No PLC output to valve when program is running

Discussion in 'Embedded Systems and Microcontrollers' started by Robesim, Sep 1, 2017.

  1. Robesim

    Thread Starter Member

    May 1, 2017
    118
    5
    Hi,

    2 hopper scale valves are controlled by a PLC (Siemens).
    The plc controls the pneumatic directional control valves and the directional control valves control the opening and closing of the large hopper scale valves. When pressing the test buttons on the touch panel display, the hopper scale valves work perfect. But when the program is running the hopper scale valves do not open or close. The touch panel with the program running is connected to the PLC which operate the DCvalves. What i wan to know is, are the test and the program signals in general go to different inputs of the PLC or do they follow the same route, because i can't figure that out from the manual.
     
  2. Reloadron

    Distinguished Member

    Jan 15, 2015
    3,740
    1,693
    Does the PLC receive a signal from the hopper scales during normal operation? I would start by seeing if during normal operation the PLC is getting the data it needs for its decision making process. Without seeing the system and any supporting documentation that is the best I can suggest.

    Ron
     
    N11778 likes this.
  3. Robesim

    Thread Starter Member

    May 1, 2017
    118
    5
    No, i a not sure if the PLC receives a signal from the scale during normal operation. I will investigate this. This installation is from a friend of mine and he asked if i can help troubleshoot, so i'm not familiar with the system. My first thoughts are also that the PLC does not get data from other sensors or so. The hopper scales weigh the color powder for coloring cement. After weighing the valves are opened and the powder is transferred to a mixer. I will look and see if ther is more documentation available.

    But in general, are test signals programmed to the same input adress on the PLC as program signals or are they programmed to another input address on the PLC than de program signals????
     
    Last edited: Sep 1, 2017
  4. N11778

    Member

    Dec 4, 2015
    163
    38
    Sounds like there will be a signal from the hopper scale going to the PLC telling the PLC the weighing is done.
    and its time to take a dump :).
     
  5. Reloadron

    Distinguished Member

    Jan 15, 2015
    3,740
    1,693
    In general test or push to test merely make something happen, they do this in the absence of any decision making inputs. For example if I have a scale it has a sensor. That sensor is measuring the weight of a batch, be it a batch of dye powder or anything else. When the batch achieves a certain weight the PLC does something or tells something else out there to do something. A test button will bypass the decision making process and energize an output to make something happen. Normally the PLC in this case is likely looking at weight, as weight gets to a pre set point the PLC makes the decision to activate something, be it a solenoid or whatever is out there. Push to test circumvents the decision making process and any inputs used in that process. If you can review the coding how all of this works should be apparent.

    Ron
     
    N11778 likes this.
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
    16,389
    4,822
    The inputs can easily be observed as to whether they receive an input signal without the need of a terminal etc, by monitoring the input LED status, same as the outputs.
    Whether the test buttons goes to a separate input is the decision of the original programmer, do you have the printed listing copy?
    Max.
     
    Last edited: Sep 1, 2017
  7. Robesim

    Thread Starter Member

    May 1, 2017
    118
    5
    No, i don't have the printed listing copy. My thoughts were just like Reloadron said, the weight sensor or another input device or the input card of the PLC. Chances there is something wrong with the PLC input card is small i think, because the PLC receives test signals. i'll ask if there is more documentation. Does this problem has something to do with power fluctuations, because a few hours before it was discovered that the valves didn't work , there were heavy power fluctuations on the electric system.
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
    16,389
    4,822
    When you ask a question such as this, you at least need the I/O listing to find out the address of each separate inputs, if this is the case.
    No one here can answer that question off the bat.
    Normally when a company installs a PLC system, a copy of the ladder at least should exist, at least that is what I do as a matter of course.
    Max.
     
  9. Reloadron

    Distinguished Member

    Jan 15, 2015
    3,740
    1,693
    If you knew what you had, exactly what you had then trying a few logical troubleshooting steps would be much easier. Being able to view the code used or having a good schematic to work from is sort of important. Less any good documentation we are just guessing.

    I see Max pretty much covered it while I took the dogs out. :)

    Ron
     
  10. Robesim

    Thread Starter Member

    May 1, 2017
    118
    5
    You're both right Max and Ron. Tomorrow i will ask if there is a copy of the ladder diagram or other documentation.
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
    16,389
    4,822
    If it is not available in the plant, then either the OEM or the company responsible for commissioning the onsite equipment should have it in their records, (IF they are still around!).
    Max.
     
  12. Robesim

    Thread Starter Member

    May 1, 2017
    118
    5
    Yes, tomorrow i will ask and get back to you guys.
     
  13. GopherT

    AAC Fanatic!

    Nov 23, 2012
    8,025
    6,786
    I'm surprised to see you posting here, I thought you'd be all tied up in the Canadian Non-disclosure agreement and mysterious source of energy.
     
  14. strantor

    AAC Fanatic!

    Oct 3, 2010
    5,010
    3,027
    I think he's using an HMI, so I wouldn't expect to see any input LEDs change status.
     
  15. N11778

    Member

    Dec 4, 2015
    163
    38
    I would think there is a way to watch the inputs on the Screen then.
     
  16. ScottWang

    Moderator

    Aug 23, 2012
    6,415
    992
    Hello, Robesim:
    Did you see the PM(inbox)?
     
  17. Robesim

    Thread Starter Member

    May 1, 2017
    118
    5
    I'm back. Yesterday I've tested the sensors of the valves of the hopper scales going to the PLC input and they seem to work correct. the lights on the input of the PLC are lit when testing the sensors. When the program is running, the valves do not open or close, but now the valves also do not open or close when we press the test buttons on the touch panel display.. So the valves do not work when the program is running and the valves also don't work when we press the test buttons on the diplay. My guess is that there is something wrong internally with the PLC. The night before this problem there were also heavy power fluctuations and the PLC is not connected to a Stabilizer/UPS system. Reloadron and Max what do you think??
     
    Last edited: Sep 3, 2017
  18. Reloadron

    Distinguished Member

    Jan 15, 2015
    3,740
    1,693
    If the PLC is getting the inputs as you describe to make decisions and is not giving an output, now no output for normal or test modes, then yes, I would be looking at the PLC or if it has specific output modules the specific output module.

    Ron
     
  19. MaxHeadRoom

    Expert

    Jul 18, 2013
    16,389
    4,822
    From my experience, if something happens to the PLC, the program does not run at all, it is possible that a Input or output supply got zapped, but if nothing appears to work, then usually the former.
    A monitor terminal or program is essential to confirm.
    Max.
     
  20. Robesim

    Thread Starter Member

    May 1, 2017
    118
    5
    Many thanks. Yes, i think an input or output is bad. I've setup a connection, so that the manufacturer can dial in on the system tomorrow to check it.
    Tomorrow i'll get back to you guys with feedback.
     
Loading...