How to Measure Analog Signal Rate of Change

[dj143]

I have an application where the end user needs to observe the rate of change in an analog signal.  In this case the signal is 4-20 mA.  A preliminary attempt to do this compares the signal between iterations within a timed while loop.  However, the result is very erratic and unpredictable.

 

I’m monitoring the 4-20 mA signal with a cFP-100, which was chosen for its relatively fast update rate.  The processor is the cFP-2110 for an RT application.  I have a host PC connected to the target via Ethernet with shared variables and I’m using LV v.8.5.

 

Is there a LabVIEW function that is intended for this type of signal processing or can any of you recommend a method to measure and display the rate of change in an analog signal?  For this application the expected rate of change to be monitored is between 0 and 1.6 mA/Sec.

Thanks,

 

Dave J.

[Norbert_B]

Dave,

there is no function in LV available which already solves your needs. There are some issues you have to keep in mind while designing an algorithm for your needs:

a) Dataacquisition (DAQ) always includes errors. You will never measure the real value. Most common errorsource is the resolution of the DAQ-device. But there are others as well: Thermal noise, impedance-mismatches,  "general" noise, ...

b) Using just two measured values will statistically create huge errors because of a). So you have to use more than two samples and think about something like averaging.

c) There are functions like curve-fitting in LV available. Maybe they can be used to get the desired value.

hope this helps,

Norbert