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unstable voltage reading?

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unstable voltage reading?

Postby aseman » Mon Mar 06, 2006 12:55 pm

Hi, I am having some problems of transfering data from picoscope to microsoft excel.
1. Is there anyway to write a macro in excel to transfer the reading from the 'meter' window in picoscope?
The values from the 'scope' is sinusoidal, and if I import it to excel and try to average it, i will never get the same result to the meter. I need to see repeatability results like what i see on the meter window. which is quite stable.

I understand that the meter reading is rms values of the results obtained from the scope window. How come the values of the meter is stable?


2. Is there any way to measure phase difference from graph obtain in channel A and B. I need this to be written in excel macro too.

3. Is it possible to measure the cyle time from channel a and b? again I need this in excel macro

thx
aseman
 

Postby Michael » Tue Mar 07, 2006 11:24 am

Hello,

PicoScope does not support data exchange from the meter window.

1.
You will need to perform a calculation in excel to extract the highest and lowest voltage measurement from an AC signal. You will need to convert this peak to peak measurement to a more meaningful measurement like root mean square(RMS) like this:
RMS Volts = 0.707 x Peak Volts

2.
You will have to use Excel to measure the pahse shift from the rising or falling 0volt point. There is no Pico macro that currently exists for this measurement.

3.
Again, you will have to build this yourself.

Best regards,
Michael
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That's what I did

Postby aseman » Tue Mar 07, 2006 1:05 pm

thx michael. That's exactly the problem. I did wrote the excel macro like what you were suggesting, but the rms voltage reading(or even the average) obtained from the data points will produce a fluctuating readings which is more than 5%.

If you don't get what I am trying to say, try measuring a calibrated voltage source.
When you read the voltage using the Picoscope 'meter' reading, you will get almost a constant value all the time say 5.75 volts over the period of say 3 minutes. But that's not the case when we try to get the values from excel data. say now the excel say the voltage is 5.89volts when the 'meter' says 5.75volts.

If I turn the voltage source off and put it back on, the 'meter' will still say 5.75 volts. No fluctuation. But if I import the data points to excel, alas, the voltage is now 6.12. turn it off and on again and the reading now maybe 5.4 volts, while the 'meter' in Picoscope still display the same values, no change or fluctuation whatsoever.




What I want to know is how the software in picoscope 'meter' maintain to keep the value constant. I intend to adopt the method in excel.
If the readings is not repeatable like what has been displayed on the 'meter', I might as well chuck the ADC212 to the bin. There must be a way to do this, I believe.

There must be a shrink in your company who design the software who can explain all this abnormalies.
aseman
 

Postby Michael » Wed Mar 08, 2006 10:57 am

Hello,

I was wrong about my rpevious post.

You will need to locate a start and finish point for the sine wave that is the same point on the rising or falling edge of the wave. I.E. the zero volt point on the rising or falling wave. You will need to square each data point and calculate the root mean of the average value.

In essence It may be easier to use DDE to share data with Excel and use PicoScope to perform the caluations for you. I have attached a short video of how to do this.

You will need to experiment to get the best results, this should get you started though.

Best regards,
Michael
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PSDDEExcel.zip
Using DDE to share data with Excel
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Postby anton » Wed Mar 08, 2006 3:50 pm

Thanks but that doesn't really help. I have included what I've obtain from the data point in excel, the video clip and your illustration of #the same point on the rising or falling edge of the wave. I.E. #

What I understand from your statement #the zero volt point on the rising or falling wave# is that you are trying to tell me to input a number directly to the excel cell. Look at the attached image #where#.
Attachments
where.JPG
Is this what you mean?
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fluctuation.xls
(20.5 KiB) Downloaded 249 times
anton
 

Postby anton » Wed Mar 08, 2006 4:01 pm

this is the video, please rename to avi.
Attachments
fluctuationavi rento avi.c
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anton
 

Postby Michael » Thu Mar 09, 2006 9:14 am

Hello Anton,

This image shows the zero point.

I am unsure how the meter calculates the value but it seems to be over several consecutive sample runs.

You also like to increase the number of samples per scope trace and employ the zero point rising across more wave cycles.

Best regards,
Michael
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ZeroRising.jpg
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Postby anton » Thu Mar 09, 2006 12:01 pm

Hello Michael,

Forgive me for saying this, but what are you talking about. If those are your points to calculate voltage, the meter would display 0 (zero) volts all the time.

My question is why the meter reading is stable almost all the time when displaying the AC RMS voltage? NOT how to read zero volt.
anton
 

Postby Michael » Thu Mar 09, 2006 2:17 pm

Hello anton,

I have adjusted your spreadsheet to reflect the RMS measurements with greater accuracy. ~please view before reading further.

RMS is calculated quite simply in this fashion:

Collect one cycle of data points. Square each data point and then average the squared value. Now square root this average to produce RMS volts.

The previous post from me is purely to indicate the start and stop point for one or many cycles. As you can see from the spreadsheet the RMS value is accurate to the meter value to within 1mv. Averaging many cycles will eliminate this error but I will leave this part to you.

Best regards,
Michael
Attachments
NEWfluctuation.xls
Ammended Excel file with RMS calculation
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