FMC involves capturing raw ultrasonic echo signals from every possible transmitter-receiver pair in an array. This allows reconstruction of standard phased array or TFM imaging through post-processing. TFM processing uses a dedicated focal law for each pixel to focus processing, but physical limits affect near-field focusing and steering angles. The large data volume of FMC requires high-speed storage and processing.
18. • Data example, 16 elements probe
• NOT to be used by human…
18
FMC – Data example
• Element 1 firing, receiving on 1
through 16• Element 2 firing, receiving on 1
through 16
• Element 16 firing, receiving on 1
through 16
• Main Bang of probe on wedge
• Signal inside wedge
• Interface signal between wedge
and component• Signal inside component
• Actual data used by TFM for image
construction
31. 3131
TFM – Amplitude
• Several phenomena can impact the precision of the amplitude
reading on a given TFM pixel within a frame
– Effect of frame resolution versus probe frequency
– Effect on interface signal
32. • The Nyquist Theorem, also known as the sampling theorem, is a principle
that engineers follow in the digitization of analog signals. For analog-to-digital
conversion (ADC) to result in a faithful reproduction of the signal, slices,
called samples, of the analog waveform must be taken frequently.
• Signal frequency can be expressed as period or wavelength
– 5 MHz signal = 200 ns period or 1.2 mm Wavelength (LW in steel)
• Waveform cannot be preserved with less than 2 samples per period
• Even when waveform is preserved, signal peak amplitude can be missed,
resulting in amplitude error in digitized signal
– Higher digitizing frequency reduce the amplitude error
• In Ultrasound methodology, an effective digitizing frequency of 5 times the
probe frequency is required to limit this amplitude error
– 5 MHz signal = 25 MHz minimum digitizing frequency (1 sample every 40 ns)
32
Digitizing Frequency of Signal
33. 33
Digitizing Frequency of Signal
Digitizing at
20 X signal
Frequency
Digitizing at
5 X signal
Frequency
5 MHZ Signal
Digitized at 100 MHz
Digitized at 25 MHz
Sample positions are variable
34. • Low digitizing frequency is subject to variable error level in
amplitude
34
Amplitude Error related to Digitizing Frequency
Peak Value = 100 %
Digitized Value = 96 %
35. • At same digitizing frequency, exact error is dependent on signal
timing
– If the sampling is synchronized with the peak signal = no amplitude error
– If the sampling is exactly halfway before and after the peak = maximum
amplitude error
Amplitude Error related to Digitizing Frequency
Synchronized signal, no amplitude error Worst case, maximum amplitude error
36. • Maximum error occurs at random occurrence, depending on signal generator location (flaw location)
• Maximum error can be computed
– Phase spread = number of degrees between two consecutive samples
– Phase error = number of degrees between a sample and peak location
– Maximum Phase error = Phase spread / 2
• Occur when samples are at same distance before and after the peak
– Amplitude error ratio in % = cos(Maximum Phase Error)
– Amplitude error in dB = 20 X log (cos(Maximum Phase Error))
Amplitude Error related to Digitizing Frequency
Synchronized signal, no amplitude error Worst case, maximum amplitude error
37. • Sample Error level
– 5 MHz signal, digitized at 25 MHz = 5 points per period
– Phase spread = 360 / 5 = 72 degrees
– Maximum Phase error = 72 / 2 = 36 degrees
– Amplitude Error ratio in % = cos(36) = 80.9%
– Amplitude Error (AE) in dB = 20 X log (cos(36)) = -1.85 dB
• Other examples
– 20 samples per period: AE = -0.1 dB
– 5 samples per period: AE = -1.85 dB
– 4 samples per period: AE = -3.0 dB
– 3 samples per period: AE = -6.0 dB
– 2 samples per period: AE = infinite loss
Amplitude Error related to Digitizing Frequency
38. 3838
TFM – Amplitude subject to resolution
• Every pixel in a TFM frame is created using a dedicated focal law
– Corollary: each pixel has ONLY one focal law to cover it
– Focal law is tuned for center of pixel
• Depending on instrument and software capability, issues may be present:
– Size ratio of a pixel versus wavelength of the ultrasonic beam is critical for
appropriate imaging
– A large pixel will statistically miss the peak amplitude of a signal
– The Nyquist Theorem must be respected for a peak signal to be detected
– Systems with low and fixed frame resolutions (i.e.: 256 * 256) are more
susceptible to this issue
– Systems with higher and flexible frame resolutions (x * y) can overcome this
issue and generate high quality images
39. 3939
TFM – Nyquist in TFM
• Given a pixel size, the acoustic wave may peak
right at its center, or elsewhere
• When the signal is at its peak elsewhere, the
amplitude used in TFM construction is lower
than expected
• When the size discrepancy between pixel and
wavelength is large, a large error in amplitude
is statistically expected.
