Two degree of freedom capacitive MEMS velocity sensor

Abstract

This research presents the design and implementation of a novel two-degree-of-freedom (2-DoF) capacitive MEMS velocity sensor for use with structural vibration measurements. The sensor comprises two mass–spring–damper systems that are connected in series. The base principal system is used as the principal sensing element, and the other system functions as the secondary sensing element for the implementation of an internal velocity feedback loop. This loop is aimed at producing damping force on the proof mass of the principal sensing system, so that the frequency response function of the velocity sensor takes on three important properties: (1) At low frequencies below the fundamental resonance of the 2-DoF sensor, the output of the sensor becomes proportional to the velocity of the sensor’s frame. (2) Around the fundamental resonance, the sensor is characterised by a flat amplitude spectrum. (3) Finally, above the fundamental resonance, the sensor is characterised by an amplitude roll-off with only a 90° phase lag. In contrast to standard accelerometer vibration sensors, this sensor produces the desired velocity output within the bandwidth up to the first resonance frequency and generates a filtering effect with a –90° phase lag after the first resonance frequency. A piezoresistive MEMS velocity sensor presented in the literature was explored to confirm the effectiveness of the concept that drives the current 2-DoF velocity sensor. Such technique, however, is susceptible to temperature changes, presents low sensitivity and requires several fabrication steps. To avoid these drawbacks, the sensor proposed in this research was specifically designed with a capacitive transducer and an actuation technique. The sensor interface and the controller are implemented on a printed circuit board. The control loop and closed-loop response were designed by a post-process intended to measure frequency response functions (FRFs) for the displacements of the two proof masses with respect to (i) base acceleration and (ii) the electrostatic actuator applied to the principal proof mass. The comparison of the simulated and measured FRFs indicates that the MEMS sensor dynamically and closely reproduces the desired 2-DoF response. The first prototype sensor was fabricated on a silicon-on-insulator (SOI) wafer with two masks. Below 1 kHz, the measured output signal of the closed-loop sensor is proportional to the velocity of the base. Above the fundamental resonance, the output signal rolls off with a phase lag of –90°. The second prototype sensor is grounded on an innovative design and fabrication process, which enabled the direct measurement of the relative displacement between the two proof masses. The measurement was conducted using a capacitive transducer and mechanical subtraction. The second prototype was fabricated on an SOI wafer with three masks. The post-process of the measured data shows that at low frequencies (between about 300 Hz and 1 kHz), the spectrum of the sensor’s output signal is proportional to the base velocity. Around the fundamental resonance frequency, the characteristic resonance peak flattens and the phase lag decreases to –90°. These three properties are of considerable interest for the implementation of vibration control systems that use feedback loops with a collocated velocity sensor and piezoelectric patch actuator pairs.

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