LogixPro elevator assistance needed

[RWS8086]

Hi everyone,

I’m new to PLC programming, this is my first post. I finished TheLearningPit’s elevator exercise #4 a week ago and it worked. Wow, that was a tough nut to crack (it was for me anyway). After I was done impressing myself; I zipped it up, tossed it in a folder, and moved on with my life.

A couple of days ago I decided to compare it with someone else’s and found very few completed examples out there. I did find an excellent one here on this website.

My question is on the elevator car’s reversal. The requirement states:

In our multi floor system, the elevator should continue in it's initial direction of travel, stopping at each intermediate floor which has a request pending for that particular direction, and continue in this same direction until the farthest request for service is reached. At this point the direction of travel should then be reversed if further requests are pending. Any requests associated with this new direction of travel should then be serviced.

Did I complete this or over-engineer the exercise? Please help, I need to know, thanks

 

[Lancie1]

You followed the directions exactly. Your elevator program works well.

 

[RWS8086]

Thanks for the feedback. I was concerned when I noticed that my solution didn't look anything like other solutions.

 

[pal]

Hi RWS8086 , in a way you are in a unique postion at the moment - you're learning PLC programming and have written a successful program ( Lancie1 said so ) and have gotten hold of other solutions to the same problem . Check first if they work correctly . If so , look at each program and see which is easiest for you to understand . The reason I suggest this is that as a programmer you're not just writing the program to do a job , but also so that it can be understood ( it might even be you in a couple of years time ) . It often happens that the program needs modifications to suit changing circumstances and , if it is easy to understand , then it is usually easier to alter and certainly less time consuming .

Paul

 

[RWS8086]

Paul,

Thanks for the words of encouragement and some good advice.

Rob

 

[Lancie1]

One reason your program looks different is that the official LogixPro Elevator exercise specifies certain latched flag bits (that you did not latch or did not provide in some cases), even though your program logic is very efficient.

One thing to keep in mind is if your "customer" gives you specific directions about how he wants the PLC program written, then you should not ignore those directions, even if you know a better way to do them.

You should always discuss any variations you want to make, because sometimes there are good financial and operational reasons why your customer wants to do programs a certain way. In real-world situations, changing those contract parameters without getting your customers approval could lead to legal problems for you.

 

[RWS8086]

Lancie1,

Once again, thanks for the feedback. Your constructive comments are much appreciated and taken with the intent of a mentor trying to impart knowledge to a student. I hope my response is interpreted in the reverse manner.

While working a solution to this challenge I had many questions. If this had been a real world exercise; I'd have been on the phone or knocking on someone's door, because the written guidance was not working for me. I agree that specific directions from the "customer" should NEVER be disregarded.

One reason your program looks different is that the official LogixPro Elevator exercise specifies certain latched flag bits (that you did not latch or did not provide in some cases), even though your program logic is very efficient.

Guilty as charged!

I admit my design using "OR/XOR" logic is in the gray area for most engineers when the word "latch" is used. As for the remaining flags/outputs, that was ignorance, misinterpretation and past experiences. While I am very new to PLC programming, this is not my first rodeo in the electronics arena. The last sentence of that exercise said something about "utilizing whatever tools you have at your disposal" and I did.

Do I need to go back to the drawing board?

Regards,

Rob

 

[Lancie1]

No, you definitely do not need to go back to the drawing board. You solved the problem, and your elevator program does the job with no faults. I was just pointing out the only little flaw that I could find, and really in most cases that is a very minor problem. Most of the time, the customer only tells you how a machine or system should perform (performance specification), not how your program logic should work.

But there are a few cases where the customer does specify exactly what he wants in the program. Usually that is where the maintenance department is very familiar with an existing machine and PLC program, and wants the new version to look like and work like the old version, to minimize the retraining time. Those are the rare times when you have to refrain from doing it your way.

 

[RWS8086]

Understood and thanks...