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Programmable Logic controller introduction
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PRESENTATION ON PROGRAMMABLE LOGIC CONTROLLER
HIMSHEKHAR DAS 11-1-3-042GNYANDEEP BISWAS 11-1-3-048
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OBJECTIVES
1. What is a PLC and Why was it developed 2. Familiarization with major components of
PLC3. PLC Operation4. Ladder Logic5. Simple Ladder Logic Program6. PLC Features7. Ladder Diagram of a DC Motor Control Circuit
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INTRODUCTION
PLC is a digital computer used for automation of electromechanical processes such as control of machinery on factory assembly lines , amusement rides.
PLC’s have multiple inputs and outputs and are designed for extended temperature ranges, immunity to electrical noise etc
The control program is stored in non-volatile memory
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WHY PLC WAS DEVELOPED
Before the PLC, control was done using relays, drum sequencers, closed loop control system. Since they could number in thousands the process of updating such facilities was time consuming and expensive.
Digital computer were now used .But these computers required the following attributes: tolerating the shop floor environment , require less training for use ,have fast response time
Hence the PLC was developed and it came into use
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DEVELOPMENT
PLC’s were designed to replace relay logicPLC’s were programmed using ladder logic
(similar to schematic diagram representation of relay logic) or instruction list programming
Mordern PLC’s are programmed using State logic ,BASIC,C
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Major Components of a Common PLC
PROCESSOR
POWERSUPPLY
I MN O P D U UT L E
O M U OT DP UU LT E
PROGRAMMING DEVICE
From SENSORS
Pushbuttons,contacts,
limit switches,etc.
ToOUTPUT
Solenoids, contactors,
alarmsetc.
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Major Components of a Common PLC
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.
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Major Components of a Common PLC
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.
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PLC
INPUTS
OUTPUTS
MOTOR
LAMPCONTACTOR
PUSHBUTTONS
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PLC Operation
Basic Function of a Typical PLC
Read all field input devices via the input interfaces, execute the user program stored in application memory, then, based on whatever control scheme has been programmed by the user, turn the field output devices on or off, or perform whatever control is necessary for the process application.
This process of sequentially reading the inputs, executing the program in memory, and updating the outputs is known as scanning.
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While the PLC is running, the scanning process includes the following four phases, which are repeated continuously as individual cycles of operation:
PHASE 2Program
Execution
PHASE 3Diagnostics/
Comm
PHASE 4Output
Scan
PHASE 1Read Inputs
Scan
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PHASE 1 – Input Status scan
· A PLC scan cycle begins with the CPU reading the status of its inputs.
PHASE 2– Logic Solve/Program Execution
· The application program is executed using the status of the inputs
PHASE 3– Diagnostics · Once the program is executed, the CPU performs diagnostics and
communication tasks
• An output status scan is then performed, whereby the stored output values are sent to actuators and other field output devices. The cycle ends by updating the outputs.
• As soon as Phase 4 are completed, the entire cycle begins again with Phase 1 input scan.
PHASE 4 - Output Status Scan
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LADDER LOGIC
Each device in the relay rack is represented by a symbol on the ladder diagram with connections between those devices
Used to develop software for programmable logic controllers used in control applications
Used in conjunction with HMI interface Ladder logic can be thought of as a rule based
language rather than procedural language. A “rung” in the ladder represents a rule
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A SIMPLE LADDER LOGIC PROGRAMPre-requisite
Ladder diagram is read from top to bottom or from left to right
The vertical lines are called rails . They provide power to the circuitry. AC or DC power may be connected
The horizontal lines are called rungs .They are composed of input or output.
Output from a PLC is called coil. A coil may be a motor , pump etc.
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Inputs/Sensors to a PLC are referred to as Contacts and may consist of switches, buttons, etc. Contacts begin in one of two states normally open or normally closed.
These contacts have an initial and follow-on state
Stop buttons are given a higher order of importance and therefore be shown ahead of other devices.
The normal condition of electrical diagrams is the circuit de-energized and with no external forces such as pressure, flow, etc. acting on the device.
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L1L2
Ladder Rail
Ladder Rung
Schematic Ladder Diagram.
INPUT OUTPUT
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OFFLogic 0
IN
PLC
InputModule
24 V dc
ONLogic 1
IN
PLC
InputModule
24 V dc
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All contacts associated with a device change state when
the device is energized Devices that perform a STOP function are normally
placed in series on a rung Devices that perform a START function are normally
placed in parallel or in a branch configuration.Some commonly used ladder symbols : -( )- : A regular coil energized when rung is closed -(\) : A “not” coil , energized when rung is open -[ ]- : A regular contact, closed whenever its
corresponding coil is energized -[\]- :A “not” contact , closed whenever its coil is not
energized -- -- :Boxes represent various functions such as
timers ,counters , etc.
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Logical AND : |--------[ ]----------[ ]-----------( )------ key 1 key 2 dc motor when the normally open contacts of both the switches close ,
electricity is able to flow and the dc motor starts Logical AND with NOT : |------------[ ]------------[\]-------------( )-------- close door obstruction door motor The above realizes the function : Door Motor The circuit shows a pushbutton that closes a door and a
obstruction detector that senses if something is in the way of closing the door. When the normally open pushbutton closes and there is no obstruction the normally closed obstruction detector is closed and current is able to flow.
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CIRCUIT 1
CIRCUIT 2
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PROGRAMMING
Early PLC’s were programmed using proprietary programming panels which had dedicated function keys representing the various logical elements of PLC programs
Recently PLC’s are programmed using application software on personal computers, the software allows entry and editing of the ladder style logic
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FUNCTIONALITY
Primary functionality includes relay control , motion control , process control , distributed control systems and networking
Programmable logic relay are used in light industry where only a few points of I/O are involved and low cost is desired
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More About PLC
FEATURES SCANSYSTEM SCALEUSER INTERFACE COMMUNICATION PLC PROGRAMMING
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Tank Used to Mix Two Liquids
A
B
C
FS
MOTOR
TIMER
FLOAT SWITCH
SOLENOIDS
SOLENOID
1 -MINUTE
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DC MOTOR STARTING CIRCUIT
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