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Basics of Programmable Logic Controllers (PLCs)Focused, To-The-Point and Practical!
Personal IntroductionEngr. Arslan Ahmed Amin is a professional Electrical and Instrumentation Engineer serving Pakistan’s pioneer Oil and Gas Organization, Pakistan Petroleum Limited. He obtained his Bachelor's degree in Electrical Engineering from the prestigious University of Engineering and Technology, Lahore in 2010 and started his professional career with Pakistan Petroleum Limited. He has served this organization for 06 years and achieved lots of accomplishments in the development of the systems of newly installed 210 MMSCFD gas compression facility. He actively contributed his services in commissioning, testing, maintenance and upgradation of the E&I systems. He completed his Master’s in Business Administration (M.B.A.) in 2014 from Virtual University of Pakistan, Lahore through distance learning and afterwards obtained Masters (M.Sc.) in Electrical Engineering from University of Engineering and Technology, Lahore in 2015.
P.H.D. Electrical Engineering (University of Engineering and Technology, Lahore) In progress.
M.Sc. Electrical Engineering (University of Engineering and Technology, Lahore)
M.B.A. Online (Virtual University of Pakistan, Lahore)
B.Sc. Electrical Engineering with Honors (University of Engineering and Technology, Lahore)
Education
06 years’ experience in industrial process controls and electrical power systems domain with Pakistan Petroleum Limited (PPL) in CMMS (SAP) environment.
Experience in commissioning, testing and maintenance roles of latest systems regarding Power Generation, Field Instrumentation, Distributed Control System, Safety Instrumented System, Gas Turbines, PLCs, Analyzers, SCADA, Well Head Control Panels and Utility packages.
Experience
‘DCS Troubleshooting Practices’ by Haward Technology Middle East from 16th to 20th May 2016.
‘Protective Relays Applications in Power System’ by Haward Technology Middle East from 04th to 08th April 2016.
‘Circuits and Electronics’ from Massachusetts Institute of Technology (MIT) USA.
‘Project Management’ from Virtual University of Pakistan (VU).
‘Production and Operations Management’ from Virtual University of Pakistan (VU).
‘Conflict Management’ from Virtual University of Pakistan (VU).
‘Crisis Management’ from Virtual University of Pakistan (VU).
Professional Courses
QMS 9001, ISO 14001 EMS and OHSAS 18001, ERP System, Cost of Quality, Productivity Improvement Techniques, Process Safety Management, Hazard Identification and Risk Assessment, HAZOP, SIL systems, Occupational health and Safety, Permit to work system, Safety Modules (Complete), Communication Skills, Team Work Skills, Decision Making Skills (Organized by PPL)
SAP System (R3P version) Maintenance Work Orders Processing, Contracts Management, Spares and Material.
‘Instrumentation and Controls Fundamentals’ from OMS Institute of Management and Technology, Lahore.
Installation, calibration and maintenance of Fire and Gas detectors by Det-tronics.
Generation, Transmission and Distribution at WAPDA Engineering Academy Faisalabad.
Professional Trainings
Among Top 10 students in the session of 240 students in B.Sc. Electrical Engineering.
Received Dean’s Honor Role award in consecutive five semesters for excellent academic performance in B.Sc. Electrical Engineering.
Overall Topped in F.Sc. in Board of Intermediate and Secondary Education, Faisalabad 2006.
Winner of Quaid-e-Azam Scholarship. Gold medal winner in District Science Quiz Competition
Faisalabad. Represented as ‘Talent of Pakistan Youth’ in China in 2007 by
Ministry of Youth, Pakistan.
Academic Achievements
P L C C O U R S E
Introduction PLC history PLC functions PLC selection Logic Gates in PLC Combination of Gates Simple Logic Examples Exercise
What is a PLC?
A PLC is a device that was invented to replace the necessary sequential relay circuits for machine control.
The PLC works by looking at its inputs and depending upon their state, turning on/off its outputs.
The user enters a program, usually via software, that gives the desired results.
What is a PLC?
For example, let's assume that when a switch turns on we want to turn a solenoid on for 5 seconds and then turn it off regardless of how long the switch is on for. We can do this with a simple external timer. But what if the process included 10 switches and solenoids? We would need 10 external timers. What if the process also needed to count how many times the switches individually turned on? We need a lot of external counters.
PLC History
In the late 1960's PLCs were first introduced. The primary reason for designing such a device
was eliminating the large cost involved in replacing the complicated relay based machine control systems.
