Design of an Integrated MEMS Magnetic Gradiometer Rejecting Vibrations and Stray Fields

Abstract

Magnetic sensors are often used near current-carrying wires or electrical motors generating significant magnetic interference. To mitigate the effects of these stray fields, the traditional design approach relies on a differential sensing scheme: multiple magnetometers are spaced apart, and the field differences are measured. Despite being rejected, stray fields still constrain the design space. Extra linear range and matched channels are required to accommodate their peak amplitude without saturation or residual common-mode leakage. On the contrary, single-point MEMS gradiometers rely on the force acting on a magnet, which is directly proportional to the magnetic field gradient. The stray field is intrinsically rejected by the magnetic transducing mechanism, even before entering the measurement chain. The range of the measurement chain can then be largely optimized for the gradient, independently of the stray field amplitude. This paper discusses the design of a single-point MEMS gradiometer. By design, it rejects magnetic stray fields and mechanical disturbances like vibrations and gravity. It is the first single-point MEMS gradiometer capable of operating unshielded and in various orientations. The prototype achieves a noise density of 4 nT/mm/ $\sqrt {\mathrm { Hz}}$ within a measurement range of ${\pm } 300~{\mu }$ T/mm. The paper demonstrates the sensor’s effectiveness in a bus-bar current sensing application. Design limitations and future design prospects are also outlined.