Low frequency Voltage measurement

Hi,
I am using PCI 6025E to control load bank and to measure voltage, current and frequency of a generator.
While current and frequency measurement is not a problem, voltage measurement is toublesome.
Since on test bench already there are few PCI boards and huge number of sensors, I do not want to add NI 9205 or any other additional boards and clutter the area (other people are also working there).
The problem with voltage measurement is that out of three generators two run from 400 RPM to 1600 RPM i.e.. somewhat 15-55 Hz.
I had two solutions for this:
1, Putting a step down transformer (230 Vac / 5 Vac) and then measure the voltage. But when testing at somthing like 15 Hz I have to deal with magnetic saturation with commercially available transformer. I do not want to take pain to go on built my own transformer.
2, Using high precision voltage Divider. The problem is that what should be the galvanic isolation? Isolation transformer, but then again low frequency problem.
Hence could someone suggest me since there is a bit confusion in my head.
Thanks.
Solved!
Go to Solution.

If you have a sine generator (15Hz migth be hard for the soundcard ) you can do a calibration of your transformer.
(If only the voltage amplitude is of interest , use a isolated DMM as reference and run your generators)
Or you buy a isolation voltage sense amplifier. (weidmueller, phoenix, wago, ...)
Greetings from Germany
Henrik
LV since v3.1
“ground” is a convenient fantasy
'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'

Similar Messages

Low frequency measurement (1 to 5 Hz) issues - Reg

Hello All,
i am facing a problem in measuring Low-Frequencies in the range of 1-5 Hz. I am using VIs used for pulse-width measurement and the card is PCI-6031E. i am generating frequency using PCI-6013 card.
Actually i am trying to develop an application for measuring the speed (RPM) of the motor which runs very slowly. so if i miss one pulse per second also, when i multiply that with 60 it will be big error.
So if any can give me a solution that will be big help to my issues.
Thanks.

Hello sathiskumar,
I'm not sure what you mean by missing a pulse but I think what is
happening is that your counter is rolling over and starting to count
from zero again. Every time the counter rolls over, it will
output a pulse. You can connect this signal to another counter
and count how many times your first counter has rolled over. That
way, if you don't receive a pulse on your first counter before your
counter rolls over, the second counter will have a record of how many
times this has happened.
I hope this helps!
Eric
DE For Life!

"How to measure very low frequency?"

I'm using Labview 5.1 and a data acquisition card PC-LPM-16PNP. How can I measure a low frequency signal like the human pulse of 72 pulses/min or 1.2 Hz. Thanks.

Virgo,
When measuring very low frequencies, I recommend measuring the period between pulses and converting to frequency.
To measure the period, there are a few methods. For something like a heartbeat, I would probably use a peak detector to locate the peaks and measure the distance between the peaks. For other signals, it may be easier to locate zero crossings. Sometimes filtering or smoothing is required to remove noise and improve the results.
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Bruce Ammons
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Low frequency pulse duration measurement

We have low frequncy pulse duration measurement to be done . We have tried with pulse measurement but it shows error for the frequency bellow 1.5 Hz. We have input signal of frequency varie from 0.5hz to 3 hz .Please suggest us is there any other method that can be used for mesureing the pulse width with low frequency mentioned above .If this can be done using tick count , please send us the example .
Regards,
Sharanu

Hello Sharanu,
I'm not sure you will be able to measure a frequency below 1.5 Hz in
the traditional way. However, I would like to ask you some more
questions about your system.
1) What hardware are you using?
2) What is the error you are receiving (please include the exact error text)?
Eric
DE For Life!

Low Frequency Measurements using counter timers

I am trying to measure multiple speeds using FP-CTR-502 modules. I am having a resolution problem. Looking at the post at the following URL (http://forums.ni.com/ni/board/message?board.id=110&message.id=3601&query.id=42252#M3601) looks like exactly what I need to do, but I'm not sure how to program this. Does anyone have any example code to get me headed in the right direction?

