1/7 www.ni.com

Ultrasonic Nondestructive Testing - Advanced Concepts and Applications

1. 2. 3. 4. 5. 6. 7. 8.

1. 2. 3. 4. 5. 6. 7. 8.

Overview

This tutorial is part of the NI Analog Resource Center. Each tutorial teaches you a specific topic by explaining the theory and giving practical examples. This tutorial describes several PC-based andexternal components that make up each ultrasonic test system.

You can also for a multimedia presentation with slides and audio. view a webcast

For more information, return to the .NI Analog Resource Center

Table of Contents

Assembling an Ultrasonic Test SystemUltrasonic TransducerPulser/Receiver (P/R)DigitizerMotion ControlSwitchingApplication SoftwareRelevant NI products

Assembling an Ultrasonic Test System

Ultrasonic test systems can take several forms, but the most common for automated test is immersion testing, as shown in Figure 1. To have good acoustical impedance matching between thecouplant and the UUT and free range over the entire surface of the UUT, many test systems use an immersion tank filled with water.

Figure 1. Example of an Ultrasonic Test System

These test systems use one or more ultrasonic transducers, which are moved over the surface of the unit under test (UUT). As the transducer is moved over the surface, it is pulsed and receivesechoes from various surfaces. This process is repeated many times a second - sometimes more than 50,000 times per second (>50 kHz). There are several pieces of the test system that mustwork together to get expected results. The following list includes the steps, and the accompanying hardware and software pieces, required to get one pulse and the subsequent echoes:

Application software - The user interacts with the application software to set up the test and presentation parameters.Motion control - The ultrasonic transducer is moved over the appropriate area over the UUT.Communication - The pulser/receiver operation parameters, such as pulse energy, pulse damping, and bandpass filtering, are set. The communication path is typically RS232 or USB.Pulser/receiver - This device generates the high-voltage pulse that is required by the ultrasonic transducer.Ultrasonic transducer - The transducer is pulsed, sending out an ultrasonic wave. The subsequent echoes generate a voltage in the transducer, which is sent back to the pulser/receiver.Pulser/receiver - The analog signal from the ultrasonic transducer is amplified and filtered before it is sent back to the digitizer within the PC.Digitizer - The waveform sent from the pulser/receiver is converted from voltage to bits using an analog-to-digital converter (ADC).Application software - Data from the digitizer is processed, analyzed, and presented according to the user-defined parameters.

If there are multiple transducers that are coupled with one digitizer/pulser/receiver combination, much be used.switches

There are several hardware and software components that must interact effectively for even the simplest ultrasonic test system to work properly. When assembling your custom ultrasonic testsystem, there are several factors to consider for each component of the system, including how well the components interact with one another. The Ultrasonic Transducer section describes eachcomponent of the test system in detail as well as the important features required for ultrasonic testing.

View the Webcast

: Document Type Tutorial: Yes NI Supported: Feb 13, 2012 Publish Date

2/7 www.ni.com

Ultrasonic Transducer

Ultrasonic transducers are built around piezoelectric ceramics that vibrate at ultrasonic frequencies when a voltage is applied, and generate voltages when vibrated. The piezoelectric ceramics canbe packaged in a variety of housings, depending on how they are used. For instance, ultrasonic transducers used in field service are commonly contact sensors, and are contoured to the surface tobe inspected. These transducers have special wear and handling requirements because of how they are used.

Ultrasonic transducers perform according to two main parameters: resolution and sensitivity. The resolution of a particular transducer is denoted by its ability to discern between two discontinuitiesthat are on top of one another. A transducer with sufficient resolution will stop ringing, or vibrating, from the first discontinuity before receiving the echo from the second discontinuity. If the ceramicdoes not stop ringing before the second echo is received, the second echo is masked from the test system. Sensitivity of an ultrasonic transducer refers to the ability to detect small discontinuities.Reference blocks with standard sized defects are used to gauge the sensitivity of a particular transducer.

The frequency of the transducer is chosen based on the required sensitivity and depth of penetration. Remember that the higher the frequency, the better the sensitivity, but lower penetrationdepth.

