Unit 2 (PLC)

UNIT 2
LOGICAL SENSORS AND ACTUATORS

STUDENTS LEARNING OUTCOME
At the end of the lesson, students will be able
• to define logical sensor used in Industry.
• To identify contact and proximity type of sensors.
• To identify limit switch and its function.
• To identify capacitive proximity sensor and its’
function.

• To identify inductive proximity sensor and its’
function.
• To identify different type of optical sensors and
its’ function.
• To identify ultrasonic sensor and its’ function.

LOGICAL SENSOR DEFINITION
Logical sensors are devices used to detect the state of a
process and return the state either in true or false condition.
Examples of physical phenomena that are typically detected are :
• inductive proximity - is a metal object nearby?
• capacitive proximity - is a dielectric object nearby?
• optical presence - is an object breaking a light beam or
reflecting light?
• mechanical contact - is an object touching a switch?

TYPE OF LOGICAL SENSOR
Generally logical sensors can be classified into :
• Contact type
• Contact implies that there is mechanical contact and a resulting force between
the sensor and the object.
• limit switch, tactile sensor
• proximity type
• Proximity indicates that the object is near, but contact is not required.
• Capacitive sensor, optical sensor

CONTACT TYPE SENSOR
CONTACT SWITCH
Contact or Limit switches are used to
indicate end and beginning position of a
moving mechanism
It is operated by depressing the lever on
the switch to activate the switching
action.
Usually the switch will provide a set of
contacts which include NC and NO type.
Roller lever type
hinge lever type

PROXIMITY TYPE SENSOR
OPTICAL ( PHOTOELECTRIC) SENSOR
Optical sensors require both a light source (emitter) and detector
Emitters will produce light beams in the visible and invisible
(infra red) spectrums using LEDs

OPTICAL ( PHOTOELECTRIC) SENSOR
Detectors are typically built with photodiodes or phototransistors
The oscillating light wave is used so that the sensor can filter out
normal light in the room. The freq is in a range of KHz

OPTICAL ( PHOTOELECTRIC )SENSOR
When the detector receives the light it checks to make sure that it is at the
same frequency as is transmitted
An advantage of the frequency method is that the sensors can be used
with lower power at longer distances

THROUGH BEAM TYPE OPTICAL SENSOR
This sensor needs two separate components; the transmitter is
separated with the detector.
Emitter is set up to point directly at a detector
An object is detected if it interrupts the beam

RETROREFLECTIVE TYPE
Transmitter and detector are housed in the same enclosure
Requires the use of a separate reflector or reflective tape mounted across
from the sensor to return light back to the detector.
An object is detected if it interrupts the beam

DIFFUSE TYPE
Diffuse sensors detect light reflected from object rather than light
interrupted by object in the case of through beam and retroreflective.
Easiest to set up, but they require well controlled conditions
For example if it is to pick up light and dark coloured objects problems
would result.

CAPACITIVE SENSOR
Act similar to a simple capacitor.
A metal plate, in the end of the sensor, is electrically
connected to the oscillator.
The object to be sensed acts as a second plate.
When power is applied to the sensor, the oscillator
senses the external capacitance
between the target and the internal sensor plate
This forms a part of the feedback capacitance in the
oscillator circuit.
If the distance is near enough, the oscillation
amplitude is higher than the threshold voltage of the
trigger circuit, the output of the sensor will switch
state from logic 0 to logic 1, indicating and object is
present nearby.

CAPACITIVE SENSOR
Capacitive sensor can detect all materials object
nearby with vary sensing distance from material to
material.
The output circuit of the sensor can be of type NPN
, PNP or contact

INDUCTIVE SENSOR
• The oscillator creates a radio frequency that is emitted
from the coil away from the face of the sensor.
• If a metal plate enters this radiated field, eddy currents
circulate within the metal
• The oscillator requires energy to maintain the eddy
currents in the metal plate.
• As the plate approaches the sensor, the eddy
currents increase and cause a greater load on the
oscillator.
• The oscillator stops when the load becomes too
great.
• The trigger circuit senses when the oscillator
stops, then changes the state of the switching device

ULTRASONIC SENSOR
Operates by sending ultrasonic sound waves (above
18,000 hertz) toward the target and measuring the
time it takes for the pulses to bounce back
The time taken for this echo to return to the sensor is
directly proportional to the distance or height of the
object because sound has a constant velocity.
Very effective for applications such as fluid levels in
tanks and crude distance measurement as well as
object orientation and sorting of object with different
height.
Solids, fluids, granular objects and textiles can be
detected by an ultrasonic sensor.

SENSOR WIRING
Introduction
When a sensor detects a logical change it must signal that
change to a controller such as PLC in a control system by
switching current or voltage ON or OFF.
The output circuit of the sensor will determine the method of
switching.

Type of Sensor output circuit
Switches
• The simplest examples of sensor
outputs are switches and relays.
• The sensor is powered
separately, by connecting power
to V+ and V- of the sensor.
• When the sensor is active ,the
internal switch (probably a relay)
will be closed allowing current to
flow and the positive voltage will
be applied to input 06.

