IIA module 6

IIA-MODULE 6
Introduction to Sequence Control
SYLLABUS
Introduction to Sequence Control, PLCs - Working,
Specifications of PLC Onboard/Inline/Remote IO’s, Comparison
of PLC & PC,
Relay Ladder Logic-
PLC Programming- realization of AND, OR logic, concept of
latching,
Introduction to Timer/Counters, Exercises based on Timers,
Counters. Basic concepts of SCADA, DCS and CNC

Sequence Control
• A control system in which the individual steps are processed in a
predetermined order, progression from one sequence step to the next being
dependent on defined conditions being satisfied.
• Many control applications do not involve analog process variables, that is,
the ones which can assume a continuous range of values, but instead
variables that are set valued, that is they only assume values belonging to a
finite set.
• The simplest examples of such variables are binary variables, that can have
either of two possible values, (such as 1 or 0, on or off, open or closed etc.).
• These control systems operate by turning on and off switches, motors,
valves, and other devices in response to operating conditions and as a
function of time. Such systems are referred to as sequence/logic control
systems.
• For example, in the operation of transfer lines and automated assembly
machines, sequence control is used to coordinate the various actions of the
production system (e.g., transfer of parts, changing of the tool, feeding of
the metal cutting tool, etc.).

Example- die stamping process
• Process consists of a metal stamping die fixed to the end of a
piston. The piston is extended to stamp a work piece and
retracted to allow the work piece to be removed.
• Actuators: an up solenoid and a down solenoid, which
respectively control the hydraulics for the extension and
retraction of the stamping piston and die.
• Sensors: an upper limit switch that indicates when the piston
is fully retracted and a lower limit switch that indicates when
the piston is fully extended.
• The process has a master switch which is used to start the
process and to shut it down.
• The control computer for the process has 3 inputs (2 from the
limit sensors and 1 from the master switch) and controls 2
outputs (1 to each actuator solenoid).
• When the master switch is turned on the die-stamping piston
is to reciprocate between the extended and retracted
positions, stamping parts that have been placed in the
machine.
• When the master switch is switched off, the piston is to
return to a shutdown configuration with the actuators off and
the piston fully retracted.

Elements of sequence control
• Expressions – They form the building blocks for statements. An
expression is a combination of variable constants and operators
according to syntax of language. Properties as precedence rules
and parentheses determine how expressions are evaluated
• Statements – The statements (conditional & iterative) determine
how control flows from one part of program to another.
• Declarative Programming – This is an execution model of
program which is independent of the program statements. Logic
programming model of PROLOG.
• Subprograms – In structured programming, program is divided
into small sections and each section is called subprogram.
Subprogram calls and co routines, can be invoked repeatedly and
transfer control from one part of program to another.

Types of sequence control
• Implicit Sequence Control
Implicit or default sequence-control structures are those defined by the
programming language itself. These structures can be modified explicitly by the
programmer.
eg. Most languages define physical sequence as the sequence in which statements
are executed.
• Explicit Sequence Control
Explicit sequence-control structures are those that programmer may optionally use
to modify the implicit sequence of operations defined by the language.
eg. Use parentheses within expressions, or goto statements and labels

Programmable Logic Controller (PLC)
• PLC is a special purpose industrial microprocessor based real-time computing
system, which performs the following functions in the context of industrial
operations
 Monitor Input/Sensors
 Execute logic, sequencing, timing, counting functions for
Control/Diagnostics
 Drives Actuators/Indicators
 Communicates with other computers

Components
• POWER SUPPLY
Provides the voltage needed to run the primary
PLC components
• I/O MODULES
Provides signal conversion and isolation
between the internal logic level signals inside
the PLC and the field’s high level signal.
• PROCESSOR
Provides intelligence to command and govern
the activities of the entire PLC systems.
• PROGRAMMING DEVICE
Used to enter the desired program that will
determine the sequence of operation and control
of process equipment or driven machine.

PLC operation
Self test: Testing of its own hardware and software
for faults.
Input scan: If there are no problems, PLC will
copy all the inputs and copy their values into
memory.
Logic solve/scan: Using inputs, the ladder logic
program is solved once and outputs are updated.
Output scan: While solving logic the output
values are updated only in memory when ladder
scan is done, the outputs will be updated using
temporary values in memory.

Advantages
• Programming the PLC is easier than wiring physical components; the only wiring required
is that of connecting the I/O terminals.
• The PLC can be reprogrammed using user-friendly programming devices. Controls must be
physically rewired.
• PLCs take up much less space.
• Installation and maintenance of PLCs is easier, and with present day solid-state technology,
reliability is grater.
• The PLC can be connected to a distributed plant automation system, supervised and
monitored.
• Beyond a certain size and complexity of the process, a PLC-based system compare
favourably with control panels.
• Ability of PLCs to accept digital data in serial, parallel and network modes imply a drastic
reduction in plant sensor and actuator wirings, since single cable runs to remote terminal
I/O units can be made. Wiring only need to be made locally from that point.
• Special diagnostic and maintenance modes for quick troubleshooting and servicing, without
disrupting plant operations.