Figure 2. Noncontact Ultrasonic Transducers (Courtesy of The Ultran Group)

Figure 3. Standard Contact Ultrasonic Transducers (Courtesy of Panametrics-NDT)

Figure 4. Standard Immersion Ultrasonic Transducers (Courtesy of Panametrics-NDT)

View the Webcast

Pulser/Receiver (P/R)

These devices provide the high-voltage pulse required by the ultrasonic transducer as well as signal conditioning before the analog signal is passed to the digitizer. For use within an automated testsystem, the P/R should be computer programmable via a standard PC bus such as RS232 or USB. The devices are typically programmed one time at the beginning of the test to set the pulsevoltage level, pulse repetition frequency, damping, band pass filtering settings, and several other parameters. After these parameters are set, these devices are passive and do not send anyinformation back to the PC during operation.

The motion controller, digitizer, and P/R must operate as one tightly timed unit during the test to ensure accuracy of results and brevity of test time. The P/R can act as the master timebase of the

3/7 www.ni.com

system or can act as a slave to the digitizer or motion controller. In the applications where motion control is implemented, it is typically the slowest part of the system and, for that reason, acts as themaster timebase.

View the Webcast

Digitizer

This portion of the test system converts the echo waveforms returned by the ultrasonic transducer into digital information using an ADC. Consider the following factors when choosing the digitizerfor your system:

Sample rateBandwidthVertical resolutionTriggering featuresMemoryBus type

For applications that require well-shaped waveforms in the time-domain, such as research, a sample rate 10 times higher than the resonant frequency of the transducers is required. In theseapplications, a transducer that has a resonant frequency of 5 MHz requires 50 MS/s to accurately represent the shape of the signal. However, for applications that require less amplitude andecho-timing accuracy, 4-5x the resonant frequency is acceptable.

Vertical resolution establishes the minimum voltage step size within a voltage range. 16-bits is equivalent to 65,536 (2^16) steps. When a 16-bit ADC is applied to a voltage range of 0-10 Volts, theminimum voltage step size is 0.153 mV (10 V / 65,536). However, when an 8-bit ADC is applied to the same range, the minimum voltage step size is 39 mV (10 V/256). In ultrasonics, the voltageamplitude is proportional to the amount of energy echoed by the discontinuity or flaw. The front face and back face of the UUT generally reflect the most energy, while flaws reflect much less. Tosee the energy reflected from small flaws, the signal from the transducer must be amplified or the digitizer must have high resolution. Amplifying the signal to detect a small flaw can cause the frontsurface and back surface reflection voltages to swing outside the voltage range of the digitizer. On the other hand, additional resolution permits the user to zoom in on small flaws and not distortmain surface reflections at the same time. High resolution also relaxes the amplification levels required by the P/R component.

Figure 5. Example of an Ultrasonic Acquisition

The digitizer typically acts as a timing slave within an ultrasonic test system, while the motion controller or the P/R acts as the master. Flexibility of internal or external timing is essential forultrasonic test applications. Programmable trigger delay is also a useful feature for ultrasonic testing. In immersion testing, the ultrasonic wave must travel through a significant distance of waterbefore arriving at the UUT. If this distance is known, a trigger delay can be implemented to minimize the amount of unnecessary data that is recorded and stored.

Timing and triggering between multiple devices can be simplified when using a bus that is designed for integrated test systems. PXI uses the PCI bus for high-speed data transfer with the additionof timing and triggering lines. The timing and triggering lines within PXI negate the need for external cabling, which reduces error and clutter. This capability also reduces the overall test timebecause all I/O is hardware synchronized, which minimizes time wasted with software timing. Learn more about the architecture and advantages of .PXI

Because of the high frequencies associated with ultrasonic test, the amounts of collected data can be staggering. This data can be handled in many ways depending on the computer bus beingused and the data collection rate. For instance, the PCI bus can realistically handle 80 MB/s (or 40 MS/s at 16-bit) of continuous data throughput to PC memory. If your application requires morethan 40 MS/s sample rate, onboard device memory must be a consideration. In either case, the speed at which the data can be transferred back to permanent PC storage, such as a hard drive,after device onboard memory is full, must be a consideration. If data is transferred over a slow bus, such as USB 1.0 at 10 MB/s, the amount of time required to transfer data can easily double ortriple the amount of time required to test one UUT. Find the that works best for your application.bus

There is a trade-off between resolution, speed, channel count, data throughput, and cost. However, as ADC technology, PC memory, and data transfer rates evolve, many of these trade-offs havebecome or will become insignificant with respect to cost.

