Design and simulation of high linearity and high-G Accelerometer Chip
Date Issued
2014
Date
2014
Author(s)
Lin, Po-Fu
Abstract
Nowadays, micro-accelerometer is widely integrated, in the devices used in our daily life, such as vehicles, mobile devices and smartosensory devices, as well as in some high-tech applications, such as aerospace technology and military weapons. Especially in the application of military aspects, the warhead of projectile must be sustainable to high impact so that it needs to install a high-G accelerometer to detect severe environment. With the development of the micro-electro-mechanical system (MEMS) in recent years, the reliability and measurement range of accelerometer has been improved. Various types of accelerometers, which could be operated up to 10,000 G or higher impacts, were developed successively. However, in the process of design an accelerometer chip, the sizes of devices, accuracy of manufacturing and cost must take into consideration. Different configurations of devices or manufacturing processes may lead to different requirements and restrictions in the design. As such, there is no simple and accurate way to predict the influence of the mechanical sizes of the device to the output signal of the sensor. We can only construct various models by computer aided design (CAD) software, then relay into computer aided engineering (CAE) software for simulation to find out the better design from the computational results. Nevertheless, it cost a lot of time.
Consequently, this thesis exploited the geometry design of a high linearity and high-G microaccelerometer by correlating the CAE computational results of various geometrical sizes of the accelerometer structure and the output signals. Besides, this thesis also put the natural frequency of the structure and sensitivity of the sensor into consideration to establish a new design guideline for forecasting the output signal. This thesis referred to a piezoresistive accelerometer chip designed by Kuells, which composed of a flat-plate cantilever beam and four piezoresistors placed symmetrically on the central upside of the cantilever beam and perpendicularly to the beam. This structure deformed and vibrated in response to the applied impact. It could be modeled as a spring-mass system. However, the configuration of the beam had three fixed edges with suspended piezoresistors which had some difficulties to manufacture by MEMS processes. This thesis relaxed this restriction by releasing the lower longitudinal edge of the beam which could be realized by etching the trenches through the wafer.
To investigate the influence of geometry on the performance of the accelerometer, this thesis implemented several models in different geometrical sizes. The cantilever beam had a length ranged from 350μm to 600μm, its depth ranged from 130μm to 500μm, and its width ranged from 12.5μm to 25μm; the piezoresistors had the length ranged from 10μm to 30μm, and the cross-sectional area ranged from 3μm2 to 24μm2. The stress responses of these models were numerically simulated subjected to a half-sine wave impact with various amplitudes and durations by the CAE software. The peak values of half-sine waves ranged from 5,000G to 100,000G with specific durations. To make the design process more simple and efficient, this thesis correlated the regression equations for the design parameters and the respondent average stresses of the piezoresistors firstly, and then converted the stresses into the output voltage signals based on the piezoresistive effect.
After properly analyzing and regressing of the resultant data, this thesis successfully concluded two correlation equations for the rapid design of the flat-plate microaccelerometer. The first one describes the relationship between all size parameters and the output voltage signals; and the second one describes the relationship between the size parameters and the maximum respondent stress of the microstructure. Before manufacturing a similar structure of the accelerometer chip, one can put the requirements of sensing device, accuracy, cost into consideration; and then sketch primitive geometry sizes of the device and plug into these two equations. The maximum stress equation can be forecasted which dictates the safety of the structure during the impact. Subsequently, the magnitude of the output signal for sensing can be predicted by the output signal equation. If the forecasting value of the output signal is not fit for the sensitivity of the sensing device, one should change the design parameters and recalculate for a new design, until a satisfactory device is resulted. The design process is programmed in an interactive interface which facilitates the user to design an accelerometer quickly and easily.
Subjects
加速感測晶片
動態響應
高G衝擊模擬
互動設計
Type
thesis
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