I'm looking for two piezo transducers between 500KHz and 1MHz for use in doppler velocity measurement. I'd like to find really cheap piezos and put them in my own plastics assembly with polyurethane epoxy to match the water's acoustic impedence. Precision is definitely not important, does anyone know where to find cheap piezos in this range with wire leads? All I am finding are high end transducers with highly focused beams, which actually the opposite of what I need. Thanks in advance.
You can get the raw custom disks from Channel Industries in Santa Barbara, although it's rather technical and you have to know what you're doing.
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Give them a call and ask what they have left around from other jobs.
Yes, and no. Most commercial transducers in this frequency range that one easily come across are intended for use in the metallurgy industry, and they have special short-range focusing structures mounted on the piezo element. If instead you simply use a small disk, say 10mm dia, the acoustic beam will diverge. The beam-width in water is a function of the frequency and the width of the disk.
The disk's acoustic vibration mode may be complex, depending on its thickness, e.g., see
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but a piston radiator is an appropriate approximation for many cases. Even so, the acoustic directivity may be complex to analyze, e.g.,
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however you can simplify and assume a 5-degree beam width for a 10mm disc at 1MHz, and scale from there.
To understand acoustic transmission calculations, beam-width and the sonar equation, attenuation and other issues, I recommend Robert Urich's excellent book, Principles of Underwater Sound. On pages 102-111 of the third edition, you'll learn that the attenuation in water can be severe, increasing with the square of frequency. At 1MHz it's about 0.4 dB/meter (that's 0.8dB/m for a round trip). This is in addition to spreading losses.
Have fun, Kelly, and report back to us as you proceed.
It is not being high end that causes them to have highly focussed beams. At these frequencies, with corresponding short wavelengths, you would find it hard to make a transducer any other way.
On Wed, 04 May 2005 05:50:34 GMT, snipped-for-privacy@pearce.uk.com (Don Pearce) wroth:
You can use most of the optical methods, lenses and mirrors, to expand or contract ultrasonic beams.
There is a good source for piezo transducers on eBay. Check out
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You will have to add your own wires, but that's extremely easy to do.
Go to their eBay store for more items. Their web site has data sheets for their transducers. I've ordered from them before and can recommend them highly.
Thanks all for the help, the resources, this is exactly what I was looking for. One more question: are there any good websites/books/tutorials that describe how to make a transducer from a piezo disk? I've it is rather difficult and thought I should try to find something with wire leads, but I'd like to know more. Thanks again.
I am taking a stab at making a flow meter using continuous wave doppler. Mainly to avoid ones on the market that are problematic and pretty expensive for what you get. If I can make a transducer successfully (and test it out with an oscilloscope), I think I can make the A/D, DSP, and interface work pretty easily. Suggestions are always appreciated, thanks again for your help.
On 6 May 2005 00:35:04 -0700, snipped-for-privacy@yahoo.com wroth:
I don't think A/D and DSP are entirely necessary. Everything you need should be "do-able" with analog devices. Take a look at the spec sheets for the LM2907 F/V converter.
On 7 May 2005 03:51:16 -0700, Winfield Hill wroth:
OK, but remember, you asked for it. 8-)
Start with a clean source of voltage to drive the transmitt element. Harmonic distortion doesn't matter much, but amplitude and frequency "noise" should be kept as small as you can. Make it easy to vary the frequency a little to hit the "sweet spot" of the transducers, ie. be able to tune for maximum smoke.
The receiver section needs some gain at the ultrasonic frequency with a little selectivity. A tuned step-up transformer feeding a JFET with an LC tank in its output should be enough.
Follow that with a simple signal diode envelope detector. The received signal will contain a lot of the un-dopplered transmitt signal with the dopplered part being 40 to 60 db lower, so most of the system gain needs to be after the envelope detector. The un-dopplered carrier will create a large DC value at the detector. You don't need that for anything, so throw it away with a coupling capacitor. Make the RC roll-off low enough to catch the slowest flow rate you are interested in.
From here on, it's a normal audio frequency signal.
Next, pass the signal through an active low-pass filter with the upper cut-off set to pass the doppler for the highest flow rate of interest. Gain can be incorporated in the filter or place a gain stage before the filter with, perhaps, another after the filter.
