plc scada for automation process control
DESCRIPTION
PLC SCADA FOR AUTOMATION PROCESS CONTROLTRANSCRIPT
PLC & SCADA for
Automation & Process
Control
Dr. Mohammad H. Salah
Introduction to
Control Strategies
Outlines
� Control Systems
� Continuous Control Systems
� Sequential (Logic) Control Systems
� Synchronous Control Systems
� Asynchronous Control Systems
� Mixed Synchronous/Asynchronous Control Systems
� Implementation of Synchronous Control Systems
� Relay Control systems
Dr. Mohammad H. Salah Page no. 1
Control Systems
� Any process contains the application (operative part) and control
system (active coordinator).
� The best way to describe a control system is to use a block
diagram.
� All control systems have, at least, three parts to them;
� An INPUT that takes information into the control system,
� A PROCESS that uses the input information to create the
output information,
� An OUTPUT that passes information out of the control system.
Control Systems - Inputs
� Input signals are provided by transducers / detectors that convert
physical quantities into electrical signals.
� Depending on transducer used, the information detected can be
discontinues (binary) or continuous (analog) representation of the
input quantity.Transducers Measured Quantity Output Quantity
Switch
Limit Switch
Thermostat
Thermistor
Strain Gauge
Photo Cell
Proximity Cell
Thermocouple
Movement / Position
Temperature
Pressure / Movement
Light
Presence of Objects
Movement / Position
Temperature
Temperature
Binary Voltage
Binary Voltage
Varying Voltage
Varying Voltage
Varying Voltage
Varying Resistance
Varying Resistance
Varying Resistance
Dr. Mohammad H. Salah Page no. 2
Control Systems - Outputs
� Output devices (like relays, pumps, motors..) are tools used by a
control system to alter certain key element or quantities within the
process.they are also transducers but contrary signals from the
control system into other necessary. There are also discontinuous
(binary) or continuous (analog) devices
Motor
Pump
Solenoid
Heater
Valve
Relay
Piston
Rational motion
Heat
Orifice variation
Elec. Switching / limited physical movement
Rational motion + product displacement
Linear motion / pressure
Linear motion / pressure
Electrical
Electrical
Electrical/Hydraulic/pneumatic
Hydraulic / pneumatic
Electrical
Electrical
Electrical
Output Device Quantity Produced Input
Control Systems
� The heater is an Open Loop control system.
� In this system the information from the output is not sent back to the
input.
� However the room gets hot, the heater keeps producing heat until
someone switches it off.
� If the heater had a thermostat, it would switch off by itself when the
room reached a set temperature (the ‘input’ to the system).
� In this case information from the output of the system (heat) has
been fed back to the input.
Dr. Mohammad H. Salah Page no. 3
Control Systems
� The control system is now a Closed Loop system.
Information from the output goes back to the input in a
Feedback loop.
� The comparison block of the system is normally represented
by the special symbol:
� This shows the place of the heater thermostat in the control
system.
� It compares the set temperature with the actual
temperature.
Control Systems
� A difference between these two temperatures is an error.
� When the control system detects an error, it tries to make itsmaller by changing the output.
� This system now has all the basic elements of any controlsystem:
� A demand - this is the set temperature shown above.
� A sensor to measure the output - a temperature sensor.This is part of the thermostat
� A controller - the thermostat
� An actuator - the heater
Dr. Mohammad H. Salah Page no. 4
Control Systems
Remember:
� A sensor is a device that converts a physical signal (such asheat, light, sound or movement) into an electrical signal.
� An actuator is a device that converts an electrical signal intoa physical signal (such as heat, light, sound or movement).
� Actuators and sensors are both devices that change one kindof signal into a different kind of signal.
Control Systems
� The controller should be designed with some objective inmind.
� Typical objectives are:
� fastest response - reach the setpoint as fast as possible(e.g., hard drive speed)
� smooth response - reduce acceleration and jerks (e.g.,elevators)
� energy efficient - minimize energy usage (e.g., industrialoven)
� noise immunity - ignores disturbances in the system (e.g.,variable wind gusts)
Dr. Mohammad H. Salah Page no. 5
Continuous Control Systems
� Continuous processes require continuous sensors and/or
actuators.
� In continuous control systems the inputs are sending information
into the system all the time and the outputs of the system are
being controlled all the time.
� A change to the input leads directly to a change in the output.
� An example of this kind of system is a security floodlight that
comes on in the dark; the level of light reaching the light sensor is
continually controlling whether or not the lamp is on.
Continuous Control Systems
� Another example is filling a washing machine with water uses acontinuous control system that monitors the water level andcontrols the water input valves.
� Continuous control systems typically need a target value, this iscalled a setpoint
Dr. Mohammad H. Salah Page no. 6
Continuous Control Systems
� Water Tank Level Control
Sequential Control Systems
� In a sequential control system a series of different events
takes place one after the other.
� The finishing of one event in the sequence provides the
signal for the next event to start.
� Examples of sequential systems are:
� the timers that control central heating systems.
�washing machines.
� traffic lights.
� lifts in buildings.
Dr. Mohammad H. Salah Page no. 7
Sequential Control Systems
� Sometimes one of the events in the sequence is itself a
continuous control system.
� However this is only one event in the series of events that
makes up the complete sequential control system for the
washing machine
Synchronous Control
� In a fully synchronous control system all of the events in thesequence take place at set points in time, regardless of anyexternal change.
� Synchronous control systems are used where the control ofa sequence of events must take place at pre-set timeintervals.
� Such a system doesn’t take any account of events outside it,only the time between events is important.
� Therefore it doesn’t need any sensors; it is an open loopcontrol system.
� Central heating timers are synchronous controllers; thepoints at which the heating and hot water systems areturned on and off are fixed in time.
Dr. Mohammad H. Salah Page no. 8
Synchronous Control
� Once the heating or hot water is turned on, that part of thesequence is usually a continuous system; temperature iscontinuouslymonitored to control the heating system.
Asynchronous Control
� In an asynchronous control system all of the events in thesequence take place as a result either due to an externalevent or because the previous event has finished,regardless of the time taken.
� Asynchronous control systems are used where the timetaken for a sequence to occur is unimportant.
� Each event happens as soon as the previous event is finishedor when something outside the system happens; suchsystems require sensors to detect the completion of anevent or an outside event and so must be closed loop.
Dr. Mohammad H. Salah Page no. 9
Asynchronous Control
� The control system for a lift is asynchronous; the sequenceof events depends entirely on external events (peoplepressing the call buttons outside the lift, and the floorbuttons inside it) or the completion of lift movements (thelift stops moving, and the doors are opened, when a switchdetects that a floor has been reached).
Mixed Syn./Asyn. Control Systems
� In most real sequential control systems there is a mixture ofsynchronous and asynchronous control
� Many modern traffic light sets have pedestrian crossing lights orsensors in the road to detect the presence of cars. These affectthe timing of the sequence of lights making the mainlysynchronous system mixed.
� In a lift automatic doors often stay open for a fixed time. Thismakes a mainly asynchronous system mixed.
� The following is a list of some devices that use sequential controlsystems:
1. A security gate.
2. A dishwasher
3. A time lock on a bank’s safe.
4. A robot arm welding parts of a car together
Dr. Mohammad H. Salah Page no. 10
Mixed Syn./Asyn. Control Systems
For each system;
� Draw a block diagram showing the sequence of events in thesystem.
� Write down whether you think it is synchronous,asynchronous or mixed.
� Explain your answer.
� If you think a system is asynchronous, explain how you thinkeach step in the sequence is triggered.
� If you think it is a mixed system, describe which parts youthink are synchronous and which asynchronous.
Implementation of Syn. Control Sys.
� The heart of a synchronous control system is some kind oftimer.
� This can be mechanical or electronic.
� The timer also needs:
� A sequencing element; this sets the times that outputsare switched on and off. Remember that there are noexternal inputs into a synchronous timer.
� An output stage that provides the start and stop signals.
Dr. Mohammad H. Salah Page no. 11
Implementation of Syn. Control Sys.
Mechanical Systems
� All mechanical timers are different kinds of cam timer.
� Here a motor turning at a constant speed is used to turn lotsof cams.
� As the cams turn they push on switches to turn them on oroff.
� Central heating and washing machine timers always used tobe made from cam timers.
� In industry too, cam timers havebeen used widely - though they arebeing rapidly replaced by electronictimers these days.
Implementation of Syn. Control Sys.
Electronic Systems
� There are a number of different electronic systems that canbe used.
� A dedicated circuit uses an oscillator to give electronic clockpulses.
� Further circuitry, often involving the use of logic gates, isthen used to control a sequence of switching.
� Programmable Logic Controllers (PLCs) are commonly usedin industry.
� A PLC contains the same kind of microprocessor as acomputer.
� However it is designed to be used in an industrial setting sois very robust.
Dr. Mohammad H. Salah Page no. 12
Implementation of Syn. Control Sys.
Electronic Systems
� The timing sequence can be programmed either through acomputer or with a small, hand held, programmer.
� PLCs are replacing cam timers in most places in industry.
Implementation of Syn. Control Sys.
Electronic Systems
� An important thing to note about these electronic systems isthat they all use low voltages and currents.
� They also need to be able switch powerful outputs.
� So they will need some kind of output interface that protectsthe circuit and provides power.
� Relays are very often used for this.
Dr. Mohammad H. Salah Page no. 13
Relay Control Systems
� The relay based systems are control systems that use relays
or contactors or both to operate the system actuators
sequentially and they are usually electromechanical devices
(some are solid state relays).
Relay Control Systems
� Contactor can handle higher load currents than relays.
� The behavior of a relay or a
contactor (electromechanical
devices) exhibits nonlinearity in
operation
Dr. Mohammad H. Salah Page no. 14
Relay Control Sys. - Features
� Group of relays with large number of contacts
� Space required
� Fixed application
� Simple control tasks
� Difficult expansion and/ or modification
� Slow action (except for solid state relays)
BUT
Relays continue to be used as output devices being ideal
for the conversion of small signals to higher current /
voltage driving signal.
Relay Ladder Logic Control
� Logic control is used with relatively simple ON/OFF systems -
like pneumatics
Pneumatic System
Relay Ladder Logic
Control
Dr. Mohammad H. Salah Page no. 15
Relay Ladder Logic Control
Relay Ladder Logic Control
Dr. Mohammad H. Salah Page no. 16
Relay Ladder Logic Control
Normally Open Schematic
Normally Closed Schematic
Relay Ladder Logic Control
Output Schematic
Dr. Mohammad H. Salah Page no. 17
Relay Ladder Logic Control
Why is it called “Logic Control”?
Relay Ladder Logic Control
Write the logic for this rung.
