According to its function, vibration sensor can be divided into: according to the principle of mechanical reception; Relative and inertial; According to the principle of electromechanical conversion, it is divided into electric type, piezoelectric type, eddy current type, inductive type, capacitive type, resistive type and photoelectric type; According to the measured mechanical quantities: displacement sensor, speed sensor, acceleration sensor, force sensor, strain sensor, torsional vibration sensor and torque sensor.
1, relative electric sensor The electric sensor is based on the principle of electromagnetic induction, that is, when a moving conductor cuts magnetic lines in a fixed magnetic field, electromotive force will be induced at both ends of the conductor, so the sensor made by this principle is called an electric sensor. The relative electric sensor is a displacement sensor based on the principle of mechanical reception. Because the electromagnetic induction law is applied in the electromechanical conversion principle, the electromotive force generated by it is proportional to the measured vibration speed, so it is actually a speed sensor.
2. Eddy current sensor Eddy current sensor is a relatively non-contact sensor, which measures the vibration displacement or amplitude of the object through the change of the distance between the sensor end and the measured object. 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 and vibration displacement measurement and vibration monitoring of rotating shaft 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 forms of inductance sensor, one is variable gap, and the other is variable magnetic permeability area.
4. Capacitive sensors Capacitive sensors are generally divided into two types. That is, the gap type and the variable common area type can be changed. Variable gap type can measure the displacement of linear vibration. The variable area formula can measure the angular displacement of torsional vibration.
5. Inertial electric sensor The inertial electric sensor consists of a fixed part, a movable part and a supporting spring part. In order to make the sensor function as a displacement sensor, the mass of its movable part should be large enough and the stiffness of the supporting spring should be small enough, that is, the sensor should have a low natural frequency. According to the law of electromagnetic induction, the induced electromotive force is: u = blx &；; Where b is the magnetic flux density, l is the effective length of the coil in the magnetic field, and r x&; Is the relative speed of the coil in the magnetic field. From the structure of the sensor, the inertial electric sensor is a displacement sensor. However, because the output electrical signal is generated by electromagnetic induction, according to the law of electromagnetic induction, when the coil moves relatively in the magnetic field, the induced electromotive force is directly proportional to the speed at which the coil cuts the magnetic field lines. So as far as the output signal of the sensor is concerned, the induced electromotive force is directly proportional to the measured vibration speed, so it is actually a speed sensor.
6. Piezoelectric acceleration sensor The mechanical receiving part of piezoelectric acceleration sensor is the principle of inertial acceleration mechanical receiving, and the electromechanical part uses the positive piezoelectric effect of piezoelectric crystal. Its principle is that some crystals (such as artificially polarized ceramics, piezoelectric chronotropic crystals, etc.). ), different piezoelectric materials have different piezoelectric coefficients, which can generally be found in the piezoelectric material performance table. ) under the action of an external force in a certain direction or when it is deformed, there will be charges on its crystal plane or polarization plane. This transformation from mechanical energy (force, deformation) to electrical energy (charge, electric field) is called positive piezoelectric effect. The conversion of electric energy (electric field, voltage) to mechanical energy (deformation, force) is called inverse piezoelectric effect. Therefore, the piezoelectric effect of crystals can be used to make force sensors. In vibration measurement, because the force on the piezoelectric crystal is the inertial force of the inertial mass, the amount of charge generated is proportional to the acceleration, so the piezoelectric sensor is the acceleration sensor.
7. In the vibration test, besides measuring the vibration, the piezoelectric force sensor often needs to measure the dynamic exciting force applied to the sample. Piezoelectric force sensor has the advantages of wide frequency band, large dynamic range, small volume and light weight, so it has been widely used. The working principle of piezoelectric force sensor is to use the piezoelectric effect of piezoelectric crystal, that is, the charge signal output by piezoelectric force sensor is proportional to external force.
8. Impedance head Impedance head is a comprehensive sensor. It integrates piezoelectric force sensor and piezoelectric acceleration sensor, and its function is to measure the excitation force and motion response of the force transmission point at the same time. So the impedance head consists of two parts, one is the force sensor and the other is the acceleration sensor. Its advantage is that the response of the measuring point is the response of the excitation point. When in use, the small head (force measuring end) is connected to the structure, and the big head (acceleration measuring end) is connected to the force applying rod of the vibration exciter. The exciting force signal is measured from the "force signal output end" and the acceleration response signal is measured from the "acceleration signal output end". Note that the impedance head can only bear light load, 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 measuring 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 kinds of sensor elements to realize this electromechanical conversion, among which the most common is the resistance strain sensor. The working principle of resistance strain gauge is that when the strain gauge is stuck on a specimen, the specimen is deformed by force, and the original length of the strain gauge changes, thus changing the resistance of the strain gauge. The experiment shows that the relative change of strain gauge resistance is proportional to the relative change of its length within the elastic range of the sample.