Interpretation of
Ultrasonic Transducer
Return Signal
Ronald Fox
November 13, 2009
Keywords: Ultrasonic Transducer, Amplifier, Butterworth, Band-Pass
Abstract: This application note describes how to process and interpret the ultrasonic
transducer return signal from a railroad spike.
Introduction
The main goal of an Ultrasonic Flaw Detector is to determine if a flaw is present within a
testing material. The core component of an Ultrasonic Flaw Detector is a piezoelectric
ultrasonic transducer. The transducer probe converts electrical energy to mechanical
energy, in the form of sound. This sound wave penetrates the testing material and is
reflected back into the transducer where the sound wave is converted back into an
electrical signal. The return electrical signal contains valuable data relating to possible
flaws within the test material.
Objective
The return signal generated from an ultrasonic transducer is in the form of electrical
pulses. The timing of the pulses is crucial in determining the presence of a flaw as well as
the depth of the flaw within the spike. The return signal must be amplified and filtered to
determine if the flaw lies within the appropriate region where flaws are commonly
present on a railroad spike.
Amplifier
The piezoelectric ultrasonic transducer is initially excited by a negative going 600 volt
fast rise time, short duration pulse. This voltage signal is converted into a longitudinal
ultrasound wave that penetrates the testing material. The longitudinal wave is a
compression wave in which the particle motion is in the same direction as the
propagation of the wave. The large negative spike in voltage will ensure that the
longitudinal ultrasonic wave does not lose all of its energy within the solid medium. The
wave must have the ability to penetrate a material and return a large enough signal from a
defect that can be easily processed.
An ultrasonic wave naturally attenuates as it travels through the solid medium. Assuming
no major reflections, there are three causes of attenuation: diffraction, scattering and
absorption. Scattering is the reflection of a sound wave in directions other than its
original direction of propagation. Absorption is the conversion of ultrasound energy to
other forms of energy. The decaying amplitude can generally be represented by the
following mathematical equation.
A = A 0 e −αz
A 0 is the amplitude of the longitudinal wave before the affects of attenuation. Z is the
distance the propagating wave has traveled from the initial location and α is the
attenuation coefficient. The signal decays exponentially with respect to the distance the
wave propagates. This equation states the importance of creating a large initial signal that
will ensure proper return signals from within the railroad spike.
Energy is also lost when the wave is transmitted and reflected across two separate
mediums. Some mediums commonly dealt with are air (flaw in the material), water
(couplant) and in the case of a railroad spike, 1020 steel.
The following is an example of amount of energy loss in the propagating sound wave.
The loss of energy on transmitting a signal from medium 1 into medium 2 is given by:
2
dB loss= 10 Log 10 [4Z 1 Z 2 / (Z 1 + Z 2 ) ]
Z1 = Acoustic impedance of material 1
Z 2 = Acoustic impedance of material 2
Transmitting a signal through a water couplant with acoustic impedance Z1 =1.48 into a
1020 steel railroad spike with acoustic impedance of Z 2 = 45.41 give a -9.13 dB loss in
the sound wave.
Attenuation within the spike and at medium boundaries causes the return signal to be
relatively low in amplitude, typically in the millivolts range. An amplifier is necessary to
increase the gain of the signal where it can be more easily analyzed and processed for the
appropriate data.
Receiving a signal with such low amplitude of voltage and at a high frequency of 2.25
MHz can be tricky to amplify. The TLC227 family of op-amps produced by Texas
Instruments is an appropriate IC for handling the small signal conditioning for high
impedance sources such as the piezoelectric transducer. Creating a simple non-inverting
operational amplifier with the TLC227CD IC will ensure that the return signal is
amplified to an appropriate voltage level. Below is a graph of the peak-to-peak output
voltage with respect to the supply voltage. According to the graph we could choose
roughly 5 volts for the supply voltage which would give 5 volts peak-to-peak amplitude
in the output signal.
Supply Voltage vs. Output Voltage
Filter
The amplified signal from the piezoelectric transducer needs to be analyzed to determine
if there is a flaw present in the solid medium. The time response of the amplified return
signal can inform if a flaw is present as well as provide its location relative to the top of
the spike. The figure shown below shows a typical 1020 steel rail spike. The average
failure point of the spike lies almost directly in the center of the spike. The region 30
millimeters from the top and bottom of the spike is the flaw detection region where flaws
will most likely occur. Knowing the length of this region as well as the frequency at
which the transducer operates at one can create a filter to detect the presence of defects in
the flaw detection region.
Flaw Detection Region of Railroad Spike
A simple second-order Butterworth band-pass filter will allow for the reflected signals
from flaws lying within the flaw detection range to be passed on for further processing.
The velocity of the 2.25 MHz signal within the 1020 steel spike is 5890 m/s. The velocity
and length of the spike allows for calculation of the time period that reflections from
flaws generally appear. The time range of possible defects is calculated to lie between
10.186 μs and 48.03 μs. This data will allow for a band-pass filter to be created passing
only signals that lie within the flaw detection range. The below schematic is a
Butterworth band-pass filter using 741 op-amps. The left side of the schematic is the lowpass filter at 98,173 KHz and the right side is the high-pass filter at 20,820 KHz. The
frequency corresponds with the time period of the flaw detection range.
Butterworth Band-Pass Schematic
The spice simulation shows that the reflected signals within the flaw detection range lie
above approximately 0.7 volts. The filter will ensure that the appropriate signals that
represent flaws are separated from the reflected signals that are not flaws such as
reflections off the front and back of the spike. The filtered signal is then passed to a
microcontroller where it is further processed to let the user know the outcome of the
spike.
Spice Simulation of Butterworth Filter
Conclusion
The result of producing an appropriate signal for interpretation of the reflected ultrasonic
waves from piezoelectric transducer is essential for the ultrasonic flaw detector. The main
purpose of the device is to accurately detect if flaws are present within a railroad spike.
An amplifier and band-pass filter are both cheap and reliable circuits that will allow for
interpretation of flaws from the received ultrasonic waves. With reliable interpretation of
the return signals of the ultrasonic transducer the user will have no problem
distinguishing defective spikes from good quality spikes.
References
[1] NDT Resource Center, Iowa State University Center for NDE, 11 Nov. 2009.
<http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/.htm>
[2] TLC2274CD Datasheet, 10 Nov. 2009.
<http://pdf1.alldatasheet.com/datasheet-pdf/view/98794/TI/TLC2274CD.html>
[3] Panametrics-NDT Ultrasonic Transducer Technical Notes, Olympus Corporation.
<http://www.olympus-ims.com/data/File/panametrics/panametrics-UT.en.pdf>