eee3420 lecture08 rev2011
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1Week © Vocational Training Council, Hong Kong.
│ Lecture 8 │
Sensors, Actuators and Applications of PLC
EEC3420 Industrial ControlDepartment of Electrical Engineering
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EEE3420 Industrial Control
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Learning Objectives
To know various types of sensors, actuators and typical applications of PLC in industrial control.
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EEE3420 Industrial Control
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Sensors
Sensors allow a PLC to detect the state of a process. Logical sensors can only detect a state that is either true or false. Examples of physical phenomena that are typically detected are listed below.
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?
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SENSOR WIRING When a sensor detects a logical change it must signal that change
to the PLC.
This is typically done by switching a voltage or current on or off. In some cases the output of the sensor is used to switch a load directly, completely eliminating the PLC.
Typical outputs from sensors (and inputs to PLCs) are listed below in relative popularity.
Sinking/Sourcing - Switches current on or off.
Plain Switches - Switches voltage on or off.
Solid State Relays - These switch AC outputs.
TTL (Transistor Transistor Logic) - Uses 0V and 5V to indicate logic levels.
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Switches In the figure a NO contact switch is
connected to input 01.
A sensor with a relay output is also shown.
The sensor must be powered separately, therefore the V+ and V- terminals are connected to the power supply.
The output of the sensor will become active when a phenomenon has been detected.
24 VdcPowerSupply
normally open push-button
PLC Input Card24V DC
00
01
02
03
04
05
06
07
COM
+
-
sensorV+
V-
relayoutput
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Transistor Transistor Logic (TTL) Transistor-Transistor Logic (TTL) is
based on two voltage levels, 0V for false and 5V for true.
The voltages can actually be slightly larger than 0V, or lower than 5V and still be detected correctly.
This method is very susceptible to electrical noise on the factory floor, and should only be used when necessary.
TTL outputs are common on electronic devices and computers, and will be necessary sometimes. When connecting to other devices simple circuits can be used to improve the signal
Vi VoVi
Vo
A Schmitt trigger will receive an input voltage between 0-5V and convert it to 0V or 5V. If the voltage is in an ambiguous range, about 1.5-3.5V it will be ignored.
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Sinking/Sourcing
Sinking sensors allow current to flow into the sensor to the voltage common, while sourcing sensors allow current to flow out of the sensor from a positive source.
For both of these methods the emphasis is on current flow, not voltage.
By using current flow, instead of voltage, many of the electrical noise problems are reduced.
Sensor
V+
V-
Active
physical phenomenon
V+
NPN
V-
sensor output
and Detector
Line
current flows in when switched on
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Sinking/Sourcing
Sinking sensors allow current to flow into the sensor to the voltage common, while sourcing sensors allow current to flow out of the sensor from a positive source.
For both of these methods the emphasis is on current flow, not voltage.
By using current flow, instead of voltage, many of the electrical noise problems are reduced.
Sensor
V+
V-
Active
physical phenomenon
V+
NPN
V-
sensor output
and Detector
Line
current flows in when switched on
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Sinking/Sourcing
Sourcing sensors are the complement to sinking sensors.
The sourcing sensors use a PNP transistor, as shown above.
PNP transistors are always drawn with the arrow pointing to the center.) When the sensor is inactive the active line stays at the V+ value, and the transistor stays switched off. When the sensor becomes active the active line will be made 0V, and the transistor will allow current to flow out of the sensor.
Sensor
V+
V-
Active
physical phenomenon
V+
PNP
V-
sensor output
and Detector
Line current flows out when switched on
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Sinking/Sourcing
Most NPN/PNP sensors are capable of handling currents up to a few amps, and they can be used to switch loads directly.
power supply
sensor V+
V- (common)
V+
V-
NPN
power supply
sensor V+
V- (common)
V+
V-
PNP
sinking
sourcing
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Sinking/Sourcing
The dashed line in the figure represents the circuit, or current flow path when the sensor is active.
PLC Input Card for Sinking Sensors
Inte
rnal
Car
d E
lect
roni
cs
PLC Data Bus External Electrical
NPN sensor
power supply
+V
-V +V
NPN -V
current flow +V
00
01
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Sinking/Sourcing
The current flow loop for an active sensor is shown with a dashed line.
