Eee3420 lecture02 rev2011

1Week© Vocational Training Council, Hong Kong.
│ Lecture 2 │
Programmable Logic Controller (PLC)
Hardware
EEC3420 Industrial Control
Department of Electrical Engineering

EEE3420 Industrial Control
Learning Objectives

Understand the basic hardware components.

Understand the operating principles of PLC.

Background of PLC

low cost, compact, versatile units based on the
standard microprocessor architecture used in the
control of machines or processes

designed for ease of programming and maintenance

replace the old relay logic control systems in
automated manufacturing

provide an easy and efficient replacements for the
bulky relay logic controllers

Definition of PLC

also known as programmable controller (PC) defined by
the National Electrical Manufacturers Association
(NEMA) in 1978 as:
"a digitally operating electronic apparatus which uses a
programmable memory for the internal storage of
instructions for implementing specific functions, such as
logic, sequencing, timing, counting and arithmetic, to
control through digital or analog input/output, various
types of machines or process".

Advantages of PLC

Cost effective

Flexible

Computational abilities allow more sophisticated
control

Trouble shooting aids make programming easier

Reliable

Typical applications of PLC

Embedded control in standard or custom-designed machines

Control of conveyor systems

Networked to manage large complex distributed control systems

Security alarm monitoring system

Elevator control

Air-conditioning control panel

Water treatment plant

Warning system on-board of ocean vessels.

Smoke candle control on-board aircraft for air-show.

Underground coal-mining machines.

Central building vacuum system.

Electromagnetic relays

a voltage applied to the input coil,
the resulting current creates a
magnetic field

magnetic field pulls a metal switch
towards it and the contacts touch,
closing the switch

contact that closes when the coil
is energized is called Normally
Open (NO) contact

contact that is closed when the
input coil is not energized is called
Normally Closed (NC ) contact

Relay circuit and ladder diagram

The first relay on the left is used
as NC, and will allow current to
flow until a voltage is applied to
the input A.

The second relay is NO and will
not allow current to flow until a
voltage is applied to the input B.

If current is flowing through the
second relay coil then current
will flow through the coil in the
third relay, and close the switch
for output C.

This circuit would normally be
drawn in the ladder diagram as
shown.

Simulated model of a PLC

there are two inputs from
push buttons

imagine the inputs as
activating 24V DC relay
coils in the PLC

in turn drives an output
relay that switches 220
VAC, that will turn on a
light

A self-hold circuit

input B will onlybe on when the output B is on

if B is off, and A is energized, then B will turn on

if B turns on then the input B will turn on, and keep output B on even if input A goes off

after B is turned on the output B will not turn off
Note: When A is pushed, the output B will turn on, and
the input B will also turn on and keep B on perma
nently - until power is removed.
A
B
B
Note: The line on the right is being left off intentionally
and is implied in these diagrams.

PLC standards - IEC 61131
Developed with the input of vendors, end-users and
academics, IEC 61131 consists of five parts:
• General information
• Equipment and test requirements
• PLC programming languages
• User guidelines
•
Communications

PLC Programming Languages Standard
IEC 61131-3 is the international standard for
programmable controller programming languages.
As such, it specifies the syntax, semantics and display for
the following suite of PLC programming languages:
• Ladder diagram (LD)
• Sequential Function Charts (SFC)
• Function Block Diagram (FBD)
• Structured Text (ST)
• Instruction List (IL)

Ladder diagram and instruction list

Sequential Function Charts (SFCs)
SFCs have been developed to accommodate the
programming of more advanced systems

PLC simulation using Personal Computer
With TRiLOGI’s simulator anyone could write ladder
programs and immediately test them using just the
keyboard and see the result of the action on screen.

PLC scan cycle
• the control loop is a continuous cycle of the PLC
reading inputs, solving the ladder logic, and then
changing the outputs
• like any computer this does not happen instantly
Read inputs
PLC program changes outputs
by examining inputs Set new outputs
Process changes and PLC pauses
while it checks its own operation
THE
CONTROL
LOOP
Power turned on

The PLC internal working cycle
• PLC accepts the inputs from the
Input Port and stores the data to
the Input Image Register
• PLC then executes the ladder
logic program with the input
image content in a sequential
manner, that is, from the left to
the right and from the top to the
bottom
• upon the execution of a rung in
the ladder, the contents in the
Component Image Register are
updated and at the end of a
cycle, the outputs are updated

PLC Hardware
Power Supply - This can be built into the PLC or be an
external unit. Common voltage levels required by the
PLC (with and without the power supply) are 24Vdc,
120Vac, 220Vac.
CPU (Central Processing Unit) - This is a computer where
ladder logic is stored and processed.
I/O (Input/Output) - A number of input/output terminals must
be provided so that the PLC can monitor the process
and initiate actions.

