Introduction to plc (s7)

SIEMENS SIMATIC S7

INTRODUCTION TO

PROGRAMMABLE
LOGIC
CONTROL

Revision 2

ASSESSMENT

» Practical Test 1 – 20%
» Practical Test 2 – 20%
» Assignment – 20%
» Final Exam – 30%
» Key Qualification – 10%

MODULE OBJECTIVES

Upon completion of this course, the participants will be able to:

» explain the basic ideas on PLC such as PLC components’
signaling, I/O addressing and program execution;

» apply PLC programming method such as LAD, FBD and STL using
Siemens STEP 7 software;

» define and explain Siemens STEP 7 PLC software such as RS,
timers, counters, load and transfer commands, comparisons and
arithmetic functions.

Handout section 1.0

Topic 1
Basic Principle of
Control Technology

PLC

PROGRAMMABLE LOGIC CONTROL (PLC):

“ A digital electronic device that uses a
programmable memory to store instructions
and to implement specific functions such as
logic, sequence, timing, counting and
arithmetic to control machines and process. “

Definition of Control

What is CONTROL?

“ CONTROL is the process in a system in which
one or several input variables influence other
variables “

DIN 19226

A Simple View of a Control System

INFORMATION C S SENSORS
O Y P
N S L
T T A
COMMANDS R E ACTUATORS N
O M T
L

Types of Control System

CONTROL
SYSTEM

OPEN-LOOP CLOSED-LOOP
CONTROL SYSTEM CONTROL SYSTEM

Open-loop Control System

In open-loop control systems, output variables are
influenced by the input variables.

L

N

Closed-loop Control System

It is characterized by continuous comparison of the
desired value (or set point) with the actual value of the
controlled variable.

L
Xi > Xs C
Xi - Required value
Xi
Xs - Actual value Xs

Xi < Xs
N

Handout section 1.1

PLC and Conventional Control System

The essential difference between programmable
control and traditional control technology may be
summed up as follows:

» The functions are no longer determined by the wiring, but
rather by the program

» Programming is simplified to enable symbols familiar to
the control engineer to be used (contacts or logic graphic
symbols)

Handout section 1.3

Hardwire and PLC Wiring Diagrams

L 24 VDC

S1 S1 S2

S2 K1
PLC

K1 K1

N 0V
Hardwire PLC

Comparison

Hardwired control systems Programmable control system

» The functions are determined » The functions are determined
by the physical wiring. by a program stored in the
memory.

» Changing the function means » The control functions can be
changing the wiring changed simply by changing
the program.

» Can be contact-making type » Consist of a control device, to
(relays, contactors) or which all the sensors and
electronic type (logic circuits) actuators are connected.

HISTORY OF PLC
» During the late 1960s, General Motors (USA) was interested in the
computer application to replace the hardwire systems.

» Bedford Associates (Modicon) and Allen Bradley responded to
General Motors.

» The name given was “Programmable Controllers” or PC.

» Programmable Logic Controller or PLC was a registered trademark
of the Allen Bradley.

» Later, PC was used for “Personal Computer” and to avoid
confusion PLC for “Programmable Controller” and PC for a
personal computer.

ADVANTAGES OF PLC COMPARED TO HARDWIRE

» Implementing changes and correcting errors

» Pilot run - trial / test run

» Visual observation - online monitoring

» Speed of operation

» Reliability

» Documentation

PLC Application Example

CONVEYOR LINE

WORKSTATION #1 WS #2 WS #3

FLOW OF MATERIAL

PLC Control System

» Input devices
◊ Sensors
M

◊ Switches etc.

