weird output behavior in RSLogix5000

[tripero01]

Hey!

I am new in this forum and in the manufacturing world. I am interning for a manufacturing company and getting my hands on their machines for the first time. Upon fixing some issues on a rollformer machine I started reviewing an aluminum oven machine that runs coils through it with the help of six conveyors that run simultaneously.

The PLC in this machine is a compact logix L43 and when I get online I only see two of the six conveyors outputs lit up when all six should be high since they are all running as I am writing this.

I am guessing this post might lack information that you guys need but I was just wondering if it is possible to see weird I/O behavior when going online with Logix.

Any info and tips will be highly appreciated!

thanks

Ramiro

 

[tripero01]

forgot to also attach this one

 

[maxketcham]

I would check to see if the drives were in manual and running from keypad, that's what it looks like, since logic is false

 

[Ken Roach]

Welcome to the Forum !

I assume you're talking about the Conveyor_A, Conveyor_B, etc that are being run by PowerFlex 40 AC drives.

This is normal behavior, but it can be confusing if you're not used to it.

Allen-Bradley AC drives typically can be started and stopped in either "2-wire" or "3-wire" modes, which are meant to emulate the way common hardwired motor start circuits work.

2-wire mode is what you might expect with a single discrete output. Whenever the output is ON, the drive runs. Whenever the output is OFF, the drive stops.

3-wire mode is what you might do with two different buttons. The Start button is momentary, and the Stop button is also momentary but is wired normally closed.

When you press the Start button in 3-wire mode, the motor starts. When you release the Start button, the motor keeps running. When you press the Stop button, the motor stops.

 

[Ken Roach]

Allen-Bradley AC drives when commanded from a network module work similar to 3-wire mode.

When the Start bit goes True, the drive Starts. The drive keeps running even if the Start bit goes false.

When the Stop bit goes True, the drive Stops.

As long as the Stop bit is True, the drive remains Stopped and cannot be started.

 

[maxketcham]

I know that, but if you look at the logic it is holding the "start" output on until a stop is given. I could be wrong, but assume a stop has been given the conveyor B and have been told it is still running. I have been to plants where the drive comms would fault or miss a stop or start, etc, and rather than reset the system, the operators would put the drive into manual and run from keypad. Ergo, check the drives first then look at PLC if there is a problem. Saved me a lot of grief in the past because there would be no indication at PLC that this had happened.

 

[Ken Roach]

And one last thing before I go too far on this: Be careful when programming your Stop bit.

I can't tell for sure if your program does this, but I've seen lots of programs that try to treat the drive like a 2-wire device, where the Start bit is maintained True, and the Stop bit is made true whenever the Start bit is false.

This is logically sound, and it might work on a hardwired circuit, but it doesn't always work when you're using drives on a network with ControlLogix.

This is because networked I/O in ControlLogix is asynchronous to the program scan. The network Requested Packet Interval (RPI) timer for any I/O connection, including a drive, can time out at any point during a logic routine, including in the middle of the rung.

In the common case where the Start command is programmed above the Stop command in a relay ladder diagram, if the Start is made true but the RPI expires before the Stop is made false, both the Start and Stop bits go out over the wire as true. The Stop bit overrides the Start bit, so the practical effect is that the drive 'fails to start when commanded'. The more frequently your drive starts and stops, the more likely this is to occur.

 

[tripero01]

wow thanks for the responses and interest! im really glad I opted to ask in this forum.

what I have to do is make sure that when an E-stop switch in the machine is pressed it will stop all six conveyors in manual mode. it is my understanding that auto mode is never used and thus irrelevant in this situation.

When this button is pressed there is a variable called emergency_stop that latches. what I was thinking was to put an XIC instruction so that whenever that variable was 'high', it would activate the rung that has the conveyor_X_stop coil. Also I was planning to use this emergency_stop variable as an XIO to break the conveyor_X_start output. Am I on the right track?

I am sorry if what I said makes no sense to you and contradicts at some point to what you guys explained me. As I stated before I am new in this and I tried to follow what you just explained up to my level of knowledge.

thanks!

 

[Ken Roach]

Let's take one short step back: you should never use a general-purpose programmable device as an 'Emergency Stop'. This includes any non-Safety-rated programmable controller.

If there is a red mushroom-head button with a yellow background and it's labeled 'Emergency Stop', you absolutely have to wire it directly to the devices it disables. For variable frequency drives, the minimum you should consider is wiring this circuit directly to the Enable terminal.

If this is really just a 'stop all in manual mode' button, just don't call it an 'Emergency Stop'.

To stop all of the conveyor drives when in Manual Mode, you have the right idea: interrupt the Start command, and assert the Stop command.

 

[tripero01]

All figured out; you were right. No one in here is familiarized with PLC so I had no reference. I will trace the outputs for the conveyors and wire those to the e-stop button so that they get forced to 0 when the switch is pressed.

Thank you so much for your tips, you guys have been great help

Ramiro