hello there timothy,


Clive here again just want to thank you for your great advice. I hopefully have gotten to the root of the problem and my program seems to work. I am enclosing a copy of it for you to see what you think of it and would you suggest any changes or what other way could it have been done? Also included for you to check out is the configuration of the cyclinders.

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Replying to:

Clive,



I wanted to make sure that I understood your project before I answered. Perhaps I am missing something, but the timer seems (at least to me)to be the easy part.

By the way, you must have the EB manual, because the number you gave (EB NJI044) is the part number for the manual. If you look on page 28, there is a very good example of how to use the timer.

As I set out to recreate your project on one of my demo, several thoughts crossed my mind that I would like to share. First ( in my humble opinion), there is never just one way to do a project. If they work, then they are good. If they don't then they are bad. If you asked 5 very proficient programmers to write your project, you would get 5 probably very different approaches. This may not sound encouraging to you at this point, but it is really meant to be. There are a lot of books, training programs, and training classes that one can take to help you learn how to program. The project you end up creating is a culmination of your life experiences and the training to which you have been exposed.   Obviously a project written by an electrician who has come up through the ranks, is going to be quite different from one written by a C++ programmer from an IT department who has been drafted into the wonderful world of Control.

I was picturing how your project would look and function in the "real world".  I envisioned two mechanical limit switches on each ram, which would indicate a fully retracted or conversely the fully extended position.  Three rams times two limit switches equals six switches (also equal to six inputs). You probably would have a "Start" or "Initiate" switch or push-button. You would undoubtedly need an "Emergency Stop" switch that would intercede no matter what stage the operation was in, and make all the rams return to the "home" or safe position.  Depending on the size of the hydraulic pump, you may need to insure that only one ram was in operation at any given point in time.  This is easily accomplished by putting in NC (normally closed - or sometimes termed as Examine Off contacts) from the other ram circuits.  Even a "simple project" as you put it, can start getting complicated when safety issues come up.  There is a good chance that in the real world,  OSHA  would require a different level of sophistication, depending on the potential for harm to humans (and/or other dumb animals).  These might include "Light Curtains" or Laser Proximity systems which would freeze the position of the rams if someone/something came within range.

You don't have to be an "old timer" to see some pretty wild things come up to complicate a system.  For example, imagine a hydraulic line breaking while a ram was mid-travel.  You happen to be in an environmentally sensitive area (aren't they all?).  So you have to design in something to safeguard the system to satify the EPA so you don't pump 100 gallons of toxic hydraulic fluid out into the nearby playground full of defenseless (and unattended, of course) babies.  Maybe this is as simple as a timer for each ram, that shuts the pump off if the ram has not reached its destination within the designated time frame.  It could also be an "excess flow" switch on the hydraulic discharge.  One can see how easily your "simple" project can get vastly complicated.

Unfortunately, there are also those code writers who build in  unnecessarily complicated circuitry in order to prove their mental superiority, and make it so that they always get the call-back - because no one else can understand what they have done.  Anyone who has had to come in behind one of these intellectual snobs knows what I am talking about.  That is why many good programmers try to make the project as simple as possible, and provide the best possible documentation (hint, hint, hint).

So, back to your timer.  How about this?

Maybe this is oversimplified, Clive, but taking in to account all the other considerations mentioned above, this part of your project should not present much of a problem, as I see it.  If I have overlooked something, please feel free to discuss this further.

Good Luck with your project!

Timothy Davis

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Replying to:

Hello


i am having a problem with a Hitachi PLC (model EB NJI044) I am doing a "simple" project with 3 cylinders where they extend one after another (i.e) 1 out 2 out 3 out. and then 1 retracts followed by 3 retracting and then number 2 retracting after a 5 second time delay, where the whole cycle runs again. I can get them all out but the time delay has me stumped and the manual is i have is missing a lot of information from it. So it anyone can help me i would be greatful. I am a mature student from the aero industry who has gone back to learn some new stuff!!! Any information with the ladder logic would be appreciated. Thanks again ,


Clive.