PLC Hardware
Indicator lights - These indicate the status of the PLC
including power on, program running, and a fault.
These are essential when diagnosing problems.
Software - A software based PLC requires a
computer with an interface card

INPUTS and OUTPUTS of PLC
• Inputs to, and outputs from, a PLC are necessary
to monitor and control a process. Both inputs and
outputs can be categorized into two basic types:
digital or continuous.
• Outputs to actuators allow a PLC to cause
something to happen in a process. A short list of
popular actuators is given below in order of relative
popularity

• Inputs come from sensors that translate physical
phenomena into electrical signals. Typical
examples of sensors are listed below in relative
order of popularity.
• Proximity Switches - use inductance, capacitance
or light to detect an object logically.
• Switches - mechanical mechanisms will open or
close electrical contacts for a logical signal.

• Potentiometer - measures angular positions
continuously, using resistance.
• LVDT (linear variable differential transformer) -
measures linear displacement continuously using
magnetic coupling.
• Inputs for a PLC come in a few basic varieties, the
simplest are AC and DC inputs.

• Outputs to actuators allow a PLC to cause
something to happen in a process. A short list of
popular actuators is given below in order of relative
popularity.
• Solenoid Valves - logical outputs that can switch a
hydraulic or pneumatic flow.
• Lights - logical outputs that can often be powered
directly from PLC output boards.

• Motor Starters - motors often draw a large amount
of current when started, so they require motor
starters, which are basically large relays.
• Servo Motors - a continuous output from the PLC
can command a variable speed or position.
• Outputs from PLCs are often relays, but they can
also be solid state electronics such as transistors
for DC outputs or Triacs for AC outputs.
• Continuous outputs require special output cards
with digital to analog converters.

Input connection
• some PLCs
require
external DC
supply
• some PLCs
have built-in
24 VDC
supply

INPUTS of PLC
• PLC inputs must
converts a variety
of logic levels to
the TTL +5V dc
logic levels used
on the data bus
• can be done with
circuits similar to
those shown.

PLC Input Trade-offs
• DC voltages are usually lower, and therefore safer (i.e., 12-24V)
• DC inputs are very fast, AC inputs require a longer on-time. For
example, a 60Hz wave may require up to 1/60s for reasonable
recognition
• DC voltages can be connected to larger variety of electrical
systems
• AC signals are more immune to noise than DC, so they are suited
to long distances, and noisy (magnetic) environments
• AC power is easier and less expensive to supply to equipment
• AC signals are very common in many existing automation devices

OUTPUTS of PLC
• PLC must convert the
+5V dc dc logic levels
on the PLC data bus to
external voltage levels
• can be done with
circuits similar to those
shown, where it may be
transistor output, or
solid state relay (triac)
output, or simply relay
output

PLC output with sinking current
• the output card
shown in Figure on
the right is an
example of a 24Vdc
output card that has
a shared common
• this type of output
card would typically
use transistors for
the outputs

PLC output with sourcing current
• the circuit shown in
Figure on the right
has the sequence of
power supply, then
device, then PLC
outputs, then power
supply
• this requires that the
outputs have a
common

PLC with relay output
• in Figure shown on
the right the 24VDC
supply is connected
directly to both relays
• when an output is
activated the output
switches on and
power is delivered to
the output devices

PLC Wiring
• Wiring Diagram for a Car-washing Machine

Double coil designation
• double or dual coiling is not a
recommended practice.
• using multiple output coils of
the same device can cause
the program operation to
become unreliable
• the coil operation designated
last is the effective coil

Input durations
• the ON or OFF duration of the PLC inputs must be longer than the
operation cycle time of the PLC
• taking a 10 ms (standard input filter) response delay into account, the
ON/OFF duration must be longer than 20 ms if the operation cycle
(scan time) is 10 ms
• in the example above, input pulses of more than 25 Hz (1/(20ms
ON+20ms OFF) = 1/40ms) cannot be sensed

Summary of PLC Hardware
• PLC input modules convert the AC or DC inputs to the suitable
format to be detected by the logic of the PLC.
• Outputs of PLC are transistors (DC), triacs (AC) or relays (AC and
DC).
• Input and output addresses are a function of the card location and
input bit number.
• The input module usually shares a common ground and so does
the output module of a PLC

PLC Hardware
End of Lecture 2
 Revision
Mitsubishi PLC FX series hardware handbook.
Allen Bradley PLC handbook

Eee3420 lecture02 rev2011

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