capture electric frequency using a computer soundcard

[nwill002]

Hello, I with 0 experience with DAQ.  I am looking for a solution that can capture the electric frequency coming out of the power socket.  In the US this should be 60Hz.  It needs to be able to interface with a computer where the end result will be an application with a visual meter of the current frequency that will send a alert if the frequency increases/decreases out of a given threshold, say +/- .5Hz from 60Hz.  Also the device needs to be completely automated where you plug it in and it will continuously measure the frequency without someone having to continuously adjust knobs or something.

I have no idea what product to look into so any suggestions of what can accomplish this will be greatly appreciated.  I know this is the hardware forum but are there any existing apps that can use the data captured and provide this monitoring and alert functionality?  If not I'm sure we can code our own.

Thanks

[labview-programmer]

Hi,

See this link for current measurement using CTs, NI hardware & LabVIEW: http://zone.ni.com/devzone/cda/tut/p/id/8198

Once you have the waveform on PC, you can easily calculate the frequency (in LabVIEW or SignalExpress) and set thresholds & alerts.

 Does it solve the issue? 

[nwill002]

thanks for the suggestion, Im not an electric engineer so it will take me a while to make sense of the whole thing.  But it looks this suggestion that its not practical to directly measure the frequency of the current coming from a standard socket right?  (ie have a single device plugged into the wall and sending measurement data to a PC)

[labview-programmer]

You can use NI 9227 or similar with a USB carrier. You can set it up yourself (or by any electrical technician). The advantage of using this type of instrument is that one can easily add/modify the functions and parameters in LabVIEW using basic functionality and can make use of the carrier for different purposes.

You can also use many devices to different PCs using a LabVIEW application. 

[nwill002]

thanks again, so you are saying I can get a NI 9227 and a USB carrier to connect to a PC? Wouldnt I need some kind of special probe to measure the circuit from the power socket?

[Henrik_Volkers]

Here is my approach: I used the soundcard mic input to catch line hum and analyses it.

I used a headphone plugged into the mic 😛 and placed it closed to a wall plug transformer (A traditional heavy one, not the switched ones 😉  , somewhere I have one of these old inductive telephone pickups, they should would work better.

The program catch 2 seconds of hum and noise, bandpass filter for the line freq and uses the Extract Single Tone Information VI to get the main frequency.

Things to add and improve:

• Record to file
• Best would be to use a wall plugged transformer with some volt AC output and a voltage divider to fit the line input... RC pre filter ... but hey that is for fun 🙂
• Frequency detection via SAM Sinus Approximation. I already tried it and it works better, however that code didn't belong to me so I can't share it. Try a 4 parameter fit with a*sin(w*x)+b*cos(w*x)+c  it is slow as an express vi , but more stable
• Add alarms if more than 2% off because at least at that moment the grid will fall apart.....  
• Run it against the HP3458 powerline freq measurement....

 here you can see that my use-the-headphone-as-magnetic-pickup has a real bad SNR 🙂 red is signal before white after bandpass filter

Message Edited by Henrik Volkers on 03-25-2010 06:29 PM

Message Edited by Henrik Volkers on 03-25-2010 06:33 PM

[Henrik_Volkers]

here is the chart I found this morning...

And don't trust the absolute value!! The time references of ordinary soundcards are BAD BAD .  

And here a traditional solution:

[nwill002]

wow so you used your headphones to listen to the AC sound frequency from an ac adapter?  very creative!

but I dont understand how the headphones captured the sound, wouldnt you have to use a mic?

[AnalogKid2DigitalMan]

The headphones have a coil of wire in them, that coil will produce a voltage in the presence of a changing magnetic field.

-AK2DM

[Henrik_Volkers]

---

nwill002 wrote:

wow so you used your headphones to listen to the AC sound frequency from an ac adapter?  very creative!

but I dont understand how the headphones captured the sound, wouldnt you have to use a mic?

---

A dynamic loudspeaker is also a dynamic microphone*  😉 

As AK2DM already mentioned: Here I (ab)used the coil of the speaker as pickup for the magnetic field. 

*: Same is also true for electrostatic speaker/ condensator mic, this is used for high precision calibration. However usual ECM mic have a build in FET - amplifier, so these  are really one way.

Yesterday we (a 12a boy and me)  light a LED with a DIY battery from copper coins, paper, aluminum foil , salt and vinegar. What a wonderful rainy Sunday project 🙂

The joy of engineering is to do what you want with the things you can get.:D