Sequencer Instructions

Hi All

I was wondering if anyone has used sequencer Instructions (SQI,SQO,SQL)

I have been studying up about them with a view of using them for machine control steps that I am writing. Currently I use a method with a "CurrentStep" Tag and a rung with "CurrentStep" EQU for each step of the sequence to evaluate the status of the step and increment "CurrentStep" Tag when all conditions are met.

I can see how this would work with Sequencer Instructions but cannot see that editing the sequence later will be very easy.

Basically has anyone used them? What for? and How?

Regards

Its not very easy to edit; thus can be a big headache for someone that has to maintain it. Could he read it as easily as your step logic that you have now?

No the step logic I have now is VERY maintenence and modification friendly, therefore I was curious to the use of these instructions

I don't know what PLC you are using, that makes a difference.

Typically though, I make my own sequencer logic, and ignore the built in instructions.

Logix L74

A word of friendly advise from someone who has been there.

if your program is user friendly now, why in the world would you change it? Most companies frown on sequencers! most maintenance people have a hard time understanding them and cannot figure them out.

if maintenance does not understand how sequencers work and cannot figure the program out, guess who gets called at 1 am in the morning?

guess who gets yelled at for changing the logic from a user friendly system to a piece of junk?


not trying to slam you in any way, a programmer i used to work with loved sequencers and used them a lot. he got calls all the time in all hours of the night, he didn't mind. When he quit, guess who got the calls, me and the owner. Neither of us could figure out what was going on and spent 3 weeks figuring out what was happening on one of his programs. we then rewrote all but one of those programs at our expense. Never again! 20+ years later, i have had to rewrite every sequencer program i have had to deal with. no matter where i go, i have to rewrite
the logic because no one knows how to deal with sequensers.

just ranting a little due to personal experience.

regards,
james

Hi James

When I started out in this career my 'Mentor' (Local System Integrator) told me that what will make you successful in this career is :
how simple your code is for the next person to understand and modify
AND your willingness to share infomation, this can be touchy as obviously you dont want to go and give away your trade secerets but if you know how a piece of kit works in your plant, dont keep it a seceret. O and be nice to Maintenence personell and give respect

OK enough of the lecture.

I am interested in these instructions as I came across them while reading the instruction set, and wanted to know more about them, however I did see that they where way more complex than my method.

However I am continiously looking to improve my knowlodge and hence why I started this thread to find out more about them.

Regards

Advantages to the SQI/SQO based sequencer:
1) Flexibility. You can "re-peg" the sequencer, even from an HMI, and completely change the machine behavior.
2) Compact code. You can control thousands of steps in an itty bitty living space.
3) Diagnostics. If you use the SQI instruction to advance the sequence, then it is very easy to provide diagnostics to show exactly which bits need to change states to advance the sequencer.

Disadvantages:
1) Requires good design and forethought. A poorly implemented sequencer can be a pain to modify and troubleshoot.
2) Troublshooting. Using the SQO without the SQI and accessory code for diagnostic display purposes wil mean that figuring out what the machine is waiting for requires good understanding of the data layout and how the instructions work. It is always a good idea to have a design document to assist with this. I prefer to do the design in Excel using DDE/OPC links to make sure the data is the same as the running machine.
3) Step manipulation. the SQO/SQI based sequence is best suited for machines that run through a linear list of steps and don't do a whole lot of decision based branching.

I have an example over at MrPLC for a SLC but, like rdrast, I don't use the built in instructions. I like to control the step number manipulation externally for maximum flexibility. The example is very basic and would need to be expanded for most real applications.

Sequencers can be very difficult, especially if you use masks. My first experience with sequencers left me baffled = I think it was an AB PLC 1.
However, there are other threads on this site that give a good overview of 'self explanatory' sequencing, which I used as a basis for my programming. A sequence can make programming easy, and easy to read. I tried to attach SEQ_TEST R2[1].pdf, an AB ladder logic print, but I can't tell if it worked. This is fully explained in another thread. MOVe sequence bits into a word. Start the SQO with a pulse. Use the inputs, or a timer, to confirm the step and pulse to the next step.

Basically has anyone used them? What for? and How?

Click to expand...

Mostly I have used Sequencer instructions to solve student problems on this site. On rare occasions I have used them to control the start sequence of real pump motors, where the owner wanted the lead pump rotated for even wear.

The Sequencer seems to be an idea that works best in theory, not in practice.

Last company went to all SQI/SQO for controlling the machine. There were hundreds and hundreds of inputs and outputs. The sequencers were cascaded to allow for that many I/O. Had to have a VERY LARGE (printed on 17x22)excel spreadsheet just to see what was going on. There were many lost hours of production trying to decipher the code.

Once you understood, changing the machine operation wasn't too hard. And being the machine just constantly cycled doing the same things over and over, it made for a consistantly operating machine with some very compact code.

Still, I am an old school person and I didn't like them.

Not to belabor the point; I put sequencers in my program partly because it was a learning process. I have been programming AB SLC 5/03 for 2 months now. Granted the sequencers are short and simple, but they work for me.

still trying to attach a file ?

GlennA said:

still trying to attach a file ?

Click to expand...

zip it first.

Let's get things into perspective....

The SQO instruction is just a "cam-timer", and is a handy tool to have if, and a big IF, what it is controlling is a "permanently" fixed sequence of controlling outputs, a few simple examples...

Basic intersection traffic lights
Rail/Road crossings
Pick-and-place machinery
Re-loading power supply breakers
...and many more.

The above examples do not require any "decision-making", the sequencer will just go through the set patterns, and do a very admirable job.

Now let's consider more complex "sequencing", take for example the "Basic intersection traffic lights" mentioned above, and add some "decision-making" in the form of filter lanes, pedestrian crossings, peak-time adjustments to timing, etc.

Very quickly the SQO is ruled out for the job, because of its inflexibility, and programmers soon decide to "roll their own" sequencing logic that enables them to introduce the "twists and turns", add sub-sequences, modify timing differences, etc.

I'm not saying that SQO/SQI couldn't be manipulated to perform the more complex tasks, but it starts to become a nightmare to debug, to modify, and worst of all, to understand.

If your application requires a "push, pull, rotate, punch, pull, push" sequence of actions on a fixed time-base, and no more, then fine, go ahead and use SQO/SQI and it does the job with ease.

Go any further than that, I would move away from the built-in sequencer instructions.