Author Topic: Generic ladder logic question  (Read 17924 times)

Joel Moore

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Generic ladder logic question
« on: January 08, 2010, 10:11:58 AM »
I'm looking for some advice on how to accomplish the following in ladder logic:

I have 3 inputs that are being used to command the PLC to move to 1 of 8 predefined positions.  I'm trying to figure out a robust way to respond to those inputs.  Ideally I want to make sure that whatever external system is controlling the inputs has plenty of time to make its changes before I start to move.  For example if the inputs are currently "0,0,0" and the external system is in the process of changing to "0,1,1" I don't want to initiate a move while the inputs are in the intermediate state of "0,0,1".

Currently I just have a 1s timer that checks the state of the inputs but that still leaves the possibility that I check the state while the inputs are in the middle of changing.

The best way, in my opinion, would be for a timer to start when the system sees an input change and the PLC won't initiate the move until the timer expires.  However there are a total of six events that could trigger the timer (3 inputs, either on or off).  I have no idea how to design this using ladder logic.

garysdickinson

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Re:Generic ladder logic question
« Reply #1 on: January 27, 2010, 02:53:24 PM »
The following logic will detect when the 3 inputs have been stable for the minimum time determined by the SetValue of TIMER, InputChange.

I'd give this logic a try,

Gary D.




  Detect Input Change.  Have any of the inputs changed
  since the previous scan through the ladder logic?

   Input1  Latched1                            InputChange
-----||-----|\|--------+--------------------------(RLY)
                       |
   Input1  Latched1    |
-----|\|-----||--------+
                       |
   Input2  Latched2    |
-----||-----|\|--------+
                       |
   Input2  Latched2    |
-----|\|-----||--------+
                       |
   Input3  Latched3    |
-----||-----|\|--------+
                       |
   Input3  Latched3    |
-----|\|-----||--------+


  When Inputs have remained stable longer than the
  SetValue of the InputStable TIMER then InputStable will
  be TRUE.

   InputChange                                 InputStable
-----|\|------------------------------------------(TIM)


  Remember Input state for comparison on
  Next scan of logic.

   Input1                                        Latched1
-----||-------------------------------------------(RLY)

   Input2                                        Latched2
-----||-------------------------------------------(RLY)

   Input3                                        Latched3
-----||-------------------------------------------(RLY)