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Basic Electronics » Ultrasonic thickness gauge with the NerdKit; interference form capacitance?

September 15, 2010
by Frozenlock
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Hello everyone,

It's been a while since I've posted here. I have a new project for the parts I gathered some months ago. As some of you may know, I was playing around with piezos and high frequencies.

Well now I'm trying to put all this to good use by making an ultrasonic thickness gauge. However I don't want to use the sound echo, but rather put something between two piezos and measure the time taken for the sound wave to travel between them.

I'm trying to use some 5MHz piezos. What I've done:

  • Generate the required frequency.
  • Use a GHz (yes, GHz) op-amp to amplify the signal.
  • Use a flip flop counter to divide the frequency of the amplified signal.
  • Connect a cheap brass piezo and "listen" to the frequency obtained.

Now my problem: Whenever I approach my hand form the Nerdkit, or if I change the pressure applied on the piezos (emitter and receiver), the frequency I hear changes. In fact, It starts to sound like a theremin. Besides being interesting, it's really not what I want to obtain. I'm starting to believe I'm mesuring some kind of capacitance rather than picking up the physical vibration.

Any suggestions?

September 15, 2010
by mongo
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I am a certified NDT level II Ultrasonic tester. The test equipment we use actually operates in the MHz ranges, even though it is still audio by nature. The transducers should not change in frequency as they are generated by the test equipment and not dependent on outside influences. The readings work just like sonar and you can see the results graphically on a screen, where the measurements can be made. These things can also be used to check the integrity of a material, like finding cracks and voids as well as liquids in a vessel. The transducers are very precisely made and tuned for the frequencies being used.

For the frequencies to change by proximity to outside influences, I would tend to think that there is a capacitive effect on the circuit.

September 16, 2010
by Frozenlock
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Thank you! It seems I was on the right track (finding what causes my circuit to fail miserably, that is...)

Would you be so kind as to tell me what voltage you think might be required? I currently power my piezos with 5V, but I'm well aware that's not uncommon to use way higher voltage. I don't need to be able to read an echo, simply the direct transmission from one piezo to another.

Again, thank you for your previous answer.

September 16, 2010
by mongo
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Years ago, I used a piezo transducer for depth and level measurements in a variety of tanks. These used pulses that were about 90V, short bursts (like 5 or 6 complete cycles) of 100 KHz going to them. They were short so there was room for the echo to return through the same transducer. Most people cannot hear them but I can hear the tick each time it fires.

September 16, 2010
by Frozenlock
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Thank you very much, I will then begin my future tests with around 90V.

Now a more general electric question: If my piezo has a resonant impedance of 6 ohm, with 90v it would use 15 A. That's huge! My Nerdkit would probably burn! Do I need to limit the current, or on the contrary should I try to give it as much power as it requires?

September 16, 2010
by mongo
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I would suggest a little caution here. These transducers were made for that voltage. The ones available in the general public may not be able to use these voltages. Keep in mind that the transducers I used were heavy duty, 1.5" across and solidly mounted in a PVC tubular housing with a cover to seal them from dust. (they were in flour silos where you cannot have a spark potential).

Don't mess with voltages that can turn hazardous unless you are absolutely certain of what you are doing. Even OSHA regulates voltages above 50V in the industrial environments.

A quick test to get an idea what kind of voltage you can use is to set up the piezo crystal across a scope. Tap on the face of the element and measure the peak value of the voltage spike it generates. Generally, you can probably go twice that voltage safely without causing damage to the piezo.

January 25, 2011
by Jalex
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I am a little interested in measuring distances too but I would like to keep it below 12 volts if posible. Could I measure a distance of say 0 to 5 feet using only one transducer for TX and RX? And what would the switching delay time be at say 20-40khz?

January 25, 2011
by mongo
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I have one of those ultrasonic yardsticks that I bought at Radio Shack about 20 years ago. It's pretty accurate but really picky about your aim. It uses the same technology from the Polaroid SX70 cameras of the '70's and '80's. I have never taken it apart since it still works and I don't want to mess that up. It operates at about 30 KHz I think.Max range is 100 feet in an open room but I never got it to read mire than about 25 feet. It sends out the initial pulse of about 5 cycles and then immediately switches to receive. It does this about 5 times a second.

January 26, 2011
by Jalex
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Hi mongo I never saw that at Radio Shack. That sounds like what I would want. How is it effected by material types? Like say a soft pillow and then a hard metal or board? Does this effect it's measurments? I would like to measure liquid in a 55 gal. drum. It could even have flamable material in it so I would not want any sparks. I would think the MCU would be able to give a very accurate measurement.

January 26, 2011
by mongo
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Soft things don't reflect the signal very well. They do work nicely in liquids and they do make sensors for liquid level detection for industry. Where I work, we have several and they are made to be used with flammable or explosive areas. Costs a lot of money as well.

you gotta be careful when mixing electronics and vapors like gas.

February 01, 2011
by Jalex
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Thanks mongo I have some cheap piezo transducers. I built an op-amp circuit for them and it picks up a refeltion pretty well up to about 6 feet. Now I would like to know how much delay I would use between the transmit and receive and could I switch it with a 4066 fet switch using a single transducer.

February 02, 2011
by Jalex
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Hi I forgot to mention that 6 feet is probably way off as I was using only the .5 volt Cal signal from me scope to power the Tx piezo transducer. With more signal voltage the distance should be much greater. Even at that it did see soft materials.

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