Hi Tcrower,
Here is the example that Carl L. was describing in his forum post along with his instructions. Also, there are thorough instructions for using the program in the front panel as well as in File >> VI Properties >> Documentation.
You can measure frequency in two ways using a counter.
1) Measure the time it takes for a set number of pulses to pass or,
2) Count the number of pulses that pass in a set amount of time.
For low-frequency signals it is best to measure the time duration of a set number of pulses. If we tried to calculate the number of pulses that passed in a set amount of time, we would need a long gate period to get accurate results.
The drawback of measuring the amount of time it takes for a set number of pulses to pass is that the sample period varies with the frequency of the signal.
To accurately measure low frequencies with a CTR module, select two counter channels, one output channel, and one gate input channel. Count Input 0 counts the pulses of the signal that we are measuring. The output of the counter is set to high for x pulses and low for x pulses, which is used as the gate of Count Input 1. The source of Count Input 1 is the internal clock reference (the example can be modified to use an external clock reference). During the high phase of Count Input 0, Count Input 1 counts the pulses of the clock reference.
During the low phase, we read channel 1 and reset the counter.
For example, if the output of Count Input 0 is set to high for 4 pulses and the signal has a frequency of 20 Hz, the output of Count Input 0 is high for 200 ms. During the high phase, we will count 200 counts if we are using the 1 kHz internal clock reference. So to calculate the frequency, we have F = #pulses/duration = timebase * terminal count/counts = 1000 Hz * 4 terminal counts/200 counts = 20 Hz.
Regards,
Hal L.
Attachments:
Low_Frequency_Measurement_(CTR).zip ‏77 KB

Low-Frequency measurements using counter/timer

I am trying to measure speed, and am using the FP-502 counter timer module to count the pulses from my sensor. The field point example that uses a fixed width gate pulse will not work for my appliation because I need a gate pulse so big that it updates way to slow.
Does anybody have any sugestions on how to measure a low frequency signal using a counter/timer?
Thanks
Dan

Hello guys
Thanks for all your sugestions I have the final results. I got it to work but I would like to add some comments for anybody who is trying to attempt this.
Triggeing on the gate is a must for low frequency measurements. I used the divide by sugestion however using a factor of 3 only gets you about 60 Hz after that you can't sample fast enough. My application required me to measure from 0 to 150 Hz. I added a "gear shifter" routine where the terminal count was set to 3 on frequencies lower then 45 Hz and 15 for frequiencies greater than 45 Hz. Using a terminal count of 15 for all measurements made the udate rate on really low frequencies way to slow i.e. .5 seconds for 20Hz (check the math on this).
I a
lso needed to cascade the counters. At low frequencies the count went higher than 6550. Using the previos channel function of the fieldpoint module enable me to get 32 bit worth of terminal count data.
Finally I needed to add a timeout condition for 0 Hz. If there are no pulses coming from the senser the program displays the last value read. That won't be 0. So I added a time out routine that if I didn't see a pule in a certian amout of time, output 0 and reset the gate count.
Thank you for you help you relly saved me. I hope my comments made sense.
Dan

Help with Frequency/voltage Ctrl settings in CMOS

I have the K7N420 Pro Mobo and the Ge4Ti4200 card. I just ran 3DMark01, after a new bare hard drive, fresh install of Win XP and running Live Update on my drivers. I got a low score of 7459. On my old hard drive yesterday I got a score of 9745. I also upgraded my PSU to a 350 W (temporarily - 400W on the way) In the CMOS under Frequency/Voltage Control, is a listing:
CPU/MEM/AGP and it is set at 100/200/66. What do those numbers mean? I can't find these numbers anywhere either in my documentation or on this site. How do we determine the proper setting for these numbers for the best performance? What can go wrong if they aren't set properly?
K7N420 mobo
Ge4Ti4200 VTD-8X 128M
AMD Athlon 1700+
SB Audigy
Win XP Pro
Direct X 9
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Lite On CD-RW
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Hi,
Your CPU and FSB are only running at 200mhz. Which you are underclocked. Try setting the FSB clock to 133 or load performance defaults.
-rob

Is it a bug or just a typo in Agilent 532XX Series\Public\Data\Low Level\Fetch Measurement (Single).vi?