Dr. Mohammad H. Salah Page no. 18
Relay Ladder Logic Control
Relay Ladder Logic Control
Dr. Mohammad H. Salah Page no. 19
Relay Ladder Logic Control
Relay Ladder Logic Control
Dr. Mohammad H. Salah Page no. 20
Relay Ladder Logic Control
Relay Ladder Logic Control
Dr. Mohammad H. Salah Page no. 21
Relay Ladder Logic Control
Relay Ladder Logic Control
Dr. Mohammad H. Salah Page no. 22
Relay Ladder Logic Control
Relay Ladder Logic Control
Dr. Mohammad H. Salah Page no. 23
Transducer
A transducer is any device that converts energy from
one form to another.
Input transducer
(microphone) converts
sound energy to electric
energy
Output transducer
(speaker) converts
electric energy to sound
energy
Amplifier
Sensors
Sensors are input transducers used for detecting and
often measuring the magnitude of something. They
convert mechanical, magnetic, thermal, optical, and
chemical variations into electric voltages and currents.
Photoelectric
sensor
Dr. Mohammad H. Salah Page no. 24
Sensors
Sensors provide the equivalent of eyes, ears, nose,
and tongue to the microprocessor brain.
Microprocessor
Optical
sensor
Gas
sensor
Microphone
Probe
Proximity Sensor
Proximity sensors or switches detect the presence of
an object without making physical contact with it.
Dr. Mohammad H. Salah Page no. 25
Proximity Sensor Applications
The object being detected is too small, lightweight, or
soft to operate a mechanical switch.
Rapid response and high switching rates are required.
An object has to be sensed through nonmetallic barriers
such as glass, plastic, and paper cartons.
Hostile environments conditions exist.
Long life and reliable service are required.
A fast electronic control system requires a bounce-free
input signal.
Inductive Proximity Sensor Operation
Barrel type
Block diagram
As the target
moves into the
sensing area,
the sensor
switches
the output ON
Dr. Mohammad H. Salah Page no. 26
Proximity Sensor Connections
The method of connecting and exciting a proximity
sensor varies with the type of sensor and its
application.
TargetL1 L2
Load
Two-wire sensor connection
Proximity Sensor Connections
Load is connected
between the
sensor and ground
Current-sourcing output (PNP)
Load
Sensor
Control
output
Dr. Mohammad H. Salah Page no. 27
Proximity Sensor Connections
Load is connected
between the positive
supply and sensor
Current-sinking output (NPN)
Load
Sensor
Control
output
Proximity Sensor Connection To Input Module
Proximity
sensor
Input
module
L1 L2
Bleeder resistor
The use of a bleeder
resistor allows enough
current for the sensor
to operate but not
enough to turn on the
input of the PLC
A proximity sensor should
be powered continuously
Dr. Mohammad H. Salah Page no. 28
Capacitive Proximity Sensor
A capacitive proximity sensor can be actuated by both
conductive and nonconductive material such as wood,
plastics, liquids, sugar flour and wheat.
Operation is similar to that of inductive
proximity sensor. Instead of a coil, the
active face of the sensor is formed by
two metallic electrodes – rather like an
"opened capacitor".
Magnetic Switch (Reed Switch)
A magnetic switch (also called
a reed switch) is composed of
flat contact tabs that are
hermetically sealed (air-tight).
Common
NO
NC
The switch is
actuated by a
magnet.
Magnet
N S
Dr. Mohammad H. Salah Page no. 29
Reed Switch Activation
Magnet
Reed switch
Proximity motion – movement
of the switch or magnet will
activate the switch
Rotary motion – switch is
actuated twice for every
complete revolution
Shielding – the shield
short circuits the magnetic
field; switch is activated
by removal of the shield
Photovoltaic Or Solar Cell
The photovoltaic cell, or solar cell, is a
common light-sensor device that
converts light energy directly into
electric energy.
Solar cell
The solar cell converts light
impulses directly into electrical
charges which can easily be
amplified to provide an input
signal to a PLC.
Dr. Mohammad H. Salah Page no. 30
Photoconductive Or Photoresistive Cell
The photoconductive cell, or
photoresistive cell, is is another
popular type of light transducer.
Light energy falling on this device
will cause a change in the
resistance of the cell.
20 Ohms Light resistance
5,000 Ohms Dark resistance
Ohms
Photoelectric Sensor Operation
Most industrial photoelectric sensors use a light-emitting
diode (LED) for the light source and a phototransistor to
sense the presence or absence of light.
Object
to be
sensed
Light detector
Light source
Light from the LED falls
on the input of the
phototransistor and the
amount of conduction
through the transistor
changes. Analog
outputs provide an
output proportional to
the quantity of light
seen by the
photodetector.
Dr. Mohammad H. Salah Page no. 31
Reflective Photoelectric Sensor
Emits a light beam (visible,
infrared, or laser) from its light
emitting element and detects
the light being reflected.
Retro-reflective type
Operating
range
Reflector
Operating
range
Diffused-reflective type
Emitter/receiver
Target
Through-Beam Type Photoelectric Sensor
A through-beam photoelectric
sensor is used to measure the
change in light quantity caused
by the target's crossing the
optical axis.
Operating
range
Target
Emitter Receiver
Dr. Mohammad H. Salah Page no. 32
Bar Code Systems
Bar code systems can be used to
enter data much more quickly
than manual methods, and are
highly accurate.
Scanner
Decoder
PLC
Diverter
The decoder receives the
signal from the scanner
and converts these data
into the character data
representation of the
symbol's code.
Ultrasonic Sensor
An ultrasonic sensor operates by
sending sound waves towards the
target and measuring the time it
takes for the pulses to bounce back.
The returning echo signal
is electronically converted
to a 4 mA to 20 mA output,
which supplies flow rate to
external control devices.
Dr. Mohammad H. Salah Page no. 33
Strain/Weight Sensors
A strain gauge transducer converts
a mechanical strain into an electric
signal.
ForceWire type The force applied to the gauge causes the
gauge to bend. This bending action also
distorts the physical size of the gauge,
which in turn changes its resistance.
The load cell provides
sensor input to the
controller, which
displays the weight
and controls the
hopper chute.
Load cell
Controller
Hopper
ChuteON/OFF
Control
Temperature Sensors
Temperature sensors convert heat into an electric
signal. There are four basic types used: thermocouple,
resistance temperature detector (RTD), thermistor, and
IC sensor.
The thermocouple consists of a pair
of dissimilar conductors fused
together at one end to form the
"hot" or measuring junction, with the
free ends available for connection to
the "cold" reference junction. A
temperature difference between the
measuring and reference junction
generates a small DC signal voltage.
Dr. Mohammad H. Salah Page no. 34
Temperature Sensors
Temperature sensors convert heat into an electric
signal. There are four basic types used: thermocouple,
resistance temperature detector (RTD), thermistor, and
IC sensor.
The resistance temperature
detector (RTD) varies in resistance
value with changes in temperature.
RTD
Temperature Sensors
Temperature sensors convert heat into an electric
signal. There are four basic types used: thermocouple,
resistance temperature detector (RTD), thermistor, and
IC sensor.
The thermistor varies in
resistance value with
changes in temperature
Dr. Mohammad H. Salah Page no. 35
Temperature Sensors
Temperature sensors convert heat into an electric
signal. There are four basic types used: thermocouple,
resistance temperature detector (RTD), thermistor, and
IC sensor.
The Integrated Circuit (IC) temperature
sensor produces changes in voltage or
current with changes in temperature.
Flow Measurement
The usual approach used in
measuring fluid flow is to
convert the kinetic energy that
the fluid has into some other
measurable form.
Flow Magnet
Turbine
Turbine Flow Meter
Coil
The turbine blades turn at
a rate proportional to the
fluid velocity and are
magnetized to induce
voltage pulses coil.
Dr. Mohammad H. Salah Page no. 36
Flow Measurement
The usual approach used in
measuring fluid flow is to
convert the kinetic energy that
the fluid has into some other
measurable form.
Electronic Magnetic
Flow MeterCan be used with electrically
conducting fluids and offers no
restriction to flow. A coil in the
unit sets up a magnetic field. If
a conductive liquid flows
through this magnetic field, a
voltage is induced and sensed
by two electrodes.
Velocity/RPM Sensors
A tachometer is a small permanent
magnet DC generator which when
rotated produces a voltage that is
directly proportional to the speed at
which it is driven.
Controller
Tach
Motor
M
Tachometers coupled
to motors are
commonly used in
motor speed control
applications to provide
a feedback voltage to
the controller that is
proportional to motor
speed.
Dr. Mohammad H. Salah Page no. 37
Velocity/RPM Sensors
The rotating speed of a
shaft is often measured
using a magnetic (inductive)
pickup sensor.
0 V
Pickup coil Pole piece
N S
MagnetSensor
output
A magnet is attached to the shaft. A
small coil of wire held near the
magnet receives a pulse each time
the magnet passes. By measuring
the frequency of the pulses, the
shaft speed can be determined.
Output Control Devices
A variety of output control devices can be operated by the
controller output module to control traditional processes.
These include:
Pilot light
Solenoid Solenoid
valveControl
relay
Alarm
HeaterMotor starter Small motor
Dr. Mohammad H. Salah Page no. 38
Actuators
An actuator is any device that converts an electrical
signal into mechanical movement. The principle types
of actuators are relays, solenoids, and motors.
AIR
Coil
Plunger
Solenoid Symbol The solenoid converts
electric current into
linear motion.
Solenoid Valve
A solenoid valve is a combination of:
� a solenoid with its core or plunger
� a valve body containing an orifice
in which a disc or plug is positioned
to restrict or allow flow
SOL A
Forward motion of piston
Directional
solenoid
valve
FWD
CR
CR
SOL A
When SOL A is energized, the valve
spool is shifted to redirect the fluid
and move the cylinder forward
Dr. Mohammad H. Salah Page no. 39
Stepper Motor
A stepper motor converts electrical
pulses applied to it into discrete
rotor movements called steps. They
are used to provide precise position
control of movement.
ModuleStepper-motor
translator
Step
motor
Stepper motor control system
Communicates
with the PLC and
responds with
pulse trains
Enables control
of the stepper motor The motor will move
one step for each pulse
received
PLC Control of a Large Motor Load
When a PLC needs to
control a large motor, it
must work in
conjunction with a
starter.
Motor starters are
available in various
standard National Electric
manufacturers (NEMA)
sizes and ratings.
Dr. Mohammad H. Salah Page no. 40
Programmable Logic
Controller
Outlines
� Introduction
� Advantages of PLC Control Systems
� PLC Versus Other Types of Control
� Typical Areas of PLC Applications
� PLC Product Application Ranges
� Structure and Hardware
� PLC Scan Process
� PLC Programming
� Modes of Operation
� PLC and Networks
Dr. Mohammad H. Salah Page no. 41
Introduction
� A programmable logic controller (PLC) is a specialized
computer used to control machines and process.
