Irrigation Controller System 1217604986108284 8

7/27/2019 Irrigation Controller System 1217604986108284 8

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Irrigation System Controllers


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A controlleris an integral part of an irrigation system.

It is an essential tool to

apply waterin the necessary quantity and at the right time to sustain agricultural production

to achieve high levels of efficiency in water, energy and

chemical uses.


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Irrigation controllers have been available for many years

in the form of mechanical and electromechanical

irrigation timers.

These devices have evolved into complex computer-

based systems that allow accurate control of water,

energy and chemicals while responding to environmental

changes and development stages of the crop.


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Types of controllers

Open control loop systems

Closed control loop systems


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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.


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Open Control Loop Systems

In an open control loop system, a decision is

made by the operator.

The amount of waterand the time at which this

water should be applied is decided by theoperator and set an irrigation controller

according the desired schedule.

Open loop control systems use irrigation

duration orapplied volume for control purposes.


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Components of an open loop

time- based irrigation controller

In open loop system, a clock is used to start irrigationand the application of a given volume to stop irrigation.

In this type of controller the parameters set by thesystem operator are how often and the volume of waterto be applied.


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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.


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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.


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Closed loop System

Closed loop controllers for irrigation systems is based onthe decisions

Direct measurement of soil-moisture using sensors

Calculations of water used by the plants based onclimatic parameters

Both soil moisture sensors and climatic parameter

measurements

The simplest form of a closed loop control system is that

of a high frequency irrigation controllerthat is interrupted

by a moisture sensor.


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The sensoris wired into the line that supplies power from

the controllerto the electric solenoid valve.


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A solenoid valve is an electromechanical valve for use

with liquid or gas controlled by running or stopping an

electrical current through a solenoid, which is a coil ofwire, thus changing the state of the valve.

Operation of a solenoid valve is similar to that of a light

switch, but typically controls the flow of air or water,

whereas a light switch typically controls the flow of

electricity.


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Closed loop system


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The sensor operates as a switch that responds to soilmoisture. When sufficient soil-moisture is available in thesoil, the sensor maintains the circuit open.

When soil-moisture drops below a certain threshold, thesensing device closes the circuit, allowing the controllerto power the electrical valve.

The controller can be set to irrigate at a very highfrequency (4 or 5 times more often than required).

When the controller attempts to irrigate, irrigation willoccur only if the soil-moisture sensor allows it, which inturn occurs only when soil-moisture has dropped belowacceptable levels.

The feedback system is very low cost and is easy toinstall and maintain.


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Limitations

Determining the best location of the sensor whichrequires

knowledge of soil-water

root dynamics

spatial variability of soil properties


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IRRIGATION TIMERS

Irrigation timers are simple controllers consisting ofclock

units capable of activating one or more subunits of theirrigation system at specified times.


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Functions of Irrigation Timer

Clock/timer : Provides the basic time measurements bywhich schedules are executed.

Calendar selector: This function allows definition ofwhich days the system is to operate.

Station time setting: This function allows definition ofstart time and duration for each station.

Master switch: This function prevents activation of any

station connected to the timer.


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Station omission : This function is used to omit any

specified number of stations from the next irrigation

cycle.

Master valve control: Provides control to a master

system valve.

Pump start lead : This feature allows a pump start

solenoid to be activated whenever a station is activated,

thus tying pump control with irrigation control.


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Types of controller designs

Electromechanical

Electronic

El t h i l C t ll


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Electromechanical Controllers

Electromechanical controllers use an electrically drivenclock and mechanical switching (gear arrays) to activatethe irrigation stations.

Very reliable and not too sensitive to the quality of thepower available.

Not affected by spikes and surges in the powerand suchmagnitude will damage the motor.

Even if there is a power outage, the programmedschedule will not be lost and is generally delayed only forthe duration of the power outage.


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Electronic Controllers

Electronic controllers rely on Integrated circuits toprovide the clock/timer, memory and control functions.

These systems are more sensitive to power line quality

than electromechanical controllers.

May require electrical suppression devices in order tooperate reliably.

Because of the inherent flexibility of electronic devices,these controllers tend to be very flexible and provide alarge number of features at a relatively low cost.


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Computer based irrigation control

systems

A computer-based control system consists of a

combination of hardware and software that acts as a

supervisor with the purpose of managing irrigation andother related practices such as fertigation and

maintenance.

This is done by the use of a closed control loop.


