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'[EE]: Piezo transducer for the evaluation of food'
2001\02\09@054256 by Peter Lau

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Hi,

For a school project, I want to investigate the use of ultrasonic waves to
determine the state of various items of food (eg. fruit).

I have done quite extensive research, and determined that I need a piston
transducer of the type used in NDT (non destructive testing) of metal
components and rails.

The problem is this, such units are ludicrously expensive. A ready-made unit
is definitely out of my reach. However, having read the authoritative volume
on NDT by the Krautkramers, I think a home made version is not out of
question, in fact, it should be quite doable, assumming pinpoint accuracy
and cutting edge efficiency is not required. Afterall, I plan on measuring
attenuation and transit-time in apples and pears, not detecting microscopic
cracks inside hunks of steel.

Such piston transducers as I have described rely on the mechanical resonant
frequency of the piece of PZT (or equivalent piezoelectric material). For a
thin disc, this is calculated easily from a simple formula, using a
frequency constant for the material and the thickness of the disc. The disc
is held in place with a knife-edge ring and dampened by either a Helmholtz
chamber (a smaller version of those found in studios) or a backing material
(eg. rubber), this is done to even out the transducer's bandwidth (decrease
Q).

I've managed to acquire free from a large manufacturer as a sample a disc
2mm thick x 20mm dia. Such a disc has a resonant frequency of around 1Mhz.
This is a pretty standard frequency for NDT in metals.

The catch is, a 1Mhz pulse would not survive very far into such a high-water
content, porous material as an apple, let alone hitting the other end and
reflecting back. Apparently, 50kHz is more reasonable. Why not build a 50kHz
transducer then? The second catch is, a piece of PZT having a natural
resonate frequency of 50kHz would have to be 50mm thick(that's two whole
inches!), with a diamter greater than that still. OK then, so what?

Well, unfortunately this isn't compatible with my concept of economy :-).
Furthermore the drive voltage would have to become impossible high.
Commercial transducers (read $$$) get around the latter limitation by
sandwiching together a stack of plates, electrically wired in parallel. This
is not feasible in my case.

And now comes the part where any help from you techno gurus would be
tremendously appreciated.
I have a few options here:

1.    Use the freebie PZT plate to build a 1Mhz transducer and use it at
that frequency. However, the odds are stacked against it ever being able to
work with my target samples, and so is the physics.

2.    Use the freebie PZT plate to build transducer that is excited at
50kHz. This essentially means operating the plate in it's 'static mode',
since the driving frequency is too far removed from the resonant freq. for
mechanical harmonic motion to play any part. I'm trying to work out the
relative acoustic output intensity of such an arrangement. I have a feeling
that, even without any additional dampening, the wave intensity might be
insufficient.

3.    Save the freebie for later, try a different alternative. You've all
seen piezo buzzers and their u-sonic siblings. They work on a totally
different principle to the piston transducer discussed above. The piezo
material is usually very thin (~0.3mm). It is mounted on a brass diaphragm.
As a voltage is applied across the material, the disc thickens as expected,
but this isn't important. The key is, it also expands/contracts sideways
(the radius changes). This causes the diaphragm to flex in and out. The
actual change in the thickness of the piezo sheet is too small to be
measured, but the diaphragm flex enough to blast your ears out.

AFAIK, this method is commonplace for air transducers but is not used in
surface contact NDT. My guess is that pressing such a TX head against any
material (even via a coupling liquid) would quench the delicate flexing
osillations, because solid matter compresses so little compared with air. I
think in acoustic terms this can be boundary reflection. There is a huge
difference between the wave velocities of air and steel (300 m/s vs. 6km/s),
hence they respond diffferently to. But what about apples? Perhaps a
bender(diaphragm) transducer can be modified to work. However I have found
no information on the quantitive design of diaphragm transducers, so I can
only make educated guesses (this subject is not-surprisingly not covered in
the Krautkramer book)


Can you give me some help on how I can go about this dilemma? I believe the
electronics to follow would be the easier part, a good challenge but
defeatable in due course, with patience.


Peter L.

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2001\02\09@094504 by Chris Eddy

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Peter;

Just becouse it is resonant at 1MHz, does that mean that you cannot drive it at
50K?  I bet it will drive, possibly with less efficiency.  Just a guess.

Chris

Peter Lau wrote:

> Hi,
>
> For a school project, I want to investigate the use of ultrasonic waves to
> determine the state of various items of food (eg. fruit).
>

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2001\02\09@105523 by Alan B. Pearce

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>Just becouse it is resonant at 1MHz, does that mean that you cannot drive it at
>50K?  I bet it will drive, possibly with less efficiency.  Just a guess.

that was my thought too, using it like a crystal microphone/speaker arrangement.
The same thing is done for echo sounders for marine use.

however the other disc transducers he described would be more suitable for this
style of operation. It sounds to me as though the sort of transducer he has is
rather specialised in design.