� PLC uses a programmable memory to store instructions and
execute specific functions that include On/Off control,
timing, counting, sequencing, arithmetic, and data handling.
� The word Programmable differentiates it from the
conventional hard-wired relay logic.
Introduction
PLCs are used in both SCADA and DCS systems as the control
components of an overall hierarchical system to provide local
management of processes through feedback control
Dr. Mohammad H. Salah Page no. 42
Introduction
� Using a PLC requiressetting up the hardwareand software
� The hardware installationconsists of wiring the PLCto all switches andsensors of the systemand to such outputdevices as relay coils,indicator lamps, or smallmotors
Introduction
� The control program is usually developed on a PC, usingsoftware provided by the PLC manufacturer
� This software allows the user to develop the controlprogram on the monitor screen
� Once the program is complete, it is automaticallyconverted into instructions for the PLC processor
� The completed program is then downloaded into the PLC
� Once the program is in the PLC’s memory, theprogramming terminal can be disconnected, and the PLCwill continue to function on its own
Dr. Mohammad H. Salah Page no. 43
Advantages of PLC Control Sys.
� Eliminates much of the hard wiring that was associatedwith conventional relay control circuits: The PLC alsosurpassed the hazard of changing the wiring.
The program takes the place ofthe external wiring that would berequired to control the process
Advantages of PLC Control Sys.
� Increased Reliability: Once a program has been written and
tested, it can be downloaded to other PLCs.
Since all thelogic is contained
in the PLC’smemory, there is
no chance ofmaking a logic
wiring error.
Dr. Mohammad H. Salah Page no. 44
Advantages of PLC Control Sys.
� Faster Response Time: PLCs operate in real-time which
means that an event taking place in the field will result in an
operation or output taking place.
Machines thatprocess thousands ofitems per second andobjects that spend only afraction of a second infront of a sensor requirethe PLC’s quick responsecapability.
Advantages of PLC Control Sys.
� More Flexibility: Original equipment manufacturers (OEMs)
can provide system updates for a process by simply sending
out a new program.
It is easier tocreate and change aprogram in a PLC thanto wire and rewire acircuit. End-users canmodify the program inthe field.
Dr. Mohammad H. Salah Page no. 45
Advantages of PLC Control Sys.
� Lower Cost: Originally PLCs were designed to replace relay
logic control. The cost savings using PLCs have been so
significant that relay control is becoming obsolete, except
for power applications.
Generally, if anapplication requiresmore than about 6control relays, it willusually be lessexpensive to install aPLC.
Advantages of PLC Control Sys.
� Communication Capabilities: PLC can communicate with
other controllers or computer equipment.
They can benetworked to performsuch functions as:supervisory control, datagathering, monitoringdevices and processparameters, anddownloading anduploading of programs.
Dr. Mohammad H. Salah Page no. 46
Advantages of PLC Control Sys.
� Easier to Troubleshoot: PLCs have resident diagnostic and
override functions that allows users to easily trace and
correct software and hardware problems.
Thecontrol programcan be watchedin real-time as itexecutes to findand fix problems
Advantages of PLC Control Sys.
� PLCs can work with the help of the HMI (Human-Machine
Interface) computer
HMI
Dr. Mohammad H. Salah Page no. 47
PLC Versus Other Types of Control
PLCs Versus Relay Control
� Today’s demand for high quality and productivity can hardly
be fulfilled economically without electronic control
equipment.
� With rapid technology developments and increasing
competition, the cost of programmable controls has been
driven down to the point where a PLC-versus-relay cost
study is no longer necessary or valid.
� When deciding whether to use a PLC-based system or a
hardwired relay system, the designer must ask several
questions. Some of these questions are:
PLC Versus Other Types of Control
PLCs Versus Relay Control
� Is there a need for flexibility in control logic changes?
� Is there a need for high reliability?
� Are space requirements important?
� Are increased capability and output required?
� Are there data collection requirements?
� Will there be frequent control logic changes?
� Will there be a need for rapid modification?
� Must similar control logic be used on different machines?
� Is there a need for future growth?
� What are the overall costs?
Dr. Mohammad H. Salah Page no. 48
PLC Versus Other Types of Control
PLCs Versus Relay Control
� Even in a case where no flexibility or future expansion is
required, a large system can benefit tremendously from the
troubleshooting and maintenance aids provided by a PLC.
� The extremely short cycle (scan) time of a PLC allows the
productivity of machines that were previously under
electromechanical control to increase considerably.
� Also, although relay control may cost less initially, this
advantage is lost if production downtime due to failures is
high.
PLC Versus Other Types of Control
PLCs Versus Relay Control
Dr. Mohammad H. Salah Page no. 49
PLC Versus Other Types of Control
PLCs Versus Computer Control
� Unlike computers, PLCs are specifically designed to survive
the harsh conditions of the industrial environment.
� A well-designed PLC can be placed in an area with
substantial amounts of electrical noise, electromagnetic
interference, mechanical vibration, and non-condensing
humidity.
� PLC’s hardware and software are designed for easy use by
plant electricians and technicians.
� the software programming uses conventional relay ladder
symbols, or other easily learned languages, which are
familiar to plant personnel.
PLC Versus Other Types of Control
Dr. Mohammad H. Salah Page no. 50
Typical Areas of PLC Applications
PLC Product Application Ranges
� The PLC market can be segmented into five groups:
1. Micro PLCs
2. Small PLCs
3. Medium PLCs
4. Large PLCs
5. Very large PLCs
The A, B, and Coverlapping areasreflect enhancements,by adding options, ofthe standard featuresof the PLCs within aparticular segment.
Dr. Mohammad H. Salah Page no. 51
PLC Control of a Large Motor Load
When a PLC needs
to control a large
motor, it must work
in conjunction with a
starter.
Motor starters are
available in various
standard National
Electric manufacturers
(NEMA) sizes and
ratings.
Structure and Hardware
� Power Supply
� Processor (CPU)
� Memories
� Input/output modules
� Programming Port
� PLC Bus
� Expansion Models
Dr. Mohammad H. Salah Page no. 52
Structure and Hardware
� The PLC bus are the wires which contains the databus, address bus, and control signals. The processoruses the bus to communicate with the modules
Structure and Hardware
Dr. Mohammad H. Salah Page no. 53
Power Supply
� PLCs are usually powered directly from 120 or 240Vac
� The power supply converts the AC into DC voltages for the internal microprocessor components
� It may also provide the user with a source of reduced voltage to drive switches, small relays, indicator lamps, and the like
Structure and Hardware
Processor (CPU)
� The processor is the brain of the
PLC
� The processor is a
microprocessor-based CPU and
is the part of the PLC that is
capable of reading and executing
the program instructions, one-
by-one (such as the rungs of a
ladder logic program)
Structure and Hardware
ProcessorModule
Dr. Mohammad H. Salah Page no. 54
Processor (CPU)
� A special program called the operating system controls
the actions of the CPU and consequently the execution
of the user’s program
� The operating system is supplied by the PLC
manufacturer and is permanently held in memory.
� A PLC operating system is designed to scan image
memory and the main memory which stores the ladder
diagram program
Structure and Hardware
Memories
� The program memory receives and holds the downloadedprogram instructions from the programming device
� This memory is usually an EEPROM (electrically erasableprogrammable ROM) or a battery-backup RAM, both ofwhich are capable of retaining data
� Data memory is RAM memory used as a “scratch pad” bythe processor to temporarily store internal and externalprogram-generated data
Structure and Hardware
� For example, it would store thepresent status of all switchesconnected to the input terminals andthe value of internal counters andtimers.
Dr. Mohammad H. Salah Page no. 55
Memories
Structure and Hardware
Input/Output Modules
Structure and Hardware
� The I/O modules are interfaces to the outside world
� These control ports may be built into the PLC unit or, moretypically, are packaged as separate plug-in modules, where eachmodule contains a set of ports
� The most common type of I/O is called discrete I/O and dealswith on-off devices
� Analog I/O modules allow the PLC to handle analog signals
Dr. Mohammad H. Salah Page no. 56
Input/Output Modules
Structure and Hardware
Fixed I/O configuration
� Is typical of small PLCs
� Comes in one package, withno separate removable units.
� The processor and I/O arepackaged together.
� Lower in cost – but lacksflexibility.
Input/Output Modules
Structure and Hardware
Modular I/O configuration
� When a module slides intothe rack, it makes anelectrical connection with aseries of contacts calledthe “backplane”.
� The backplane is located atthe rear of the rack.
Dr. Mohammad H. Salah Page no. 57
Discrete Input Modules (DIM)
Structure and Hardware
� DIM connect real-world
switches to the PLC and
are available for either
AC or DC voltages
(typically, 240 Vac, 120
Vac, 24 Vdc, and 5 Vdc)
� circuitry within the
module converts the
switched voltage into a
logic voltage for the
processor
Discrete Output Modules (DOM)
Structure and Hardware
� DOM provide on-off signals to
drive lamps, relays, small
motors, motor starters, and
other devices
� Several types of output
� ports are available: Triac
outputs control AC devices,
transistor switches control DC
devices, and relays control AC
or DC devices (and provide
isolation as well)
Dr. Mohammad H. Salah Page no. 58
Analog Input Modules (AIM)
Structure and Hardware
� An analog input module has one or more ADCs (analog-to-
digital converters), allowing analog sensors, such as
temperature, to be connected directly to the PLC
� Depending on the module, the analog voltage or current is
converted into an 8-, 12-, or 16-bit digital word
Analog Output Modules (AOM)
Structure and Hardware
� An analog output module contains one or more
DACs (digital-to-analog converters), allowing the PLC
to provide an analog output—for example, to drive a
DC motor at various voltage levels
Dr. Mohammad H. Salah Page no. 59
Input/Output Modules
Structure and Hardware
Specialized modules that perform particular functions areavailable for many PLCs. Examples include:
� Thermocouple module — Interfaces a thermocouple to thePLC.
� Motion-control module — Runs independently to controlmuti-axis motion in a device such as a robot
� Communication module — Connects the PLC to anetwork
� High-speed counter module — Counts the number ofinput pulses for a fixed period of time
� PID module — An independently running PID self-contained controller (PID control can also be implementedwith software, as described later in this chapter)
Input/Output Modules
Structure and Hardware
Dr. Mohammad H. Salah Page no. 60
Interpreting I/O Specificaions
Structure and Hardware
Electrical:
� I/O Voltage Rating.
� I/O Current Rating.
� Input Threshold Voltage.
� Input Delay.
� Off-State Leakage Current.
� Output Power Rating.
� Surge Current (Max).
� Output On-Delay.
� Output Off-Delay.
� Digital Resolution.
Interpreting I/O Specificaions
Structure and Hardware
Mechanical:
� Points Per Module.
