relative type
The electrodynamic sensor is based on the principle of electromagnetic induction, that is, when the moving conductor cuts the magnetic force line in a fixed magnetic field, the electromotive force is generated at both ends of the conductor. Therefore, the sensor produced by using this principle is called electrodynamic sensor.
Relative to the principle of mechanical reception, the electric sensor is a displacement sensor. Because the electromagnetic induction law is applied in the principle of mechanical-electrical conversion, the electromotive force generated by it is proportional to the vibration speed measured, so it is actually a speed sensor.
Eddy current type
Eddy current sensor is a kind of relative non-contact sensor. It measures the vibration displacement or amplitude of the object by the distance change between the end of the sensor and the measured object. Eddy current sensor has the advantages of wide frequency range (0-10 kHZ), wide linear working range, high sensitivity and non-contact measurement. It is mainly used in static displacement measurement, vibration displacement measurement and vibration measurement of rotating shaft in rotating machinery.
Inductance type
According to the relative mechanical receiving principle of the sensor, the inductance sensor can convert the change of the measured mechanical vibration parameters into the change of the electrical parameters signal. Therefore, there are two forms of inductance sensor, one is variable gap, and the other is variable permeability area.
Capacitive type
Capacitive sensors are generally divided into two types. Variable clearance type and variable common area type. Variable clearance type can measure the displacement of linear vibration. Variable area type can measure angular displacement of torsional vibration.
inertial-type
The inertial electric sensor consists of a fixed part, a movable part and a supporting spring part. In order to make the sensor work in the state of 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 to say, the sensor has a sufficiently low natural frequency.
According to the law of electromagnetic induction, the induced electromotive force is u=Blx&r.
In the formula B is the flux density, L is the effective length of the coil in the magnetic field, and R x is the relative velocity of the coil in the magnetic field.
In terms of the structure of the sensor, the inertial electric sensor is a displacement sensor. However, because the output electric signal is generated by electromagnetic induction, according to the law of electromagnetic induction, when the coil moves relative in the magnetic field, the induced electromotive force is proportional to the speed of the coil cutting the magnetic force line. 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.
Piezoelectric type
The mechanical receiving part of piezoelectric accelerometer is the principle of inertial accelerometer, and the mechanical and electrical part uses the positive piezoelectric effect of piezoelectric crystal. The principle is that some crystals (such as artificial polarized ceramics, piezoelectric quartz crystals, etc.), different piezoelectric materials have different piezoelectric coefficients, which can generally be found in the performance table of piezoelectric materials. Charge will be generated on the crystalline or polarized surface of a certain direction under external force or deformation. This transformation from mechanical energy (force, deformation) to electrical energy (charge, electric field) is called positive piezoelectric effect. The transformation from electric energy (electric field, voltage) to mechanical energy (deformation, force) is called inverse piezoelectric effect.
Therefore, the piezoelectric effect of crystal can be used to make a force sensor. In vibration measurement, the piezoelectric sensor is an acceleration sensor because the force acting on the piezoelectric crystal is the inertia force of the inertia mass, and the number of charges generated is proportional to the magnitude of acceleration.
Piezoelectric force
In vibration test, besides measuring vibration, it is often necessary to measure the dynamic excitation force on the specimen. Piezoelectric force sensors are widely used because of their wide frequency range, large dynamic range, small size and light weight. The working principle of piezoelectric force sensor is to use piezoelectric effect of piezoelectric crystal, that is, the output charge signal of piezoelectric force sensor is proportional to the force.
Impedance head
Impedance head is a comprehensive sensor. It integrates piezoelectric force sensor and piezoelectric acceleration sensor. Its function is to measure the excitation force at the force transfer point and the motion response at the same time. Therefore, the impedance head consists of two parts, one is force sensor and the other is acceleration sensor. Its advantage is to ensure 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 large head (acceleration measuring) is connected to the force rod of the exciter. The signal of exciting force is measured from the output end of force signal, and the response signal of acceleration is measured from the output end of acceleration signal.
It should be noted that the impedance head can only bear light loads, so it can only be used for the measurement of light structures, mechanical components and material samples. Whether it is a force sensor or an impedance head, its signal conversion elements are piezoelectric crystals, so its measuring circuit should be a voltage amplifier or a charge amplifier.
Resistance strain gauge
Resistance strain sensor converts the measured mechanical vibration into the variation of the resistance of the sensing element. There are many kinds of sensors to realize the 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 pasted on a specimen, the stress and deformation of the specimen and the original length of the strain gauge change, so that the resistance of the strain gauge changes. Experiments show that the relative change of the resistance of the strain gauge is proportional to the relative change of its length in the range of elasticity of the specimen.