Current sinking output
• This type of output
allow current to flow
into the sensor to
the voltage common.
• NPN transistor is used
for the sinking output
to switch current flow
into the sensor.
(sinking)
• When sensor is active, it turns on the transistor and allow
current to flow into the sensor
• When sensor is inactive, it turns off the transistor and thus
disallow current to flow into sensor.

Connect sinking output to PLC
• The positive terminal
of the supply is
connected to the
sensor positive
terminal and the
common terminal of
PLC.
• The negative terminal of the supply is connected to the sensor
negative terminal only.
• The output terminal of the sensor is connected to the input
terminal of PLC ( 00)

Connect sinking output to PLC
• The dashed line in
the figure
represents current
flow path when the
sensor is active
• This current will flow through optocoupler and use light to turn
on a phototransistor to tell the computer in the PLC that the
input current is flowing

Current sourcing output
• This type of output
allow current to flow
out from the sensor to
the voltage common
through the load .
• PNP transistor is used
for the sourcing output
to switch current flow
out from the sensor to
the load
(sourcing)
• When sensor is active, it turns on the transistor and allow current
to flow out from V+ through the transistor and to the load.

Connect sourcing output to PLC
• The negative
terminal of the
supply is connected
to the sensor
negative terminal
and the common
terminal of PLC.
• The positive terminal of the supply is connected to the sensor
positive terminal only.
• The output terminal of the sensor is connected to the input
terminal of PLC ( 00)

• To describe function of solenoid, valves, and cylinder used in
Industry.
• To explain the operation of solenoid, valves and cylinder
• To describe function of motor.
• To identify type of motor
• To describe the operation of dc motor, induction motor
, synchronous motor.
• To describe the operation of stepper motor and its various drive
method.

LOGICAL ACTUATOR
DEFINITION
Actuators converts electrical ,pneumatic
or hydraulic energy into some form of
mechanical motion.

LOGICAL ACTUATOR
SOLENOID
• The principle of operation is a moving ferrous core (a piston) that will
move inside wire coil.
• Normally the piston is held outside the coil by a spring.
• When a voltage is applied to the coil, a magnetic field is built up that
attracts the piston and pulls it into the center of the coil.
• The piston can be used to supply a linear force.
• Applications of these include pneumatic valves and car door openers.

LOGICAL ACTUATOR
VALVES
1. Valve Body
2. Inlet Port
3. Outlet Port
4. Coil / Solenoid
5. Coil Windings
6. Lead Wires
7. Plunger
8. Spring
9. Orifice

LOGICAL ACTUATOR
VALVES
• The flow of fluids and air
can be controlled with
solenoid controlled valves
• The solenoid is mounted on
the side, When actuated it
will drive the central spool
left.
• The top of the valve body has
two ports that will be
connected to a device such as
a hydraulic or pneumatic
cylinder
• The bottom of the valve body has a single
pressure line in the centre with two exhausts to
the side

LOGICAL ACTUATOR
VALVES
In the top drawing the power
flows in through the centre to
the right hand cylinder port.
The left hand cylinder port is
allowed to exit through an
exhaust port.
In the bottom drawing the
solenoid is in a new position
and the pressure is now
applied to the left hand port
on the top, and the right
hand port can exhaust.

LOGICAL ACTUATOR
VALVES
A 3 ports 2 positions Normally close
valve controlling a single acting spring
return cylinder

LOGICAL ACTUATOR
CYLINDERS
• Cylinder uses pressurized fluid or
air to create a linear force/motion
• Fluid is pumped into one side of
the cylinder under
pressure, causing that side of the
cylinder to expand , and
advancing the piston.
• The fluid on the other side of the
piston must be allowed to escape

LOGICAL ACTUATOR
CYLINDERS
The force the cylinder can exert is
proportional to the cross sectional
area of the cylinder.
For Force:
P=pressure on the fluid
F=force acting on the piston
A=area of the piston
A
F
P
PAF

LOGICAL ACTUATOR
CYLINDER
Single acting cylinders apply force when
extending and typically use a spring to
retract the cylinder.
Double acting cylinders apply force in
both direction.

ELECTRIC MOTOR
Introduction
• Motor is a continuous actuator allow a system to position or adjust outputs
over a wide range of values.
• Electric motor is composed of a rotating centre core, called the rotor and a
stationary outside, called the stator.
• These motors use the attraction and repulsion of magnetic fields to induce
forces, and hence motion.
• Typical electric motors use at least one electromagnetic coil, and sometimes
permanent magnets to set up opposing fields.
• When a voltage is applied to these coils the result is a torque and rotation of an
output shaft.

