םיאמדקאהו םילכירדאה , םיסדנהמה תכשל
לארשיב םייגולונכטה תועוצקמב
אל תוקידב ףנע תונוכמ יסדנהמ תדוגא
תוסרוה
February 24, 2011
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Sensors
•
AE Sensor Design
•
Piezoelctricity Effect and Piezoelectric Manufacturing Process
• Modes of Vibration and Temperature Effect
• Frequency response
• Modification of AE Waves by Medium and Sensor
• Aperture Effect
• Resonant Sensors
•
Wide Band Sensors
• Differential Sensors
• Capacitive Sensors
• Laser Interferometer Sensors
•
Couplants, Bonds and Sensitivity
• Installation of Sensors on Structures, Waveguides
Preamplifiers and Cables
• Preamplifiers
•
Frequency Filters
•
Important Aspects of Preamplifier Performance
• Coaxial Cables/Connectors
• Other Cables
•
Electric Properties of Cables/Impedance Matching
Sensor Calibration
• Primary Calibration
• Reciprocity
•
Reproducibility
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• Digital Signal Proccesing
• AE Data Acquisition Devices and Block Diagram
• AE Systems and Signal Noise Generator
• AE Digital Signal Processing Features
•
•
•
•
Hit Based Features
Time Driven Features
Combination of Features
HDT,PDT and HLT
• Frequency Filters
• Thresholds Used in AE
• Background Noise
• Waveform Based Processing
• Burst and Continues Signals
• E 1316: Standard Terminology for Nondestructive Examinations
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 An AE sensor consists of several parts:

A piezoelectric ceramic element with electrodes on each face.
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One electrode is connected to an electric ground, the other to a signal lead.
A backing material behind the element is designed to minimize reflections back to the element and to damp the signal around the resonance frequency.
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The case provides an integrated mechanical package and may also serve as a shield to minimize electromagnetic interference.
 Sensors may have an internal preamplifier (integral sensors).
Typical AE sensor mounted on a test object.

 Two principal dimensions associated with AE sensors are the piezoelectric element ’s thickness and diameter.
 Element thickness controls the frequencies at which the sensor has the highest electrical output (sensitivity).
 Active element diameter defines the area over which the sensor averages surface motion.
 The piezoelectric and elastic constants of the piezo also effects the resonance frequency.
Case
Preamplifier
Electrical
Lead
Piezoelectric element
Couplant layer
Wear Plate element and preamplifier.

 Acoustic Emission (AE) sensors usually use piezoelectric elements for transduction.
 The element is coupled to the test item ’s surface, so that dynamic surface motion propagates into the piezoelectric element.
 The dynamic strain produced in the element produces a voltage-vs.-time signal as the sensor ’s output.
Fig. 1: When pulled, compressed or twisted, and electric charge is generated.
The reverse is also true, when applying an electric charge to the element.
Fig. 2: Piezoelectric element under compression/tensile stress.
http://greencleen.wordpress.com/2010/09/22/talk-powered-cell-phones/ http://www.newelectronics.co.uk/article/Defa ult.aspx?articleid=22782&img=1
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 Principal steps in a piezoelectric crystal fabrication:
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
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Powder Mixing- Fine powders of the component metal oxides are mixed in specific proportions.
Calcination- The mixed powder is heated to a temperature of about 1000 °C in order to remove volatile elements and create thermal decomposition.
Pressing- The powder is then pressed in the desired shape of element.
Firing- The element is then sintered according to a specific time and temperature program.
Electroding- Electrodes are attached to the desired faces of the element
Poling- The element is then subjected to a strong direct current electric field just below the curie temperature
Before (left), during (middle) and after (right) poling.
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Effect of Currie Temperature:

