Programmable Logic Controller PLC is a microprocessor based system that uses programmable memory to - PowerPoint PPT Presentation

1 / 23
About This Presentation

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


Programmable Logic Controller (PLC) is a microprocessor based system that uses ... is the so-called Carrier Sense Multiple Access with Collision Detection (CSMA/CD) ... – PowerPoint PPT presentation

Number of Views:2454
Avg rating:3.0/5.0
Slides: 24
Provided by: ognjenma


Transcript and Presenter's Notes

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

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
  • 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
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).

Programmable Logic Controller Architecture
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
Programmable Logic Controller Architecture
External Computer
Communication Module
Output Module
Input Module
Programmable Logic Controller (PLC)
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
  • 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
  • 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 System Architecture
Operator Workstation 1
Operator Workstation 2
Operator Workstation 3
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
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
  • 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
  • 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 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.
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
  • 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.

  • 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
  • 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
  • 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.

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
  • 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
  • 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,
  • 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
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
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
This is the actual response made by the sending
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
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

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

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).

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
  • 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
  • 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
  • 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.

Communication in Industrial Control Example
Office Computer
Operator Workstation
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
Communication in Industrial Control Example
Office Computer
Operator Workstation
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
Communication in Industrial Control Example
Office Computer
Operator Workstation
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
Programmable Logic Controller
Communication in Industrial Control Example
Office Computer
Operator Workstation
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
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
  • 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.
Write a Comment
User Comments (0)