engg3490 plc l3

1

ENGG3490-PLCL3L3

PLCs Ref. [3];1. Developing

Fundamental PLC

ENGG3490: Mechatronics W07. Addapted From Programmable Logic Controllers By F. D. Petruzella, McGraw-Hill

1

Fundamental PLC Wiring Diagrams and Ladder Logic Programs

Electromagnetic Control Relay

The PLC's original purpose was the replacement of electromagnetic relays with a solid-state switching system that could be programmed.

Th bl t ll d i dThe programmable controller was designedto replace physically small control relays that make logic decisions but are notdesigned to handle heavy current or high voltage.

ENGG3490: Mechatronics W07. Addapted From Programmable Logic Controllers By F. D. Petruzella, McGraw-Hill 2

Electromagnetic relays, such as thelighting contactor shown, are still used as auxiliary devices to switch I/Ofield devices.

2

Electromagnetic Relay Operation

An electromagnetic relay is a magnetic switch. It useselectromagnetism to switch contacts.

A relay will usually have only one coil but may have any number of different


number of different contacts.

Electromagnetic Relay Operation

With no current flow through the coil (coil de-energized),( g ),the armature is held away from the core by spring tension.

When the coil is energized,the electromagnetic field


the electromagnetic field moves the armature causing the contact pointsof the relay to open orclose.

3

Relay Symbol

CR1-1 CR1 2CR1

Coil

Normally closed (NC) contact

CR1 1 CR1-2

Normally open (NO) contact


Contacts are open whenno current flows throughthe coil but close as soonas the coil is energized.

Contacts are closed whenno current flows throughthe coil but open as soonas the coil is energized.

Relay Circuit Operation

L1 L2S

CR1CR1

CR1-1OFF

R

With switch S open: coil CR1 is de-energized contacts CR1-1 are open light R is off contacts CR1-2 are closed light G is on


CR1-2 ON

G

4

Relay Circuit Operation

L1 L2

SCR1CR1

CR1-1

R

ON With switch S closed: coil CR1 is energized contacts CR1-1 are closed light R is on contacts CR1-2 are open


CR1-2 OFF

G

p light G is off

Magnetic Contactor

A contactor is a special type of relay designed tohandle heavy power loads that are beyond thecapability of control relayscapability of control relays.

Contactors are designed to operate such loads as lights, heaters, transformers, capacitors, and electric

f hi h l d


motors for which overload protection is provided separatelyor not required.

5

Magnetic Contactor


PLC Used in Conjunction with a Contactor

Pump

L2

Contactorpower contacts

Programmable controllershave I/O capable of operating

L2

L1

High-currentwiring

Coilt i l


the contactor but they do nothave the capacity to operateheavy loads directly.

PLC output module

L1L2

Low-currentwiring

terminals

6

Magnetic Motor Starter

A magnetic motor starter is a contactor with anoverload relay attached physically and electrically.They are electromagnetically operated switches thatprovide a safe method for starting large motor loads.

The overload relay will open the supply voltage to the starter if itdetects an overload on a motor.


detects an overload on a motor.Motor overload relay contacts arenormally hardwired in series with the magnetic starter coil.

Magnetic Motor Starter

Overload heaters areconnected in series with the contactor. If the motor becomes overloaded they cause a mechanical latch totrip. Tripping this latch opens a set of

h


contacts that are wired in series with the voltage supply and motor.

7

Across the Line AC Starter Operation

When the start button is pressed, coil M energizes to close all M contacts.

The OL contact opens automatically when an overload condition is sensed to de

Low-currentcontrol circuit


is sensed, to de-energize the M coil and stop the motor.

High-currentpower circuit

Across the Line AC Starter Operation

The M contacts, in seriesLow-currentcontrol circuit

The M contacts, in serieswith the motor, close tocomplete the high-current path to the motor.

Control contact M alsocloses to seal-in the coil


High-currentpower circuit

circuit when the STARTbutton is released.

