Ultrasonic Transmitter(S)-Receiver(E) Sensors
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Ultrasonic transmitter(S)-receiver(E) sensors
General information
Ultrasonic S/E sensors, also known as transmitter/receiver sensors, consist of a transmitter unit and a receiver unit that are electrically and vibrationally separated, i.e. separate piezoelectric transducers. Such sensors are used when, due to the low thickness of the test piece or surface imperfections, the close-up resolution of the standard sensor is insufficient even at increased frequencies or when using a delay line, i.e. the back wall or defect echoes cannot be separated from the transmission pulse in time. These sensors therefore contain two electrically and acoustically decoupled standard sensors in one housing, resulting in a combination of the transmission and pulse-echo methods for measurement purposes.
Schematic setup of an S/E sensor
Due to the necessary dual-channel design of the ultrasonic measuring system, the transmission pulse does not overlap with the reception echo in the display. This allows defects located just below the surface of the test piece to be detected or very accurate results to be achieved in wall thickness measurement [1, 2].
The two transducers, which operate continuously in the transmit or receive modes, are inclined at a certain angle to the normal or separation plane, which is referred to as the roof angle β (Fig. 1). Depending on the roof angle implemented, inclined delay lines of length l are glued under the transducers, resulting in maximum sensitivity in a specified depth range [1].
| Fig. 1: | Schematic arrangement of an ultrasonic S/E sensor |
The delay lines correspond to wedges made of plastics with good sound conductivity, such as polymethyl methacrylate (abbreviation: PMMA) or polystyrene (abbreviation: PS). An acoustic adaptation layer is usually installed between the transducer and the attachment wedge. Its thickness corresponds to the quarter wavelength (λ/4) of the sensor, and serves in particular to adapt the impedance and optimise sound transmission between the transducer and the delay line on the transmitter and receiver side. In addition to a low reflection factor, a suitable damping body also provides high mechanical and acoustic damping of the S/E sensor [3]. If the transducers or delay wedges are only inclined by an angle β, these are referred to as standard S/E sensors. If the transducers are additionally inclined by an angle γ (γ > β) in the 90° plane, then an angle S/E sensor is present, with which transverse waves can be generated in the test object. The conditions and statements defined under the term angle beam sensor apply to the angle γ.
The application or working range of S/E sensors is in the test piece area where the sound fields of the transmitter (S) and receiver (E) overlap (Fig. 2)
| Fig. 2: | Sound field of the ultrasonic S/E sensor (a) with large and small roof angle β and (b) sensitivity–distance diagram |
As can be seen in Fig. 2a, the near-field resolution can be varied over a wide range by selecting the roof angle, the distance between the transmitter and receiver, and the length of the delay wedge, although this also affects the dead zone of the sensor. The sensitivity therefore varies with the path of the ultrasound l and can be represented graphically in the diagram corresponding to Fig. 2b. The dark red field marks the zone of maximum sensitivity, and the light red triangle marks the working range. With large roof angles, the ultrasound is already inclined and not perpendicular to the surface due to the design. As a result, an increase in the roof angle causes a so-called detour error, which is reflected in an increase in the transit time and thus, for example, in an error in the wall thickness measurement. This error can be compensated for by adjusting the ultrasonic system or mathematically in digital devices.
In addition to the roof angle, the test frequency, the type and shape of the oscillator, the dimensions of the oscillator and the attachment wedge, and the distance between the transmitter and receiver also influence the quality and reliability of the test result [2, 4, 5].
Examples of S/E sensor designs
Examples of different S/E sensors are shown in Fig. 3, where you can see the separate connections for the transmitter and receiver.
| Fig. 3: | Examples of S/E test sensors (handheld test sensors) of different sizes and frequencies (a) from GE Measurement & Control Solutions, Alzenau, and (b) from GAZ-Prüftechnik GmbH, Alpen |
See also
- Ultrasound testing
- Ultrasonic direct coupling
- Ultrasonic composite sensors
- Ultrasonic weld inspection
- Ultrasonic standard sensors
References
| [1] | Steeb, S.: Zerstörungsfreie Werkstoffstück- und Werkstoffprüfung. 5th Edition, Expert Verlag, Renningen (2016), (ISBN 978-3-81693-261-1); 2nd Edition (1993) (ISBN 3-8169-0964-7; see AMK-Library under M 41) |
| [2] | Matthies, K.: Dickenmessung mit Ultraschall. DVS-Verlag GmbH, Berlin, 2nd Edition, (1998), (ISBN 3-87155-940-7; see AMK-Library under M 44) |
| [3] | Deutsch, V., Platte, M., Vogt, M.: Ultraschallprüfung – Grundlagen und industrielle Anwendungen. Springer, Berlin (1997), (ISBN 3-540-62072-9; see AMK-Library under M 45) |
| [4] | Schuster, V., Lach, M., Platte, M.: Die Qual der Wahl: Welcher Prüfkopf für welchen Einsatz. DGZfP-Jahrestagung „Zerstörungsfreie Werkstoffprüfung“ (2004), Salzburg, Österreich, Sonderdruck Karl Deutsch, SD 1/51 |
| [5] | Krautkrämer, J.; Krautkrämer, H.: Werkstoffprüfung mit Ultraschall. Springer, Berlin Heidelberg (2013) p. 43 (ISBN 978-3-662-10910-6) |