� Wire Size.
Environmental:
� Ambient Temperature Rating.
� Humidity.
Dr. Mohammad H. Salah Page no. 61
Programming Port and Device
Structure and Hardware
� The programming port receives the downloaded program
from the programming device (usually a PC)
Programming Port and Device
Structure and Hardware
� The PLC does not have a front panel or a monitor; thus, to
“see” what the PLC is doing (for debugging or
troubleshooting), you must connect it to a PC
Dr. Mohammad H. Salah Page no. 62
Programming Port and Device
Structure and Hardware
� A personal computer (PC) is the most commonly used
programming device.
� The computer monitor is used to display the logic on the
screen.
� The personal computer communicates with the PLC
processor via a serial or parallel data communications link.
� The software allows users to create, edit, document, store
and troubleshoot programs. If the programming unit is not
in use, it may be unplugged and removed. Removing the
programming unit will not affect the operation of the user
program.
Programming Port and Device
Structure and Hardware
� Hand-held programming devices are sometimes used to program
small PLCs.
� They are compact, inexpensive, and easy to use, but are not able
to display as much logic on screen as a computer monitor.
� Hand-held units are often used on the factory floor for
troubleshooting, modifying programs, and transferring programs
to multiple machines.
Dr. Mohammad H. Salah Page no. 63
Expansion Modules
Structure and Hardware
� Most PLCs are expandable
� Expansion modules contain additional inputs and outputs
� These are connected to the base unit using a ribbon
connector
BIG PICTURE
Dr. Mohammad H. Salah Page no. 64
PLC Scan Process
PLC Scan Process
Dr. Mohammad H. Salah Page no. 65
PLC Scan Process
PLC Scan Process
� The scan time is dependent on the clock frequency of the processor.
� Misunderstanding the way the PLC scans can cause programming
bugs!
Dr. Mohammad H. Salah Page no. 66
PLC Scan Process
Data Flow Overview
PLC Scan Process
Dr. Mohammad H. Salah Page no. 67
PLC Scan Process
PLC Programming
� The term PLC programming language refers to the method by
which the user communicates information to the PLC.
� A PLC program is not actually a wiring diagram but a way to
describe the logical relationship between inputs and outputs
� The PLC programming languages are:
– Sequential Control and State Graph (Graph)
– Sequential Function Chart (SFC)
– Structured Text (ST)
– Instruction List (IL)
– Function Block Diagram (FBD)
– Ladder Diagram (LD)
The most common is LD, FBD, and IL but the most use is the LD.
Dr. Mohammad H. Salah Page no. 68
Sequential Functional Chart
PLC Programming
� Sequential functional chart, or SFC, is a graphical “language” that
provides a diagrammatic representation of control sequences in a
program.
� Basically, sequential function chart is a flowchart-like framework
that can organize the subprograms or subroutines (programmed
in LD, FBD, IL, and/or ST) that form the control program.
� SFC is particularly useful for sequential control operations, where
a program flows from one step to another once a condition has
been satisfied (TRUE or FALSE).
� The SFC programming framework contains three main elements
that organize the control program: steps, transitions, and
actions.
Sequential Functional Chart
PLC Programming
Dr. Mohammad H. Salah Page no. 69
Sequential Functional Chart
PLC Programming
Structured Text
PLC Programming
Dr. Mohammad H. Salah Page no. 70
Instruction List
PLC Programming
Function Block Diagram
PLC Programming
Dr. Mohammad H. Salah Page no. 71
Ladder Diagram
PLC Programming
Ladder Diagram
PLC Programming
� A LAD (special kind of wiring diagram) was developed to
document electromechanical control circuits.
� Ladder diagram programs are highly symbolic and are the
result of years of evolution of industrial control circuit
diagrams
� This type of diagram has two vertical wires (rails) on either
side of the drawing to supply the power
� Each rung of the ladder diagram connects from one rail to
the other and is a separate circuit, which typically consists
of some combination of switches, relay contacts, relay coils,
and motors
� It is common for the coil of a relay to be in one rung and the
contacts to be in another
Dr. Mohammad H. Salah Page no. 72
Ladder Diagram
PLC Programming
Ladder Diagram
PLC Programming
Control scheme is drawn
between two vertical
supply lines.
Ladder rungLadder rail
Dr. Mohammad H. Salah Page no. 73
Ladder Diagram - Comparison
PLC Programming
Ladder Diagram - Comparison
PLC Programming
Dr. Mohammad H. Salah Page no. 74
Relay-Type Instructions
PLC Programming
Examine if Closed (XIC) Instruction
PLC Programming
Dr. Mohammad H. Salah Page no. 75
Examine if Closed (XIC) Instruction
PLC Programming
Examine if Closed (XIC) Instruction
PLC Programming
Dr. Mohammad H. Salah Page no. 76
Examine if Closed (XIC) Instruction
PLC Programming
Examine if Open (XIO) Instruction
PLC Programming
Dr. Mohammad H. Salah Page no. 77
Examine if Open (XIO) Instruction
PLC Programming
Examine if Open (XIO) Instruction
PLC Programming
Dr. Mohammad H. Salah Page no. 78
Output Energize (OTE) Instruction
PLC Programming
Output Energize (OTE) Instruction
PLC Programming
Dr. Mohammad H. Salah Page no. 79
Output Energize (OTE) Instruction
PLC Programming
Status Bit Example
PLC Programming
Dr. Mohammad H. Salah Page no. 80
Status Bit Example
PLC Programming
Ladder Rung
PLC Programming
Dr. Mohammad H. Salah Page no. 81
Rung Continuity
PLC Programming
Rung Continuity
PLC Programming
Dr. Mohammad H. Salah Page no. 82
Example
PLC Programming
Parallel Input Branch Instruction
PLC Programming
Dr. Mohammad H. Salah Page no. 83
Parallel Output Branching
PLC Programming
Nested Input and Output Branches
PLC Programming
Dr. Mohammad H. Salah Page no. 84
Nested Contact Program
PLC Programming
PLC Matrix Limitation Diagram
PLC Programming
Dr. Mohammad H. Salah Page no. 85
Programming of Vertical Contacts
PLC Programming
Programming for Different Scan
Patterns
PLC Programming
Dr. Mohammad H. Salah Page no. 86
Internal (Auxiliary) Control Relay
PLC Programming
Internal (Auxiliary) Control Relay
PLC Programming
Dr. Mohammad H. Salah Page no. 87
Operation of XIC/XIO Instruction
PLC Programming
Operation of XIC/XIO Instruction
PLC Programming
Dr. Mohammad H. Salah Page no. 88
Ladder Diagram - Timers
PLC Programming
� The Timer instruction provides a time delay, performing the
function of a time-delay relay (e.g., controlling the time for
a mixing operation or the duration of a warning beep)
� The length of time delay is determined by specifying a
preset value
� The timer is enabled when the rung conditions become
TRUE
� Once enabled, it automatically counts up until it reaches the
Preset value and then goes TRUE (and stays TRUE)
� There are two types of time delay (On and Off)
Ladder Diagram - Timers
PLC Programming
Dr. Mohammad H. Salah Page no. 89
Ladder Diagram - Timers
PLC Programming
Off-Delay Timer
On-Delay Timer
Ladder Diagram - Timers
PLC Programming
Dr. Mohammad H. Salah Page no. 90
Ladder Diagram - Timers
PLC Programming
Ladder Diagram - Timers
PLC Programming
Dr. Mohammad H. Salah Page no. 91
Ladder Diagram - Timers
PLC Programming
Ladder Diagram - Counters
PLC Programming
� A Counter instruction keeps track of the number of timessome event occurs (e.g., the count could represent thenumber of parts to be loaded into a box)
� Counters may be either count-up or count-down types. TheCounter will increment (or decrement) every time the rungmakes a FALSE-to-TRUE transition
� The count is retained until a RESET instruction (with thesame address as the Counter) is enabled
� The Counter has a Preset value associated with it. When thecount gets up to the Preset value, the output goes TRUE.This allows the program to initiate some action based on acertain count
Dr. Mohammad H. Salah Page no. 92
PLC Programming
Ladder Diagram - Counters
PLC Programming
Ladder Diagram - Counters
Dr. Mohammad H. Salah Page no. 93
PLC Programming
Ladder Diagram - Counters
Ladder Diagram - Counters
PLC Programming
Dr. Mohammad H. Salah Page no. 94
Ladder Diagram - Sequencers
PLC Programming
� The Sequencer instruction is used when a repeating
sequence of outputs is required
� Traditionally, electromechanical sequencers (Figure 12.10)
were used in this type of application (where a drum rotates
slowly, and cams on the drum activate switches)
� The Sequencer instructionallows the PLC to implementthis common control strategy
Ladder Diagram - Sequencers
PLC Programming
Dr. Mohammad H. Salah Page no. 95
Ladder Diagram - Sequencers
PLC Programming
Ladder Diagram - Sequencers
PLC Programming
Dr. Mohammad H. Salah Page no. 96
Ladder Diagram - Comparators
PLC Programming
The temperature in an electric oven
is to be maintained by a 16-bit PLCat approximately 100°C, using two-
point control (actual range: 98-102°). An oven with an electric
heating element driven by acontactor (high-current relay), an
LM35 temperature sensor (produces10 mV/°C), an operator on-off
switch, and the PLC. The PLC has aprocessor and three I/O modules: a
discrete input module (slot 1), a 16-bit analog input module (slot 2), and
a discrete output module (slot 3).Draw the ladder diagram for this
system
Ladder Diagram - Comparators
PLC Programming
Dr. Mohammad H. Salah Page no. 97
Equivalent Ladder / Logic Symbols
PLC Programming
Equivalent Ladder / Logic Symbols
PLC Programming
Dr. Mohammad H. Salah Page no. 98
Equivalent Ladder / Logic Symbols
PLC Programming
Equivalent Ladder / Logic Symbols
PLC Programming
Dr. Mohammad H. Salah Page no. 99
Equivalent Ladder / Logic Symbols
PLC Programming
Equivalent Ladder / Logic Symbols
PLC Programming
Dr. Mohammad H. Salah Page no. 100
Equivalent Ladder / Logic Symbols
PLC Programming
Equivalent Ladder / Logic Symbols
PLC Programming
Dr. Mohammad H. Salah Page no. 101
Equivalent Ladder / Logic Symbols
PLC Programming
Equivalent Ladder / Logic Symbols
PLC Programming
Dr. Mohammad H. Salah Page no. 102
Equivalent Ladder / Logic Symbols
PLC Programming
Ladder Diagram – Programming Comments
PLC Programming
Arranging Instructions for Optimum Performance
There is more than one way to correctly implement the ladder logic. Insome cases one arrangement may be more efficient in terms of theamount of memory used and the time required to scan the program.