4/7 www.ni.com

Performance Comparison of NI Digitizers

Family Model Bus Max. on-board memory(samples)

# of channels Max. sample rate perchannel (S/s)

Input resolution(bits)

Bandwidth (Hz)

Digitizer 5124 PXI 512 M 2 200 M 12 150 M

Digitizer 5122 PCI, PXI 256 M 2 100 M 14 100 M

Digitizer 5112 PCI, PXI 32 MB 2 100 M 8 100 M

S Series 6115 PCI, PXI 64 M 4 10 M 12 7.2 M

S Series 6133 PXI 32 M 8 3 M 14 1.3 M

M Series 6259 PCI, PXI 4095 32 1.25 M 16 1.7 M

M Series 6289 PCI, PXI 2047 32 625 k 18 725 k

View the Webcast

Motion Control

Most automated ultrasonic test systems use to gather multiple points of data with one transducer. For instance, acquiring b- or c-scans requires movement of the ultrasonicmotion control systemssensor over the UUT surface to create a surface map. There are a few basic features that are required for ultrasonic test motion control such as servo and/or stepper control, multi-axis control, aposition feedback interface, and so on. However, there are several features that can make ultrasonic test much faster and more efficient. Because the motion control system typically acts as themaster trigger, triggering flexibility is the key to ensure interoperability and efficiency with your particular test needs. A combination of position, velocity, and acceleration triggering, which arecommonly referred to as breakpoints, is required to ensure the test is accurate and repeatable. Also, the ability to share motion breakpoints with other I/O without external cabling, as available withPXI, guarantees all I/O are synchronized in a repeatable fashion.

Feature Comparison of NI Motion Controllers

Feature 7330 Series 7340 Series 7350 Series

Maximum number of axes 4 2,4 2,4,6,8

Servo control -

Closed-loop stepper control

Linear interpolation

Configurable move complete criteria

On-board programming functionality -

Number of axes per 62.5 microsecond PIDrate

1 1 2

Static PWM outputs 2 2 2

DIO lines 32 32 64

Digital-to-analog converter - 16 bit 16 bit

Analog-to-digital converter 12 bit 12 bit 16 bit

Maximum step output rate 4 MHz 4 MHz 8 MHz

Encoder rate 20 MHz 20 MHz 20 MHz

View the Webcast

Switching

For ultrasound applications that have one digitizer and pulser/receiver for multiple ultrasonic sensors, switching is required to route the signals properly. This topology is common in applications thatuse arrays of sensors to create images. Arrays of ultrasonic sensors are common in biomedical and nondestructive test applications because the sound energy can be steered in multiple directionswithout moving the sensor array. Multitransducer applications are also common when speed of test is an issue.

The three main consideration when choosing a switch for ultrasound applications are voltage rating, bandwidth, and switch topology. Common switch topologies include matrix and multiplexing. Inlarge test systems, it is common to use a matrix topology that connects multiple instruments with multiple test points. In ultrasound applications, it is more common to use the multiplexing topology,which connects one digitizer/pulser/receiver combination to multiple sensors. NI offers multiplexing switches, which range extend from 4x1-wire up to 256x1-wire.

Ultrasound pulser/receivers can create very high voltages. If this is the case, your switching devices should have the clearances and creepages necessary to withstand these transient voltages.Your switching devices should also have the appropriate amount of bandwidth compared to the ultrasonic transducer and should be impedance matched to the rest of the system. If the switches donot have enough bandwidth, the signal generated by the pulser or the echo returning from the transducer will be attenuated. Proper impedance matching, typically 50 ohm, is required to minimizereflections and keep the signal clean.

NI offers a full line of that offer RF bandwidths and transient voltage protection necessary for ultrasound applications.switches

View the Webcast

Application Software

Ultrasonic test application software combines many types of I/O, analysis algorithms, and presentation techniques to form one software interface. The application software can be separated intothree basic parts: acquisition/control, analysis, and presentation. Acquisition/control refers to the interface between the application software and the hardware that you have assembled for yourultrasonic application. For the most flexibility as test parameters change, make certain that a wide variety of hardware is supported by the application software.

Some of the required analysis algorithms for ultrasonic test are peak detection, computation of distances based on material properties, wave rectification, statistics, fast Fourier transforms (FFTs),level crossing, and filtering. Some of these algorithms are simple while others are complex and computationally intensive. Computationally intensive algorithms included in the application softwareshould be optimized, especially in ultrasonics applications where the presentation is graphically intensive. To ensure that the application software is not the limitation for your application, make

5/7 www.ni.com

certain that the analysis libraries will grow with your application needs.

