Autonomous bistable microsensors for noninvasive AC electrical current measurements

An autonomous bistable MEMS sensor for noninvasive monitoring of AC electrical currents is proposed in this work. The main idea is presented together with simulations and feasibility study. The sensor is composed of a piezoelectric cantilever beam equipped with two permanent magnets having opposed magnetization (one on its free end and one external and fixed) in order to achieve bistable behaviour. The beam snaps between its two stable equilibrium states by means of the magnetic force due to the interaction of the magnetic fields generated by the permanent magnets and the electrical current to be measured. The measurement of AC electrical currents is performed by counting the number of commutations of the bistable structure that are related to the magnitude of the current to be measured. An adjustable threshold allows for tuning of the device sensitivity, of its operative range and of its robustness to external noise. The piezoelectric bistable device is also able to harvest the energy needed to make the system autonomous. The proposed current sensor provides intrinsic digital output, the bistable behaviour allows for obtaining good performances at the industrial frequency even if the mechanical resonance frequency of the beam is larger due to the small dimension of the sensor and to the material properties. The sensor has been modeled and several simulation results are provided in order to assess the device working principle.