What Are the Different Types of Vibration Sensors?

In the highly developed modern industry, the development of modern test technology towards digitalization and informatization has become an inevitable development trend. The forefront of the test system is the sensor, which is the soul of the entire test system and is listed as cutting-edge technology by countries around the world. It is a rapidly developing 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 getting better every day, and digital, multifunctional and intelligent are the important characteristics of the development of modern sensors.

Chinese name: Vibration sensor
testing method: Mechanical, optical, electrical

Classification: Relative type, inductive type, current type, etc.
nickname: Transducer, vibration pickup
Belong to: Electromechanical conversion device

Development Trend of Vibration Sensor

1. Introduce new technology and develop new functions ^[1]

As people deepen their understanding of nature, new physical effects, chemical effects, and biological effects will be discovered. Using these new effects, corresponding new sensors can be developed, which provides new possibilities for improving sensor performance and expanding the application range of sensors. Turk Marketing Product Manager and Technical Support Supervisor Yang Deyou told reporters that "the biggest feature of the current sensor industry is the continuous introduction of new technologies and new functions." For example, inductive proximity switches that detect the position of metal products, it uses metal objects to approach The position of the metal product can be detected by the eddy current effect formed on the metal to be tested when the electromagnetic induction head can generate an electromagnetic field. Due to the different effects of eddy current effects of different metals, the detection distances of different metals are different. Especially when facing various types of alloys, ordinary inductive proximity switches become inadequate. This requires manufacturers to work hard to improve product functions. . Because the internal structure of an inductive proximity switch is a coil wound on a ferrite core as an inductive coil, and the limitations of the ferrite core itself make it impossible for the inductive sensor to develop under the existing design concept, then only Technically developed products that can replace ferrite coils to improve product performance. Turck's inductive proximity switches abandon ferrite cores, thereby removing the core limitation. In this way, when detecting different metals, the detection distance of the product can be increased through circuit adjustment, and the detection distance of all metals is not attenuated, and the anti-interference ability is also improved.

2. Use new materials to develop new products

Sensor materials are an important foundation of sensor technology. With the advancement of materials science, people can make a variety of new sensors. For example, a polymer film is used to make a temperature sensor. Optical fibers can be used to make pressure, flow, temperature, and displacement sensors, and ceramics are used to make pressure sensors. High molecular polymers can adsorb and release water molecules in proportion to the relative humidity of the surrounding environment. A polymer dielectric is used as a capacitor, and the change in capacitance is measured to obtain the relative humidity. The plasma polymerized polystyrene film temperature sensor made by this principle has the characteristics of wide humidity measurement range, wide temperature range, fast response speed, small size, can be used for small space humidity measurement, and small temperature coefficient. Ceramic capacitive pressure sensor is a dry pressure sensor without intermediary liquid. Adopting advanced ceramic technology and thick film electronic technology, its technical performance is stable, the full-scale error of the annual drift does not exceed 0.1%, the temperature drift is small, and the anti-overload can reach hundreds of times of the range.

The application of optical fiber is a major breakthrough in sensing materials. Compared with traditional sensors, fiber optic sensors have many characteristics: high sensitivity, simple structure, small size, corrosion resistance, good electrical insulation, flexible optical paths, and easy telemetry. The combination of fiber optic sensors and integrated optical path technology has accelerated the development of fiber optic sensor technology. The integrated optical circuit device will replace the original optical components and passive optical devices, and the fiber optic sensor has the characteristics of high bandwidth, low signal processing voltage, high reliability, and low cost.

Vibration sensor test method

In the field of engineering vibration testing, there are various testing methods and methods, but they can be divided into three categories according to the measurement methods of various parameters and the physical properties of the measurement process.

Mechanical vibration sensor

The engineering vibration parameters are converted into mechanical signals, and then amplified and measured by the mechanical system for measurement and recording. The commonly used instruments are the lever type vibrometer and the Geiger vibrometer, which can measure lower frequencies and have higher accuracy. difference. However, it is simpler and more convenient in the field test.

Vibration sensor optical

The engineering vibration parameters are converted into optical signals, which are displayed and recorded after being amplified by the optical system. Such as reading microscopes and laser vibrometers.

Electrical measurement of vibration sensor

Vibration sensor ^[2] The engineering vibration parameters are converted into electrical signals, which are displayed and recorded after being amplified by electronic circuits. The main point of the electrical measurement method is to convert the amount of mechanical vibration into electricity (electromotive force, electric charge, and other electricity), and then measure the electricity to obtain the mechanical quantity to be measured. This is currently the most widely used measurement method.

