How to set up parallel SoftMotion axes on a single FPGA?

JKSH · ‎07-10-2013

I'm trying to drive 5 independent stepper motors through 1 CompactRIO controller, but am encountering strange behaviours (I think it might be a timing issue).

This is my hardware:

Controller: cRIO-9081 (scan period = 5ms)
Modules for reading encoders: NI 9411
Modules for driving steppers: NI 9474
Modules for limit detection: NI 9421

Here's what I've tried so far.

For software, I took the code from official examples (LabVIEW 2012\examples\Motion\UnboundAxis\Stepper Interface (9401)), and cloned them for each axis.

In the FPGA software, I simply cloned the various loops and hooked them to the same synchronization loop:

In the RT software, I made all the axis interface VIs and subVIs reentrant (preallocated clone), and ran 5 axis interface VIs in parallel:

The two big issues I've noticed so far are:
1) I can't start all 5 copies of the axis interface VI simultaneously; if an axis is started within 300ms of another, its Loop Timer VI in the Position Loop (FPGA) will say "Fault Occurred?" == TRUE, and "Scan Running?" == FALSE indefinitely. I worked around this by staggering their start times:

2) If I use the asynchronous SoftMotion Express VIs, there are times when the Express VIs' outputs don't appear when they should. E.g. I call Stop Move while a Straight Line Move is in progress. The Straight Line Move VI is supposed to produce an "aborted" signal, but sometimes it doesn't. (http://forums.ni.com/t5/Motion-Control-and-Motor-Drives/Do-all-of-SoftMotion-s-express-VIs-produce-a...) This is a reproducible behaviour. I can't think of any good workarounds, except to use redundant checking so as to not rely on a single signal only to drive logic. This is obviously not ideal.

I guess my questions are: