The overall goal of
section control is to reduce pass-to-pass overlap as well as prevent the spraying on
nontarget areas. Within section control spraying systems, one or more sections of the
sprayer boom or tower are individually controlled in an on/off fashion. Each section
is controlled independently of the rest of the system based on the section's location
within the field or canopy. These systems rely on a task computer to control the state
of each section based on the sections' location within the field or canopy, taken from
accompanying sensors and/or onboard maps (Section 10.4.3).
Map-based boom section control systems are commonly used in row crops. GIS
(geographical information system) maps are the central components in this system,
which contain no-spray zones determined by the operator before spraying, as well as
