How to select the sensitivity and range of the sen

2022-08-01
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How to select the sensitivity and range of the sensor

piezoelectric accelerometer is the most important sensor for vibration test. Although the measurement range of piezoelectric accelerometers is wide, there are many kinds of accelerometers in the market, so it is difficult to select them correctly. As a general principle for selecting vibration sensor, the correct selection should be based on the analysis and estimation of the following three aspects of the measured signal. a. Piezoelectric accelerometer is the most important sensor for vibration test. Although the measurement range of piezoelectric accelerometers is wide, there are many kinds of accelerometers in the market, so it is difficult to select them correctly. As a general principle for selecting vibration sensor, the correct selection should be based on the analysis and estimation of the following three aspects of the measured signal

a. the magnitude of the measured vibration momentum

b. the frequency range of the measured vibration signal

c. the vibration test site environment

the following will further discuss the specific selection based on the above three aspects and with reference to the relevant technical indicators of the sensor

the sensitivity and range of the sensor

the sensitivity of the sensor is one of the most basic indicators of the sensor. The sensitivity directly affects the measurement of vibration signal by 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 momentum (acceleration value). However, since the piezoelectric acceleration sensor measures the acceleration value of vibration, and the acceleration value is proportional to the frequency square of the signal under the same displacement amplitude, the magnitude of acceleration signals in different frequency bands varies greatly. The acceleration value of low-frequency vibration of large structures may be quite small. For example, when the vibration displacement is 1mm and the frequency is 1 Hz, the acceleration value of the signal is only 0.04m/s2 (0.004g); However, for high-frequency vibration, when the displacement is 0.1mm and the frequency is 10 kHz, the acceleration value can reach 4 x 10 5m/s2 (40000 g). Therefore, although the piezoelectric acceleration sensor has a large measurement range, for the vibration signals used to measure the frequencies of high and low ends, the signal should be fully estimated when selecting the sensitivity of the acceleration sensor. The most commonly used vibration measurement piezoelectric accelerometer sensitivity is 50-100 mv/g for voltage output type (IEPE type) and 10-50 pc/g for charge output type

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

iepe voltage output piezoelectric acceleration sensor's measuring range is determined by the maximum allowable output signal voltage within the error range, and the maximum output voltage is generally ± 5V. The maximum range of the sensor can be obtained by conversion, 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 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 technical indicators of the sensor, the maximum output signal of the sensor will be distorted. Therefore, whether the bias voltage of IEPE acceleration sensor is stable or not will not only affect the low-frequency measurement, but also may distort the signal; This phenomenon needs special attention in high and low temperature measurement. 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 continuously and slowly, causing the measurement signal to fluctuate

the measuring 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 sensing core subject to the nonlinear restriction of the mechanical elastic range is much larger than that of the IEPE type sensor, and most of its values need to be determined through experiments. In general, when the sensor has high sensitivity, the mass of its sensitive core is larger, and the range of the sensor is relatively small. At the same time, because the mass block is large, its resonant frequency is low, 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, 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 in February this year should also be considered to avoid the resonance component of the sensor. At the same time, there shall be enough safety space on the measuring range to ensure that the signal does not produce distortion

the calibration method of acceleration sensor sensitivity is usually verified by comparison method. The ratio between the output of the calibrated sensor vibrating at a specific frequency (usually 159 Hz or 80 Hz) and the acceleration value read by the standard sensor is the sensor sensitivity. The sensitivity of the impact sensor is measured by measuring the output response of the calibrated sensor to a series of different impact acceleration values, and the corresponding relationship between the input impact acceleration value and the electrical output of the sensor within its measurement range is obtained by measuring the output response of the calibrated sensor to a series of different impact acceleration values. Then, the line with the minimum difference between each point is obtained through numerical calculation, The slope of this line is the impact sensitivity of the sensor

the nonlinear error of impact sensor can be expressed in two ways: full range deviation or linear error according to subsection range. The former refers to the error percentage based on the full range output of the sensor, that is, the error is 4.2 "based on the full range regardless of the size of the measured value 2. The error value calculated by the percentage of lower pass. The calculation method of the linear error of the segmented range is the same as that of the full range, but the reference does not use the full range, but uses the segmented 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 in the full range; However, when the linear error of the sensor is measured according to the segmented measuring ranges of 5000g, 10000g and 20000g, and the error is still 1%, the linear error of the sensor in three different measuring ranges is 50g, 100g and 200g respectively

measurement frequency range of the sensor

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 cut-off frequencies respectively. The cut-off frequency is directly related to the error. If the allowable error range is large, the frequency range will be wide. 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 comprehensive electrical parameters of the sensor and the subsequent circuit. The sensor with high cut-off frequency must be small in size and light in weight. On the contrary, the sensor with high sensitivity used for low-frequency measurement is relatively large in size and heavy in weight

high frequency measurement range of the sensor

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 should not only look at the cut-off frequency, but also 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 in the sensor manufacturing process. In addition, because the frequency band of the vibration signal of the measurement object is wide, or the natural resonance frequency of the sensor is not high enough, the excited resonance signal wave may be superimposed on the signal in the measurement frequency band, resulting in large measurement error. Therefore, when selecting the high-frequency measurement range of the sensor, in addition to the high-frequency cut-off frequency, the influence of the resonant frequency on the measurement signal should also be considered; Of course, the signal outside the measurement frequency band can also be eliminated by a filter in the measurement system

generally, the high-frequency cut-off frequency of the sensor is independent of the form of the output signal (i.e. charge type or low resistance voltage type); It is closely related to the structural design, manufacturing, installation form and installation quality of the sensor. The following table is a qualitative classification of the high frequency response of different types of acceleration sensors

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