Irrigation System ControllersIrrigation System Controllers

A A controllercontroller is an integral part of an irrigation system. is an integral part of an irrigation system.

It is an essential tool to It is an essential tool to

apply waterapply water in the in the necessary quantitynecessary quantity and and at the right timeat the right time to to sustain agricultural productionsustain agricultural production

to to achieve high levels of efficiencyachieve high levels of efficiency in in water, energy and water, energy and chemical uses.chemical uses.

Irrigation controllersIrrigation controllers have been available for many years have been available for many years in the form of in the form of mechanicalmechanical and and electromechanical electromechanical irrigation timersirrigation timers. .

These devices have evolved into These devices have evolved into complex computer-complex computer-based systemsbased systems that allow that allow accurate control of water, accurate control of water, energy and chemicalsenergy and chemicals while while responding to environmental responding to environmental

changes changes andand development stages of the crop development stages of the crop..

Types of controllers Types of controllers

Open control loop systemsOpen control loop systems

Closed control loop systemsClosed control loop systems

DifferenceDifference

Open control loop systems Open control loop systems apply a preset actionapply a preset action, as is , as is done with done with irrigation timersirrigation timers. .

Closed control loops have Closed control loops have feedback from sensorsfeedback from sensors, , make make decisionsdecisions and and apply decisions to the irrigation systemapply decisions to the irrigation system. .

Open Control Loop SystemsOpen Control Loop Systems

In an open control loop system, a In an open control loop system, a decision is decision is made by the operator. made by the operator.

The The amount of wateramount of water and the and the time at which this time at which this water should be appliedwater should be applied is is decided by the decided by the operatoroperator and and set an irrigation controller set an irrigation controller according the desired schedule. according the desired schedule.

Open loop control systems use Open loop control systems use irrigation irrigation durationduration or or applied volume for control purposesapplied volume for control purposes. .

Components of an open loop Components of an open loop time- based irrigation controllertime- based irrigation controller

In open loop system, a In open loop system, a clockclock is used is used to start irrigationto start irrigation and and the application of a given volume to stop irrigationthe application of a given volume to stop irrigation..

In this type of controller the In this type of controller the parametersparameters set by the set by the system operator are system operator are how oftenhow often and and the volume of water the volume of water to be applied. to be applied.

Advantages of open loop systemAdvantages of open loop system

Low cost Low cost

Readily available Readily available

The devices are manufactured with The devices are manufactured with different degrees of different degrees of flexibility related to the number of stationsflexibility related to the number of stations and and schedule schedule specification. specification.

Disadvantages of open loop systemDisadvantages of open loop system

Open loop system Open loop system do not respond automatically to do not respond automatically to changing conditions in the environmentchanging conditions in the environment

Require Require frequent resettingfrequent resetting to to achieve high levels of achieve high levels of irrigation efficiency. irrigation efficiency.

Closed loop SystemClosed loop System

Closed loop controllers for irrigation systems is based on Closed loop controllers for irrigation systems is based on the decisions the decisions

Direct measurement of soil-moistureDirect measurement of soil-moisture using using sensorssensors Calculations of water used by the plantsCalculations of water used by the plants based on based on

climatic parametersclimatic parameters Both soil moisture sensorsBoth soil moisture sensors and and climatic parameterclimatic parameter

measurementsmeasurements

The simplest form of a closed loop control system is that The simplest form of a closed loop control system is that of a of a high frequency irrigation controllerhigh frequency irrigation controller that is that is interrupted interrupted

by a moisture sensor.by a moisture sensor.

The The sensorsensor is wired into the is wired into the lineline that that supplies power from supplies power from the controllerthe controller to the to the electric solenoid valveelectric solenoid valve. .

A A solenoid valvesolenoid valve is an is an electromechanical valveelectromechanical valve for use for use with with liquidliquid or or gas gas controlled by controlled by runningrunning or or stopping an stopping an electrical current through a solenoidelectrical current through a solenoid, , which is a coil of which is a coil of wire, thus changing the state of the valve.wire, thus changing the state of the valve.

