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Offline SIMONNTopic starter
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« on: August 26, 2010, 09:14:14 am »
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Hello together,

I am looking for help to build up coils for a VLF-detector. So, I don't use the a whole metal detector because I use LabVIEW to generate signals. At least I wanna descriminate between different metals by the help of LabVIEW. But this is not my question. My basic set-up is: Signal-generator --> Transmitter coil    -->   receiver coil --> Oscilloscope.

I can't really find electrical propertys for a VLF-Detector coil. Which values of resistence (Ohm) and inductance (Hendry) should be achieved in a transmitter- and a receiver-coil?

In my opinion, the values depend on the resonant frequency and the value of the tuning capacitor. However, I can't calculate the turns of both coils at least...

Another question:
If I create a sinodial signal to the transmitter coil, which signal should be expected from the receiver coil to the oscilloscope?

Thank's for all advices and examples!

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Offline GoldDigger1950
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« Reply #1 on: August 26, 2010, 04:49:11 pm »
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Labview should have formulas for calculating tuned LCR (Inductance/Capacitance/Resistance) circuits based on two fixed values (CR) and one variable (L). You need to know the Fo (Frequency of Operation) for the circuit. Keep your Q as low as you can make it and remember to specify that this is an air core coil.

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« Reply #2 on: August 26, 2010, 05:27:13 pm »
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Thank you! Surely, LabVIW can calculate it. But my problem is the coil. There are so many questions:

- What do you think about 11kHz frequeny? Is that a good point to start?
- Then I can calculate the capacity. But what about the resistance? Shouldn't be the value between 1.8 - 3.0 (Ohm)?
- What do you mean with "Q"?

Thank's for your advice!

Simon

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Offline GoldDigger1950
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« Reply #3 on: August 27, 2010, 04:33:51 pm »
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The 11khz frequency is as good as any other. The resistance in a coil is the result of the wire size and number of turns to achieve the proper inductance. The Q of a circuit is the efficiency. Low Q means high efficiency and is related to very low resistance and a rapid field establishment in a tuned tank circuit - which is what a coil is. Too many turns on a coil mean the field builds up very slowly. This is bad for a metal detector because that field has to collapse quickly and if it takes a long time to build to 100% it will take an equal amount of time to collapse. The longer it takes, the later the receiver coil will be active. The low Q of a receiver coil also means it reacts quickly to fields in the metal underground.

Do you have a technical background or are you trying to learn this all from a forum?

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« Reply #4 on: August 27, 2010, 06:44:34 pm »
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Of corse, I have knowledge to implement the phase shifter (for ground adjust) and the demodulator. I currently build up the whole circuit in LabVIEW but my first coil isn't really good enough for me. Regarding to that, I try to get "better" information here to design a great coil for detecting purposes.
You wrote about inductance. Sorry for that, but nobody answered my question accurately. I also found value's for inductance between 300-400 uH but I think the resistance has also an important influence. That's all what I need. I can calculate the number of turns by myself, but I actually got 5 different approximation formulary...
The problem is this project must be done until the next week. I created the housings and also the complete circuit depending to the frequency...

What I need are checked values about inductance and resistance.

Simon

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