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How To Choose The Right Vibration Sensor - Vibration Sensor Science

How To Choose The Right Vibration Sensor - Vibration Sensor Science

November 06 , 2023 0

I.What Is A Vibration Sensor

Vibration sensor is one of the key components in the test technology, its role is mainly to receive the mechanical amount and convert it into a proportional amount of electricity. Because it is also an electromechanical conversion device. So we sometimes call it a transducer, a vibration pickup, and so on.
Generally speaking, vibration sensors in terms of mechanical reception principle, there are only two types of relative and inertial, but in terms of electromechanical transformation, due to the different conversion methods and properties, there are a wide variety of applications
The sensor used in modern vibration measurement is no longer an independent mechanical measurement device in the traditional concept, it is only a link in the entire measurement system, and is closely related to the subsequent electronic circuit.

II.The Principle OF Vibration Sensor Receiving Signal

Its role is mainly to receive the mechanical amount, and the vibration sensor, which is one of the key components in the test technology, is converted into a proportional amount of electricity. Because it is also an electromechanical conversion device. So we sometimes call it a transducer, a vibration pickup, and so on.
The vibration sensor does not directly convert the original mechanical quantity to be measured into electricity, but the original mechanical quantity to be measured is used as the input quantity of the vibration sensor, and then received by the mechanical receiving part to form another mechanical quantity suitable for transformation, and finally converted into electricity by the electromechanical transforming part. Therefore, the working performance of a sensor is determined by the working performance of the mechanical receiving part and the electromechanical conversion part.

III.Classification Criteria For Vibration Sensors

Due to the different electromechanical transformation principle inside the sensor, the output power is also different. Some transform the change of mechanical quantity into the change of electromotive force and charge, and some transform the change of mechanical vibration quantity into the change of electrical parameters such as resistance and inductance. Generally speaking, these quantities cannot be directly accepted by subsequent display, recording, and analysis instruments. Therefore, sensors with different electromechanical transformation principles must be attached with special measurement lines. The function of the measurement line is to change the output power of the sensor into a general voltage signal that can be accepted by the subsequent display and analysis instrument. Therefore, vibration sensors can be classified according to their functions in the following ways:

According to the mechanical receiving principle, it is divided into relative vibration sensor and inertial vibration sensor
According to the principle of electromechanical transformation: electric vibration sensor, piezoelectric vibration sensor, eddy current vibration sensor inductive vibration sensor, capacitive vibration sensor, resistance vibration sensor, photoelectric vibration sensor
According to the measured mechanical measurement: displacement sensor, speed sensor, acceleration sensor, force sensor, strain sensor, torsional vibration sensor, torque sensor

IV.How To Choose The Right Vibration Sensor

Each type of sensor has a particularly suitable application scenario, so the most suitable vibration sensor must be selected according to the test requirements. In the selection of vibration sensors, mainly from the sensor performance, environmental factors, electrical characteristics and physical characteristics to consider four aspects.

Performance Index:

Range/sensitivity: Each sensor has a measurement range, usually a large range of sensors, low sensitivity, small range of sensors, high sensitivity. Usually the upper limit of the output voltage of the sensor is 5V, so the sensor sensitivity multiplied by the range gets the maximum output voltage of the sensor 5V. If the sensitivity of a certain type of sensor is 50mV/g, the range of the sensor is 100g. Generally, ICP-type acceleration sensors meet this law, while other types, such as zero-frequency acceleration sensors, do not meet this law. On the other hand, the higher the sensor sensitivity, the greater the sensor mass, the larger the sensor output voltage, the higher the signal-to-noise ratio, and the stronger the resolution. For testing different structures, the matching sensor range should be selected. Generally, the acceleration vibration magnitude of civil Bridges and very large mechanical structures is about 0.1g~10g, and the vibration of mechanical equipment is about 10g~100
Resonant frequency: The sensor itself is also a structure, therefore, there is also a natural frequency, usually, the first natural frequency of the sensor is called the resonant frequency. The smaller the sensor size, the higher the resonant frequency. The upper frequency of the accelerometer depends on the resonant frequency in the amplitude-frequency curve. The operating frequency range of the general sensor is less than 1/3 of its own resonant frequency.
Frequency response characteristics: The upper limit of the operating frequency of the general acceleration sensor is about 1/3 of the resonant frequency of itself. On the other hand, the low frequency characteristic of the acceleration sensor is usually poor, and the signal attenuation is serious, while the linearity is poor in the high frequency band, and the nonlinear influence is serious. Figure 2 shows the frequency response curve of a certain type of accelerometer. It can be seen from the curve that the signal attenuation is serious below 2Hz and the frequency response performance is poor. When the linearity is poor above 12KHz, the resonant frequency is about 38KHz. Therefore, the sensor operates at a frequency below 12KHz. When selecting the accelerometer, the upper limit of the frequency of the accelerometer is slightly higher than the vibration frequency of the structure under test. In general, the frequency range of civil engineering structures is about 0.2~1KHz, and mechanical equipment is in the middle frequency band, and the frequency range is about 0.5~5KHz. In addition, the mounting stiffness of the sensor also affects the frequency range that the sensor can measure

