Skip to main content

What is Sequential Function Chart (SFC?)

 As per IEC 61131-3, there are five PLC programming languages, three are graphical. The other two are textual languages.


The three graphical languages are ladder diagram (LD), Function Block Diagram (FBD), Sequential Function Chart (SFC). Two textual languages are Standard Text (ST) and Instruction List (IL).


Out of all the above, various languages have different strengths for different tasks.


In this article let us discuss Sequential Function Chart (SFC). It is a flow chart-like programming language. The execution of process operations is structured sequentially. It can encapsulate ladder diagram (LD), Function Block Diagram (FBD), Instruction List (IL), and Standard Text (ST) structures to control subroutines.


Sequential Function Chart

Sequential Function Chart (SFC) can be used to structure the internal organization of a program consisting of sub-programs and are also written in other languages of the standard. It can also be used in its pure form of specifying the logical control algorithms.


The SFC programming structure consists of three main elements that organize the control program.


A program in SFC consists of


Steps (Stages)

Transitions

Actions.

Stages


Each stage indicates the state of the process. A step can be active or inactive.


As shown in the figure below, a transition not only can have one input step and one output step but also can have more input steps and more output steps, connected by the directed links.


These sequential flow chart diagrams show the stages in sequential order and are widely used to present algorithms, workflow, or process flow.


Typically, a sequential flow chart displays the steps as charts of various kinds. Their order is established by connecting them with lines.


The stages involve associated actions.


Only some of the stages can be found active in each cycle of the process.


Each stage is identified with a number that must be unique and not it must necessarily keep an order consistent with the sequence of the process.


Transitions

In between two steps, a transition is a step represented by a horizontal line across a vertical line. It is containing the logic condition allowing the transfer from one step to another.

Transitions are associated with a condition, these can be a logical function of variables of input and internal system.

Actions


The actions associated with the stages are represented by a rectangle sidebox where the type of action to be carried out is indicated (only the active ones). A stage can have one or more actions associated with it.


The process is broken down into stages, which will be activated sequentially.


One or more actions are associated with each stage. These actions will only be active when the stage is active.



 

The activation of a transition condition indicates the activation of the next stage and the deactivation of the preceding stage.


Program operation

Sequential Flow Chart (SFC) steps are connected with each other same as the flow chart. SFC is conducive to Boolean operation AND or OR decisions. AND decisions are simply placed in series and OR decisions are placed in parallel.


There are two ways to indicate conditions


Symbolic and

Literal

Between stage and stage, there must be a transition. The Transition conditions are written to the right of the line that joins them. There may be a jump or step backward seen same as flow charts.


A condition is associated with every transition. If all the input steps of a transition are active, and the transition condition is satisfied then the transition is fired, which means deactivation of all its inputs steps and activation of all its output steps.


Merits of Sequential Function Charts (SFC)

It is a graphical language that provides a diagrammatical representation of control sequences in a program.


Similar to a flow chart.

Useful for sequential control operations.

Shows the main states of a system.

SFC shows all possible status changes.

Sequence Function Chart (SFC) is suggested by many programming and modeling platforms for logic controllers such as step 7 by SIEMENS and control builder by ABB.

Labels:

Comments

Post a Comment

Popular posts from this blog

Ferrules and Cross Ferruling

 Ferrules are identification labels provided for every wire terminations in an instrument, equipment, or electrical/instrumentation control panels. These tube-shaped sleeves can be inserted easily on each individual wire in a multi-core cable. In earlier days fixed digits/letters are used as ferrules, but now Instrumentation engineers/technicians prints out desired ferrules by using a ferrule printing machine. Typical Ferrule The numbers/ letters on the ferrules will be given as per the approved electrical hook up or loop diagrams. This helps technicians to easily identify a particular loop/wiring from a series of terminal blocks and to troubleshoot the desired terminal connection. Separate numbers on the ferrules distinguish the positive and negative polarities of wires, thus ensure the polarity protection of the instrument. Cross Ferruling  As a wire is connected on its both ends, it is quite useful to use a cross reference method for wire identification. Unlike normal ferru...
1 comment
Read more

What is a Torbar? – Averaging Pitot Tubes

 The Torbar is employed for flow measurement of liquids, gases, or steam in circular, square, or rectangular section ducts for large flow rates. The Torbar is an insertion type multi-port self-averaging primary sensor for flow measurement. Torbar TORBAR is a set of Pitot tubes mounted on a bar across the pipeline with no moving parts. An averaging Pitot tube is a technology, while TORBAR is a manufacturing brand name. There are several brands available in the market with VERABAR, ANNUBAR, etc. Averaging Pitot Tube Principle Purpose Averaging Pitot tube can be employed when the average velocity of the flow profile, rather than the velocity in a specific point in the cross-section is desired. Averaging Pitot Tubes Principle It measures the differential pressure between the static pressure tap and the tap of full pressure of a stream. Thus such magnitude of differential pressure is directly proportional to the square of the flow rate. Working The TORBAR is designed in such a way that ...
7 comments
Read more

Dissolved Oxygen Analyzer Working Principle

 What is dissolved oxygen ? Dissolved oxygen refers to oxygen dissolved in water. Its concentration is expressed as the amount of oxygen per unit volume and the unit is mg/L. Biologically, oxygen is an essential element for respiration of underwater life and also acts as a chemical oxidizer. The solubility of oxygen in water is affected by water temperature, salinity, barometric pressure, etc. and decreases as water temperature rises. Measurement of dissolved oxygen by the membrane electrode method The membrane electrode method measures a diffusion current or reduction current generated by the concentration of dissolved oxygen or partial pressure of oxygen to obtain the concentration of dissolved oxygen. This method is not affected by the pH value of water being measured, oxidation and reduction substances, color, turbidity, etc. and the measurement method offers good reproducibility. When a sensor is inserted into water, an air layer forms on the membrane (Teflon membrane). The ox...
1 comment
Read more