Ultrasonic Sensors
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Ultrasonic sensors
General information
Ultrasonic sensors, also known as ultrasonic transducers or probes, are used to generate and receive ultrasonic signals, primarily in non-destructive material testing and medical diagnostics. In conjunction with analogue or digital ultrasonic testing equipment, they can be used for a wide variety of testing tasks, provided that the sensor selected is correct and optimal for the application in question. Since there is no universal sensor suitable for all testing tasks, they must be adapted to the respective practical application, just like the testing and evaluation devices, e.g. defectoscopy on thin or thick components of varying geometry (pipes, plates), weld seams (fillet, butt or V-seams) or wall thickness measurement. This also applies to the application for different materials (metal alloys, grey cast iron structures, filled or reinforced plastics and concrete), where strong attenuation or scattering at internal inhomogeneities sometimes requires the centre frequency fm and the frequency band Δf as well as the impedance to be adapted to the test task (transmission behaviour), In each case, despite differing resolution or defect detectability, the quality and reliability of the test result must be guaranteed in relation to the error limit [1−4].
Ultrasound generation
Depending on the application, different physical principles are used to generate the ultrasound. For metallic materials, which are required to be electrically conductive, the electrodynamic or electromagnetic method (e.g. EMUS sensor) can be used, with which longitudinal or transverse waves can be generated in the test piece without contact [5].
Commercial ultrasonic sensors (e.g. standard, angle or S/E sensors) mostly use the piezoelectric effect for a wide range of materials. The sensors essentially consist of a piezoelectric ceramic transducer, which generates mechanical stress waves of varying voltage and pulse repetition frequency as a result of electrical excitation by the ultrasonic testing device, an electrical matching circuit (inductance, resistance) and an encapsulated housing with the cable connection.
To generate the ultrasound, the ultrasonic sensor has a transducer made of piezoelectric ceramic, which is circular in the case of the standard sensor, for example, or rectangular in most cases for the angle sensor, or several transducers as in the case of phased array sensors (see: ultrasonic phased array sensors). The vibration mode of the ultrasonic signal always depends on the acoustic impedance ratio between the transducer and the damping body contained in the housing. If the impedance is almost identical, broadband sensors with very short pulses and high damping (shock waves) are available; otherwise, the sensor tends to be narrowband and exhibits longer vibration modes with low signal damping [1]. For impedance matching and abrasion protection of the surface of the sensor, the underside is usually coated with a protective layer of ceramic materials or ceramic-filled plastics and often with additional wear protection films. These measures serve to couple the ultrasound into the test piece with as little loss or reflection as possible, even with rough and untreated surfaces. In conjunction with the coupling agent and the lead-in distance (delay line), the protective layer and the wear film represent an additional frequency filter in direct coupling, which can influence the centre frequency and the frequency response and should therefore be optimised for the test task [2, 6, 7].
Types of ultrasonic sensors
The most important types of sensors used in ultrasound testing are:
- standard sensors (also known as vertical or delay line sensors) [1]
- angle beam sensors
- transmitter(S)-receiver(E) sensors
- composite sensors
- phased array sensor
- shock wave sensors and
- ultrasonic immersion bath sensors
These different types of sensors are mostly operated using pulse-echo technique, but are also partially suitable for transmission testing and are designed for a wide frequency spectrum depending on their construction size. With the implementation of appropriate design measures, the sensors can also be used in immersion bath and squirter techniques. Figure 1 shows different types of commercially available sensors.
| Fig. 1: | Ultrasonic sensors for different areas of application (a) Standard and (b) Angle beam sensor from Optimess Engineering GmbH, Gera, (c) Trnsmitter (S)-receiver (E) sensor from GAZ-Prüftechnik GmbH, Alpen, (d) Composite sensor from Sonaxis, Besançon (France), (e) phased array sensor from GE Measurement & Control Solutions, Alzenau |
See also
- Non-destructive polymer testing
- Ultrasound testing
- Ultrasonic runtime measurement
- Ultrasonic immersion bath technique
- Ultrasonic wall thickness measurement
- Ultrasonic weld inspection
References
| [1] | Deutsch, V., Platte, M., Vogt, M.: Ultraschallprüfung – Grundlagen und industrielle Anwendungen. Springer, Berlin, (1997), (ISBN 978-3-642-63864-0; see AMK-Library under M 45) |
| [2] | Krautkrämer, J., Krautkrämer H.: Werkstoffprüfung mit Ultraschall. Springer, Berlin (1986), (ISBN 978-3-662-10909-0) |
| [3] | Sirch, C., Bierögel, C., Grellmann, W., Rufke, B., zur Horst-Meyer, S.: Akustisches Verhalten von Kunststoffbauteilen. Tagung Problemseminar "Deformation und Bruchverhalten von Kunststoffen" Merseburg (2001), Proceedings pp. 326–336 |
| [4] | Deutsch, V., Vogt, M.: Ultraschallprüfung von Schweißverbindungen. Schweißtechnische Praxis, DVS-Verlag, Düsseldorf, Vol. 28, (1995) |
| [5] | Matthies, K. u. a.: Dickenmessung mit Ultraschall. DVS-Verlag GmbH, Berlin, 2nd Edition, (1998), (ISBN 3-87155-940-7; see AMK-Library under M 44) |
| [6] | Deutsch, V., Platte, M., Vogt, M., Deutsch, W. A. K., Schuster, V.: Die Ultraschallprüfung. Castell Publishing, Wuppertal, Vol. 1, (1999) |
| [7] | Deutsch, V., Platte, M., Schuster, V., Deutsch, W. A. K.: Messtechnik mit Ultraschall. Castell-Verlag, Wuppertal, Vol. 2, (2002) |