Problems with Ladder Programming
From a software engineering perspective, programming PLC using
conventional ladder diagrams suffers from the following deficiencies:
ˆ The ladder symbols and facilities vary between different PLC
products.
ˆ Poor facilities for structured or hierarchical program decomposition.
ˆ Limited facilities for software re-use.
ˆ Poor facilities for addressing and manipulating data structures (i.e. no
support for grouping data into a structure).
ˆ Limited facilities for building complex sequences
ˆ Limited control over program execution
ˆ Facilities for arithmetic operations are cumbersome.
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Unclear Sequence: An Example
Step1
Equip_OK
Trans1_2
Step2
Speed_Stable
Trans2_3
Step1
Step2
Step3
/
/
/
Ready
Transition rungs
Step1
Step rungs
Trans1_2
Step2
Step2
Step1
Speed1
Step actions
Step2
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Speed2
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Programming Languages in IEC 1131-3
•
5 languages (3 graphical + 2 textual)
•
Structured Text (ST)
•
Instruction List (IL)
•
Ladder Diagram (LD)
•
Function Block Diagram (FBD)
•
Sequential Function Chart (SFC)
•
Today almost all PLC programming systems, to a certain
extent, support all these 5 languages.
•
1 or 2 more language: Flowchart ? Statechart ?
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Software Model of IEC 1131-3
A layered model - each layer hides many of the features of the layers beneath it.
Configuration
Resource
Resource
Task
Task
Task
Task
Program
Program
Program
Program
var
FB
FB
FB
var
FB
var
Global and directly represented variables
Access paths
Communication functions defined in IEC1131-5
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Configurations and Resources
Configurations
•
• The entire software that defines the behaviour of a PLC for a specific
application. For most PLC systems, it means the hardware platform that
contains one or more resources.
• A configuration is able to communicate with other IEC configurations
within different PLC systems via defined interfaces (e.g. Ethernet,
Fieldbus, proprietary backplane bus, etc).
Resources
•
• A resource provides support for all of the features needed for the
execution of programs (e.g. interface between programs and physical
I/O channels). An IEC program cannot function unless it is loaded into a
resource.
• A resource may be simulated within a personal computer instead of a
“PLC” to support program testing.
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Programs and Tasks
•
Programs
• A logical assembly of all programming elements and constructs
necessary for the intended processing required for the control of a
machine or a process.
• A program may consist of 1) a number of interconnected FBs; 2) a
ladder diagram or 3) a SFC with its underlying action and transition subprogram or ...
•
Tasks
• A task can be configured to control a set of programs and/or FBs either
to execute periodically or upon the occurrence of a specified trigger. FBs
can be assigned to a specific task. Some PLC programming systems
e.g. ISaGRAF, do NOT directly support configurable tasks.
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Functions, FB and POUs
•
Functions
• A software element which when execute, with a particular set of input
values, produce one result (e.g. SIN( ), COS( ) ).
•
Function Blocks
• Support hierarchical design - Use FB to contain program defined with
other languages. A FB may itself be built up with smaller FBs.
• When compared with functions, FB may have numerous outputs. A FB
is able to store values declared as internal variables.
•
Program Organisation Units (POUs)
• The standard describes programs, function blocks and functions as
POUs. POUs can be used repeatedly in different parts of an application.
• FB instances from a particular type have to be created before use.
• IEC 1131-3 specifically forbids the use of recursion within POUs.
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Variables and Access Paths
•
Local and global variables
• Variables can be declared in a configuration, resource, program,
FB or function.
• Variable may be declared as global which may be accessed to
all software elements within the resource.
•
Access Paths
• Provide facilities for transferring data and information between
different configurations. Each configuration may have a number
of designated named variables which can be accessed by other
remote configurations.
• Lower level communication facilities are beyond the scope of
IEC1131-3.
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Some Common Elements in the Languages
•
Language keywords
•
Comments: (* …… *)
•
Data types
•
Initial values
•
Constants
•
Derived data types: e.g. structured data types or array data types
•
Variables:
• Input variables
• Output variables
• Internal variables
• Retain values - values will be retained from, when the PLC loses power,
to when the power is returned
•
Functions and FBs
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Basic IEC Data Types
• Strings have a size. It represents the maximum number of characters that the
string can contain.
• Timers contain values less than one day and cannot be used to store dates.
• Users can define their own types based on the above basic types: user type.
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Structured Text (ST) (1)
•
Although there are some minor similarities to PASCAL, ST is a
distinct language that has been specifically developed for industrial
control applications.
•
Assignment:
• X:=Y;
•
Operators:
• -, NOT, +, -, *, /, MOD, <, >, <=, >=, =, <>, AND, &, XOR, OR
•
Conditional statements:
• IF...THEN...ELSIF...ENDIF;
• CASE … OF … ELSE … END_CASE;
• FOR … TO … BY … DO… END_FOR;
• WHILE … DO … END_WHILE;
• REPEAT … UNTIL … END_REPEAT
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Structured Text (ST) (2)
•
EXIT - can be used within iteration statements and allows iteration
loops to be ended prematurely.
•
RETURN - can be used only within function and FB bodies to return
from the code body prematurely.