WS #1 WS #2 WS #3
» Output devices
◊ Relays PLC
0032
◊ Lamps etc

» PLC

Handout section 2.0 ( Topic 2 )

Familiarization with STEP 7

Handout section 1.4

Basic Structure of a PLC

POWER PG/
SUPPLY PC

INPUT CENTRAL OUTPUT
MODULES PROCESSING MODULES
UNIT (CPU)

MEMORY
(EPROM/RAM)

PLC Inputs / Outputs (I/Os)

USER
PROGRAM
Input Output
Devices (LOGIC) Devices

PLC

Input Connections

» Input card

» Converter
Input ◊ field voltage to 5V
Devices acceptable by the CPU

Handout section 1.4.1

Input Interface / Module
From field wiring

Detection
Bridge

Signal
Conditioning

Logic Status
Threshold
Light
Decision

Opto-Isolation

Logic

To CPU / Memory

Output Connections

» Output card

» Converter
◊ 5V to field voltage
Output
to drive field devices Devices


Output Interface / Module
From CPU / Memory

Logic Status
Logic Light

Opto-Isolation

Switching
Circuitry

Protection
Circuitry

To field wiring

Input/output Connections

I O
N PLC U M

P T
P
U Logic U
WS #1 WS #2 WS #3

T T
S S 0 0 32 PLC

Input / Output Modules
» Digital input modules adapt digital signals e.g. from proximity
sensors

» Digital output modules convert the internal signal level of PLC into
digital process signals e.g. relays

» Analog input modules adapt analog process signals e.g. from
transducers

» Analog output modules convert internal digital values of the PLC to
analog process signals e.g. temperature controller


Central Processing Unit (CPU)

What is a CPU?

» The “brain” of a PLC

» Controlled by a program called the executive or operating
system (OS)

» The executive is a collection of supervisory programs
permanently stored in memory

CPU

Four basic types of CPU operations:

» Input and output operation

» Arithmetic and logic

» Reading or changing contents of memory locations

» Jump operations

CPU

INTERNAL MEMORY
PROGRAM SUBMODULE
ACCUMULATOR MEMORY (EPROM/
(RAM) EEPROM/
RAM)

TIMERS,
COUNTERS,
Memory SERIAL
INTERFACE
PROCESSOR

PII PIQ

CPU
» The CPU reads in input signal states, processes the control
program and controls the outputs.

» The CPU provides internal Memory, timers and counters.

» Restart procedure can be preset and errors can be diagnosed
using the CPU’s LEDs.

» The overall Reset on the CPU is used to delete the contents of the
RAM.

» A PG or a Memory submodule is used to transfer the control
program to the CPU.


Program Memory

Program memory

RAM (Random Access Memory) ROM (Read Only Memory)

• the memory contents can be • the memory contents can be
read and written (modified) read, but cannot be modified

• memory contents will be lost
when the supply voltage fails

Types of Program Memory

Program memory

Programmable Non-programmable
(Read-write memory)

Non-alterable
Alterable ROM / PROM

UV erasable Electrically erasable
EPROM / REPROM EEPROM / EAPROM

Semiconductor RAM Semiconductor
EEPROM / EAPROM

Memory Submodules
» EPROM SUBMODULE
An ultraviolet erasing device is used to delete the contents of the
submodule

» EEPROM SUBMODULE
EEPROM submodule can be programmed or erased using a
programmer

» RAM SUBMODULE
Can be used in addition to program storage; and used to test a
control program during system startup


Power Supply Module
» The power supply module supplies the operational voltage for the
PLC and provides backup for the RAM with a battery

» Backup battery

» The backup battery maintains the program and data when the PLC
is switch off

» The backup battery has a service life of approximately 2 years

Hardware Summary

PG External power supply

PS951 Input Output
CPU module module

Input Output
devices devices

Handout section 1.5

How Does a Programmable Controller Work?

24 VDC

Sensors

Program
Processor
Memory Input modules
Power
Supply
Output modules

Actuators / Annunciators

GND

Steps of Operation

» The sensors are connected to the INPUT MODULES

» The processor in the CPU MODULE executes the program and
scans the individual input for presence or absence of voltage

» Depending on the state of the inputs, the processor directs the
OUTPUT MODULES to switch voltages

» The ACTUATORS or ANNUNCIATORS are switched “ON” or
“OFF” according to the voltage states

Handout section 1.6

Signal States and Sensor Contacts
» There are only two different states:

SIGNAL STATE “0” = voltage not present = OFF
SIGNAL STATE “1” = voltage present = ON

» The sensor is a The sensor is Voltage at input Signal state

NO contact activated present 1
NO contact not activated not present 0
NC contact activated not present 0
NC contact not activated present 1

