Powerflex Start/Stop logic on Controlnet / Ethernet

Ravioli · Aug 17, 2016

I have read several posts where Ken Roach warns of using not-start, as your stop condition, and how asynchronous IO updates may result in a failed start. The recommendation is to assert a start until the drive is active, and assert a stop until the drive is not active.

I have two issues with this approach:

If the PLC logic commands the drive to start while it is in the middle of ramping to a stop, the drive will first come to a stop, before starting again, rather than accelerating back up from whatever speed it's at.
If the drive is configured for ramp and hold, the active bit stays high when the drive is sitting at DC injection, meaning the drive won't get a start command.

Bad practice

Best practice

My suggestion

My thinking with this method is when the PLC wants to run the drive (CMD_FWD/ CMD_REV), the first scan stops asserting the stop, second scan will enable the forward/reverse bits, and the third scan will enable the start command. Would this be sufficient to keep the race condition from happening? If I moved the start rung below the forward/reverse rungs it should execute it in two scans (drop the stop in the first scan, then send start and direction in the second scan)

Bonus question: Has anyone heard or seen a missed start phenomenon on PF700 on RIO on PLC-5? I'm trying to track down an issue where once every few months the drive doesn't seem to start. Although the RIO outputs are updated synchronously, is it possible that they are asynchronous between the 20-COMM-R and the Drive? I was thinking of trying to change the logic to something similar to the above.

Ravioli · Aug 18, 2016

Something curious I found is Drive Ready operates differently on Powerflex 700 than on PF40 and 525.

On 700 ready looks at start inhibits (parameter 214), which means the drive won't be ready if a stop is asserted. By comparison on 40/525 it will still be ready when stop is asserted.

This would really cause grief if the logic is arranged like the bad practice example.

otw · Aug 31, 2018

Is it possible to start PowerFlex 525 over ethernet just in one rung?

iggy · Aug 31, 2018

This works

otw · Sep 3, 2018

Thanks for help.

I found solution on Rockwell site as well:

https://rockwellautomation.custhelp.com/app/answers/detail/a_id/714666

rupej · Sep 3, 2018

To the OP:

Yep, I was thinking the same thing as you from a recent conversation we had here. I used it on my last project and it works perfectly.

On another note, for the goal of simplification, why even bother with the XIC for the ready bit? If the drive isn't ready it won't start won't work anyways. Or does the drive not automatically restart if being commanded to start and the ready bit comes back?

ASF · Sep 3, 2018

rupej said:
To the OP:

Yep, I was thinking the same thing as you from a recent conversation we had here. I used it on my last project and it works perfectly.

On another note, for the goal of simplification, why even bother with the XIC for the ready bit? If the drive isn't ready it won't start won't work anyways. Or does the drive not automatically restart if being commanded to start and the ready bit comes back?

I'm not sure about that specific question, but to me it's more a question of good practice and ease of troubleshooting. If you're looking at the logic to see why your drive won't start, it's a lot simpler to see that the drive is not being commanded to run because it's waiting for a "drive ready" condition, instead of seeing that your drive is being told to run but for some reason is not. Ultimately any halfway decent programmer will get to the same destination either way, but the easier I can make it for them, the less often my phone rings

rupej · Sep 3, 2018

ASF said:
I'm not sure about that specific question, but to me it's more a question of good practice and ease of troubleshooting. If you're looking at the logic to see why your drive won't start, it's a lot simpler to see that the drive is not being commanded to run because it's waiting for a "drive ready" condition, instead of seeing that your drive is being told to run but for some reason is not. Ultimately any halfway decent programmer will get to the same destination either way, but the easier I can make it for them, the less often my phone rings

Ahhh yeah that makes total sense. Good point. I sometimes forget about putting things in the code just to help the viewer, and may not have a "logic" purpose. Like MOV'ing a number into itself just so it's visible in logic so you don't have to scroll to another routine to see the value.

janner_10 · Sep 4, 2018

rupej said:
Ahhh yeah that makes total sense. Good point. I sometimes forget about putting things in the code just to help the viewer, and may not have a "logic" purpose. Like MOV'ing a number into itself just so it's visible in logic so you don't have to scroll to another routine to see the value.

Or put it as an alarm on the HMI, then nobody has to drag the laptop around and go-online. KISS.

otw · Sep 4, 2018

Notice that parameter A574 has to be changed to achive proper behavior of "Ready" bit. According to the manual instruction of Powerflex 525 if parameter A574 is set to 1 the inverter's input "Ready" is off when the inverter receives "Stop" command over Ethernet.

rdrast · Sep 5, 2018

rupej said:
To the OP:

Yep, I was thinking the same thing as you from a recent conversation we had here. I used it on my last project and it works perfectly.

On another note, for the goal of simplification, why even bother with the XIC for the ready bit? If the drive isn't ready it won't start won't work anyways. Or does the drive not automatically restart if being commanded to start and the ready bit comes back?

Another issue with ignoring the ready status, is that while the drive won't start, it will also ignore other commands like fault reset, and will be stuck in nowhere land.

ASF · Sep 5, 2018

janner_10 said:
Or put it as an alarm on the HMI, then nobody has to drag the laptop around and go-online. KISS.

Obviously, I do that as well. But the amount of phone calls I've had that go something like this:

Operator: The motor won't run!
Me: are there any alarms on the SCADA that look like they might be related?
Op: No!
Me: *logs in remotely*
Me: OK, I've got a drive "not ready" status. Are you sure there are no alarms?
Op: Yes!
Me: Press the motor graphic on the SCADA and read out the text at the top of the popup that appears
Op: It all looks normal, it just says "Motor Such-and-Such"
Me: Yes...that's the window header...what does it say just below that?
Op: ...um...Not Ready - Safety Circuit Not Reset?
Me: Okay. So. Do you maybe have a guard open?
Op: ...ah yes, there is, let me close that
Me: Cool, cool. Can you also just double check the alarms screen for me to see if there's an alarm that says so?
Op: Oh yes. Motor Such-and-Such Guard Open
Me: ...great. So when I asked you to look for relevant alarms a moment ago, did you actually read any of the alarms on the alarm screen, or did you just open it up and say "yes. This is an alarms screen."?

Keep It Simple Stupid is 100% my mantra, but sometimes when the Stupid is particularly talented, something slightly more than Simple can be helpful

willxfmr · Sep 5, 2018

ASF said:
Obviously, I do that as well. But the amount of phone calls I've had that go something like this:

Operator: The motor won't run!
Me: are there any alarms on the SCADA that look like they might be related?
Op: No!
Me: *logs in remotely*
Me: OK, I've got a drive "not ready" status. Are you sure there are no alarms?
Op: Yes!
Me: Press the motor graphic on the SCADA and read out the text at the top of the popup that appears
Op: It all looks normal, it just says "Motor Such-and-Such"
Me: Yes...that's the window header...what does it say just below that?
Op: ...um...Not Ready - Safety Circuit Not Reset?
Me: Okay. So. Do you maybe have a guard open?
Op: ...ah yes, there is, let me close that
Me: Cool, cool. Can you also just double check the alarms screen for me to see if there's an alarm that says so?
Op: Oh yes. Motor Such-and-Such Guard Open
Me: ...great. So when I asked you to look for relevant alarms a moment ago, did you actually read any of the alarms on the alarm screen, or did you just open it up and say "yes. This is an alarms screen."?

Keep It Simple Stupid is 100% my mantra, but sometimes when the Stupid is particularly talented, something slightly more than Simple can be helpful

How is it we work at the same place but have never run into each other?

Bubba.

Powerflex Start/Stop logic on Controlnet / Ethernet

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