ELECTRIC MOTOR
Type of electric motors
AC motors - rotate with relatively constant speeds proportional to
the frequency of the supply power.
induction motors - squirrel cage, wound rotor -
inexpensive, efficient.
synchronous motor- fixed speed, efficient
DC motors - have large torque and speed ranges
permanent magnet - variable speed
wound rotor and stator - series, shunt and
compound (universal)
Hybrid
brushless permanent magnet -
stepper motors

ELECTRIC MOTOR
Basic brushed DC motor
• The structure of a dc motor contains a current
carrying armature(rotor) which is connected
to the supply through commutator segments
and brushes and placed within the north
south poles of a permanent or an electro-
magnet(stator )
• The magnetic field produced by current flows
through the armature interacts with the field
of the stator and produces a force acting on
the armature in a direction that can be
determined by Fleming’s left hand rule.
• This force rotates the rotor in clockwise or
anticlockwise direction according to the apply
voltage polarity

ELECTRIC MOTOR
Basic brushed DC motor
• The speed of rotation will vary with the
applied voltage amplitude. A higher voltage
will make the rotation faster.
.
• The motor can be started or stopped through
using contactor or relay. The speed of
rotation can be control by using Pulse Width
Modulation(PWM) technique.

ELECTRIC MOTOR
Speed control using PWM
• (PWM) signal produces an effective
voltage that is relative to the time that
the signal is on.
• The percentage of time that the signal is
on is called the duty cycle
• When the voltage is on all the time the
effective voltage delivered is the
maximum voltage.
• if the voltage is only on half the
time, the effective voltage is half the
maximum voltage.
• The frequency of these waves is
normally above 20KHz

AC MOTOR
Induction Motor
• in a three phase induction motor, when
three phase supply is given to three
phase stator winding, a rotating
magnetic field is produced.
• The rotor of an induction motor is either
wound type or squirrel cadge type.
• Whatever may be the type of rotor, the
conductors on it are shorted at end to
form closed loop.
• Due to rotating magnetic field, the flux
passes through the air gap between
rotor and stator, cuts the rotor
conductor. Hence induced current in the
closed rotor conductors.

AC MOTOR
Induction Motor
• As per Lenz law the rotor will try to
reduce the every cause of producing
current in it.
• Hence the rotor rotates and tries to
achieve the speed of rotating magnetic
field to reduce the relative speed
between rotor and rotating magnetic
field.
• The speed of rotation of the rotor is
always slower than the rotating
magnetic field. The difference is called
the SLIP.

Sine wave current in each
of the coils produces
sine varying magnetic
field on the rotation
axis.
Vector sum of the magnetic
field vectors of the stator
coils produces a single
rotating vector of resulting
rotating magnetic field.
Rotating magnetic field

• The operation of a synchronous motor is due to the interaction of the
magnetic fields of the stator and the rotor.
• The stator winding, when excited by a poly-phase (usually 3-phase)
supply, creates a rotating magnetic field inside the motor.
• The rotor locks in with the rotating magnetic field and rotates along with it.
Once the rotor locks in with the rotating magnetic field, the motor is said to
be in synchronization.
• The speed of rotation is same as the speed of the rotating magnetic filed
N
S
S
N
Synchronous Motor

HYBRID MOTOR
STEPPER MOTOR
• The stepper motors consists of a stator an a rotor
• The rotor carries a set of permanent magnets, and
the stator has the coils.
• Basic design of a stepper motor have 4 coils with 90o
angle between each other fixed on the stator
• The motor has 90o rotation step. The coils are
activated in a cyclic order, one by one
• The rotation direction of the shaft is determined by
the order that the coils are activated.
• Have the advantage in position control, no feedback
is needed because the motor rotates with a fixed
degree of step. Some step angle can be as small as
1.8o.

TYPE OF STEPPER MOTOR DRIVE
• Only one coil is energized each time
• This method is only when power saving
is necessary.
• It provides less than half of the nominal
torque of the motor, therefore the
motor load cannot be high.
• This motor will have 4 steps per full
cycle.
Wave drive or Single-Coil Excitation

• Most often used method
• The coils are energized in pairs.
• Depend on the connection of the coils
(series or parallel) the motor will
require double the voltage or double
the current to operate than operation
in Single-Coil Excitation.
• it produces 100% the nominal torque
of the motor.
• This motor also have 4 steps per full
cycle.
Full step drive

• A way to achieve double the accuracy of a
positioning system, without changing anything
from the hardware
• The coils are energized in the sequence of first a
single coil, then adjacent coil and the first coil
together.
• This will cause the rotor to rotate to position in
between the energized coils.
• Next the first coil is de-energized, leaving only
the second coil remains in energized state.
• The rotate will advance another half step to align
with the second coil.
• The sequence is repeated to cause the rotor to
rotate a cycle with 8 steps instead of 4 steps.
Half stepping ( single coil excitation )

• Another way to achieve double the
accuracy of a positioning system, without
changing anything from the hardware
• Instead of one two, one two, the coils
are energized in sequence of 2,4,2,4.
• It consumes more power than the single
coil excitation method.
• This motor also have 8 steps per full
cycle.
Half stepping ( double coils excitation )

Using a sorting device, parts are to be transferred from
conveyor belt. By pressing the pushbutton switch, the piston
rod of a single-acting cylinder pushes the part off the
conveyor belt. When the pushbutton is released, the piston
rod returns to the retracted end position
Application

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