Operation of a solenoid valve is similar to that of a Operation of a solenoid valve is similar to that of a light light switchswitch, but typically , but typically controls the flow of air or watercontrols the flow of air or water, , whereas a light switch typically whereas a light switch typically controls the flow of controls the flow of

electricity.electricity.

Solenoid valves may have two or more ports: Solenoid valves may have two or more ports:

in a two-port valve the in a two-port valve the flow is switched on or offflow is switched on or off; ;

in a three-port valve, the in a three-port valve, the outflow is switchedoutflow is switched between the between the two outlet portstwo outlet ports. .

Multiple solenoid valves can be placed together on a Multiple solenoid valves can be placed together on a manifoldmanifold

Closed loop systemClosed loop system

The The sensorsensor operates as a operates as a switch that responds to soil switch that responds to soil moisturemoisture. When . When sufficient soil-moisturesufficient soil-moisture is available in the is available in the soil, the soil, the sensor maintains the circuit opensensor maintains the circuit open. .

When When soil-moisture drops belowsoil-moisture drops below a certain threshold, the a certain threshold, the sensing device closes the circuitsensing device closes the circuit, allowing the , allowing the controller to controller to power the electrical valve. power the electrical valve.

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

When the When the controller attempts to irrigatecontroller attempts to irrigate, irrigation will occur , irrigation will occur only if the only if the soil-moisture sensor allows it,soil-moisture sensor allows it, which in turn which in turn occurs only when occurs only when soil-moisture has droppedsoil-moisture has dropped below below acceptable levels. acceptable levels.

The feedback system is The feedback system is very low costvery low cost and is and is easy to install easy to install and maintain. and maintain.

LimitationsLimitations

Determining the Determining the best location of the sensorbest location of the sensor which which requires requires

knowledge of soil-waterknowledge of soil-water root dynamicsroot dynamics spatial variability of soil propertiesspatial variability of soil properties

IRRIGATION TIMERSIRRIGATION TIMERS

Irrigation timers are Irrigation timers are simple controllerssimple controllers consisting of consisting of clock clock unitsunits capable of capable of activating one or more subunits of the activating one or more subunits of the

irrigation system at specified times.irrigation system at specified times.

Functions of Irrigation TimerFunctions of Irrigation Timer Clock/timer :Clock/timer : Provides the Provides the basic time measurementsbasic time measurements by by

which schedules are executed. which schedules are executed.

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

Station time setting:Station time setting: This function allows This function allows definition of definition of start time and duration for each stationstart time and duration for each station. .

Master switch:Master switch: This function prevents This function prevents activation of any activation of any station connected to the timer. station connected to the timer.

Station omissionStation omission : : This function is used to This function is used to omit any omit any specified number of stations from the next irrigation specified number of stations from the next irrigation cycle. cycle.

Master valve controlMaster valve control:: Provides control to Provides control to a master a master system valve. system valve.

Pump start leadPump start lead : : This feature This feature allows a pump start allows a pump start solenoid to be activated whenever a station is activated, solenoid to be activated whenever a station is activated, thus tying pump control with irrigation control. thus tying pump control with irrigation control.

Types of controller designsTypes of controller designs

Electromechanical Electromechanical

ElectronicElectronic

Electromechanical ControllersElectromechanical Controllers

Electromechanical controllers use an Electromechanical controllers use an electrically driven electrically driven clockclock and and mechanical switching (gear arrays)mechanical switching (gear arrays) to activate to activate the irrigation stations. the irrigation stations.

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

Not affected by spikes and surges in the Not affected by spikes and surges in the powerpower and such and such magnitude will damage the motor. magnitude will damage the motor.

Even if Even if there is a power outagethere is a power outage, the , the programmed programmed schedule will not be lost and is generally delayed only for schedule will not be lost and is generally delayed only for the duration of the power outage. the duration of the power outage.

Electronic ControllersElectronic Controllers

Electronic controllers rely on Integrated circuits to Electronic controllers rely on Integrated circuits to provide the clock/timerprovide the clock/timer, , memory memory and and control functionscontrol functions. .

