Problem with linear Position measurment( Counter quadrature signal)

[laiiiiii]

Hi guys,

I am trying to sample the  AquadB (Quadrature) singal from a interferometer to measure the linear position of an objective.  The clock and the resolution of this quafrature signal could be adjusted through the software of the interferometer like in the picture that attached below.  

Through the DAQ card PCI6221, i use the counter with X4 encoder to sample the signal form interferometer continuously. And the shape of the signal that i acquired through the NI card is the same as the signal form the interferometer's software. But the problem now is the amplitude of the singal that i got from the NI card is not the same like as the software.  I think this is because the parameter of "puls distance" in the counter that i setted.  I just used the same number as the resolution of signal from quadrature signal interface. Does anyone can help me to understand the relationship between the resolution of quadrature signal from interferometer and the puls distance in the setting of counter?   Thanks a lot!!!!!!!

In addition:

-the minimal period of the quadrature signal is 4 times the clock in the software.(please see the picture in attachment).  

- the clock that i used in the software is 100ns

- sample rate of the counter 100kHz , number of samples is 10000

sincerely

laiiiiiiiii

[Kevin_Price]

When reading mfgr specs, there can be some ambiguity about encoder resolution due to a little fuzziness in the common terminology.  It generally comes down to a possible factor of 4, depending on whether they are talking about *cycles* of *each* of the A,B quadrature signals, or whether they are talking about post-decode *total transitions* of *both* A,B signals combined.  

Unfortunately, NI has brought a similar fuzziness into its own terminology for configuring position measurement.  They use the term "pulses per rev".  Until you dig down into the advanced help, it wouldn't be clear that they mean the # *cycles per rev* for A or B individually rather than the available quadrature *counts per rev* that people are usually thinking about.  

   In my opinion, they should have called these "cycles per rev" to be more clear.  Part of the industry includes external quadrature decoder circuits that output 4x quadrature *pulses* and a direction bit.  So the term "pulse" has a common but different meaning in the encoder industry.  Searching the forums a bit brings up a similar complaint by me in the early days of DAQmx about 10 years ago.

Anyway, I advocate a little experimentation to see whether the scaling is off by a factor of 4.  If so, it's very likely related to this ambiguity in resolution specs.

BTW, I'd recommend dialing down on the interferometer resolution settings while troubleshooting.  The screenshot shows you set for 1 pm, but most such systems will have measurement uncertainty that's many orders of magnitude greater.  Such resolution specs are often fool's gold.

-Kevin P