8

PLC Control of A Large Motor Load

When a PLC needs to control a large motor, itmust work in conjunctionwith a starter.

Motor starters areavailable in various


standard NationalElectric Manufacturers (NEMA) sizes and ratings.

Manually Operated Switches

Manually operated switches are controlled by hand. Pushbutton switches are the most common form of manual control found in industry.

Normally Open (NO) pushbuttonmakes a circuit when it is pressed


and returns to its open positionwhen the button is released.

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N ll Cl d (NC) hb ttNormally Closed (NC) pushbutton opensthe circuit when it is pressed and returns to the closed position when the button is released. The abbreviations NO and NCrepresent the state of the switch when it isnot actuated.


not actuated.


The break-make pushbutton is used for interlocking controls. In this switch the topsection is NC, while the bottom section is NO When the button is pressed theNO. When the button is pressed, the bottom contacts are closed as the top contacts open.

R

ONOFF

NOT PRESSEDPRESSED


L1 L2

R

G

L1 L2

R

G

ONOFF

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Selector Switch

Selector switch positions are made by turning the operator knob – not pushing it.p g

Contacts

Position A BOffHand Auto

Selector switchpositions may havetwo or more selectorpositions with either maintained contactHand X


A

B

maintained contact position or springreturn to givemomentary contactoperation.

Hand X

Off

XAuto

Selector Switch Motor Reversing


Selector switch used in conjunction with a reversingmotor starter to select forward or reverse operationof the motor.

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Dual In-Line Package (DIP) Switches

ON Are small switch assemblies designed for mounting on printed

OFF circuit board modules.

Switch settings are seldom changed and the changes occur


changed, and the changes occurmainly during installation orconfiguration of the system.

Mechanically Operated Switches

A mechanically operated switch is controlled automaticallyby factors such as pressure, position, and temperature.

The limit switch is a type of mechanically operated switch designed to operate only when apredetermined limit is reached,


p ,and is usually actuated by contact with an object such as a cam.

12

Limit Switch Operation

SymbolsyNO Contact

NC Contact

Limit switches takethe place of a humanoperator.


They are often used in the control of machine processesto govern the starting, stopping, or reversal of motors.

Typical Limit Switch Circuit

L1 L2Stop Forward

FR

FWD Limit

F

F

R

R

R

Reverse

OLsLimit switches

REV Limit


Control circuit for starting and stopping a motor inforward and reverse with limit switches providingover travel protection.

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Temperature Switch

The temperature switch or thermostat is used to sense temperature changes and is actuated by some specific environmental temperature change.

Responds to changes in temperature by opening orclosing an electric circuit.

Symbols


NO Contact NC Contact

Temperature Switch Control of a Motor

Closing ONClosing ON

Rising


Motor

gtemperature

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Pressure Switch

Pressure switches are used to control the pressure ofliquids and gases and are activated when a specific pressure is reached.

Opens or closes an electric circuit in response to a change in pressure.

Switch

Bellows


Symbols


Starter Operated By a Pressure Switch

Closing ONRising


pressure

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Level Switch

Level or float switches are used to sense the height of a liquid.

Opens or closes an electric Switch

pcircuit in response to a change in liquid level.

Float

Symbols



Two-wire level switchcontrol of starter.

Transducer

A transducer is any device that converts energy fromone form to another.

Amplifier


Input transducer (microphone) convertssound energy to electric energy

Output transducer (speaker) convertselectric energy to sound energy

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Sensors

Sensors are input transducers used for detecting andoften measuring the magnitude of something. Theyconvert mechanical, magnetic, thermal, optical, andh i l i ti i t l t i lt d tchemical variations into electric voltages and currents.


Photoelectricsensor

Sensors

Sensors provide the equivalent of eyes, ears, nose,and tongue to the microprocessor brain.

