Programmable Logic
Controller
Transducer
A transducer is any device that converts energy from
one form to another.
Amplifier
Input transducer
(microphone) converts
sound energy to electric
energy
Output transducer
(speaker) converts
electric energy to sound
energy
Sensors
Sensors are input transducers used for detecting and
often measuring the magnitude of something. They
convert mechanical, magnetic, thermal, optical, and
chemical variations into electric voltages and currents.
Photoelectric
sensor
Sensors
Sensors provide the equivalent of eyes, ears, nose,
and tongue to the microprocessor brain.
Microprocessor
Optical
sensor
Gas
sensor
Microphone
Probe
Proximity Sensor
Proximity sensors or switches detect the presence of
an object without making physical contact with it.
Proximity Sensor Applications
The object being detected is too small, lightweight, or
soft to operate a mechanical switch.
Rapid response and high switching rates are required.
An object has to be sensed through nonmetallic barriers
such as glass, plastic, and paper cartons.
Hostile environments conditions exist.
Long life and reliable service are required.
A fast electronic control system requires a bounce-free
input signal.
Inductive Proximity Sensor Operation
Barrel type
Block diagram
As the target
moves into the
sensing area,
the sensor
switches
the output ON
Capacitive Proximity Sensor
A capacitive proximity sensor can be actuated by both
conductive and nonconductive material such as wood,
plastics, liquids, sugar flour and wheat.
Operation is similar to that of inductive
proximity sensor. Instead of a coil, the
active face of the sensor is formed by
two metallic electrodes – rather like an
"opened capacitor".
Magnetic Switch (Reed Switch)
A magnetic switch (also called
a reed switch) is composed of
flat contact tabs that are
hermetically sealed (air-tight).
Magnet
N
S
NO
Common
NC
The switch is
actuated by a
magnet.
Reed Switch Activation
Reed switch
Magnet
Proximity motion – movement
of the switch or magnet will
activate the switch
Rotary motion – switch is
actuated twice for every
complete revolution
Shielding – the shield
short circuits the
magnetic field; switch is
activated by removal of
the shield
Photovoltaic Or Solar Cell
The photovoltaic cell, or solar cell, is
a common light-sensor device that
converts light energy directly into
electric energy.
Solar cell
The solar cell converts light
impulses directly into electrical
charges which can easily be
amplified to provide an input
signal to a PLC.
Photoconductive Or Photoresistive Cell
The photoconductive cell, or
photoresistive cell, is is another
popular type of light transducer.
Light energy falling on this device
will cause a change in the
resistance of the cell.
Ohms
20 Ohms Light resistance
5,000 Ohms Dark resistance
Photoelectric Sensor Operation
Most industrial photoelectric sensors use a lightemitting diode (LED) for the light source and a
phototransistor to sense the presence or absence of light.
Light detector
Object
to be
sensed
Light source
Light from the LED
falls on the input of the
phototransistor and the
amount of conduction
through the transistor
changes. Analog
outputs provide an
output proportional to
the quantity of light
seen by the
photodetector.
Reflective Photoelectric Sensor
Emits a light beam (visible,
infrared, or laser) from its light
emitting element and detects
the light being reflected.
Diffused-reflective
Retro-reflective
typetype
Emitter/receiver
Operating
Operating
range
range
Target
Reflector
Through-Beam Type Photoelectric Sensor
A through-beam photoelectric
sensor is used to measure the
change in light quantity caused
by the target's crossing the
optical axis.
Emitter
Operating
range
Target
Receiver
Bar Code Systems
Bar code systems can be used to
enter data much more quickly
than manual methods, and are
highly accurate.
Diverter
Decoder
Scanner
PLC
The decoder receives
the signal from the
scanner and converts
these data into the
character data
representation of the
symbol's code.
Ultrasonic Sensor
An ultrasonic sensor operates by
sending sound waves towards the
target and measuring the time it
takes for the pulses to bounce
back.
The returning echo signal
is electronically converted
to a 4 mA to 20 mA output,
which supplies flow rate to
external control devices.
Strain /Weight Sensors
A strain gauge transducer converts
a mechanical strain into an electric
signal.
Hopper
Wire type Force
The load cell provides
sensor input to the
controller, which
displays the weight
and controls the
hopper chute.
Controller
The force applied to the gauge causes the
gauge to bend. This
ON/OFF
bending action also
Chute
distorts the physical
size
of
the
gauge,
Control
which in turn changes its resistance.
