Agriculture Reference
In-Depth Information
thus regulating the application rate. Jeon et al. (2011) developed an over-the-row,
dual-sided, platform sprayer for tree crops. Each nozzle was controlled by a micro-
controller using a PWM solenoid valve. The application rate was calculated in real
time and adjusted accordingly. These VRA systems have been evaluated in research
as well as commercial operations with both air-blast and tower sprayers in specialty
crops and with boom sprayers in row crops.
Response time is a critical parameter of rate controllers used in variable rate
chemical application. VRA systems generally use either real-time sensing or
location-based info to update rate set point several times a second. The controller
then follows the set points through actuation of control valves. Delay time involved
in these physical systems will cause some lag between when the set point is changed
and the actual rate is achieved. For a typical variable rate applicator with a com-
mercially available rate controller, response time may vary between 0.5 and 2.5 s
(Bennur and Taylor, 2010). The response time of the rate control system to stabilize
nozzle flow during boom section control on different point rows can be up to 34.0 s
(Sharda et al., 2011b). Information about response time can be used to define optimal
spatial resolution of VRA while minimizing the error between target and actual
application rate.
Flow rate control is the fundamental control system used to achieve VRA. A rate
controller will try to bring applied output as close to the command input as pos-
sible in a minimal amount of time. Commonly, an
ad hoc
proportional controller
is applied in commercially available systems, where users have to tune controller
parameters to achieve a desired dynamic response (Bennur and Taylor, 2010; e.g.,
Raven Controllers, Raven Industries, Sioux Falls, SD). Some researchers have also
investigated the use of proportional controls to develop a solenoid valve-based spray
control system (Solanelles et al., 2006; Steward and Humburg, 2000). Even though
less frequently, full PID control has also been used in chemical application systems
(e.g., FALCON controller, AGCO Corporation, Duluth, GA; Yang, 2000). In offline
map-based application, look-ahead control has been beneficial to compensate for
lag that occurs in the VRA system. However, for real-time sensor based application,
look ahead is challenging, and components with faster response time are essential to
achieve higher application rate accuracy.
10.4.3 S
ELECTIVE
A
PPLICATION
Selective application can be viewed as a special case of VRA system discussed in
Section 10.4.1. In this system, the chemical is applied selectively by turning on the
nozzles only when there is a presence of pest or target crop canopy in the field.
Machine vision-based selective patch spraying has been investigated for row crops
that could deliver herbicide onto weed patches while traveling at typical operational
speeds (Tang et al., 2000a; Steward et al., 2002). Investigators have also developed
and tested a prototype pulsed-jet, microdosing actuator capable of applying micro-
liter herbicide dose rates to small target areas 6.3
×
12.5 mm in size (Zhang et al.,
2009; Downey et al., 2004; Crowe et al., 2005).
This approach of turning sprayer nozzles on and off based on presence and
absence of canopy in perennial crops is one of the simplest but practical automation
