Program Testing

I have another beginner question for the experienced members of the forum. While performing operational testing of a program during startup, what would be the best method to halt the operation of the program in the event that something goes wrong. This will be a wetwell pump down operation using ControlLogix processors and RS Logix 5000. The pumps are VFD driven. Thank you.

One method

Instead of your logic acting directly on the outputs, use internal bits.
Then map the bits to the outputs. You can place an AFI in front of the mapping.
Then when you test the logic, you can see which outputs should be made and when and determine if this is correct. You can then remove the AFI on each output, one by one, and gradually bring the entire system on line.

One method

Instead of your logic acting directly on the outputs, use internal bits.
Then map the bits to the outputs. You can place an AFI in front of the mapping.
Then when you test the logic, you can see which outputs should be made and when and determine if this is correct. You can then remove the AFI on each output, one by one, and gradually bring the entire system on line.

Plus there's always the E-stop. (I hope)

William Martin said:

... what would be the best method to halt the operation of the program in the event that something goes wrong. ....

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E-stop Button. These should be hard wired and should kill power is activated.

I know it's not the answer you are looking for, but this should be the first order of checkout, hardwired safety items.

I always isolate my circuits by removing fuses, breakers, air, etc . Then begin testing the E-circuit/s. Once the E-circuit/s are verified I begin energizing the individual loops starting with the control circuits (ie sensors and outputs). After that if there is no priority I work my way up to the most expensive parts.
Personal insight: Some people frown on using an EMO stop because they cause problems, My belief is if it can't handle or easily recover from an EMO people will be afraid to use them.

brucechase said:

E-stop Button. These should be hard wired and should kill power is activated.

I know it's not the answer you are looking for, but this should be the first order of checkout, hardwired safety items.

Click to expand...

We call it the "OMG" button.
It kills power to all the I/O but NOT the cpu.

I'm sure you'll have to use it a few times before testing is done. Anyone that says they can write code that tests out first time with no changes required is a liar!!

This project is the headworks of a waste water treatment plant using VFD driven submirsible pumps in a dry pit application. There are no E-stops on the VFDs or control panels. The AFI instruction is a testing option that I did not consider. TND would be another. Thank you for the replies!

lessons from the Boot Camp ...

Greetings William ...



first let’s start with the original question:

what would be the best method to halt the operation of the program in the event that something goes wrong?

Click to expand...

I take that to mean that you’re planning to start the machinery running - and then you might need to STOP that machinery in a hurry ...



now let’s look at what seems to be the outcome (so far) of the discussion ...

The AFI instruction is a testing option that I did not consider. TND would be another.

Click to expand...

let’s be careful here ... I might just be misunderstanding your question ... but ... in the event that “something goes wrong” while the system is running, programming in an AFI might not be the best and/or quickest way to “halt the operation” of things ... (I’ll bet that I’m just misunderstanding your original question) ...



but just for discussion, you should know (and maybe already do) that an AFI does NOT “disable” the rung ... specifically, the AFI just causes the processor to execute the rung with FALSE logic ... now in most cases (and maybe in yours) that’s exactly what you would want to happen ... because if the controlling element of that rung happens to be an OTE instruction, then using the AFI would cause the OTE to send a ZERO status to the output bit ... and in most cases that would cause a piece of machinery to shut down ...



but (again, just for discussion) let’s suppose that the controlling element of that rung happens to be an MOV (Move) instruction which sends a speed command out to the VFD ... in that case using the AFI would prevent the MOV from sending any signal at all ... and in most situations that would leave the VFD running right along at the last speed command it received ... that could be bad ...



now (again just for discussion) let’s talk about the TND (Temporary End) instruction ... in the vast majority of cases, putting that particular instruction into your program will NOT bring things (like pumps) to a stop ... in fact, it will usually have exactly the opposite effect and leave things running in their last controlled condition ... specifically, if the PLC processor is no longer executing part of the program (due to the TND) then that part of the program won’t usually “shut off” things ... on the contrary, things which have already been “turned on” in those parts of the program are usually left running right along ...