There are several ways to look at ultrasonic test data ranging from TOF to surface scan. The most common scans are referred to as A-, B-, and C-scans. The TOF scan, or A-scan, is analogous tothe display on an oscilloscope, which displays voltage amplitude versus depth. The depth is calculated by multiplying the speed of sound through the medium by the time of flight.

Figure 6. Unit Under Test (UUT) Used for this Example

The UUT used in the following examples is a rectangular block. Four features of various shapes have been machined into this block at various depths.

Figure 7. Example of an A-Scan

In the A-scan, shown in Figure 7, the first echo is from the front surface of the material and the second echo is from the rear surface of the material. Using the information above, it would be simpleto calculate the thickness of the material. If there was a flaw within the material, you would see a small peak somewhere between the frontwall and backwall.

Imagine the voltage peaks in the A-scan moving back and forth over time. This movement is caused by the varying thickness of the UUT as the transducer is moved over the surface. In manyapplications, which map the surfaces/flaws of a material, motion control plays a key role in collecting data. Put all of those A-scan images together and the result is a B-scan. The B-scan belowshows the echo peaks moving over time. B-scans display depth versus linear position along the UUT.

6/7 www.ni.com

Figure 8. Example of a B-Scan

Looking at a C-scan shown in Figure 9, is analogous to looking through an opaque material in the direction of the ultrasonic wave. C-scans display x- and y-position, while the color representsdepth.

Figure 9. Example of a C-Scan

Graphical Representation of the UUT

One of the main trends in nondestructive test, and ultrasonic test in particular, is full automation of test. Automation not only includes automation of the data collection and presentation, but alsoincludes automation of pass/fail for any particular UUT. Setting pass/fail templates for A-, B-, and C-scans increases statistical accuracy and eliminates much of the subjectivity that is commonlyfound when performing NDT. One such method that can be used on C-scans is blob analysis. With blob analysis, you can analyze objects in your image and classify objects according to size,location, and quantity. This type of analysis, and many more, is available in .NI Vision Builder for Automated Inspection

Because of the wide variety of requirements in ultrasonic test, it is difficult to find turn-key software that uses the hardware components, specialized algorithms, and unique displays that yourapplication requires. The alternative to turn-key software is application development software, or programming. With custom application development, you can assemble all of the hardware,analysis, and presentation components that your application requires. Furthermore, graphical application development environments, such as , allow a novice programmer to createNI LabVIEWadvanced ultrasonic test programs easily and quickly.

NI LabVIEW has connectivity to , regardless of bus connectivity or instrument type. In addition, software packages such as , thousands of instruments NI Vision Builder for Automated Inspection NI, and the let you to interactively configure your ultrasonic test and subsequently generate LabVIEW code to use in your custom application.Motion Assistant NI DAQ Assistant

LabVIEW has hundreds of , including FFT, peak detection, Hilbert Transforms, or anything your ultrasonic test application could require. Also, if you applicationanalysis and mathematics functionsinvolves more than just an ultrasonic test portion, LabVIEW and extend to other types of test and control. Regarding the presentation abilities of LabVIEW, all of the A-, B-, and C-scan figuresabove were all created in LabVIEW; however, they are referred to as graphs, intensity charts, and intensity graphs. LabVIEW is a general-purpose test and measurement tool that applies extremelywell to ultrasonic test, whether it is acquisition/control, analysis, or presentation.

Download the . This starter kit contains example programs of A-, B-, and C-scans. It also features the building blocks, including common analysis and presentationLabVIEW Ultrasonic Starter Kitobjects, to make you more productive when building your own ultrasound applications in LabVIEW.

For more information, return to the page.Ultrasonic Test System Resource

View the Webcast

7/7 www.ni.com

Relevant NI products

Customers interested in this topic were also interested in the following NI products:

LabVIEW Graphical Programming EnvironmentOscilloscopesMotion ControlData Acquisition

For the complete list of tutorials, return to the .NI Analog Resource Center

LegalThis tutorial (this "tutorial") was developed by National Instruments ("NI"). Although technical support of this tutorial may be made available by National Instruments, the content in this tutorial maynot be completely tested and verified, and NI does not guarantee its quality in any way or that NI will continue to support this content with each new revision of related products and drivers. THISTUTORIAL IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND AND SUBJECT TO CERTAIN RESTRICTIONS AS MORE SPECIFICALLY SET FORTH IN NI.COM'S TERMS OF USE (

).http://ni.com/legal/termsofuse/unitedstates/us/