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At a certain temperature, Curie Temperature, piezoelectric ceramics undergo permanent change and loose there piezoelectricity.
Piezoelectric ceramics have been used successfully within 50 0 c of their cuie temperature.
Testing limitation are encountered in environments with high temperature due to loss in piezoelectric characteristics.
Effect of Fluctuating Temperature:
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Special problems are encountered when sensors encounter fluctuating temperatures.
Piezoelectric ceramics consist of domains which a regions which the electric polarization is in one direction.
Temperature changes can cause some of these domains to flip, resulting in a spurious electric signal that is not easily distinguished to an AE event.
Sensors should be allowed o reach thermal equilibrium before data is taken at different temperatures
Single crystal piezoelectric (quartz) are recommended for application with fluctuating temperatures or the use of wave guides.
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 The majority of emission testing is based on the processing of signals with a frequency range from 30kHz to 1MHz.
 Attenuation of the wave motion increases rapidly with frequency, and for materials with high attenuation, it is necessary to sense lower frequencies to detect AE hits.
 For materials with low attenuation, the background noise will be higher, so
AE hits will be easier to detect at high frequencies.
 AE sensors can be designed to sense a portion of the whole frequency spectrum by changing the piezoelectric dimensions, which account for the popularity of this transduction mechanism.
Fig. 6: typical Voltage vs. Time and Power [dB] vs. Frequency spectrum.
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 When the displaced surface is a sine wave, then there are occasions when one or more full wavelength will match the diameter of the piezoelectric element.
 When this occurs, the average movements may give a zero output.
 This effect, called the Aperture Effect, has been carefully measured and theoretically modeled.
 For an element larger than the wavelengths of interest, the sensitivity will vary with the properties of the test material, depending on frequency and on direction of wave propagation.
 Because of these complications, it is recommended that the element ’s diameter will be as small as other constraints allow.
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
These sensors have one or more preferred frequencies of oscillation governed by crystal size and shape.
 Enables to make select a suitable trade-off between the desired detection range and the noise environment.
 Higher amplitude output in response to broadband excitation.
 Low fidelity: Output is not similar to the motion of the original wave.
 Most practical AE testing employs resonant type sensors that are more sensitive and less costly than wideband sensors.

In practice, the vast majority of AE testing is done with sensors that are resonant at about
150 kHz.
 Sensor response is determined by the :
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Piezoelectric crystal
The way the element is backed and mounted inside the senor housing
Coupling and the mounting of the sensor.
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 Wideband sensors are typically used in research applications or other applications where a high fidelity AE response is required.
 Frequency response I relatively smooth and flat.
 High fidelity: Good reproduction of original wave motion.
 In research applications, wideband AE sensors are useful where frequency analysis of the AE signal is required.
 Helps determine the predominant frequency band of AE sources for noise discrimination and selection of a suitable lower cost, general purpose AE sensor.
 In high fidelity applications, various AE wave modes can be detected using wideband sensors, providing more information about the AE source.
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 Used mainly in laboratories.
 Flat frequency response.
 Capacitance is a property that exists between any two conductive surfaces within some reasonable proximity.
 Changes in the distance between the surfaces changes the capacitance.
 It is this change of capacitance that capacitive sensors use to indicate changes in position of a target.
 Capacitive sensors are basically position measuring devices.
 Their outputs always indicate the size of the gap between the sensor's sensing surface and the target.
 When the probe is stationary, any changes in the output are directly interpreted as changes in position of the target.
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 In recent years the laser based ultrasonic (LBU) method has been developed to detect flaws in materials, which consists of two techniques:
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The generation of ultrasonic waves by laser.

The detection of surface motion by laser interferometer.

Advantages:
Non-contact measurement is practicable. Can be accurately calibrated.

Measurements are possible in hostile environments.
 Disadvantages:
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Most laser interferometers require a reflective surface for sufficient sensitivity.
Handling or setting-up a laser interferometer equipment is not very easy.
Fig. : Methods of AE Interferometer.
Fig. : Interferometer used for AE and displacement measuring.

