Different ladder programming concept

[ETJ 1]

Hi all, I was just playing with different concept of ladder programming and I would like to know what do you guys think about it. The programming is divided into 2 sections which are: 1) Stage transition and stage dependent output/action - Transition from what stage to what stage when a certain input is triggered as well as what will happen to a stage dependent output/action. (Rung 1 - 4) 2) Output dependent - what will happen to a certain multistage/"stage independent" output. (Rung 7 - 9) I just thought about it yesterday so I decided to do a simple programming using Logixpro Door (Exercise 4) and post it up for some feedback. Please do share your thoughts about it. Attached below are the .rsl files and .bmp of the sequence. Thanks. Door_Stage based.rsl

[Andy_P 7]

Yeah, I have seen (and used) that kind of thing before. A benefit from this method is that you have easy access to your step variable for display on an HMI for example. Makes it easy for fault-finding because you can instantly know what step the sequence is stuck at. Be sure to leave gaps between your steps, as you have done, to allow for the inevitable time when you have to add intermediates.

[Chris Elston 79]

This is my style of programming. Somewhat.... As mentioned before, I used the STEP variable to drive HMI animations. I also am able to set a range of outputs at any given step. So if there is a change in sequence. Say today..I wanted output #1 to be on during steps 10-40. Tomorrow I change it to 10-50. It's easy by changing a limit statement, and no ladder logic has to be edited. I evolved from bit -latch and bit -unlatch concepts to a method similar to what you have described. I think it's easy to change sequence and easy to understand.

[ETJ 1]

Yeah.. Thats great.. I like to have the ability to view/know what current step my plc is running just incase i need to troubleshoot it. I also like to have the ability to control which part of the program that is able to run based on the "Stage Counter" N7:0. That will help to seperate out the program into stages. Another thing I like about that is to have the ability to plan the whole process in a sequential kind of way. That way, based on the "Stage Counter" I can know what stage/process will happen next. I know that another common way of programming is by setting/resetting the latch, but I prefer the "Stage Counter" way because it is able to "kill 3 birtds in 1 stone". Having said that, I think that it also has its own limitations. For a large complex project where there are hundreds of different stations/processors, you will need the equal amount of "Stage Counters" because no 2 stations/processors can share the same "Stage Counters". So if each "Stage Counters" uses 1 integer then 100 "Stage Counters" use 100 integers. I'm do not know if a typical PLC has that muuch memory.. Anyway, since i'm quite new to this kind of programming, please do share more of your experiences. The pros and cons of it. Thanks. Edited 19 Jun 2011 by ETJ

[ETJ 1]

Cool.. do you mind sharing some of the code of what you have done? Like, how you assign the sequence, change the sequence, changing the limit statements etc..? Thanks.

[Chris Elston 79]

Sure here is one I did awhile ago. example.pdf example.RSS

[Chris Elston 79]

Here is one from RS Logix 5000. A simple Walking Beam. walk-beam-sample.pdf

[Peter Nachtwey 15]

I am a big believer in state machines. In fact we make it easy to use STEP programming. The picture below is a 10 step state machine but the picture only shows the first few steps. The program itself simply changes the set point for a temperature PID simulator that I wrote for temperature control webinars. This state machine will restart a what ever step it was on last. The ability to restart where the machine left off is often necessary so that material on the production line doesn't need to be run off the line or scrapped.