Monolithic Accelerometer for 3D Measurements
Bulk micromachining low-g accelerometers for single axis measurements are today’s state of the art technology for measuring low-g range signals in automotive, medical and other applications — also referred as “human scale motion”. The latest trend in sili
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Monolithic Accelerometer for 3D Measurements
T. Lehtonen, J. Thurau, VTI Technologies Oy Abstract Bulk micromachining low-g accelerometers for single axis measurements are today’s state of the art technology for measuring low-g range signals in automotive, medical and other applications – also referred as “human scale motion”. The latest trend in silicon micromachining is smoothening the border between bulk and surface micromachining. A combination of SOI (Silicon On Insulator) wafers and DRIE (Deep Reactive Ion Etching) as new technologies is leading to smaller and more cost efficient micromachined inertial sensors. A novel capacitive 3-axis accelerometer element is the first example of this combination by VTI Technologies. The key task was to overcome the physical limitation to measure a 3-dimensional acceleration signal with a 2-dimensional silicon element that suits to modern packaging constraints. It will be shown how the new sensor element is working and that the similarity of the manufacturing process with today’s mass produced VTI single axis accelerometer, will lead to a fast commercialization of the novel 3-axis design. Additionally the utilization of this technology platform for further projects will be sketched.
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Starting Position
Automotive inertial sensor road maps show a six degrees-of-freedom inertial measuring unit, where a three-axis accelerometer is needed for measuring linear motion. The three-axis accelerometer is also expected to open up new markets for accelerometer based motion sensing, navigation and inclination measurement in portable electronic devices [1]. Several previous attempts to design and manufacture a three-axis accelerometer are reported in the literature [2-5]. VTI Technologies has been developing and producing single axis low-g bulk micromachined capacitive accelerometers since the early 90’s and has achieved a leading position with over 10 million annual production [6, 7]. Great deal of recent research and development work has been done to develop a new concept for a single element multiple axis accelerometer.
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Safety
The approach described in this paper is a 3-axis accelerometer which requires a high sensitivity system consisting of springs and a proof mass or several masses, whose position is observed capacitively and further mapped to acceleration information by an electrical circuit. Several requirements for the sensor element are existing such as: mass movement, conversion to an electrical signal and the mapping algorithm should yield a high signal to noise ratio and a linear acceleration response with negligible cross axis sensitivity. In the ideal case each measured axis should be mechanically equal in sensitivity and frequency response – or even better, freely adjustable over a wide range. The next important specifications are temperature dependence and shock resistance. It isn’t less important that the accelerometer should be manufacturable based on feasible low cost processes, have small size and should enable low cost interconnecting and packaging. VTI i
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