Why do I get signals ghosts with the meter?

Hi, I am able period using the NI 9401 on a cDAQ9174 chassis.  I have a sensor whose output is 50mV LO and HI 4500mV, and this is easily measured with a scope and a DMM.  Furthermore, the scope shows very clean transition, no noise near the 2, 5V TTL threshold.  The probe monitors pattern reflecting on a spinning wheel.  Measures the time of the order of less than 0.3 seconds, but, curiously, the measurement period seems to be good for a few cycles, and then suddenly these values close to ZERO are observed. (Excel data show these values literally '0'.)  What is c?  Where are these unwanted readings from?

Also, see the code that I use to record data from the period, and a typical example of the data plotted in Excel.  One thing I'm confused about is the timer in the WHILE loop.  How the meter may return a value that is smaller than the function parameter WAITING in the WHILE loop?  The WAIT function is set to 100ms, but the meter will return to values far less than 50ms.

Is there a way to remove these values in the software?  For example, can I just throw any value less than 0.050 seconds?  This kind of fix would be perfect for our application; It's a little gross, but she would get me to the next step in our project.

I really appreciate your comments!  This photointerrupter is supposed to have the advantage of being a camera and free of noise in the environment, up to now, the only noise in our environment is the small DC motor spin our wheel on our lab bench... not a harsh environment at all.

Dave

Hi Dave,.

That's what I think is happening here.  In the configuration of the measurement of a meter task period DAQmx, the source of the signal is connected to the door of the meter and the internal time base is used as a source.  When the input signal is HI at the entrance to the Terminal, a count is saved for each rising edge of the timbase to the source of the Terminal.  When the signal of door to LO County is drawn from the register in the buffer and legumes subsequent source are ignored until the door is HI again.

Now assume that the signal of your door is swinging very quickly prior to staying at the State HI or LO.  Whenever a HI LO oscillation occurred, a count would be drawn from the register of the meter.  If this rise and fall of the door happened quickly enough, it is quite possible that very few or none of the timebase impulses were actually counted to the source terminal, thus recording a very low double value in a buffer of the computer.  When these sets of values has been extracted from the buffer and written into the worksheet, accuracy of scripture to the worksheet VI may have been set to the default 3 decimal, so write "0" s to the file.

The speed of the swing may be the order of 10 to 100 ns, fast enough at your fingertips, do not register.  The quote that I copied the help file describes the behavior with the digital filter enabled.  When you apply the digital filter, a county is not from the register unless the signal of the door remains HI for a fixed minimum amount of time.  With the filter, the period 0 counts are not sent to the buffer because the door is not HI for the minimum period of time.

Brian

Tags: NI Hardware

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