scada system basic plc for fundamental and basic knowledge

1. • Programmable Logic Controller (PLC) is a microprocessor based system that uses programmable
memory to store instructions and implement functions such as logic, sequencing, timing, counting and
arithmetic in order to control machines and processes.
• The first PLC was developed in 1969 by General Motors. A microprocessor-based PLC was introduced
in 1977 by Allen Bradley. It was based on 8080 microprocessor with circuitry to handle bit logic
instructions at high speed.
• Nowadays, PLC is viewed as a solid-state, digital, industrial computer that is capable of both logic and
PID control. It is made to fit an industrial environment and for exposure to hostile conditions, such as
heat, humidity, unreliable power and mechanical shocks and vibrations.
• Unlike Personal Computer, PLC does not contain peripherals, such as display or keyboard, that allow
user to directly interact with PLC. In order to facilitate interaction, separate computer is provided,
normally taking form of a standard PC. Through this external computer, operator can re-program PLC,
provide set-points and view trends of process variables that are controlled and manipulated by PLC.
Programmable Logic Controller (PLC)
PLC Actuator Process
Sensor
External
Computer

2. • PLC consists of the following components:
• Microprocessor – This is the brain of PLC. It reads input signals, executes control program and
communicates results (decisions) of control program as action signals to the outputs.
• Memory – It stores control program that is to be executed at a prescribed rate.
• Power Supply – This component is used to convert the mains AC voltage to the low DC voltage
(e.g. from 240V AC to 5V DC). This unit powers the processor and the circuits in the input and
output modules.
• Input Module – This component receives information from external devices (sensors). It contains
circuitry that provides electrical isolation and signal conditioning functionalities. Input module can
be analogue input (AI) or discrete input (DI) module. AI module receives continuously changing
signal whose amplitude is proportional to the current value of the measured process variable. DI
module receives discrete/digital (ON/OFF) information from discrete sensors, for example push
button (ON if button is pressed, OFF if button is not pressed). Note that DI is much more frequently
used than AI.
• Output Module – This module communicates control actions to external devices (actuators). It
contains circuitry required to interface PLC with actuators (e.g. digital-to-analogue converter and
power amplifier). Like input module, output module can be analogue output (AO) or discrete output
(DO) module depending on the type of actuator used.
• Communication Module – This component allows PLC to communicate with external devices using
sophisticated multiple-bit digital communication protocols (e.g. Ethernet).
Programmable Logic Controller Architecture

3. Power Supply
Communication Module
Microprocessor + Memory
Analogue Input (AI) Module
Discrete Output (DO) Module
Discrete Input (DI) Module
Analogue Output (AO) Module
Analogue Sensor
Discrete Sensor
Analogue Actuator
Discrete Actuator
Operator
Workstation
Programmable Logic Controller Architecture
PLC

4. Communication
Module
Microprocessor
Input Module
External
Computer
Programmable Logic Controller Architecture
PLC
Output Module Actuator Process
Sensor

5. Programmable Logic Controller (PLC)

6. • Distributed Control System (DCS) refers to control system architecture in which control elements are not
centrally located but are rather distributed across manufacturing process. More specifically, control
functions are performed by a number (tens, hundreds, thousands) of distributed microprocessor-based
units (controllers) situated near to the devices being controlled or the instruments from which data is
being gathered.
• First DCS systems appeared around 1975. These were TDC 2000 (from Honeywell) and CENTUM (from
Yokogawa). Their development was largely due to the increased availability of microcomputers and
proliferation of microprocessors in process control.
• DCS normally consists of the following units:
• Input/Output Modules (interface between sensors/actuators and controllers)
• Controllers (perform control functions such as PID algorithm, logic control or sequential control)
• Operator Workstations (PC-like computers that allow users to interact with DCS controllers)
• Database (collects and stores all the data related to DCS operations history)
• Communication Network (allows all of the above elements of the DCS to communicate information
between each other)
Distributed Control Systems (DCS)

7. Operator
Workstation 1
Operator
Workstation 2
Operator
Workstation 3
Controller 1 Controller 2 Controller 3 Controller 4
Sensor 1 Actuator 1 Actuator 2 Sensor 3 Actuator 3 Sensor 4 Actuator 4
Database
Distributed Control System Architecture
Input
Module
Output
Module
Sensor 2
Input
Module
Input
Module
Input
Module
Output
Module
Output
Module
Output
Module

