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Driving Piezoelectric Transducers

Discussion in 'Electronic Equipment' started by George Gabriel, Nov 15, 2004.

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  1. Hello to everybody,
    as a part of my thesis work I'm trying to design a device for driving
    piezoelectric transducers, to be used as a portable non-destructive
    testing device.

    I have lots of troubles in finding some good technical documentation
    about probes. I'm quite confused so I think it's a good idea to ask for
    help, before I'll go brain-dead.

    I tried looking on several web sites of some serious producers of
    piezo-ultrasonic transducers (like Panametrics-ndt) but I haven't found
    detailed *electrical* information regarding a model I can use to
    represent the device as bipole. Asking by email was hopeless too.

    In a more detailed way:

    The transducer is a wafer of piezoelectric material between two
    metalized layers used as electrode for polarizing the active one (so
    applying a voltage we can produce a thickness-deformation, in this case.)

    The first-order equivalent model could be a capacitor; if we want a more
    detailed characterization, we have to assume some RLC-series branch in
    parallel with this capacitor to account for the multiple resonance
    frequecies that the device shows.

    The transducer will absorb a very low power from the driving device. I
    read somewere (I can't remember) about 250mW.
    Piezoelectric properties could be lost forever if the device dissipate
    too much power in heat (If the temperature is over the "Curie
    temperature" for too much time, we'll assist to a irreversible
    depolarization of the piezoelectric layer.)

    I wasn't able to find these informations and characteristics parameters
    on the datasheet (ehr, I wasn't able to find the datasheet too) of the
    probes I found, because producers usually assume that you'll plug these
    device in yet-built pulser/receivers. So you'll not need these information.

    I tried to do some "reverse-engineering work" to discover something
    regarding those devices, looking at the characteristics of some
    pulser/receiver (like those about Ritec or Panametrics-ndt) or power
    amplifiers usually used to drive transducers.

    I discovered that all those devices are assumed to be closed on 50 ohm
    impedance.

    Here is the origin of all my doubt!

    Why they assume to close the pulser on a 50 ohm bipole, if the
    transducer is modeled as a capacitor?

    Have I to worry myself about that or it's just a normal "loading condition?"

    Perhaps in the transducer is included a matching network so the
    impedance we see looking in the bnc is just 50 ohm?

    But it's quite strange, to me, because if this is the case applying a
    200 V peak voltage, the power absorbed by the transducer is

    P = (V^2)/R = 200*200/50=800 W

    a rather huge power...

    Perhaps the 50 ohm load is just a standard way to describing the
    characteristics of the pulser, so I haven't any matching network?
    But in this way we haven't any matching condition realized (nor maximum
    power transfer, nor minimum distortion)

    I'm really confused...

    Is there anybody that knows links or articles about the information I need?

    Thanks in advance for your help,

    Pierpaolo
     
  2. MK

    MK Guest

    Your simple capacitor model is no good at all when you are driving at high
    power and want reasonable efficiency since this will usually involve
    operating the piezo transducer at or near it mechanical resonant frequency.

    There is a lot of tutorial stuff on

    http://www.morganelectroceramics.com

    start at

    http://www.morganelectroceramics.com/piezoguide1.html


    which may help you.

    Good luck.

    Michael Kellett
    www.mkesc.co.uk
     
  3. John Fields

    John Fields Guest

     
  4. Here's a link to an Apex app note that might help.


    http://eportal.apexmicrotech.com/mainsite/pdf/an25u.pdf?guid=&item_id=&f_page=

    Don Cleveland
     
  5. Joerg

    Joerg Guest

    Hello George,

    They need to be driven like transducers in a medical ultrasound system.
    Basically there are two schemes, unipolar or bipolar. The lower cost
    version is unipolar and it works well. A FET that can withstand the
    required pulse voltage has its drain connected to a large positive
    voltage via a choke. The drain connects to the transducer. Another LC
    (or LR) combination can be used to get rid of the DC component on the
    transducer. Now a brief pulse or a burst of pulses is applied to the
    gate. It is important to drive this gate hard. The pulse length is
    roughly compliant with the expected center frequency of the piezo. In
    case of materials such as PZT-5 you can calculate that from the thickness.

    As for voltage level it is not uncommon to pulse PZT-5 in excess of
    100V. Please be cautious with these voltage levels. Especially at DC
    these voltages can be very hazardous or even lethal.

    Also, when sending bursts you have to mind the dissipation in the FET.
    While Rdson might be low enough much of the losses there will be caused
    by the slopes of the drive signal where the FET has to transit through
    its linear region.

    Regards, Joerg
     
  6. Thanks to all people answered me, now I'm reading the material with much
    interest.
    Expect new questions soon!
    :)
     
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