Converting Diagram to Ladder Logic

[mj3]

Hi there! I am new to PLC's, and I have undertaken the task of converting a primarily pneumatically-operated machine to electrically & pneumatically controlled/operated.

As far as I have gathered, there are about 6 inputs, 6 corresponding outputs, and the machine will utilize limit switches to control actuators -- both electrical and pneumatic actuators. I am wondering if there is a way to do this, having the machine be electrically controlled but still maintain the pneumatic actuators.

I'm struggling to make much sense of the diagram here, and I'm not sure how to take anything from the diagram and apply it to a ladder logic PLC program.

 

[PLCnovice61]

Hi mj3, welcome to the forum.
I would first ask someone how this physical works, get them to explain it out as a scope of works (written down as a step by step thru how the device works), then redraw it as a single line diagram, When you have that, you pretty much have your PLC code.
Now l imagine you really wanted someone to start or complete the code for you, but unfortunately l am a big believer that people should not touch things they don't understand, keep in mind, people here have no idea what safety issue's maybe involved re this device. Also the most likely first question to be asked next will be what type of PLC?

 

[drbitboy]

I second @PLCnovice61's sentiment about safety, especially when I see the phrase "Two-Hand Module" in there, which usually refers to an operator-protection device. Another question to ask is why this was done pneumatically in the first place e.g. are electric circuits a Bad Thing to introduce into this (explosive?) process environment?


My brother told me that someone in HR once set up an area for smokers outside his plant's building, with a picnic table and other such accoutrements. The only problem was that it was located by the discharge of unused oxygen from the PSA unit that the plant used to generate nitrogen for blanketing.


That said, and again with the caveat that you need a LOT more information than what is in that diagram, here is my GUESS at what this is:

The stack of {1/2A-DCB-...-DCB-1/2A} in the middle is a pneumatic sequencer that will be replaced by PLC logic. Each DCB layer is modular element equivalent to a rung using the canonical Start/Stop pattern summat like this:

       Output(N-1)  LimSw(N)   Output(N+1)  Output(N)
---+-----] [-----------] [----+----]/[----------( )---
   |                          |
   |  Output(N)               |
   +-----] [------------------+

In other words,

1. When the process is at Step N-1 (i.e. Output(N-1) value is 1), and Limit Switch N makes contact
   1. Then transition into Step N i.e. assign a value of 1 to Boolean Output(N)
2. As long as the process is in Step N and there is no transition to Step N+1,
   1. Remain in Step N i.e. maintain the value of 1 in Output(N).
3. When the process is at Step N, and a similar rung transitions to Step N+1,
   1. Then transition away from Step N i.e. assign a value of 0 to Output(N)

The logic for the first value of N (1?) as well as the last value (5?) may be a bit different; that may be what the {E} and/or {1/2A} layers are about.

In the modified image below

• the solid lines are "PLC" inputs, LimSw(N), coming in the left side of the DCB stack;
• the dotted lines are "PLC" outputs, Output(N), going out the right side of the DCB stack and activating pneumatic devices.

So if the red [LOWER LIMIT SW.], just below the upper right of the diagram makes, then that triggers the red DCB, which in turn puts air to the red dotted line, which activates (sends air to) the lone {B} device and possibly the right-hand {OR VALVE SWITCH}*.


Activating {B} and/or that right-hand {OR VALVE SWITCH] eventually causes the green [LIMITE SWITCH], at the lower-left of the diagram, to make, which triggers the green DCB, which in turn both cancels the red DCB, and sends air to the green dotted line, etc.

Again, this is only a wild guess, mostly based on the word "SEQUENCER" showing up twice in the notes; this diagram is in no way enough to actually do anything.

I imagine asking The Google to look for the "pressure sequencer" and similar terms will return some useful links, interesting videos, etc.


Update: check this link out.

 

[GaryS]

That would be n intersting project to work on but as was said befor we need more informatin on what the machine dose and how it should work.

 

[parky]

That looks like something I came across some years ago, I was given the task of converting it using a PLC. Looking at it, it is some sort of glue system where an operator places something in a jig, presses a two hand safety device & it applies glue, compresses it (possibly using a vacuum), it's only a guess but converting that to ladder without having in-depth knowledge of pneumatic logic elements is difficult, I had the same problem, the system was not working but speaking to the operator he gave me quite a good description of what happened, I managed to force it to run in a sort of way, wrote down what the system did and came up with the sequence of operation, documented what would become the inputs & outputs (note: on mine some of the limits as such where pneumatic not electrical so had to add sensors in their place). It turned out to be quite a simple bit of logic cannot remember now but no more than about 30 rungs of code, could probably have reduced it but at the time although worked out the sequence I was being careful to have enough interlocks to do the job. So as virtually every OP has stated you need to know the sequence of operation, check that what seem to be sensors are electrical (watch out for pneumatic ones, these will probably need replacing), size the the control valves correctly as you may need to replace them.
One other major thing is safety, remember any alteration to an existing system must comply to the latest legislation, failure to do so will make you liable to prosecution, any safety device you intend to use must meet current regulations.

 

[JLand]

100% your best bet is to get a functional description from someone familiar with the process. An electrical diagram does not tell the whole story and this also opens up the opportunity for added functionality.

For something as simple as this looks, this is a 15 minute conversation. I'm going to go out on a limb here and guess you've spent many multiples of this trying to sort out this diagram already.

Once you have your description of how the machine should run (be sure to get a description of every input and output!), something as simple as this all you need to do is go output by output and program in the necessary logic.