At the frequency of interest (50khz) I would be tempted to see if you could get
a pair of dome tweeter speakers and set them up one on each side of the piece of
fruit with the dome of the speaker just in contact with the fruit. If you then
drive one with 50khz, what sort of pickup does the other have? Do bear in mind
that there will also be some acoustic coupling around the outside of the fruit
as well. The advantage of this is it would be low voltage drive using well known
speaker amplifier technology, and the receive side will work into an ordinary
low impedance microphone input.

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2001\02\09@131416 by rottosen

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Peter, see my comments in your text.

-- Rich


Peter Lau wrote:
{Quote hidden}

I don't know anything about the piston type of transducer so bear that
in mind when I make my other comments.


>
> I've managed to acquire free from a large manufacturer as a sample a disc
> 2mm thick x 20mm dia. Such a disc has a resonant frequency of around 1Mhz.
> This is a pretty standard frequency for NDT in metals.
>
> The catch is, a 1Mhz pulse would not survive very far into such a high-water
> content, porous material as an apple, let alone hitting the other end and
> reflecting back.

The water content is not a problem, in reality it helps. The speed of
sound is about 1500 meters/second in water. This gives a wavelength of
about 1500m/s / 1MHz = 1.5mm. The resolution will always be worse than
the wavelength. Maybe 5mm resolution is possible with some care in
design.

Sound is attenuated at about 1db/cm/MHz. As I remember, this is
round-trip attenuation. For an apple 10cm in diameter, that would give
1db * 10cm * 1MHz = 10db of attenuation. Even if I am wrong, 20 db of
attenuation is not a problem with a low noise amplifier. Since the
amplifier can be narrow band, the noise should not be a problem.

I don't know how much the porous nature of the apple will affect the
attenuation. My gut feel is that the porousness is not a problem.


>Apparently, 50kHz is more reasonable. Why not build a 50kHz
> transducer then? The second catch is, a piece of PZT having a natural
> resonate frequency of 50kHz would have to be 50mm thick(that's two whole
> inches!), with a diamter greater than that still. OK then, so what?
>
> Well, unfortunately this isn't compatible with my concept of economy :-).


I think that the wavelength at 50KHz (about 30mm) is WAY too long. In
fact you may need to have more like 10MHz to get the resolution to see
fine details of the fruit's interior.


> Furthermore the drive voltage would have to become impossible high.
> Commercial transducers (read $$$) get around the latter limitation by
> sandwiching together a stack of plates, electrically wired in parallel. This
> is not feasible in my case.


Since the attenuation is only 10db to 20db I don't see that you need
very high drive voltages. A few volts should be more than enough.


>
> And now comes the part where any help from you techno gurus would be
> tremendously appreciated.
> I have a few options here:
>
> 1.    Use the freebie PZT plate to build a 1Mhz transducer and use it at
> that frequency. However, the odds are stacked against it ever being able to
> work with my target samples, and so is the physics.


I think that you must start with this transducer.

You may want to try your hand at making your own high frequency
transducer with the Kynar film materials at a later time. Another
possibility is to find some old or surplus diagnostic ultrasound
transducers.


>
> 2.    Use the freebie PZT plate to build transducer that is excited at
> 50kHz. This essentially means operating the plate in it's 'static mode',
> since the driving frequency is too far removed from the resonant freq. for
> mechanical harmonic motion to play any part. I'm trying to work out the
> relative acoustic output intensity of such an arrangement. I have a feeling
> that, even without any additional dampening, the wave intensity might be
> insufficient.


See comments about wavelength.


>
> 3.    Save the freebie for later, try a different alternative. You've all
> seen piezo buzzers and their u-sonic siblings. They work on a totally
> different principle to the piston transducer discussed above. The piezo
> material is usually very thin (~0.3mm). It is mounted on a brass diaphragm.
> As a voltage is applied across the material, the disc thickens as expected,
> but this isn't important.


I believe that the thickening _is_ the sound producing mechanism that
you _do_ want.



> The key is, it also expands/contracts sideways
> (the radius changes). This causes the diaphragm to flex in and out. The
> actual change in the thickness of the piezo sheet is too small to be
> measured, but the diaphragm flex enough to blast your ears out.
>
> AFAIK, this method is commonplace for air transducers but is not used in
> surface contact NDT.

With a transducer that changes in thickness you will couple from the
flat surface into the material being tested.


>My guess is that pressing such a TX head against any
> material (even via a coupling liquid) would quench the delicate flexing
> osillations, because solid matter compresses so little compared with air.

