Acceleration sensor selection
Piezoelectric acceleration sensors are widely used because of their wide frequency response, large dynamic range, high reliability, and ease of use. In general general vibration measurement, the technical indicators that users are mainly concerned with are: sensitivity, frequency range, internal structure, difference between built-in circuit type and pure piezoelectric type, field environment and subsequent instrument configuration, etc.
First, the choice of sensitivity
The manufacturer generally gives the sensitivity and reference range of the sensor in the product introduction or manual. The purpose is to allow the user to easily select the appropriate product when selecting the acceleration sensor with different sensitivity. The minimum acceleration measurement value is also called the minimum resolution. In consideration of the noise problem of the post-amplifier circuit, it should be far away from the minimum available value to ensure the best signal-to-noise ratio. The maximum measurement limit must consider the nonlinear effect of the acceleration sensor itself and the maximum output voltage of the subsequent instrument.
Estimation method: maximum measured acceleration × sensor charge (voltage) sensitivity, whether its value exceeds the maximum input charge (voltage) value of the supporting instrument. It is recommended that if the measured acceleration range is known, it can be selected in the "reference range" in the sensor index (taking into account the frequency response and weight). At the same time, if the frequency response and quality allow, try to select a high-sensitivity sensor to improve Subsequent instrument input signals improve signal-to-noise ratio. While taking into account the frequency response and quality, the sensor sensitivity can be selected with reference to the following range: Take the charge output piezoelectric acceleration sensor as an example:
1. The vibration of civil engineering and super large mechanical structures is about 0.1g-10g (1g = 9.81m / s2), and the piezoelectric acceleration sensor with charge sensitivity of 300pC / ms-2 to 30pC / ms-2 is optional, which belongs to charge Output piezoelectric acceleration sensor
2. The vibration of special civil engineering structure (such as pile foundation) and mechanical equipment is 100ms-2 ～ 1000ms-2, and 20pC / ms-2 ～ 2pC / ms-2 acceleration sensor can be selected.
3. The measurement range of impact and collision is generally 10000ms-2 ~ 1000000ms-2, and the sensor sensitivity is optional. The acceleration sensor is 0.2pC / ms-2 ~ 0.002pC / ms-2.
2. Frequency selection
The frequency response curve of the acceleration sensor given by the manufacturer is installed with a rigid connection of screws.
The curve is generally divided into two sections: resonance frequency and use frequency. The frequency of use is given in terms of sensitivity deviation, which is ± 10%, ± 5%, ± 3dB. Resonance frequency is generally avoided, but there are special cases, such as bearing failure detection. The frequency range of the acceleration sensor should be higher than the vibration frequency of the test piece. The frequency response of the acceleration sensor required by frequency doubling analysis should be higher. Civil engineering is generally low-frequency vibration. The frequency response range of the acceleration sensor can be selected from 0.2Hz to 1kHz. The mechanical equipment is generally a medium frequency band. The vibration frequency can be comprehensively estimated according to factors such as equipment speed and equipment rigidity, and an acceleration sensor of 0.5Hz to 5kHz can be selected. If the generator speed is 3000rms, divide it by 60s and its main frequency is 50Hz. Collision and impact measurements are mostly high-frequency.
The different installation methods of the acceleration sensor will also change the use frequency response (it has little effect on the vibration value).
The installation surface should be flat and smooth, and the installation selection should be based on the principles of convenience and safety. We give the use frequency of different installation methods of the same RC6500S acceleration sensor: screw rigid connection (± 10% error) 10kHz; epoxy glue or "502" adhesive installation 6kHz; magnetic suction seat installation 2kHz; double-sided adhesive installation 1kHz. It can be seen that the different installation methods have a great influence on the response of the test frequency and should be selected carefully. The quality and sensitivity of the acceleration sensor are inversely proportional to the frequency of use. High sensitivity, high quality, and low frequency of use are also techniques for selection.
3. Internal structure
Internal structure refers to the way and installation form of sensitive material crystal piece to feel vibration.
There are two major categories of compression and shearing, common ones include center compression, plane shearing, triangle shearing, and ring shearing.
