Irrigation Controller System 1217604986108284 8
Transcript of 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.