16-04-2012, 05:10 PM
Irrigation System Controllers
irrigation-controller-system-1217604986108284-8.ppt (Size: 473 KB / Downloads: 55)
A controller is an integral part of an irrigation system.
It is an essential tool to
apply water in the necessary quantity and
at the right time to sustain agricultural production
to achieve high levels of efficiency in water, energy and chemical uses.
Types of controllers
Open control loop systems
Closed control loop systems
Difference
Open control loop systems apply a preset action, as is done with irrigation timers.
Closed control loops have feedback from sensors, make decisions and apply decisions to the irrigation system.
Open Control Loop Systems
In an open control loop system, a decision is made by the operator.
The amount of water and the time at which this water should be applied is decided by the operator and set an irrigation controller according the desired schedule.
Open loop control systems use irrigation duration or applied volume for control purposes
Advantages of open loop system
Low cost
Readily available
The devices are manufactured with different degrees of flexibility related to the number of stations and schedule specification.
Disadvantages of open loop system
Open loop system do not respond automatically to changing conditions in the environment
Require frequent resetting to achieve high levels of irrigation efficiency.
Closed loop system
The sensor operates as a switch that responds to soil moisture. When sufficient soil-moisture is available in the soil, the sensor maintains the circuit open.
When soil-moisture drops below a certain threshold, the sensing device closes the circuit, allowing the controller to power the electrical valve.
The controller can be set to irrigate at a very high frequency (4 or 5 times more often than required).
When the controller attempts to irrigate, irrigation will occur only if the soil-moisture sensor allows it, which in turn occurs only when soil-moisture has dropped below acceptable levels.
Limitations
Determining the best location of the sensor which requires
knowledge of soil-water
root dynamics
spatial variability of soil properties
Software Components
Software is used to implement procedures as they apply to the controlled system.
These procedures are usually very elaborate, but in a well-engineered piece of software, they are transparent to the user.
Because the user is more concerned with ease of use and performance of the system, good quality software has an interface that allows easy definition of the characteristics of the system to be controlled and simplifies the assignment of hardware resources.
Performance is measured by how well the computer control system maintains the desired state.
Actuators (Automatic valves)
Valves specifically designed for remote control are an important component of the irrigation control system.
Valves are constructed using different materials.
Typically brass, stainless steel and a wide array of plastics.
Different designs are used to operate the valves:
1) diaphragm type
2) piston type
3) electric thermal motor type.