PLC Input Card for Sourcing Sensors
Inte
rnal
Car
d E
lect
roni
cs PNP
sensor
power supply
+V
-V
+V
PNP
-V
current flow 00
01
com
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Two Wire Sensors
A two wire sensor can be used as either a sourcing or sinking input.
In both of these arrangements the sensor will require a small amount of current to power the sensor, but when active it will allow more current to flow.
This requires input cards that will allow a small amount of current to flow (called the leakage current), but also be able to detect when the current has exceeded a given value.
PLC Input Card two wiresensor
powersupply
+V
-V
+V
-V
00
01
comNote: These sensors require a certain leakage
current to power the electronics.
PLC Input Card
two wiresensor
powersupply
+V
-V
+V
-V
00
01
V+
for Sourcing Sensors
for Sinking Sensors
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Contact Switches
Contact switches are available as normally open and normally closed.
Their housings are reinforced so that they can take repeated mechanical forces.
These often have rollers and wear pads for the point of contact. Lightweight contact switches can be purchased for less than a dollar, but heavy duty contact switches will have much higher costs.
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Reed Switches
Reed switches are very similar to relays, except a permanent magnet is used instead of a wire coil.
When the magnet is far away the switch is open, but when the magnet is brought near the switch is closed as shown
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Optical (Photoelectric) Sensors
Optical sensors require both a light source (emitter) and detector. Emitters will produce light beams in the visible and invisible spectrums using LEDs and laser diodes.
oscillator
+V +V
lens lens
square wave
light
LED
phototransistor
amplifierdemodulatordetector andswitching circuits
smaller signal
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Capacitive Sensors
• Capacitive sensors are able to detect most materials at distances up to a few centimeters.
• The dielectric constant of the space around them will vary as different materials are brought near the sensor.
electric field
object electrode
electrode
oscillator
detector
load switching
+V
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Capacitive Sensors
• These sensors work well for insulators (such as plastics) that tend to have high dielectric coefficients, thus increasing the capacitance.
• But, they also work well for metals because the conductive materials in the target appear as larger electrodes, thus increasing the capacitance
electrode
electrode
electrode
electrode
metal dielectric
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Inductive Sensors
Inductive sensors use currents induced by magnetic fields to detect nearby metal objects. The inductive sensor uses a coil (an inductor) to generate a high frequency magnetic field
oscillator and level detector
output switching
inductive coil metal
+V
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Inductive Sensors
• These work by setting up a high frequency field. If a target moves near the field will induce eddy currents.
• These currents consume power because of resistance, so energy in the field is lost, and the signal amplitude decreases.
• The detector examines field magnitude to determine when it has decreased enough to switch
shieldedunshielded
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Ultrasonic An ultrasonic sensor emits a sound above the
normal hearing threshold of 16KHz.
The time that is required for the sound to travel to the target and reflect back is proportional to the distance to the target. The two common types of sensors are;
electrostatic - uses capacitive effects. It has longer ranges and wider bandwidth, but is more sensitive to factors such as humidity.
piezoelectric - based on charge displacement during strain in crystal lattices. These are rugged and inexpensive.
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Hall Effect • Hall effect switches are basically transistors that can be
switched by magnetic fields.
• Their applications are very similar to reed switches, but because they are solid state they tend to be more rugged and resist vibration.
• Automated machines often use these to do initial calibration and detect end stops.
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Fluid Flow
As the fluid flow rate increases the pressure forces the float upwards.
The tapered shape of the float ensures an equilibrium position proportional to flowrate. An inductive proximity sensor can be positioned so that it will detect when the float has reached a certain height, and the system has reached a given flow-rate.
fluid flow in
fluid flow out
metal inductive proximity sensor
As the fluid flow increases the float is forced higher. A proximity sensor can be used to detect when the float reaches a certain height.
float
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Actuators
Actuators Drive provides motions in mechanical systems.
Most often this is by converting electrical energy into some form of mechanical motion
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SOLENOIDS
• Solenoids are the most common actuator components. The basic principle of operation is there is a moving ferrous core (a piston) that will move inside wire coil as shown
• Normally the piston is held outside the coil by a spring. When a voltage is applied to the coil and current flows, the coil builds up a magnetic field that attracts the piston and pulls it into the center of the coil.
