Flowchart is a useful method employed by programmer that it simply is a tool that describes a sequence of events or processes. The two most important symbols used in the flowcharts are a block for a single process and a diamond for a decision. The number of Flowchart pages, as with block diagrams, is strictly a function of the complexity of the program. The flowcharting languages not only have improved organisation, modularity, and readability, over free-form programming, it reduced the problems when a small change in one section of the program is carried out. In addition, this leads to faster program debugging, easier modifications, and improved software reliability.
In most PCs that use sequential function charts, each process
or function block is written in ladder logic so that the system
designer is able to use a top-down approach to directly transform
a control problem into programming solution. First, the control
logic is subdivided into smaller and smaller blocks until each
block or process step is directly represented in the function
chart. Then each step is programmed using standard ladder logic.
This method has greatly reduces the effective program scan time.
The processor scans only the active steps and transition (decisions)
of the sequential function chart that correspond to the particular
stages of the process or machine operation that are currently
active. The other advantage of the sequential function chart programming
is that it is self-documenting. As the programmer builds the software
on a programming terminal, it is displayed on a CRT (cathode ray
terminal) for monitoring and debugging, and it can be easily printed
out for future reference.
Sequential Function Chart Program Symbols
Start block is at top of the program that defines the beginning of the program.
Initial block is the next block after start block. This is where the programmable controller starts function chart execution and returns to this step from the end of the program unless directed otherwise by the program logic. This block is identified by a double-sided box.
Step block is the function chart's basic unit and contains ladder logic for each independent stage of the process or machine operation. It is identified by a single-sided box.
Transition is the logic condition that the processor checks after completing the active step. When the transition logic is true, the step preceding the transition is disabled, and the step following it becomes active. The transition is normally a single logic
OR path is identified by a single horizontal line at the beginning and end of a logic zone. The processor selects one of several parallel paths depending on which transition goes true first. Normally, the number of parallel paths is limited to seven.
AND path is identified by a horizontal double line at the beginning and end of a zone. The processor can normally execute up to seven paths at the same time.
In sequential function chart programs, steps and transitions are arranged in series and parallel paths, and they are numbered with the file numbers that contain their ladder logic. The programmable controller scans the logic of a step repeatedly until its transition logic goes true. Then the program scan moves to the next step or steps, and the previously active steps is turned off.
1. The initial step is always activated at start-up. When restarting from the beginning of the chart and on subsequent passes through the flowchart program, the programmer does have the option of restarting from the beginning of the last active step(s), following or changing the programmable controller's mode from test or run to program.
2. A transition is tested after its associated step, and operations pass from one step to the next through a transition when the transition goes true.
3. After a true transition, the processor scans the step once more to reset all timer instructions and then executes the next step. This extra processor scan is called postscan. It is important to note that the processor never postscans a transition file, so timers probably should not be used in transition files.
The processor scans a sequential function chart program from left to right and top to bottom. When the sequential function chart program scan encounters active parallel steps, it executes the ladder logic in the leftmost step first, then moves to the ladder logic in the next parallel step across the screen from left to right.
This programming is an example of sequential function chart programming for PCs and is a French national standard called Grafcet. Grafcet is a graphical function chart programming language adapted into a programmable controller language by Telemechanique. Grafcet is intended as a universal sequential control language to be used on all types of control systems: electronic, pneumatic, hydraulic, or mechanical. It describes all control functions and produces a structured, logical approach to control system design and analysis. It also has the advantage of being self-documenting and is easy to follow for simple control problems.
