Agriculture Reference
In-Depth Information
account for these difficulties in maintaining a constant ground speed (Al-Gaadi and
Ayers, 1993), electronic sprayer control systems adjust application components to
maintain a consistent application rate regardless of speed primarily in one of two
ways listed in the text box in the right.
10.3.2.1 RateControl
Constant pressure systems do not use any control system to minimize application
error and rely on the user to calibrate the system. The operator measures and mixes
the correct amount of chemical with the correct amount of water, checks for worn
nozzles, and maintains a known near-constant ground speed to apply the correct
application rate of chemical. The second method in producing a consistent applica-
tion rate through rate control is to change the nozzle delivery rate by controlling
the nozzle
inlet pressure
. In this method, nozzle inlet pressure is measured, and a
control valve is adjusted to keep the pressure at a set point. Knowing nozzle charac-
teristics and keeping them calibrated is crucial to achieve accurate application rates.
Variation of pressure will cause variation in droplet size and spray pattern created
by the nozzle, but it still can be done to vary the delivery rate within a limited range
without loss of accuracy if nozzles are consistently inspected for wear (Ayers et al.,
1990). The third method is to use a flow meter to measure the volume delivery rate
and adjust the control valve to achieve a target delivery rate. Nozzle characteristics
are not the source of error in this
variable flow rate
method of rate control.
10.3.2.2 ChemicalInjection
Another method of rate control is called
direct chemical injection
or just chemical
injection, in which concentration of the chemical is changed in the carrier while
holding the nozzle delivery rate constant (Reichard and Ladd, 1983). Direct chemical
injection systems limit the handling of leftover active ingredient by the applicator/
worker (a safety concern discussed in Section 10.2.4) and reduce human error in pre-
mixing a tank solution. In a direct injection system, at least two tanks are needed on
the sprayer to hold the carrier and chemicals separately (more than one chemical can
be injected at the same time). The carrier is placed in one large tank on the sprayer,
and the chemical is placed in smaller tank(s) on the sprayer before application (see
Figure 10.4 for direct injection system components).
During the chemical application process in the field, the chemical is precisely
injected into the carrier stream and is mixed in the line carrying the mixture to
the nozzles (Gebhardt et al., 1974; Vidrine et al., 1975; Reichard and Ladd, 1983;
Larson et al., 1982; Peck and Roth, 1975). These systems have the capability of
reducing much of the error that is associated with variation in the application vehicle
ground speed. Several studies have been done to characterize direct injection sys-
tems (Budwig et al., 1988; Tompkins et al., 1990; Hou et al., 1993). A potential limi-
tation of direct chemical injection systems is the transport delay from the injection
point to the application nozzles (Koo et al., 1987; Way et al., 1992; Sudduth et al.,
1995). Incorporation of carrier control into the direct chemical application system
will minimize the effect of the transport delay. Controlling carrier flow rate (water
or air) is equally important to achieve optimal coverage with reduced chemical use
(Steward and Humburg, 2000).
