Programming Logic Controllers

Input/Output Devices & Motor Controls - Chapter 4

PLC ControlOutput 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

PushbuttonsMomentary• Must hold open/closed• Return to normal position when

releasedLocked-position• Latch until state changed again

physically

SwitchesManual• 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 ControlMotor 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

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