Passive Wireless Strain Sensors Using Microfabricated Magnetoelastic Beam Elements

This paper describes resonant wireless strain sensors fabricated from magnetoelastic alloys. The transduction mechanism is the ΔE effect-the change in stiffness of magnetoelastic materials with applied strain or magnetic field. This is measured as a shift in the resonant frequency and is detected wirelessly using pick-up coils utilizing the magnetoelastic coupling of these materials. The sensors are fabricated from a 28-μm-thick foil of Metglas 2826 MB (Fe₄₀Ni₃₈Mo₄B₁₈), a ferromagnetic magnetoelastic alloy, using microelectrodischarge machining. Two sensor types are described-single and differential. The single sensor has an active area of 7 × 2 mm², excluding the anchors. At 23°C, it operates at a resonant frequency of 230.8 kHz and has a sensitivity of 13 × 10³ ppm/mstrain; the dynamic range is 0.05-1.05 mstrain. The differential sensor includes a strain independent reference resonator of area 2 × 0.5 mm² in addition to a sensing element of area 2.5 × 0.5 mm² that is divided into two segments. The sensor resonance is at 266.4 kHz and reference resonance is at 492.75 kHz. The differential sensor provides a dynamic range for 0-1.85 mstrain with a sensitivity of 12.5 × 10³ ppm/mstrain at 23°C. The reference resonator of the differential sensor is used to compensate for the temperature dependence of the Young´s modulus of Metglas 2826 MB, which is experimentally estimated to be -524 ppm/°C. For an increment of 35°C, uncompensated sensors exhibit a resonant frequency shift of up to 42% of the dynamic range for the single sensor and 30% of the dynamic range of the differential sensor, underscoring the necessity of temperature compensation. The geometry of both types of sensors can be modified to accommodate a variety of sensitivity and dynamic range requirements.