Instruction MOST
likely to be FALSE
Instruction LEAST
likely to be FALSE
Once a processor sees a FALSE input instruction in series, it
executes the remaining instructions FALSE, even if they are
TRUE
Sequence series instructions from the most likely to be FALSE
(far left) to least likely to be FALSE (far right)
Dr. Mohammad H. Salah Page no. 103
Ladder Diagram – Programming Comments
PLC Programming
Arranging Instructions for Optimum Performance
If your rung contains parallel branches, place the path that
is most often TRUE on the top. The processor will not look
at the others unless the top path is FALSE.
Path most likely to be TRUE
LESS likely
LEAST likely
Modes of Operation
Dr. Mohammad H. Salah Page no. 104
Modes of Operation
PLC and Networks
� Physically, a network is a wire acting as an “electronic
highway” that can pass messages between nodes (PCs and
other electronic devices)
� Each node on the network has a unique address, and each
message called a data packet (includes the address of
where it’s going and where it came from)
� All data on the network is sent serially (one bit at a time) on
one wire
� The most common type of network uses the bus topology,
which means that all the nodes tap into a single cable
Dr. Mohammad H. Salah Page no. 105
PLC and Networks
PLC and Networks
� There are three good reasons for using a network:
� A device network simplifies wiring. Clearly the network is a
simpler system that uses less wire. This reduces the amount
of wiring needed
� With a network, the sensor data arrives in better shape. In
the traditional system, a low-level analog voltage may have
to travel many feet. The signal is subject to attenuation and
noise and other losses
� Network devices tend to be more intelligent. For example,
a photo cell could send a message saying the light level has
diminished, (indicating that the lens may be getting dirty or
that someone has bumped it out of position)
Dr. Mohammad H. Salah Page no. 106
PLC and Networks
PLC and Networks
Three levels
of networks
Dr. Mohammad H. Salah Page no. 107
PLC and Networks
Dr. Mohammad H. Salah Page no. 108
PLC ExercisesLadder Diagram
Programming
Steps for Building a Ladder Diagram
1. Determine the No. of digital I/O
2. Determine the No. of analog I/O (if needed)
3. Determine if there are special functions in the process
4. Estimate program capacity depending on the process
5. Choose a suitable PLC series
6. Prepare the wiring diagram
7. Draw flowchart or control diagram (Optional)
8. Program the PLC using the ladder diagram
Dr. Mohammad H. Salah Page no. 109
Exercise #1: Moving a Pneumatic Piston
Control ProblemThe PLC task is to move thepiston in and out. Whenswitch SW1 is momentarilyturned on, piston A is tomove out of the cylinder inA+ direction. When switchSW2 is momentarily turnedon, piston A is to move intothe cylinder in A- direction.
Exercise #1: Moving a Pneumatic Piston
Dr. Mohammad H. Salah Page no. 110
Exercise #1: Moving a Pneumatic Piston
The two solenoid valves will be tuned off
If SW1 and SW2 arepressed together,what would happen?
Use the contacts of the main relays instead of the input contacts
How can we make an electrical interlock?
Exercise #2: Sequencing of Pneumatic Pistons
Control ProblemThe PLC task is to operate piston A followed bypiston B followed by piston C. The sequence is A+,A-, B+, B-, C+, C- is to be repeated when switchSW1 is turned on
Dr. Mohammad H. Salah Page no. 111
Exercise #2: Sequencing of Pneumatic Pistons
Exercise #2: Sequencing of Pneumatic Pistons
• Solenoid valves do not work• The wiring of solenoid valves is not correct or not in
the correct order (wiring problem)• The ladder diagram is not properly written (sequence
in not correct)
If the system does not work or sequence in notcorrect, what would be the possible reasons?
Dr. Mohammad H. Salah Page no. 112
Exercise #3: Batching Machine
Control ProblemThe PLC task is to control a simplemachine which counts and batchescomponents moving along a conveyor. Itis required that ten components bechanneled down route A and twentycomponents down route B. A reset facilityis required
Exercise #3: Batching Machine
Dr. Mohammad H. Salah Page no. 113
Exercise #3: Batching Machine
Exercise #3: Batching Machine
• The reset switch is always on• The microswitch does not work• The flap solenoid does not work• The ladder diagram is not properly
written
If the system does not batch and/orcount, what would be the possiblereasons?
Dr. Mohammad H. Salah Page no. 114
Exercise #4: Reject Machine
Control ProblemThe PLC task is to detect and reject faulty components. Components aretransported on a conveyor past a retro-reflective type photoelectric switch. Thephotoelectric switch is positioned at a height (H) above the conveyor where (H)represents a tolerance value for component height. Good components passunderneath the photoelectric switch and no signal is generated. Faultycomponents break the light beam twice as they pass the photoelectric switch.
Exercise #4: Reject Machine
Dr. Mohammad H. Salah Page no. 115
Exercise #4: Reject Machine
Exercise #4: Reject Machine
• The photoelectric switch is too high (H is too big)• The photoelectric switch does not work• The pneumatic blower does not work• The ladder diagram is not properly written• The faulty components is not as described in the drawing
If the system does not reject faulty components, what would be thepossible reasons?
Dr. Mohammad H. Salah Page no. 116
Exercise #5: Pick and Place Unit
Control ProblemThe PLC task is to:a) move the gripper to X+ positionb) close the gripper so that it takes hold of a componentc) rotate the gripper through 180o to the Θ+ positiond) Release the componente) Rotate the gripper back to the Θ- position so that the pick and place
operation may be repeated
Exercise #5: Pick and Place Unit
Dr. Mohammad H. Salah Page no. 117
Exercise #5: Pick and Place Unit
• Wiring problem• Some solenoid valves do
not work• Timing is not correct• The ladder diagram is not
properly written(sequence in not correct)
If the system does notwork or sequence is notcorrect, what would bethe possible reasons?
Use position sensors for feedback but that would be expensive compared tousing timers but more accurate and reliable in case the mechanical systemstarts to have some problems
How can we get rid of the timers in the ladder diagram/program?
Exercise #6: Production Line
Control ProblemThe PLC task is to organize the production process. Cans filled with fluid andcapped before passing into a conveyor. The photoelectric switches P1 and P2 areused to check that each can has a cap. Photoelectric switch P3 provides a triggerfor the ink jet printer which prints a batch number on each can. Photoelectricswitch P4 is used to count three cans into the palletizing machine that transportsthree cans through a machine which heat shrinks a plastic wrapping over them.All photoelectric switches on the production line are of the retro reflective type.
Dr. Mohammad H. Salah Page no. 118
Exercise #6: Production Line
Exercise #6: Production Line
Dr. Mohammad H. Salah Page no. 119
Exercise #6: Production Line
• The height of the photoelectric switch needs to be readjusted• The photoelectric switch does not work (transmitter or receiver)• The photoelectric transmitter is not aligned with the receiver• The ladder diagram is not properly written (or timer is not set properly)
If the system allows uncapped cans to pass, what would be thepossible reasons?
Exercise #7: Star-Delta Connection
Dr. Mohammad H. Salah Page no. 120
Exercise #7: Star-Delta Connection
Exercise #7: Star-Delta Connection
PLC system layout – Wiring diagram
Dr. Mohammad H. Salah Page no. 121
Exercise #7: Star-Delta Connection
Exercise #8: Drilling Process
A simple drilling operation requires
the drill press to turn on only if
there is a part present and the
operator has one hand on each of
the start switches. This precaution
will ensure that the operator's
hands are not in the way of the
drill.
PB1 PB2Drill
motor
Part sensor
Switches
Dr. Mohammad H. Salah Page no. 122
A simple drilling operation requires the drill press to turn on only if there is a
part present and the operator has one hand on each of the start switches. This
precaution will ensure that the operator's hands are not in the way of the drill.
Exercise #8: Drilling Process
A motorized overhead garage door is to be operatedautomatically to preset open and closed positions.
Exercise #9: Motorized Door
Dr. Mohammad H. Salah Page no. 123
Exercise #9: Motorized Door
Continuous filling operation requires boxes moving on a conveyor to be
automatically positioned and filled.
HooperPL
PL
PL
Run
Standby
FullSolenoid
Level
switch
MotorPhoto
switch
START
STOP
Exercise #10: Continuous Filling Machine
Dr. Mohammad H. Salah Page no. 124
Exercise #10: Continuous Filling Machine
Exercise #11: Transporting Process 1
Need to do the PLC hardware layout + ladder diagram
Dr. Mohammad H. Salah Page no. 125
Exercise #11: Transporting Process 1
Exercise #12: Transporting Process 2
Need to automate the system using a PLC(hardware layout +
ladder diagram)
Dr. Mohammad H. Salah Page no. 126
Industrial Control
Systems
Outlines
� Introduction
� ICS Operation
� ICS Key Components
� SCADA Systems
� DCS Systems
� RTU
� Telemetry
� Modems
� SCADA Systems Examples
Dr. Mohammad H. Salah Page no. 127
Introduction
� Industrial Control Systems is a general term that includesseveral types of control systems:
�Supervisory Control And Data Acquisition (SCADA) systems.
�Distributed Control Systems (DCS).
�Other control system configurations such as ProgrammableLogic Controllers (PLC).
� ICS are typically used in industries such as electrical, waterand wastewater, oil and natural gas, chemical,transportation, pharmaceutical, food and beverage, anddiscrete manufacturing (e.g., automotive).
� These control systems are used for critical infrastructuresthat are often highly interconnected and mutuallydependent systems
Introduction
� SCADA systems are highly distributed systems used to
control geographically dispersed assets, often scattered
over thousands of square kilometers, where centralized
data acquisition and control are critical to system operation.
� They are used in distribution systems such as water
distribution and wastewater collection systems, oil and
natural gas pipelines, electrical power grids, and railway
transportation systems.
� A SCADA control center performs centralized monitoring
and control for field sites over long-distance
communications networks, including monitoring alarms and
processing status data.
Dr. Mohammad H. Salah Page no. 128
Introduction
� Based on information received from remote stations,
automated or operator-driven supervisory commands can
be pushed to remote station control devices, which are
often referred to as field devices.
� Field devices control local operations such as opening and
closing valves and breakers, collecting data from sensor
systems, and monitoring the local environment for alarm
conditions.
Introduction
� DCS are used to control industrial processes such as electric
power generation, oil refineries, water and wastewater
treatment, and chemical, food, and automotive production.
� DCS are integrated as a control architecture containing a
supervisory level of control overseeing multiple, integrated
sub-systems that are responsible for controlling the details
of a localized process.
� Product and process control are usually achieved by
deploying feedback or feedforward control loops whereby
key product and/or process conditions are automatically
maintained around a desired set point.
