gards88

mitsubishi question (newbie to this)

12 posts in this topic

hi to who reads this, I've been given a question today - (Explain the sequencing system operated by a Mitsubishi Fx1 PLC and suggest possible problems that may be caused by this.) if anyone can point me in the direction of were i could get this answer it would be greatly appreciated .

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Hi You have to give lot more details on what you want to achieve and which PLC. Rodney

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thats all I've been given in the question. I'm struggling to find any answers to it

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I would presume they are referring to STL (Step Ladder Programming) Google JY992D48301 to get the programming manual. Chapter 3 describes it in detail. And quoting the manual... "STL is really a method of sequencing a series of events" Best Regards

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cheers for the info will go through it and try to get my head around it. thanks for the advice.

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STL (or STep Ladder)is Mitsubishi's own brand of sequencing instructions. Signified by S coils or STL insructions It is a program within a program - starting with an STL instruction and ending with a FEND instruction. When called only the active step is scanned. And when you go from one step to the next the last step is automatically reset and not scanned. It is an effective way of implementing a sequence one step at a time. Disadvantages: care has to be taken with timers within steps because if the step moves on before the timer has timed out it will stop timing. Although double coiling is allowed within steps, it is not recommended as it can be confusing to future technicians.

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Are there any other disadvantages you have come across?

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Are you on the same course as the OP? If so, the question is not a very good one. How could you know (or find out) the disadvantages or problems without having any experience of actually programming in STL. I cannot give you a list of disadvantages as there are not many.... and none that aren't easily avoided (as in my post above) STL is an excellent method of separating 'auto' control from manual control. With proper interlocking neither can interfere with the other.

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I have not use STL myself, however one problem I find with standard "step type" or "state machine" programming as I see it is that you transition to the next step by a condition. Now if you loose this condition after you have move on to the next step then the logic will just continue on regardless (Ron_S touched upon this earlier). In many situations this is undesirable. A simple example would be... You want to put a round bolt into a hole and your machine uses a sensor to detect when the hole and the bolt line up. The step logic starts moving the machine into position and waits for the input (X0) to indicate the bolt and the hole line up. It detects X0, stops the machine and then transitions to the next step. On this step we want to actual insert the bolt - so we start moving the bolt into the hole. However, for some reason the machine actually overshot the X0 sensor and the bolt and the hole are not actually lined up. Regardless of the fact the bolt and hole are not lined up the machine continues to try and insert the bolt. (because we are not looking for X0 in the logic anymore as we have moved on from that step). Step programming is common and there are lots of ways to do it. STL is just Mitsubishi's inbuilt system. Many people just roll there own using set/reset, shift registers or even switch/case statements (in structured text). Then they put extra code in where required to check for things like the above (and act accordingly, stopping the machine or jumping back to previous steps etc). As I said earlier I have not used STL before - just my thoughts on this type of programming. Edited by Nightfly

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I would say: Disadvantages compared to what exactly (what are you refering to when you say disadvantage)??? Everything is relative to something...

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I`m guessing Gards88 is on the same course otherwise it`s a massive coincidence. I think the question we are asking boils down to what do you not like about the Mitsubishi Fx1`s sequencing system. Thanks for your help, I`ve got a feeling I be on here quite often.

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And I still think the answer boils down to "compared to what"... Asking anyone for a personal opinion regarding a software implementation do not reflect actual life. E.g. If I were to ask what is the disadvantage about a PLC, what would you answer?? It's impossible to say. However if I were to ask what is the disadvantage about a PLC compared to a computer, the answer could be that PLC's don't naturally implement event-based execution... And the other way around; if you ask what is the advantage of a PLC compared to a computer, one could say that the PLC is pretty much deterministic in it's execution cycle time. But then, the answer could change if the computer has a realtime OS... As you can see above, it all depends on what we are comparing. Asking what is the disadvantage about a FX1S sequencing program is like asking what is the disadvantage of driving a car. Again; what are you comparing with (driving a car vs. walking, vs. cycling, vs. bus, vs. train etc. etc.)? The answers will change depending on what you are comparing with. Feel free to ask the question again, with more details. I'm sure that the course you are taking, you are learning about different approaches (e.g. different software/hardware) to solve different problems?

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