Linearity: Because the sensor can only input a single sensitivity when measuring, it is used to describe whether the sensitivity of the sensor meets the actual sensitivity within a certain frequency response range, that is, linearity. Relatively speaking, in low frequency bands (such as below 5Hz), the sensitivity of the sensor will be less than the actual sensitivity, and in high frequency bands (such as greater than the upper limit of the operating frequency), the sensitivity will be greater than the actual sensitivity. Only in the intermediate frequency band, the sensitivity meets the linear relationship, as shown in Figure 2. If the sensor is not measured in the linear interval, the amplitude error obtained by the measurement is large, and the nonlinear sensor is generally required. One percent.
Lateral effect: When the vibration in a certain direction is measured, the signal output should all be the vibration perception direction, but in fact there is also a signal output in the direction perpendicular to that direction, this effect is called the lateral effect. Lateral effect The lower the sensitivity, the better the performance, but relatively speaking, there is a certain lateral effect of the sensor, usually nominal lateral effect. 5%.

Environmental Factor

Use environment: The use of the sensor is affected by environmental factors such as temperature, humidity, and dust. Any kind of sensor has its own operating temperature range, so it is necessary to choose the appropriate sensor according to the temperature of the actual measuring point location and the ambient temperature. In addition, when there are influence factors such as moisture, corrosion and electromagnetic field in the test environment, these factors should also be considered in the selection of sensors.
Temperature response: The sensitivity of the sensor will be affected by the temperature, when the temperature has changed, if we still use the sensitivity at room temperature, it will bring error to the measurement. Figure 3 shows the temperature response curve of a sensor. It can be seen from the figure that the deviation of sensing sensitivity becomes larger and larger at room temperature. Therefore, the sensitivity of the sensor's operating temperature response is not biased, but when the temperature is far from room temperature, it is consistent with the temperature in the temperature response curve where the sensitivity is not biased
Impact limit: indicates the instantaneous impact limit that the sensor can withstand, usually expressed by the peak value, such as the impact limit of a sensor is +7000g pk.

Electrical Characteristics:

Excitation voltage/current: Active sensors need to provide excitation voltage/current to work properly, like lCP sensors need to provide 20-30VDC excitation voltage and 2-20mA constant current excitation. Today's data acquisition instruments generally have such a power supply device built in, so that the ICP sensor can be directly powered. But there are many other types of acceleration sensors, such as MEMS acceleration sensors, force balance acceleration sensors, etc., if the acquisition instrument can not provide the corresponding excitation voltage/current, you need to choose an external power supply method
Stability time: For ICP-type sensors, due to the existence of a discharge constant, when the sensor is powered, the sensor output signal will slowly stabilize from infinity to near the baseline, this time is called the stability time. When we measure, we should wait until the sensor output signal is stable before measuring. Usually this time only takes a few seconds

Physical Characteristics:

Sensitive materials: For piezoelectric and ICP-type sensors, quartz crystals and piezoelectric ceramics are mostly used as sensitive materials. The dielectric and piezoelectric constants of quartz crystal have good temperature stability and are suitable for sensors with wide operating temperatures. The piezoelectric ceramics with piezoelectric effect are synthetic, and the original piezoelectric ceramics have no piezoelectric effect. Due to the advantages of more convenient manufacturing process, moisture resistance, high temperature resistance and so on, most of today's piezoelectric sensors use piezoelectric ceramics as sensitive materials.
Size and mass: The shape of the acceleration sensor is mostly cylindrical and hexahedron, and the cylindrical accelerometer is divided into top line and side line. When choosing the dimensions of the accelerometer, it is mainly affected by the installation position space. For the measuring points with limited installation position space, the appropriate dimensions of the sensor must be selected. On the other hand, when selecting the sensor type, it is also necessary to consider the impact of the additional mass brought by the weight of the sensor itself, especially when testing lightweight structures, the weight of the sensor itself has a significant impact. May treat the total mass of the measured structure, the total mass of the sensor is very small, but the vibration is not all the mass of the structure, but the part of the mass involved in the vibration, called the effective mass, at this time, the total mass of the sensor may be relative to the effective mass of the structure will be very large, at this time the impact of the additional mass of the sensor will be obvious. In addition, when the sensor is installed, the tooling may also be used, and the quality of the tooling will have an impact on the vibration amplitude of the structure. For some small and lightweight structural vibrations or when measuring vibration parameters on thin plates, the "extra" load caused by the sensor and fixture mass may change the original vibration of the structure, thus rendering the measured results invalid. Therefore, a small and light sensor should be used in this case to estimate the effect of the accelerometer mass load.