MicroprocessorMicroprocessor

Opticalsensor

Gassensor


sensorMicrophone

Probe

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Proximity Sensors

Proximity sensors or switches detect the presence ofan object without making physical contact with it.


Proximity Sensors Applications

The object being detected is too small, lightweight, orsoft to operate a mechanical switch.

Rapid response and high switching rates are requiredRapid response and high switching rates are required.

An object has to be sensed through nonmetallic barrierssuch 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-freeinput signal.

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Inductive Proximity Sensor Operation

Barrel typeBarrel typeBlock diagram

As the targetmoves into the


moves into thesensing area, the sensorswitchesthe output ON

Proximity Sensor Connections

The method of connecting and exciting a proximity sensor varies with the type of sensor and its application.pp

TargetL1 L2

Load


Two-wire sensor connection

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Current-sourcing output (PNP)Sensor

Controloutput


Load is connectedbetween the sensor and ground

Load


Current-sinking output (NPN)Sensor

Load is connectedbetween the positive supply and sensor

Load


Controloutput

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Proximity Sensor Connection to Input Module

L1 L2A proximity sensor should be powered continuously

Proximitysensor

Inputmodule

The use of a bleeder resistor allows enough current for the sensor to operate

ENGG3490: Mechatronics W07. Addapted From Programmable Logic Controllers By F. D. Petruzella, McGraw-Hill 39Bleeder resistor

but not enough to turn on the input of the PLC

Capacitive Proximity Sensor

A capacitive proximity sensor can be actuated by bothconductive and nonconductive material such as wood,plastics, liquids, sugar flour and wheat.

Operation is similar to that of inductiveproximity sensor. Instead of a coil the


proximity sensor. Instead of a coil, theactive face of the sensor is formed by two metallic electrodes – rather like an"opened capacitor".

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Magnetic Switch (Reed Switch)

A magnetic switch (also called a reed switch) is composed of flat contact tabs that are hermetically sealed (air-tight).

NO The switch is

Magnet

N S


Common

NO

NC

The switch is actuated by a magnet.

Reed Switch ActivationReed switch

Proximity motion – movementof the switch or magnet willactivate the switch

Magnetactivate the switch

Rotary motion – switch is actuated twice for every complete revolution

S i i i


Shielding – the shield short circuits the magnetic field; switch is activated by removal of the shield

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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


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.popular type of light transducer. Light energy falling on this device will cause a change in the resistance of the cell.


20 Ohms Light resistance5,000 Ohms Dark resistance

Ohms

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Photoelectric Sensor Operation

Most industrial photoelectric sensors use a light-emitting diode (LED) for the light source and a phototransistor to sense the presence or absence of light.

Object to be

Light detectorLight from the LED falls on the input of the phototransistor and the amount of conduction through the transistor changes Analog


sensed

Light source

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 detectsemitting element and detects the light being reflected.

Retro-reflective typeOperatingrange

Operatingrange

Diffused-reflective type

Emitter/receiver


Reflector

g

Target

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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.

OperatingrangeEmitter Receiver


g

Target

Bar Code Systems

Bar code systems can be used to enter data much more quicklythan manual methods, and arethan manual methods, and arehighly accurate.

DiverterThe decoder receives the signal from the scanner and converts


ScannerDecoder

PLC

scanner and converts these data into the character data representation of the symbol's code.

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Ultrasonic Sensor

An ultrasonic sensor operates by sending sound waves towards the target and measuring the time it g gtakes for the pulses to bounce back.

The returning echo signal


is electronically convertedto a 4 mA to 20 mA output,which supplies flow rate toexternal control devices.

Strain/Weight Sensor

A strain gauge transducer converts a mechanical strain into an electric signalsignal.

ForceWire type The force applied to the gauge causes the gauge to bend. This bending action also distorts the physical size of the gauge,

The load cell providessensor input to the

ControllerHopper

ChuteON/OFF

Control


p y g gwhich in turn changes its resistance.

pcontroller, which displays the weight and controls the hopper chute.

