PC-Based Ultrasonic Test System Basics

Several PC-based and external components make up each ultrasonic test system. Each of these components has a unique set of features and considerations. Some considerations are limited to the component itself while others depend on the various components in the system. This document describes each component in the system, some of the features/issues that you should consider, and how these components can be put together to build a custom, flexible ultrasonic test system.

Assembling an Ultrasonic Test System

Ultrasonic test systems can take several forms, but the most common for automated test is immersion testing. To achieve effective acoustical impedance matching between the couplant and the UUT and free range over the entire surface of the UUT, many test systems use an immersion tank filled with water.

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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 receives echoes 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 must work 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:

1. Application software – The user interacts with the application software to set up the test and presentation parameters.
2. Motion control – The ultrasonic transducer is moved over the appropriate area of the UUT.
3. 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.
4. Pulser/receiver – This device generates the high-voltage pulse that is required by the ultrasonic transducer.
5. 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.
6. Pulser/receiver – The analog signal from the ultrasonic transducer is amplified and filtered before it is sent back to the digitizer within the PC.
7. Digitizer – The waveform sent from the pulser/receiver is converted from voltage to bits using an analog-to-digital converter.
8. Application software – Data from the digitizer is processed, analyzed, and presented according to the user-defined parameters.

If you have multiple transducers that are combined with one digitizer/pulser/receiver combination, you must use switches.

Several hardware and software components must interact effectively for even the simplest ultrasonic test system to work properly. When assembling your custom ultrasonic test system, you have several factors to consider for each component of the system, including how well the components interact with one another. The ultrasonic transducer section describes each component of the test system in detail as well as the important features required for ultrasonic testing.

Ultrasonic Transducer

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

These transducers perform according to two main parameters: resolution and sensitivity. The resolution of a particular transducer is determined by its ability to discern between two discontinuities that are on top of one another. A transducer with sufficient resolution stops ringing, or vibrating, from the first discontinuity before receiving the echo from the second discontinuity. If the ceramic does not stop ringing before the second echo is received, the second echo is masked from the test system. The sensitivity of an ultrasonic transducer refers to the transducer's 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 the less the depth of penetration.

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Noncontact Ultrasonic Transducers – Courtesy of The Ultran Group


Standard Contact Ultrasonic Transducers – Courtesy of Panametrics-NDT


Standard Immersion Ultrasonic Transducers – Courtesy of Panametrics-NDT

Pulser/Receiver 

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 test system, the pulser/receiver 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 pulse voltage 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 any information back to the PC during operation.

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

PC-Controlled Pulser/Receiver – Courtesy of Panametrics-NDT

PC-Controlled Pulser/Receiver – Courtesy of Imaginant

Digitizer

This portion of the test system converts the echo waveforms returned by the ultrasonic transducer into digital information using an analog-to-digital converter (ADC). Consider the following factors when choosing the digitizer for your system:

• Sample rate
• Bandwidth
• Vertical resolution
• Triggering features
• Memory
• Bus 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 these applications, 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 and echo-timing accuracy, four to five times the resonant frequency is acceptable.

Vertical resolution establishes the minimum voltage step size within a voltage range. Sixteen bits is equivalent to 65,536 (2^16) steps. When a 16-bit ADC is applied to a voltage range of 0 to 10 V, the minimum voltage step size is 0.153 mV (10 V/65,536). However, when an eight-bit ADC is applied to the same range, the minimum voltage step size is 39 mV (10 V/256). In ultrasonics, the voltage amplitude 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. To see 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 front surface and back surface reflection voltages to swing outside the voltage range of the digitizer. On the other hand, with additional resolution, you can zoom in on small flaws without distorting main surface reflections. High resolution also relaxes the amplification levels required by the P/R component.

The digitizer typically acts as a timing slave within an ultrasonic test system, while the motion controller or the pulser/receiver acts as the master. Flexibility of internal or external timing is essential for ultrasonic 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 water before 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.

You can simplify timing and triggering between multiple devices when using a bus that is designed for integrated test systems. PXI leverages the PCI bus for high-speed data transfer with the addition of 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 time because 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 amount of data you collect can be staggering. You can handle his data in many ways depending on the computer bus you are using and your data collection rate. For instance, the PCI bus can realistically handle 80 MB/s (or 40 MS/s at 16 bits) of continuous data throughput to PC memory. If your application requires more than a 40 MS/s sample rate, you must consider onboard device memory. In either case, the speed at which you can transfer the data 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 or triple the amount of time required to test one UUT. Find the bus that works best for your application.

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 have become or will become insignificant with respect to cost.

