2. AUTOMATION
AUTOMATION OR AUTOMATIC
CONTROL, IS THE USE OF
VARIOUS CONTROL SYSTEMS
FOR OPERATING EQUIPMENT
SUCH AS MACHINERY,
PROCESSES IN FACTORIES,
BOILERS AND HEAT TREATING
OVENS, SWITCHING IN
TELEPHONE NETWORKS, AND
AIRCRAFT AND OTHER
APPLICATIONS WITH MINIMAL
OR REDUCED HUMAN EFFORT.
SOME PROCESSES HAVE BEEN
COMPLETELY AUTOMATED.
3. RELAY
A RELAY IS AN ELECTRICALLY
OPERATED SWITCH. MANY
RELAYS USE AN ELECTROMAGNET
TO MECHANICALLY OPERATE A
SWITCH, BUT OTHER OPERATING
PRINCIPLES ARE ALSO USED,
SUCH AS SOLID-STATE RELAYS.
RELAYS ARE USED WHERE IT IS
NECESSARY TO CONTROL A
CIRCUIT BY A LOW-POWER SIGNAL
(WITH COMPLETE ELECTRICAL
ISOLATION BETWEEN CONTROL
AND CONTROLLED CIRCUITS), OR
WHERE SEVERAL CIRCUITS MUST
BE CONTROLLED BY ONE SIGNAL.
4. COMMON TYPES OF RELAY
SPST:–THESE HAVE TWO TERMINALS
WHICH CAN BE CONNECTED OR
DISCONNECTED. INCLUDING TWO
FOR THE COIL, SUCH A RELAY HAS
FOUR TERMINALS IN TOTAL..
SPDT:– A COMMON TERMINAL
CONNECTS TO EITHER OF TWO
OTHERS. INCLUDING TWO FOR THE
COIL.
DPST:– THESE HAVE TWO PAIRS OF
TERMINALS. EQUIVALENT TO TWO
SPST SWITCHES OR RELAYS
ACTUATED BY A SINGLE COIL
DPDT:- THESE HAVE TWO ROWS OF
CHANGE-OVER TERMINALS.
EQUIVALENT TO TWO SPDT SWITCHES
OR RELAYS ACTUATED BY A SINGLE
COIL.
5. FUNCTIONING OF RELAY
RELAYS ARE SWITCHES THAT OPEN AND CLOSE
CIRCUITS ELECTROMECHANICALLY OR
ELECTRONICALLY. RELAYS CONTROL ONE
ELECTRICAL CIRCUIT BY OPENING AND CLOSING
CONTACTS IN ANOTHER CIRCUIT. AS RELAY
DIAGRAMS SHOW, WHEN A RELAY CONTACT IS
NORMALLY OPEN (NO), THERE IS AN OPEN
CONTACT WHEN THE RELAY IS NOT ENERGIZED.
WHEN A RELAY CONTACT IS NORMALLY CLOSED
(NC), THERE IS A CLOSED CONTACT WHEN THE
RELAY IS NOT ENERGIZED. IN EITHER CASE,
APPLYING ELECTRICAL CURRENT TO THE
CONTACTS WILL CHANGE THEIR STATE.
6. RELAYS ARE GENERALLY USED
TO SWITCH SMALLER
CURRENTS IN A CONTROL
CIRCUIT AND DO NOT USUALLY
CONTROL POWER CONSUMING
DEVICES EXCEPT FOR SMALL
MOTORS AND SOLENOIDS THAT
DRAW LOW AMPS.
NONETHELESS, RELAYS CAN
"CONTROL" LARGER VOLTAGES
AND AMPERES BY HAVING AN
AMPLIFYING EFFECT BECAUSE
A SMALL VOLTAGE APPLIED TO
A RELAYS COIL CAN RESULT IN
A LARGE VOLTAGE BEING
SWITCHED BY THE CONTACTS.
7. ADVANTAGES OF RELAY
Electromagnetic relays have fast operation and fast
reset
They can be used for both ac and dc systems for
protection of ac and dc equipments
Electromagnetic relays operating speeds which has
the ability to operate in milliseconds are also can be
possible
They have the properties such as simple, robust,
compact and most reliable
These relays are almost instantaneous. Though
instantaneous the operating time of the relay varies
with the current.
