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Effect of Low Pressure Environment on Quartz Flexible Accelerometer

  • AlishaBridges

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    Spritz 15 мая 2024 г. 12:30

    The quartz flexure accelerometer has the characteristics of high sensitivity and low noise, making it suitable for measuring both static and dynamic acceleration. It can be used as an acceleration-sensitive sensor for monitoring micro-vibration environments in spacecraft orbits. This article mainly introduces effect of low pressure environment on quartz flexible accelerometer.

    The sensitive diaphragm of the quartz accelerometer experiences membrane damping effects when in motion in the air environment, which could potentially cause changes in the sensor’s performance (scale factor and noise) in low-pressure environments. This could affect the accuracy and precision of measuring on-orbit micro-vibration acceleration. Therefore, it is necessary to analyze this effect and provide a feasibility analysis conclusion for the long-term use of quartz flexible accelerometers in high vacuum environments.

    1.Damping analysis in low-pressure environments

    The longer the quartz flexure accelerometer operates in orbit, the more air leakage occurs inside the package, resulting in lower air pressure until it reaches equilibrium with the space vacuum environment. The average free path of air molecules will continuously lengthen, approaching or even exceeding 30μm, and the airflow state will gradually transition from viscous flow to viscous-molecular flow. When the pressure drops below 102Pa, it enters the molecular flow state. The air damping becomes smaller and smaller, and in the molecular flow state, the air damping is almost zero, leaving only electromagnetic damping for the quartz flexible accelerometer diaphragm.

    For quartz flexure accelerometers that need to operate for a long time in low-pressure or vacuum environments in space, if there is significant gas leakage within the required mission life, the membrane damping coefficient will significantly decrease. This will change the characteristics of the accelerometer, making scattered free vibrations ineffective in attenuation. Consequently, the scale factor and noise level of the sensor may change, potentially affecting measurement accuracy and precision. Therefore, it is necessary to conduct feasibility tests on the performance of quartz flexible accelerometers in low-pressure environments, and compare the test results to assess the extent of the impact of low-pressure environments on the measurement accuracy of quartz flexible accelerometers.

    2.Impact of low-pressure environments on the scale factor of quartz flexure accelerometers

    Based on the analysis of the working principles and application environments of quartz flexible accelerometer products, it is known that the product is encapsulated with 1 atmosphere pressure, and the application environment is a low Earth orbit vacuum environment (vacuum degree approximately 10–5 to 10–6Pa) at a distance of 500km from the ground. Quartz flexible accelerometers typically use epoxy resin sealing technology, with a leakage rate generally guaranteed to be 1.0×10–4Pa·L/s. In a vacuum environment, the internal air will slowly leak out, with the pressure dropping to 0.1 atmosphere pressure (viscous-molecular flow) after 30 days, and dropping to 10–5Pa (molecular flow) after 330 days.

    The impact of air damping on quartz flexure accelerometers mainly manifests in two aspects: the impact on the scale factor and the impact on noise. According to design analysis, the impact of air damping on the scale factor is approximately 0.0004 (when the pressure drops to vacuum, there is no air damping). The calculation and analysis process is as follows:

    The quartz flexure accelerometer uses the gravity tilt method for static calibration. In the accelerometer’s pendulum assembly, in an environment with air, the normal force on the pendulum assembly is: mg0, and the buoyant force fb is: ρVg0. The electromagnetic force on the pendulum is equal to the difference between the force it experiences due to gravity and the buoyant force, expressed as:

    f=mg0-ρVg0

    Where:

    m is the mass of the pendulum, m=8.12×10−4 kg.
    ρ is the density of dry air, ρ=1.293 kg/m³.
    V is the volume of the moving part of the pendulum assembly, V=280 mm³.
    g0 is the gravitational acceleration, g0=9.80665 m/s².
    The percentage of the buoyant force to the gravitational force on the pendulum assembly itself is:

    ρVg0/mg0=ρV/m≈0.044%

    In a vacuum environment, when the air density is approximately zero due to gas leakage causing the pressure inside and outside the instrument to balance, the change in scale factor of the quartz flexible accelerometer is 0.044%.

    3.Conclusion:

    Low-pressure environments can affect the scale factor and noise of the quartz flexible accelerometer. Through calculation and analysis, it’s shown that the maximum impact of the vacuum environment on the scale factor is not more than 0.044%. Theoretical analysis indicates that the influence of low-pressure environments on the sensor’s scale factor is less than 0.1%, with minimal impact on measurement accuracy, which can be neglected. This demonstrates that low-pressure or vacuum environments have minimal effects on the scale factor and noise of the quartz flexure accelerometer, making it suitable for long-term on-orbit applications.

    It’s worth noting that the ER-QA-01A series quartz flexible accelerometers are designed specifically for aerospace applications. Among them, the ER-QA-01A3 has the highest precision, with zero bias repeatability ≤10μg, a scale factor of 1.1~1.mA/g, and scale factor repeatability ≤10μg. It is fully suitable for monitoring micro-vibration environments of spacecraft in orbit. Additionally, it can be applied to inertial navigation systems and static angle measurement systems with high precision requirements.

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