Level application on this site

simplelogix

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Sep 2003
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Hi enlightened plc engineers,

Please help me! I am learning plc programming and came across the tutorials on this site. I have a specific question regarding the level application mentioned on this site.

www.plcs.net/chapters/level12.htm

Although Phil Melore says that the level-sensing inputs are normally closed, he has used normally open inputs in the ladder diagram. Am I too dumb to understand what he is trying to explain? Please enlighten me.

Many thanks for your suggestion
Simplelogix
 
This is a common early learning stumbling block – real world inputs N/C – ladder logic N/O.
(just about all learners struggle with this concept at the beginning)

In that example, imagine the tank had no oil in it at all.

The 2 inputs from the sensors would be N/C and so the 2 inputs at the plc would be ON (the input lights would be lit)

So the N/O contacts in the program would be closed. (0000 &0001)

This would cause the pump to run, to fill the tank with oil until the top level sensor was covered by the oil.

The top sensor would then go N/O breaking the latch circuit (1000) and turning off the pump.

The pump would stay off until the oil had lowered below the low level sensor thus starting the sequence again.

Hope this helps a little, I know its tough grasping this first concept :cool:
 
Thanks a lot Goody,

I think I get it. The way I understand it is as you have said is...the NO contact representing the sensor input on the ladder logic is not a physical representation of the input. Only what we need to achieve. Am I right?
 
One thing to consider on your subject. A reason for using N/C devices in this type of application. If for some reason the switch and/or wiring where to fail,(ie. lack of signal), your pumps would not start and/or keep running without control. depending on your application the lack of signal can become a fail-safe. Much like the lack of current in a 4-20mA loop does. This is especially important in safety operations where a signal would indicate "safe" and a lack would indicate "not safe".
 
One thing to consider on your subject. A reason for using N/C devices in this type of application. If for some reason the switch and/or wiring where to fail,(ie. lack of signal), your pumps would not start and/or keep running without control. depending on your application the lack of signal can become a fail-safe. Much like the lack of current in a 4-20mA loop does. This is especially important in safety operations where a signal would indicate "safe" and a lack would indicate "not safe".
 
Simplelogix,

The --| |-- and --|/|-- symbols in ladder do not indicate the type of contact in the device... at least, not directly.

If a contact is properly named then you should be able to deduce the type of contact that is in the device. If not properly named, then who knows.

The following was something I wrote in response to a slightly different question... but it applies to your question.

This description indicates the relationship between the bit-name, the actual contact type in the device and the TRUE/FALSE result.




In programming logic, we use one bit per condition.

If we use one-bit to indicate the "Up Condition" of a roll-up door, what can we ascertain from that bit?

We can only determine whether the door IS UP or IS NOT UP. We can not deduce, infer or determine in any other manner that the door is DOWN. We simply don't have enough information to make that determination. All we can say is, either, the door IS UP or, the door IS NOT UP.

Of the two possible states that a particular condition might exhibit, one is the "reference state". The "reference state" is "declared" in the "name" describing the "condition". This is not necessarily the condition being sought at some particular part of the program.

There can be only one name for any given bit. However, that name could be one of two possible names. Referring back to the UP bit, the name of the bit could be "Door IS UP". It could just as easily be "Door IS NOT UP".

You can look at the door and see that the door IS NOT up. Is that an indication of TRUE? Or, is that an indication of FALSE?

The presence of a signal is not necessarily required for a TRUE response. Sometimes we are looking for the absence of a signal.





If we declare the name of the bit to be "Door IS UP" then the signal comes from the Normally Open contact on a limit switch.


Door
IS UP
--| |-- < --- This signal comes from the
Normally Open contact on a limit switch.


.
This piece of logic is asking if the "Door is UP" signal is ON.
If the door is up, then the signal is ON and the response is TRUE.
If the door is not up, then the signal is OFF and the response is FALSE.




Door
IS UP
--|/|-- < --- This signal comes from the
Normally Open contact on a limit switch.


.

This piece of logic is asking if the "Door is UP" signal is OFF.
If the door IS up, then the signal is ON and the response is FALSE.
If the door is NOT up, then the signal is OFF and the response is TRUE.




If the name is "Door is NOT UP" then the signal comes from the Normally Closed contact on a limit switch.


Door IS
NOT UP (typical fail-safe method)
--| |-- < --- This signal comes from the
Normally Closed contact on a limit switch.


.

This piece of logic is asking if the "Door is NOT UP" signal is ON.
If the door IS up, then the signal is OFF and the response is FALSE.
If the door is NOT up, then the signal is ON and the response is TRUE.




Door IS
NOT UP (typical fail-safe method)
--|/|-- < --- This signal comes from the
Normally Closed contact on a limit switch.


.

This piece of logic is asking if the "Door is NOT UP" signal is OFF.
If the door IS up, then the signal is OFF and the response is TRUE.
If the door is NOT up, then the signal is ON and the response is FALSE.

In human terms, the double negation cancels and we are asking if the "Door is UP".




Another version of the explanation...

The --| |-- symbol asks the program to see if the signal is ON.

If the signal is ON then the RESULT is TRUE.

If the signal is OFF then the RESULT is FALSE.
.
.
The --|/|-- symbol asks the program to see if the signal is OFF.

If the signal is ON then the RESULT is FALSE.

If the signal is OFF then the RESULT is TRUE.
 

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