Micromachined Three-Axis Accelerometers and Sensor Readout Electronics

Doktorsavhandling, 2005

Miniaturized, MEMS-based, inertial navigation systems are used to track the position of moving objects without an external reference system. Several existing and future applications such as indoor navigation, stability control of vehicles, active prostheses or training aids for sportsmen would benefit from the small size and the low cost of micromachined inertial sensors. The system performance depends on the performance of the inertial components, gyroscopes and accelerometers. Today true navigation with MEMS components is limited to short time periods, typically less than a minute. This thesis focuses on the development of better performing MEMS accelerometers and gyroscopes. The design, fabrication and characterisation of micromachined three-axis accelerometers are described. These accelerometers are batch fabricated using silicon technology suitable for low-cost mass production. In contrast to most other monolithic silicon accelerometers slanted-beam accelerometers inherently show direction independent resolution and frequency response. Analytical models are presented that accurately model the sensitivity and the frequency response of the devices from the device geometry, the surrounding atmosphere and the readout circuitry. Different fabrication processes using bulk micromachining are proposed. Measurements on fabricated accelerometers over a wide range of temperatures, pressures and shaking conditions verify the models and demonstrate proper operation for both DC and AC measurements. These accelerometers are a promising alternative for future inertial navigation applications. Switched electronic circuits for sensor readout implemented in CMOS processes are proposed and verified. First, a capacitive readout circuit that is suitable for sensors with multiple capacitors connected to a common node, like the three axis accelerometer, is investigated. It is easy to implement since the need of fully differential amplifiers is eliminated. Second, a circuit for resonance frequency readout is investigated that eliminates crosstalk from the excitation signal to the detection circuit. Digital implementation of traditionally analogue parts of sensor systems is proposed. The digital technology is more flexible and allows more complex algorithms to be used. Gyroscope systems with digital excitation and detection loops as well as digital demodulation of the rate signal are demonstrated. Compared to previous analogue implementations these gyroscope systems show more stable rate signals since the low frequency noise from analogue circuits is avoided.