Handout section 1.7

Addressing of Inputs and Outputs

» The addressing of inputs and outputs are identified by an operand
identifiers and the parameter

» Operand identifiers:
I - Input
Q - Output

» Parameter: (consists of a byte and a bit address)
0.0 … 0.7 (where 0. is the byte; 0…7 are the bit addresses)
1.0 … 1.7

Types of Addressing
Absolute Symbolic

» example: » example:
» A I 0.0 » A “System_On”
» = Q 8.0 » = “System_On”
» A I0.4 » A “M_FORW”
» = Q20.5 » = “MOTOR_FOR”
» Call FC18 » Call “COUNT”

Symbol Address Data Type Comment
MOTOR_FOR Q20.5 BOOL Motor moves forward
COUNT FC18 FC18 Count bottles
SYSTEM_ON I0.0 BOOL Switch system ON
SYSTEM_ON Q8.0 BOOL Indicator: “System is ON”
M_FORW I0.4 BOOL Pushbutton: Motor forward

Max. 24 character Max. 80 character


Program Representation - LAD

LAD - Ladder Diagram
I 0.0 I 0.1 Q 4.0
( )

» The graphical representation of a control task using symbols to
DIN 19239

» Very similar to traditional circuit diagrams, but the current paths are
arranged horizontally instead of vertically


Program Representation - FBD

FBD - Function Block Diagram
I 0.0

I 0.1
& Q 4.0

» The graphical representation of a control task using symbols to
DIN 40700 and DIN 19239

» Inputs are arranged on the left side while outputs on the right


Program Representation - STL

STL - Statement List
A I 0.0
A I 0.1
= Q 4.0

» The control statement describes the task with mnemonic
abbreviations of function designation (DIN 19239)

» Each method of representation has special characteristics and
specific limits

» If certain rules are followed, translation into all three methods of
representation is possible


Operation And Operand

Operation;
Describes the function to be carried out (what is to be done)
e.g Binary operations, Digital operations and Organizational operations

Operand;
START FROM HERE


Operation And Operand

LAD FBD STL
OPERATION + OPERAND OPERAND + OPERATION OPERATION + OPERAND

I 0.0 M 80.0 I 0.0 A I 0.0
M 80.0 & A M 80.0

OPERATION + OPERAND

Q 4.0
( ) = Q 4.0 = Q 4.0

Handout section 1.9

Program Execution

PLC Scan Function:

» Read the status of all inputs and outputs

» Examine the application program instructions

» Execute the control program


Linear Program Scanning

» Statements are scanned linearly

» At the end of the program, scanning starts again from the
beginning

» This is also referred to as cyclical scanning

» Linear program scanning is used mainly for simple, small-scale
control schemes

OB1
Linear program scanning

» OB = Organization Block

» Every program must have OB1 OB1

A I 0.0
A I 0.1
» When the PLC is set to run, the = Q 4.0
PLC will look for OB1 only in the :
:
user memory and execute it :
BE

» Other blocks can be called from
OB1 with the “jump” command
Cyclic program execution


Structured Program Scanning
Operating FC1
system
A I 0.0
» Complex tasks are subdivided A I 0.1
= Q 4.0
into clearly differentiated sub- OB1 :

Cyclic program execution
tasks :
JU FC 1 :
BE
JU FC 4
» i.e. the program is divided into :
small, easy-to-follow program :
FC4
:
blocks, organized according to BE
A Q 4.0
different functions A I 0.2
= Q 5.0
:
:
:
BE
Structured program scanning

Linear programming Structured programming

OB1 FC 1
Network 1 Network 1
A I 0.6 A I 0.6
A I 0.7
A I 0.7 = Q 4.2
OB 1
= Q 4.2 Network 2
A I 0.7
Network 2 Network 1
A I 0.5
A I 0.7 JU FC 1 = Q 4.3
A I 0.5 BE
JU FC 4
= Q 4.3
Network 3 BE FC 4
A Q 4.2 Network 1
A I 0.2 A Q 4.2
= Q 5.5 A I 0.2
= Q 5.5
BE BE


Program Execution
Input Process Program in Process Output
24 VDC module input image the RAM output image module GND

