Research On Bistable Piezoelectric Cantilever-based Current Sensor

In recent years,with the advent of the Internet of Things and the information age,the first thing to be solved is to obtain accurate and reliable information,and sensors as the main way to obtain information have become increasingly important.Among them,current is closely related to our daily life and production.In order to automatically detect the current under different circumstances or have automatic protection when dangerous situations occur,we need to apply different current sensors.In order to realize passive detection without the need for external power,non-contact measurement,no need for surrounding wires,two-wire detection,etc.,piezoelectric cantilever-based current sensors have emerged,but traditional oscillating piezoelectric cantilever-based current sensors have low sensitivity and narrow measurement range.Meanwhile,in order to realize low-powered and even self-powered current alarming system for a large number of sensor nodes,a current sensor capable of realizing threshold-triggered characteristics through a physical triggered mechanism is needed.Therefore,this thesis proposes a novel piezoelectric cantilever-based current sensor on a basis of bistable interwell oscillations.By investigating the bistable vibration energy harvester,we analyze the influence of the bistable mechanism on the output of the cantilever.A spring-assisted adaptive bistable energy harvester with two degrees of freedom is proposed and the governing electromechanical equations and potential function are derived.We find that subjected to both harmonic and random excitations,higher outputs from proposed harvester with suitable spring stiffness,compared to those from conventional one,are demonstrated by both experimental and analytical results,especially in low-excitation.It is proved that the spring stiffness can not only contribute to broadening the harvesting bandwidth,but also influence the system characteristics.Theoretical models of oscillating cantilever-based current sensor are developed to analyze the relationship between the relative change in the maximum value of output voltage and the applied current.It can be used for constant measurement in two-wire appliances.We find that as the applied current increases,the oscillating type sensing system exhibits three response areas: linear increase area,abrupt increase area and decrease area.Linear increase area is defined as the sensing area where the equations on the linear relationship and the sensitivity are derived.Besides,theoretical considerations are emphasized on the nonlinear response,the restoring forces and the factors affecting sensitivity.A bistable piezoelectric cantilever-based current sensor is proposed.When used for constant measurement,by changing the positions of magnet pole and current core without extra component,the proposed bistable sensor can realize wide measurement range and high sensitivity utilizing the concept that bistable interwell oscillations can dramatically increase output.The magnetic forces and displacement responses of both oscillating type and bistable type are derived.The output displacement amplitude is approximately proportional to the applied current in the interwell oscillations area of bistable type and the increase area of oscillating type.Therefore,we define these two areas as the sensing areas.The gradient of magnetic fields,vibrational trajectories and the relative output displacements of both types have been investigated.On the same conditions the measurement range of the oscillating type is only 0-0.3 A,while that of bistable type is 0-3.3 A,and the sensitivity of oscillating type is 0.3548 mm/A while that of bistable type is 0.8846 mm/A.When used for triggered measurement,by adding external magnet and changing the positions of magnet and current core,the proposed bistable sensor can utilize snapthrough to monitor the current whether reaches a critical level.The output voltage and the time histories of displacement are investigated to demonstrate the thresholdtriggered performance.Below the threshold,the output voltage is pretty low while the voltage rises abruptly when the current reaches or exceeds the threshold.