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Magnetically levitated accelerometer design

Ceylan, İlke
This thesis proposes the utilization of magnetic levitation for designing an acceleration sensor, taking the advantage of up-to-date contactless displacement sensing technology. The accelerometer is expected to have long-term robustness by isolating the proof mass from the rest of the accelerometer body, virtually eliminating mechanical friction and wear. Furthermore, levitated sensors have a great potential to achieve high precision. In this context, this study presents designing a levitated accelerometer, which suspends the proof mass and controls its position relative to the sensor body. When the sensor body moves, the shift of the proof mass with respect to the sensor body is detected and the control system produces feedback forces on the proof mass to keep the proof mass stationary with respect to the sensor body. In this study, a magnetically levitated accelerometer is designed, constructed and tested. Permanent magnets are used to offset the weight of the levitated proof mass. However, that is not enough to keep it steady and stable afloat. Hence, active magnetic actuation is utilized not only for levitation but also for position control of the proof mass. In the design phase, a mathematical model of the system is developed and a simulation model is built by using MATLAB®/Simulink®. Magnetic analyses are performed by using finite element method. PID controllers run independently in a digital microcontroller for position control in two axes and two rotations. Eddy current sensors are installed on the system to measure the relative position along those axes. Moreover, motor drivers are used to feed the proportional current to electromagnets and evaluate the current values on the solenoids by built in sensors. Tests are conducted in order to tune the controllers and finally compare the acceleration measurements with a commercial sensor. The measurement limit of accelerometer is measured as ± 0.6 g. Bias instability and velocity random walk values is calculated as 0.174 mg and 0.182 m/s/√h respectively.