 Differential sensors are identical to general purpose sensors in physical, electrical and response characteristics, except that two signal leads
(instead of one as for general purpose sensors) are brought out to attach to a differential pre-amplifier.
 By using a differential preamplifier, common mode noise is eliminated, resulting in a lower noise output from the preamplifier, and a higher electrical noise rejection in difficult and noisy environments.
 Noise improvements in the range of 2dB or higher can be expected using a differential sensor and preamplifier over a single ended general purpose sensor.
 Differential sensors are used in environments where very low level AE signals need to be processed and is also very applicable in high noise environments.
 Differential sensors are slightly higher in price than their general purpose sensor counterparts.
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 The purpose of the couplant is to provide a good acoustic path from the test material to the sensor.
 The mounting has a significant effect on the performance of the sensor (sensitivity and frequency band).
 Optimum and reproducible detection of AE requires both appropriate sensor-mounting methods and procedures (see ASTM E-650)
 Mounting Methods:

Compression (Use of mechanical force, couplant is strongly advised)

Bonding (direct attachment with adhesive that also acts as the couplant).
 Virtually any fluid will act as a good couplant. Fluid will not transmit shear waves.
 It is necessary for the couplant to have chemical compatibility, to fully wet the surface but not to corrode it.
 Mounting Requirements
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Sensor Selection (size, sensitivity, frequency response environmental and material compatibility)
Structure Preparation (mechanical preparation and cleaning)
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Couplant or Bonding Agent Selection to suite with the environment as well as acoustical conductivity.

General mounting techniques (amount, selection of couplant and mounting fixture)
 After mounting verification of sensor sensitivity should be verified (standard requirement)
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Type of installation and choice of couplant material is defined by a specifics of application.
 Glue (superglue type) is commonly used for piping inspections.
 Magnets usually used to hold sensors on metal pressure vessels. Grease and oil then used as a couplant.
 Bands used for mechanical attachment of sensors in long term applications.
 Waveguides (welded or mechanically attached) used in high temperature applications.
 Rolling sensors are used for inspection rotating structures.
 Special lead (Pb) blankets used to protect sensors in nuclear industry.
Sensor attached with magnet Pb blanket in nuclear applications
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Waveguide Rolling sensor produces by PAC

 The function of an preamplifiers is to increase the strength of the input signal.
 Amplifying and frequency filtering are the two components of signal conditioning.
 The typical AE amplifieir is a linear, voltage amplifier with the property:
Output Voltage= Input Voltage x Gain
Vo(t) = G x Vi(t)
 Gain - the ratio of output voltage to input voltage- can also be expressed in decibels B). The decibel is a logarithmic unit: dB = 20 logG
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 To bring the voltage to a higher level, so that any electromagnetic noise picked up on the long cable will have relatively less effect.
 To provide circuitry that can deliver the signal down long lengths of cable with Minimum loss.
 Frequency filters are often package as an integral part of the preamplifier.
Amplifiers in the main system:
 To bring the signal to the desired level for measurement. This gain can be either a fixed feature of the system, or an operator-controlled test variable.
Preamplifier 60 dB
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Noise
 The generation of electrical “Johnson“ noise is a thermodynamic process associated with all resistive component.
 The actual input resistance at the preamp is chosen to optimize the signal-to-noise ratio.