Load cell

Control

26

Temperature Sensors

Temperature sensors convert heat into an electric signal. There are four basic types used: thermocouple, resistance temperature detector (RTD), thermistor,

d ICand IC sensor.

The thermocouple consists of a pairof dissimilar conductors fused together at one end to form the "hot" or measuring junction, with the free ends available for


connection to the "cold" reference junction. A temperature difference between the measuring and reference junction generates a small DC signal voltage.

Temperature Sensors


d ICand IC sensor.

The resistance temperature detector (RTD) varies in resistance value with changes in temperature.


RTD

27

Temperature Sensors


d ICand IC sensor.

The thermistor varies in resistance value with h i t t


changes in temperature

Temperature Sensors


d ICand IC sensor.

The Integrated Circuit (IC) temperaturesensor produces changes in voltage or


p g gcurrent with changes in temperature.

28

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.

Turbine Flow Meter

CoilThe turbine blades turn at


Flow Magnet

Turbine

e u b e b des ua rate proportional to the fluid velocity and are magnetized to induce voltage pulses coil.

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.

Electronic Magnetic Flow Meter

Can be used with electrically conducting fluids and offers no restriction to flow. A coil in the unit sets up a magnetic field If


unit sets up a magnetic field. If a conductive liquid flows through this magnetic field, a voltage is induced and sensed by two electrodes.

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Velocity/RPM Sensors

A tachometer is a small permanent magnet DC generator which when rotated produces a voltage that is directly proportional to the speeddirectly proportional to the speed at which it is driven.

Controller

M

Tachometers coupledto motors are commonly used in motor speed control


Tach

Motor

M motor speed controlapplications to providea 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 (i d ti ) i k(inductive) pickup sensor.

Pickup coil Pole piece A magnet is attached to the shaft. A small coil of wire held near the

t i l h ti


0 V

N SMagnetSensor

output

magnet receives a pulse each time the magnet passes. By measuring the frequency of the pulses, the shaft speed can be determined.

30

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

Solenoid Solenoidvalve

Controlrelay

Alarm

ENGG3490: Mechatronics W07. Addapted From Programmable Logic Controllers By F. D. Petruzella, McGraw-Hill 59Heater Motor starter Small motor

Actuator

An actuator is any device that converts an electrical signal into mechanical movement. The principle types of actuators are relays, solenoids, and motors.

Plunger

Solenoid Symbol The solenoid converts electric current into linear motion.


AIR

Coil

31

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 positionedin which a disc or plug is positionedto restrict or allow flow

Forward motion of piston

Directionalsolenoid

When SOL A is energized, the valve spool is shifted to redirect the fluid and move the cylinder forward


SOL A

valve

FWDCR

CR

SOL A

Stepper Motor

A stepper motor converts electrical pulses applied to it into discrete rotor movements called steps. They are used to provide precise positionare used to provide precise position control of movement.

M d lStepper-motor Step

Stepper motor control system


Module translator motor

Communicates with the PLC and responds with pulse trains

Enables controlof the stepper motor The motor will move

one step for each pulsereceived

32

Seal-In Circuits

A seal-in circuit is a method of maintaining current flow after a momentary switch has been pressed and released. p

StopStartMotor starter coil

M

Hardwired Seal-In Circuit


M

Seal-in contact

The seal-in auxiliary contact of thestarter is connected in parallel withthe Start button to keep the startercoil energized when the Start button is released.

Programmed Seal-In Circuits

Inputs Output

StopMotorstarter

Ladder logic program

L1 L2p

Start

Stop PBStart PB coil (M)

Motorstarter coil (M)

M

M


Both the Start and Stop buttons are examined for a closed condition because both buttons must be closed to cause the motor starter to operate.