These systems are These systems are more sensitive to power line qualitymore sensitive to power line quality than electromechanical controllers. than electromechanical controllers.

May require May require electrical suppression deviceselectrical suppression devices in order to in order to operate reliably. operate reliably.

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

Computer based irrigation control Computer based irrigation control systemssystems

A computer-based control system consists of a A computer-based control system consists of a combination of combination of hardwarehardware and and softwaresoftware that that actsacts as a as a supervisor supervisor with the purpose of with the purpose of managing irrigationmanaging irrigation and and other other related practicesrelated practices such as such as fertigationfertigation and and maintenance.maintenance.

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

Closed control loop consists of:Closed control loop consists of:

1)1) Monitoring Monitoring the state of variables the state of variables

2) 2) ComparingComparing the state of variables with their the state of variables with their desired or target desired or target statestate

3) Deciding 3) Deciding what actions are necessarywhat actions are necessary to change the state of the to change the state of the systemsystem

4) 4) Carrying outCarrying out the necessary actions the necessary actions

To Perform these functions it To Perform these functions it requires a combination of requires a combination of hardware and softwarehardware and software that must be implemented for each that must be implemented for each

specific application.specific application.

Hardware ComponentsHardware Components

SensorsSensors

A A sensor sensor is a is a devicedevice placed in the system that placed in the system that produces an electrical signal directly related to produces an electrical signal directly related to the parameter that is to be measured. the parameter that is to be measured.

Types of sensorsTypes of sensors

ContinuousContinuous

DiscreteDiscrete

Sensors are an Sensors are an extremely important componentextremely important component of the of the control loop because they control loop because they provide the basic dataprovide the basic data that that drive an drive an automatic control systemautomatic control system. .

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

Example, for Example, for soil moisture measurementsoil moisture measurement, the sensor , the sensor must be able to must be able to "keep up""keep up" with the with the changes in soil changes in soil moisturemoisture that are caused by that are caused by evapotranspiration.evapotranspiration.

Proper Proper selection of the sensorsselection of the sensors and and understanding the understanding the principle of operationprinciple of operation is critical to the success of a is critical to the success of a control system.control system.

Discrete / Continuous sensorsDiscrete / Continuous sensors

ContinuousContinuous

Continuous sensors Continuous sensors produce a continuous electrical produce a continuous electrical signalsignal, such as a , such as a voltage, current, conductivity, voltage, current, conductivity, capacitancecapacitance, or any other , or any other measurable electrical propertymeasurable electrical property. .

Example, sensors of different kinds can be used to Example, sensors of different kinds can be used to measure measure temperaturetemperature, such as , such as thermistorsthermistors and and thermocouplesthermocouples. A thermocouple will produce a . A thermocouple will produce a voltage voltage difference that increases as the temperature increasesdifference that increases as the temperature increases. .

Continuous sensors are used Continuous sensors are used where values taken by a where values taken by a state variablestate variable are required and are required and an on/off statean on/off state is not is not sufficient, for example, to sufficient, for example, to measure pressure drop across measure pressure drop across a sand filter. a sand filter.

DiscreteDiscrete

Discrete sensors are Discrete sensors are basically switchesbasically switches, , mechanical or mechanical or electronicelectronic, that indicate , that indicate whether an on or off condition whether an on or off condition exists. exists.

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

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

Examples of discrete sensors are Examples of discrete sensors are

a a float switchfloat switch to detect if the level in a storage tank is to detect if the level in a storage tank is below a minimum desirable levelbelow a minimum desirable level

a a switching tensiometerswitching tensiometer to detect if to detect if soil moisture is soil moisture is above a desired threshold above a desired threshold

a a thermostatthermostat to indicate if a to indicate if a certain temperature has certain temperature has been reachedbeen reached

When When combined with timecombined with time, pulses from switches can be , pulses from switches can be used to used to measure ratesmeasure rates. .

