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Programming Logic Controllers
Input/Output Devices & Motor Controls - Chapter 4
PLC Control
Output control• Turn on/off a circuit• Increase/decrease
motor speed or torqueInput parameters• Temperature• Pressure• Humidity • Time
Pushbuttons and Switches
• Discrete input devices • 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
releasedLocked-position• Latch until state changed again
physically
Switches
Manual• Require human interventionAutomatic• Mechanical or electrical• Thermostat• Limit switch
Switches cont’d
• Temperature Switch• Pressure Switch• Liquid level Switch• Flow Switch• Float Switch• Foot Switch• Proximity Switch
• Led + phototransistor
Sensors• 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• 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
• 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
• Capacitive • Longer range but sensitive to
environment
• Piezoelectric up to 1MHz• Rugged• Crystal lattice produces voltage
RADAR Sensors
• Radio Detection and Ranging• Detect metal
objects• Not used in
factories
Cameras & Object Recognition Software
• Can identify defects• Detecting which parts• Quality assurance• Getting better
Indicators
• Pilot lights• Red yellow green for states/conditions
• Ammeters & Voltmeters• Looking for undervoltage or overcurrent
conditions
Relays & Contactors• Relay
• like a reed switch but with a coil• energized (magnetized) • de-energized (demagnetized)• Power relay (contactor) or
control relay (relay)
• Contactor• Power relay• Uses a small amount of voltage to
control a large amount
• Solid state relays• 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• 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