# New Magnetstrictive Transducer Designs for Emerging Application Areas of NDE
## 7 MsTs with Special Functionality: Unidirectional Operation
This is because a strip that is much larger than one-quarter of the wavelength that is being generated would self-cancel during generation. Thus, the wider active region should produce a larger signal amplitude while having a lower operating frequency range than in the narrower active region. This test confirmed the unidirectional operation of this sensor concept. The higher (>300 kHz) and lower (<150 kHz) frequency information propagated toward the nearest plate boundary, while the central frequency information around 270 kHz propagated predominately toward the far plate boundary. Furthermore, after applying band-pass filtering to the waveform shown in Figure 11, it was determined that the amplitude ratio between the higher frequency signal traveling in the wanted  
Figure 11: Reflections obtained from the ends of the stainless steel plate with the arrangement shown in Figure 10 (upper image), and the spectrogram of signals obtained from the opposite plate edges (lower image). Figure 10: Unidirectional MST probe arrangement—the ferromagnetic strip is magnetized by a permanent magnet offset from the strip center. (higher frequency signal) and unwanted (lower frequency signal) directions was near 25 dB. Another test conducted on a 0.25-inch thick carbon steel plate produced an amplitude ratio of the order of 16 dB in the frequency range of 200-430 kHz without adjustment of the magnet position. Figure 12a shows the test arrangement and test results using a 1.5-inch wide MsT transducer. The magnet placement divides the strip into two areas, one that is 6 mm wide, and another that is 18 mm wide. The narrow magnetized region was closest to the near edge of the plate as with the prior example, so the expectation is that the higher frequency components will propagate primarily toward the near edge. Data at a 200 kHz center frequency are shown in Figure 12b, and 430-kHz data are shown in Figure 12d. The spectrograms of the received signals are shown in Figure 12c at 200 kHz and Figure 12e at 430 kHz to confirm the frequency of the probe operation. The signals presented at high frequency (430 kHz in Figure 12d,e) indicate that this directional operation is observed, and the waves propagated in the expected direction. Furthermore, the directionality of the propagating wave works with both the SH0 and SH1 modes. On the other hand, the lower frequency 200-kHz signals still predominately propagated toward the near edge of the plate. Finally, additional experiments not shown here demonstrated that unidirectional transduction is feasible in a wider frequency range (150-700 kHz) with some minor adjustments of the permanent magnet position relative to the strip edge. At the present stage, the unidirectional transducers have the following advantages:  
* 50% lower power consumption because only one channel pulser is needed
* Broadband (200-700 kHz) signal generation/ reception without changing directionality
* Reduced sensitivity to variations in coupling when used for SHM applications, because only one sensing element with a relatively small footprint is coupled instead of two in alternative designs
* Reduced instrument complexity, because only one channel is required. Figure 12: Demonstration of unidirectional probe operation at 200-kHz and 430-kHz center frequencies on a 0.25-inch thick carbon steel plate: (**a**) test arrangement using 1.5-inch wide MsT transducer, (**b**) 200-kHz data, (**d**) 430-kHz data. The spectrograms of signals in panels (**b,d**) are shown in (**c,e**).