Family	Model	Bus	Maximum onboard memory (samples)	Number of channels	Maximum sample rate per channel (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

Performance Comparison of NI Digitizers

Motion Control

Most automated ultrasonic test systems use motion control to gather multiple data points with one transducer. For instance, acquiring B- or C-scans requires movement of the ultrasonic sensor over the UUT surface to create a surface map. Ultrasonic test motion control must have a few basic features such as servo and/or stepper control, multiaxis control, a position feedback interface, and so on. However, several features can make ultrasonic test much faster and more efficient. Because the motion control system typically acts as the master 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 are commonly 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 with PXI, guarantees all I/O lines are synchronized in a repeatable fashion.

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
Onboard programming functionality	-
Number of axes per 62.5 µs PID rate	1	1	2
Static PWM outputs	2	2	2
DIO Lines	32	32	64
Digital-to-analog converter	-	16 bits	16 bits
Analog-to-digital converter	12 bits	12 bits	16 bits
Maximum step output rate	4 MHz	4 MHz	8 MHz
Encoder rate	20 MHz	20 MHz	20 MHz

Feature Comparison of NI Motion Controllers

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 that use 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 directions without moving the sensor array. Multitransducer applications are also common when speed of test is an issue.

The three main considerations when choosing a switch for ultrasound applications are voltage rating, bandwidth, and switch topology. Common switch topologies include matrix and multiplexing. Large test systems often 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 provides multiplexing switches that range 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 do not have enough bandwidth, the signal generated by the pulser or the echo returning from the transducer is attenuated. Proper impedance matching, typically 50 Ω, is required to minimize reflections and keep the signal clean.

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

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 comprises three 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 your ultrasonic 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 (FFT), level crossing, and filtering. Some of these algorithms are simple while others are complex and computationally intensive. Computationally intensive algorithms included in the application software should be optimized, especially in ultrasonic applications where the presentation is graphically intensive. To ensure that the application software does not limit your application, make 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 to the display on an oscilloscope, which shows voltage amplitude versus depth. The depth is calculated by multiplying the speed of sound through the medium by the time of flight.

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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.

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In the A-scan above, 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, you can simply calculate the thickness of the material. If there is a flaw within the material, you see a small peak somewhere between the front wall and back wall.

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 many applications that 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 below shows the echo peaks moving over time. B-scans display depth versus linear position along the UUT.

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Looking at a C-scan is analogous to looking through an opaque material in the direction of the ultrasonic wave. C-scans display x- and y-positions while the color represents depth.

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Graphic Representation of the UUT

One of the main trends in nondestructive test, and ultrasonic test in particular, is full test automation. Automation  includes not only data collection and presentation automation but also pass/fail automation 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 commonly found when performing NDT. One such method you can use 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 others, is available in NI Vision Builder Automated Inspection.

Because of the wide variety of requirements in ultrasonic test, it is difficult to find turnkey software that uses the hardware components, specialized algorithms, and unique displays that your application requires. The alternative to turnkey 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 NI LabVIEW, allow a novice programmer to create advanced ultrasonic test programs easily and quickly.

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

LabVIEW has hundreds of analysis and mathematics functions, including FFT, peak detection, and Hilbert Transforms. Also, if your application involves more than just an ultrasonic test portion, LabVIEW can extend to other types of test and control. Regarding the presentation abilities of LabVIEW, all of the A-, B-, and C-scan figures above were 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 well to ultrasonic test, whether it is acquisition/control, analysis, or presentation.

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

For more information, see the Ultrasonic and Nondestructive Test (NDT) page.

Reader Comments | Submit a comment »

Be aware from inaccuracy
I am sorry but the example of the software is like as cheat. The images of geometrical shapes which presented at the figures above could not be achieved by UT equipment and was composed by developer of ultrasonic software kit.
- Andrew Bazoulin, SPC ECHO+. android@echoplus.ru - Jan 21, 2007

About Acoustic Impedance Ratio
About acoustic impedance ratio, the more acurate depiction for the energy reflection should be: the further from 1 the ratio, the more energy is replected (instead of "The higher the ratio, the more energy is reflected").
- Lintao Wang, U-Systems, Inc.. lintao.wang@u-sys.com - Feb 7, 2005

Typing Mistakes?
The descriptions of the 2 parameters (incident angle and refraction angle ) should be swapped, the original looks like: Where: θR = incident angle from normal of beam in the wedge or liquid θI = angle of the refracted beam in the UUT
- Lintao Wang, U-Systems, Inc.. lintao.wang@u-sys.com - Feb 7, 2005