Piezoelectric accelerometer low frequency application guide - Solutions - Huaqiang

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Charge output accelerometers are not suitable for low frequency measurements

Since the acceleration signals of low-frequency vibration are very small, the high-impedance small-charge signal is very susceptible to interference; when the volume of the measurement object is larger, the lower the measurement frequency, the problem of the signal-to-noise ratio of the signal is more prominent. Therefore, in the current situation where the built-in circuit acceleration sensor is becoming more and more popular, a low-impedance voltage output type piezoelectric acceleration sensor with relatively small electric noise and excellent low-frequency characteristics should be selected.

Low frequency cutoff frequency of the sensor

Similar to the high frequency cutoff frequency of the sensor, the low frequency cutoff frequency is the lowest frequency signal that the sensor can measure within the specified sensor frequency response amplitude error (±5%, ±10% or ±3dB). The larger the error value, the lower the low frequency cutoff frequency. Therefore, the low frequency cutoff frequency indicators of different sensors must be compared under the same error conditions.

The low frequency characteristics of the low impedance voltage output type sensor are determined by the integrated electrical parameters of the sensor sensitive core and the built-in circuit. The frequency response characteristics can be described by the first-order high-pass filter characteristics of the analog circuit, so the low-frequency response and cut-off frequency of the sensor can be completely determined by the time constant of the first-order system. From a practical point of view, since the calibration of the VLF frequency response of the sensor is difficult, the time constant of the sensor can be measured by the response of the sensor to the step signal in the time domain; therefore, the low frequency response of the sensor and the first-order high-pass filter are utilized. The characteristics are almost consistent, and the low-frequency response of the sensor and its corresponding low-frequency cutoff frequency can be easily obtained by calculation.

Sensor sensitivity, low frequency noise characteristics and dynamic response range

Sensors used for low frequency measurements generally require relatively high sensitivity to meet the measurement of low frequency small signals. But the increase in sensitivity is often limited. Although the sensitivity of the accelerometer can reach 10V/g or higher, the high sensitivity often brings other negative effects, such as sensor stability, overload resistance, and sensitivity to surrounding environment interference. Therefore, the pursuit of excessive sensitivity does not necessarily solve the measurement of small signals. On the contrary, high-resolution and low-noise sensors are often easier to solve practical problems in engineering applications. Therefore, the selection of sensors with low electrical noise is especially important in low frequency measurements.

To demonstrate the minimum signal that the sensor can measure, most commercial accelerometers also provide resolution or electrical noise specifications. The broadband electrical noise indicators of most domestic sensors are generally marked as 20μV, while the broadband electrical noise index of BW-sensor has been reduced to 10μV. However, for low-frequency small-signal measurements, providing only wide-band electrical noise does not fully reflect the resolution of the sensor's acceleration measurement in the low-frequency range; this is because the low-frequency noise caused by the built-in circuit is proportional to the reciprocal of the frequency, ie The so-called 1/f noise, when the measurement frequency is very low, the sensor's electrical noise output increases exponentially. Therefore, the value of the low-frequency electrical noise of the sensor is completely different from the broadband electrical noise index, and the lower the frequency, the more obvious the difference. Therefore, the resolution of a sensor used for very low frequency measurement is usually expressed by the power spectral density of the sensor output electrical noise. The practical significance of this indicator is the noise level of the sensor at a specific frequency, and its unit is generally expressed in μV/√Hz or μg/√Hz. The typical value of the electrical noise power spectral density of the BW-sensor built-in circuit is 3μV/√Hz@ 10 Hz.

The effect of the sensor's transient temperature response on low frequency measurements

Due to the characteristics of piezoelectric ceramics, piezoelectric accelerometers produce varying degrees of charge output for sudden changes in temperature. The sensor's transient temperature response is a measure of how sensitive the sensor is to temperature changes. This is especially important for low frequency measurements. Since the signal measured at low frequencies is small, the sensor is likely to produce errors comparable to low-frequency vibration signals due to changes in ambient temperature; these two signals are difficult to distinguish in the very low frequency range, so how to reduce the influence of ambient temperature changes on the sensor output It is very important in low frequency measurements. The sensor's transient temperature response is expressed in g/oC, which is the acceleration output corresponding to each change in transient temperature. The value is obtained by conversion between voltage (charge) output and sensor sensitivity.

The transient temperature response of the sensor is directly caused by the piezoelectric material, so the piezoelectric ceramic determines the quality of the indicator by the magnitude of the charge output caused by the sudden change in temperature. BW-sensor selects the current piezoelectric ceramics with the best comprehensive performance index and combined with memory metal for the low-frequency measurement of the acceleration sensor. It has been verified by the use of defense weapons, aerospace and large-scale structures for many years. The sensor has superior low-frequency output stability. And anti-interference performance. In the actual very low frequency measurement, in order to reduce the influence of the ambient temperature change on the low frequency signal output of the sensor, the outer casing of the sensor should be insulated with a protective cover as much as possible.

Sensor mounting base and base strain on the measurement

Since the low frequency measurement sensor does not require high frequency response, the sensor can generally meet the requirements using any type of installation. However, there are two issues to be aware of. One is that the sensor should consider using an insulated base as much as possible to avoid any measurement signals that are affected by noise caused by the ground loop. The second is to consider the effect of the measured structural strain on the sensor installation at the sensor installation, ie the sensor strain sensitivity. Piezoelectric accelerometers in the form of shear structures have good susceptibility to susceptibility and generally meet the usual low frequency structural tests. If the structural strain is too large to affect the sensor's measurement signal, the effect of structural strain on the sensor measurement can be reduced by reducing the contact area between the sensor and the structure being tested.