1 0
I 0.0 A I 0.0
0 Q 4.0
A I 0.1
I 0.1
P = Q 4.0 P
I I 1
O I 0.5
I O I 0.7
Q Q 4.3

1
I 0.5 = Q 4.3
BE:
1
I 0.7

Input cycle Program execution Output cycle

PII - Process Input Image
Update PII
» A buffer of input signals

» Update just before program
execution starts Execute
Program
» Not updated during program
Logic
execution

» Logic executed based on status in PII
Update Output
» Prevent signal transition during
program cycle to affect the program

PIQ - Process Output Image

» Updated by the
program logic during
program execution
OB1 PIQ
» The contents of PIQ
are transferred to the
output module at the
end of OB1
Copy PIQ to Output Module


BLOCK TYPES
» ORGANISATION BLOCKS (OB) – Interface between the operating system
and the user program

» FUNCTIONS (FC) - Contains a partial functionality of the program

» DATA BLOCKS (DB) – Are data areas of the user program in which user
data are managed in a structured manner

» SYSTEM FUNCTION BLOCKS (SFB), SYSTEM FUNCTIONS (SFC) -
SFBs and SFCs are integrated in the S7 CPU and allow you access to some
important system functions
» FUNCTION BLOCKS (FB) - FBs are blocks with a “memory” which you can
program yourself

» INSTANCE DATA BLOCKS (DB) - Instance DBs are associated with the
block when an FB/SFB is called. They are created automatically during
compilation

Block Nesting Depth

FC 7
FC 4 A I ....
FC 1 ..
OB1 JU FC 7 ..
JU FC4 .. ..
JU FC 1 .. ... BE
.. ... BE
... BE
..
BE


The Operand Areas (for Siemens S5-95U PLC)

» I (Input)
Interface from the process to the programmable controller
» Q (Output)
Interface from programmable controller to the process
» M (Memory/Flag)
Memory for intermediate results of binary operations
» T (Timer)
Memory for implementing timers
» C (Counter)
Memory for implementing counters


The Addressing of Siemens S7
Operand Areas Addressing
Input (I) 0.0 to 0.7
1.0 to 1.7
2.0 to 2.7
3.0 to 3.7
Output (Q) 4.0 to 4.7
5.0 to 5.7
8.0 to 8.7
9.0 to 9.7
Counters (C) 0 to 63
Timers (T) 0 to 127

Handout section 3.0

Topic 3
Programming Basic
Functions

Handout section 3.1

The Stages of Project Planning

Description of the Problem

Assignment Lists

Rough Structure of the Control System

Program Structure

Detailed Structure of the Control System


Problem Description
» it consists of process schematic, a short description of the task
definition, and a list of the sensors and actuators

Assignment List
» the sensors and actuators are allocated to the parameters of the
programmable controller
» it contains a short functional description as well as the device
identifier


Rough Structure of the Control System
» it contains all sub-functions of the process with relevant sensors,
actuators and indicators

Program Structure
» it determines the order in which the LAD, FBD or STL diagram to
be drafted

Detailed Structure of the Control System
» using the assignment list and the program structure, the flow chart
contained in the rough structure is refined

Handout section 3.2

Programming AND Operation

LAD
I 0.0 I 0.1 Q 4.0
( )

FBD STL

I 0.0 A I 0.0
I 0.1
& Q 4.0
A I 0.1
= Q 4.0

Handout section 3.3

OR Operation

LAD
I 0.0 Q 4.0
( )
I 0.1

FBD STL

I 0.0 O I 0.0
>= 1 Q 4.0 O I 0. 1
I 0.1 = Q 4.0

Handout section 3.4

AND - before - OR Operation
LAD
I 0.0 I 0.1 Q 4.0
( )
I 0.0 I 0.2
I 0.2 I 0.3

I 0.1 I 0.3 FBD STL
A I 0.0
I 0.0
A I 0.1
& O
I 0.1
>= 1 Q 4.0 A I 0.2
I 0.2 A I 0.3
& = Q 4.0
I 0.3

Handout section 3.5

OR - before - AND Operation
LAD
I 0.0 I 0.1 Q 4.0
( )
I 0.0 I 0.2 I 0.2 I 0.3 STL
A(
I 0.1 I 0.3 FBD O I 0.0
O I 0.2
I 0.0 )
>= 1 A(
I 0.1 O I 0.1
& Q 4.0
O I 0.3
I 0.2
>= 1 )
I 0.3 = Q 4.0