This noise is somewhat influenced by the bandwidth of the filters used. The narrower the Bandwidth, the less the noise.
 Theoretically it is possible to reduce the level of noise, by decreasing the temperature of the resistor, although not always practical.
Dynamic Range
This is simply the range from the smallest manageable signal to the largest manageable signal.
 In other words, the range from the electronic noise level to the saturation level.
 Depending on just how you specify its measurement, it is on the order of 80dB for AE systems.
 Note that the 16-bit digitization process used in today's DSP-based systems has a nominal dynamic range of 96dB( each bit gives a factor of 2, or 6dB); thus, it can give an adequate rendering all the way from the highest signals down to electronic noise and AE signals just emerging from it.
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 The cables function is to transmit an electric signal from the source to the main load and to connect points together electrically.
 For AE work, coaxial cable is almost universal because of its superior electrical shielding.
 RG-58 is most commonly used due to its sturdiness with BNC connectors.
 BNC (Bayonet Neill-Concelman) connector is a common type of RF connector used for the coaxial cable.
 The connector is the most vulnerable point.
 As a rule cables should be handled carefully.
Fig. : BNC Connector Fig. : RG-58 Coaxial Cable.
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
There are a number of not so common cable types used for AE such as Twisted Pair cables and Optical
Fiber cables.
 Twisted Pair cable consists of a pair of insulated wires twisted together.
 Cable twisting helps to reduce noise pickup from outside sources.
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Optical Fiber cable is a cable containing one or more optical fibers.

Optical fiber refers to the medium associated with the transmission of information as light pulses along a glass or plastic strand or fiber.
 Optical fiber carries much more information than conventional copper wire and is in general not subject to electromagnetic interference.
 The glass fiber requires more protection within an outer cable than copper.
Fig. : Multi-Twisted Pair cable.
http://wapedia.mobi/en/Twisted_pair
Fig. : Optical Fiber cable.
http://www.easychinasupply.com/produc ttrade/last_pt_sel/512876.html
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 While using long cables in AE applications, one must recognize their nature as long transmission lines, carrying an electrical wave with a definite speed
(somewhat less than the speed of light).
 The specified impedance of a coaxial cable (e.g. 50 ohms for RG-58 or RG-
174) refers to the voltage/current ratio in a transmission line.
 This property becomes important only in great lengths of cable.
 Impedance the total opposition that a circuit presents to the flow of an alternating current.
 When the cable is connected into a load, it is advisable to have impedance matching between load and cable so there is no reflection.
 The resistance of a cable (proportional to its length) will cause loss of signal as the voltage is divided between the cable and the load.
 The resistance of cables used in AE work is very small, a few ohms at the most.
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The step function force calibration is based on the fact that a known and measureable input displacement can be generated on the surface of the test block.
The units of calibration are output voltage per unit mechanical input (displacement, velocity acceleration).
A step function force input initiates an elastic wave that propagates through the test block.
Given step function source, the free displacement of the test block can be calculated by elastic theory (transfer function of the test block).
The displacement is also measured by a capacitive transducer with a known absolute sensitivity (wide band).
It is essential that the theoretical displacement and the capacitive measurement agree.
The calibration reveals the frequency response of a sensor to waves at a surface.
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Calibration Results
D(f m
) = U(f m
) / S(f m
)
U(f m
) - FFT of sensor under test S(f m
) - FFT of capacitive sensor r m
- magnitude of D(f m
) = |D(f m
)|
A - Sensitivity of the reference sensor in V/m.
In absolute units: S o
(f m
)= A*r m
FFT of capacitive sensor FFT of sensor under test
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D(f m
) = U(f m
) / S(f m
)

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
Reproducibility of AE Sensor Response
Intended to provide a reliable and precise way of comparing a set of sensors and telling whether an individual sensor’s sensitivity is degrading during its service life.
More economical and simple than primary or secondary calibration.
Recommended for routinely checking the sensitivity of AE sensors
This method is not an absolute calibration technique.
The essential elements of apparatus for this method are:
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The AE sensor under test
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The test block (acrylic rod, steel block, nonresonant blocks).
A signal source (Pencil lead break, pulse generator, white noise generator, sweep generator, gas jet)
Measuring and recording equipment
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 Today, AE systems use digital signal processing techniques with low costs per channel and more improved real time performance.
 AE systems have been expanded to include time-based or continuous features, hit-based based time domain features, frequency and combination based features.
 The basic system is composed of one or more AE transducers connected to a processor.
 A computer stores al AE features and combines them with process and control inputs to form outputs that can be printed, replayed or analyzed.
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Fig. : Typical AE system block diagram.
ASNT- Acoustic Emission Testing Handbook, Vol 6.