• The piston can be used to supply a linear force. Well known applications of these include pneumatic values and car door openers.
current off current on
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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 cylinder.
solenoid
solenoid
power inexhaust out
power in exhaust out
The solenoid has two positions and when actuated will change the direction that fluid flows to the device. The symbols shown here are commonly used to represent this type of valve.
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CYLINDERS• A cylinder uses pressurized
fluid or air to create a linear force/motion as shown
• A fluid is pumped into one side of the cylinder under pressure, causing that side of the cylinder to expand, and advancing the piston.
Fluid pumped
in at pressure Fluid flows out
F
Fluid pumped
in at pressure Fluid flows out
F
advancing
retracting
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HYDRAULICS • Hydraulics use incompressible fluids to supply very
large forces at slower speeds and limited ranges of motion.
• If the fluid flow rate is kept low enough, many of the effects predicted by Bernoulli’s equation can be avoided.
• The system uses hydraulic fluid (normally an oil) pressurized by a pump and passed through hoses and valves to drive cylinders.
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HYDRAULICS Hydraulic systems normally contain the
following components;
1. Hydraulic Fluid
2. An Oil Reservoir
3. A Pump to Move Oil, and Apply Pressure
4. Pressure Lines
5. Control Valves - to regulate fluid flow
6. Piston and Cylinder - to actuate external mechanisms
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PNEUMATICS• Pneumatic systems are very common, and have much
in common with hydraulic systems with a few key differences.
• The reservoir is eliminated as there is no need to collect and store the air between uses in the system.
• Also because air is a gas it is compressible and regulators are not needed to re-circulate flow.
• But, the compressibility also means that the systems are not as stiff or strong.
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PNEUMATICS Some basic characteristics of pneumatic systems are,
- stroke from a few millimeters to meters in length (longer strokes have more springiness
- the actuators will give a bit - they are springy
- pressures are typically up to 85psi above normal atmosphere
- the weight of cylinders can be quite low
- additional equipment is required for a pressurized air supply- linear and rotatory actuators are available.
- dampers can be used to cushion impact at ends of cylinder travel.
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MOTORS• Motors are common actuators, but for logical
control applications their properties are not that important.
• Typically logical control of motors consists of switching low current motors directly with a PLC, or for more powerful motors using a relay or motor starter.
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Other Types of Actuators Heaters
The are often controlled with a relay and turned on and off to maintain a temperature within a range.
Lights
Lights are used on almost all machines to indicate the machine state and provide feedback to the operator. most lights are low current and are connected directly to the PLC.
Sirens/Horns
Sirens or horns can be useful for unattended or dangerous machines to make conditions well known. These can often be connected directly to the PLC.
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Application of PLC in Industrial Control Motor Control
The following example involves a motor start and stop circuit.
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Application of PLC in Industrial Control Fluid Mixer
In the following example a tank will be filled with two chemicals, mixed, and then drained.
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Application of PLC in Industrial Control Counter Applications
A bottling machine shown below uses a counter to count bottles into groups of six for packaging
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Application of PLC in Industrial Control Position Control
Positioning is one example of an application that can use high-speed counters. .
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Application of PLC in Industrial Control Weight Differentiator – appication of analog input
As packages move along a conveyor they are weighed by a load cell whose output is transferred to a PLC.
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Application of PLC in Industrial Control Weight Differentiator – appication of analog input
A package that weighs at or greater than a specified value is routed along one conveyor path. A package that weighs less than a specified value is routed along another conveyor path.
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Summary Sourcing sensors allow current to flow out from the V+
supply.
Sinking sensors allow current to flow in to the V- supply.
Photo-optical sensors can use reflected beams (retroreflective), an emitter and detector (opposed mode) and reflected light (diffuse) to detect a part.
Capacitive sensors can detect metals and other materials.
Inductive sensors can detect metals.
Hall effect and reed switches can detect magnets.
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Summary Ultrasonic sensors use sound waves to detect parts up
to meters away.
Solenoids can be used to convert an electric current to a limited linear motion.
Hydraulics and pneumatics use cylinders to convert fluid and gas flows to limited linear motions.
Solenoid valves can be used to redirect fluid and gas flows.
Pneumatics provides smaller forces at higher speeds, but is not stiff. Hydraulics provides large forces and is rigid, but at lower speeds.
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Sensors, Actuators and Applications of PLC
End of Lecture 8
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