Sigma-delta electronic interface for a levitated disc accelerometer

Abstract

This thesis is focused on the design of an electronic interface for the levitated disc accelerometer.  Besides the advantages offered by the system there are also several challenges in the design of the electronic interface.  Since the disc is electrostatistically floating a pick-off circuit with no ohmic connection to the proof mass needs to be designed.  This distinguishes the circuit from typical sensing circuits for MEMS capacitive accelerometers.  Moreover, in the start-up phase of the systems, since the disc is not mechanically attached, it lies on the bottom plate of the sensor.  This determines a very large value for the differential sensing capacitance and its sign needs to be detected correctly by the electronic interface to bring that the disc into the middle position.  Also, the accelerometer operates only in the closed loop which, makes the testing of the system more difficult compared to other accelerometers.  Finally, the equivalent model for the sensor is represented by a complex capacitive network with the disc as a common electrode.  This can determine the transfer function of the sensing circuit to become nonlinear, thus influencing the system stability.  These design and testing issues are analyzed throughout the thesis and solutions to the problems are presented.

First, the electronic interface was implemented on a printed circuit board, PCB.  The sensing element embedded in the ∆-∑ modulator introduces a phase lag in the signal which causes instability of the system.  Therefore, to ensure closed loop stable operation, a phase compensator was introduced in the system.  PSpice simulates were used to analyze the performance of the electronic interface in open loop and closed loop operation.  The PCB design of the interface went in parallel with the sensor fabrication and hence some of the sensor parameters, such as the nominal sensing capacitance, could only be estimated which, made the design of the interface more difficult.  Since the PCB implementation of the interface was completed well before the sensor to be fabricated, first a capacitive network was used instead of the sensor to test the interface.  After the fabrication of the sensor, an open loop testing of the PCB electronic interface together with the fabricated sensor was performed and demonstrates the movement of the disc by electrostatic forces when feedback voltages larger than 20V are applied.

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