Signal conditioning technique for magnetoelastic sensors

Abstract

Magnetoelastic materials are obtaining an increasing interest in the last few years. Magnetoelastic wave resonant frequency is the sensory parameter generally used to reliably obtain critical information, nevertheless, it was shown that wave amplitude also is very sensitive to measure displacements or magnetic fields. Both parameters, amplitude and resonance frequency could be used as the "sensitive parameter" in no-contact vibration sensors or stress, flux or magnetic field sensors. To make them advantageous to be used, some problems should be solved: the excitation problem (to induce resonance) and the conversion problem (to transduce frequency or amplitude information into a convenient electric signal); in other words, the signal conditioning problem. The paper describes a signal conditioning technique for magnetoelastic sensors developed by using metallic glass materials. Analysis and synthesis phases of design for the signal-conditioning prototype are presented. From the frequency response analysis of a magnetoelastic metallic glass ribbon, a very low damping factor is pointed out. This means that a stationary oscillation could be easily induced into the ribbon and it was seen that the oscillation frequency changed when the boundary conditions (mechanical stress or magnetic field) changed. So, the ribbon can be used as a sensor. The proposed device (PLG 3) for the sensor signal conditioning is described in detail. Finally, PLG 3 plus a magnetoelastic sensor is used for a no contact vibration measurement and compared to the results from traditional tools.

OLC: a signal conditioning and calibration technique for magnetoelastic sensors

“…The first tests pointed out that the MES, even though it showed a sufficiently low distortion level and a good linearity (comparable to that of traditional devices, such as LVDT or accelerometer -see [1]) it could not be practically used due to its poor repeatability: measurements were sufficiently accurate just in the same set of data, and a calibration by a reference sensor would be required before each set of measurements. The low MES accuracy seems to be dependent on an already known time varying behavior of the magneto-elastic sensing element of the sensor.…”

“…As described in [1], also amplitude can be used as the sensor sensitive parameter. Energizing the excitation coil by a signal wave (excitation signal) at a frequency close to the free ribbon resonance frequency, the electric signal induced in the pick-up coil shows an amplitude (pseudo-resonance amplitude) depending on the real resonance frequency of the structure, and consequently, depending on the H field.…”

“…As proved in [1], the magnetoelastic properties of the ribbon are maintained into a closed loop system, such as that shown in figure 3, called MagnetoStrictive Resonator system (MSR). It was designed for self-tuning on the resonance frequency of magnetoelastic ribbons.…”

“…The cycle is repeated again, inverting the phase each time, so the vibration stays on and the oscillation frequency is very close to the open loop characteristic frequency of the ribbon (ref. [1]). This frequency will be referred in the following as closed loop resonance frequency.…”

“…PLG 3 can be considered the signal-conditioning prototype for magnetoelastic sensors. In fact, it is able to give an output signal proportional to the magnetic field variations (see [1]). It can operate in the two modes mentioned in paragraph 3: a. by inducing sensor resonance, as a frequency follower (locking mode), giving as output a voltage level proportional to the frequency variation; in this case the output is given by a "frequency modulation" demodulator.…”