Voltage Measurement using PCI-MIO-16E4 and CB-68LP

[venkatgill]

Hello,

I want to measure voltage from a single phase power supply panel using PCI-MIO-16E4 DAQ card and I am using LabView 7.1, DAQMx s/wres. I am using the transducer VT7-010D for scaling of input, as our DAQ cannot take high voltages. I am not sure if I can use this transducer as it says input 0-600V Dc and output 0-10V Dc, but I am using it and applying AC voltage from 0-100V.

Connection is from Power Panel to transducer and then from transducer to CB-68LP and from there to DAQ card inside PC. I use multimeter and it says right at i/p side and o/p shows for 10V i/p ac = 0.1V o/p dc initially and from 50V i/p shows 0.6Vdc and continues increasing. But Labview shows 0.1V = 0.5V (p-p) instead. I would like to seek help from experts in measuring this simple task. I am using inbuilt VI to do this task.

Thanks and appreciate any help

[venkatgill]

Please can someone have a look into my issue and suggest me some help

[sergesl]

Hello,

I am not sure I understand your problem. You are measuring .1V DC voltage using the DMM, but LabVIEW says it is .5V peak to peak?
Is the transducer outputing DC or AC? LabVIEW will show the waveform that the transducer sends to the SCB-68. The DMM on the other hand either gives you RMS value of the AC voltage if you have it set to measure AC or give you the DC component of your waveform if you have it set to DC.
Hope this helps..

[venkatgill]

Hello Sergesl,

Thanks for your reply. Now I understood that I am getting the signal which is rms value of the voltage from the transducer. Now I am getting another problem i.e the signal is getting distorted after a couople of seconds. I am attaching  the distorted signal. I verified the measurements from all the channeld of the DAQ but its same and I feel the problem with sampling rate. Please look into this issue and suggest me the possible solution.

Thanks

[johnsold]

It looks as though your signal is going off scale. It certainly goes off the graph. If the signal exceeds the ratings of any device in the chain (transducer, DAQ card, etc.), the result will be bad data and possible damage to the devices.

That sort of waveform can result from rapid switching of currents through an inductor. Does your system switch inductive current or is the spike coming in externally? Is the spike part of what you are trying to measure or is it something you want to eliminate from your data?

Is your transducer isolated from ground? Ground currents, sometimes called ground loops, can produce erroneous measurments and can be dangerous to equipment and personnel.

Lynn

[venkatgill]

Hello Johnsold,

Thanks for your instant response, I was checking and testing all your suggestions. The DAQ card I have is PCI-MIO-16E4 and it can handle AC siganls upto 10 volts and I never went more than 4.5V (p-p) to the device. I want to eliminate the spike from the plot which is unusual and for no reason it appears. All I am doing is measure the voltage from the supply panel using VARIAC (0-120V) connected to Transducer VT7-010D.

This transducer measure  high voltages in the range of -500/+500 volts and produce an output signal propotional to the input. The output  range is -10/+10 Volts and is connected to channnel 6 of CB-68LP which therefore connected to our DAQ which can handle between (-10/+10V). Everything is grounded well in the lab and no inductor is present in the process.

Will this appear beacuse of buffer problems? I take sampling rate @10000 HZ and Samples to read @1000. Probably the buffer might get filled because of this high rates. Check my 'vi' file attached.

Can you suggest me how to see the peak-peak value using a numeric indicator on the front panel which i was unable to do from block diagram.

Thanks

[johnsold]

I doubt that buffer overflow would synchronize with the signal frequency. A buffer error would produce an error message through the error handler.

The peak to peak voltage could be obtained by extracting the voltage values array from the waveform and running it to the Array Min Max function. Peak to peak is then Maximum - Minimum. Any noise and those big spikes will be included in the peak to peak measurement.

Lynn

[venkatgill]

Thanks Lynn, the problem would be because of some triggering timings too which I am unaware of as I am not getting any error signal of buffer overflow. Since the signal initially is good and correct but after a couple of minutes it gets distorted. I could now measure P-P value using Amplitude and Level Measurement block.

I want now to Fourier analysis to this signal and observe the harmonics (especially 3rd harmonic)  which would finish my experiment. Any help is appreciated.

Thanks

[sergesl]

I would agree with Lynn that this is not a buffer error. Your VI would not run if you had a buffer overflow.  For your frequency domain analysis there are express VIs on the express palette for FTT. That will give you the harmonics if you drop the FFT express VI and select power spectrum.

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@venkatgill wrote:
Thanks Lynn, the problem would be because of some triggering timings too which I am unaware of as I am not getting any error signal of buffer overflow. Since the signal initially is good and correct but after a couple of minutes it gets distorted. I could now measure P-P value using Amplitude and Level Measurement block.

I want now to Fourier analysis to this signal and observe the harmonics (especially 3rd harmonic)  which would finish my experiment. Any help is appreciated.

Thanks

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