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Latching Relay

Latching relays are used where it is necessary for contacts to stay open and/or closed, even though the coil is energized momentarily

L1 L2ON

OFF

L

Electromagnetic latching relay circuit

When the ON button is momentarily actuated, the l t h il i i d t t

is energized momentarily.


U

PL

Contact shown with relayin the unlatched position

latch coil is energized to set the relay to its latched position. The relay does nothave to be continuously energized to hold the contact closed.

Programmed Latching Relay Instruction

Command Name Symbol Description

OTL sets the bit to “1” when the rung becomes true, and

OTL Output Latch,

retains its state when the rung loses continuity or a power cycle occurs

OTU Output Unlatch

OUT resets the bit to “0” when the rung becomes true and retains it


Latch and unlatch coils have the sameaddress

XXX

XXX

34

Latching Relay ProgramON

OFF

Latch rung

ON

OFF

Unlatch rung

Output rung


When the ON button is momentarily actuated, the latch rungbecomes true and the latch status bit (10) is set to 1, and so theoutput is switched on. This status bit will remain on (1) when logic continuity of the latch rung is lost.

1

Latching Relay ProgramON

OFF

Latch rung

ON

OFF

Unlatch rung

Output rung


When the unlatch rung becomes true (OFF button actuated),the status bit (10) is reset back to 0 and so the output is switchedoff.

0

35

PLC Water Level Program

Pump running

Lowlevel

Highlevel

The program for this processis designed to control the level of water in a storage tank by turning a discharge pump on and off.

High sensor switch

discharge pump on and off.

Off Position: the pump will stop if it is running and will not start if it is stopped.

Modes of Operation


Low sensor switch

Pump motor

pp

Manual Mode: the pumpwill start if the water in thetank is at any level exceptlow.


Pump running

Lowlevel

Highlevel


High sensor switch


Automatic Mode:- if the level of water in the tank reaches a high point, the water pump will start so that water can be removed from

Modes of Operation


Low sensor switch

Pump motor

water can be removed from the tank, thus lowering the level- when the water level reaches a low point, the pump will stop

36


Pump running

Lowlevel

Highlevel


High sensor switch


Modes of Operation

Status Indicating Lights:- water pump running light

(green) - low water level status light


Low sensor switch

Pump motor

- low water level status light (red)

- high water level status light (yellow)



OFF /Man Positions ( water at a low level)

37



Manual Mode ( water between high and low levels)



Automatic Mode ( water at high level)

38

Sequential Control Process

Sequential controls are required for processes that demand certain operations to be performed in specific order.

Filling operation Capping operation


Bottle movement

In this filling and capping operation, the tasks are:(1) fill bottle(2) press on capThese tasks must be performed in the proper order.

Combination Control Process

Combination control processes require that operations be performed without regard to the order in which they are performed.

Labeling processLabel solenoid 1

Label solenoid 2


In this labeling operation, the tasks are:(1) place label 1 on bottle(2) place label 2 on bottleThe order in which the tasks are preformed does not really matter.

39

Automatic Control

Automatic control involves maintaining a desired set-point.

Automatic furnace temperature

Furnace

PLC

In this operation the set-point is selected by means of the touch screen interface. If thereis a deviation from that set point, an error isdetermined by comparing the output against th t i t d i thi t k

pcontrol


Touch screeninterface

the set point and using this error to make a correction. This requires some form offeedback from the output back to the controlfor the input.

Hardwired Sequential Process

The sequential task is as follows:1. Start button is pressed.2. Table motor is started.3. Package moves to the limit switch

and stops.

