Programmable Logic Controller PLC is a microprocessor based system that uses programmable memory to

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Programmable Logic Controller (PLC)

• 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.

External Computer
PLC
Actuator
Process
Sensor
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Programmable Logic Controller Architecture

• 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).

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Programmable Logic Controller Architecture
PLC
Power Supply
Microprocessor Memory
Operator Workstation
Communication Module
Discrete Input (DI) Module
Discrete Sensor
Analogue Input (AI) Module
Analogue Sensor
Discrete Output (DO) Module
Discrete Actuator
Analogue Output (AO) Module
Analogue Actuator
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Programmable Logic Controller Architecture
External Computer
Communication Module
PLC
Microprocessor
Output Module
Actuator
Process
Input Module
Sensor
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Programmable Logic Controller (PLC)
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Distributed Control Systems (DCS)

• 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)

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Distributed Control System Architecture
Operator Workstation 1
Operator Workstation 2
Operator Workstation 3
Database
Controller 1
Controller 2
Controller 3
Controller 4
Input Module
Output Module
Input Module
Input Module
Input Module
Output Module
Output Module
Output Module
Sensor 1
Actuator 1
Actuator 2
Sensor 3
Actuator 3
Sensor 4
Actuator 4
Sensor 2
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Human Machine Interface (HMI)

• 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.

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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.
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Supervisory Control and Data Acquisition (SCADA)

• 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.

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SCADA Versus DCS

• 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.

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Communication in Industrial Control

• 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.

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Communication in Industrial Control

• 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).

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

• 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

PLC3 (SLAVE)
PLC2 (SLAVE)
PLC1 (SLAVE)
Operator Workstation (MASTER)

• Operator Workstation -gt PLC1 Change set-point to
  the value of 3.2
• PLC1 -gt Operator Workstation I have changed
  set-point to the value of 3.2
• Operator Workstation -gt PLC2 Change set-point to
  the value of 6.7
• PLC2 -gt Operator Workstation I have changed
  set-point to the value of 6.7
• Operator Workstation -gt PLC3 Change set-point to
  the value of -1.2
• PLC3 -gt Operator Workstation I have changed
  set-point to the value of -1.2

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Master-Slave Communication

• 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 masters 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).

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Peer-To-Peer Communication

• 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.

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Peer-To-Peer Communication

• 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.

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Communication in Industrial Control Example
Office Computer
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.
Programmable Logic Controller
Actuator
Sensor
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Communication in Industrial Control Example
Office Computer
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 PLCs 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.
Programmable Logic Controller
Actuator
Sensor
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Communication in Industrial Control Example
Office Computer
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.
Programmable Logic Controller
Actuator
Sensor
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Communication in Industrial Control Example
Office Computer
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.
Programmable Logic Controller
Actuator
Sensor
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System Integration

• 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.