Programming Logic
Controllers
Input/Output Devices & Motor Controls - Chapter 4
PLC Control
Output control
• Turn on/off a circuit
• Increase/decrease
motor speed or torque
Input parameters
• Temperature
• Pressure
• Humidity
• Time
Pushbuttons and Switches
• Discrete input devices
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either on or off
Connected to PLC inputs
When ON they place a positive voltage
When OFF they apply zero voltage to the port
(NO) Normally open – on until switched off
(NC) Normally closed – off until switched on
Pushbuttons
Momentary
• Must hold open/closed
• Return to normal position when released
Locked-position
• Latch until state changed again physically
Switches
Manual
• Require human intervention
Automatic
• Mechanical or electrical
• Thermostat
• Limit switch
Switches cont’d
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Temperature Switch
Pressure Switch
Liquid level Switch
Flow Switch
Float Switch
Foot Switch
Proximity Switch
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Led + phototransistor
Sensors
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Digital – high low voltage
Analog – varying voltage levels
Proximity Sensors
Inductive Sensor – detects current
induced from magnetic fields to
detect metal
Capacitive Sensor – measures change in
sensors dielectric caused by nearby objects
Optical sensors
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Emitter + detector
Visible, ultraviolet, infrared, or laser
Reflection varies with object type
Hall Effect Sensor
• Reed switch
• Switched by
magnetic fields
pull or push
• Solid state
• Resist vibration
• Relay used wire coil to generate magnetic field
• Reed switch used permanent magnate
Ultrasonic Sensor
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Above 16kHz sound ping (like submarine)
Time for reflected sound to return proportional to
object’s distance
Accurate over short distances and cheap
Electrostatic up to 50kHz
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Capacitive
Longer range but sensitive to
environment
Piezoelectric up to 1MHz
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Rugged
Crystal lattice produces voltage
RADAR Sensors
• Radio Detection and Ranging
• Detect metal
objects
• Not used in
factories
Cameras & Object Recognition Software
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Can identify defects
Detecting which parts
Quality assurance
Getting better
Indicators
• Pilot lights
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Red yellow green for states/conditions
• Ammeters & Voltmeters
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Looking for undervoltage or overcurrent conditions
Relays & Contactors
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Relay
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Contactor
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like a reed switch but with a coil
energized (magnetized)
de-energized (demagnetized)
Power relay (contactor) or
control relay (relay)
Power relay
Uses a small amount of voltage to
control a large amount
Solid state relays
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No moving parts to wear out
perform well in harsh environments
Solenoids
• Convert electrical energy into mechanical
motion
• Movable plunger at core with spring
• When energized
plunger is pulled
• When de-energized
spring reverses action
Motor Control
Motor Starters
• Constant speed motors (on/off)
• Contactors and Overload protection overload
relays
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Temperature or thermal overload relay - common
Magnetic overload relay - faster but $$$
Motor Drives
• Soft start stand still to operating speed
• Plugging when motor reverses direction
Electrical Symbols
http://www.techtransfer.com/resources/wiki/entry/1829/#Contacts
Electrical Symbols
http://www.wadeinstruments.com/ez_schematics/description.htm
http://www.wadeinstruments.com/ez_schematics/description.htm
http://www.industrial-electronics.com/image/15-5.jpg