# New Magnetstrictive Transducer Designs for Emerging Application Areas of NDE
## 7 MsTs with Special Functionality: Unidirectional Operation  
The second design discussed here is a single winding unidirectional MsT transducer. This type of transducer is desirable because direction control currently requires the use of at least two transducer positions or two transducers with an active phasing of the excitation signal and/or signal processing after data collection. When using two transducers with active phasing, a typical amplitude ratio between signals traveling in the wanted and unwanted direction is of the order of 12-15 dB. Each of the two transducers is typically further split into multiple segments that are spaced a half wavelength apart. This is similar to a meander coil EMAT; each meander repetition reinforces the generation of guided waves at a specific wavelength and increases the rejection of other wavelengths. Combining two transducers--each with multiple segments--further improves the direction control capability and the amplitude ratios between the signals traveling in the wanted and unwanted directions; with this arrangement, the amplitude ratio can reach 25-30 dB. Figure 9a shows a direct Wiedemann effect probe with two quarter-wavelength spaced meander coils (each coil has two channels separated by one-half of the wavelength). Figure 9b shows an MsT probe (reversed Wiedemann effect) with two-quarter wavelength spaced transducers (each transducer has two segments that are also separated by half of the wavelength to reject unwanted directions). The major disadvantage of these probes is that the best direction control performance is achieved only at the correct wavelength. Switching the frequency range (and thus wavelength) requires changing the spacing between segments; this can be difficult when an access to a probe is limited. In addition, active phasing requires multi-channel equipment for operation, which increases power consumption and electronics complexity. Thus, there is a motivation to develop a unidirectional MsT for special applications where broader frequency operation and/or simpler electronics are required. The basic concept explored here for the development of a unidirectional MsT was to offset the permanent magnet to one side of the magnetostrictive strip. Normally, the biasing magnet on the strip uniformly biases the strip; in contrast, the offset divides the magnetostrictive strip into two regions with opposite bias polarization. The expectation is that the strains produced in one region will be in the opposite direction of the other region because of the static field reversal. A conceptual illustration of the setup is shown in Figure 10. This illustration uses a 19-mm wide magnetostrictive strip; the magnet placement divides the strip into two areas, one that is 6 mm wide, and another that is 13 mm wide. Figure 9: Directional transmission of guided waves: (**a**) utilizing two quarter-wavelength spaced meander coils (each coil has two conductors that are separated by one half of the wavelength); (**b**) utilizing two quarter-wavelength spaced MsT type transducers (each transducer in turn has two channels separated by one half of the wavelength and connected to reject the unwanted direction). While not completely understood at this point, the intent is for the waves that are generated in each section to interfere with one another to control the wave propagation direction. A sensor was built following this conceptual drawing and coupled to a 6.3-mm thick stainless steel plate using a shear wave couplant, as shown in the Figure 10. A broadband pulse centered at 300 kHz was applied to the solenoid coil. This sensor was used for both the generation and reception of the guided waves. At this input frequency, shear-horizontal guided waves will be generated. The probe produced waves traveling in both directions, but their frequency content and amplitudes were different, as shown in Figure 11. The current theory is that the waves generated from each active region will have different characteristics in terms of frequency and amplitude.