Agriculture Reference
In-Depth Information
parallel to the irrigation system lateral (Omary et al. 1997; Camp et al. 1998; Stone
et al. 2006). A second approach for achieving variable sprinkler flow rate is to pulse
sprinklers on and off using a control valve with an appropriate duty cycle, normally
of 60-s duration (Fraisse et al. 1992, 1995; Evans et al. 1996, 2010; Han et al. 2009;
Chávez et al. 2010). The primary disadvantage of this approach is cost and reli-
ability of the control valves in terms of plugging and longevity. The plugging issue
is often addressed using normally open-air actuated control valves rather than the
common normally closed electric solenoid pilot operated control valves. Longevity
of air actuated control valves remains an issue (Evans et al. 2010). Use of air actuated
control valves requires the addition of an irrigation system mounted air compressor
and solenoid valves to control air pressure to the air actuated control valves (Han et
al. 2009; Chávez et al. 2010; Evans et al. 2010). Use of normally open-air actuated
control valves alleviates the issue of catastrophic failures associated with freezing
temperatures and normally closed electric solenoid pilot operated control valves.
The number of control valves required can be minimized by grouping sprinklers
on manifolds mounted parallel to the irrigation system lateral (Fraisse et al. 1992;
Omary et al. 1997; Camp et al. 1998), but requires the use of check valves to keep
the manifolds from draining between on/off cycles and makes the system susceptible
to freeze damage.
King and Kincaid (2004) developed a pseudo-variable rate sprinkler by cycling
insertion of a concentric pin in a sprinkler nozzle bore to obtain time-averaged flow
rates of 36% to 100% of normal flow rate for the nozzle sizes tested. The variable
rate sprinkler was field tested on a three-span lateral-move irrigation system and
measured application uniformity equaled or exceeded 90%. The variable rate sprin-
kler allowed site-specific water application to vary over a 36% to 100% range with
minimal effect on water application uniformity. The variable rate sprinkler concept
was patented (King et al. 1998) but has not generated commercialization interest.
11. 5. 3 C
ONTROL
S
YSTEM
H
ARDWARE
The control system executes the software logic that links the control parameter(s)
with a map of spatial water application to achieve SSI management. Hence, the con-
trol system is some type of programmable logic device. Logic devices that have
been used include programmable logic controller (PLC) (Fraisse et al. 1992; Camp
et al. 1998; Evans et al. 2010), single board computer (SBC) (McCann et al. 1997;
King et al. 1998; Coates et al. 2004, 2005, 2006), embedded microcontroller (Evans
et al. 2000), and portable computer (Han et al. 2009). When a PLC or embedded
microcontroller is used, a master computer is used to download control programs to
the PLC and monitor system status and update instructions to the PLC in real time.
The master computer is normally stationary and linked to the PLC using a wireless
modem (Camp et al. 1998; Evans et al. 2000; Coates et al. 2006; Han et al. 2009;
Chávez et al. 2010; Evans et al. 2010). When an SBC is used, a master computer
is used for control program development, downloading control maps to the SBC
(McCann et al. 1997; King et al. 1999) and uploading data (King et al. 2005b).
The communication media used by the control system can vary widely. Initial
SSI developments used open-loop control media such as individual voltage lines to
