I have a few piezo transducers acquired from surplus. They were made by Plessy for use in Sonobouys.
Cylindrical form, about 40mm dia x 65mm long. Barium Titanate core with silver plating inside and out as separate electrodes.
Can anyone tell me what would be the best way to determine the resonant frequency of these devices using standard test equipment?
What kind of fun things (projects) are these good for?
Bob Lipton
Coupling lightly with a small capacitor and sweeping with a signal generator will get you the resonance(s) in air, for a start. Observe the peak(s) with a 'scope.
Cheers, James Arthur
Well, you could build an oscillator and look at it with a scope or a frequency counter.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
snipped-for-privacy@radiodyne.com wrote in news:pl90s7l7pfh5oppk05qm5qfnd1f86r0q34@
4ax.com:
Ultra-sonic cleaning? The size suggests high power capacity and a reasonable resonance frequency......... Also, it would be fun, to see whether a pair of them might be able to produce a transmitter/reciever pair using air, water or even the soil as medium. In water you might be surprised about the range you might get. In air or water, putting them in the centre of a parabolic mirror would give you an opportunity of some nice research.
Connect it to an oscilloscope and tap it with a small screwdriver.
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Ultrasonic fishing?
Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
I have a similar transducer. Mine is 37 mm in diameter by 37 mm long with a wall thickness of 3 mm, a little smaller than yours.
The standard piece of test equipment that works best for determining the resonant frequency(s) is an impedance analyzer.
This image:
shows a scan of the impedance magnitude from 10 kHz to 10 Mhz.
As you can see, there are several resonances. I think the one at about 66 kHz is the useful one. Your unit will probably have a resonance around there.
Assuming you have a driver, which you'll need anyway, and if it has variable frequency, then you can connect up the transducer, touch a small metal object such as a knife blade lightly against the active face then sweep the frequency slowly. When the acoustic output peaks you generally hear a squeal due to subharmonics of the vibration. The advantage of this method is that it measures the acoustic output directly, transducers can display a variety of electrical poles and zeros that don't always align with the maximum sound output.
If it's a transducer designed for water, then it's going to be next to useless in air, the coupling factor is absolutely miniscule. Attenuation of high frequency sound through the ground is also generally very high, unless you're talking solid rock.
I've worked with u/s for a long time with nothing more than a scope. It is useful to have a couple of sensors with a Fr that's different to what you're mainly working with, as tell-tales. If you're lucky, their frequency response will be relatively flat where you're working, you can then use them to adjust for peak output, that sort of thing. I use unmounted piezo disks with wires attached, or magnetostrictives with permag biasing inbuilt and a few turns round them.
Hook up a couple to an amp as mic and speaker. Crank it up until it feeds back.
High-tech tin can telephone?
In that band, you can use your sound card as a spectrum analyzer with tracking generator option. A 24 bit EMU-1212 will sample at 192000 S/s which gets you resolution above 82kHz, and you can squeak up to 87kHz.
Produce sound, and 'read' the response. A little bit of characterizing all the electronics and you can create a Spice model for your transducer - well not be a bad model. Armed with that you can then determine the best driver.
These transducers probably have a preamp in the transducer cavity. I'm basing this off the sonobuoy units I futzed with in the 1980s for dolphin research. The ones I dealt with were two-wire, so you need to mux power on the signal with a 2.2k resistor. Yours may be different.
These things generally go up to 30 kHz. They are very sensitive and make nice underwater listening devices. Fun for the whole family. Near shore in warmer climates, snapping shrimp are common. Out in the ocean, clicks from dolphins and groans from whales. In the toilet, a flush is a fun way to entertain your beer buddies. Extrapolate the toilet stuff with beer buddies, and the fun is endless. You may need to seal the top portion of the transducer since the seals aren't that good. They're only meant to last a few hours.
To measure the resonant frequency, you need to remove the preamp and connect the transducer to an impedance analyzer. Do an admittance measurement with the xdcr in water and see where the conductance peaks. That will be the resonant point. I'm guessing it will be around
30 to 50kHz. Be sure to short the terminals of the transducer before connecting to your analyzer as there can be significant DC voltage (hundreds of volts) across the xdcr terminals due to mechanical deformation (temperature). There is enough capacitance to give you a surprise jolt if you have a couple hundred volts across the terminals.
Resonant frequency in the radial mode of vibration should be
f = sqrt(E/rho)/(pi * D)
where E is Young's modulus and rho is the density and D is the OD
if we assume that material is BaTiO3, then using Matweb numbers
E = 107E9 Pa , rho = 5500 kg/m^3 (and D = 0.04m)
f~= 35 kHz.
The numbers are all over the place (MEMsnet says 67GPa and 5800 kg/m^3) which yields 27kHz, so probably somewhere around 31kHz +/-20%.
Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Sonobuoys you say, they may be designed to not have much resonance instead.
They are designed to operate in fluids , maybe ultrasonic cleaners?
?-)
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