NON-DESTRUCTIVE TESTING – SAM Functional principle (Scanning Acoustic Microscope)
An Acoustic Microscope (Ultrasonic Microscope) is based on the same working principle as
the well known medical ultrasonic examinations: A probe is moved (within a coupling fluid)across the area of interest. An image comes up showing the interior without violating the “object” under investigation.
The heart of the Acoustic Microscope is the probe (=Transducer; loudspeaker and microphone in one piece). It converts the electrical signal into the acoustic signal. The sound waves are focussed and transmitted to the sample by the coupling fluid (normally water). The ultrasonic waves interact with the sample; one part is reflected back to the transducer, the other part is transmitted.
Basically there are 2 methods of ultrasonic imaging: In the majority of cases the “Pulse-Echo”-Mode is used. Amplitude, phase and time of flight of the reflected soundwave are analysied to create the pixel-by-pixel image information. This mode operates with one transducer.
The couterpart of this mode is the “Transmission Mode” (=Throgh Scan Mode). In this process a second transducer underneath the sample receives the transmitted part of the soundwaves. This transmitted signal is the base for the acoustiv throgh scan image.
At both methodes (Puls-Echo- and Transmission- Mode) the inspected sample is scanned pixel by pixel and line by line. The movement of the transducer is realised by a x-y-mechanic, the so called Scanner.
The time of a sample inspection is depending on the size of the sample and the chosen resolution and can last a few seconds till several minutes.
The features of the transducer (frequence and focal length)must be aligned with the application.
For this reason there are a huge number of transducers; comparable to the multiplicity of objects used for high quality cameras.
To choose the right Transducer some things have to be regarded: Transducers with high frequence and short focal length produce a high resolution in comparison with the penetration depth.
The choice of the correct Transducer for the respective application is very important. An agreement between favoured resolution and penetration depth have to be found.
How are contrasts created in an acoustic image?
Ultra sonic waves need instead of light a propagation medium. The higher the mediums density the better sonic waves can expand. This can also be recognized in everyday life: in air (comparatevely “light” medium) ultrasonic waves can expand slow and not so far. For example: loud music in a building can be heard only 2-3 rooms further. Dense materials carried sound much better. For example: metal pipes carry sound excellent; the heaters at the end of a pipe attend to “loud speakers”.
Sonic acoustic microscopy make use of the physical circumstances. After leaving the transducer the ultrasonic waves are transported by the couple medium (generaly water) to the sample. Inside the sample it strikes a lot of different materials and fringe areas that affect the amplitude, the phase and the elapsed time of the ultrasonic signal. The evaluation unit of the sonic acoustic microscope recognized these partly lowest changes and creates an image out of this inforamtions.
In summary: ultrasonic waves react very sensitive to changes and inhomogeneity of the medium it moves in. Primarily differences in density and elasticity are mentioned.
Patented based FCT Transducer by KSI
Due to the Transducers new, optimized design turbulences (blisters) and wave formation in the coupling medium ars reduced. This is necessary, because the maximum scan speed of the new scanning acoustic microscope v-400 rises from 1 m/s up to 2.0 m/s.
- Increase of scan speed from 1,0 m/s to 2.0 m/s is only possible with the brandnew FCT Technology
- Heavy reduction of turbulences in the coupling fluid
- Air bubbles an cavitations underneath the active element are a thing of the past
- Transducer with frequency range of 1 MHz – 230 MHz
- Each transducer is checked and measured thoroughly in terms of bandwith, frequency range, etc., with specially designed measurement
- equipment by KSI-Germany, prior to shipment.
- Highest quality ultrasonic images
Patented based FCT Transducer by KSI
In this video you can compare the ‘new Transducer’ with the ‘old Transducer’.
Due to the Transducers new, optimized design turbulences (blisters) and wave formation in the coupling medium ars reduced. This is necessary, because the maximum scan speed of the new scanning acoustic microscope KSI v400 rises from 1 m/s up to 2 m/s.
You archieve better results in less time!
What is a Tranducer?
The ultrasonic signal of a scanning acoustic microscope is produced inside a transducer. This transducer is assembled to the scanning mechanic of the ultrasonicmicroscope. Inside a transducer an electric signal is transformed into an ultrasonic signal by a piezoelement. After the ultrasonic signal is generated it is focussed by an acoustical lense on the bottom end of the transducer and conducted into the coupling medium (water). The coupling medium is needed to transfer the ultrasonic signal with higher energy into the specimen. As the attenuation of the ultrasonic signal inside the coupling medium is much lower than in air.
Due to physical reasons every transducer has its own frequency-range. Therefore it is necessary to change the transducer when switching to significantly higher or lower frequencies. On the SAM-systems of KSI the change of a transudcer is easy and quick by turning one transducer out and another in. The available frequency-range for KSI-products is 1 MHz to 2000 MHz.
In addition to the generation of the ultrasonic signal the transducers of KSI contain another feature: The new patented shape for highest scanspeeds. Due to the highest scanspeed of 1500 mm/s of the new v-Series air-blisters can be caused by an usual transducer when moving through the coupling medium. These blisters can cause a partly blackout of the ultrasonic picture when flowing between transducer and specimen.
To solve this problem KSI developed a special transducer-shape that highly reduces turbulences and air-blisters inside the coupling medium for faultless ultrasonic pictures created at highest scanspeeds.
V(z)-Curve-Rayleigh wave lens
V(z)-Curves are produced with the help of special transducers that create not only longitudinal-waves but also Rayleigh-waves. V(z)-Curves contain all the material parameters determinable with the help of the acoustic imaging. To produce a V(z)-Curve the focal point is software-controlled displaced from the surface into the sample, the intensity of the reflected ultrasonic signals is measured (V=voltage) and plotted against the distance the acoustic lens is lowered (z in µm).
The result is a frequency dependent curve of the material or layer combination, characteristic for the sample investigated: V(z)-Curves allow a completely new, non-destructive type of material characterisation in the micron range as they are the key to a wide range of quantitative measurements. V(z)-Curves are influenced by varieties in film thickness and reveal defect structures under the surface.
V(f)-Curve – Lamb wave lens
The lamb – wave lens is a special lens adapted to a transducer. Lamb wave lenses are proposed for imaging and characterization of layered structures. They differ from a conventional transducer-lens as they have a conical refracting element to selectively excite surface waves. With this special lens images can be obtained that are easy to interpret, with a higher accuracy and higher penetration power of the soundwaves.
To improve the achieved resolution the effect of the central beam (see picture on the bottom left; array 2) can be eliminated by using very short ultrasonic-pulses and time gating.
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