PLC Programming
Relay Ladder Logic (RLL)
• A RLL diagram, is a visual and logical method of
displaying the control logic.
• The ladder is made up of a series of “rungs” of
logical expressions expressed graphically as series
and parallel circuits of relay logic elements such as
contacts, timers etc.
• Rails- These are vertical lines and provide the
sources of energy to relays and logic system
• Rungs- These are horizontal and contains the
branches ,inputs and outputs. Each rung consist of
a set of inputs on the left end of the rung and a
single output at the right end of each rung.
• Branches
• Inputs
• Outputs
• Timer
• Counter

Latching
• Latching is designed to keep the relay
closed after power has been
disconnected , the usual relay logic
opens the contact instantaneously after
the coil is released.
• Latching is used where it is necessary for
contacts to stay open and/or closed even
though the coil is energised momentarily.
• The program elements that are assigned
as latching relays will remain on once
they are energised
• Only relays and outputs may be assigned
as latching.

Latching
The latching uses two coils to accomplish the function
as shown in fig.
• When the ON button is momentarily actuated the
latch coil is energised to set the relay to its latched
position.
• The contacts close, completing the circuit to the
pilot light and so the light is switched on.
• The relay coil does not have to be continuously
energised to hold the contacts closed and keep the
light on.
• The only way to switch the lamp off is to actuate the
OFF button , which will energise the unlatch coil
and return the contacts to their open, unlatched state.
• In cases of power loss, the relay will remain in its
original latched or unlatched state when power is
restored

Relay Latch
• Latching relay function can be programmed on a PLC to work like its real
world counter parts
• They have a small metal strip that, in essence revolves between two terminals
• Solenoids or small coils can be found on either side of a magnetized switch
that has one input and two outputs at these terminals
• The switch can be used to toggle one circuit on and off or it can be used to
switch power between two circuits. It’s the coils that control the relay action.
• when an electrical flow goes into the coils that current generates a magnetic
field which turns off.
• The magnetic strip between the two coils is also exposed to magnetic field ,
so that when circuit causes a pulse of electrical current through these coils, it
then pulses the switch mechanism from side to side.
• The metal strip remains in that position until it receives another magnetic
pulse, but this time in the opposite direction.
• This action will push the switch back to the other terminal

Timers
• A timer is device that introduce a time delay in a circuit or system
during its ON or OFF condition.
• PLC timer, the time delay is introduced by programming
• The contacts on the left side of the timer function block are the timer
enable contacts
• When they are closed, power passes to the left terminal of the timer, its
clock is enabled and it starts timing.
• When they are open, power stops flowing through this terminal, and the
timer stops functioning
• A timer function block has three output contacts.
• When the timer is timed out, DONE BIT(DN) is set.
• The ENABLE BIT follows the input enable contact status.
• If the enable contact is true then output ENABLE BIT(EN) is true.
• The timer timing(TT) bit is set when the timer is operating

Timer
• Variety of time base is available. The most common time bases are 0.01
sec, 0.1 sec and 1 sec
• Accumulator value(ACC)- This is the time that has elapsed, since the
timer was last reset.
• When enabled, a timer updates this continuously
• Preset Value(PRF)- This specifies the value that the timer must reach
before the controller sets the done bit
• The programmer determines the preset time.
• When the accumulator value becomes equal to or greater than the preset
value, the timer stops operating and the done bit is set
• This bit can be used to control an output device

Types of Timers
TIMER ON DELAY
• The instruction is used to delay turning an output ON or OFF. The TON
instruction begins to count time base intervals when the rung condition
become true.
• As long as the rung condition remains true the time increments its
accumulator value, over each scan until reaches the preset value. The
accumulator value is reset when the rung condition becomes false,
regardless of whether the timer has timed out

• TIMER OFF DELAY
The TOFF instruction begins t count time base intervals when the rung
condition makes a true to false transition. As long as the rung condition
remains false the timer increments its accumulator vale over each scan until
it reaches the preset value. The controller resets the accumulated value
when the rung conditions becomes true regardless of whether the timer has
timed out
• RETENTIVE AND NON RETENTIVE TIMERS
Retentive refers to the device's ability to remember its exact status such that
when the circuit is again activated, the timer continues from the previous
point. RTO - Retentive Timer. Counts time base intervals when the
instruction is true and retains the accumulated value when the instruction
goes false or when power cycle occurs. The Retentive Timer instruction is a
retentive instruction that begins to count time base intervals when rung
conditions become true. Non-retentive timers reset to zero and start from
zero each time the timer function block is energized.