– 20% (-2 dB) drop is measured when the
pixel size is smaller than 1/5 the wavelength
of the ultrasound beam
– 100% of the amplitude can be lost when the
pixel size is 1/2 the wavelength of the
ultrasound beam
Probe
0 LW
Example of signal at 2 different depth. Amplitude will
vary depending on location and Nyquist theorem
40. 4040
TFM – Nyquist Example
Example A
• V Weld, Carbon steel (LW)
• Thickness 25mm
• Frame depth with coverage of first
and second leg require 50mm
• 50mm / 256 pixel = 0.2 mm per pixel
• 5MHz probe in steel = 1.2mm
• Factor: 1.2 / 0.2 = 6
• Maximum phase error: 360°/ 6 / 2 =
30
• Amplitude error:
– 20X log(cos(30)) = -1.2 dB
Example B
• Steel Bolt (LW)
• Thickness 100mm
• Frame depth with coverage of first leg
require 100mm
• 100mm / 256 pixel = 0.4 mm per pixel
• 5MHz probe in steel (LW) = 1.2mm
• Factor: 1.2 / 0.4 = 3
• Maximum phase error: 360° / 3 / 2 =
60
• Amplitude error:
– 20X log(cos(60)) = -6.0 dB
41. 4141
TFM – Nyquist Example
6 Pixels per wavelength
• Maximum Amplitude: 97.3%
• Width (at -6 dB): 3.3mm
• Echo Dynamics Curve
– Regular amplitude distribution
2 Pixels per wavelength
• Maximum Amplitude: 69.4%
• Width (at -6 dB): 2.1 mm
• Echo Dynamics Curve
– Jagged amplitude distribution
Tilted Notch
• Single FMC data
collection
• TFM processed twice
• Only parameter modified
is pixel size in TFM
frame
• Every other parameter is
kept constant
• Same indication
visualized!
42. 4242
TFM – Nyquist Example
6 Pixels per wavelength 2 Pixels per wavelength
Tilted Notch
• Same notch
• Same parameters
• Probe position is moved 0.5 mm
• Amplitude pattern changes
drastically when under sampling
43. 4343
TFM – Amplitude and Interface/Dead zone
• As opposed to STF, where all beams transit through a small area and
cause saturation interference, TFM has a relatively smooth interface
signal
• In TFM, even when the frame is located below the probe, the
contribution to the top row of pixels is performed in such way that the
energy is distributed, and usually prevents saturation
• In fact, top rows of a frame dedicated to corrosion mapping will have an
interface signal that is less disturbing than with an equivalent phased
array approach
• This effectively reduces the dead-zone in the top section of the frame
compared to regular phased array
46. • Currently, no system supports TFM calibration; this creates
issues for amplitude-based inspection techniques
• Draft proposal for amplitude calibration is “in course of
preparation”, scheduled mid-2017; No proposal yet for wedge or
velocity calibration
• Subject to change as technology evolves …
46
Calibration