Hi, All
I am using Agilent 532XX Series driver to read data from a frequency counter. In the low level subVi : \Public\Data\Low Level\Fetch Measurement (Single).vi, "Scan From String" is used to convert the data.
however, if I check the error of this function, it always show me a error with current format string "%.;%g;". after I changed it to "%;%g", I don't get the error anymore.
If anyone use this funtion, please check the error out of that fuction and see what you get.
Regards
CQ

I don't have the instrument, so I can't reproduce it completely, but I've done some research and if there is an error I don't think it's in the string conversion itself. I say this because:
From what I can gather, a typical output string would be the following: +4.27150000E-03
I took just a "scan from string" node and gave it that string as a string input and %.;%g; as the format specifier string. This yielded a numeric value of 0.0042715
So, as far as actually finding the root of the problem, there could be a few things going on. My first thought is some form corruption causing the data received to not be valid for conversion. There are two ways of testing this:
Open up a test panel in MAX and perform a loopback test, as described here: http://www.ni.com/white-paper/3450/en/#toc4
-or-
Install and open NI IO Trace and view the bus traffic as detailed here: http://digital.ni.com/public.nsf/allkb/282C5D41E2BA04F2862574BA007803B9
If the loopback tests works properly, go ahead and run the IO trace and post the results here.
Christopher S. | Applications Engineer
Certified LabVIEW Developer
"If in doubt... flat out." - Colin McRae

Best method for collecting low frequency data

Hello everyone,
I'm looking for suggestions on the best way to collect relatively low frequency data (about 1 Hz). I know there are a few different ways to do so in labview such as the DAQ assistant or making DAQ mx and making your own virtual channel. Also there are an abundence of different settings to choose from. I'm using an NI 9215 DAQ card and am collecting voltages. I would be interested to here any opinions on a method for doing so and maybe the settings that they would use.
The reason I'm asking is because I'm just using the DAQ assistant but I'm really not sure if that's what I want to be using. I feel like there is a better way.
Thank you all!

winterfresh11 wrote:
Is this different from triggering? Because this particular DAQ card can't be triggered.
There is a big difference between triggering and sample clock. The trigger tells the DAQ to start acquiring data. The sample clock tells the DAQ when to take a sample. You trigger once per acquisition. The sample clock just keeps on going until the acquisition is complete (either aborted or desired number of samples is acquired).
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Frequency response at low frequency

I'm working on a bandpass filter, and I'd like to get the frequency response showing that the frequencies outside the lower and higher cutoff frequencies are being cut off. However, the correct plot is shown only when my cutoff frequencies are high (roughly from 1000-8000 hertz). When I use low cutoff frequences(roughly 4-5 hertz), the plot is incorrect. So how can I get the frequency response to my low cutoff frequencies? Thanks.
P.S. In the code, some parts are irrelevent. In the front panel, the only relevent part is the frequency response plot at the lower right corner, and the specs above it; in the block diagram, only the upper half(with IIR and FRF) is relevent. Thanks.
Attachments:
BME_Pressure_Sensor_V1.00.vi ‏591 KB

Hi Manson
There is a bug in your diagram since you connected the number of samples where you should have connected the sampling frequency.
The sampling frequency is related to the pace at which you take the measurement.
Usually, Fs = 1 / dT
where Fs is the sampling frequency and dT is the time interval.
It should work better.
In any case, to have a better resolution in the low frequency range of your spectrum computations, you have to increase the number of points of your data because there exist the following relationship between dF (space between 2 points in you spectrum), dT, and N (number of data points) :
dF = 1 / (2 x dT x N)
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Low frequency measurement from Parallel Port