Dr. Mohammad H. Salah Page no. 129
Introduction
� To accomplish the desired product and/or process tolerance
around a specified set point, specific PLCs are employed in
the field and proportional, integral, and/or derivative
settings on the PLC are tuned to provide the desired
tolerance as well as the rate of self-correction during
process upsets.
� DCS are used extensively in process-based industries.
Introduction
� PLCs are computer-based solid-state devices that control
industrial equipment and processes.
� While PLCs are control system components used throughout
SCADA and DCS systems, they are often the primary
components in smaller control system configurations used
to provide operational control of discrete processes such as
automobile assembly lines and power plant soot blower
controls.
� PLCs are used extensively in almost all industrial processes.
Dr. Mohammad H. Salah Page no. 130
Introduction
� While control systems used in distribution and
manufacturing industries are very similar in operation, they
are different in some aspects.
� One of the primary differences is that DCS or PLC-controlled
sub-systems are usually located within a more confined
factory or plant-centric area, when compared to
geographically dispersed SCADA field sites.
� DCS and PLC communications are usually performed using
local area network (LAN) technologies that are typically
more reliable and high speed compared to the long-distance
communication systems used by SCADA systems.
Introduction
� In fact, SCADA systems are specifically designed to handle
long-distance communication challenges such as delays and
data loss posed by the various communication media used.
� DCS and PLC systems usually employ greater degrees of
closed loop control than SCADA systems because the
control of industrial processes is typically more complicated
than the supervisory control of distribution processes.
Dr. Mohammad H. Salah Page no. 131
Introduction
� SCADA systems are generally used to control dispersed
assets using centralized data acquisition and supervisory
control.
� DCS are generally used to control production systems within
a local area such as a factory using supervisory and
regulatory control.
� PLCs are generally used for discrete control for specific
applications and generally provide regulatory control.
Introduction
� ICS have unique performance and reliability requirementsand often use operating systems and applications that maybe considered unconventional to typical IT personnel.Furthermore, the goals of safety and efficiency sometimesconflict with security in the design and operation of controlsystems.
� ICS implementations were susceptible primarily to localthreats because many of their components were inphysically secured areas and the components were notconnected to IT networks or systems.
� However, the trend toward integrating ICS systems with ITnetworks provides significantly less isolation for ICS from theoutside world than predecessor systems, creating a greaterneed to secure these systems from remote, external threats.
Dr. Mohammad H. Salah Page no. 132
Introduction
� The increasing use of wireless networking places ICS
implementations at greater risk from adversaries who are in
relatively close physical proximity but do not have direct
physical access to the equipment.
� Threats to control systems can come from numerous
sources, including hostile governments, terrorist groups,
disgruntled employees, malicious intruders, complexities,
accidents, natural disasters as well as malicious or
accidental actions by insiders.
� ICS security objectives typically follow the priority of
availability, integrity and confidentiality, in that order.
Introduction
� It is essential for a cross-functional cyber (internet) security
team to share their varied domain knowledge and
experience to evaluate and mitigate risk to the ICS.
� The cyber security team should consist of a member of the
organization’s IT staff, control engineer, control system
operator, network and system security expert, a member of
the management staff, and a member of the physical
security department at a minimum.
� For continuity and completeness, the cyber security team
should consult with the control system vendor and/or
system integrator as well.
Dr. Mohammad H. Salah Page no. 133
Introduction
� The cyber security team should report directly to site
management (e.g., facility superintendent) or the company’s
CIO/CSO, who in turn, accepts complete responsibility and
accountability for the cyber security of the ICS. An effective
cyber security program for an ICS should apply a strategy
known as “defense-in-depth”, layering security mechanisms
such that the impact of a failure in any one mechanism is
minimized.
CIO = Chief Information Officer or IT Director
CSO = Chief Security Officer
ICS Operation
Dr. Mohammad H. Salah Page no. 134
ICS Operation
1. Control Loop. A control loop consists of sensors for
measurement, controller hardware such as PLCs, actuators
such as control valves, breakers, switches and motors, and
the communication of variables.
� Controlled variables are transmitted to the controller from
the sensors.
� The controller interprets the signals and generates
corresponding manipulated variables, based on set points,
which it transmits to the actuators.
� Process changes from disturbances result in new sensor
signals, identifying the state of the process, to again be
transmitted to the controller.
ICS Operation
2. Human-Machine Interface (HMI). Operators and engineers
use HMIs to monitor and configure set points, control
algorithms, and adjust and establish parameters in the
controller.
� The HMI also displays process status information and
historical information.
3. Remote Diagnostics and Maintenance Utilities. Diagnostics
and maintenance utilities are used to prevent, identify and
recover from abnormal operation or failures.
A typical ICS contains a propagation of control loops, HMIs, and remote
diagnostics and maintenance tools built using an array of network
protocols on layered network architectures
Dr. Mohammad H. Salah Page no. 135
ICS Key Components
1. Control Components
�Control Server: hosts the DCS or PLC supervisory control
software that is designed to communicate with lower-
level control devices. The control server accesses
subordinate control modules over an ICS network.
�SCADA Server or Master Terminal Unit (MTU): The
SCADA Server is the device that acts as the master in a
SCADA system. Remote terminal units (RTUs) and PLC
devices located at remote field sites usually act as slaves.
ICS Key Components
1. Control Components
�Remote Terminal Unit (RTU): It is also called a remote
telemetry unit. It is a special purpose data acquisition and
control unit designed to support SCADA remote stations.
RTUs are field devices often equipped with wireless radio
interfaces to support remote situations where wire-
based communications are unavailable. Sometimes PLCs
are implemented as field devices to serve as RTUs; in this
case, the PLC is often referred to as an RTU.
Dr. Mohammad H. Salah Page no. 136
ICS Key Components
1. Control Components
�Programmable Logic Controller (PLC): The PLC is a small
industrial computer originally designed to perform the
logic functions executed by electrical hardware. Other
controllers used at the field level are process controllers
and RTUs; they provide the same control as PLCs but are
designed for specific control applications. In SCADA
environments, PLCs are often used as field devices
because they are more economical, multipurpose,
flexible, and configurable than special-purpose RTUs.
ICS Key Components
1. Control Components
�Programmable Logic Controller (PLC): PLCs are used in
both SCADA and DCS systems as the control components
of an overall hierarchical system to provide local
management of processes through feedback control. In
the case of SCADA systems, they provide the same
functionality of RTUs. When used in DCS, PLCs are
implemented as local controllers within a supervisory
control scheme.
Dr. Mohammad H. Salah Page no. 137
ICS Key Components
1. Control Components
�Programmable Logic Controller (PLC): PLCs are also
implemented as the primary components in smaller
control system configurations. PLCs have a user-
programmable memory for storing instructions for the
purpose of implementing specific functions such as I/O
control, logic, timing, counting, three mode proportional-
integral-derivative (PID) control, communication,
arithmetic, and data and file processing.
ICS Key Components
1. Control Components
�Intelligent Electronic Devices (IED): An IED is a “smart”
sensor/actuator containing the intelligence required to
acquire data, communicate to other devices, and
perform local processing and control. An IED could
combine an analog input sensor, analog output, low-level
control capabilities, a communication system, and
program memory in one device. The use of IEDs in
SCADA and DCS systems allows for automatic control at
the local level.
Dr. Mohammad H. Salah Page no. 138
ICS Key Components
1. Control Components
�Human-Machine Interface (HMI): The HMI is softwareand hardware that allows human operators to monitorthe state of a process under control, modify controlsettings to change the control objective, and manuallyoverride automatic control operations in the event of anemergency. The HMI also allows a control engineer oroperator to configure set points or control algorithmsand parameters in the controller. The HMI also displaysprocess status information, historical information,reports, and other information to operators,administrators, managers, business partners, and otherauthorized users.
ICS Key Components
1. Control Components
�Data Historian: The data historian is a centralizeddatabase for logging all process information within anICS. Information stored in this database can be accessedto support various analyses, from statistical processcontrol to enterprise level planning.
�Input/Output (IO) server: The IO server is a controlcomponent responsible for collecting, buffering andproviding access to process information from control sub-components such as PLCs, RTUs and IEDs. An IO servercan reside on the control server or on a separatecomputer platform. IO servers are also used forinterfacing third-party control components, such as anHMI and a control server.
Dr. Mohammad H. Salah Page no. 139
ICS Key Components
2. Network Components
�Fieldbus Network: It links sensors and other devices to a
PLC or other controller. Use of fieldbus technologies
eliminates the need for point-to-point wiring between
the controller and each device. The sensors communicate
with the fieldbus controller using a specific protocol. The
messages sent between the sensors and the controller
uniquely identify each of the sensors.
�Control Network: The control network connects the
supervisory control level to lower-level control modules.
ICS Key Components
2. Network Components
�Communications Routers: A router is a communications
device that transfers messages between two networks.
Common uses for routers include connecting a LAN to a WAN,
and connecting MTUs and RTUs to a long-distance network
medium for SCADA communication.
�Remote Access Points: Remote access points are distinct
devices, areas and locations of a control network for remotely
configuring control systems and accessing process data.
Examples include using a personal digital assistant (PDA) to
access data over a LAN through a wireless access point, and
using a laptop and modem connection to remotely access an
ICS system.
Dr. Mohammad H. Salah Page no. 140
ICS Key Components
2. Network Components
� Firewall: A firewall protects devices on a network by monitoring
and controlling communication packets using predefined filtering
policies. Firewalls are also useful in managing ICS network
isolation strategies.
�Modems: A modem is a device used to convert between serial
digital data and a signal suitable for transmission over a
telephone line to allow devices to communicate. Modems are
often used in SCADA systems to enable long-distance serial
communications between MTUs and remote field devices. They
are also used in SCADA systems, DCS and PLCs for gaining remote
access for operational and maintenance functions such as
entering commands or modifying parameters, and diagnostic
purposes.
SCADA Systems
� SCADA is not a full control system, but rather focuses on thesupervisory level.
� SCADA is used for gathering, analyzing and to storage realtime data.
� SCADA systems consist of both hardware and software.
� Typical hardware includes an MTU placed at a controlcenter, communications equipment (e.g., radio, telephoneline, cable, or satellite), and one or more geographicallydistributed field sites consisting of either an RTU or a PLC,which controls actuators and/or monitors sensors.
� The MTU stores and processes the information from RTUinputs and outputs, while the RTU or PLC controls the localprocess
Dr. Mohammad H. Salah Page no. 141
SCADA Systems
� The communications hardware allows the transfer ofinformation and data back and forth between the MTU andthe RTUs or PLCs.
� The software is programmed to tell the system what andwhen to monitor, what parameter ranges are acceptable,and what response to initiate when parameters changeoutside acceptable values.
� An IED, such as a protective relay, may communicate directlyto the SCADA Server, or a local RTU may poll the IEDs tocollect the data and pass it to the SCADA Server.
