Functional Spec / Test Documentation of Interlocks

[moistcat]

Hi all,

I was wondering if anyone had any good methods for developing functional spec / test sheets for equipment interlocks.
I find the best method to describe a particular process / equipment interlock is the ladder logic itself.

I have also seen the method with defining signals: A = Tank High Level, B = Mains Phase Failure
Then expressing as: PMP005_INTERLOCK = A | B

Any ideas /comments would be awesome!

 

[Ken Moore]

I always used a cause and effect matrix via a spread sheet as a programming/documentation tool. However, it would not be a good function test document. Test plans take a lot more effort, and a lot of process knowledge.

 

[cardosocea]

I've seen similar in Excel, but it gets very clunky very quickly when you have chained conditions like A OR (B and (D AND NOT C)) OR (F AND C).

Because of this we moved from Excel into a Word document with a portrait format and revert to text. However in some instances where the person specifying the interlocks actually understands the whole process, he may define them as the output of an interlocked block, which will split the complexity of the interlock logic in different process elements.

 

[JesperMP]

You could use Structured Text.
Even if the ST program is dormant and not actually used, you can enjoy the benefit of accessing the symbols of the PLC program.
And you can embed the ST program in the same PLC project as the 'normal' PLC program, so you automatically syncronize the versioning of both.

edit: If you can get the spec in text pseudocode, you can add that to the ST program as a comment header.

I have also seen the method with defining signals: A = Tank High Level, B = Mains Phase Failure
Then expressing as: PMP005_INTERLOCK = A | B

Technically not wrong, but people that are not used to notation of logics will be confused.
Write the logic in plain text is better.
In the above I am also missing what state both the incoming and outgoing signals must be for TRUE or FALSE states.
I am guessing that the signals are to be fail-safe, so Tank High Level is an N.C. signal, i.e. it must be TRUE to release the other part of the process.