Example of AE device parameters:
 16 bit, 10 MHz A/D converter.
 Maximum signal amplitude 100 dB AE.
 4 High Pass filters for each channel with a range from 10 kHz to 200 kHz (under software control).
 4 Low Pass filters for each channel with a range from 100 kHz to 2.1 MHz (under software control).
 32 bit Digital Signal Processor.
 1 MB DSP and Waveform buffer.
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Handheld AE - Computerized instrument for AE testing applications.
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Due to its portable nature, the system can be used in any remote application.
It can perform traditional AE feature extraction based AE signal processing, as well as advanced waveform based acquisition and processing.
Wireless Remote Monitoring - This monitoring mode allows remote inspection.
Handheld measuring and processing system.
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Wireless monitoring system.

 Noise generator produce AE signals necessary to verify the correct operation of AE sensors, preamplifiers and AE systems.
 Can also be used as a pulse or waveform generator for Acousto-Ultrasonics or guided wave applications.
 According to the desired application, the noise generator signals output can be modified by amplitude, frequency range and other features.
 Noise generator can also produce white noise for calibrating purposes.
Typical Noise Generator.
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Features that are collected form the waveform:
 Peak amplitude - The maximum of AE signal. dB=20log10(Vmax/1µvolt)-preamlifier gain
 Energy – Integral of the rectified voltage signal over the duration of the AE hit.
 Duration – The time from the first threshold crossing to the end of the last threshold crossing.
 Counts – The number of AE signal exceeds threshold.
 Average Frequency –Determines the average frequency in kHz over the entire AE hit.

[
]
 Rise time - The time from the first threshold crossing to the maximum amplitude.
 Count rate - Number of counts per time unit.

 The time driven AE features are collected and recorded on a timed basis, referred to as the time driven data rate.
 Time driven data report on the changing of background or continuous AE activity during time, showing trends, leaking or faulty components in continuous processes.
 Typical time driven features include root mean square, average signal level, external parametrics and absolute energy.
 Absolute energy is also a valuable parameter because it provides a summation of energy over time, independently of hit activity.
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Start of Hit
Starting the Hit

Threshold setting is used to adjust the sensitivity

The first crossing of the threshold starts the hit.
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Long HDT
Short HDT
Threshold Hit 1
Short HDT
Hit 2
Long HDT
Time
Hit 1
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Peak Definition Time :

The function of the Peak Definition Time (PDT) is to enable determination of the time of the true peak (risetime).
Hit Lockout Time :


A lockout time which starts at the end of the hit during which the system does not respond to threshold crossing .
Used to inhibit the measurement of reflections and late arriving signals.
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 Threshold Level - The setting of an instrument that causes it to register only those changes in response greater or less than a specified magnitude.
 Floating Threshold - Any threshold with amplitude established by a time average measure of the input signal.
 System Examination Threshold - The electronic instrument threshold which determines which data will be detected.
 Voltage Threshold - A voltage level on an electronic comparator such that signals with amplitudes larger than this level will be recognized. The voltage threshold may be user adjustable, fixed, or automatic floating.
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Background Noise: Signals produced by causes other than acoustic emission and are not relevant to the purpose of the test
Types of noise:
 Hydraulic noise –Cavitations, turbulent flows, boiling of fluids and leaks.
 Mechanical noise –Movement of mechanical parts in contact with the structure e.g. fretting of pressure vessels against their supports caused by elastic expansion under pressure.
 Cyclic noise – Repetitive noise such as that from reciprocating or rotating machinery.
 Electro-magnetic noise.
Control of noise sources:
 Rise Time Discriminator – There is significant difference between rise time of mechanical noise and acoustic emission.
 Frequency Discriminator – The frequency of mechanical noise is usually lower than an acoustic emission burst from cracks.
 Floating Threshold or Smart Threshold – Varies with time as a function of noise output. Used to distinguish between the background noise and acoustic emission events under conditions of high, varying background noise.
 Master – Slave Technique – Master sensor are mounted near the area of interest and are surrounded by slave or guard sensors. The guard sensors eliminate noise that are generated from outside the area of interest.
150
100
50
0
-50
Floating threshold
-100
-150
0 200 400 600 800 1000 Time 1200 1400 1600 1800
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 Frequency filters are used to reduce low-frequency mechanical noise and highfrequency electronic noise.
 Recommended frequency range for AE work was 100-300kHz.
 This range was high enough to escape most mechanical noise, but low enough to be able to detect AE at a great enough distance for most practical work.
 Frequency filters come in high pass, low pass and band pass.
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 Consider filters in both preamplifier and main instrument. Together the define the pass band.
 The larger the Db/octave roll-off rate, the more effective the filter.
 Integral-preamp sensors have filters but they do not have a high roll-off rate.
 Example of software switchable filters (MISTRAS, DiSP etc) have software