Handout section 3.6

Programming of NC Contacts and NO Contacts

» Physical connection PLC programming The sensor is Signal state

NO contact NO contact activated 1
NO contact NO contact not activated 0
NO contact NC contact activated 0
NO contact NC contact not activated 1
NC contact NO contact activated 0
NC contact NO contact not activated 1
NC contact NC contact activated 1
NC contact NC contact not activated 0

Handout section 3.7

Latching Output

S3 K2 S1 K1

S4 S2

K2 K1

SET Priority / Dominant SET RESET Priority / Dominant RESET

Handout section 3.8

RS Memory Function

S3 K2 S2

R
S4

= S1 K1

K2 S Q ( )

SET Priority / Dominant SET

RS Memory Function

S1 K1 S3

S
S2

= S4 K2

K1 R Q ( )

RESET Priority / Dominant RESET

Try This !

Will the output Q 4.0 be
LAD activated when you
I 0.0 I 0.1 Q 4.0 activate:
( ) » I 0.0 and I 0.1 ?

I 0.2 I 0.3 Q 4.0
( ) » I 0.2 and I 0.3 ?

I 0.4 I 0.5 Q 4.0
( ) » I 0.4 and I 0.5 ?

The Answer

» I 0.0 and I 0.1 = NO!

» I 0.2 and I 0.3 = NO!

» I 0.4 and I 0.5 = YES …… but why ?

When I0.0 and I0.1 Are Activated...

LAD
I 0.0 I 0.1 Q 4.0
» the PLC registers in the PIQ
( ) that Q 4.0 is “1”
I 0.2 I 0.3 Q 4.0
( ) that Q 4.0 is “0”

I 0.4 I 0.5 Q 4.0 » the PLC registers in the PIQ
( ) that Q 4.0 is “0”

so, Q 4.0 = “0”


LAD
I 0.0 I 0.1 Q 4.0
( ) that Q 4.0 is “0”
I 0.2 I 0.3 Q 4.0
( ) that Q 4.0 is “1”

( ) that Q 4.0 is “0”

so, Q 4.0 = “0”


LAD
I 0.0 I 0.1 Q 4.0
( ) that Q 4.0 is “0”
I 0.2 I 0.3 Q 4.0
( ) that Q 4.0 is “0”

( ) that Q 4.0 is “1”

this time, Q 4.0 = “1”

The Problem of Repetitive Outputs
» Therefore, when the same output is used more than once in the
program, only the last state of the output will be valid due to the
PLC dynamically updating the PIQ (Process Output Image)

» MEMORY = Memory for intermediate results of binary
operations

» Memory can be treated as flags/variables

» Memory can be used to solve the problem of repetitive outputs

Using Memory…...

I 0.0 I 0.1 M 100.0
( )
I 0.2 I 0.3 M 100.1
( )
I 0.4 I 0.5 M 100.2
( )
M 100.0 Q 4.0
( )
M 100.1

M 100.2

Result of Logic Operation (RLO)

Q 4.0 RLO STAT
A Q 4.0 …… ……
& A( …… ……
I 0.0 Q 5.0
O I 0.1 …… ……
I 0.1 >=1 O I 0.2 …… ……
I 0.2 O I 0.3 …… ……
)
= Q 5.0 …… ……

Parenthesized Function

Mathematics Logic Operation
Multiplication Before Addition
AND before OR
4 X 8 + 3 X 2 = 38
RLO STAT
A I 0.0 1 1
A I 0.1 1 1
O 1
A I 0.2 0 0
A I 0.3 0 1
= Q 4.0 1 1

Parenthesized Function

Mathematics Logic Operation
Addition Before Multiplication
OR before AND
4 X (8 + 3 ) X 2 = 88
RLO STAT
A I 0.0 1 1
A( 1
O I 0.1 1 1
O I 0.2 1 0
) 1
A I 0.3 1 1
= Q 4.1 1 1

