Vibration Testing and Anti-Vibration Design of High-Performance Quartz Accelerometer
Quartz flexible accelerometers are widely used in aerospace, inertial navigation, precision measurement and other fields due to their advantages of high precision and stability. However, its core sensitive component - quartz flexible pendulum - is very fragile and extremely sensitive to vibration environments. Vibration may cause performance degradation, zero shift, and even structural damage. Therefore, strict vibration testing and effective anti vibration measures are crucial.
The purpose of vibration testing is to evaluate the performance, structural integrity, and reliability of accelerometers in actual or expected vibration environments. The main types of testing include sine vibration testing and random vibration testing.
Sine vibration testing is to find the resonant frequency of the structure and evaluate its response and tolerance at specific frequencies. Typically, within the specified frequency range (e.g., 5 Hz – 2000 Hz or higher, according to the specification), a linear or logarithmic sweep is performed along three mutually perpendicular axes. Frequencies exhibiting an abnormal increase in accelerometer output (response amplification) are recorded. Special attention should be paid to the first-order and second-order flexural resonant frequencies of the quartz flexure, as these frequencies are most prone to damage. The relationship between the input vibration and output signal is analyzed. Perform fixed frequency vibration for a specified duration (such as several minutes) on the identified resonant frequency points (especially the pendulum resonance points) to test their fatigue life and stability.
Random vibration testing is to simulate wideband, random vibration excitations in actual environments (such as rocket launches, engine noise, aerodynamic turbulence, vehicle driving), and evaluate their comprehensive performance under statistically distributed vibrations. Apply vibrations with specific power spectral density patterns along three mutually perpendicular axes within the specified frequency range. The scale of testing is usually higher than that of sine testing, which can better reflect the real environment. PSD spectra and Grms values are developed based on specifications or measured data.
The key monitoring parameters in the test include output signals, physical status, and performance verification. The output signal mainly observes zero offset, scale factor changes, increased noise levels, abnormal outputs (saturation, oscillation), etc; The physical state mainly involves checking whether the accelerometer has any abnormal noise, whether the structure is loose, and whether there is any damage to the appearance (after testing, the cover needs to be opened to check the swing plate); Performance validation mainly involves comprehensive static performance testing of accelerometers before, during, and after vibration (zero bias, scale factor, second-order nonlinearity, threshold, resolution, repeatability, etc.), comparing the effects of vibration.
To address the vulnerability of quartz flexure accelerometers, vibration resistance measures must be implemented at multiple levels.
Firstly, it is necessary to optimize the design of the oscillating plate. While meeting the requirements of sensitivity and bandwidth, the thickness of the oscillating plate should be appropriately increased (balancing sensitivity), the shape and support structure should be optimized, and the lowest order resonant frequency should be maximized to be higher than the expected main vibration environment frequency (ideally higher than twice). Avoid resonance frequencies falling in the main vibration energy concentration zone. Secondly, it is precision manufacturing and assembly, strictly controlling the machining accuracy and symmetry of key components such as swing plates, torque converter coils, differential capacitor plates, etc., to ensure uniform stress distribution and reduce internal stress concentration points. Reliable processes such as laser welding are used to connect the swing plates to the base. Finally, the selection of materials should be based on high-strength and high stability quartz materials, and the stiffness and damping characteristics of the base and shell materials should also be considered.
Firstly, apply a small amount of special damping adhesive (such as silicone rubber base) on the non sensitive area of the swing plate or the supporting beam. This is one of the most commonly used and effective measures, which can significantly reduce the resonance Q value and decrease the resonance amplification factor. However, extreme caution should be taken to avoid affecting sensor performance (such as introducing thermal stress, affecting symmetry, and increasing mass). Secondly, a micro isolation system (such as low stiffness, high damping rubber pads, metal rubber, micro wire rope isolators, etc.) should be designed between the core of accelerometer (including the core components of the swing plate assembly) and the housing base to isolate the transmission of high-frequency vibrations to the sensitive core. The stiffness, damping, and load-bearing capacity need to be carefully designed.
Encapsulating silicon gel or silicone rubber with low modulus and high damping in the inner cavity of the accelerometer (usually between the core and the shell) is another very effective anti vibration measure (especially anti shock and high-frequency vibration). Silicon gel or silicone rubber provides damping and absorbs vibration energy; Support internal structure and suppress relative motion; Protect internal components from contamination and moisture. The sealing adhesive needs to have extremely low shrinkage stress, excellent thermal stability, good adhesion, low air release, and low permeability (especially for space applications). The sealing process (excluding bubbles and curing) is crucial, as poor sealing can actually introduce stress or contamination.
Optimize the installation interface to ensure that the installation base is flat and rigid. Tighten the connecting bolts evenly to the specified torque to avoid introducing local stress or reducing overall stiffness due to improper installation. At the same time, install appropriate external shock absorbers between the accelerometer and vibration sources such as engine mounts and aircraft structures. Select the type of isolator (such as metal rubber isolator, wire rope isolator, viscoelastic damping isolator) based on the characteristics of the main vibration frequency, and its natural frequency should be much lower (usually <1/√ 2 times) than the lowest main vibration frequency that needs to be isolated. The load-bearing capacity of the isolator, environmental adaptability (temperature, vacuum), and the impact on the low-frequency performance of the sensor also need to be considered.
Taking the AC-9 series high-precision quartz accelerometer produced by Micro-Magic Inc as an example, the influence of vibration environment on the K0 index of the accelerometer is tested. Before vibration, the quartz accelerometer is in a 0g state, the product is powered on, data is collected for 1 minute, and take the average of the data, denoted as 
; Start the vibration table under the following vibration conditions: 6g, random vibration at 200~2000Hz, wait for the vibration table to stabilize, collect data for 1 minute, and take the average of the data, denoted as 
; Rotating vibration fixture, quartz accelerometer in 0g state, then power on the product, collect data for 1 minute, and take the average of the data, denoted as 
. By calculation, it can be concluded that:
The vibration problem of quartz flexible accelerometers is a system engineering issue that needs to be addressed throughout the entire process of design, manufacturing, testing, and application. When selecting and designing anti vibration measures, careful trade-offs must be made to ensure that the core performance indicators of the accelerometer are not significantly sacrificed while improving the anti vibration capability, and other environmental adaptability requirements are met.
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