week 10 1_introduction_to_plc
DESCRIPTION
hydraulic subtopicTRANSCRIPT
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Industrial computer in which the hardware and software have been specifically adapted to the industrial environment
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Inp
ut
Power Supply
CPU
Memory
Ou
tpu
t Switches
Sensors
Etc.
Valves
Motor starter
Lights
Etc.
Programming device
(PC, PLC, etc.)
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Contains one or more microprocessors – brain for PLC
Same type of microprocessor in microcomputer -except PLC microprocessor accommodate ladder logic instead of other languages
Functions – executes the tasks, manages memory, monitors input, evaluate logic (ladder diagram) & turn on appropriate output.
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ROM
◦ Operating System (OS) is burnt into ROM by manufacturer
◦ Contains OS control function (software..e.g. Windows in PC)
◦ Nonvolatile memory-when the electricity shut off, the memory is retained
◦ Users cannot change ROM
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RAM
◦ User memory
◦ User can read or write program to RAM
◦ Volatile – when electricity shut off, the data in memory lost
◦ Use battery to avoid ‘memory loss’
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Device that are used to write the program and trouble shoot the PLC
Example
◦ PC, handheld programmers, microcomputers
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Power Supply
To supply power for CPU
Most PLCs operate on 115 V AC
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Performs 2 tasks – taking input from outside world & protecting CPU from outside world
Convert real-world logic (eg. 250 V AC input module) to require logic level by CPU (eg. Low-level DC signal for CPU)
Input devices – switches, sensors, etc.
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Provide connection to real-world output
Eg. – motor starters, lights, coils, valves etc.
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Ladder Logic
◦ Symbolic representation of electrical circuit
◦ Symbols in ladder logic is divided into contacts (input) and coils (output)
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Most input to PLC are either on (true) or off (false)
Input – sensors and switches
Common symbols
◦Normally open--------
◦Normally closed------
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Coils are output symbols
Real output device – motor, light, pump, counter, timer, relay
PLC examine contact (input) in ladder and turn the coil (output) on or off
Coil symbol -------
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Problem: Draw a ladder diagram for a bell circuit. When a switch is pressed, the bell will ring
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Modify the diagram, so that when the switch is pressed once and released, the bell is ringing continuously
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Add another button to stop bell ringing
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A drill press machine only works if there is a part present and the operator has one hand on each start switches (2 start switches)
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Using single solenoid with spring return & 2 sensor for each cylinder
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Using single solenoid with spring return & 1 sensor for each cylinder
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