The sensitivity of the sensor is one of the most basic indicators of the sensor. The sensitivity directly affects the vibration signal measurement of the sensor.

Sensitivity and range

sensitivity

The sensitivity of the sensor is one of the most basic indicators of the sensor. The sensitivity directly affects the vibration signal measurement of the sensor. It is not difficult 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 addition measures the acceleration value of the vibration, and under the same displacement amplitude condition, the acceleration value is the square of the signal frequency It is proportional, so the magnitude of the acceleration signal of different frequency bands is very different.

The acceleration value of the vibration of the low-frequency vibration of a large structure may be quite small. For example, when the vibration displacement is 1mm, the acceleration value of the signal with a frequency of 1Hz is only 0.04m/s2 (0.004g); but for the high-frequency vibration, when the displacement is 0.1mm, the signal with a frequency of 10kHz has an acceleration value of 4x105m/s2 (40000g).

Therefore, although the piezoelectric accelerometer has a larger measurement range, it is necessary to fully estimate the signal when selecting the sensitivity of the accelerometer to measure the vibration signal at the high and low frequencies. The most commonly used vibration measurement piezoelectric accelerometer sensitivity, the voltage output type (IEPE type) is 50-100mV/g, and the charge output type is 10-50pC/g.

Measuring range

The measurement range of the acceleration sensor refers to the maximum measurement value that the sensor can measure within a certain range of non-linear error. The nonlinear error of general-purpose piezoelectric acceleration sensors is mostly 1%. As a general principle, the higher the sensitivity, the smaller the measurement range, otherwise the smaller the sensitivity, the larger the measurement range.

Voltage/charge output type

The measurement range of the IEPE voltage output piezoelectric acceleration sensor is determined by the maximum output signal voltage allowed within the linear error range. The maximum output voltage 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 the IEPE piezoelectric sensor is not only affected by the nonlinear error, but also restricted by the supply voltage and the sensor bias voltage. When the difference between the supply voltage and the bias voltage is less than the range voltage given by the indicator, the maximum output signal of the sensor will be distorted. Therefore, whether the bias voltage of the IEPE accelerometer is stable or not will not only affect low-frequency measurement, but also may cause signal distortion; this phenomenon requires special attention when measuring high and low temperatures. When the built-in circuit of the sensor is unstable under non-room temperature conditions, The bias voltage of the sensor is likely to drift slowly and cause the measurement signal to fluctuate.

The measurement range of the charge output type is restricted by the mechanical rigidity of the sensor. Under the same conditions, the maximum signal output of the sensor core subject to the non-linearity of the mechanical elastic interval is much larger than the range of the IEPE type sensor, and its value is mostly required. Determined by experiment. In general, when the sensor has high sensitivity, the mass of the sensitive core is larger, and the range of the sensor is relatively small. At the same time, because the mass is larger, the resonance frequency is lower, which makes it easier to excite the resonant signal of the sensor's sensitive core. As a result, the resonant wave is superimposed on the measured signal, resulting in signal distortion output. Therefore, when selecting the maximum measurement range, the frequency composition of the measured signal and the natural resonance frequency of the sensor itself should also be considered to avoid the generation of resonance components of the sensor. At the same time, there should be enough safety space in the range to ensure that the signal does not produce distortion.

sensitivity calibration

The calibration method of the sensitivity of the acceleration sensor is usually verified by the comparison method. The ratio of the output of the calibrated sensor at a specific frequency (usually 159Hz or 80Hz) to the acceleration value read by the standard sensor is the sensor sensitivity. The sensitivity of the shock sensor is measured by measuring the output response of the calibrated sensor to a series of different shock acceleration values ​​to obtain the corresponding relationship between the input shock acceleration value and the electrical output of the sensor within its measurement range, and then obtain the corresponding relationship with each The line with the smallest difference between the points, and the slope of this line is the impact sensitivity of the sensor.

Non-linear error indication

The non-linear error of the impact sensor can be expressed in two ways: the full-scale deviation or the linear error according to the segmented range. The former refers to the error percentage based on the full-scale output of the sensor, that is, regardless of the measurement value, the error is the error value calculated according to the full-scale percentage. The calculation method of the linear error according to the segment range is the same as the full range deviation, but the reference does not use the full range but uses the segment range to calculate the error value. For example, for a sensor with a range of 20000g, if the full range deviation is 1%, its linear error is 200g within the full range; but when the sensor measures its linear error according to the segmented range 5000g, 10000g, 20000g, the error is still 1% , The linear error of the sensor in the 3 different ranges is 50g, 100g, 200g.

Measuring frequency range

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

As a general principle, the high-frequency response of the sensor depends on the mechanical characteristics of the sensor, while the low-frequency response is determined by the integrated electrical parameters of the sensor and the subsequent circuit. Sensors with high high-frequency cut-off frequency must be small in size and light in weight, while high-sensitivity sensors used in low-frequency measurement are relatively large and heavy.

high frequency measurement range

The high-frequency measurement index of the sensor is usually determined by the high-frequency cut-off frequency, and a certain cut-off frequency is related to the corresponding amplitude error. Therefore, when selecting the sensor, you can't just look at the cut-off frequency, you must know the corresponding amplitude error value. The small frequency amplitude error of the sensor not only improves the measurement accuracy, but more importantly reflects the ability to control the installation accuracy deviation in the sensor manufacturing process.

In addition, due to the wide frequency band of the vibration signal of the measurement object, or the natural resonant frequency of the sensor is not high enough, the excited resonant signal wave may be superimposed on the signal in the measurement frequency band, causing larger measurement errors. Therefore, in addition to the high-frequency cut-off frequency when selecting the high-frequency measurement range of the sensor, the influence of the resonance frequency on the measurement signal should also be considered. Of course, the signal outside the measurement frequency band can also be eliminated by the filter in the measurement system.

Under normal circumstances, the high-frequency cut-off frequency of the sensor has nothing to do with the form of the output signal (ie charge type or low resistance voltage type); it is closely related to the sensor’s structural design, manufacturing, installation form and installation quality.

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