The F/V converter, if you use a LM2907, only needs a 25 to 50 mV P-P signal, so distribute the overall gain to get that value. The output of the F/V converter is, of course, an analog version of the flow rate.
I have done the job by putting the raw amplified ultrasound through a fast A/D into a DSP, but I can't recommend that for a DIY project.
Jim/Winfield, Thanks both for the help. The problem with Jim's approach is I am much more of a programmer than an electrical enginner and really only understood the parts that pertained to signal processing. Here's my planned approach, let me know what you think.
If you sample at the transmit frequency (F), exactly, a velocity of 0 corresponds to no frequency exactly. A doppler shift of 100 Hz will result in a strong frequency around 100Hz in the FFT. You can theoretically detect a shift of F/2, but that would be an unrealistically high velocity. The other problem is that with a relatively small FFT, say 2048, one bin corresponds to a large change in velocity.
Therefore, if you sample at F/2, a doppler shift of 100HZ will result in a strong frequency around 50Hz in the FFT, and the maximum detectable doppler shift is F/4 (Still very fast in terms of water velocity). So my plan is to sample at somewhere around 50KHz, which will still give me span of F/20, which in practical terms is about 30 m/s, depending on the angle. A benefit of this approach is that the resolution of each bin is quite, which is desireable.
Naturally, the signal will have to be amplified before DSP, I will determine that gain using an oscilloscope. Any thoughts? I am going to try using a rabbit core module
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as they are pretty easy to program. Please let me know if you see any problems, or have suggestions. Thanks again.
Isn't it often worse than that? Jim, for your nice return signals have you been using well-chosen send-receive transducer placements, with a shield, and with dirty water?
On 8 May 2005 04:58:43 -0700, snipped-for-privacy@yahoo.com wroth:
That's fine as far as it goes. Did you notice that the doppler shifted energy will be 40 to 60 db lower than the total signal? That places very harsh requirements on the A/D process and the digital processing subsequent to that.
No, I suspect even dirty water is a poor backscatterer compared to red blood cells. Clean water, even worse unless there are turbulence cells. Ahem, the O.P. did say, "with polyurethane epoxy to match the water's acoustic impedance," so I'd imagine poor backscatter signal strength compared to the transmit signal could be a significant issue.
On 8 May 2005 08:19:38 -0700, Winfield Hill wroth:
My major effort so far has been with blood flow using reflection from red blood cells. Dirty water? I guess that's a pretty good description. The transducers are side-by-side and oriented so that the beams converge on an artery about 10mm under the skin's surface.
The water is dirty, wastewater (i.e sewage) if you really want to know, which should help a bit with the recieve. I am aware the return signal will be pretty faint compared to the transmit, I'm not sure how faint but am prepared to invest the time to make a transducer, connect to an oscilloscope and play around with it until I find out if it is possible. My guess is the first few bins of FFT will be all bleed from the transmit, so near zero velocity detection will be impossible. I'm also trying to read up on transducer design to get a good recieve signal. Thankfully it is not as critical as blood flow, but should be a good way to learn and possibly produce something useful.
Nice project, Jim. I hope it works out to be usable for continuous monitoring of blood flow to the brain, usable by first responders to detect the reduction of brain blood flow which often occurs after head trauma. The current method used by first responders, the Glasgow scale, is applied correctly less than half of the time (detects less than half of the instances of high Intra-Cranial Pressure (ICP) causing loss of blood flow to the brain, in a timely manner). Since loss of brain blood flow always results in death or severe brain damage if not treated promptly, such a device has the potential to save many thausands of lives. High ICP, typically caused by hemorrhage of small blood vessels when the brain moves in the skull due to impact or brain "bruising" and swelling, can be properly treated only at major trauma centers having MRI and a surgical team prepared to enter the skull, and sucessful outcome is dependent on prompt recognition of the problem and rapid transit to an appropriate facility. Since there is often no immediate outward sign of this problem, and there may not be any other serious injury, appropriate treatment is often not provided even if readily available.
This is just the sort of thing that NIH likes to fund.
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