Hi there...
I need to calculate the "on" and "off" time and duty cycle in pulse form from a parallel port. By making the circuit in 5 or 0 V, I just simply put it in my parallel port. The problem comes when I need to measure a very very low frequency. In this case, I want to measure the duty cycle from my operated refrigerator. I need to know when the thermostat goes "on" and when it comes to "off". In my experience, the thermostat will be "on" in about 5-10 minutes and "off" in about 20-30 minutes. So, the pulse might be take for a long periode each.
I've tried with Timing and Transition Measurement wizard or even by using Pulse Measurement.vi which is included in Waveform Measurement category. It only works for 2 Hz and . If I try to set it with 1 Hz or below, it comes the message :
"Error -20308 occurred at Timing and Transition Measurements -> Untitled 1
:4" (waveform index 0 of 1)
Possible reason(s):
Analysis: The waveform did not cross the mid reference level enough times to perform this measurement. Check the signal length, reference levels, and ref level units."
Could someone help me please ?
Regards,
Ricki

here is a quick shot to give you an idea
Greetings from Germany
Henrik
LV since v3.1
“ground” is a convenient fantasy
'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'
Attachments:
port logger.vi ‏22 KB

Slow AC voltage measurement readings with hp34401a

Hi,
I am currently using the hp34401a to record an input and output signal voltage (this is then converted programmatically to frequency response). This measurement however I consider to take exceptionally long for what it is (approx 3 seconds per measurement). I am using Configure measurement.vi (set to AC voltage on auto range) and Read(single point).vi. If anyone can give me some ideas how to speed up these readings that would be appreciated.
I have had a look at a similar test done with Labview 6.2.1, where the testing is much faster. The same instrument is being used but with Traditional NI-DAQ.
Thanks
Tania
Note: Labview 8.6

TanWal wrote:
I have tried all the possible/ suggested ways I can think of to improve the reading. There seems to be little that can be done in AC. Even applying the fast filter did nothing to improve the time. I tried to do DC voltage measurement, then take a multipt read and use the Basic rms.vi to obtain an AC value but this is very inaccurate. It seems that this DMM is very limited in its speed for this type of measurment.
I took a look in the spec. Forget the DC way if you have frequencies higher 80Hz....however if you have sampled waveforms do a tone detection (FFT based) or better a SAM (sinus approximation via min MSE) to get the best out of your data ( For FRF with sinus excitation). With RMS measurements you will measure hum and noise too, however you didn't tell us the uncertainty goal nor your DUT.
I don't use this instrument but from the spec and my experience with HPs(agilent) you should be able to get 10 reading/s in AC mode BUT still have to wait for the settling of the AC filter after switching (I assume you use the front and rear input)
I would encourage you to write a small state machine for the task and post it here.
Greetings from Germany
Henrik
LV since v3.1
“ground” is a convenient fantasy
'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'

Frequency response measurements with pxi-5922

I’ am using signal express and the pxi-5922 digitizer together with the AWG pxi-5441 to analyse the frequency response of a buffer amplifier. See the attached signal express file. Many different ways to measure the frequency response have been tried and this is the best I came up with. It is basically two tone extract steps in a sweep loop. But I’ am still uncertain if this is the best way to do this kind of measurement. The fact that the detected frequency differs between the two channels worries me, even when the two channels of the pxi-5922 are looped. Is there a more accurate way to determine the frequency response?
Best wishes
Stefan Johansson, SP
Attachments:
sweep.JPG ‏397 KB
Frequency Sweep funkar.seproj ‏81 KB

