How to choose the sensitivity and range of the sensor?

Sensitivity and range range

sensitivity

 The sensitivity of the sensor is one of the most basic indicators of the sensor. The size of the sensitivity directly affects thesensorMeasurement of the vibrational signal. It is not difficult to understand that the sensitivity of the sensor should be determined according to the amount of measured vibration (acceleration value), but because piezoelectric addition is the acceleration value of measured vibration, and the acceleration value is proportional to the frequency square of the signal, so the acceleration signal size of different frequency bands is very different.

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

Therefore, although the piezoelectric acceleration sensor has a large measurement range, the sensitivity of the acceleration sensor selects the vibration signal used to measure the frequencies of both ends. The most commonly used vibration measurement piezoelectric accelerometer sensitivity, voltage output type (IEPE type) is 50 ~ 100 mV / g, charge output type is 10 ~ 50 pC / g.

range ability

The measurement range range of the acceleration value sensor refers to the maximum measurement value measured by the sensor within a certain nonlinear error range. The nonlinear error of the universal piezoelectric acceleration sensor is mostly 1%. As a general principle, the higher the sensitivity, the smaller the measurement range, and 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, and the maximum output voltage value is generally ± 5V. The conversion can obtain the maximum range of the sensor, 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 size, but also restricted by the power 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 index, the maximum output signal of the sensor is distorted. Therefore, the bias voltage stability of the IEPE acceleration sensor not only affects the low frequency measurement but also causes the signal distortion; this phenomenon requires 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 slowly and cause the measurement signal to fluctuate large and small.

However, 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 sensing sensitive core under the nonlinear restriction of the mechanical elastic range is much larger than that of the IEPE type sensor, and most of its value needs to be determined by experiment. In general, when the sensor sensitivity is high, the mass block of the sensitive core is larger, and the range of the sensor is relatively small. At the same time, because the resonance frequency of the mass block is low, it is easier to stimulate the resonance signal of the sensor sensitive core, and the resonance wave is superimposed on the measured signal to cause the signal distortion output. Therefore, in the selection of the maximum measurement range, the frequency composition of the measured signal and the spontaneous resonance frequency of the sensor itself should also be considered, so as to avoid the resonant component of the sensor. At the same time, there should be enough safe space in the range to ensure that the signal is not distorted.

 Sensitivity calibration

The calibration method of acceleration sensor sensitivity is usually determined by comparative method. The ratio of the output of the corrected sensor vibration at a specific frequency (usually 159Hz or 80Hz) to the acceleration value read by the standard sensor is the sensor sensitivity. And the sensitivity of the impact sensor is measured by measuring the output response of a series of different impact acceleration value, the sensor within its measurement range and the corresponding relationship between the electrical output, and then through the numerical calculation and the minimum difference between the minimum line, and the slope of the line is the impact sensitivity of the sensor.

 Nonlinear errors are represented

The nonlinear error of impact sensor can be expressed in two ways: full range deviation or linear error. The former refers to the error percentage of the full range output of the sensor, that is, the error is the error value calculated according to the percentage of the full range. The linear error of the segment range is the same as the full range deviation, but the benchmark does not calculate the error value instead of the full range. For example, for the sensor with a range of 20000g, if the full range deviation is 1%, the linear error is 200g; but when the linear error of the sensor is measured by the range of 5000g, 10000g and 20000g, the error is still 1%, the linear error of the sensor in different ranges is 50g, 100g and 200g respectively.

 Measure the 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 end frequencies, respectively. As the frequency is directly related to the error, and if the allowable error range is large, the frequency range is 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 integrated electrical parameters of the sensor and the subsequent circuit. The sensor with high frequency cut-off frequency must be small in size and light in weight. Otherwise, the high sensitivity sensor used for low frequency measurement is relatively large in size and heavy in weight.

 Range of high frequency measurement

 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 correlated with the corresponding amplitude error. Therefore, the sensor selection can not only look at the end frequency, must understand the corresponding amplitude error value. The small frequency amplitude error of the sensor is not only for the improvement of the measurement accuracy, but more importantly, it reflects the ability to control the installation accuracy deviation in the sensor manufacturing process.

 In addition, because the vibration signal frequency band of the measurement object is wide, or the intrinsic resonance frequency of the sensor is not high enough, the excited resonant signal waves may be superimposed on the signal in the measurement frequency band, resulting in a large measurement error. Therefore, in addition to the high frequency measurement range of the sensor, the influence of the resonance frequency on the measurement signal should also be considered. Of course, this signal outside the measurement band can also be eliminated by a filter in the measurement system.

 In general, 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); and the structural design, manufacturing, installation form and installation quality of the sensor.

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