•
Function call:
• result := myfunc(param1, param2);
•
Accessing FB in ST:
• MyFBinstance(inparam1, inparam2);
• result1:=MyFBinstance.outparam1;
• result2:=MyFBinstance.outparam2;
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Structure of a ST Program
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Instruction List (IL)
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Introduction to Petri nets
Carl Adam Petri (1960), Kommunikation mit
Automaten, University of Darmstadt
2
P1
T1
P2
Legend
P3
Net structure
2
2
P1
P1
Place
T1
Transition
P3
P2
T1
P2
a. Initial marking
Directed Arc
Token
P3
b. T1 enabled
2
2
P1
P1
T1
P3
P2
T1
P3
P2
c. T1 firing (t
0)
d. Fired
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Modelling Features
Sequence
Sychronization
Concurrency
Conflict
Shared resource
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Sequential Function Chart (SFC)
•
Date back to the early 1960s, Petri nets by C.A. Petri. Special case
of binary interpreted Petri nets.
•
A graphical PLC language based on the French national standard Grafcet.
•
Standard IEC848 published in 1988.
•
Most of the definitions in IEC848 have been directly imported into the
IEC1131-3 standard to form the SFC. Mainly enhanced to integrate
with other IEC1131-3 languages.
•
Important and useful to show the main states of a system. Can be
used in high level to organise other language elements and low level
to control devices directly.
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SFC Basic Chart Structure
S0
Initial Step
Direction of
control flow
Step
N
Action0
StartSwitch
S1
N
Action1
Temperature > 30
S2
N
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N
Transition
with
transition condition
Action2
S2.T > T#10s
S3
Action
ST, IL, LD, FBD
or SFC
Action qualifier
Action3
Amos Ng
SFC Step Executing and Time Variables
•
Every step is associated with two variables:
•
The step active flag (.X). Boolean variable that is only true while the
corresponding step is active. Used to test step state from other part
of the SFC using syntax <Stepname>.X
•
Elapse time (.T) variable of data type TIME. It records how long a
step was active the last time it was activated. Can be accessed in
the form of <Stepname>.T, e.g. in a transition condition:
•
.X and .T are not supported in GX IEC Developer
S1
N
Action1
S1.T > T#10s
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SFC Divergent Paths
S0
Branch
or Link
Divergent
Path (OR)
Left-to-right
or user-defined
Precedence
S1
S2
S3
S4
Convergent
Path
Jump
S0
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SFC Simultaneous Sequences
S0
Simultaneous
Sequence
Divergence
S1
Simultaneous
Sequence
Convergence
S2
S3
S5
S6
S4
Enabled only
when S4, S5 & S6
are active
S7
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SFC Action Qualifiers (1)
Non stored
action qualifier
S1.X
N
S1
Action1
Action1
T1
T1
S
S1
Set and Reset
action qualifiers
Action1
S1.X
T1
T1
Action1
SN.X
R
SN
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Action1
TN
TN
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SFC Action Qualifiers (2)
Pulse on
activation
S1.X
P1
S1
Action1
Action1
T1
T1
Pulse on
deactivation
S1.X
P0
S1
Action1
Action1
T1
T1
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Child SFC - Macro Steps
N
S1
SFC2
S0
T1
S1
Action qualifier
N, S or R
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S2
N
Action0
N
Action1
N
Action2
Amos Ng
Function Block Diagram (FBD)
•
Can be used to express the behaviour of functions, FBs and
programs as a set of interconnected graphical blocks. It can be used
within the SFC to express the behaviour of steps, actions and
transitions (NOT directly in GX IEC Developer).
•
Used when the problem involves the flow of signal between control
blocks or processing elements. E.g. describing control loop.
•
Input and output parameters can only use data types defined within
the standard.
•
Instance must be created before use.
•
A list of fairly rudimentary FBs are defined in the standard.
•
Can be implemented with other general-purpose languages provided
that the block has input and output parameters that are defined using
IEC1131-3 data types.
•
Fundamental building blocks for other standard IEC1131 Part 5, Part
7 and IEC 61499.
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FB Re-use (1)
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FB Re-use FB (2)
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Benefits of using IEC 1131-3
•
Above all, it is an international standard
•
Theoretically, inter-system portability (still a dream!)
•
Inter-language portability and Reuse
• e.g. FB can be programmed in any of the standard languages.
• e.g. FBs can be re-used as the basic building blocks of a control system if
a library of ‘off-the-shelf’ FBs provided by different vendors is available.
•
OMAC (Open, Modular Architectural Controllers) Specification
• Laid down the requirements for an open, modular architecture controller
known to the supplier and technology development communities, from the
perspective of the automotive industry.
• Under the section Discrete Event Control: “The IEC1131-3 standard
languages must be used to program discrete I/O logic”
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PLCopen (www.plcopen.org)
•
NOT a standards-governing or -setting up body.
•
A supplier and vendor organisation that helps and guides
users of PLCs, mainly concerning the IEC 1131 standard.
Based in the Netherlands and now has more than 100
members from Europe, USA and Japan.
•
Actively defining different compliance levels with the objective
that products that have the same compliance level will
support a known level of software portability.
•
Actively developing concepts for common implementation.
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