8. • HMI is the system that presents process data to the operator and through which the human operator
controls the process.
• It allows the user (operator/engineer) to interact (“talk/listen”) with the controlled process.
• HMI is a software package that is normally installed on the Operator Workstation.
• DCS vendors provide their own HMI software. Also, PLC vendors sometimes provide their own HMI
software that can interact with PLC.
• There are HMI software providers that are not associated with any particular PLC or DCS product but
instead provide generic system that can interact with various DCS and PLC products through generic
“open” interfaces.
• Main functionality of HMI system:
• Recording and trending of measured process variables. This allows the operator to view time-
domain trajectories of recorded process variables.
• Configuration of controller parameters. This allows the operator to modify controller parameters
and then communicate them down to the actual process controller.
• Display mimic of the actual process. This allows the operator to see in real-time a schematic
representation of the plant being controlled.
Human Machine Interface (HMI)

9. Human Machine Interface System Example
Screen view of the HMI that interacts with the control system of the penicillin
production vessel. Note that in the centre of this display is the mimic diagram of
the controlled process. Also in the right section of this display are two trends of a
measured process variable.

10. • SCADA system performs the following tasks
• Collection of data from field devices, which can be sensors, actuators and controllers.
• Transfer of field devices’ information via communication link to the central site (master station)
• Execution of any necessary analysis and supervisory control calculations, all of which are taking
place at the master stations.
• Display process information on a number of operator screens.
• Convey any required supervisory control actions back to the field devices.
Supervisory Control and Data Acquisition (SCADA)

11. • In the past SCADA and DCS were generally thought of as separate entities. However, in recent years
these two technologies have converged to a great extent. From a big-picture perspective, SCADA and
DCS have become more or less synonymous with each other. However, there are some crucial
differences between DCS and SCADA:
• DCS is process oriented. Its primary role is to control a given process. By-product of DCS activity
is to present data to process operators. SCADA is data-gathering oriented. Its primary function is to
provide, analyse and record/display process information to operators. SCADA does not generally
execute closed-loop control.
• SCADA is designed to operate over large physical distances and is therefore capable of
maintaining safe operation even when communication between operator workstation and field
devices breaks down. This is not necessarily true for DCS systems which require at least one
operator workstation to be functioning properly in order for controllers to maintain satisfactory
process control.
• Operator workstation within DCS is intimately linked to field devices (actuators, sensors and
controllers) over short distances. On the other hand, operator workstation within SCADA may be
connected to field devices over long-distance communication link.
SCADA Versus DCS

12. • Critical prerequisite for the realisation of a control system is the establishment of communication
between the components of the control loop:
• Controller
• Actuator
• Sensor
• In order to implement control system it is necessary to interface sensor with a controller so that
measurements of controlled variable can be communicated from a sensor to a controller. Also, it is
necessary to interface controller to an actuator so that control actions (values of manipulated variable)
can be communicated from controller to an actuator.
• Simplest form of communication between sensor and controller or actuator and controller consists of
transmitting signal whose amplitude is either:
• Proportional to the value of measured process variable or manipulated variable. This is the so-
called analogue communication.
• Dependent on a status of measured process variable or manipulated variable. For example, signal
amplitude is HIGH if a storage tank is full, or it is LOW if a storage tank is not full. This is the so-
called single-bit digital communication.
• These two types of communication are very simple but the information content that is communicated is
very limited (only the value of a variable is communicated). For example, it is not possible to
communicate status of sensor/actuator using these simple communication types.
Communication in Industrial Control

13. • In recent years, sensors and actuators have started to be equipped with microprocessors, which allow
them to communicate with controllers or operator workstations using sophisticated communication
protocols (e.g. Foundation Fieldbus, Industrial Ethernet).
• These protocols allow sensors, actuators, controllers and operator workstations to exchange large
amounts of data that include:
• Values of process variables (values of controlled variables, manipulated variables, set-points)
• Device status (normal, busy, faulty)
• Configuration parameters of devices (sensor resolution, PID controller gains)
• Messages exchanged using these sophisticated digital communication protocols consist of multiple bits
and are therefore referred to as multiple-bit digital communication messages.
• Communication protocols are analogous to human languages and represent rules of communication
between different devices. Protocols specify length of a message and format of a message. They also
specify which device is in control of communication (i.e. which device has a right to initiate
communication).
Communication in Industrial Control

