How to locate the (physical) z index of encoder

Question

Hello,
I am trying to locate the Z-Index of a rotary encoder. I need the counter to reset to 0 everytime the encoder's shaft reaches that same point.
My problem is that when I run the VI, the angular position starts at 0 no matter where the shaft is located, and this point is used to reset the counter to 0 once a 360 degree revolution occurs. Hence, everytime I run the VI, the current shaft position is used as the Z-Index.
Is it possible to locate the actual physical Z-Index of the encoder which is at a specific location of the shaft, so that everytime the shaft reaches that point the counter is reset to 0 and this specific point becomes the Z-Index everytime I run the VI (and not be at the location of the shaft at the instant the VI is run)?
Basically, I am trying to accomplish the homing technique to be at a fixed location of the encoder's shaft and not at arbitrary locations which are at the location of the shaft when the VI is run, which may be different everytime I run the VI.
Thank you,
r15
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Answer 1

One way out might indeed be to use absolute encoders, but it isn't yet clear to me that it's the only way or even the best way.  So let's make sure we first cover the basics very clearly.
If your encoder has a z-index and your task is configured to use it properly, the z-index pulse will cause a reset at a specific shaft angle.  The very first such reset will usually occur in less than 360 degrees of motion, depending how far away you started from.  The remaining resets will always be at the same physical angle, thus every 360 degrees thereafter.
If you want to perform z-index reset with an encoder task, it's important that the z-index *signal* is coming from the quad encoder.  The task config depends on a z-index signal that is sync'ed to the A & B quad signals and requires you to define which 1 of the 4 possible A quad B states should be used for the reset.  This helps NI support both full-cycle and quarter-cycle gated index pulses.  (Full-cycle means the index pulse lasts one full cycle of the A channel, quarter-cycle means that it lasts for one A quad B state, i.e. 1/4 of a full cycle.)
A z-index signal that isn't sync'ed to the encoder may cause multiple resets per pulse or none at all depending on how & whether the pulse overlaps the A & B quad state changes.
Having said all that, the symptoms you describe sound more software-ish.  If you aren't explicitly doing a modulus function, I would start to wonder whether something about the task config is doing something similar behind the scenes.  I wasn't aware that DAQmx would or could do that, but I haven't done much encoder work since before X-series existed and that was many DAQmx versions ago.  Perhaps it now supports modes for non-cumulative angular position?
-Kevin P