In vibration tests, vibration values at control points and monitoring points are mostly obtained through sampling by accelerometers . The accuracy and reliability of these values directly affect the judgment of the test results. Besides the sensor's installation location and the specimen's installation, the accurate acquisition of vibration values in vibration tests is also related to the sensor's technical specifications; it is one of the most direct and important components for obtaining vibration values. This article, combining theory and practical experience, introduces the selection of piezoelectric accelerometers in vibration tests.
1. Sensitivity
The sensitivity of a piezoelectric accelerometer can be expressed in two ways: one is charge sensitivity Sq, and the other is voltage sensitivity Sv. The equivalent circuit of its electrical characteristics is shown in Figure 1.
Figure 1 shows the equivalent circuit of the electrical characteristics of a piezoelectric accelerometer.
The pressure exerted on the piezoelectric element is F1 = ma, and the qa generated on the working surface of the piezoelectric element is proportional to the acceleration a of the measured vibration: that is...
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Qa=Sqa
Wherein, the proportionality coefficient Sq is the charge sensitivity of the piezoelectric accelerometer , with dimensions [pC/ms²]. The open-circuit voltage of the sensor:
Ua=Qa/Ca
In the formula, Ca is the internal capacitance of the sensor , and for a specific sensor, Ca is a fixed value. Therefore, the open-circuit voltage Ua of the accelerometer is also proportional to the measured acceleration a, and the proportionality coefficient Sv is the voltage sensitivity of the piezoelectric accelerometer, with dimensions [mV/ms²].
Ua=(Sq/Ca)*a
The voltage sensitivity specified in the instruction manual of a piezoelectric accelerometer generally refers to the voltage sensitivity Sv within a defined frequency range. Under normal conditions, all other things being equal, accelerometers with larger geometric dimensions tend to have higher sensitivity. The instruction manual will also provide the minimum acceleration measurement value, also known as the minimum resolution. Considering the noise issues of the subsequent amplifier circuit, this value should be kept as far away as possible from the minimum possible value to ensure the best signal-to-noise ratio. The maximum measurement limit must take into account the nonlinear effects of the accelerometer itself and the maximum output voltage of the subsequent instruments.
Estimation method:
The maximum measured acceleration × sensor charge (voltage) sensitivity, and whether its value exceeds the maximum input (voltage) value of the matching instrument.
If the range of the acceleration to be measured is known, it can be selected from the reference range in the sensor specifications (taking into account frequency response and weight). At the same time, if the frequency response and weight allow, try to select a sensor with high sensitivity to improve the input signal of subsequent instruments and improve the signal-to-noise ratio.
When considering both frequency response and quality, the following range can be used as a reference for selecting sensor sensitivity:
Vibration acceleration is around 1ms² to 100ms², and an acceleration sensor of 300pC/ms² to 30pC/ms² can be selected;
For vibration acceleration in the range of 100ms² to 1000ms², an acceleration sensor with a range of 20pC/ms² to 2pC/ms² can be selected;
Collision and impact measurements typically range from 10,000 ms² to 1,000,000 ms², and acceleration sensors with a range of 0.2 pC/ms² to 0.002 pC/ms² can be selected.
2. Frequency
Figure 2 Frequency response curve of the piezoelectric accelerometer
Therefore, for the sensor itself, the natural frequency fn is its main parameter. Generally, sensors with smaller geometric dimensions have higher natural frequencies but lower sensitivity. How to balance the trade-off between sensor sensitivity and operating frequency range? This depends on the measurement requirements. However, for a precise measurement, it is preferable to choose an accelerometer with lower sensitivity to ensure a sufficiently wide effective frequency range.
The frequency range of the selected accelerometer should be higher than the vibration frequency of the test specimen, and the frequency response of the accelerometer should be even higher for octave analysis requirements.
Low-frequency vibration: The frequency response range of the accelerometer can be selected from 0.2Hz to 1kHz;
Mid-frequency vibration: Mechanical equipment is generally in the mid-frequency range. The vibration frequency can be estimated by comprehensively considering factors such as equipment speed and equipment stiffness, and an accelerometer with a range of 0.5Hz to 5kHz can be selected.
Collisions and impacts: mostly measured at high frequencies.
The installation method of an accelerometer will also change its frequency response. The mounting surface should be flat and smooth, and the installation method should be chosen based on the principles of convenience and safety. Different installation methods have a significant impact on the test frequency response, so careful selection is necessary.
3. Internal structure
The internal structure refers to the way the sensitive material crystal sheet senses vibrations and its mounting configuration. There are two main categories: compression and shearing. Common types include central compression, planar shearing, triangular shearing, and ring shearing.
Center compression has a higher frequency response than shear type, while shear type is more adaptable to different environments. When using an integrating charge amplifier to measure velocity or displacement, it is best to choose a shear type product, as this will result in less signal fluctuation and better stability.
4. Built-in circuitry
Built-in refers to placing the amplifier circuit inside the accelerometer, making it a sensing element with voltage output function. It can be divided into two types: dual-power supply (four-wire) and single-power supply (two-wire, with bias, also known as ICP). Built-in circuit sensors are generally used with data acquisition instruments to directly collect data. ICP type accelerometers share a single wire for power supply and signal output. Their characteristics include: low impedance output, anti-interference, low noise, high performance-price ratio, easy installation, especially suitable for multi-point measurement, stable and reliable, and resistant to moisture, dust, and harmful gases.
Selection calculation for built-in circuit sensor sensitivity:
Measured acceleration value (g) = Maximum output voltage (mV) / Sensor sensitivity (mV/g)
If the most commonly used 100mV/g is selected, vibrations up to 50g can be measured; if 100g is to be measured, an accelerometer with 50mV/g is used, and so on.
5. Environmental Impact
The environment in a mechanical vibration test chamber is generally harsh, and there are many factors to consider, such as high temperature, strong magnetic field and ground circuit, which can all have a great impact on the measurement. In addition, cable noise and base strain can cause false data.
In general mechanical vibration testing laboratories, the primary environmental factor affecting sensitivity is high temperature. Most manufacturers provide sensor temperature ranges that are usable values, not sensitivity under high-temperature conditions. In reality, sensitivity deviations are significant at high temperatures. Because sensitivity calibration is performed at room temperature (20°C), the sensitivity can be corrected according to the temperature correction curve provided by the manufacturer for each sensor (or obtained from the manufacturer) depending on the ambient temperature. Sensitivity specifications are crucial for ensuring test accuracy. When using accelerometers, the allowable temperature must not be exceeded, otherwise it will damage the piezoelectric elements. Furthermore, sudden temperature changes can also cause measurement data drift and errors.
