Piezoelectric accelerometers are widely used due to their wide frequency response, large dynamic range, high reliability, and ease of use. In general vibration measurement, the main technical indicators of the user are: sensitivity, frequency range, internal structure, difference between built-in circuit type and pure piezoelectric type, field environment and subsequent instrument configuration.
First, the choice of sensitivity
The sensitivity and reference range of the sensor are generally given by the manufacturer in the product introduction or the 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 is also called the minimum resolution. Considering the noise problem of the post-amplifier circuit, it should be kept away from the minimum usable value to ensure the best signal-to-noise ratio. The maximum measurement limit takes into account the nonlinear effects of the acceleration sensor itself and the maximum output voltage of subsequent instruments.
Estimation method: Maximum measured acceleration × sensor charge (voltage) sensitivity, whether its value exceeds the maximum input charge (voltage) value of the instrument. It is recommended that if the measured acceleration range is selected, it can be selected in the “reference range range†of the sensor index (considering the frequency response and weight). At the same time, in the case of frequency response and quality, try to select a high sensitivity sensor to improve Subsequent instrument input signals to improve the 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 type piezoelectric acceleration sensor as an example:
1. The vibration of civil engineering and super large mechanical structure is about 0.1g-10g (1g=9.81m/s2), and the piezoelectric acceleration sensor with charge sensitivity of 300pC/ms-2～30pC/ms-2 is the electric charge. Output type piezoelectric acceleration sensor
2. The vibration of special civil structures (such as pile foundation) and mechanical equipment is between 100ms-2 and 1000ms-2, and acceleration sensors of 20pC/ms-2 to 2pC/ms-2 can be selected.
3, impact, collision measurement range is generally 10000ms-2 ~ 1000000ms-2, optional sensor sensitivity is 0.2pC / ms-2 ~ 0.002pC / ms-2 acceleration sensor.
Second, the frequency selection
The frequency response curve of the accelerometer given by the manufacturer is mounted rigidly with screws.
The curve is generally divided into two segments: the resonant frequency and the frequency of use. The frequency of use is given by the sensitivity deviation, which is ±10%, ±5%, ±3dB. The resonant frequency is generally avoided, but there are special cases, such as bearing fault detection. The frequency range of the acceleration sensor should be selected to be higher than the vibration frequency of the test piece. The acceleration sensor frequency response required for multiplier 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, and the mechanical equipment is generally in the middle frequency band. The vibration frequency can be comprehensively estimated according to factors such as equipment speed and equipment stiffness, and an acceleration sensor of 0.5 Hz to 5 kHz is selected. If the generator speed is 3000rms, divide it by 60s and its main frequency is 50Hz. Collision and impact measurement are mostly high frequency.
Different installation methods of the accelerometer will also change the frequency response (has little effect on the vibration value).
The mounting surface should be smooth and clean, and the installation should be based on the principle of convenience and safety. We give the frequency of use of the same installation of the same RC6500S accelerometer: screw rigid connection (±10% error) 10kHz; epoxy glue or "502" bonding installation 6kHz; magnetic suction mount 2kHz; double-sided adhesive installation 1kHz. It can be seen that the difference in the installation method has a great influence on the response of the test frequency, and attention should be paid to the selection. The quality and sensitivity of the accelerometer are inversely proportional to the frequency of use. The sensitivity is high, the quality is high, and the frequency of use is low. This is also the skill of choice.
Third, the internal structure
The internal structure refers to the way in which the crystal piece of sensitive material senses vibration and the form of installation. There are two major categories of compression and shear, common are center compression, plane shear, triangular shear, and ring shear.
The center compression type frequency response is higher than the shear type, and the shear type has better environmental adaptability than the center compression type. If the integral type charge amplifier is used to measure the speed and displacement, it is better to use a shear type product, so that the obtained signal has small fluctuation and good stability.
Fourth, 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 offset, also known as ICP). The built-in circuit referred to below refers to the ICP type. At present, the built-in circuit sensor is generally matched with the data acquisition instrument. It is used for mechanical failure and pile foundation detection in many aspects in China. Many online monitoring projects also use this type of product. The power supply and signal output of the ICP type accelerometer share a single line. Its characteristics are: low impedance output, anti-interference, low noise, high performance and price ratio, easy to install, especially suitable for multi-point measurement, stable and reliable, anti-humidity, anti-dust, anti-harmful gas. Selection calculation of built-in circuit sensor sensitivity:
The measured acceleration value (g)=maximum output voltage (mV)/sensor sensitivity (mV/g) can be measured within 50g if the most common 100mV/g is used. If measuring 100g, use 50mV/g accelerometer. The rest is like this.
V. Environmental impact
The environment at some test sites is relatively harsh, and many factors are considered, such as waterproofing, high temperature, installation location, strong magnetic field and ground circuit, which will have a great impact on the measurement.
Waterproof: 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 water seepage, wire protection, long-term reliability. Sex and many other issues.
High temperature: The temperature range of the sensor given by most manufacturers is the usable value, not the sensitivity of the high temperature condition. In fact, the sensitivity deviation is high at high temperature. Special users should request the special high temperature sensitivity index from the manufacturer. The sensitivity index is guaranteed. The key to accuracy.
Position limitation: The acceleration sensor is permanently installed on the site and will be subject to human collision. The industrial long-term monitoring acceleration sensor should be selected. It adopts an external protective cover and triangular flange mounting, which has the function of insulating and dustproofing to the ground. The requirements for the direction of the outlet can be made to the manufacturer. For inaccessible parts, a hand-held accelerometer (with a long probe) is available.
Insulation, geoelectric circuit and magnetic field: For the test site with strong magnetic field, the acceleration sensor and special wire of special casing material should be selected. Such research is still rare in China. For two-point grounding, humidity, etc., to solve the test interference, floating or insulated acceleration sensors can be used, and the protection of the wire joints should be considered. In order to overcome the influence of the ground loop current generated by two or more groundings on the test, a floating or insulated sensor may be selected. Insulated acceleration sensors are available without special requirements and with little interference, while floating type is used for permanent monitoring or high interference conditions. The difference between these two names is that the outer casing of the insulated product is a signal ground, and the outer casing of the floating type product is a shielding layer.
Additional mass: The mass of the accelerometer mounted on the vibrating structure is less than 1/10 of the dynamic mass of the measured point. The effect on the measured signal is considered negligible.
Six, supporting equipment
Piezoelectric accelerometers, such as charge output, 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, single, multi-channel, integral, quasi-static. This is determined by the measurement requirements. There are also special cases, such as directly connecting the output signal of the piezoelectric sensor to a three-meter instrument (such as an oscilloscope) with a certain high resistance performance, and the signal can also be measured, but because the impedance matching is not enough, it can only qualitatively understand the dynamic condition. Built-in IC amplifier accelerometer (ICP type) specializes in constant current adapter, a constant current adapter for constant current supply and signal output of multiple accelerometers. For data acquisition instruments that provide constant current source power, this type of sensor can be directly connected to the data acquisition instrument. Dual-supply accelerometers can be powered by a dual supply or with a dual DC regulated supply.
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