14. Example of message format used in industrial control application for communication between operator
workstation and PLC.
Communication in Industrial Control
1. Hello
2. I am Operator Workstation A1
3. I want to talk to PLC1
4. I want you to change set-point to 3
5. I requested change of set-point
from PLC1
6. Good Bye
This segment signals
beginning of the
message
This segment signals
end of the message
This segment provides
address of a message
sender
This segment provides
address of a message
receiver
This is the actual
request made by
the sending
device
This segment is used
by the receiving device
to check if any
corruption of message
has occurred during
transmission.
Request Message
1. Hello
2. I am PLC1
3. I want to talk to Operator
Workstation A1
4. I changed set-point to 3
5. New set-point is equal to 3
6. Good Bye
Response Message
This is the
actual response
made by the
sending device

15. • Master-Slave has been predominant type of communication in industrial control.
• One device, called MASTER, initiates all of the communication. Master device is typically operator
workstation and sometimes process controller such as PLC.
• Other devices on the network are called SLAVES. They do not initiate communication. Instead, slaves
listen for the requests made by the master device and then send their response messages. Slave
devices are typically controllers as well as “smart” sensors and actuators (sensors and actuators with
their own microprocessor that enables them to communicate using multiple-bit digital communication
protocols).
• Master station periodically makes request to each slave on a network:
Master-Slave Communication
1. Operator Workstation -> PLC1: Change set-point to the value of 3.2
2. PLC1 -> Operator Workstation: I have changed set-point to the value of 3.2
3. Operator Workstation -> PLC2: Change set-point to the value of 6.7
4. PLC2 -> Operator Workstation: I have changed set-point to the value of 6.7
5. Operator Workstation -> PLC3: Change set-point to the value of -1.2
6. PLC3 -> Operator Workstation: I have changed set-point to the value of -1.2
PLC3 (SLAVE)
PLC2 (SLAVE)
PLC1 (SLAVE)
Operator Workstation
(MASTER)

16. • Advantages:
• Communication failure between master and any of the slaves is detected fairly quickly. This is
because master regularly requests information from each slave.
• Collisions (two devices “talk” at the same time) CANNOT occur. Therefore the data throughput is
predictable and constant, which is a critical requirement in real-time control applications.
• Disadvantages
• Variations in the data requirements of each slave cannot be handled. In other words, each slave is
required to use the same response format even though some slave devices may be much more
sophisticated than others.
• Emergency requests from a slave, requesting urgent master’s action, cannot be handled.
• Slaves needing to communicate with each other have to do so through the master. This leads to
added complexity when designing master.
• Due to the predictable data throughput, this communication method is referred to as deterministic
communication method. This fact is the predominant factor for prevalence of master-slave protocols in
control applications. This is particularly true for the low-level regulatory control (controller-sensor and
controller-actuator communication) where the sampling rates are much higher than in supervisory control
applications (MPC controller-PID controller communication link).
Master-Slave Communication

17. • In the case of peer-to-peer communication, all devices on a network are allowed to initiate
communication (i.e. make a request). They are all equal in their rights to make requests, hence the name
peer-to-peer. Ethernet is an example of peer-to-peer communication protocol.
• Due to the fact that any device can start sending message at any point in time it is highly possible that
the so-called collisions will occur. Collisions occur when two devices start transmitting their messages
simultaneously.
• Management of these collisions is an important issue in peer-to-peer communication.
• Typically used collision management scheme is the so-called Carrier Sense Multiple Access with
Collision Detection (CSMA/CD). This scheme is used in the Ethernet protocol for example. Description of
this scheme is as follows:
• All devices on a network “listen” to the common communication link in order to detect if some other
device is transmitting its message.
• If there is no communication going on at the moment then a device starts transmitting its message.
• If by accident two or more devices start transmitting their messages simultaneously, they then
detect that the collision has occurred and each of them stops transmitting their messages.
• Each of the devices involved in collision waits for a short and random time before re-transmitting its
message.
Peer-To-Peer Communication

