Vasco António Lourenço Lima finaliza o seu Doutoramento
Tema da Tese: ‘Low-pressure closed-loop MEMS accelerometers for automotive applications’
Autor: Vasco António Lourenço Lima
Programa Doutoral: Programa Doutoral em Sistemas Avançados de Engenharia para a Indústria
Orientadores: Jorge Miguel Nunes dos Santos Cabral; João Carlos Azevedo Gaspar; Luís Alexandre Machado da Rocha
Abstract: The current trends in the automotive industry are automation and electrification. Nowadays, a typical vehicle has around one hundred sensors, from which at least thirty use MEMS technology, amounting to an annual market value of 11 billion US dollars. A large number of these devices are inertial sensors, representing an annual market value of 1.6 billion US dollars.
Typical inertial applications, like inertial measurement units (IMU), require more than one type of inertial sensor, such as accelerometers and gyroscopes. Yet, MEMS gyroscopes based on the Coriolis force must operate in low-pressure, while accelerometers are typically operated at atmospheric pressure since they must be critically damped. Thus, vacuum encapsulation of MEMS accelerometers can be advantageous since it enables integration in a single common silicon substrate with other MEMS inertial sensors, leading to size reduction and to a decrease in fabrication and packaging costs. However, the high quality factor resultant from vacuum encapsulation causes undesirable high settling times for accelerometers.
Electromechanical sigma-delta modulators can provide the electrical damping necessary for low-pressure accelerometers while delivering high resolution and linearity, and low thermal dependency and susceptibility to process variations, representing one of the most attractive architectures for achieving high-performance MEMS inertial sensors. Vacuum encapsulated MEMS accelerometers using sigma-delta modulation can be found in the literature, but all present a common issue: large proof-mass. This has
several drawbacks, such as reduced full-scale, added complexity of the manufacturing process, and integration limitations, ultimately leading to cost increase.
In this thesis, a small-size and low-pressure MEMS accelerometer operated in a closed-loop sigma-delta modulator is presented. The sigma-delta modulator loop was digitally implemented in a FPGA, enabling a fast and accurate tuning of the loop parameters at the final trim of the devices. This aims to achieve loop stability, more difficult for low-damping devices, and to improve the sensor overall performance.
The implemented accelerometer achieved a noise figure of 173 μg/√Hz, for a bandwidth of 400 Hz, a measurement range of at least ±5 g, and a nonlinearity lower than 0.66 %. Lower noise values (123 μg/√Hz) are also attainable at the expense of lower dynamic range (±1.5 g), verifying the flexibility of the accelerometer. To the best of the author’s knowledge, this work shows the best relation between noise, device size, and bandwidth, when comparing similar devices presented in the literature to date.
Keywords: accelerometer; MEMS; sigma-delta modulation; vacuum encapsulation.