Agriculture Reference
In-Depth Information
control valves (Fraisse et al. 1992; Camp et al. 1998; Han et al. 2009; Evans et al.
2010) or unidirectional digital communications (King et al. 1995, 1999; McCann et
al. 1997). The need to monitor irrigation system operational parameters and field
location led to the use of bidirectional communications with multidrop capability
(Evans et al. 1996, 2000; King and Wall 1998). The bidirectional communication
medium used includes power line carrier (King and Wall 1998), RS-485 (Evans et
al. 1996, 2000), RS-232 (Chávez et al. 2010), one-wire (Coates et al. 2006), and RF
(Coates and Delwiche 2009). The use of bidirectional communications has resulted
in control systems with master-slave networks (King and Wall 1998; King et al.
2005b; Coates et al. 2006) and mesh networks (Coates and Delwiche 2009). The
need for real-time soil and plant water status information to make SSI management
decisions led to the development of control system networks that can also collect
data from in-field sensors. Data collection from in-field monitoring equipment has
been integrated into the controls system network by either using the control system
bidirectional communications media (King et al. 2005b) or using a parallel bidirec-
tional communications media such as RF (Han et al. 2009; Chávez et al. 2010; Evans
et al. 2010).
Digital control outputs from the programmable logic device (0, +5 VDC) are
converted to 24 VDC needed to actuate control valves and power relays using relay
interface hardware. Most PLC and SBC manufacturers provide expansion boards
with relays that interface directly (Fraisse et al. 1992; Camp et al. 1998; Han et al.
2009; Chávez et al. 2010; Evans et al. 2010). Relay interface circuitry can be included
on printed circuit boards (PCB) manufactured specifically for SSI (King and Wall
1998; King et al. 2005b; Coates et al. 2006; Coates and Delwiche 2009). The relay
interface hardware is used to power control valves for water application and power
control relays used for manipulating the speed of the irrigation system (Camp et al.
1998; Han et al. 2009). Serial communication interface and analog-to-digital and
digital-to-analog conversions with PLCs and SBCs is provided directly or through
expansion boards provided by manufacturers (Camp et al. 1998; Han et al. 2009;
Chávez et al. 2010; Evans et al. 2010) and can be included on PCBs manufactured
specifically for SSI (King and Wall 1998; King et al. 2005b; Coates et al. 2006;
Coates and Delwiche 2009).
11. 5.4 C
ONTROL
M
AP
The control map contains coding the control system uses to apply the prescribed
volume of water everywhere on the land area covered by the SSI system. In control
system software, the control map usually consists of a two-dimensional array where
each element represents a control zone. The size of the array is determined by the
size of the control zones and irrigated area. For lateral move irrigation systems, a
rectangular coordinate system is usually used, and each array row represents con-
trol zone dimension in the direction of travel and columns represent control zone
dimension parallel to the system lateral. For center pivot irrigation systems, a polar
coordinate system is usually used where each row of the element represents angular
control zone dimension and the columns represent control zone dimension parallel to
the system lateral. Using control zone dimensions and a specified reference location,