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Closed control loop consists of:

1) Monitoring the state of variables

2) Comparing the state of variables with their desired or

target state

3) Deciding what actions are necessary to change the state

of the system

4) Carrying out the necessary actions

To Perform these functions it requires a combination of

hardware and software that must be implemented for each

specific application.


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Hardware Components


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Sensors

A sensor is a device placed in the system thatproduces an electrical signal directly related tothe parameter that is to be measured.

Types of sensors

Continuous

Discrete


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Sensors are an extremely important component of thecontrol loop because they provide the basic data thatdrive an automatic control system.

An important factor related to the sensor is its timeresponse. A sensor must deliver a signal that reflects thestate of the system within the frame of time required by

the application.

Example, for soil moisture measurement, the sensormust be able to "keep up" with the changes in soilmoisture that are caused by evapotranspiration.

Proper selection of the sensors and understanding theprinciple of operation is critical to the success of a

control system.


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Discrete / Continuous sensors


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Continuous

Continuous sensors produce a continuous electricalsignal, such as a voltage, current, conductivity,capacitance, or any othermeasurable electrical property.

Example, sensors of different kinds can be used tomeasure temperature, such as thermistors andthermocouples. A thermocouple will produce a voltagedifference that increases as the temperature increases.

Continuous sensors are used where values taken by astate variable are required and an on/off state is notsufficient, for example, to measure pressure drop acrossa sand filter.


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Discrete

Discrete sensors are basically switches, mechanical orelectronic, that indicate whether an on or off conditionexists.

Discrete sensors are useful for indicating thresholds,such as the opening and closure of devices (vents,doors, alarms, valves, etc.).

They can also be used to determine if a threshold of animportant state variable has been reached.


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Examples of discrete sensors are

a float switch to detect if the level in a storage tank isbelow a minimum desirable level

a switching tensiometer to detect if soil moisture is

above a desired threshold

a thermostat to indicate if a certain temperature hasbeen reached

When combined with time, pulses from switches can be

used to measure rates.


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Variables measured in computer based

control systems

1) Flow rate

2) Pressure

3) Soil-moisture

4) Air temperature

5) Wind speed

6) Solar radiation

7) Relative humidity8) Total salts in irrigation water

9) pH of irrigation water

A/D i t f


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A/D interface

Computer systems work internally with numbers (digits),

the electrical signals resulting from the sensors must beconverted to digital data.

This is done through specialized hardware referred to astheAnalog-to-Digital (A/D) interface.

Discrete signals resulting from switch closures andthreshold measurements are converted to 0 and 1.

Continuous electrical (analog) signals produced by thesensors signals are converted to a numberrelated to thelevel of the sensed variable.

Accuracy of the conversion is affected by the resolutionof the conversion equipment. In general, the higher theresolution the better the accuracy.


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A/D interface


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Computer system

The A/D conversion hardware is directly connected to

the computer system.

Computer system may be a PC (personal computer), a

minicomputer, or a specially designed machine that is

solely dedicated to the control task.


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The type of machine depends on the type of application,and is greatly affected by factors such as environmentcharacteristics, complexity of the controlled system, andthe speed with which conversions need to take place.

Many agricultural applications can be economicallycarried out using personal computers (PC), as is evident

by the increasing number of system integrators andequipment manufactures that are marketing PC-basedcontrol systems.

Many manufacturers of control equipment have designedand manufactured specialized computer control systems.


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Control Interface

Using control software, decisions may be made tomodify the controlled system. The changes are achieved

by having devices within the system that will affect the

controlled variables.

These devices are controlled through actuators that

respond to signals from the control interface.

For example, the extension of a robot arm of a citrusharvestingrobot requires the use of a continuous signal

from the computer, while a fan or a valve requires only

an on/off (discrete) signal from the computer.


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Software Components

Software is used to implement procedures as they applyto the controlled system.

These procedures are usually very elaborate, but in awell-engineered piece of software, they are transparent

to the user.

Because the user is more concerned with ease of useand performance of the system, good quality software

has an interface that allows easy definition of thecharacteristics of the system to be controlled andsimplifies the assignment of hardware resources.

Performance is measured by how well the computer

control system maintains the desired state.


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Actuators (Automatic valves)

Valves specifically designed for remote control are animportant component of the irrigation control system.

Valves are constructed using different materials.

Typically brass, stainless steel and a wide array ofplastics.

Different designs are used to operate the valves:

1) diaphragm type

2) piston type

3) electric thermal motor type.

Irrigation Controller System 1217604986108284 8

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