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

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