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An improvement of micromechanical accelerometers (MMA) is relevant due to the constant expansion of their applications and the use of such devices in increasingly demanding conditions. In the paper the sandwich designs of the sensitive element (SE) of a capacitive micromechanical accelerometer (MMA) with folded springs have been studied from the point of view of ensuring the high sensitivity to acceleration, resistance to temperature change and the presence of residual mechanical stresses in the structures while providing the relative simplicity of their manufacturing technology. The modeling has been executed in ANSYS program. The gaps between the movable and stationary electrodes are increased compared to analogs up to 20 microns. It has been shown that the high sensitivity to changes of the acceleration is provided due to the optimization of the sensitive element design, using folded springs with lower values of stiffness coefficient. It has been determined that the capacity change under the action of acceleration along the working axis ( Z ) is almost 20 times more than changes in capacity along axes X and Y and the effect of temperature in the range of -40 ºC to 85 ºC on changes in capacitances along the working axis ( Z ) is small ±0.0025 - 0.003 pF. The mechanical stresses, which occur in constructive elements of the sensitive element under acceleration to 50 g, do not exceed 2.29 MPa, while silicon has strength 440 MPa. The natural frequency of oscillation of the second mode of the sensitive MMA does not affect the natural frequency of oscillation of the first mode of the sensitive element due to a significant difference of these frequencies approximately by 2 kHz. The analysis has shown that the studied sandwich constructions are characterized by high sensitivity and stability of parameters with relatively simple manufacturing.
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In high-efficient MMA-accelerometers the capacitive principle of measurement is used. In addition, in this connection a low level of noises and energy consumption, economical efficiency and reliability are provided. The capacitive sensitive elements, based on changing the clearance, require, as a rule, the control with feedback, which increases the complexity of the measurement circuit and energy consumption. The capacitive sensitive elements with changing the area of the electrodes overlapping have a good linearity of the capacitance dependence on movement and large range of measurements, but their fabrication is more complicated. In the paper a model of a sensitive element of a microelectromechanical capacitive accelerometer with sandwich construction has been presented and analyzed. The operation of the sensitive element is based on using the changes in the relative permittivity of the dielectric capacitors due to the introduction of a moving inertial mass between the moving capacitor electrodes under the action of acceleration. As a result, there is a change of capacitance in the output measuring circuit. It has been shown that the model considered provides high sensitivity to acceleration, resistance to temperature changes and low residual mechanical stress in the sensitive element. Modeling and calculations have been performed with using the Ansys and SolidWorks programs. It has been obtained that the movement of movable mass along the axis of sensitivity X 5 times exceeds the movement of the movable mass along the non-working axes, and the capacitance changes between the electrodes along the X-axis is 2500 times greater than the capacitance changes between the electrodes on the non-working axes Z and Y. The calculations have shown that for all values of acting acceleration (up to 30 g) the mechanical stress in the sensitive element is significantly less than the strength limit of silicon, equal to 440 MPa. It has been determined that the temperature variations in the range from -40 to +85 C have led to insignificant changes of capacitance along the working axis (0.0025 - 0.003pF). This demonstrates the temperature stability of work of the sensitive element. The analysis has shown that the developed and studied model of the sensitive element sandwich construction provides the high sensitivity of MMA accelerometer and stability of its parameters.
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In microelectromechanical devices and systems (MEMS), capacitive micromechanical accelerometers (MMA) are used in airbag systems, machine vibration monitoring, navigation, seismology, microgravity measurements, etc. The most important structural elements of the sensing element are suspensions, through which the inertial mass is connected to a fixed frame. To ensure reliable operation and stability of sensor parameters, it is necessary to take into account the results of external factors already at the design stage of the sensor structure, especially its sensitive element, and at subsequent stages of the life cycle. In this work, the characteristic structures of suspension elements, which are made of silicon with different crystallographic orientations, are investigated and the results of modeling their most important parameters are presented. The simulation has been performed using the ANSYS program. The natural frequencies of inertial mass oscillations, residual mechanical stresses in the structural elements of silicon sensitive element with different crystallographic orientations have been calculated upon impact (up to 10 000 g). The natural vibration frequency changes and the dynamics of the change in the residual mechanical stress in the suspension elements with a temperature change in the range from +150 to -150 °C for a short time interval of 10 s, which corresponds to thermal shock, investigated. The results of studies of residual mechanical stresses arising upon impact and natural frequencies of inertial mass oscillations have been made it possible to develop recommendations on the choice of the design of suspension elements made of silicon, providing high sensitivity and stability of MMA parameters. It has been established that the use of folded springs with a rectangular or round cross-sectional shape with a suspension element thickness of 40 μm provides the highest temperature stability of the parameters. The results obtained are useful for developing real designs of MMA and other micromechanical devices
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Various types of parasitic influences must be considered at the stage of designing sensitive element of a micromechanical capacitive accelerometer (MMA) and the whole MMA construction, and throughout its lifecycle stages, to ensure consistent performance and parameter stability of MMA. Due regard to actual operating conditions is a primary consideration for designers. In this work, the influence of random vibration on the functioning of a 2-electrode sensitive element of an MMA with a sandwich construction using the principle of operation based on a change in the relative dielectric constant of a capacitor dielectric under acceleration was investigated. In the structure of the investigated sensitive element with one axis of sensitivity, the inertial mass is suspended on folded springs and is located between two fixed electrodes. Using the Ansys program, the deformations of the inertial mass of the sensitive element and the change in the capacity in the sensitive element were calculated, which occur when it is exposed to acceleration up to 5 g along the working axis and up to 50 g along the non-working axes and random vibration along the working axis X and along the non-working axes Y and Z . The random vibration affecting the sensitive element had a profile, the frequency of which was 20 Hz at 0.01 g / Hz, the frequency from 80 to 350 Hz at 0.04 g / Hz, 2000 Hz at 0.01 g / Hz. The changes in the capacity in the sensitive element of MMA under the influence of random vibration on it were calculated. Results have been obtained that confirm the performance of the sensitive element of MMA under conditions of significant impacts along non-working axes. The investigated sensitive element model requires further refinement to be used under conditions of exposure to random vibration.
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