Counters
• Counters are used to count the number of items produced, and the number of
operations performed.
• PLC counter utilizes a sensor to count operations, which is processed by software
execution in the PLC. Thus the failure rate is reduced and the accuracy level is
increased in a PLC counter.
• The major difference between the counter and the timer is that timer instructions
will continually increment its accumulative value at a rate determined by the time
base when the enable contact is on.
• Counter must see a complete contact transition from 0 to 1 each time it
increments the accumulative value.
• This means that the contact must returns to its zero state before it can have a
transition for a second time.

Counters
UP COUNTER
• PV (preset value) is the count value to be
stored in the counter. Up counter CTU
increases CV (current value) by 1 when
input CU changes from 0 to 1 (i.e.
positive edge).Output Q is 1 when CV ≥
PV. When reset input R is 1, CV is
cleared (becomes 0)
DOWN COUNTER
• PV is loaded to CV (current value) using
the input LD. Each time a positive edge
occurs on CD (count down) terminal the
CV is decremented by one.If CV <= 0, Q
turns on

SCADA (Supervisory Control And Data
Acquisition)
• A SCADA system is an automated centralized monitoring
and control function/system for processes with data logging
capability used to control and monitor geographically
dispersed sites over long distance communications
networks, including monitoring alarms and processing status
data.
• 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= control local operations such as
opening and closing valves and breakers, collecting data
from sensor systems, and monitoring the local environment
for alarm conditions
• SCADA systems are typically used in industries such as
electric, water, oil and gas, transportation, chemical,
pharmaceutical, pulp and paper, food and beverage, textile
industry and other discrete manufacturing processes.
• Also used in distribution systems such as water distribution
and wastewater collection systems, oil and gas pipelines,
electrical power grids, and railway transportation systems.

SCADA -functions
A SCADA system performs four functions:
1. Data acquisition
2. Networked data communication
3. Data presentation
4. Control
These functions are performed by four kinds of SCADA components:
1. Sensors (digital or analog) and control relays that directly interface with the managed system.
2. Remote telemetry units (RTUs). These are small computerized units deployed in the field at
specific sites and locations. RTUs serve as local collection points for gathering reports from
sensors and delivering commands to control relays.
3. SCADA master units. These are larger computer consoles that serve as the central processor
for the SCADA system. Master units provide a human interface to the system and automatically
regulate the managed system in response to sensor inputs.
4. The communications network that connects the SCADA master unit to the RTUs in the field.

Data Acquisition
• SCADA system needs to monitor hundreds or thousands of sensors.
Sensors measure:
1. Inputs and outputs e.g. water flowing into a reservoir (input), valve pressure as
water is released from the reservoir (output).
2. Discrete inputs (or digital input) e.g. whether equipment is on or off, or tripwire
alarms, like a power failure at a critical facility.
3. Analog inputs: where exact measurement is important e.g. to detect continuous
changes in a voltage or current input, to track fluid levels in tanks, voltage levels in
batteries, temperature and other factors that can be measured in a continuous range
of input.
• For most analog factors, there is a normal range defined by a bottom and top level
e.g. temperature in a server room between 15 and 25 degrees Centigrade. If the
temperature goes outside this range, it will trigger a threshold alarm.
• In more advanced systems, there are four threshold alarms for analog sensors,
defining Major Under, Minor Under, Minor Over and Major Over alarms.

Data Communication
A communications network is required to monitor multiple systems from a
central location.
•TREND: put SCADA data on Ethernet and IP over SONET.
• SECURITY: Keep data on closed LAN/WANs without exposing sensitive data to
the open Internet.
• Encode data in protocol format (use open, standard protocols and protocol
mediation)
• Sensors and control relays can’t generate or interpret protocol communication - a
remote telemetry unit (RTU) is needed to provide an
interface between the sensors and the SCADA network.
• RTU encodes sensor inputs into protocol format and forwards them to the SCADA
master;
• RTU receives control commands in protocol format from the master and transmits
electrical signals to the appropriate control relays.

Data Presentation
SCADA systems report to human operators over a master station, HMI
(Human-Machine Interface) or HCI (Human-Computer Interface).
SCADA master station has several different functions:
• continuously monitors all sensors and alerts the operator when there is an
“alarm”
• presents a comprehensive view of the entire managed system,
• presents more detail in response to user requests
•performs data processing on information gathered from sensors
• maintains report logs and summarizes historical trends.