Normal ranges: high pass: 10,20,100,200 kHz low pass: 100,200,400,1200 kHz
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 Set the high pass filter based on noise and detection range requirements.

Use 100 kHz for general purpose

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200 kHz for high noise situations
20 kHz for long-rate testing in quiet situation and or in highly attenuating materials
10 kHz in extreme or research cases.
 Set the high-frequency end to get adequate bandwidth.

Consider requirements for frequency analysis or waveform capture.

To prevent “aliasing”, the low pass filter must be less than the sampling rate.
 The top end of the calculated frequency spectrum will be at one half of the sampling frequency.
 Verify that the preamplifier pass band is compatible with the pass band of the main instrument and in the analysis.
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Scope: This standard defines the terminology used in the Nondestructive Testing standards. These nondestructive testing (NDT) methods include: acoustic emission, electromagnetic testing, gamma- and X radiology, leak testing, liquid penetrant testing, magnetic particle testing, neutron radiology and gauging, ultrasonic testing, and other technical methods.
Section A: Common NDT Terms
Nondestructive Testing - the development and application of technical methods to examine materials or components in ways that do not impair future usefulness and serviceability in order to detect, locate, measure and evaluate flaws; to assess integrity, properties and composition; and to measure geometrical characteristics.
Indication - the response or evidence from a nondestructive examination.
Interpretation - the determination of whether indications are relevant or nonrelevant.
relevant indication - an NDT indication that is caused by a condition or type of discontinuity that requires evaluation.
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Evaluation - determination of whether a relevant indication is cause to accept or to reject a material or component.

 acoustic emission (AE)—the class of phenomena whereby transient elastic waves are generated by the rapid release of energy from localized sources within a material, or the transient waves so generated.
 AE rms —the rectified, time averaged AE signal, measured on a linear scale and reported in volts.
 attenuation —the decrease in AE amplitude per unit distance, normally expressed in dB per unit length.
 count, acoustic emission (emission count) —the number of times the acoustic emission signal exceeds a preset threshold during any selected portion of a test.
 energy, acoustic emission signal—the energy contained in an acoustic emission signal, which is evaluated as the integral of the volt-squared function over time.
 effective velocity — velocity calculated on the basis of arrival times and propagation distances determined by artificial AE generation; used for computed location.
 floating threshold—any threshold with amplitude established by a time average measure of the input signal.
 hit—the detection and measurement of an AE signal on a channel.
 location, continuous AE signal —a method of location based on continuous AE signals, as opposed to hit or difference in arrival time location methods.
 sensor, acoustic emission—a detection device, generally piezoelectric, that transforms the particle motion produced by an elastic wave into an electrical signal.
 signal strength—the measured area of the rectified AE signal with units proportional to volt-sec.
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