|
FeGa-based alloys (Fe1-xGax, Galfenol) belong to a branch of magnetic materials called “magnetostrictive” materials, in which their dimensions change in response to changes in the magnetization. Magnetostrictive materials also experience an inverse effect, called the Villari effect, where magnetization and permeability changes occur in response to changes in applied stress/strain. In this study, an active mode water level sensor has been developed. The sensor has been designed to work when the water is both in motion and still. A Galfenol-brass unimorph beam has been constructed, with Galfenol as the active layer. The beam is clamped at one end, and, when it is flexed, there will be a stress concentration near the clamped region. This change in stress can be measured by a magnetic field sensor, which can detect local fluctuations in magnetic field due to the Villari effect. Two magnetic coils are used, one for alternating current (AC) magnetic field generation and the other for measuring the magnetic field response of the strip. The resonance frequency of the beam in air is higher than in water. By choosing an operating vibrational frequency higher than the resonance frequency in air, we can separate the pick-up coil impedance responses when the beam is surrounded by air or water, or even by sediment. The vertical response of the beam has also been measured; as the beam is covered by more water, its resonance frequency should incrementally change. The experimental results are compared with a simulation using a theoretical vibration model for a cantilever beam under water. The beam was tested both as the water level increased and decreased, showing a 10% increase from not submerged to fully submerged and the results were verified by the simulation. The relative sensor impedance between air and water was also evaluated from room temperature (25°C) to 80 °C to verify signal differentiation.
|