PLC History
Bedford Associates (Bedford, MA) proposed something called a Modular Digital Controller (MODICON) to a major US car manufacturer.
The MODICON 084 brought the world's first PLC into commercial production.
Programmable Logic Controller ( P L C ) – T Y P E S
Modular:Capacity and functionality can be
added as required and then removed to be used elsewhere.
Industrial:Suitability for the temperature,
humidity, vibration and voltage variation and design for 24 hour operation.
Benefits of PLC
Safe operation of plant In emergency, automatically shutdown the plant
in fail safe condition In startup check the prerequisite & then permit to
start Logging the event.
Benefits of PLC
High reliability Flexible control Easily modified Easy troubleshooting Reduced space requirement Revisable Lower cost Modularity
PLC Selection
Safety Reliability and maintainability Availability Flexibility Cost Speed of operation Security Space Events logging
Safety
For the term of safety, there are numerous definitions that have been worked out the different boards. All definitions have in common that safety means a sufficient protection form danger. In the DIN 31 000 standard document, part 2 safety is defined as a situation in which the risk is not higher than the risk limit. This also means that absolute safety cannot be achieved in technology.
Safety
Safety related controls are needed for trains, lifts, escalators, burners etc. the safe controls must be designed in a way that any component fault and other imaginable influences do not cause dangerous states in the plant.
Safety
The safe state is the state to which a system can be put out of its current operational state and which has a system specific lower hazard potential than the operational state. The absolutely safe state is always the state with the lowest amount of energy involved.
Reliability
Reliability is the ability of a technical device to fulfill its function during its operation time. This is often no longer possible if one component has a failure. So the MTBF (Mean time Between Failure) is often taken as a measurement of reliability. It can either be calculated statistically via systems in operation or via the failure rates of the components applied.
Note
The reliability does not say anything about the safety of a system! Unreliable systems are safe if an individual failure put the plant to the safe state each time.
PLC Components
The PLC mainly consists of a CPU (Processor) Memory areas Input/output data devices Separate relays Counters Timers
Data Storage locations Power Supply Programming unit
Processors
Processor is the brain of PLC 5 basic parts CPU control the operation of PLC. Memory instruction & data that tell the cpu
what to do. Communications processor: handles
communication from outside world. Battery :- keeps time & maintain the
programme. Power supply: supply to run the processor.
Power supply
Two sources of power are used in PLC. Battery power Local power from plant
Battery power used to maintain the user programme / keep the time of day, clock running, maintain the status of certain bits and register.
PLC Components
Do these counters, timers, etc. really exist? No, they don't "physically" exist but rather they are simulated and can be considered software counters, timers, etc.
These internal relays are also simulated.
PLC Components
INPUT RELAYS-(contacts) Connected to the outside world. They physically exist and receive signals from
switches, sensors, etc. Typically they are not relays but rather they are
transistors.INTERNAL UTILITY RELAYS-(contacts)
Do Not Receive Signals From the Outside World nor Do They Physically Exist.
They Are Simulated Relays and Are What Enables a PLC to Eliminate External Relays.
PLC Components
COUNTERS Do not physically exist. They are simulated counters
and they can be programmed to count pulses. Counters can count up, down or both up and down. Since they are simulated they are limited in their
counting speed. Increments vary from 1ms through 1s.
PLC Components
TIMERS Do not physically exist. They come in many varieties and increments. The
most common type is an on-delay type. Others include off-delay and both retentive and non-
retentive types. Increments vary from 1ms through 1hr.
PLC Components
OUTPUT RELAYS Connected to the outside world. They physically exist and send on/off signals to solenoids, lights, etc.
PLC Components
DATA STORAGE Typically there are registers assigned to simply store data. They are usually used as temporary storage for math or data manipulation. They can also typically be used to store data when power is removed from the PLC. Upon power-up they will still have the same contents as before power was removed. Very convenient and necessary
Logic gates in PLC
Logic gates in a PLC are implemented in software as compared to conventional hard-wired logic.
Any size of the logic can be implemented using same PLC. It will not increase the size of PLC as it was the case with conventional systems.
Logic gates in PLC
AB
AB
O
O
OA
AB
O
(OR Gate)
(AND Gate)
(NOT Gate)
(XOR Gate)
= A . B
= A + B
= A_
= A.B + A.B_ _
Combination of Logic gates
A
A
B
B
C
C
O
Two out of three logic with digital Gates
Timer
On-Delay timer:-This type of timer simply "delays turning on".
