PXI 6224 linear position encoder resolution question

[JSInc]

Hello,

I am using Quadrature Encoder connected to the counter 0 of NI PXI 6224. As shown in the picture, the decoder is working at X4 mode, I set Distance per Pulse as 2. The measured waveform looks like the resolution is 0.5.

My question is:

                         Isn't the measurement resolution equal to Distance per Pulse?

Thanks,

JSI

[Kevin_Price]

It's a case of somewhat ambiguous terminology.  I've never been real happy with their choice of the term "pulse" for configuring quadrature encoders -- I would prefer a term like "cycle."

The value you wire in should be the distance represented by 1 full quadrature cycle.  (Since this will be the distance from rising edge to rising edge on one of the quadrature signals, you can kinda see how someone decided to refer to that span as a "pulse.")   If you measure in X4 mode, you'll produce 4 increments of measurement per cycle.  That's why you see the 0.5 quantization when wiring a value of 2. 

Generally, when I find such ambiguities in meanings of inputs, I run little test cases to help me figure out how things really work.  I've found this to be more reliable than counting on my memory. 

-Kevin P.

[JSInc]

Kevin,

Thank you for the comment.

I feel confused because if I create an X4  linear position task in Measurement and Automation Explorer, the distance/pulse will be interpreted to 1 increments as shown in attached picture.

[Kevin_Price]

Hmm, that's a new one on me, as I don't use MAX to define global tasks.  I have used it a couple times to define a global channel on a rotary quadrature encoder and I'm pretty sure it worked as I described it.  I put in the value corresponding to cycles per rev, and if I picked X4 decode mode I'd get an actual resolution of 1/4 cycle.

Maybe there's an inconsistency in either:

A. global task vs. global channel

B. rotary vs. linear encoder

C. MAX vs. DAQmx calls

Again, regardless of how it should behave I'd advise that you run test cases, make sure the scaling gives consistent and predictable results, then live with how it actually behaves.

-Kevin P.