Transcript Programmable Logic Controllers

Chapter
4
Input/Output Devices and
Motor Controls
Objectives
• Identify the electrical symbols for various
switches.
• Describe the operation of proximity, Hall
effect, ultrasonic, and RADAR sensors.
• Name the common indicators used in PLC
systems.
• Explain the purpose of using indicators in a
PLC system.
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Objectives
• Explain the difference between a power
relay and a control relay.
• Describe the operation of a relay.
• Describe the operation of a solenoid.
• Name the two major types of motor control
devices.
• Name two types of overload relays.
• Explain the difference between temperature
and thermal overload relays.
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Pushbuttons and Switches
• Also called discrete input devices.
• Independent devices that either allow
or disallow electric current to flow
through them.
• Can be physically changed to the ON
or OFF position.
• Remains in that condition until its state
is changed.
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Pushbuttons and Switches
(Cont.)
• Normally open
(NO):
– Always open—
until they are
forced to close.
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Pushbuttons and Switches
(Cont.)
• Normally closed
(NC):
– Always closed—
until they are forced
to open.
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Pushbuttons and Switches
(Cont.)
• Poles:
– Shown in schematics as those contacts
through which current enters the switch.
– Connected to the movable contacts.
– Number of places in which the switch
opens or breaks the circuit.
• Switches:
– Single-break.
– Double-break.
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Pushbuttons and Switches
(Cont.)
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Pushbuttons
• Momentary pushbuttons:
– Also called spring-loaded pushbuttons.
• Locked-position pushbuttons:
– Also called latch pushbuttons.
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Switches
• Manual switch:
– Requires an operator to change their state.
• Automatic switch:
– Controlled by a mechanical or electrical
device.
– Do not have to turn an automatic switch on
or off manually.
– Limit switches.
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Switches (Cont.)
• Switches are designed to work in
harsh environments.
• Industrial switches may be exposed to:
– Extreme humidity.
– Splashing liquid.
– Gas or liquid immersion.
– Dust.
– Vibration.
– High electrical noise.
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Switches (Cont.)
• Temperature switches:
– Detect a specific preset temperature.
– Normally open switch closes and normally
closed temperature switch opens for
temperatures above set point.
– Use vapor pressure technology to sense
changes in temperature.
– Offer many features and modifications,
allowing them to be used in a variety of
applications.
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Switches (Cont.)
• Pressure switches:
– Used to detect low and high pressure in
hydraulic and pneumatic systems.
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Switches (Cont.)
• Liquid level switches:
– Used to detect the liquid level in a
chemical tank or water well.
– Typically, placed at a specific height
within a tank or well.
– Normally open liquid level switch closes,
normally closed liquid level switch opens
above the set point.
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Switches (Cont.)
• Flow switches:
– Set to detect specific flow rates.
– Normally open flow switch closes,
normally closed flow switch opens above
set point.
– In pipes and heating, ventilation, and air
conditioning (HVAC) air ducts.
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Switches (Cont.)
• Float switches:
– Used to open and close contacts in
response to changes in liquid level.
– Used in water treatment plants,
wastewater treatment plants, and
storage tanks.
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Switches (Cont.)
• Foot switches:
– Used as emergency or contact
switches.
– Placed in factory environments in
which workers are using both hands.
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Switches (Cont.)
• Limit switches:
– Physically touched by the part.
– Open or close when objects physically hit
their actuator.
– Consists of an actuator that is
mechanically linked to a set of contacts.
– Used in a variety of applications for
detecting the presence of parts.
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Proximity Switches
• Proximity switches use:
– Light-emitting diode (LED).
– Phototransistor.
(Siemens)
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Proximity Switches (Cont.)
• Light-emitting diode:
– Transmitter that generates an infrared
light beam.
• Phototransistor:
– Receiver that detects the presence of the
light beam.
– Switch triggered when the light beam is
interrupted by the objects placed in the
light beam’s path.
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Sensors
• Electronic sensors:
– Used in PLC control systems to detect
changes in the environment and industrial
settings.
– Can emulate the five human senses and
convert the changes to electronic signals.
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Proximity Sensors
• Can detect the existence of an object.
• Electronic signal used to detect an
object so object does not have to be
touched.
• Can only have a state that is either
true or false.
• Can either be inductive-, capacitive-,
or optical-type sensors.
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Inductive Sensors
• Detects the presence of nearby
metallic objects by measuring changes
in magnetic fields.
• Proved by Faraday in the 1800s.