� IEDs provide a direct interface to control and monitorequipment and sensors.
SCADA Systems
� IEDs may be directly polled and controlled by the SCADA Server and inmost cases have local programming that allows for the IED to actwithout direct instructions from the SCADA control center.
� SCADA systems are usually designed to be fault-tolerant systems withsignificant redundancy built into the system architecture.
SCADA System General Layout (Components and General Configuration)
Dr. Mohammad H. Salah Page no. 142
SCADA Systems
� The control center houses a SCADA Server (MTU) and thecommunications routers.
� Other control center components include the HMI,engineering workstations, and the data historian, which areall connected by a LAN.
� The control center collects and logs information gathered bythe field sites, displays information to the HMI, and maygenerate actions based upon detected events.
� The control center is also responsible for centralizedalarming, trend analyses, and reporting.
� The field site performs local control of actuators andmonitors sensors.
SCADA Systems
� Field sites are often equipped with a remote accesscapability to allow field operators to perform remotediagnostics and repairs usually over a separate dial upmodem or WAN connection.
� Standard and proprietary communication protocols runningover serial communications are used to transportinformation between the control center and field sites usingtelemetry techniques such as telephone line, cable, fiber,and radio frequency such as broadcast, microwave andsatellite.
Dr. Mohammad H. Salah Page no. 143
SCADA Systems
� MTU-RTU communication architectures vary amongimplementations. The various architectures used, includingpoint-to-point, series, series-star, and multi-drop.
� Point-to-point is functionally the simplest type; however, itis expensive because of the individual channels needed foreach connection.
� In a series configuration, the number of channels used isreduced; however, channel sharing has an impact on theefficiency and complexity of SCADA operations.
� Similarly, the series-star and multi-drop configurations’ useof one channel per device results in decreased efficiency andincreased system complexity.
SCADA Systems
Basic SCADACommunication Topologies
Dr. Mohammad H. Salah Page no. 144
SCADA Systems
� The four basic architectures can be further augmented usingdedicated communication devices to managecommunication exchange as well as message switching andbuffering.
� Large SCADA systems, containing hundreds of RTUs, oftenemploy sub-MTUs to alleviate the burden on the primaryMTU. This type of topology is shown in the following figure.
SCADA Systems
Large SCADACommunication Topology
Dr. Mohammad H. Salah Page no. 145
SCADA Systems
SCADA System
Implementation
Example
(Distribution
Monitoring And
Control)
SCADA Systems
� This particular SCADA system consists of a primary controlcenter and three field sites.
� A second backup control center provides redundancy in theevent of a primary control center malfunction.
� Point-to-point connections are used for all control center tofield site communications, with two connections using radiotelemetry.
� The third field site is local to the control center and uses thewide area network (WAN) for communications.
� A regional control center resides above the primary controlcenter for a higher level of supervisory control.
Dr. Mohammad H. Salah Page no. 146
SCADA Systems
� The corporate network has access to all control centersthrough the WAN, and field sites can be accessed remotelyfor troubleshooting and maintenance operations.
� The primary control center polls field devices for data atdefined intervals (e.g., 5 seconds, 60 seconds) and can sendnew set points to a field device as required.
� In addition to polling and issuing high-level commands, theSCADA server also watches for priority interrupts comingfrom field site alarm systems.
SCADA Systems
SCADA System ImplementationExample(Rail Monitoring and Control)
Dr. Mohammad H. Salah Page no. 147
SCADA Systems
� The previous example includes a rail control center thathouses the SCADA system and three sections of a railsystem.
� The SCADA system polls the rail sections for informationsuch as the status of the trains, signal systems, tractionelectrification systems, and ticket vending machines.
� This information is also fed to operator consoles at the HMIstation within the rail control center.
� The SCADA system also monitors operator inputs at the railcontrol center and disperses high-level operator commandsto the rail section components.
SCADA Systems
� In addition, the SCADA system monitors conditions at theindividual rail sections and issues commands based on theseconditions (e.g., shut down a train to prevent it fromentering an area that has been determined to be flooded oroccupied by another train based on condition monitoring).
Dr. Mohammad H. Salah Page no. 148
SCADA Systems
� There are several common media of communication:- Fiber optics
- Electrical cable.
- Leased lines from a telephone utility.
- Satellite telecommunications.
� The communications method used by most SCADA systemsis called “master–slave”, where only one of the machines (inthis case the MTU) is capable of initiating communication.
� The MTU talks to each RTU then returns to the first. This iscalled "scanning".
� The time required for the MTU to scan ALL its RTUs is calledthe MTU Scan Time (Scan Interval).
� Factors that determine scan interval are: number of RTUs,amount of data, data rate, and communications efficiency.
SCADA Systems
Calculate a scan interval for a SCADA system that:
- Has 20 RTUs
- Every RTU has a point count of 180 status points, 30 alarmpoints, 10 meters (at 16 bits each), and 10 analog points (at16 bits each).
- The MTU sends information to the RTU of 150 discretecontrol signals to valves and motors, 6 stepping motors (16bits each), and 10 valve controller set points (16 bits each)
- Data rate for communication is 1200bps.
- Communication efficiency is 40%.
Solution
Total Points is 920, therefore the total amount of data is 20 x920 = 18,400bits and the data rate is 18,400b/1200bps =~15sec at 100% efficiency but at 40% efficiency, the scaninterval is 15sec/0.4 =~ 38sec.
Example
Dr. Mohammad H. Salah Page no. 149
DCS
� In a DCS, the data acquisition and control functions are
performed by a number of distributed microprocessor-based
units, situated near to the devices being controlled or, the
instrument from which data is being gathered.
� DCS systems have evolved into providing very sophisticated
analogue (e.g. loop) control capability. A closely integrated
set of operator interfaces (or man machine interfaces) is
provided to allow for easy system configurations and
operator control. The data highway is normally capable of
high speeds - typically 1 Mbps up to 10 Mbps.
DCS
Dr. Mohammad H. Salah Page no. 150
DCS
The PLC is still one of themost widely used controlsystems in industry. Asneeds grew to monitorand control more devicesin the plant, the PLCswere distributed and thesystems became moreintelligent and smaller insize. PLCs and DCS areused as shown
Remote Terminal Units (RTUs)
� RTU (sometimes referred to as a remote telemetry unit) asthe title implies, is a microprocessor controlled electronicdevice which interfaces objects in the physical world to adistributed control system (DCS) or SCADA system bytransmitting telemetry data to the system and/or alteringthe state of connected objects based on control messagesreceived from the system
� RTU is a standalone data acquisition and control unit,generally microprocessor based, which monitors andcontrols equipment at some remote location from thecentral station.
� Its primary task is to control and acquire data from processequipment at the remote location and to transfer this databack to a central station.
Dr. Mohammad H. Salah Page no. 151
Remote Terminal Units (RTUs)
� It generally also has the facility for having its configuration
and control programs dynamically downloaded from some
central station.
� There is also a facility to be configured locally by some RTU
programming unit.
Remote Terminal Units (RTUs)
� Although traditionally the RTU communicates back to some
central station, it is also possible to communicate on a peer-
to-peer basis with other RTUs.
� The RTU can also act as a relay station (sometimes referred
to as a store and forward station) to another RTU, which
may not be accessible from the central station.
� Small sized RTUs generally have less than 10 to 20 analog
and digital signals, medium sized RTUs have 100 digital and
30 to 40 analog inputs.
� RTUs, having a capacity greater than this can be classified as
large.
Dr. Mohammad H. Salah Page no. 152
Remote Terminal Units (RTUs)
� RTU is a device installed at a remote location that:
1. Collects data
2. Codes data into a format that is transmittable
3. Transmits the data back to a central station (master)
4. Collects information from the master device andimplements processes that are directed by the master
� RTU is equipped with input channels for sensing and outputchannels for control or alarms and communications port.
RTU - Types
� There are two basic types of RTU
1. The “single board RTU” which is compact, and contains allI/O on a single board
2. The “modular RTU” which has a separate CPU module, andcan have other modules added, normally by plugging into acommon “backplane” (a bit like a PC motherboard and plug inperipheral cards).
� The single board RTU normally has fixed I/O (e.g., 16 digitalinputs, 8 digital outputs, 8 analogue inputs, and say 4 analogueoutputs). It is normally not possible to expand its capability.
� The modular RTU is designed to be expanded by addingadditional modules. Typical modules may be a 8 analog inmodule, a 8 digital out module. Some specialized modules suchas a GPS time stamp module may be available.
Dr. Mohammad H. Salah Page no. 153
RTU - Sizes
� Tiny stand-alone systems that run off batteries for an entireyear or more. These systems log data into EPROM or FLASHROM and download data when physically accessed by anoperator. Often these systems use single chip processors withminimal memory and might not be able to handle asophisticated communications protocol.
� Small stand-alone systems that can power up periodically andapply power to sensors (or radios) to measure and/or report.Usually run off batteries that are maintained by solar energy.The batteries are large enough to maintain operation for atleast 4 months during the darkness of the winter in the farnorthern hemisphere. These systems generally have enoughcapability for a much more complex communications scheme.
RTU - Sizes
� Medium Systems that are dedicated single board industrialcomputers, including IBM-PC or compatible computerseither in desk-top enclosures or industrial configurationssuch as VME, MultiBus, STD bus, PC104, etc….
� Large Systems for complete Plant control with all the bellsand whistles. These are usually in Distributed ControlSystems (DCSs) in Plants, and often communicate over highspeed LANs. Timing may be very critical.
Dr. Mohammad H. Salah Page no. 154
RTU – Architecture and Communications
The SCADA RTU has the following hardware features:
1. CPU and volatile memory
2. Non volatile memory for storing programs and data
3. Communications capability either through serial port(s)
or sometimes with an on board modem
4. Secure Power supply (with battery backup)
5. Watchdog timer (to ensure the RTU restarts if something
fails)
6. Electrical protection against "spikes"
7. I/O interfaces to DI/DO/AI/AO's
8. Real time clock
RTU – Architecture and Communications
Dr. Mohammad H. Salah Page no. 155
RTU – Architecture and Communications
� RTU monitors the field digital and analog parameters and
transmits all the data to the Central Monitoring Station.
� RTU can be interfaced with the Central Station with
different communication media (usually serial (RS232,
RS485, RS422) or Ethernet)
� RTU can support standard protocols (Modbus, DNP3,
ICCP…etc.) to interface any third party software.
� In some control application, RTU drives high current
capacity relays to a digital output board to switch power on
and off the devices in the field
RTU – Architecture and Communications
� RTU can monitor Analog inputs that can be of different
types like (4 to 20mA), (0 to 10V), (-2.5 to 2.5V), (1 to
5V)…etc
� RTU then translates this raw data into the appropriate units
(e.g., gallons of water or temperature) before presenting
the data to the user via the Human Computer Interface (HCI)
or Man-Machine Interface (MMI)
Dr. Mohammad H. Salah Page no. 156
RTU – Applications
� Oil and Gas remote instrumentation monitoring, (offshoreplatforms, onshore oilwells)
� Networks of remote pump stations
� Hydro-graphic monitoring and control, (water supply,reservoirs, sewerage systems).
