question about integral x(t) PtbyPT

[bbm12345]

when i use this function .the integral x(t) is decreasing.

the smapling rate is 10000. dt = 0.0001

input signal is +- 2.5v sinewave

Please tell me what is the problem.

[johnsold]

I cannot be sure, but in general a decreasing output from a integral function indicates a non-zero constant term at the input. Is it possible that your input signal has a few millivolts of DC bias? To test this remove the DAQ stuff and put a sine wave signal generator (simulated) in the loop and see if you get the same result.

Also I noticed that your data does not start at a zero crossing, so this introduces a DC bias until a large number of cycles have passed.

Lynn

Message Edited by johnsold on 11-05-2007 04:17 PM

[bbm12345]

when I use simulate signal as input in Integral x(t) PtByPt the integral x(t) is wrong ( = constant ) but in Integral x(t) is right.

Simulate Signal:

f = 10 Hz, Amp = 1, rate = 1000 sinewave

what different between PtByPt the integral x(t) and Integral x(t) ???

<<Also I noticed that your data does not start at a zero crossing, so this introduces a DC bias until a large number of cycles have passed

actually, the input is a contiunous signal, it should start at zero.

[johnsold]

The PtbyPt VIs have an uninitialized shift register internally to accumulate point until they have an array of dat to feed to the standard array-input VI. The diagram you show only uses one point (probably the first) of the signal output from the simulator.

Extract the array from the simulated signal and feed it by autoindexing to the PtbyPt integral VI inside a loop.

Lynn

[bbm12345]

this is the output after add loop but it still wrong

[johnsold]

The Simulate Signal needs to be outside the loop so that you feed the points in one by one. These are fundamental LV operations. If you have not done so, I recommend you look at the tutorials for LV that NI has available on line or through the help menu.

Lynn

[bbm12345]

I think what you used is Integral x(t) not Integral x(t) PtByPt.

I tried both of them in same loop but the results are different

.

where can I find the tutorials on line ?

can you give me the link ?

[johnsold]

The icons are different in my version of LV, but I did use the PtbyPt Integral.vi. The key is to move the conversion to array outside the loop and to use autoindexing at the tunnel where the wire enters the loop.

Getting Started with LabVIEW: http://www.ni.com/swf/demos/us/labview/getting_started_lv/

Lynn