• Uses a coil to generate a magnetic
field.
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Capacitive Sensors
• Detects an object through the change
in the sensor’s dielectric.
• Value of a capacitor is directly
proportional to the plate area and
dielectric constant used between the
plates.
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Capacitance
• Inversely proportional to the distance of the plates.
C = (8.85  10–12) A  k
d
• Area of the plates (A) and the distance (d) between
them are fixed.
• Dielectric constant (k):
– Space around them varies as different materials
are brought near the sensor.
– Constant value that depends on the material is
used to isolate the capacitor plates.
• Oscillator is used to generate the electric field.
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Optical Sensors
• Can use visible, ultraviolet, infrared, or laser lights.
• Require both an emitter to generate the light beam
and a detector.
• Modern versions are very sophisticated and can
even detect the type of object.
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Hall Effect Sensors
• Semiconductor devices (transistors) that
can be switched by magnetic fields.
• Applications are similar to those of reed
switches and relays.
• Solid state, more rugged, and resists
vibration.
• Used in automated machines to
complete initial calibration and detect
end stops.
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Ultrasonic Sensors
• Produces sound above the normal
human hearing threshold of 16 kHz.
• Uses this sound to detect the distance
to an object.
• Relatively accurate for short distances.
• Economical.
• Time required for sound to travel to the
target and reflect back is proportional
to the distance to the target.
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Electrostatic Ultrasonic
Sensors
• Use capacitive effects.
• Short sound wave is transmitted to hit
an object.
• Longer ranges and wider bandwidths
than piezoelectric ultrasonic sensors.
• More sensitive to factors such as
humidity.
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Piezoelectric Ultrasonic
Sensors
• Work by charge displacement strain on
crystal lattices.
• Rugged and inexpensive.
• Effective for applications such as fluid
levels in tanks and crude distance
measurement.
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Piezoelectric Ultrasonic
Sensors (Cont.)
• When the reflected sound wave hits a
crystal, the crystal produces a small
AC voltage signal and the signal is
then measured.
• Frequency of the sound wave can be
up to 1 MHz.
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Radio Detection and Ranging
(RADAR) Sensors
• Used to detect metallic objects.
• Several cycles of high-frequency waves
are transmitted into the environment
and the reflection is picked up via a
receiver.
• Speed of traveling waves is too fast to
use RADAR in factory for object
detection.
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Indicators
• Pilot lights, ammeters, and voltmeters.
• Help an operator to visually inspect the
operation of an output device.
• Different colored pilot lights show each
stage or operating condition of an
output device.
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Relay
• Magnetized or
demagnetized coil
causes relay’s
contacts to either
close or open.
• Electrically-operated
control switches.
• Classified according
to use as power relay
or control relay.
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Contactors
• Contactor:
– Uses a relatively small amount of
electrical power to control the switching of
a large amount of power.
– Used to control power in heavy power
cables that are run to motors and other
power devices.
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Contactors (Cont.)
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Solid-State Relays
• Similar to Hall effect
switches
• Uses transistors that can
be switched on in the
presence of a magnetic
field.
• Switching design uses no
moving parts or contacts
that can wear out.
• Performs well in harsh
environments.
(Siemens)
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Solenoids
• Control devices that use electromagnetism to convert electrical energy
into mechanical motion.
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Solenoids (Cont.)
• Movement is used to:
– Close a set of electrical contacts.
– Cause the movement of a mechanical
device.
– Or do both at the same time.
• Used to open and close a valve to
control the flow of a liquid or gas.
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Motor Control Devices
• Motor starter:
– Turns a motor on and off.
– Incorporates a motor protective device to
protect the motor from:
• Low voltage.
• High temperature.
• Overload (high load current) conditions.
• Motor drive:
– Controls velocity, acceleration, and
deceleration.
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Motor Control Devices (Cont.)
• Overload relays:
– Overload protective devices in the motor
starter.
– Placed in series between the contacts and
the motor.
– Used to detect excess motor current.
– Contact opens when excess current is
detected, breaking the motor starter coil
circuit, and opening the motor starter
contactors.
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Motor Control Devices (Cont.)
• Temperature overload relay:
– Also called a thermal overload relay.
– Detects over current through the rise in
excessive temperature generated by the line
current.
– Used in most motor control systems.
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Motor Control Devices
(Cont.)
• Magnetic overload relay:
– Detects over current through the
magnetization the line current generates.
– Have faster response time than temperature
overload relays.
– More expensive.
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