8. PLC (PROGRAMMABLE LOGIC
CONTROLLER)
A PROGRAMMABLE LOGIC CONTROLLER, PLC OR
PROGRAMMABLE CONTROLLER IS A DIGITAL COMPUTER USED
FOR AUTOMATION OF TYPICALLY INDUSTRIAL
ELECTROMECHANICAL PROCESSES, SUCH AS CONTROL OF
MACHINERY ON FACTORY ASSEMBLY LINES, OR LIGHT FIXTURES.
PLCS ARE USED IN MANY INDUSTRIES AND MACHINES. PLCS ARE
DESIGNED FOR MULTIPLE ANALOGUE AND DIGITAL INPUTS AND
OUTPUT ARRANGEMENTS, EXTENDED TEMPERATURE RANGES,
IMMUNITY TO ELECTRICAL NOISE, AND RESISTANCE TO
VIBRATION AND IMPACT. PROGRAMS TO CONTROL MACHINE
OPERATION ARE TYPICALLY STORED IN BATTERY-BACKED-UP OR
NON-VOLATILE MEMORY. A PLC IS AN EXAMPLE OF A HARD REAL-
TIME SYSTEM SINCE OUTPUT RESULTS MUST BE PRODUCED IN
RESPONSE TO INPUT CONDITIONS WITHIN A LIMITED TIME. IT IS A
MULTIPURPOSE, CLOCK DRIVEN, MEMORY BASED DEVICE
WHICH DEALS WITH THE DIFFERENT LEVEL OF COMPLEXITY AND
CONTROL SYSTEM.
10. PLC ADVANTAGES AND BENEFITS
HIGHER RELIABILITY:ONCE THE PROGRAM WRITTEN AND TESTED IT CAN
BE EASILY DOWNLOADED INTO OTHER PLC'S MEMORY.
MORE FLEXIBILITY:IT IS EASIER TO CREATE A NEW PROGRAM MODULE OR
CHANGE AN EXISTING PROGRAM IN PLC COMPARED TO HARD WIRED
CIRCUITRY SYSTEM.
LOWER COST:PLCS WERE ORIGINALLY DESIGNED TO REPLACE RELAY
CONTROL LOGIC WHICH IS NOT ECONOMICAL AND COMPLEX ESPECIALLY
FOR LARGE CONTROL CIRCUITS.
COMMUNICATION CAPABILITY:COMMUNICATION CAPABILITY OF PLC WITH
THE OTHER CONTROLLERS AND COMPUTERS IN THE SYSTEM IS ONE OF
THE MAIN ADVANTAGES COMPARED TO RELAY CONTROL CIRCUIT.
FASTER RESPONSE:PLCS ARE DESIGNED FOR HIGH SPEED AND FOR THE
REAL TIME APPLICATIONS.RESPONSE TIME FOR PLCS ARE MUCH
SMALLER COMPARED TO RELAY LOGIC CIRCUITS.
EASY TO TROUBLESHOOT:PLCS HAVE INBUILT DIAGNOSTICS AND
OVERRIDE FUNCTIONS THAT HELPS THE USER TO EASILY TRANCE THE
SOFTWARE AND HARDWARE ERRORS.
11. COMPONENTS OF PLC
PROCESSOR: IT IS THE CENTRAL PROCESSING
UNIT (CPU) OF THE PROGRAMMABLE
CONTROLLER, IT EXECUTES THE VARIOUS
LOGIC AND SEQUENCING FUNCTIONS BY
OPERATING ON THE PLC INPUTS TO DETERMINE
THE APPROPRIATE OUTPUT SIGNALS.
THE PROCESSOR ACCEPTS THE INPUT DATA
FROM VARIOUS SENSING DEVICES,
EXECUTES THE STORED USER PROGRAM AND
SENDS APPROPRIATE OUTPUT COMMANDS TO
CONTROL DEVICES
12. MEMORY : MEMORY UNIT IS CONNECTED TO THE
CPU, WHICH CONTAINS THE PROGRAMS OF LOGIC,
SEQUENCING AND I/O OPERATIONS .
IT ALSO HOLDS DATA FILES ASSOCIATED WITH
THESE PROGRAMS, INCLUDING I/O STATIONS BITS,
COUNTER AND TIMER CONSTANTS, AND OTHER
VARIABLE AND PARAMETER VALUES.
