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مواقع
مفضله
A
PLC works by continually scanning a program. We can think of this scan cycle as
consisting of 3 important steps: checking input status, executing the program,
and updating output status. Step
1-CHECK INPUT STATUS-The PLC takes a look at each input to determine if it is on
or off. In other words, is the Sensor
connected to the first input on? How about the second input? How about the
third? etc. It records this data into its memory to be used during the next
step. Step
2-EXECUTE PROGRAM-The PLC executes your program one instruction at a time.
Maybe your program said that it should turn on the first output if the first
input was on. It already knows which inputs are on/off from the previous step.
Therefore, it will be able to use the state of the first input to decide whether
the first output should be turned on. It will store the execution results for
use later during the next step. Step
3-UPDATE OUTPUT STATUS-Finally, the PLC updates the status of the outputs based
on which inputs were on during the first step and the results of executing your
program during the second step. Using the
example in step 2, it would turn on the first output because the first input was
on and your program said to turn on the first output when this condition is
true. After the third step, the PLC goes back to step one and repeats the steps
continuously. .
Let's
say that we have the following program in our PLC, where M is a motor Starter
that controls a conveyor motor Action:
The operator pushes the start button to
start the conveyor. Step
One: The PLC will see that the Start
button, an input, has been activated. (The diagram below illustrates the status
of the system after this action.) Step
Two: The PLC will run the logic and see
that if the Start button has been pushed, there is a complete path to the motor
starter. Step
Three: Because there is now a complete path
or circuit to the motor starter, the PLC turns the motor starter (an output) on. (Because
the Start pushbutton is traditionally a momentary pushbutton, a latching
contactor maintains a closed circuit path.) When
the Stop button is pushed, the PLC will see that the path to it is broken and
turn the motor starter off.
Figure
7. Example Program Figure
8. Status of the System After Step One Figure
9. Status of the System When Start Pushbutton is Released
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تصميم المهندس : محمد صبري محمود فهيم إستعداد تام لتصميم المواقع إستعداد تام للعمل داخل أو خارج مصر E-Mail: Mohamedmsm@Masrawy.com |
How
PLC work?
Example
