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

PLC Program for Mixing Tank

 Create a ladder diagram for controlling a batch mixing process. Implement a PLC program for mixing tank or Mixing Process using PLC Ladder Logic. PLC Program for Mixing Tank Fig : Mixing tank A tank is used to mix two liquids. The required control circuit operates as follows: A. When the START button is pressed, solenoids A and B energize. This permits the two liquids to begin filling the tank. B. When the tank is filled, the float switch trips. This de-energizes solenoids A and B and starts the motor used to mix the liquids together. C. The motor is permitted to run for 1 minute. After 1 minute has elapsed, the motor turns off and solenoid C energizes to drain the tank. D. When the tank is empty, the float switch de- energizes solenoid C. E. A STOP button can be used to stop the process at any point. F. If the motor becomes overloaded, the action of the entire circuit will stop. G. Once the circuit has been energized, it will continue to operate until it is manually stopped. Solution...
1 comment
Read more

What is Relay? How it Works? Types, Applications, Testing

 We use relays for a wide range of applications such as home automation, cars and bikes (automobiles), industrial applications, DIY Projects, test and measurement equipment, and many more. But what is Relay? How a Relay Works? What are the Applications of Relays? Let us explore more about relays in this guide. What is a Relay? A Relay is a simple electromechanical switch. While we use normal switches to close or open a circuit manually, a Relay is also a switch that connects or disconnects two circuits. But instead of a manual operation, a relay uses an electrical signal to control an electromagnet, which in turn connects or disconnects another circuit. Relays can be of different types like electromechanical, solid state. Electromechanical relays are frequently used. Let us see the internal parts of this relay before knowing about it working. Although many different types of relay were present, their working is same. Every electromechanical relay consists of an consists of an Elect...
1 comment
Read more

Chlorine dioxide Analyzer Principle

 Chlorine dioxide measurement Chlorine dioxide (ClO2) is an instable, non-storable, toxic gas with a characteristic scent. The molecule consists of one chlorine atom and two oxygen atoms – represented in the chemical formula ClO2. It is very reactive. To avoid the risk of spontaneous explosions of gaseous chlorine dioxide or concentrated solutions, it is generally handled in dilution with low concentrations. ClO2 is soluble in water, but tends to evaporate quickly. Typically it is prepared on site, for example from hydrochloric acid and sodium chlorite. The procedure provides solutions with approx. 2 g/l ClO2 that can be safely handled and stored for several days. Image Credits : krohne Sensor Parts : Reference electrode Applied chlorine dioxide specific potential Current needed to maintain the constant potential Counter electrode Measuring electrode The disinfection effect of ClO2 is due to the transfer of oxygen instead of chlorine, so that no chlorinated byproducts are formed. C...
6 comments
Read more