Eee3420 lecture08 rev2011

1Week © Vocational Training Council, Hong Kong.

│ Lecture 8 │

Sensors, Actuators and Applications of PLC

EEC3420 Industrial ControlDepartment of Electrical Engineering

2© Vocational Training Council, Hong Kong.

EEE3420 Industrial Control

Week

Learning Objectives

To know various types of sensors, actuators and typical applications of PLC in industrial control.



Week

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?



Week

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.



Week

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



Week

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.



Week

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



Week

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



Week

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



Week

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



Week

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



Week

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



Week

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.



Week

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



Week

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



Week

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



Week

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



Week

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



Week

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



Week

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.



Week

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.



Week

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



Week

Actuators

Actuators Drive provides motions in mechanical systems.

Most often this is by converting electrical energy into some form of mechanical motion



Week

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



Week

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.



Week

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



Week

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.



Week

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



Week

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.



Week

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.



Week

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.



Week

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.



Week

Application of PLC in Industrial Control Motor Control

The following example involves a motor start and stop circuit.



Week

Application of PLC in Industrial Control Fluid Mixer

In the following example a tank will be filled with two chemicals, mixed, and then drained.



Week

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



Week

Application of PLC in Industrial Control Position Control

Positioning is one example of an application that can use high-speed counters. .



Week

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.



Week

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.



Week

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.



Week

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.



Week

Sensors, Actuators and Applications of PLC

End of Lecture 8

Revision

Eee3420 lecture08 rev2011

Engineering

Transcript of Eee3420 lecture08 rev2011