The sensitivity of the sensor is one of the most basic goals of the sensor. The sensitivity of the sensor directly affects the measurement of the oscillation signal. It is not difficult to understand that the sensitivity of the sensor should be determined according to the size of the measured oscillation (acceleration value), but because piezoelectric acceleration measures the acceleration value of the oscillation, and the acceleration value is proportional to the frequency square of the signal under the same displacement amplitude value, the acceleration signal of different frequency bands is very different.
For example, when the oscillation displacement is 1 mm and the frequency is 1 Hz, the acceleration value of the signal is only 0.04 m/s2 (0.004 g). However, for high-frequency oscillations, when the displacement is 0.1 mm, the acceleration value of the signal with a frequency of 10 kHz can reach 4x105 m/s2 (40,000 g).
Therefore, although piezoelectric accelerometers have a large measuring range, the sensitivity of the accelerometer should be fully estimated when selecting the oscillating signal used to measure the frequency of the concave and convex ends. The most commonly used oscillation measures the sensitivity of piezoelectric accelerometers, which are 50-100mV/g for voltage output type (IEPE type) and 10-50pC/g for charge output type.
Measuring range
The measurement range of the accelerometer sensor refers to the maximum measurement value that the sensor can measure within a certain nonlinear error scale. General-purpose piezoelectric accelerometers have a nonlinear error of 1% in general-purpose piezoelectric accelerometers. As a general criterion, the higher the sensitivity, the smaller the measurement scale, and vice versa, the smaller the sensitivity, the larger the measurement scale.
Voltage/charge output type
The measurement scale of IEPE voltage output piezoelectric accelerometer is determined by the maximum output signal voltage promised within the linear error scale, and the maximum output voltage is generally ±5V. The maximum range of the sensor is obtained, which is equal to the ratio of the maximum output voltage to the sensitivity. The requirement points out that the range of IEPE piezoelectric sensors is limited by the supply voltage and the sensor bias voltage, in addition to the size of the nonlinear error. When the difference between the supply voltage and the bias voltage is less than the range voltage given by the target, the maximum output signal of the sensor is distorted. Therefore, whether the bias voltage of the IEPE accelerometer is stable or not not only affects the low-frequency measurement, but also distorts the signal. 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 form a measurement signal that fluctuates from one size to another.
Under the same conditions, the maximum signal output of the sensitive core limited by the nonlinearity of the mechanical elastic interval is much larger than that of the IEPE sensor, and its value needs to be determined by experiments. In general, when the sensor is sensitive, the mass of its sensitive core is larger, and the range of the sensor is relatively small. In addition, due to the large mass, its resonant frequency is low, so it is easier to excite the resonant signal of the sensitive core of the sensor, and the result is that the resonant wave is superimposed on the measured signal to form a signal distortion output. Therefore, when selecting the maximum measurement scale, it is also necessary to consider the frequency composition of the measured signal and the natural resonance frequency of the sensor itself to prevent the resonant component of the sensor from being generated. Together, there should be enough safe space in the measuring range to ensure that the signal is not distorted.
Sensitivity calibration
The calibration method of accelerometer sensitivity is generally verified by the comparative method, and the ratio of the output of the oscillating sensor at a specific frequency (generally 159Hz or 80Hz) to the acceleration value read by the standard sensor is the sensor sensitivity. The sensitivity to the impact sensor is measured by measuring the output of the calibrated sensor to a series of different impact acceleration values, and the correspondence between the input impact acceleration value and the electrical output of the sensor in its measurement scale is obtained, and then the straight line with the smallest difference between each point is obtained through numerical calculation, and the slope of the straight line is the impact sensitivity of the sensor.
Nonlinear error indications
Nonlinear errors in shock sensors can be indicated in two ways: full-scale errors or piecewise range-based errors. The former refers to the error percentage based on the full-scale output of the sensor, that is, no matter how large or small the measurement value is, the error is calculated according to the full-scale percentage. Linear errors in the segmented range are accounted for in the same way as full-scale errors, but the benchmark does not have to be in the full scale but in the segmented range. For example, for a sensor with a range of 20,000g, if the full-scale error is 1%, the linear error is 200g in the full-scale range; However, when the linear error of the sensor is measured by the segmented range of 5000g, 10000g, and 20000g, and the error is still 1%, the linear error of the sensor in different three range segments is 50g, 100g, and 200g respectively.
Measure the scale of the frequency
The frequency measurement scale of the sensor refers to the frequency scale that the sensor can measure within the specified frequency response amplitude error (±5%, ±10%, ±3dB). The high and low limits of the frequency scale are called high and low frequency to frequency, respectively. When the frequency is directly related to the error, the larger the scale of the promised error, the wider the frequency scale.
As a general guideline, the high-frequency response of a sensor depends on the mechanical characteristics of the sensor, while the low-frequency response is determined by the combined electrical parameters of the sensor and the subsequent circuitry. Sensors with high frequency cut-off frequencies are necessarily small in size and light in weight, while high-sensitivity sensors for low-frequency measurement must be relatively large and heavy.
High-frequency measurement scale
The high-frequency measurement target of the sensor is generally determined by the high-frequency cut-off frequency, and the cut-off frequency must be related to the corresponding amplitude error. Therefore, it is necessary to know the corresponding amplitude error value when selecting a sensor not only to see the frequency. The small error of the frequency amplitude of the sensor not only improves the measurement accuracy, but more importantly, it reflects the ability of the accuracy error of the control device in the process of sensor production.
In addition, because the frequency band of the oscillation signal of the measurement target is wide, 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, forming a large measurement error. Therefore, in addition to the high frequency to frequency, the influence of the resonant frequency on the measurement signal should also be considered when selecting the high-frequency measurement scale of the sensor. Of course, such signals outside the measurement frequency band can also be eliminated by filters in the measurement system.
In general, the high-frequency cut-off frequency of the sensor is independent of the way the signal is output (i.e., charge type or low-impedance voltage type); It is closely related to the structural design and production of the sensor, as well as the device method and device quality.
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