Claudia-
Thanks for the response.
Regarding the CJC- When I switch it on, the temperature readings I get are very random, roughly negative 1 degrees. (I am operating right now at room temperature, and will be using J-type TC's to measure ~43 degrees C). Also, when I use the built-in CJC, the aquisition rate seems to slow down considerably. When I use the "user specified" everything seems to be ok, including the aquisition rate.
I measured the resitance of the Thermistor on the TBX-68T and it was about 5000 Ohms, as expected.
Just to make sure: When using the TBX-68T, do I need to hard-wire a thermocouple to Channel 1/auto-zero and another to channel 0/CJC? Because I connected a TC to channel 0 right now, but I wasn't 100%
sure.
I've attached my main vi and two sub vi's that I am using for the voltage aquisition part of my project. (Note:the current measurements are just voltage measurements multiplied by the recipricol of the resistance it was measured across, ie. 10).
I would like to keep this file as is because it writes to a file exactly the way I want it to. I'd like to have the temperature aquisition with the 4351 in the same vi as the 6030E so that they both stop and start at the same time. I am just not sure how and where to log the temperature data since there will be fewer data points than the voltage data. Any suggestions? Should I write two separate files? can I somehow append them?
Thanks again. Hope to here from you soon.
Attachments:
EBlackMainDAQ.vi ‏107 KB
Save_Data8.vi ‏45 KB
Build_String_Array5.vi ‏33 KB

Is it possible to detect low frequency signals with a high sampling rate?

Hello everyone,
I'm having an issue detecting low frequency signals with a high sampling rate. Shouldn't I be able to detect the frequencies as long as the sampling rate is at least 2 times the highest frequency I will measure? The frequency range I am measuring is 5-25 Hz, and I use Extract Single Tone.vi to measure the frequency. The sampling setting I am using is 2 samples at 10 kHz. Is there a method I can use to make this work?
Attached is the vi.
Attachments:
frequencytest.vi ‏21 KB

You are sampling at 10Ks/S, but only taking two samples. What do you expect to see? If your signals are binary (On or Off) you would only see either an on or an off, or if the rise/fall time was fast and you were Extremely lucky, one of each. If you want to see a waveform you have to sample for at least the period of a waveform. So you should take samples for at least 0.2 seconds to capture an entire waveform at 5Hz, ideally longer. Think of looking at a tide change at a dock. If you want to see the entire tide change you will probably have to measure repeatedly over 24 hours, not just run out on the dock, measure the height twice and leave. That wouldn't tell you anything other than at that precise moment the tide height was X, but not that it was at high tide, low tide, in between, etc.
I type too slowly, I see that a more technical answer has been given, so mine will be the philosophical one!
Putnam
Certified LabVIEW Developer
Senior Test Engineer
Currently using LV 6.1-LabVIEW 2012, RT8.5
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Very low frequency caused by sample frequency in FFT analog input?

I'm measuring a very low frequency on my analog input, this frequency is in connection with the sample frequency of the Analog Input. At a sample frequency of 1000Hz I see a frequency of 0,05Hz in my FFT, at a sample frequency of 500Hz I see a frequency of 0,02Hz.
Attached is a screenshot of an example how I see this very low frequency.
My hardware: NI USB 6008 --> measuring on AI-0 (in this example the input is unwired). But in my real measurement I see the same FFT + signals I want to see (about 2 Hz).
In my real measurement I windowed the FFT (1-3Hz) so I see only the FFT I want to see. But I suspect that my complete signal moves along with this very low frequency of 0,05Hz. I saw this in my measerement.
What did I do wrong?
Attachments:
screenshot.JPG ‏66 KB

First, do you live in Europe? If so, that 50Hz could be power-line pick up.
Antialias filtering must be done in hardware before the DAQ. Because of the way aliasing works if you have sampled the signal it's already too late, you're hosed and no amount of digital filtering can remove the aliased signal. In terms of filter specifications, the filter cutoff needs to be at twice the highest frequency you are interested in seeing. For example, if you are looking for signals in the 2- to 5-Hz range, your antialiasing filter should cutoff at around 10Hz.
Obviously good signal management is also needed: shielding, appropriate signal termination, proper lead dress and spacing from known noise sources, etc...
Mike...
PS: There were no attachments to your last post.
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Low frequency Voltage measurement

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