Piezoelectric accelerometers stand out as the primary choice for vibration testing. Despite their broad measurement range, choosing the right one from the vast array available on the market can be challenging. Generally speaking, the proper selection of a vibration sensor hinges on analyzing and estimating three key aspects of the measurement signal: the level of vibration being measured, the frequency range of the vibration signal, and the environmental conditions where the test takes place.
When considering these factors, it's important to evaluate the sensitivity and range of the sensor. The sensitivity of a sensor fundamentally determines its ability to detect vibration signals. Since the sensitivity of a piezoelectric accelerometer relates directly to the acceleration values of the vibration, and because the relationship between acceleration and displacement varies across different frequency bands, it’s crucial to estimate the sensitivity appropriately. Commonly, the sensitivity of piezoelectric accelerometers ranges from 50 to 100 mV/g for voltage output types (IEPE), and from 10 to 50 pC/g for charge output types.
The measurement range of an accelerometer refers to the maximum measurable value within a certain nonlinear error limit, typically around 1%. Generally, higher sensitivity correlates with a smaller measurement range, while lower sensitivity allows for a broader range. For IEPE voltage output accelerometers, the range is determined by the maximum allowable output voltage within the linear error range, usually ±5V. The range is calculated as the ratio of the maximum output voltage to the sensitivity. However, the stability of the bias voltage is critical, as any instability could lead to signal distortion, particularly noticeable in extreme temperatures.
For charge output accelerometers, the measurement range is defined by the mechanical stiffness of the sensor. These sensors often have a higher maximum output compared to IEPE types, but determining the exact range usually requires experimentation. High sensitivity often accompanies a larger mass for the sensing core, which reduces the overall range and lowers the resonant frequency. This can make the sensor more prone to resonant distortions, especially if the resonant frequency of the sensor coincides with the tested signal's frequency components. Thus, when selecting a sensor, it's essential to account for the natural resonant frequency and maintain adequate range margins to prevent signal distortion.
Calibrating the sensitivity of an accelerometer typically involves comparing its output at a specific frequency (like 159 Hz or 80 Hz) to a standard sensor. Impact sensors' sensitivities are gauged by exposing them to various impact acceleration levels and deriving the relationship between input acceleration and electrical output, followed by calculating the line with the least error variation.
The nonlinearity error of an impact sensor can be expressed either as a full-scale deviation or segmented range linear error. Full-scale deviation calculates the error as a percentage of the total output, while segmented range linear error uses smaller ranges for more precise error calculations.
Moving on to the frequency range, the sensor’s ability to measure within a specific frequency range is determined by the amplitude error tolerance, typically ±5%, ±10%, or ±3dB. The upper and lower limits define the high and low-frequency cutoffs. High-frequency responses depend on the sensor's mechanical properties, whereas low-frequency responses are influenced by the integrated electrical parameters and post-circuitry.
When selecting a high-frequency sensor, the cutoff frequency and corresponding amplitude error are both vital considerations. Smaller amplitude errors enhance measurement accuracy and help manage manufacturing tolerances. Moreover, the wide bandwidth of vibration signals or the sensor's insufficient resonant frequency might introduce resonant interference, necessitating careful consideration of resonant frequencies alongside filtering options in the measurement system.
In summary, the high-frequency cutoff frequency is shaped by the sensor's structural design, fabrication, and mounting quality, rather than its output type. Below is a qualitative categorization of different accelerometer types regarding their high-frequency response.
This comprehensive approach ensures optimal performance and reliability in vibration testing scenarios, balancing sensitivity, range, and environmental adaptability.
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