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Programming Logic
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
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
• Must hold open/closed
• Return to normal position when released
• Latch until state changed again physically
• Require human intervention
• 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
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
Longer range but sensitive to
Piezoelectric up to 1MHz
Crystal lattice produces voltage
RADAR Sensors
• Radio Detection and Ranging
• Detect metal
• Not used in
Cameras & Object Recognition Software
Can identify defects
Detecting which parts
Quality assurance
Getting better
• Pilot lights
Red yellow green for states/conditions
• Ammeters & Voltmeters
Looking for undervoltage or overcurrent conditions
Relays & Contactors
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
No moving parts to wear out
perform well in harsh environments
• Convert electrical energy into mechanical
• 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
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
Electrical Symbols