Load cell
Temperature Sensors
Temperature sensors convert heat into an electric
signal. There are four basic types used: thermocouple,
resistance temperature detector (RTD), thermistor,
and IC sensor.
The thermocouple consists of a pair
The
resistance temperature
detector
of dissimilar
conductors fused
(RTD)
varies
resistance
value
together
at in
one
end to form
the
The Circuit
thermistor
in
The Integrated
(IC) varies
temperature
with
changes
in temperature.
"hot"
or measuring
junction, with
resistance
value
with or
sensor produces
changes
in
voltage
the free ends available for
changes
inintemperature
current with
changes
temperature.
connection to the
"cold" reference
junction. A temperature difference
between
RTD the measuring and
reference junction generates a small
DC signal voltage.
Flow Measurement
The usual approach used in
measuring fluid flow is to
convert the kinetic energy that
the fluid has into some other
measurable form.
Flow
Electronic Magnetic Turbine
Can beFlow
used Meter
with electrically
Flow
conducting fluids and offers no
Coil Meter
restriction
to flow.
A coil
in at
the
The turbine
blades
turn
unita sets
a magnetic to
field.
rateup
proportional
the If
Magneta conductive
liquid
fluid velocity
andflows
are
this magnetic
field, a
Turbinethrough
magnetized
to induce
voltage
is induced
and sensed
voltage
pulses coil.
by two electrodes.
Velocity/RPM Sensors
A tachometer is a small permanent
magnet DC generator which when
rotated produces a voltage that is
directly proportional to the speed
at which it is driven.
Controller
Lo
ad
M
Motor
Tach
Tachometers coupled
to motors are
commonly used in
motor speed control
applications to provide
a feedback voltage to
the controller that is
proportional to motor
speed.
Velocity/RPM Sensors
The rotating speed of a
shaft is often measured
using a magnetic
(inductive) pickup sensor.
Pickup coil
Sensor
output
0V
Pole piece
N S
Magnet
A magnet is attached to the shaft.
A small coil of wire held near the
magnet receives a pulse each time
the magnet passes. By measuring
the frequency of the pulses, the
shaft speed can be determined.
Output Control Devices
A variety of output control devices can be operated by
the controller output module to control traditional
processes. These include:
Pilot light
Heater
Control
relay
Solenoid
Motor starter
Alarm
Solenoid
valve
Small motor
Actuators
An actuator is any device that converts an electrical
signal into mechanical movement. The principle
types of actuators are relays, solenoids, and motors.
Solenoid
The solenoid converts
electric current into
linear motion.
Symbol
Plunger
AIR
Coil
Solenoid Valve
A solenoid valve is a combination of:
 a solenoid with its core or plunger
 a valve body containing an orifice
in which a disc or plug is positioned
to restrict or allow flow
Forward motion of piston
Directional
solenoid
valve
When SOL A is energized, the
valve spool is shifted to redirect
the fluid and move the cylinder
forward
CR
SOL A
FWD
SOL A
CR
Stepper Motor
A stepper motor converts electrical
pulses applied to it into discrete
rotor movements called steps. They
are used to provide precise position
control of movement.
Stepper motor control system
Module
Communicates
with the PLC and
responds with
pulse trains
Stepper-motor
translator
Enables control
of the stepper motor
Step
motor
The motor will move
one step for each pulse
received
11. Sensors convert mechanical, magnetic,
thermal, optical, and chemical variations into:
(a) electric voltages and currents.
(b) sound
(c) movement
(d) all of these
12. A proximity switch can be actuated without
any physical contact.
(True/False)
13. A thermocouple, when heated, increases its
resistance value. (True/False)
14. The force applied to a strain wire gauge causes
it to bend and change resistance. (True/False)
15. A capacitive proximity sensor may be actuated
by nonconductive materials. (True/False)
16. When a magnetic pickup sensor is used to
indicate the speed of a shaft, the speed is usually
determined by measuring the:
a. voltage output produced
b. frequency of the pulses produced
c. current output produced
d. change in resistance that occurs
17. A through-beam photoelectric sensor is used
to measure the change in ______ caused by the
target crossing the optical axis.
a. resistance
c. current
b. voltage
d. light quantity
18. Which of the following devices would “not”
be classified as an actuator?
a. relay
c. solenoid
b. tachometer d. motor
19. Solenoid valves are available to control:
a. oil flow
c. water flow
b. air flow
d. all of these
20. A(n) ______ converts electrical pulses applied
to it into discrete rotor movements.
a. tachometer c. stepper motor
b. solenoid
d. electronic magnetic flowmeter