Off-Delay timer:- This timer simply "delays turning off".
t
t
0
0
Timer
Timed:-Pulse with defined length after signal change from L to H at the input.Output carries H-signal for preset time.
Pulse:- Generate a pulse of 1 second at the output when input signal change from L to H.
t
Flip Flop
To understand this SR flip flop we need its truth table which is below,
Q
QSET
CLR
S
RS R Q Q
1 0 1 0
0 0 1 0
0 1 0 1
0 0 0 1
1 1 X X
Relays
PLC processes inputs, outputs, and the actual program we are almost ready to start writing a program.
But first lets see how a relay actually works. After all, the main purpose of a plc is to replace "real-world" relays.
We can think of a relay as an electromagnetic switch. Apply a voltage to the coil and a magnetic field is generated.
Relays
This magnetic field attracts the contacts of the relay in, causing them to make a connection.
These contacts can be considered to be a switch. They allow current to flow between 2 points
thereby closing the circuit.
Relays Consider example. Here we simply turn on a bell Whenever a switch is closed. We have 3 real-world parts.
A switch A relay A bell
Whenever the switch closes we apply a current to a bell causing it to sound.
Notice in the picture that we have 2 separate circuits. The bottom indicates the DC part. The top indicates
the AC part.
PLC Operation
PLC works by continually scanning a program. We can think of this scan cycle as consisting of 3 most important steps.
PLC Operation
PLC operate very much just like a computer. PLC read input devices, execute its program using the
status of input devices, writes the appropriate values to the output devices.
Principle of Operation Computer execute keyboard instruction while PLC follow the
program. PLC scan time varies from mS to Seconds. Step 1- CHECK INPUT STATUS
PLC takes a look at each input to determine if it is on or off. Rerecords this data into its memory to be used during the next
step.
Principle of Operation
Step 2-EXECUTE PROGRAM PLC executes your program one instruction at a
time. It already knows which inputs are on/off from the
previous step it will be able to decide whether the program output should be turned on based on the state of the input.
It will store the execution results for use later during the next step.
Principle of Operation
Step 3-UPDATE OUTPUT STATUS PLC updates the status of the outputs. It updates the outputs based on which inputs were
on during the first step and the results of executing your program during the second step.
PLC and other type of controller No. of input / output handling
Micro PLC ( Relay Replacer ) Small Medium Large
Response Time
The total response time of the PLC is a fact we have to consider when purchasing for a PLC. Just like our brains, the PLC takes a certain amount of time to react to changes.
INPUT It takes certain amount of time for the
processor to notice the input signal. EXECUTION
Certain amount of time to process the information received from the input.
OUTPUT Output receives a signal from the processor
Processor Scan
Scan time PLC execute only one programme and it keeps on
doing it over and over again. The time out PLC takes to execute this programme once is called the scan time.
Scan time can be reduced by stopping the solution of a line of logic once it has determined it is False, regard less of the rest of the line.
House keeping Checking battery stakes. Verify memory integrity. Checking power supply. Vesting the watchdog timer.
Start up Procedure Operation Test Place processor in “Program mode” and
turn on main power switch. Load the pre-tested control program
into the programmable controller. Disable all outputs, select the run mode
on the processor, and verify that the run light on the processor is activated.
Start up Procedure
Check each rung of logic for proper operation by simulating the inputs, proper time and sequence in the program.
Make any required changes to the control program.
Enable output modules & place processor in “Run” mode.
Test control system as per process operating procedure.
Problem sources in PLC Applications. Field devices Wiring I/O modules Program Processor, racks, etc. Heat protection, loose connections Accumulation dirt, dust, failed ventilation.
Maintenance
PLC Preventive Maintenance
Routine Maintenance Perform Weekly Change Filters. Check error log and scan time. Check diagnostic lights.
Maintenance
Visual Inspection Perform Quarterly Check for proper operation of ventilation system.
Check for buildup of dust or dirt. Is any extraneous material in the cabinets. Look for loose connections. Are there signs of temporary modifications, jumps
etc.? Look for signs of heating, burning, or water
infiltration. Is all the documentation present and up to date?
Maintenance
Maintenance Perform Yearly Replace batteries. Backup Program
Maintenance
Programming device
Copy the program to same secure media. Monitor the programme which it is running. Reload a programme from the storage media. Make changes to the programme.