Handout section 4.0

Topic 4
Numerical Systems and
Data Formats

Handout section 4.1

Comparison of Number Systems

Handout section 4.2

Bit, Byte and Word Addresses

Handout section 4.3

Force Variable and Data Format

Force Variable

» Display the signal status from memory (PII, PIQ and flag) of the
CPU

» Used to access the system data area of the CPU and modify the
data

Force Variable and Data Format

Data Format

» KM - bit pattern
» KH - hexadecimal
» KF - sign number ( - 32768 to +32767 )
» KT - time value
» KC - counter value
» KY - left hand and right hand byte (high / low byte)
» KS - alphanumeric character

Handout section 4.4

Load and Transfer Operations

Characteristics:
» They are used to perform operations on a whole byte or word in
memory
» They are unconditional operations i.e. They are performed by the
processor in each cycle

Functions:
» Exchange information between various operand areas
» Prepare times and counts for further processing
» Load constants for program processing

Load Operation
L IB 0
ACCUM 2 ACCUM 1 L IB 1

Byte d Byte c Byte b Byte a PII

IB 0
Byte b Byte a 0 IB 0

IB 1
0 IB 0 0 IB 1
Information from PII

Transfer Operation

T QB 0
ACCUM 2 ACCUM 1

Byte d Byte c Byte b Byte a PIQ

Byte a QB 0

Byte d Byte c Byte b Byte a

Information in the PIQ

Handout section 4.5

Arithmetic and Assignment of Accumulator

Handout section 4.6

Binary Coded Decimal (BCD)

Handout section 5.0

Topic 5
Timer Operations

Handout section 5.0

Fault Indication with Timer Function

Handout section 5.1

Inputs and Outputs of a Timer


Types of Timer - Pulse Timer (SP)


Extended Pulse Timer (SE)


On Delay Timer (SD)


Stored On Delay Timer (SS)


Off Delay Timer (SF)

Handout section 5.3

Specifying the Time Period

Time Value and Accuracy

Example:

KT 500.1 500 X 0.1S 49.9s …….. 50.0s

KT 050.2 50 X 1S 49s ………... 50s

KT 005.3 5 X 10S 40s ………... 50s

Handout section 5.4

Return Operations
» BE (Block End)
» the return operation is performed unconditionally
» it is always the last statement in the block

» BEU (Block End Unconditional)
» the return operation is performed unconditionally
» statements can follow BEU, but they will not be executed
» BEU is often used during commissioning so that individual parts of
the program can be tested

» BEC (Block End Conditional)
» the return is made dependent on a condition and is only performed
if the condition is satisfied

Block End Operations BEC, BEU and BE
FC1
: is always executed
OB1 :A I 0.6
:BEC
System : is executed only
:
:JU FC1 when I 0.6 = “0”
:BE
:A I 0.0
:JC FC 2 FC2
: :
:BEU is executed only
:
: when I 0.0 = “1”
:BE
:JU FC3 is not
:BE executed FC3
:
is not executed
:
:BE

Handout section 6.0

Topic 6
Counter Operations

Handout section 6.0

Counter

Counter Operations

CU - count up
CD - count down
S - set counter to the count value (CV)
CV - the count value
R - reset the counter (count value = 0)
BI - counter output as binary number
DE - counter output as BCD number
Q - counter status
Q = 0 when count value = 0
Q = 1 when count value > 1

Handout section 6.1

Load and Transfer for Counter

Handout section 6.2

Timing Diagram

Assign an Initial Value to a Counter (S)

Assign Value (CV)

» constant KC 0 to 999
» input word IW ….....
» output word QW …...
» flag word FW …....
» data word DW …...

Handout section 6.3

Counter Output

Handout section 6.4

Comparator

Types of comparison:

!=F compare for equal to
><F compare for not equal to
>F compare for greater than
>=F compare for greater than or equal to
<F compare for less than
<=F compare for less than or equal to

Comparison Operations

» The comparison operations compare two digital values in
accumulator 1 and accumulator 2

» The result of comparison produces an RLO:

» Comparison satisfied RLO = “1”

» Comparison not satisfied RLO = “0”

Handout section 6.4

Comparator

Introduction to plc (s7)­

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