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Classification and basis of vibration sensors

baijiahao | 03/07/2021

In the highly developed modern industry, it has become an inevitable development trend for modern test technology to develop towards digital and informatization. The forefront of the test system is the sensor, which is the soul of the entire test system and is listed as a cutting-edge technology by countries all over the world. It is the rapid development of IC technology and computer technology in recent years, which provides a good and reliable scientific and technological foundation for the development of sensors. The development of sensors is changing with each passing day, and digitization, multi-function and intelligence are important characteristics of the development of modern sensors.

The vibration sensor does not directly convert the original mechanical quantity to be measured into electricity, but uses the original mechanical quantity to be measured as the input quantity of the vibration sensor, which is then received by the mechanical receiving part to form another mechanical quantity suitable for transformation. Finally, the electromechanical conversion part converts it into electricity. Therefore, the working performance of a sensor is determined by the working performance of the mechanical receiving part and the electromechanical transforming part.

Commonly used vibration sensors are as follows:

1. Piezoelectric Resonant

The piezoelectric plate is used to receive the vibration signal. The resonant frequency of the piezoelectric plate is higher. In order to reduce the resonant frequency, the mass of the piezoelectric plate vibrating body is used to realize it, and the spring ball is used to replace the attachment to reduce the two resonance frequencies and enhance Vibration effect. Its advantages are high sensitivity and simple structure. However, the signal needs to be amplified and sent to the TTL circuit or the single-chip circuit, but a single-stage transistor can be used for single-stage amplification.

2. Mechanical vibration

In the traditional vibration detection method, after being subjected to vibration, the spring ball performs amplitude-reduction vibration for a long time, and this vibration is easy to be detected by the detection circuit. The vibration output switch signal is determined by the output impedance and the resistance value of the matched output resistance. According to the input impedance of the detection circuit, it can be made into a high impedance output mode.

3. Mini Vibration Sensor

The mechanical vibration sensor is miniaturized, the vibrating body is carbonized and sealed, and its working performance is more reliable. The output switch signal is directly connected with the TTL circuit and or the single-chip input circuit, and the circuit structure is simple. The output impedance is high and the quiescent current is small.

According to the principle of mechanical receiving, vibration sensors are divided into:

1. Relative electric sensor

Electric sensors are based on the principle of electromagnetic induction, that is, when a moving conductor cuts magnetic lines of force in a fixed magnetic field, electromotive force is induced at both ends of the conductor. Therefore, sensors produced using this principle are called electric sensors. The relative electric sensor is a displacement sensor from the principle of mechanical reception. Since the electromagnetic induction law is applied in the principle of electromechanical transformation, the electromotive force generated by it is proportional to the measured vibration speed, so it is actually a speed sensor .

2. Eddy current sensor

The eddy current sensor is a relative non-contact sensor, which measures the vibration displacement or amplitude of the object through the change of the distance between the end of the sensor and the measured object. The eddy current sensor has the advantages of wide frequency range (0~10 kHZ), large linear working range, high sensitivity and non-contact measurement. It is mainly used for static displacement measurement, vibration displacement measurement, and vibration measurement of monitoring shafts in rotating machinery.

3. Inductive sensor

According to the relative mechanical receiving principle of the sensor, the inductive sensor can convert the change of the measured mechanical vibration parameter into the change of the electrical parameter signal. Therefore, there are two types of inductive sensors, one is a variable gap, and the other is a variable magnetic area.

4. Capacitive sensor

Capacitive sensors are generally divided into two types. Namely variable gap type and variable common area type. The variable gap type can measure the displacement of linear vibration. The variable area type can measure the angular displacement of torsional vibration.

5. Inertial electric sensor

The inertial electric sensor is composed of a fixed part, a movable part and a supporting spring part. In order to make the sensor work in the displacement sensor state, the mass of its movable part should be large enough, and the stiffness of the support spring should be small enough, that is, the sensor has a sufficiently low natural frequency. From the structure of the sensor, the inertial electric sensor is a displacement sensor. However, since the output electrical signal is generated by electromagnetic induction, according to the law of electromagnetic induction, when the coil moves relative to the magnetic field, the induced electromotive force is proportional to the speed at which the coil cuts the lines of magnetic force. Therefore, as far as the output signal of the sensor is concerned, the induced electromotive force is proportional to the measured vibration speed, so it is actually a speed sensor.

6. Piezoelectric acceleration sensor

The mechanical receiving part of the piezoelectric acceleration sensor is based on the inertial acceleration mechanical receiving principle, and the electromechanical part uses the positive piezoelectric effect of the piezoelectric crystal. The principle is that certain crystals (such as artificial polarized ceramics, piezoelectric quartz crystals, etc., different piezoelectric materials have different piezoelectric coefficients, which can generally be found in the piezoelectric material performance table.) External force in a certain direction Under the action or deformation, there will be charges generated on its crystal surface or polarization surface. This transformation from mechanical energy (force, deformation) to electrical energy (charge, electric field) is called the positive piezoelectric effect. The transformation from electrical energy (electric field, voltage) to mechanical energy (deformation, force) is called the inverse piezoelectric effect.

Therefore, the piezoelectric effect of the crystal can be used to make a load cell. In vibration measurement, since the force of the piezoelectric crystal is the inertial force of the inertial mass, the number of charges generated is proportional to the acceleration, so the pressure The electric sensor is an acceleration sensor.

7. Piezoelectric force sensor

In the vibration test, in addition to measuring vibration, it is often necessary to measure the dynamic excitation force applied to the test piece. Piezoelectric force sensors have the advantages of wide frequency range, large dynamic range, small size and light weight, so they are widely used. The working principle of the piezoelectric force sensor is to use the piezoelectric effect of the piezoelectric crystal, that is, the output charge signal of the piezoelectric force sensor is proportional to the external force.

8. Impedance head

The impedance head is a comprehensive sensor. It integrates a piezoelectric force sensor and a piezoelectric acceleration sensor, and its function is to measure the excitation force at the force transfer point while measuring the motion response of that point. Therefore, the impedance head is composed of two parts, one is a force sensor, and the other is an acceleration sensor. Its advantage is to ensure that the response of the measuring point is the response of the excitation point.

When in use, connect the small head (force measuring end) to the structure, and the big head (measure acceleration) to the force rod of the vibration exciter. Measure the excitation force signal from the "force signal output terminal", and measure the acceleration response signal from the "acceleration signal output terminal". Note that the impedance head generally can only withstand light loads, so it can only be used to measure light structures, mechanical parts, and material samples. Whether it is a force sensor or an impedance head, its signal conversion element is a piezoelectric crystal, so its measurement circuit should be a voltage amplifier or a charge amplifier.

9. Resistance strain sensor

The resistance strain sensor converts the measured mechanical vibration into the change of the resistance of the sensing element. There are many forms of sensing elements to achieve this electromechanical conversion, the most common of which is a resistance strain sensor. The working principle of the resistance strain gauge is: when the strain gauge is pasted on a certain specimen, the specimen is deformed by force, and the original length of the strain gauge changes, so that the resistance value of the strain gauge changes. The experiment proves that within the elastic change range of the specimen, the strain The relative change of sheet resistance is proportional to the relative change of its length.

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