G
George Gabriel
Guest
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
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
