Teach you how to choose the right accelerometer

Accelerometers are widely used due to 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 they are concerned about are: sensitivity, frequency range, internal structure, on-site environment and subsequent instrument configuration.

　　1. Selection of sensitivity

　　The product introduction of Yangzhou Jingming gives the reference range, the purpose is to allow users to easily select the appropriate product from many accelerometers with different sensitivities. The minimum acceleration measurement value is also called the minimum resolution. Considering the noise problem of the post-amplifier circuit, it should be kept as far away from the minimum available value as possible to ensure the best signal-to-noise ratio. The maximum measurement limit should take into account the nonlinear influence of the accelerometer itself and the maximum output voltage of the subsequent instrument. The estimation method is: the maximum measured acceleration × the charge/voltage sensitivity of the sensor. Whether the above values ​​exceed 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" of the sensor indicators (taking into account frequency response and weight). At the same time, if the frequency response and weight allow, the sensitivity can be considered higher to increase the input signal of the subsequent instrument and improve the signal-to-noise ratio.

　　While taking frequency response and weight into consideration, the sensor sensitivity can be selected according to the following range: the vibration of civil engineering prototypes and super-large mechanical structures is around 0.1g~10g, and an accelerometer of 3000pC/g~300pC/g can be selected. The vibration of mechanical equipment is around 10g~100g, and an accelerometer of 20pC/g~200pC/g can be selected. For impact, an accelerometer of around 0.1pC/g~20pC/g can be selected.

　　2. Frequency selection

　　The frequency response curve provided by the manufacturer is installed with screws, and the curve is generally divided into two sections: resonant frequency and operating frequency. The value of the operating frequency is given according to the sensitivity deviation, which is ±10%, ±5%, and ±3dB. The resonant frequency is generally avoided, but there are exceptions, such as bearing fault detection.

　　The frequency of the selected accelerometer should be higher than the vibration frequency of the object being measured. The frequency response of the accelerometer with the requirement of frequency multiplication analysis is higher. Civil engineering is low frequency, and the accelerometer can be selected around 0.2Hz~1kHz. Mechanical equipment is generally in the medium frequency band. The frequency can be comprehensively estimated based on factors such as equipment speed and equipment stiffness, and an accelerometer of 0.5Hz~5kHz can be selected. Most of the impact measurements are high frequency.

　　Different installation methods of accelerometers will also change the frequency response (with little effect on vibration values). The installation surface should be flat and smooth, and the installation selection should be based on the principles of convenience and safety. Here are the usage frequencies of the same KD1005 accelerometer with different installation methods: 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 influence on the test frequency response, so you should be careful when choosing.

　　The weight and sensitivity of the accelerometer are inversely proportional to the frequency of use. The higher the sensitivity, the heavier the weight and the lower the frequency of use. This is also a selection technique.

　　3. Internal structure

　　Internal structure refers to the way the sensitive material crystal piece senses vibration and the installation form. There are two major categories: compression and shear. The most common ones are center compression, plane shear, triangle shear, and ring shear. The frequency response of center compression is higher than that of shear type, and the environmental adaptability of shear type is better than that of center compression type. If an integral charge amplifier is used to measure speed and displacement, it is best to use a shear type product, so that the obtained signal has small fluctuations and good stability.

　　4. Built-in circuit

　　The concept of built-in is to place the charge/voltage conversion amplifier circuit inside the accelerometer, which becomes a sensor 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 called ICP). The built-in circuit referred to below refers specifically to the ICP type.

　　At present, built-in circuit sensors are widely used in China for mechanical failure and pile foundation detection, and such products are also used in many online monitoring projects.

　　The core wire of the ICP sensor is used for power supply and signal output channel. Selection and calculation of the sensitivity of the built-in circuit sensor:

　　If you choose the most common 100mV/g, you can measure vibrations within 50g, because the dynamic range of the sensor is ±5Vp. If you measure 100g, use a 50mV/g accelerometer, and so on.

　　The advantages of built-in circuits are low price, good anti-interference, and long-term use, but their high temperature resistance and reliability are not as good as charge output products, and the dynamic range is also limited by the output voltage and bias voltage.

　　5. Environmental impact

　　The environment of some test sites is relatively harsh, and there are many factors to consider, such as waterproofing, high temperature, installation location, strong magnetic and electric fields, and ground loops, all of which will have a great impact on the measurement.

　　Waterproof: There are two concepts of waterproofing, shallow waterproofing and deep waterproofing. Deep waterproofing is particularly difficult. For example, the vibration monitoring of the permanent ship lock gates of the Three Gorges Project has a water depth of nearly 100 meters. It involves many problems such as ground loop interference, high-pressure water seepage, wire protection, and long-term reliability.

　　High temperature: The temperature range given by most manufacturers is the available value, not the sensitivity under high temperature conditions. In fact, the sensitivity deviation is large at high temperatures. Special users should ask the manufacturer for special high temperature sensitivity indicators. The sensitivity indicator is the key to ensure accurate testing.

　　Location restrictions: Accelerometers permanently installed on site will be subject to human collisions, so industrial products should be selected and a protective cover should be installed outside the accelerometer, which can also serve as insulation and dustproof. Any requirements for the direction of the outlet should also be proposed to the manufacturer. For parts that cannot be touched, a handheld accelerometer (with a long probe) can be used.

　　Insulation, ground loop and magnetic and electric field radiation: For test sites with strong radiation, accelerometers with special shell materials and special wires should be selected. Such research is rare in China. For sites with multiple grounding points and humidity, floating or insulated accelerometers can be used to solve the test interference.

　　In order to overcome the impact of ground loop current caused by multi-point grounding on the test, floating or insulated sensors can be used. Insulated accelerometers can be used for working conditions without special requirements and little interference. Floating type should be used for permanent monitoring or working conditions with large interference. The difference between the two names is that the shell of the insulated type sensor is the signal ground and the base is insulated, while the shell of the floating type product is the shielding layer and a three-wire method is used.

　　Added mass: As long as the mass of the accelerometer mounted on the vibrating structure is less than 1/10 of the mass of the structure itself, it is considered to have no significant effect on the measured signal.

　　6. Supporting instruments

　　If the piezoelectric accelerometer has charge output, it can be matched with any charge amplifier with high impedance input or collector with charge pre-stage function. There are many types of charge amplifiers, including single, multi-channel, integral, and quasi-static. The selection should be based on the measurement requirements.

　　There are also special cases. For example, directly connecting the output signal of the piezoelectric sensor to a cubic instrument with certain high-impedance performance (such as an oscilloscope) can also measure the signal, but due to insufficient impedance matching, only a qualitative understanding of the dynamic situation can be obtained.

　　ICP type built-in accelerometers have a special constant current adapter. One instrument can provide constant current power supply and signal output for multiple accelerometers. Some data acquisition instruments also have a constant current function and can be used directly with ICP sensors.

　　If ordinary charge output sensors are matched with data acquisition devices with constant current output, the JM3861 constant current adapter can be used.

　　The dual-power accelerometer can be powered by a dual power supply provided by the collector or by a dual-channel DC regulated power supply.