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2. Measurement with PPM-EMAT
A PPM-EMAT is connected to the superheterodyne phase-sensitive detector (manufactured by Ritec, Inc.) via a diplexer and a high-impedance preamplifier. The EMAT consists of a spiral, long-track-shaped coil and a permanent magnet array. The active area is 20 mm wide and 40 mm long. It contains 12 pieces of Nd–Fe–B magnets magnetized in the opposite directions, being normal to the surface (Fig. 1). The period of the magnet array, d, is 7.53 mm, which equals the wavelength of the excited SH waves. A flat coil of 28 turns was wound with enamelled copper wire. The EMAT is placed inside the pipe to generate SH waves propagating circumferentially with axial polarization. The same EMAT detects a series of round-trip signals. The first
arriving signal is gated to obtain amplitude and phase at the current operating frequency.

When the EMAT is driven by an rf burst, the coil induces a dynamic field Hv parallel to the surface, which is superimposed on the static field H0 acting perpendicular. The resultant field, H0 1 Hv , then acts in an oblique direction, along which the magnetostrictive effect causes an elongation of the surface element. As the driving current alternates, the resultant field oscillates about the normal direction, resulting in dynamic shear deformation within the surface skin. The shearing motion occurs in the same directions beneath the neighboring magnet pairs (see Fig. 1). This is the dominant excitation mechanism of SH waves with the PPM-EMAT for a ferromagnetic metal [5,6], although the Lorentz force mechanism also exerts a shear wave source in the same direction as the magnetostrictive force. In the receiving stage, the static field is perturbed by the incoming
ultrasound due to the inverse magnetostrictive and Lorentz force mechanisms. A dynamic field arises to stabilize this disturbance and is picked up by the coil in the EMAT.

The transmitter subsystem of the superheterodyne spectrometer drives the EMAT with the high-power, coherently gated square bursts of frequency f. The frequency f is produced by mixing outputs of the fixed IF (25 MHz) oscillator and the variable synthesizer. On the receiver subsystem, a gated phase-sensitive detection circuitry mixes the received signal with the reference signals (in-phase and quadrature) and filters out the components of the sum frequencies, giving outputs of Acosf and Asinf into two channels after analog integration. A is the amplitude of the probing signal and f the phase lag due to the propagation of the SH wave. All the operating parameters were set through a computer, including the frequency f (in 0.1 Hz steps), the rf burst duration time ( , 200 ms), the amplification factors, and the integrator gate. We typically used the rf bursts of 12 cycles. The computer also samples the integrator outputs digitized in a 12-bit resolution and calculates their amplitude and phase. A single set of measurement and calculation takes several milliseconds. Further details of electronics and functions of such system can be found in the literature [7– 9].