18. • Advantages:
• Device does not have to repetitively report its status, which may not have changed over the
significant amount of time. Device sends a message only when some consequential event has
occurred. This minimises communication traffic.
• Emergency requests, made by any device over the communication link, can be processed.
• Any two devices connected to the same network can communicate with each other without a need
for a mediator.
• Disadvantages
• Communication link failure cannot be quickly detected because regular requests to each device
are not made in peer-to-peer communication.
• Collisions (two devices start “talking” at the same time) CAN occur. Therefore data throughput
cannot be predicted which can be a serious limitation in real-time control applications.
• Due to the unpredictability of data throughput, this communication method is referred to as probabilistic
communication method. This fact was until recently the predominant factor in choosing not to employ
peer-to-peer communication in real-time control applications. However, data transmission speeds of
these communication networks are continuously increasing and have allowed protocols such as
Industrial Ethernet to be employed in process control applications.
Peer-To-Peer Communication

19. Communication in Industrial Control: Example
Office
Computer
Programmable
Logic
Controller
Actuator
Sensor
Operator
Workstation
Database
Actuator and sensor are directly linked to PLC.
Communication between actuator, sensor and PLC
can be analogue , single-bit digital or multiple-bit
digital communication. This depends on the
sophistication of sensor and actuator.
If actuator and sensor contain their own
microprocessors (“smart” actuator and “smart”
sensor) then multiple-bit communication is possible.
In the case of multiple-bit digital communication it is
most likely that master-slave communication protocol
would be used rather than peer-to-peer with sensor
and actuator being slaves while PLC is a master.

20. Communication in Industrial Control: Example
Office
Computer
Programmable
Logic
Controller
Actuator
Sensor
Operator
Workstation
Database
Operator Workstation and PLC communicate with
each other using multiple-bit communication protocol.
Communication between these two devices can be
accomplished by either master-slave or peer-to-peer
digital communication protocol.
If master-slave protocol is used then operator
workstation would act as a master while PLC would
act as a slave.
Operator Workstation would request values of
controlled and manipulated variables from PLC as
well as providing PLC with set-point changes and
PLC’s control algorithm changes. PLC may also
provide information regarding operational status of
sensor, actuator and itself (idle, busy, faulty).
Operator workstation will contain HMI software
package, standard computer screen, mouse and
keyboard. These would then allow the operator to
view trends of process variables and to modify control
system parameters.
Also, software package containing advanced process
control (e.g. MPC) would be installed on the operator
workstation providing set-points to PLC.

21. Communication in Industrial Control: Example
Office
Computer
Programmable
Logic
Controller
Actuator
Sensor
Operator
Workstation
Database
Operator Workstation and database would most
probably communicate using peer-to-peer
communication protocol.
The purpose of this communication link is to store
current information regarding controlled process into
a database.
Information regarding controlled process is provided
by operator workstation, which in turn has obtained
this information from PLC.

22. Communication in Industrial Control: Example
Office
Computer
Programmable
Logic
Controller
Actuator
Sensor
Operator
Workstation
Database
Database and office computer would communicate
using peer-to-peer communication protocol (e.g.
Ethernet).
The purpose of this communication link is to provide
current or historical information regarding controlled
process to office computer.
Office computer may then perform analysis of this
data to establish control system performance. Also,
office computer may provide display or trend of
controlled and manipulated process variable or some
other key performance indicator variable, which was
derived from measured process variables.
Office computer is generally disabled from writing
values into database. This ensures security of control
system. This means that supervisory controller would
NOT be implemented on office computer since it is
disabled from interacting with operator workstation
and, therefore, with PLC.

23. • One of the current trends in industrial control implementation is to design the overall control system
using components provided by different companies. For example, manufacturing facility would purchase
DCS from company AAA, 3 PLCs from company BBB, 1 PLC from company CCC, HMI software from
company DDD and MPC control software from company EEE.
• The task of interfacing these components so that they function as a whole is the so-called system
integration.
• Because each of these components or sub-systems performs different functions and manipulates
information using different formats, it is necessary for a system integrator to understand input/output
specification of each of them and to know the methods by means of which output or input of one sub-
system can be connected to input or output of another sub-system.
• Note that system integrators are generally not involved in designing and tuning of the control loops and
associated control algorithms.
System Integration
Source : University of Manchester, UK 2009