Transcript Programmable Control Chapter 15

Programmable Control
Chapter 15
Justin B. Brown
[email protected]
Programmable Logic Controller
Device used to control manufacturing
processes by means of a stored
program and feedback from various
input/ output devices.
Introduction
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Same basic concepts as solid-state control
Only decision-making (logic) section of
control generally affected
Used extensively in industrial machine control
applications
Abbreviated PLC to differ from PC (Personal
Computer)
Can perform any operation state-control can
Controlled by GUI or PC
Further Introduction
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First developed in late 1960’s to replace large
wiring panels containing electromechanical
relays.
Main Components
Signals
from
Process
Programming
Tool
Programming
Language
Input
Interface
Processor
Memory
Output
Interface
Signals
to
Process
Benefits
Relay Logic
Programmable
Controller
Benefit of PLC
Relays
Solid-state
High
Hard
components
Programming
Small Size
Programmed
functions
Microcomputer
based
I/O Interface
Diagnostic
indicators
wiring
Large size
Timer/counter
reliability
Easy to implement
Easy to change
Save floor space
Easy to change
presets
General functionality
Adjust to most field
devices
Simplify
troubleshooting
Advantages Over Hard-wiring
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More flexible and reliable
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allows for frequent control logic changes
no wiring modifications
minimal downtime for that purpose
Offers reduced space requirements
Organized troubleshooting procedures
Little difficulty in applying similar logic
control to different models of machines
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Logic programs can be easily copied
Three Sections of Control
Information/ Inputs
Decision Making/
Logic
Pushbuttons
Central
&
selector switches
Limit switches or
linear transducers
Pressure switches or
hydraulic transducers
Temperature
switches or
thermocouples
Motor starter
contacts
Relay contacts
Photoelectric eyes
Work/ Output
processor
Solenoid valves
unit (CPU)
(analog)
Includes processor, Relays
memory, and power Contactors
unit
Lights
Horns
Heating elements, fans
Motor starters
Alarm messages
Status information
Signal amplifiers
Input
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Information comes from discrete inputs
Voltage converted to DC if necessary
Analog signals converted to binary-coded
decimal (BCD) through analog to digital
converter
Conversion to BCD
Bit
Analog Input
from field
sensors
Analog
to
Digital
Converter
7
6
5
4
3
2
1
0
0
1
1
0
1
0
0
1
Digital
Output
to CPU
“Noise” Isolating Input Devices
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Optoisolator
Reed Relay
Transformer
Optoisolator
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Most common
Mostly used with LED
Other types include neon lamp and
incandescent lamp
LED supplied with current from source
voltage and series resistor
When light from LED hits photodiode, reverse
current set up in output
Voutput=Voutput supply-Rdrop
Vin~LED light~Vout
Optoisolator
24 Vdc Supply
PLC input
terminal
Signal to
CPU
Input
Limit
Switch
Protection
diode
Transistor
switch
Optoisolator
Output Device
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Reverse order of input device
Fuse used to protect from inrush currents
Transform from low power signal to high
power signal
Processor: “Brain”
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Parts: Microprocessor and memory chips
Supports certain number of I/O ports
Determines:
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Variations in instruction
Fixed math functions
Communication link with other processors
Serves as status monitor of I/O devices
Memory
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Two functions:
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Remembers information processor needs to make
decisions (Storage or data table)
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Status (I/O) of all discrete inputs and outputs stored
Numerical values of times stored
Remembers instruction given by user telling PLC
what to do (User program memory)
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Contains ladder-diagram instruction
Larger than storage table
Memory Types
Volatile
1)
Loses contents if power lost
Backup power needed
RAM one type
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Two types include: MOS and CMOS
MOS: metal-oxide semiconductor
CMOS: complimentary metal-oxide semiconductor
Memory Types (con.)
Nonvolatile
2.
Retains information when power lost
ROM common type (Read-only memory)
Several types:
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PROM: Programmable ROM
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Programmed once by manufacturer
EPROM: Erasable Programmable ROM
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Program can be erased by UV light source
EAROM: Electrically Alterable ROM
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Erasing voltage applied to pin completely erases
EEPROM: Electronically Erasable ROM
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Like RAM; changed with PC
Memory Sizes
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Depends on user’s application
Generally expressed in (K)
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Represents words available
Range from 256 words to 64K
Power Supplies
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Source must be as free as possible of:
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heavy loads from industrial machinery
Noise
Voltage fluctuations
Takes incoming line power and rectify and
filter
Lithium batteries mostly used for RAM
Ladder Logic Devices
Normally
open contact
Output devices
Normally
closed contact
A Special
Instruction
END
The end rung
of a cycle
Pictures
Allen Bradley PLC
Toshiba PLC
Any Questions?