The frequency response of the center compression type is higher than that of the shear type, and the shear type has better adaptability to the environment than the center compression type. If the integral charge amplifier is used to measure the speed and displacement, it is best to choose the shear type product, so that the obtained signal has small fluctuation and good stability.
4. Built-in circuit
The built-in concept is to place the amplifier circuit in the acceleration sensor and become a sensing element with a voltage output function. It can be divided into dual power supply (four-wire) and single power supply (two-wire, with bias, also known as ICP). The internal circuit referred to below refers specifically to the ICP type. At present, the built-in circuit sensors are generally matched with data acquisition instruments, which are mainly used for mechanical failure and pile foundation detection in China. Many online monitoring projects are also using this type of product. The power supply and signal output of the ICP-type acceleration sensor share a single line. Its characteristics are: low impedance output, anti-interference, low noise, high cost performance, easy installation, especially suitable for multi-point measurement, stable and reliable, anti-humidity, anti-dust, anti-harmful gas. Calculation of sensor selection for built-in circuit:
Measured acceleration value (g) = Maximum output voltage (mV) / Sensor sensitivity (mV / g) If the most commonly used 100mV / g is selected, vibration within 50g can be measured, if measuring 100g, use 50mV / g accelerometer , And so on.
V. Environmental impact
The environment at some test sites is harsh, and many factors are considered, such as waterproofing, high temperature, installation location, strong magnetic field, and geoelectric circuit, which will have a great impact on the measurement.
Waterproofing: There are two concepts of waterproofing, shallow waterproofing and deep waterproofing, especially deep waterproofing. For example, the vibration monitoring of the permanent ship lock gate of the Three Gorges Project, the water depth is nearly 100 meters, it involves ground loop interference, high pressure seepage, wire protection, long-term reliability Sex and many other issues.
High temperature: The sensor temperature range given by most manufacturers is the usable value, not the sensitivity of the high temperature condition. In fact, the sensitivity deviation at high temperature is large. Special users should ask the manufacturer for the special sensitivity index at high temperature. The sensitivity index is to guarantee the test The key to accuracy.
Position limitation: The acceleration sensor is permanently installed on the site and will be subject to man-made collision. The industrial long-term monitoring acceleration sensor should be selected. It uses an additional protective cover and a triangular flange to install. It has the effects of ground insulation and dust prevention. Those who have requirements on the direction of the outlet can propose to the manufacturer. For inaccessible parts, a handheld acceleration sensor (with a long probe) can be used.
Insulation, geoelectric circuit and magnetic electric field: For test sites with strong magnetic electric field, acceleration sensors and special wires of special housing materials should be selected. Such research is still rare in China. For two-point grounding, damp and other sites, to solve the test interference, floating or insulated acceleration sensors can be used, and the protection of wire joints must be considered. In order to overcome the influence of two or more points of grounding electrical circuit current on the test, floating or insulated sensors can be used. If there are no special requirements and the interference is not large, insulated acceleration sensors can be used, while the permanent monitoring or interference conditions should use floating type. The difference between the two types of naming is that the outer shell of the insulated product is the signal ground, and the outer shell of the floating product is the shielding layer.
Additional mass: The mass of the acceleration sensor installed on the vibrating structure should be less than 1/10 of the dynamic mass of the measured point, and the impact on the measured signal can be ignored.
Six, supporting equipment
If the piezoelectric acceleration sensor is charged output, it can be matched with any type of high-impedance input charge amplifier or collector with charge pre-function. There are many types of charge amplifiers, including single, multiple, integral, and quasi-static. This should be determined according to the measurement requirements. There are also special cases, such as directly connecting the output signal of the piezoelectric sensor to a tertiary meter (such as an oscilloscope) with certain high resistance performance, which can also measure the signal, but because the impedance matching is not enough, it can only understand the dynamic status qualitatively. The built-in IC amplifier acceleration sensor (ICP type) has a constant current adapter, a constant current adapter can be used for constant current power supply and signal output of multiple acceleration sensors. For data acquisition instruments that provide constant current power supply, this type of sensor can be directly connected to the data acquisition instrument. The acceleration sensor with dual power supply can be supplied with dual power by the collector or powered by dual DC stabilized power supply. At present, the typical power supply mode of the ICP acceleration sensor is: 24V 4mA constant current.
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