Variables measured in computer based Variables measured in computer based control systemscontrol systems

1) Flow rate1) Flow rate

            2) Pressure2) Pressure            3) Soil-moisture3) Soil-moisture            4) Air temperature4) Air temperature            5) Wind speed5) Wind speed            6) Solar radiation6) Solar radiation            7) Relative humidity7) Relative humidity            8) Total salts in irrigation water8) Total salts in irrigation water            9) pH of irrigation water9) pH of irrigation water

A/D interfaceA/D interface

Computer systemsComputer systems work internally with work internally with numbers (digits),numbers (digits), the the electrical signalselectrical signals resulting from the resulting from the sensors sensors must be must be converted to digital data.converted to digital data.

This is done through This is done through specialized hardwarespecialized hardware referred to as referred to as the the Analog-to-Digital (A/D) interfaceAnalog-to-Digital (A/D) interface. .

Discrete signalsDiscrete signals resulting from resulting from switch closuresswitch closures and and threshold measurements are threshold measurements are converted to 0 and 1converted to 0 and 1. .

Continuous electrical (analog)Continuous electrical (analog) signals produced by the signals produced by the sensors signals are sensors signals are converted to a numberconverted to a number related to the related to the level of the sensed variable.level of the sensed variable.

Accuracy of the Accuracy of the conversionconversion is affected by the is affected by the resolution resolution of the conversion equipmentof the conversion equipment. In general, the . In general, the higherhigher the the resolutionresolution the the better the accuracybetter the accuracy. .

A/D interfaceA/D interface

Computer systemComputer system

The The A/D conversion hardwareA/D conversion hardware is is directly connecteddirectly connected to to the the computer systemcomputer system. .

Computer system may be a Computer system may be a PC (personal computer),PC (personal computer), a a minicomputerminicomputer, or a , or a specially designed machinespecially designed machine that is that is

solely dedicated to the control tasksolely dedicated to the control task..

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

Many Many agricultural applicationsagricultural applications can be can be economically economically carried out using personal computers (PC),carried out using personal computers (PC), as is evident as is evident by the increasing number of system integrators and by the increasing number of system integrators and equipment manufactures that are marketing PC-based equipment manufactures that are marketing PC-based control systems. control systems.

Many manufacturers of control equipment have Many manufacturers of control equipment have designed designed and manufactured specialized computer control systems.and manufactured specialized computer control systems.

Control InterfaceControl Interface

Using Using control softwarecontrol software, decisions may be made to , decisions may be made to modify the controlled systemmodify the controlled system. The changes are achieved . The changes are achieved by having by having devices within the system that will affectdevices within the system that will affect the the controlled variables. controlled variables.

These devices are controlled throughThese devices are controlled through actuators actuators that that respond to respond to signals from the control interfacesignals from the control interface. .

For example, the extension of a For example, the extension of a robot arm of a citrus robot arm of a citrus harvestingharvesting robot requires the use of a continuous signal robot requires the use of a continuous signal from the computerfrom the computer, while a , while a fan or a valve requires only fan or a valve requires only

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

Software ComponentsSoftware Components

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

These procedures are These procedures are usually very elaborateusually very elaborate, but in a , but in a well-engineered piece of softwarewell-engineered piece of software, they are , they are transparent transparent to the user. to the user.

Because the user is more concerned with ease of use Because the user is more concerned with ease of use and performance of the system, and performance of the system, good quality softwaregood quality software has an interface that allows has an interface that allows easy definitioneasy definition of the of the characteristics of the system characteristics of the system to be controlledto be controlled and and simplifies the assignment of hardware resources.simplifies the assignment of hardware resources.

Performance is measured by Performance is measured by how well the computerhow well the computer control system maintains the desired statecontrol system maintains the desired state. .

Actuators (Automatic valves)Actuators (Automatic valves)

Valves specifically designed Valves specifically designed for remote controlfor remote control are an are an important component of the irrigation control system.important component of the irrigation control system.

Valves are constructed using different materials. Valves are constructed using different materials.

Typically Typically brass, stainless steelbrass, stainless steel and a wide array of and a wide array of plastics.plastics.

Different designs are used to operate the valves: Different designs are used to operate the valves: 1) diaphragm type1) diaphragm type2) piston type2) piston type3) electric thermal motor type3) electric thermal motor type..