� Environmental monitoring systems (pollution, air quality,emissions monitoring).
� Minesite monitoring applications.
� Protection supervision and data logging of Powertransmission network
� Air traffic equipments such as navigation aids.
RTU – Comparison
� A PLC is a small industrial computer which originally replacedrelay logic. It had inputs and outputs similar to those an RTUhas.
� It contained a program which executed a loop, scanning theinputs and taking actions based on these inputs.
� Originally the PLC had no communications capability, but theybegan to be used in situations where communications was adesirable feature.
� So communications modules were developed for PLC's,supporting ethernet (for use in DCSs) and the Modbuscommunications protocol for use over dedicated (wire) links.
� As time goes on we will see PLC's support more sophisticatedcommunications protocols.
Dr. Mohammad H. Salah Page no. 157
RTU – Comparison
� RTU's have always been used in situations where thecommunications are more difficult, and the RTU's strengthwas its ability to handle difficult communications.
� RTU's originally had poor programmability in comparison toPLC's. As time has gone on, the programmability of the RTUhas increased.
� We are seeing the merging of RTU's and PLC's, but it will bea long time (if ever) before the distinction disappears..
� RTUs, PLCs and DCS are increasingly beginning to overlap inresponsibilities, and many vendors sell RTUs with PLC-likefeatures and vice versa. The industry has standardized forcreating programs to run on RTUs and PLCs
RTU – Comparison
� RTU differs from a PLC in that RTUs are more suitable forwide geographical telemetry, often using wirelesscommunications, while PLCs are more suitable for local areacontrol (plants, production lines, etc.) where the systemutilizes physical media for control
� Some vendors now supply RTUs with comprehensivefunctionality pre-defined, sometimes with PLC extensionsand/or interfaces for configuration
� Some suppliers of RTUs have created simple Graphical UserInterfaces (GUI) to enable customers to configure theirRTUs easily
Dr. Mohammad H. Salah Page no. 158
Telemetry
� Telemetry refers to the transfer of remote measurement
data to a central control station over a communications link.
� This measurement data is normally collected in real-time
(but not necessarily transferred in real-time).
� The terms SCADA, DCS, PLC and smart instrument are all
applications of the telemetry concept.
Modems
� The telephone system, landline communication systems, and
radio systems cannot directly transport digital information
without some distortion in the signal due to the bandwidth
limitation inherent in the connecting medium.
� A conversion device, called a modem (modulator/demodulator),
is thus required to convert the digital signals into an analog form
suitable for transmission over a telephone network.
� This converts the digital signals generated by a computer into an
analog form suitable for long distance transmission over the
cable or radio system.
� The demodulation portion of the modem receives this analog
information and converts it back into the original digital
information generated by the transmitting computer.
Dr. Mohammad H. Salah Page no. 159
Modems
Modems - Types
There are two types of modem available today:
� Dumb (or non-intelligent) modems depend on the
computer to which they are connected, to instruct the
modem when to perform most of the tasks such as
answering the telephone.
� Smart modems have an on-board microprocessor enabling
them to perform such functions as automatic dialing and
the method of modulation to use.
Dr. Mohammad H. Salah Page no. 160
Modems – Communication Protocols
� Modems can be either synchronous or asynchronous.
� In asynchronous communications each character is
encoded with a start bit at the beginning of the character
bit stream and a parity and stop bit at the end of the
character bit stream.
� The receiver then synchronizes with each character
received by looking out for the start bit.
� Once the character has been received, the communications
link returns to the idle state and the receiver watches out
for the next start bit (indicating the arrival of the next
character).
Modems
Dr. Mohammad H. Salah Page no. 161
Modems
� Synchronous communication relies on all characters being sent in
a continuous bit stream.
� The first few bytes in the message contain synchronization data
allowing the receiver to synchronize onto the incoming bit
stream.
� Hereafter synchronization is maintained by a timing signal or
clock.
� The receiver follows the incoming bit stream and maintains a
close synchronization between the transmitter clock and receiver
clock.
� Synchronous communications provides for far higher speeds of
transmission of data, but is avoided in many systems because of
the greater technical complexity of the communications
hardware.
Modems
Dr. Mohammad H. Salah Page no. 162
Modems – Modes of Operation
Modems can operate in three modes:
� Simplex
� Half-Duplex
� Full-Duplex
A simplex system in data communications is one that is
designed for sending messages in one direction only and
has no provision for sending data in the reverse direction.
Modems – Modes of Operation
� A duplex system in data communications is one that is designed
for sending messages in both directions.
� Duplex systems are said to be half-duplex when messages and
data can flow in both directions but only in one direction at a
time.
� Duplex systems are said to be full-duplex when messages can
flow in both directions simultaneously.
� Full-duplex is more efficient, but requires a communication
capacity of at least twice that of half-duplex.
Dr. Mohammad H. Salah Page no. 163
Modems – Interface Standards
� RS-232, RS-422 and RS-485 standards form the key element in
transferring digital information between the RTUs (or operator
terminals), and the modems, which convert the digital information
to the appropriate analog, form suitable for transmission over
greater distances.
� The RS-232 standard was initially designed to connect digital
computer equipment to a modem where the data would then be
converted into an analog form suitable for transmission over greater
distances.
� The RS-422 and RS-485 standards can perform the same function
but also have the ability of being able to transfer digital data over
distances of over 1200 m.
� The most popular (but probably technically the most inferior
(poorer)) of the RS standards is the RS-232C standard.
Modems – RS232C Interface Standard
� The RS-232 interface standard was developed to interface
between data terminal equipment (DTE) and data
communications equipment (DCE) employing serial binary
data interchange.
� The EIA-RS-232 standard consists of 3 major parts, which
define:
- Electrical signal characteristics
- Interface mechanical characteristics
- Functional description of the interchange circuits
Dr. Mohammad H. Salah Page no. 164
Modems – RS232C Interface Standard
� Electrical signal characteristics: Electrical signals such as the
voltage levels and grounding characteristics of the interchange
signals and associated circuitry.
� Interface mechanical characteristics: It dictates that the
interface must consist of a ‘plug’ and Modems 183 ‘receptacle’
(socket) and that the receptacle will be on the DCE. In RS-232-
C, the pin number assignments are specified but, originally,
the type of connector was not.
� Functional description of the interchange circuits
This defines the function of the data, timing, and control
signals used at the interface between DTE and DCE.
Modems – RS232 Limitations
� The restriction of point-to-point communications is a drawback
when many devices have to be multi-dropped together.
� The distance limitation (typically 15 meters) is a limitation when
distances of 1000m are needed.
� The 20 kbps baud rate is too slow for many applications.
� The voltages of –3 to –25 volts and +3 to +25 volts are not
compatible with many modern power supplies (in computers) of
+5 and +12 volt.
� The standard is an example of an unbalanced standard with high
noise susceptibility.
Two approaches to deal with the limitations of RS-232 are the
RS-422 and RS-485 standards.
Dr. Mohammad H. Salah Page no. 165
Modems – RS422 Interface Standard
� The RS-422 standard introduced in the early 70s defines a
differential data communications interface using two
separate wires for each signal
� This permits very high data rates and minimizes problems
with varying ground potential because the ground is not
used as a voltage reference (in contrast to RS-232) and
allows reliable serial data communication for:
- Distances of up to 1200 m
- Data rates of up to 10 Mbps
- Only one line driver is permitted on a line
- Up to 10 line receivers can be driven by one line driver
Modems – RS422 Interface Standard
� The line voltages range between –2 V to –6 V for Logic 1 and +2 V to +6
V for Logic 0 (using terminals A and B as reference points). The line
driver for the RS-422 interface produces a ±5 V differential voltage on
two wires.
� The two signaling states of the line are defined as follows:
- When the ‘A’ terminal of the driver is negative with respect to the ‘B’
terminal, the line is in a binary 1 (Mark or Off) state.
- When the ‘A’ terminal of the driver is positive with respect to the ‘B’
terminal, the line is in a binary 0 (Space or On) state.
Dr. Mohammad H. Salah Page no. 166
Modems – RS422 Interface Standard
� The differential voltage signal is the major feature of the RS-
422 standard, which allows an increase in speed and
provides higher noise immunity.
� Each signal is transferred on one pair of wires and is the
voltage difference between them.
� A common ground wire is preferred to aid noise rejection.
� Consequently, for a full-duplex system, five wires are
required (with 3 wires for half-duplex systems).
� The RS-422 standard does not specify the mechanical
connections or assign pin numbers and leaves this aspect
optional.
Modems – RS485 Interface Standard
� The RS-485 standard is the most adaptable and flexible.
� It is an expansion of RS-422 and allows the same distance
and data speed but increases the number of transmitters
and receivers permitted on the line.
� RS-485 permits multi-drop network connection on two
wires and provides for reliable serial data communication
for:
- Distances of up to 1200 m (same as RS-422)
- Data rates of up to 10 Mbps (same as RS-422)
- Up to 32 line drivers permitted on the same line
- Up to 32 line receivers permitted on the same line
Dr. Mohammad H. Salah Page no. 167
Modems – RS485 Interface Standard
� The line voltages are similar to RS-422 ranging between –1.5V to
–6V for logic ‘1’ and +1.5V to +6V for Logic ‘0’.
� As with RS-422, the line driver for the RS-485 interface produces
a 5 volt differential voltage on two wires. For full-duplex systems,
five wires are required.
� For a half-duplex system, only three wires are required.
� The major enhancement of RS-485 is that a line driver can
operate in three states (called tri-state operation) logic ‘0’, logic
‘1’ and ‘high-impedance’, where it draws virtually no current and
appears not to be present on the line.
� This latter state is known as the ‘disabled’ state and can be
initiated by a signal on a control pin on the line driver integrated
circuit.
Modems – RS485 Interface Standard
� The RS-485 interface standard is useful where distance and
connection of multiple devices on the same pair of lines is
desirable.
� Special care must be taken with the software to coordinate which
devices on the network can become active.
� In most cases, a master terminal, such as a PLC or computer,
controls which transmitter/receiver will be active at any one
time.
Dr. Mohammad H. Salah Page no. 168
SCADA Examples –Water Treatment
SCADA Examples –Wind Farms 1
Dr. Mohammad H. Salah Page no. 169
SCADA Examples –Wind Farms 2
SCADA Examples – Cement Mill
Dr. Mohammad H. Salah Page no. 170
SCADA Examples – Level Control
Dr. Mohammad H. Salah Page no. 171