Application analysis of various vibration sensors

Electrodynamic sensor

The electrodynamic sensor is based on the principle of electromagnetic induction, that is, when the moving conductor cuts the magnetic line of force in a fixed magnetic field, the electromotive force is induced at both ends of the conductor, so the sensor produced by using this principle is called an electrodynamic sensor.

Relative electric sensor from the mechanical reception principle, is a displacement sensor, due to the application of electromagnetic induction law in the electromechanical transformation principle, the electromotive force generated is proportional to the measured vibration speed, so it is actually a speed sensor.

Eddy current sensor

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 sensor end and the object to be measured. Eddy current sensor has the advantages of wide frequency range (0 ~ 10 kHZ), large linear working range, high sensitivity and non-contact measurement, which is mainly used in static displacement measurement, vibration displacement measurement, and vibration measurement of rotating machinery.

Inductive sensor According to the relative mechanical receiving principle of sensor, inductive sensor can convert the change of mechanical vibration parameter to the change of electrical parameter signal. Therefore, the inductive sensor has two forms, one is variable gap, and the other is variable magnetic permeability area.

Capacitive sensor

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

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 the moving part should be large enough, and the stiffness of the supporting spring should be small enough, that is, the sensor has a low enough natural frequency.

According to the law of electromagnetic induction, the induced electromotive force is: u=Blx& r

Where B is the 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 its output number 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 magnetic field 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 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 some crystals (such as artificially polarized ceramics, piezoelectric quartz 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 under deformation, its crystal surface or polarized surface will have a charge generated, 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 made into a force sensor, in the vibration measurement, because the force received by the piezoelectric crystal is the inertial force of the inertial mass block, the number of charges generated is proportional to the size of the acceleration, so the piezoelectric sensor is an acceleration sensor.

Piezoelectric force sensor

In vibration test, in addition to measuring vibration, it is often necessary to measure the dynamic exciting force applied to the specimen. Piezoelectric force sensor has the advantages of wide frequency range, large dynamic range, small size and light weight, so it has been 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.

Impedance head

Impedance head is a comprehensive sensor. It is a combination of piezoelectric force sensor and piezoelectric acceleration sensor, and its function is to measure the excitation force at the force transfer point and measure the motion response of the point. Therefore, the impedance head is composed of two parts, one is a force sensor, the other is an acceleration sensor, its advantage is to ensure that the response of the measurement point is the response of the excitation point. When used, the small head (force measuring end) is connected to the structure, and the big head (measuring acceleration) is connected to the force applying rod of the exciter. Measure the signal of the exciting force from the "force signal output" and measure the response signal of the acceleration from the "acceleration signal output".

Note that the impedance head can generally only withstand light loads, and therefore can only be used for lightweight structural, mechanical components, and material sample measurement. Whether it is a force sensor or an impedance head, the signal conversion element is a piezoelectric crystal, so its measurement line should be a voltage amplifier or a charge amplifier.

Resistance strain gauge sensor

The resistance strain gauge sensor converts the measured mechanical vibration into the change of the resistance of the sensing element. The sensing element to achieve this electromechanical conversion comes in many forms, the most common of which is the resistance strain gauge sensor.

The working principle of the resistance strain gauge is: when the strain gauge is pasted on a specimen, the specimen is stressed and deformed, the original length of the strain gauge changes, and the resistance value of the strain gauge changes. The experiment proves that within the elastic range of the specimen, the relative change of the resistance of the strain gauge is proportional to the relative change of its length.