Remote telemetry units (RTU)
RTUs need to:
• communicate with all on-site equipment
• survive an industrial environment. Rugged construction and ability to withstand extremes of
temperature and humidity (it needs to be the most reliable element in your facility).
• have sufficient capacity to support the equipment at a site (though should support expected
growth over a reasonable period of time).
• have a secure, redundant power supply for 24/7 working, support battery power and, ideally,
two power inputs.
• have redundant communication ports e.g. secondary serial port or internal modem to keep
the RTU online even if the LAN fails (multiple communication ports easily support a LAN
migration strategy)
• have non volatile memory (NVRAM) for storing software and/or firmware. New firmware
downloadable over LAN to keep RTU capabilities up to date without excessive site visits
• control local systems by themselves (Intelligent control) according to programmed responses
to sensor inputs
• have a real-time clock to accurately date/time stamp reports
• have a watchdog timer to ensure that the RTU restarts after a power failure.

SCADA Master
A SCADA master should display information in the most useful ways to human operators and
intelligently regulate managed systems.
It should :
• have flexible, programmable soft controls to respond to sensor inputs
• allow programming for soft alarms (reports of complex events that track combinations of sensor
inputs and date/time statements).
• automatically page or email directly to repair technicians and provide detailed information
display in plain English, with a complete description of what activity is happening and how to
manage it.
• have tools to filter out nuisance alarms (to prevents operators from loosing confidence and stop
responding even to critical alarms)
• support multiple backup masters, in separate locations (primary SCADA master fails, a second
master on the network automatically takes over, with no interruption of monitoring and control
functions)
• support multiple open protocols to safeguard the SCADA system against unplanned
obsolescence.

DCS - Distributed control system
• 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 provide very sophisticated
analog (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 - Distributed control system
• DCS is a system of dividing plant or process control into several areas of
responsibility, each managed by its own controller, with the whole system
connected to form a single entity, usually by means of communication
buses.
• It refers to a control system usually of a manufacturing system, process or
any kind of dynamic system, in which the controller elements are not
central in location (like the brain) but are distributed throughout the
system with each component sub-system controlled by one or more
controllers.
• The entire system of controllers is connected by networks for
communication and monitoring.

Functions
• Physical Distribution - Nodes or Subsystems can be Distributed i.e.
located physically apart
• Functional Distribution - Specific Functionality is imparted for a Node
basing on the combination of hardware and software used. For e.g.
Application work-processor with Historian, Application work-processor
with control configuration software
• Structural Distribution - Different Structural hardware platforms
(Application Workstation processor, Workstation processor, Control
processor etc.) are used to achieve the required functionality.

DCS System components
• DCS System consists minimum of the
following components.
• Field Control station (FCS): It consists of
input/output modules, CPU and communication
bus.
• Operator station: It is basically human interface
machine with monitor, the operator man can
view the process in the plant and check if any
alarm is presents and he can change any
setting, print reports etc...
• Engineering station: It is used to configure all
input & output and drawing and any things
required to be monitored on Operator station
monitor.

CNC- (Computer Numeric Control)
• A numerical control system in which the
data handling, control sequences, and
response to input is determined by an on
board computer system at the machine
tool.
• Numerical control (NC) refer to control of
a machine or a process using symbolic
codes consisting of characters and
numerals.

CNC
A CNC machine consist of following 6 major
elements:
• Input Device
• Machine Control Unit
• Machine Tool
• Driving System
• Feedback Devices
• Display Unit

CNC
Part Program
A series of coded instructions required to produce a part
Controls the movement of the machine tool and on/off control of auxiliary functions such as spindle
rotation and coolant.
The coded instructions are composed of letters, numbers and symbols.
Program input device
The program input device is the means for part program to be entered into the CNC control.
Three commonly used program input devices are punch tape reader, magnetic tape reader, and
computer via RS-232-C communication.
Machine Control Unit
The machine control unit (MCU) is the heart of a CNC system. It is used to perform the following
functions:
• To read the coded instructions.
• To decode the coded instructions
• To implement interpolations (linear, circular, and helical) to generate axis motion commands.
• To feed the axis motion commands to the amplifier circuits for driving the axis mechanisms.
• To receive the feedback signals of position and speed for each drive axis.
• To implement auxiliary control functions such as coolant or spindle on/off and tool change.

CNC
Machine Tool
CNC controls are used to control various types of machine tools.
Regard less of which type of machine tool is controlled, it always has a slide table and a
spindle to control position and speed.
The machine table is controlled in the X and Y axes, while the spindle runs along the Z axis
Feed Back System
The feedback system is also referred to as the measuring system.
It uses position and speed transducers to continuously monitor the position at which the
cutting tool is located at any particular instant.
The MCU uses the difference between reference signals and feedback signals to generate the
control signals for correcting position and speed errors.

CNC
Advantages:
• High Repeatability and Precision e.g.
Aircraft parts.
• Volume of production is very high.
• Complex contours/surfaces can be easily
machined.
• Flexibility in job change, automatic tool
settings, less scrap.
• More safe, higher productivity, better
quality.
• Less paper work, faster prototype
production, reduction in lead times.
Disadvantages:
• Costly setup, skilled operators.
• Computer programming knowledge
required.
• Maintenance is difficult.

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