Sensor sensitivity is one of the most basic indicators of a sensor. The magnitude of sensitivity directly affects the sensor's measurement of vibration signals.

　　Sensitivity and Measurement Range

　　Sensitivity

　　Sensor sensitivity is one of the most basic indicators of a sensor. The magnitude of sensitivity directly affects the sensor's measurement of vibration signals. It is easy to understand that the sensitivity of the sensor should be determined according to the magnitude of the measured vibration (acceleration value), but because the piezoelectric accelerometer measures the acceleration value of the vibration, and under the same displacement amplitude condition, the acceleration value is proportional to the square of the signal frequency, so the acceleration signal magnitudes of different frequency bands differ greatly.

　　The acceleration value of the vibration quantity of low-frequency vibration of large structures may be quite small. For example, when the vibration displacement is 1 mm and the frequency is 1 Hz, the acceleration value is only 0.04 m/s² (0.004 g); however, for high-frequency vibration, when the displacement is 0.1 mm and the frequency is 10 kHz, the acceleration value can reach 4 x 10⁵ m/s² (40000 g).

　　Therefore, although piezoelectric accelerometers have a large measurement range, when selecting the sensitivity of the accelerometer for measuring vibration signals at both high and low frequencies, a sufficient estimate of the signal should be made. The most commonly used sensitivity of piezoelectric accelerometers for vibration measurement, voltage output type (IEPE type) is 50~100 mV/g, and charge output type is 10~50 pC/g.

　　Measurement Range

　　The measurement range of an acceleration value sensor refers to the maximum measurable value that the sensor can measure within a certain nonlinear error range. The nonlinear error of general-purpose piezoelectric accelerometers is mostly 1%. As a general rule, the higher the sensitivity, the smaller the measurement range, and vice versa, the smaller the sensitivity, the larger the measurement range.

　　Voltage/Charge Output Type

　　The measurement range of an IEPE voltage output piezoelectric accelerometer is determined by the maximum output signal voltage allowed within the linear error range, and the maximum output voltage value is generally ±5V. Through conversion, the maximum range of the sensor can be obtained, which is equal to the ratio of the maximum output voltage to the sensitivity. It should be pointed out that the range of IEPE piezoelectric sensors is not only affected by the magnitude of nonlinear error but also by the supply voltage and sensor bias voltage. When the difference between the supply voltage and the bias voltage is less than the range voltage given by the index, the maximum output signal of the sensor will be distorted. Therefore, the stability of the bias voltage of the IEPE accelerometer not only affects low-frequency measurement but may also cause signal distortion; this phenomenon needs special attention during high and low temperature measurements. When the built-in circuit of the sensor is unstable under non-room temperature conditions, the bias voltage of the sensor may drift slowly, causing the measured signal to fluctuate.

　　The measurement range of the charge output type is restricted by the mechanical stiffness of the sensor. Under the same conditions, the maximum signal output of the sensor sensitive core constrained by the nonlinearity of the mechanical elastic range is much larger than the range of the IEPE sensor, and its value mostly needs to be determined through experiments. In general, when the sensor sensitivity is high, the mass of the sensitive core is also larger, and the range of the sensor is relatively small. At the same time, because the mass is larger, its resonant frequency is lower, so it is easier to excite the resonant signal of the sensor sensitive core, resulting in the superposition of the resonant wave on the measured signal, causing signal distortion output. Therefore, when selecting the maximum measurement range, the frequency composition of the measured signal and the self-resonant frequency of the sensor itself should also be considered to avoid the generation of the resonant component of the sensor. At the same time, there should be sufficient safety margin in the range to ensure that the signal does not produce distortion.

　　Sensitivity Calibration

　　The calibration method of accelerometer sensitivity usually adopts the comparison method. The ratio of the output of the sensor to be calibrated at a specific frequency (usually 159 Hz or 80 Hz) to the acceleration value read by the standard sensor is the sensitivity of the sensor. The sensitivity of the impact sensor is obtained by measuring the output response of the sensor to be calibrated to a series of different impact acceleration values, obtaining the correspondence between the input impact acceleration value and the electrical output within its measurement range, and then obtaining the straight line with the smallest difference between each point through numerical calculation, and the slope of this straight line is the impact sensitivity of the sensor.

　　Nonlinear Error Representation

　　The nonlinear error of an impact sensor can be represented in two ways: full-scale deviation or linear error in segmented ranges. The former refers to the percentage error based on the full-scale output of the sensor, that is, regardless of the magnitude of the measured value, the error is calculated as a percentage of the full scale. The calculation method of linear error in segmented ranges is the same as that of full-scale deviation, but the reference is not the full scale but the segmented range. For example, for a sensor with a range of 20000 g, if the full-scale deviation is 1%, the linear error is 200 g within the full scale; however, when the sensor measures its linear error in segmented ranges of 5000 g, 10000 g, and 20000 g, and the error is still 1%, the linear error in the three different range segments is 50 g, 100 g, and 200 g, respectively.

　　Measurement Frequency Range

　　The frequency measurement range of a sensor refers to the frequency range that the sensor can measure within the specified frequency response amplitude error (±5%, ±10%, ±3dB). The upper and lower limits of the frequency range are called the high and low cutoff frequencies, respectively. The cutoff frequency is directly related to the error; the larger the allowable error range, the wider the frequency range.

　　As a general rule, the high-frequency response of a sensor depends on the mechanical characteristics of the sensor, while the low-frequency response is determined by the comprehensive electrical parameters of the sensor and the subsequent circuit. Sensors with high high-frequency cutoff frequencies are necessarily small and light, while high-sensitivity sensors used for low-frequency measurements are relatively large and heavy.

　　High-Frequency Measurement Range

　　The high-frequency measurement index of a sensor is usually determined by the high-frequency cutoff frequency, and a certain cutoff frequency is related to the corresponding amplitude error. Therefore, when selecting a sensor, you cannot only look at the cutoff frequency; you must understand the corresponding amplitude error value. The small frequency amplitude error of the sensor not only improves the measurement accuracy but also reflects the ability to control the installation accuracy deviation during the sensor manufacturing process.

　　Additionally, due to the wide frequency band of the vibration signal of the measured object, or the insufficiently high inherent resonant frequency of the sensor, the resonant signal waves that are excited may be superimposed on the signal within the measurement band, causing a large measurement error. Therefore, when selecting the high-frequency measurement range of the sensor, in addition to the high-frequency cutoff frequency, the influence of the resonant frequency on the measurement signal should also be considered. Of course, this signal outside the measurement frequency band can also be eliminated by filtering in the measurement system.

　　Generally, the high-frequency cutoff frequency of the sensor is irrelevant to the form of the output signal (i.e., charge type or low-impedance voltage type); however, it is closely related to the sensor's structural design, manufacturing, installation method, and installation quality.

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