finally ... if you’re already fully aware of these “issues” then don’t let me worry you ... but you did start off your first post by saying that you had a “beginner question” ... “beginner questions” are fine ... but I’d just hate to see you make a “beginner mistake” and mess things up by misinterpreting something that has been posted here ... remember, we don’t know anything more about your system than what you’ve told us ... so regardless of how you think the PLC is going to control things at start-up time, it’s ALWAYS a good idea to have someone standing by to “pull the plug” just in case something goes really wrong ...



feel free to ignore all of this if I’ve misunderstood what you’re trying to accomplish ... just remember that MANY beginners are reading over our shoulders as we write these posts ... it’s easy for inexperienced people to think that “one size fits all” when it comes to questions like yours ...



good luck with your project ...

Thanks Ron, I should have been clearer. My original question was meant for the situation where someone shrieks "Turn it off". Everyone hates to be in that position!

In my next reply I was thinking of enabling the program control in steps using the AFI or TND instructions.

Ron, your PID tutorials are great!

With CLX, a trick I like is to use a UDT for all of my outputs, then put a CPS of my UDT to the output card.

During start up, after the CPS command for nomral operation, I put an XIC on a shutdown bit that CPS the proper output state to control the machine for a shutdown state.

If I need to shut it down, I toggle the one bit & I'm putting my outputs to the proper state that I define.

You could start off by making a HMI screen to watch all of your IO directly from the processor. That way you can simulate your program very thouroghly.

Then when testing one of my programs that involve VFD's I simply turn off the 3 phase power and continue to send the outputs to the VFD. That I can watch independantly ont he HMI.

I've used HMI's as debugging tools for quite a while now. It's always good to see visually what is going on and you don't really need to spend that much time on them to test out everything.

Just my 2 cents.

Aren't there hardwired HOA switches for these pumps? A hardwired OFF position would be my first suggestion. Failing that, there's gotta be breakers somewhere. When I am debugging a program, usually I don't want to halt the PLC. How can you troubleshoot a halted program? Sure, you can look at the last state of things, but that's about it. I also don't like putting a lot of extra junk in the program that I'll forget to remove later. That's just me. I would rather put the equipment in a safe state and have the program running. Besides, they are pumps... they turn on, they turn off, and they run at some speed. Not too much to worry about there. Water is going from one well to another.. where's the safety concern? I guess you could run the pumps dry or overflow the wells, but at a WWTP this would typically take longer than it takes for someone to throw a breaker or put a pump in HAND. Good luck!

jedft said:

Aren't there hardwired HOA switches for these pumps? A hardwired OFF position would be my first suggestion. Failing that, there's gotta be breakers somewhere. When I am debugging a program, usually I don't want to halt the PLC. How can you troubleshoot a halted program? Sure, you can look at the last state of things, but that's about it. I also don't like putting a lot of extra junk in the program that I'll forget to remove later. That's just me. I would rather put the equipment in a safe state and have the program running. Besides, they are pumps... they turn on, they turn off, and they run at some speed. Not too much to worry about there. Water is going from one well to another.. where's the safety concern? I guess you could run the pumps dry or overflow the wells, but at a WWTP this would typically take longer than it takes for someone to throw a breaker or put a pump in HAND. Good luck!

Click to expand...

What if you run the logic and the contactor chatters in and out or starts and stops the pump rapidly enough that it burns up?

I would have a nice big e-stop button right there that kills all the power to the IO, JUST IN CASE!

SLC_Integrator said:

What if you run the logic and the contactor chatters in and out or starts and stops the pump rapidly enough that it burns up?

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If the contactors chatter, you turn the hardwired HOA to OFF and fix your program. A VFD is not going to start and stop that rapidly, unless you have NO ramp. If that's the case the problem won't be fixed by the program anyway. Where are your thermals for the pump? They should also be disabling them thus preventing pump burnup. All I'm saying is there should be enough hardwired safety that you shouldn't have to worry what your program is going to do to the system, within reason.