Will compactlogix L3x support a virtual axis

Hello,

I'm working on an application where, as usual, the customer is trying to cut costs with hardware. I'm stuck with a CompactLogix L3X controller communicating to a couple of VFDs for motion control. I see that the L3X doesn't outwardly support motion control (for that you need the L4X). However, if I want to program a virtual axis and control the drives indirectly, will it work? The only evidence I found that it will is from some release notes for version 17 indicating that some anomaly has been fixed if you use a virtual axis with an L3X processor. This sounds good, but I can't find anything else that positively states it can be done or is recommended. Any thoughts?

Thanks,
DT

I wonder if that was a misprint in the release notes. As far as I know, the L3x controllers don't have a motion task at all.

All the motion control I've done with L3x and L2x controllers has been with standalone single-axis controllers like the Ultra 3000i or PowerFlex 40P. The CompactLogix calculates index targets, but all the position and velocity loop work is done onboard the drives.

Thanks for the response Ken. That's exactly what I'm afraid of (programming the virtual axis only to discover it doesn't work). I have always done it the way you suggest, letting the drives worry about the velocity and positioning loops. The drives being used are Powerflex 700VC drives and they have a whole positioning matrix built in - I could just send the values over Enet. The customer has gone one step further however, and used Devicenet absolute encoders that are networked to the PLC so I can't close the loop to the drive (properly, anyway). I've advised them otherwise but they're still sticking to the original plan. They want a low tech "robot". I guess I'm going to have to program it that way.

low tech doesn't always equal low cost does it? We have all been down that road once or twice.

This is not going to turn out well

A low tech robot? I bet the expectations are low performance. Interfacing the encoders over Ethernet is a mistake for motion control.

This will not turn out well.

Yes, this is a very slow speed application with point to point "positioning". The encoders are actually interfaced over Devicenet not ethernet. If they connected the encoders to the drives directly, then I would just send the position parameters over enet to the drives and let them do the work (and tell the PLC when they're done, faulted, etc.). I believe what they're looking to do, and what I'll probably end up doing, is giving the drive a speed reference and ramping down as it gets to its position (monitoring the encoder connected to the PLC). Definitely not positioning, but as it is really slow speed (a few inches per second), it will probably work OK. I was just hoping to be able to do it properly. At this point they're still not open to upgrading even a few bits of hardware to make that possible. I'll see how it goes and maybe they'll have no choice (or I can convince them otherwise).

plccontrols said:

I'm stuck with a CompactLogix L3X controller communicating to a couple of VFDs for motion control. I see that the L3X doesn't outwardly support motion control (for that you need the L4X).

Click to expand...

Maybe, this is what your´re looking for:

http://samplecode.rockwellautomatio...ers=mms&&PageNumber=6&StartRow=126&EndRow=140

Converting movement of Virtual Axis in real movement over PowerFlex Drives without any motion module. It can be used with a L3X processor.

I expect to see a fair bit of hunting from those devicenet encoders. Just as skeptical as Peter.