Noncompressablity means that the sound will be transmitted better! To
get good transmission of the high vibrations you will have to have a
fluid filling the air gap from the transducer face to the surface of the
fruit. Putting both the transducer and the piece of fruit in a fluid is
one way of doing this. Another advantage of having the transducer and
fruit in a tank of water is that the transducer can be father away from
the fruit to put the fruit at the focal point of the transducer. This
allows the transducer to be larger than the fruit. A large transducer
can have a tighter focus than a smaller transducer.



{Quote hidden}

Good luck with your project. It sounds like fun.


>
> Peter L.
>
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2001\02\09@131838 by Olin Lathrop

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> For a school project, I want to investigate the use of ultrasonic waves to
> determine the state of various items of food (eg. fruit).

Sounds like you've done your homework.  It also sounds like you now know a
lot more about ultrasonic transducers than I do, so I can't help there.

However, what about hydrophones?  I don't know if they go to the frequency
range you want, but they are certainly intended to be coupled to water.  You
probably don't want the ones the Navy uses.  At $1,000 per toilet seat, I
can't even imagine what the taxpayers are shelling out for a hydrophone.
There must be consumer applications that use hydrophones.  Cheap fish
finders, maybe?  Maybe you can find something useable as surplus.

I imagine you've looked into this, but I'm surprised you need 50KHz to
measure an apple.  I would have thought audible frequencies would be
sufficient.  Why not emit a step or impluse, then look at the frequency
content of what comes back or goes thru.  I bet you could tell a lot by
looking at the absorption spectrum up to 20KHz.

When I was little, my father taught me how to tell what was inside a
watermellon without hurting it.  You hold one end up to your ear, then tap
on the other end with your fingernail.  With a little practise you can tell
how ripe it is and whether it has cavities in the middle.  People look at
you funny when you do this in the store, but I've never bought a bad
watermellon yet.  My sense is that all the information is well under 5KHz in
this case.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, olinspamspam_OUTembedinc.com, http://www.embedinc.com

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2001\02\09@224035 by rottosen

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Peter:
I may have jumped too quick.  :-)

I have worked with ultrasonic imaging so I just assumed that you would
want enough resolution to get an image.

I agree that you may get useful information at lower frequencies. I will
be interested in the results of your investigation.

If my attenuation formula applies to apples then you may not get very
much loss at 50KHz. It sure is hard to resist grabbing an apple and a
transducer (or 2) and trying it myself...

-- Rich


Peter Lau wrote:
{Quote hidden}

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2001\02\10@055233 by Scott Stephens

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>For a school project, I want to investigate the use of ultrasonic waves to
>determine the state of various items of food (eg. fruit).

I want to use ultrasound to explode pumpkins 8^)

>The catch is, a 1Mhz pulse would not survive very far into such a
high-water
>content, porous material as an apple, let alone hitting the other end and
>reflecting back. Apparently, 50kHz is more reasonable. Why not build a
50kHz
>transducer then? The second catch is, a piece of PZT having a natural
>resonate frequency of 50kHz would have to be 50mm thick(that's two whole
>inches!), with a diamter greater than that still. OK then, so what?

Why not use a cheap piezo disk vibrating in the shear mode? If you look up
acoustics on the web, their are pages that show the drum-head or disk mode.
The best I found is Maple's animations of the Bessel functions. If you buzz
a piezo disk, you will find multiple resonances of these various modes.
Scratch or electrochemicaly etch off the disk's plating to drive these mode
more effectively, and you should be able to get up to a watt out of the
disk. You can find a powerfull mode up around 30 KHz on a 2 KHz piezo disk.

>3.    Save the freebie for later, try a different alternative. You've all
>seen piezo buzzers and their u-sonic siblings. They work on a totally
>different principle to the piston transducer discussed above. The piezo
>material is usually very thin (~0.3mm). It is mounted on a brass diaphragm.
>As a voltage is applied across the material, the disc thickens as expected,
>but this isn't important. The key is, it also expands/contracts sideways
>(the radius changes). This causes the diaphragm to flex in and out. The
>actual change in the thickness of the piezo sheet is too small to be
>measured, but the diaphragm flex enough to blast your ears out.

If you use a laser you can well see it. Or you can sprinkle a dust and drive
it with a few watts to image the mode.

> My guess is that pressing such a TX head against any
>material (even via a coupling liquid) would quench the delicate flexing
>osillations, because solid matter compresses so little compared with air.

Solid is much better than air- you will have a much better acoustic match.

Are you going to try to measure the complex acoustic impedance of food, or
just attenuation? You might need to examine over a broad bandwidth. Kynar
film would be great for that, I doubt much power would be required.

I strongly recommend a kilo-joule shock to disintegrate the vegitable under
test, then spatter-pattern analysis 8^)

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