voltage measurements

[tmdar]

Hi,

I am trying to measure the voltage across my resistor which is in series with a dc motor -- I am using the vi attached but problem is the votage is jumping all over the place like if my multimeter is telling me voltage to be 0.25V but lab view code shows ranging from 0 to 1V and does not stablize. What is wrong ?

or what all can be wrong. I have also checked  on automation and explorer  and same problem persists there too.

[johnsold]

Multimeters usually average the voltage to be measured over a time of about one second. Your data acquisition board acquires a sample that is approximately instantaneous (by comparison, anyway). Thus you see fluctuations on the computer that do not show up on the meter. If you take all the samples measured for one second and average them, your reading will likely be much closer to the multimeter reading.

Other things to consider are ground loops and whether the instantaneous voltage spikes on the motor current line can be high enough to exceed the rating of the DAQ device.

Lynn

[tmdar]

Hi,

what modification can I do in my code--- I dont want to place a clock in the code but are you suggesting to get no of samples and specify sample rate or you want me to gather data and the see what is the avg of that data.

Thnaks

[tbob]

The first thing I would do is to put a scope on the resistor lead to look at what is there.  Is it a DC voltage with ripple in small magnitude or is it some kind of waveform with large peak to peak amplitude?  Then you decide what you want to do.  Do you want an average of the peak to peak amplitude?  Do you want a list of peak voltages?  This will help you decide how to procede.  You could sample the voltage with a DAQ every millisecond or so (depends on the waveform) and take an average of all data, and you could display the actual waveform in a graph.  Motors are notorious for causing spikes and ripple on a DC voltage line.  Look at the signal on a scope and tell us what you would like to do in more detail.

[tmdar]

Hi,

what I look at the scope is kind of spikes and ramping effect togather but not particular waveform mostly ripples one can say. see my shots attached

[tmdar]

here are more shots attached

[johnsold]

As tbob said you must decide what you want to do. Your multimeter reads a fairly steady voltage but your scope photos show that the voltage is changing significantly in a few microseconds. If you want to follow the microsecond details you will need a fast A/D converter board. If you want something like the multimeter reading, you should filter the signal before it gets to your DAQ device to avoid aliasing problems.

Let us know what you are going to do with the data and we can suggest an approach.

Lynn

[tmdar]

Hi,

I want the multimeter like readings-- so what I need to do -- how to filter the signal before it goes into DAQ

Thanks

[tbob]

You could sample the signal with your DAQ at a high rate (experiment with it), and then in Labview run the captured data signal through a low pass filter, found in the functions palette - Analyze - Waveform Conditioning - Filter (express vi).  Set the filter for Low Pass, and set the cutoff frequency to 10 Hz.  That will take all the ripple out and give you a DC signal.  Then you can extract the data points from the signal and average them by adding and then dividing by the number of data points, or if the data points are all very similar you can use any one point as your DC value.  There is also a function called Basic Averaged DC-RMS found in Analyzed - Waveform Measurements, but this is not included in the Base package for Labview.  This function will give the DC component of your signal, similar to the DMM.

Another alternative is to use a hardware low pass filter (resistor and capacitor) between the signal line and the DAQ input.  Then you can just capture one value, or capture several values and average them.  This is pretty much what the DMM does.

[tmdar]

Hi ,

I am trying with other suggestions but I was not able to understand your  statement

"You could sample the signal with your DAQ at a high rate (experiment with it)"

How can I do that how to increase the rate.

Thnaks for the suggestions