To improve the performance of anisotropic magnetoresistance (AMR) sensor, a low-noise driving circuit for the AMR sensor was developed and the magnetic field noise spectral densities of 12 pT/root(Hz) at 1 kHz and 20 pT/root(Hz) at 100 Hz were achieved. The driving circuit could operate in amplifier mode or feedback mode. For the driving circuit with feedback, the distortion of the system was reduced and the AMR sensor was suitable for the applications in the environment without shielding. The Set/Reset method was used to reduce the low frequency noise of the AMR sensor. Due to the low noise of AMR sensor, the eddy current testing (ECT) system with the AMR sensor had the advantage of detecting deep and small defects in metal structures. The dual frequency ECT system was developed to reduce the influence of lift-off variance. Using the ECT system with the AMR sensor, we successfully detected the small defects in the combustion chamber of liquid rocket.
Part of the book: Magnetic Sensors
The corrosion of steel reinforcing bar (rebar) is the leading cause of deterioration of concrete. In Japan, many railway bridges were built 40 years ago. It is necessary to develop easy-operation method to evaluate the corrosion of steel rebar. A project about the corrosion evaluation of steel rebar was started in 2015. In this project, we have two objectives: one is to evaluate the depth and the diameter of steel rebar in concrete; another is to evaluate the corrosion of steel rebar in concrete. We developed electromagnetic methods to do nondestructive evaluation of the steel reinforcing bar (rebar) in concrete. Using two probes and lower excitation frequencies of 3.8 and 4.2 kHz, the depth and the diameter of the steel reinforcing bar can be evaluated. Using higher excitation frequency of about 80 kHz, and the X, Y signals of the lock-in amplifier, where the X signal is the same phase signal with the AC excitation magnetic field and Y signal is the 90° phase different signal with the AC excitation magnetic field, we could evaluate the corrosion of steel rebar. A compact system with low power consumption of 0.5 W was developed, and we also did some field experiments using this system.
Part of the book: Failure Analysis