Auxiliary Features: An emergency stop button that will stop the table, for any reason, before the package reaches the limit switch

STOP

RUNPackage

Motor

ppositionA red pilot light to indicate the table has stoppedA green pilot light to indicate the table is running


Limit switch(LS1)

EmergencySTOP

START

40

Hardwired Sequential ProcessSummary of the control task:• Start button is actuated. • CR1-1 closes to seal in CR1 • CR1-2 opens, switching the

red stop pilot light off• CR1-3 closes, switching the

green run pilot light on

StartPB1

EmergencyStop PB2 CR1 g p g

• CR1-4 closes to energize the motor starter and motor

• The package moves to the limit switch to actuate it and de-energize coil CR1

• CR1-1 opens to open the seal-in contactCR1 2 l it hi th

LS1

CR1-1STOP

R

RUNCR1-2

1

2


• CR1-2 closes, switching the red pilot light on

• CR1-3 opens, switching the green pilot light off

• CR1-4 opens to de-energize the starter coil, stop the motor, and end the sequence

G

M

OL

CR1-3

CR1-4

3

4

Programmed Sequential Process


41

Arranging Instructions for Optimum Performance

There is more than one way to correctly implement the ladder logic. In some cases one arrangement may be more efficient in terms of the amount of memory

d d th ti i d t thused and the time required to scan the program.

Instruction MOSTlikely to be FALSE

Instruction LEASTlikely to be FALSE

Sequence series instructions from the most likely to be FALSE (far left) to least likely to be FALSE (far right)


Once a processor sees a FALSE input instruction in series, it executes the remaining instructions FALSE, even if they are TRUE

Arranging Instructions for Optimum Performance

If your rung contains parallel branches, place the path that is most often TRUE on the top. The processor will not look at the others unless the top path is FALSE.p p

Path most likely to be TRUE

LESS likely


LEAST likely

42

Writing a Program from a Narrative Description

Description :A simple drilling operation requires the drill press to turn on only if there is apart present and the operator has one hand on each of the start switches. Thisprecaution will ensure that the operator's hands are not in the way of the drill.

PB1 PB2Drillmotor

Switches


Part sensor

Writing a Program from a Narrative Description

Description :A simple drilling operation requires the drill press to turn on only if there is apart present and the operator has one hand on each of the start switches. Thisprecaution will ensure that the operator's hands are not in the way of the drill.


PB1 and PB2 and the partsensor must be activated to make the drill motor operate.

43

Example: Motorized Overhead Garage Door

A motorized overhead garage door is to be operated automatically to preset open and closed positions.pos t o s

Devices used: see next slides Solution: sequence of operations when the up button is pushed, the up motor contactor

energizes and the door travels upwards until the up limit switch is actuated;

When the down button is pushed, the down motor t t i d th d t l d til th


contactor energizes and the door travels down until the down limit switch is actuated;

when the stop button is pushed, the motor stops. The motor must be stopped before it can change direction

Description :A motorized overhead garage door is to be operated automatically to presetopen and closed positions.

Motorized Door Program


44

Motorized Door Program


Example: Continuous Filling Operation

A continuous filling operation requires boxes moving on a conveyor to be automatically positioned and filled. Solution:pos t o ed a d ed So ut o start the conveyor when the START button is

momentarily pressed; stop the conveyor when the STOP button is momentarily pressed; energize the RUN status light when the process is operating; energize the STANDBY status light when the process is stopped; stop the conveyor and energize the STANDBY light when the right edge of the box is first sensed by the photosensor; with the box in position and the conveyor stopped open


with the box in position and the conveyor stopped, open the solenoid valve and allow the box to be fill; filling should stop when the LEVEL sensor goes true; energize FULL light when the box is full; the FULL should remain energized until the box is moved clear of the photosensor.

45

Continuous Filling Operation Program

Description :A continuous filling operation requires boxes moving on a conveyor to beautomatically positioned and filled.

HooperPL RunPL

PL

PL

Run

Standby

FullSolenoid

Levelswitch


MotorPhotoswitch

STARTSTOP

Continuous Filling Operation Program


46

Assignments

Chapter 6 (PLC) book: Problems

See PLC-Problem-Set1 pdf file on the course See PLC Problem Set1 pdf file on the course web page.


engg3490 plc l3

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