Xi'an Gavin Electronic Technology Co., Ltd
Vibration is one of the most common phenomena in nature. From the universe to the atomic particles, there is vibration. In the field of engineering technology, vibration phenomena abound, but in many cases vibration is harmful. For example, vibration reduces machining accuracy and smoothness, exacerbates fatigue and wear of structural parts, and vibration of body and structural parts in vehicles and aerospace. It will not only affect the driver's operation and comfort, but also cause the body and structural parts to break or even disintegrate under severe conditions.
A vibration sensor is a sensor for detecting an impact force or an acceleration. A piezoelectric device that generates a charge by applying a stress is generally used, and a sensor that can be detected by other materials and methods is also used.
Vibration sensors can be used for vibration and displacement in machinery, long-term monitoring of thermal expansion of rotors and casings; on-line automatic detection and automatic control of production lines; measurement of various small distances and small movements in scientific research. Vibration sensors are widely used in energy, chemical, medical, automotive, metallurgy, machine manufacturing, military, scientific research and teaching.
There are many ways in which vibration sensors measure vibration, but in summary, most of the principles use the following three:
Mechanical measurement method: converts the change of engineering vibration into mechanical signal, and then performs measurement and recording after being amplified by mechanical system. Commonly used instruments include lever type vibrometer and Geiger vibrometer. This method measures frequency. The accuracy is poor, but it is very convenient to operate.
Optical measurement method: converts the amount of change in engineering vibration into an optical signal, which is displayed and recorded after being amplified by an optical system. This method is used like a laser vibrometer.
Electrical measurement method: converts the amount of change in engineering vibration into an electrical signal, which is displayed and recorded after being amplified by the line. It first converts the amount of mechanical vibration into electricity, then measures it, and knows the amount of vibration according to the corresponding relationship. This is the most widely used vibration measurement method.
It can be seen from the above three measurement methods that they are all completed by three parts: vibration sensor, signal amplification circuit and display recording.
In terms of mechanical receiving principle, the vibration sensor has only two kinds of relative and inertial types. However, in terms of electromechanical transformation, due to different transformation methods and properties, there are many kinds of applications, and the application range is extremely wide. The sensors used in modern vibration measurement are not traditionally independent mechanical measurement devices. They are only one part of the entire measurement system and are closely related to subsequent electronic circuits.
Due to the different internal electromechanical conversion principle of the sensor, the output power is also different. Some convert the change in the mechanical quantity into the electromotive force and the change in the electric charge, and some convert the change in the mechanical vibration amount into a change in the parameters such as resistance and inductance.
In general, these amounts of electricity are not directly accepted by subsequent display, recording, and analytical instruments. Therefore, sensors for different electromechanical conversion principles must be accompanied by a dedicated measuring circuit. The function of the measuring circuit is to finally change the output power of the sensor into a general voltage signal that can be accepted by the analysis instrument and analyzed by the instrument.
In general, the vibration sensor can have the following several methods according to its function:
The relative vibration sensor is mainly used to measure the movement of the vibrating body relative to its vibration reference point (for example, the vibration of the machine shaft relative to the machine base);
The inertial vibration sensor is mainly used to measure the movement of the vibrating body relative to the earth or the inertial space (such as the vibration of the machine base, the vibration of the ground, the vibration of the aircraft in the sky, etc.). Absolute vibration sensors are also called inertial vibration sensors because they contain inertial masses inside.
The inertial vibration sensor must be installed in contact with the vibration body to be tested. The relative sensor can be contact type or non-contact type.
Electric vibration sensors are further divided into relative electric sensors and inertial electric electric sensors.
The relative electric sensor is based on the principle of electromagnetic induction, that is, when a moving conductor cuts a magnetic line in a fixed magnetic field, an electromotive force is induced at both ends of the conductor, and thus the sensor produced by this principle is called an electric sensor.
The inertial electric sensor is composed of a fixed portion, a movable portion, and a support spring portion. In order for the sensor to operate in the state of the displacement sensor, the mass of the movable portion should be sufficiently large, and the stiffness of the support spring should be sufficiently small that the sensor has a sufficiently low natural frequency.
According to the law of electromagnetic induction, the induced electromotive force is: u=BLX&r where B is the magnetic flux density, which 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.
From the structural point of the sensor, the inertial electric sensor is a displacement sensor. However, since the electrical signal outputted by the electromagnetic induction is generated by electromagnetic induction, when the coil is relatively moved in the magnetic field, the induced electromotive force is proportional to the speed at which the coil cuts the magnetic flux.
Piezoelectric vibration sensors can also be divided into piezoelectric acceleration sensors, piezoelectric force sensors and impedance heads.
The mechanical receiving part of the piezoelectric acceleration sensor is the inertial acceleration mechanical receiving principle, and the electromechanical part utilizes 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.)
When a certain direction of external force or deformation, there will be a charge on its crystal face or polarization surface, this transformation from mechanical energy (force, deformation) to electrical energy (charge, electric field) is called positive piezoelectric effect . The transformation from electrical energy (electric field, voltage) to mechanical energy (deformation, force) is called the inverse piezoelectric effect.
Therefore, by using the piezoelectric effect of the crystal, a load cell can be fabricated. In the vibration measurement, since the force received by the piezoelectric crystal is the inertial force of the inertial mass, the amount of charge generated is proportional to the magnitude of the acceleration, so the pressure is The electrical sensor is an acceleration sensor.
In the vibration test, in addition to measuring vibration, it is often necessary to measure the dynamic exciting force applied to the test piece. Piezoelectric force sensors are widely used because of their wide frequency range, large dynamic range, small size and light weight. The working principle of the piezoelectric force sensor is to utilize the piezoelectric effect of the piezoelectric crystal, that is, the output charge signal of the piezoelectric force sensor is proportional to the external force.
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