Help on re-filling tank automatic!!

hrohibil · Jan 5, 2009

Hi guys..

So I need help on a tank issue.
I attached a simpel drawing of my ladderdiagram and a drawing of the tank with the inputs/outputs.

The ladderdiagram works fine as it is now.

It is a tank. When I push the start button, the whole process begins, and in the end when the liquid reaches the LL low-level, I want the process to begin all over without the need to press start again.

So when you look at the drawing.

When START is pushed, V1 opens for filling the tank. when LH is reached, it stops V1 filling, and starts the V2 heating and the M agitator.
When TH has reached high temperatur, it stops V2 heating and opens V3 drain. When drain has reached LL low level, it stops V3 drain and M agitator.

Here the program ends. I want the process to repeat it self again when it reaches LL, so I dont have to press on start again.

I have tried severals things, but is just messes up my other parameters. Please advice without making to many changes in the ladder.

Best regards Hamid

P.S sorry for my bad hand-drawing. I was so busy trying to figure out the problem that I might got instant alzheimer..

Kidblue · Jan 5, 2009

Hamid,

Not sure if this is a PLC application or you are using hard wired relays? You will need to use the START button to latch some sort of SYSTEM ENABLED flag and then use an o/p contact from this in place of the START button. I'm not sure how you want to STOP the system though (you don't mention a STOP button).

Hope this helps.

Rob

hrohibil · Jan 5, 2009

Hi Kidblue.

This is only PLC application. Regarding the STOP button, I will put that on later, because this is only for testing on PLC programming. I would offcourse in real life set an STOP function.

Im not sure I understand your answer?

Can I latch the start button? I dont want to remove my START button.

Please advise..

Hamid

twu026 · Jan 5, 2009

How about try adding this to your code.



   Start_PB     Stop_PB               Start 

---| |----------|/|-------------------( )

|       |

| Start |

---| |---

Where 'Start_PB' & 'Stop_PB' is a pushbutton input. And 'Start' is an internal bit which is used to trigger the filling.

daba · Jan 5, 2009

Hamid, your approach is too simplistic, and you will not get away without more than minimal changes to the logic you have drawn.

Things don't happen in the real-world as they do on simulators.

Example 1 : As soon (instantly) as your LH is wetted, it will de-latch the V1 rung, and stop filling, but inevitably the level will settle down with LH uncovered, consequently the heating circuit will do nothing. The agitator will start and latch-in OK, but the tank will sit there forever not heating up.

Example 2 : As soon (instantly) as your LL is uncovered, it will de-latch the V3 and M rungs, and stop emptying and agitation, but inevitably the level will RISE again due to drainage down the tank walls, consequently LL will get re-covered, and prevent you starting again.

Good programming practice is to separate the Procedural control from the Equipment control.

As a minimum, I would suggest you re-write your Procedure using a series of successive bits that latch themselves on. Use the procedure memory bits to drive your Equipment V1,2,3 & M.

I would also suggest your procedure tests the "truth" of the inputs using slaved bits, e.g. LL_Wet, LH_Wet, TH_On etc. That always makes the code easier to read and understand. Buffer your physical inputs to the internal bits elsewhere in the code, incorporating any inversions there as necessary.

Think along the following lines, improve where you wish :-



|Buffer the physical inputs to internal bits, invert as

|required to give "true" signals to the procedure

|    LL                                                                                                                                               LL_Wet

|----| |---------------------------------------------( )

|

|         LH                                              LH_Wet

|----|/|---------------------------------------------( )

|

|          TH                                                                                                                                              TH_On

|----|/|---------------------------------------------( )

|

|Procedure Start/Stop Control

|         Start_PB  Stop_PB                                                                                                   Run

|-+--| |--+---|/|------------------------------------( )

| |                  |

| |  Run   |

| +--| |--+

|

|Procedure 

|         Run                   End                                                                                                              B1

|----| |-------|/|-----------------------------------( )

|

|          LH_Wet          B1                                                                                                                B2

|-+--| |--+----| |-----------------------------------( )

| |                   |

| |    B2      |

| +--| |--+

|

|          TH_On             B2                                                                                                                 B3

|-+--| |--+----| |-----------------------------------( )

| |                  |

| |    B3      |

| +--| |--+

|

|       LL_Wet         B3                                                                                                                B4

|-+--|/|--+----| |-----------------------------------( )

| |                  |

| |  B4      |

| +--| |--+

|

|          B4                                              End

|----| |---------------------------------------------( )

|

|

|Equipment Control

|         B1                      B2                                                                                                                V1

|----| |-------|/|-----------------------------------( )

|

|          B2        B3                                    V2

|----| |-------|/|-----------------------------------( )

|

|          B3              B4                                                                                                                V3

|----| |-------|/|-----------------------------------( )

|

|          B2        B4                                                                                                                M

|----| |-------|/|-----------------------------------( )

|

Hope this helps

daba · Jan 5, 2009

can't get the formatting right - !!
align the addresses with the rungs and all will be well

hrohibil · Jan 6, 2009

Twu026:

Thank you so much. It worked fine!! And I also got a STOP function.

Daba:

Thank you for that good long description. I understand from a real-world perspective that things dont act the same. But I unfuntenly dont got a good plc simulator, so the ones I test one, are the simulators I find on the net, and most of them lack of options,especially regarding internal bits and so on.
It is just to train my self and to get better understanding of plc programming.

I will get an siemens S7-200 plc in 2 months from my work. I am really looking forward to that. My first plc.

I actually like the way you seperate the different values.It gives a much more overview of the work and you can see what goes on and off.

I will try to better at organizing my program and make it as realistic as possible.

Best regards Hamid

Help on re-filling tank automatic!!

hrohibil

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Kidblue

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hrohibil

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twu026

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daba

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daba

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hrohibil

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