Accelerometers are widely used because of their wide frequency response, large dynamic range, high reliability, and ease of use. When users make general vibration and shock measurements, the main technical indicators of concern are: sensitivity, frequency range, internal structure, field environment and configuration with subsequent instruments.
1. Selection of sensitivity
The product introduction of Yangzhou Jingming gives a reference range, the purpose is to allow users to easily select the appropriate product among many accelerometers with different sensitivities. The minimum acceleration measurement value is also called the minimum resolution, considering the post-amplifier circuit Noise problems should be kept away from the smallest available value as much as possible to ensure the best signal-to-noise ratio. The maximum measurement limit should take into account the non-linear effect of the accelerometer itself and the maximum output voltage of the subsequent instrument. The estimation method: maximum measured acceleration × sensor charge / voltage sensitivity, whether the above values ​​exceed the maximum input charge / voltage value of the supporting instrument, it is recommended 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 weight allow, the sensitivity can be considered higher to improve the subsequent instrument Input signal to improve signal-to-noise ratio.
While taking into account the frequency response and weight, the sensor sensitivity can be selected with reference to the following range: the vibration of civil engineering prototypes and super-large mechanical structures is about 0.1g to 10g, and the accelerometers of 3000pC / g to 300pC / g are optional Vibration is about 10g ~ 100g, you can choose 20pC / g ~ 200pC / g accelerometer, shock can choose about 0.1pC / g ~ 20pC / g accelerometer.
2. Frequency selection
The frequency response curve given by the manufacturer is installed with screws, and the curve is generally divided into two sections: resonance frequency and use frequency. The given value of the use frequency is given according to the sensitivity deviation, which is ± 10%, ± 5%, ± 3dB. The resonance frequency is generally avoided, but there are special cases, such as bearing failure detection.
The frequency of the accelerometer should be higher than the vibration frequency of the measured object, and the frequency response of the accelerometer with higher frequency analysis is higher. Civil engineering is low frequency, the accelerometer can choose about 0.2Hz ~ 1kHz, mechanical equipment is generally in the middle frequency band, the frequency can be comprehensively estimated according to factors such as equipment speed, equipment stiffness, etc., select the accelerometer 0.5Hz ~ 5kHz. Most of the high-frequency impact measurement.
The different installation methods of the accelerometer will also change the frequency response of use (it has little effect on the vibration value). The installation surface should be flat and smooth. The installation selection should be based on the principles of convenience and safety. Here are the use frequencies of different installation methods of the same KD1005 accelerometer: screw 5kHz, epoxy or "502" 4kHz, magnetic chuck 1.5kHz, double-sided tape 0.5kHz, it can be seen that the installation method has a great impact on the test frequency response , Should pay attention to selection.
The weight and sensitivity of the accelerometer are inversely proportional to the frequency of use. The sensitivity is high, the weight is large, and the frequency of use is low. This is also a selection technique.
3. Internal structure
The internal structure refers to the way and installation form of sensitive material crystal pieces to feel vibration. There are two types of compression and shear. Common ones include center compression, plane shear, triangle shear and ring shear. The frequency response of the center compression is higher than that of the shear type, and the environmental adaptability of the shear type is better than that of 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 resulting signal has small fluctuation and good stability.
4. Built-in circuit
The built-in concept is to place the charge / voltage conversion and amplifying circuit in the accelerometer to become a sensor element with a voltage output function. It can be divided into dual power supply (four-wire) and single power supply (two-wire with offset called ICP). The internal circuit referred to below refers specifically to the ICP type.
At present, the sensors with built-in circuit are more used in China for mechanical failure and pile foundation detection. Many online monitoring projects are also using such products.
The core wire of the ICP sensor is used for power supply and is also a signal output channel. Calculation of sensor selection for built-in circuit:
If you choose the most common 100mV / g at present, you can measure the vibration within 50g, because the dynamic range of the sensor is ± 5Vp. If you measure 100g, use an accelerometer of 50mV / g, and so on.
The built-in circuit has the advantages of low price, good anti-interference, and long-term use, but its high temperature resistance and reliability are not as good as those of charge output products, and the dynamic range is also limited by the effect of output voltage and bias voltage.
5. 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 ground loop, etc., which will greatly affect 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 temperature range given by most manufacturers is the usable value, not the sensitivity of the high temperature situation. 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 ensure the accuracy of the test key.
Position limitation: The accelerometer is permanently installed on the site and will be subject to human collision. Industrial products should be selected. A protective cover should be installed outside the accelerometer. This can play the role of insulation and dust prevention at the same time. For hand-held accelerometers (with long probes) for inaccessible parts.
Insulation, ground loop and magnetic electric field radiation: In the test site with strong radiation, an accelerometer with special shell material and special wires should be selected. Such research is rare in China. For multi-point grounding, humidity and other sites, to solve the test interference, you can use a floating or insulated accelerometer.
In order to overcome the influence of the ground loop current generated by multi-point grounding on the test, floating or insulated sensors can be used. There are no special requirements and the interference is not large, the insulation type accelerometer can be used. The permanent monitoring or interference conditions should use floating type. The difference between the two types of naming is that the housing of the insulated sensor is the signal ground, the base adopts the insulating method, and the housing of the floating type product is the shielding layer, and the three-wire method is adopted.
Additional mass: The mass of the accelerometer installed on the vibrating structure should be less than 1/10 of the mass of the structure itself, which is considered to have no significant impact on the measured signal.
6. Supporting instruments
If the piezoelectric accelerometer is a charge output, it can be matched with any kind 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. All must be selected 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 ICP built-in accelerometer has a constant current adapter, and one instrument can provide constant current power supply and signal output for multiple accelerometers. Some data acquisition instruments also have a constant current function, which can be directly used with ICP sensors.
If the common charge output type sensor is matched with the data collector with constant current output, the JM3861 constant current adaptor can be used.
The dual-power accelerometer can be supplied with dual power from the collector or powered by dual DC regulated power supplies. (Organized by China Educational Equipment Purchasing Network)
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