Re: Is PVDF for achiving high resolution the best choice? (for thin plastic layers) Rolf
Some thoughts on laser-based ultrasound in response to the comments by R.A. Day.
Laser-based ultrasound is definitely not a solution to all problems, but it can be very powerful in
the right application. For the same bandwidth, laser detection of ultrasound is less sensitive than transducer
detection by as much as two orders of magnitude. However, lasers can be very efficient generators
of ultrasound, especially if surface ablation is allowed.
With regard to cost, it is clear that laser systems are more expensive than transducer systems, but this
comparison is academic, because laser systems are only interesting for applications where transducers can
not be used. Well-known examples include scanning of large surfaces and measurement of hot and/or moving parts.
It should also be kept in mind that the price of laser-based systems is coming down, as improvements are made,
especially to interferometric receivers.
Laser-generated ultrasound can be focused using phased array techniques well-known in the rf region. These
techniques are not as efficient as the use of a curved transducer. Nevertheless, significant energy can be
delivered into a specific region in a part. For example, an annular laser beam will produce a maximum in
ultrasonic intensity along the center line of the annulus. More complex multiple-beam geometries can produce
a maximum of ultrasonic energy in a single region of the part.
With regard to temporal resolution, a 10 ns pulse from a Q-switched Nd:YAG laser will produce broadband
ultrasound up to frequencies of about 100 MHz. Picosecond-pulse lasers can produce much higher frequencies.
In many materials the higher frequencies will be attenuated, but for thin film measurements, the short ultrasonic
pulses are very useful. Film thicknesses on the order of microns can be measured with these pulses.
: Rolf -
: Actually PVDF is the best material for achieving resolution but laser generated sound is often better resolution. Problems with lasers are:
: - difficult to focus (needed I think for your application?).
: - poor signal to noise of receive.
: - very expensive.
: You don't say in your post whether you are using a
: focused 15 MHz probe but I would assume you are. One
: possible way to improve performance in such a difficult
: material and geometry is to use a cylindrical focus
: probe whose radius and position is adjusted to match
: the outer tubing. This makes the geometry more plane
: wave like and may improve signals a little. Because you
: have at least two materials with different properties it
: may not improve as much as you need.
: You might be able to get more bandwidth with a PVDF (you
: don't mention transducer type) but to improve the
: performance dramatically you need to increase the
: frequency to 20 or 25 MHz. Do you know if these
: frequencies will penetrate? Can you send samples to try
: things on. Transducer makers like samples!
: You mentioned detecting the positive or negative spike.
: Are you doing this in analog electronics or digital? One
: good way to do wall thickness measurements is to take
: the Hilbert transform of the signal which produces a
: kind of video signal but without the phase distortion
: conventional RF detectors produce. This method may be to
: slow for production applications (100s of milliseconds
: Multiple layer pipe and hose is getting very popular so
: I expect we will be called on to NDE more of these types
: of materials in the future
: : In R. A. Day's article it is pointed out:
: : "The bandwidth of PVDF is actually the best of any material so far discovered and
: : routinely produces transducers with 0.5 to 1 cycle
: : transmitted and received pulses".
: : Is it right that there is no doubt about that PVDF should be used for
: : gaining the best resolution. Especially for excellent near to surface resolution
: :or thin wall thickness measurement it should be applied.
: : Just in a case we would need maximal amplitude as well, we
: : should consider also other transducer materials.
: : Finding a solution for a very thin wall thickness measurement is always a challenge.
: : Here is an example I assume that a PVDF transducer should be used.
: : What would be the best transducer design (Dia, Freq, Focus) for the application of
: : Coextrusion Layer Thickness Measurement
: : for polyamid gasoline pipes.
: : Especially for an In-Line measurement by immersion technique,
: : however to measure also the thickness of some layers.
: : Product dimensions:
: : Dia: about 8mm
: : Total wall: about 1mm
: : 1-5 Layers: each in a range of 0.1mm - 0.3mm
: : I am locking forward your suggestion.
: : Rolf Diederichs