THIS MEMORY UNIT IS REFERRED TO AS THE
USER OR APPLICATION MEMORY BECAUSE ITS
CONTENTS ARE ENTERED BY THE USER AND ALSO
DIRECTS THE EXECUTION OF CONTROL PROGRAM
AND COORDINATES I/O OPERATIONS WITH THE AID
OF SYSTEM MEMORY.
13. POWER SUPPLY:
A POWER SUPPLY OF 240V ALTERNATING CURRENT(AC)
IS TYPICALLY USED TO DRIVE THE PLC (SOME UNITS
OPERATE ON 120 V AC).
THE POWER SUPPLY CONVERTS THE 240V AC INTO
DIRECT CURRENT (DC) VOLTAGES OF ±5V.
THESE LOW VOLTAGES ARE USED TO OPERATE
EQUIPMENT THAT MANY HAVE MUCH HIGHER VOLTAGE
AND POWER RATINGS THAN THE PLC ITSELF.
THE POWER SUPPLY OFTEN INCLUDES A BATTERY
BACKUP THAT SWITCHES IN AUTOMATICALLY IN THE
EVENT OF AN EXTERNAL POWER SOURCE FAILURE.
14. I/O MODULUS:
THE INPUT/OUTPUT MODULE PROVIDES THE
CONNECTIONS TO THE INDUSTRIAL EQUIPMENT OR
PROCESS THAT IS NOT TO BE CONTROLLED.
INPUTS TO THE CONTROLLER ARE SIGNALS FROM
LIMIT-SWITCHES , PUSH-BUTTONS, SENSORS , AND
OTHER ON/OFF DEVICES.
OUTPUTS FROM THE CONTROLLER ARE ON/OFF
SIGNALS TO OPERATE MOTORS, VALVES, AND OTHER
DEVICES REQUIRED TO ACTUATE THE PROCESS.
MANY PLCS ARE CAPABLE OF ACCEPTING
CONTINUOUS SIGNALS FROM ANALOG SENSORS AD
GENERATING SIGNALS SUITABLE FOR ANALOG
ACTUATORS.
16. LANGUAGE USED
THE LANGUAGE USED FOR PLC
PROGRAMMING IS LADDER LOGIC.
LADDER LOGIC IS A PROGRAMMING
LANGUAGE THAT REPRESENTS A PROGRAM
BY A GRAPHICAL DIAGRAM BASED ON
THE CIRCUIT DIAGRAMS OF RELAY
LOGIC HARDWARE.
19. TIMER
• TIMERS ARE USED TO PERFORM THE TIMING
OPERATIONS.
• TIME BASE IS THE MINIMUM VALUE OF TIME IN SECOND
THAT CAN BE TAKEN BY THE TIMER.
• PRESET VALUE IS THE TOTAL NUMBER OF THE
SECONDS FOR WHICH THE TIMING OPERATION HAS TO
BE DONE
• ACCUMULATOR STARTS INCREASING THE TIME IN
SECONDS UP TO THE PRESET VALUE.
• UP TO THE PRESET VALUE OF THE ACCUMULATOR THE
ENABLE BIT OF TIMER IS HIGH & THE TIMER RUNS.
• WHEN ACCUMULATOR REACHES THE PRESET VALUE
THEN THE TIMER STOPS AND THE DONE BIT OF THE
TIMER BECOMES HIGH.
20. COUNTER
•COUNTERS ARE USED TO COUNT THE
NUMBER OF OPERATIONS.
•ITS FUNCTION IS SAME AS THE TIMER
EXCEPTS THAT THE TIMER COUNTS THE
NUMBER OF SECONDS AND THE
COUNTER COUNTS THE NUMBER OF
OPERATIONS OR PULSES.
•AT EACH OPERATION THE VALUE OF THE
ACCUMULATOR INCREASES AND WHEN
THE VALUE OF THE ACCUMULATOR
COMES TO THE PRESET VALUE OF THE
COUNTER THEN THE COUNTER STOPS
21. COMPARE FUNCTION
COMPARE FUNCTIONS ARE INPUT INSTRUCTIONS.
PREVIOUS FUNCTION BLOCKS WERE OUTPUTS, THESE
REPLACE INPUT CONTACTS.
List of compare function:
EQU
NEQ
LES
LEQ
GRT
GEQ
LIM
