Phase shift-urgent

[blackbear]

Dear all

I am beginner and im using DAQ assistant to compute my 2 incoming signal. I was wondering how can do a phase shift comparsion between them?I tried using the frequecy response function (mag-phase) model unfortunately it does not work.Can send 1 pls guide me?

cheers
Shawn

[johnsold]

Shawn,

More specific information is needed to be of much help. What is the exact problem? Does the DAQ work? Do both channels of data look as you would expect them to look?

What kinds of waveforms do you have? How noisy are the signals? How stable are the frequencies? How fast can the phase shift? What is the range of phase shifts you want to measure? What are you going to do with the result - display for the user, save to a file, control physical system?

What did not work about the FRF? Did it generate an error? If so, what was the error code? Were the results off just a little from what you expect or were they way off?

Can you verify independently that your signal is what you expect it to be?

Lynn

[blackbear]

Dear Lynn,

thankyou so much for reply.I managed to see see both of my waveform using a simple DAQ to the waveform graphs. Both signal are expected from my simulated result via an amplifed to the SCB 68. My waveform is off transient type (fault)

For analysis of both waveform i would wanna see; the phase difference between them (mabe the start point of both,and between cycles)
i would also like to see the amplitude of the waveforms. Im trying to do as what our normal laboratory oscillscope would do,such as using the curzor (2 lines) between 2 waveform to see their phase shirt and the amplite of the waveform.

I ran the model it seens like they say my FRF could not pass through my phase and magnitude graphs. Would it be easier if i could send u my model?


Message Edited by blackbear on 07-16-2008 02:42 PM

[johnsold]

Please post what you have tried. Most people reading these posts will not have your hardware so run your system and save the data as default so we can test with your data without hardware.

Lynn

[blackbear]

Dear Lynn,

could you guide me as in what kind of details u need to help me resolve my problems??and what do u mean by saving as default?do u mean i attached my file in this forum?a bit newbie here.hehe

cheers
Shawn


Message Edited by blackbear on 07-16-2008 04:16 PM

[blackbear]

Dear lynn,

i just ran the program in my lab and the errorrs if i were to put a FFT model to see my magnitude and phsae response is :
"Measurements:Attempted to read samples that are no longer available.The requested sample was previously available, but has since been overwritten. Increasing the buffer size,reading the data more frequently or specifiying a fixed number of samples to read instead of reading all available samples might correct the problem

and if i were to use FRF model as well, it has the same error msg.Could you advise me.thankyou.

Cheers
Shawn

[johnsold]

Shawn,

That error is a data acquisition error. So something in your data acquisition is not working right. Either you are not reading the data fast enough or the buffer is too small. Without details about how your acquisition is set up, it would be guesswork to try to fix it.

It is meaningless to pursue analysis of your data until you are getting reliable data.

To attach a VI click the Choose File button below the Submit post button. Do not do Preview Post after attaching a file or the attachment gets removed.

Lynn

[blackbear]

Dear Lynn,

i managed to solve the problem by increasing the size of my buffer at my DAQ.Unfortunatey I still cant solve the phase shifting problem.

I try using a 2 signal generator with 1 set at 0 degree and the other set at 60 degree and using FFT or FRF model, the graphs doesnt seen to be rite.need your advise.I have attached my model.hope u can advise me.thankyou.

Cheers
Shawn

[johnsold]

Shawn,

I think your data are OK. Voltages clearly shows the phase difference. Waveform Graph shows that both signals have spectral peaks at 50 Hz. Waveform Graph 4 shows that the phase of signal 1 is -1.57 = pi/2 and the phase of signal 2 is -.52 = pi/6 at 50 Hz. Expand the X-axis to show 50 Hz +/- 10 Hz and you can see this. The phase relationship is meaningless at any frequency where there is no energy. Change the Y-axis mapping on the Waveform Graph (FFT Magnitude) to logarithmic and set the lower axis value to 1E-18. You will see that the spectral energy is not zero, mostly due to finite calculation effects, I suspect. But the variations are enough to produce what appears to be random phase.

Lynn

[blackbear]

Dear Lynn,

thankyou so much for your patient.Can i ask you how u accurately determind the vaules signal 1 is -1.57 = pi/2 and the phase of signal 2 is -.52 = pi/6 at 50 Hz ampltude by reading the graphs?Also how do i change my waveform into Log format?

cheers
Shawn


Message Edited by blackbear on 07-17-2008 09:31 AM