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The IEC 61131-3 Programming Languages

1) Instruction List

2) Structured text

3) Function Block Diagram

4) Ladder program

Ladder Diagram :

 Advantages – Easy to understand – It could be learn intuitively

• Disadvantages

 – The ladder symbols and facilities vary between different PLC products (still there areNON-61131 Ladder Editors)

 – Poor facilities for structures or hierarchical program decomposition

 – Limited facilities for software re-use

 – Poor facilities for addressing and manipulating data structures

 – Limited facilities for building complex sequences

 – Limited control over program execution

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  – Facilities for arithmetic operations are cumbersome

Instruction List :

Single Accumulator based execution model

 Advantages

 – IL in some circumstances may be used to write tight, optimised code for performance critical section ofa program.

• Disadvantages

 – It is not possible to check the structure of a section of an IL program with the same rigor than a sectionwritten in ST.

 – It is harder to follow the program flow in IL than in ST.

Structured Text :

High level language, block structured

• Syntax resembles PASCAL

• Complex statements and nested instructions possible

• Support for 

 – Iteration loops (REPEAT-UNTIL; WHILE-DO)

 – Conditional execution (IF-THEN-ELSE; CASE)

 – Functions (SQRT(), SIN())

Function Block Diagram :

Graphical language, widely used in Europe

 Allows program elements which appear as blocks to be "wired" together in a form analogous to a circuitdiagram

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Used in many applications that involve the flow of information or data between control components

SFC: Sequential Function Chart

  • Powerful graphical technique for describing the sequential behaviour of a control program

• Used to partition a control problem

• Shows overview, also suitable for rapid diagnostics

• The basic elements are STEPS with ACTION BLOCKS and TRANSITIONS

Conclusion :

Ease of maintenance by the final user: SFC

 – Universal language acceptance: LD

 – Acceptance in Europe: IL or ST

 – PLC speed of execution: IL or ST

 – Applications mainly using digital I/O and basic processing: LD or FBD

 – Ease of changing code: LD

 – Ease of use by newer engineers: ST

 – Ease of implementing complex mathematical operations: ST

 – Applications with repeating processes or processes requiring

 – interlocks and concurrent operations: SFC

Methodologies :

- Process Sequence BITS

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  -State machines  - Flowchart diagrams

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  • Program development process can be divided into four main phases – Design – Writing – Debugging and testing

 – Documenting

Methodology for programming :

why to use : 1) structured and specific way of solving a problem

2) reusable

Methodologies :

1) sequence bit

2) timing diagram

3) state diagram

4) flow chart

1) sequence bit : its useful for repetitive steps

2) timing diagram : its useful for processes that time dependent

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3) State Diagram :

its useful for processes with clear modes of execution (eg manual and automatic)

4) flowchart : Useful for implementing processes that have :

 – A single flow of execution with clear sequence of operation

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choosing plc :

  the final choice of supplier for our PLC will depend

upon functionality, support available, customer preferences, user knowledge... and price

• The objective of a Human-Machine Interface (HMI) is to allow an interaction between humans and

machines for effectively operate and control processes

Scada : SCADA is stand for Supervisory Control And Data Acquisition and this is the technology thatenables a user to collect data from one or more distant facilities and to send limited controlinstructions to those facilities.

SCADA technology is best applied for the to processes which are extended over large areas;are relatively simple to control and monitor; and require frequent, regular or immediateintervention

Element of Scada : 1)Operator console 2)Master Terminal Unit (MTU) 3)Remote Terminal Unit(RTU) 4)Modem 5)Communication media

RTU : The RTU gathers information from the field about analog values, alarm and status points and

metered amounts. RTU keeps this information available in memory until the MTU asks for theinformation. It then codes and transmits the information to the MTU. RTUs are now being offeredhave the ability to communicate with field devices in serial format, usually RS23

MTU : At the centre of each SCADA system is the device that

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 – issues all the commands

 – gathers all the data

 – stores some information

 – passes other information on to associated systems

 – interfaces with people who operate the process and actually seems to be in charge

this device is called Master Terminal Unit (MTU)

* The RTU is required to store certain critical pieces of data such as meter values.MTU supplies information to the central database.

Determining scan interval :

Factors that determine scan interval are:

 – Number of RTUs that have to be scanned.

 – Amount of data must be passed on each conversation.

 – Data rate (Number of bits per second)

 – Communications efficiency

*  Amount of data to be gathered varies from one status point to several hundred status andalarm points.

 Alarms and status points 1-2 bits

Data from meters about 16 bits. (analouge point, motor positions, valve controller SP,..)

* SCADA system can not be developed if a communication link between remote sites and central sitecan not be established.

In SCADA system all data transferred between the MTU and RTUs is binary.

- UHF radios have been developed specifically for SCADA. They offer flexibility, low cost and highreliability.

Duplex communication allows information to be transmitted and received at the same time. Duplex islike having two simplex systems running parallel to each other but in opposite directions.

It is possible to use one pair of wires or one radio frequency with the data going sometimes in onedirection and some times in other direction, This is called Half Duplex .

To reduce the scan interval should concentrate not only on data rate but also on the communicationsequipment.

One way to improve the communications equipment is to take advantage of Full Duplex.If the second frequency is available, the MTU can transmit on one and all the RTUs can transmit on theother.

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Standards :

 ANSI-ISA-5.1-1984

The purpose of this standard is to establish a uniform means of designating instruments andinstrumentation systems used for measurement and control.

• Benefits:

 – A decrease in operator errors

 – A shortening of operator training

 – Better communication of the intent of the control system designer to the system users

• The standard is suitable for use whenever any reference to an instrument or to a control systemfunction is required for the purposes of symbolization and identification.

• The standard is intended to provide sufficient information to enable anyone reviewing anydocument depicting process measurement and control (who has a reasonable amount of processknowledge) to understand the means of measurement and control of the process.

IEEE-37.1

• The purpose of this standard is to provide guidance to the engineer responsible for the designand specification of SCADA and automation systems.

• Defines the requirements for SCADA and automation systems.

HMI : Used as an operator control panel instead of using an excessive amount of hardware

Provide almost unlimited control and status of a fully automated machine cell.

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 Display data, alarms, process charts

HMI Design Principles : 

Principle 1 - Be Consistent: Output consistency often takes the form of using the same wordsand phrase structures to communicate the particular message.

Principle 2 - Controls and displays should function the way people expect them to function.

Principle 3 - Arrange controls and displays so they follow the flow of reading: left to right, and topto bottom (European language based cultures).

Principle 4 - Minimize what the user has to remember.

Principle 5 - Operations that occur most often or have thegreatest impact on safety should be the easiest to perform.

Principle 6 - Controls, displays, and information elements that are used together should be neareach other.

Principle 7 - Use metaphors and conceptual models to simplify operation. In this context oneshould code information/warning for example with simple and already known icons instead of longtext messages.

Principle 8 - Provide support for expert and novice users of interfaces.

Principle 9 - Keep the user in control.

  In the control system, two types of signal should NOT be designed to depend on SCADA : Thefirst are safety instrumented systems and the second are product measurement systems.

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-manufacturing of finished products that are distinct items capable of being easily counted, touched or

seen.

-a discrete product (in theory) can be broken down at the end of its lifecycle so its basic components canbe recycled.

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