Although the physical properties of the above three measurement methods are different, the measurement systems are basically the same. They all include three links: vibration pick-up, measurement and amplification, and display and recording.

1. Vibration pick-up. The measured mechanical vibration is converted into a mechanical, optical or electrical signal. The device that performs this conversion is called a sensor.

2. Measurement line. There are many types of measurement circuits, and they are all designed for the conversion principle of various sensors. For example, the measurement circuits specially equipped with piezoelectric sensors include voltage amplifiers, charge amplifiers, etc. In addition, there are integral circuits, differential circuits, filter circuits, normalization devices, and so on.

3. Signal analysis and display and recording. The voltage signal output from the measurement line can be input to a signal analyzer or sent to a display instrument (such as an electronic voltmeter, oscilloscope, phase meter, etc.) and a recording device (such as a light oscilloscope, tape recorder, X-Y) Recorder, etc.) and so on. It can also be recorded on magnetic tape when necessary, and then input to the signal analyzer for various analysis processing to obtain the final result.

Vibration sensor receiving principle

Vibration sensor is one of the key components in the test technology, its role is mainly

Vibration sensor principle It receives the mechanical quantity and converts it into a proportional electric quantity. Because it is also an electromechanical conversion device. So we sometimes call it a transducer, a vibrator, etc.

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, and then is received by the mechanical receiving part to form another mechanical quantity suitable for conversion. , Finally, it will be converted into electricity by the electromechanical conversion part. Therefore, the working performance of a sensor is determined by the working performance of the mechanical receiving part and the electromechanical conversion part.

1.Relative mechanical receiving principle

Since mechanical motion is the simplest form of material motion, the first thing that people think of is to measure vibration by mechanical methods, so that mechanical vibrometers (such as Geiger vibrometers, etc.) are manufactured. The mechanical receiving principle of the sensor is based on this. The working receiving principle of the relative type vibrometer is that during the measurement, the instrument is fixed on a stationary support so that the vibration direction of the contact rod is consistent with the measured object, and the elastic force of the spring is in contact with the surface of the measured object. When the object vibrates, the stylus moves with it, and pushes the recording pen to draw the change curve of the displacement of the vibrating object over time on the moving paper tape. According to this recording curve, the size and frequency of the displacement can be calculated.

It can be known that the result measured by the relative mechanical receiving part is the relative vibration of the measured object relative to the reference body. The absolute vibration of the measured object can be measured only when the reference body is absolutely motionless. In this way, a problem occurs, when the absolute vibration needs to be measured, but no fixed reference point can be found, such instruments are useless. For example: testing the vibration of a diesel locomotive on a moving diesel locomotive, measuring the vibration of the ground and buildings during an earthquake ... there is no fixed reference point. In this case, we must use another vibrometer to measure, that is, use an inertial vibrometer.

2.Inertial mechanical receiving principle

The inertial mechanical vibrometer measures the vibration directly by fixing the vibrator to the measuring point of the measured vibration object. When the sensor housing moves with the measured vibration object, the elastically supported inertial mass will oppose the housing. Movement, the stylus pen mounted on the mass can record the relative vibration displacement amplitude of the mass element and the shell, and then use the relational expression of the relative vibration displacement of the inertial mass and the shell to determine the absolute of the measured object Vibration displacement waveform.

Electromechanical conversion of vibration sensors

Generally speaking, in terms of mechanical receiving principles, vibration sensors have only two types: relative and inertial. However, in terms of electromechanical conversion, due to different conversion methods and properties, there are many types of vibration sensors and their application range is extremely wide.

The sensor used in modern vibration measurement is no longer a traditional independent mechanical measurement device. It is only a link in the entire measurement system and is closely related to subsequent electronic circuits.

Due to the different electromechanical conversion principles inside the sensor, the output power is also different. Some change the change of mechanical quantity into the change of electromotive force and electric charge, and some change the change of mechanical vibration quantity into the change of electrical parameters such as resistance and inductance. Generally speaking, these electric quantities cannot be directly accepted by subsequent display, recording, and analysis instruments. Therefore, for sensors with different electromechanical conversion principles, special measurement circuits must be attached. The function of the measuring circuit is to finally change the output power of the sensor into a general voltage signal that the subsequent display and analysis instruments can accept. Therefore, vibration sensors can be classified according to their functions in the following ways:

Divided according to the principle of mechanical reception: relative type, inertial type;

According to the principle of electromechanical transformation: electric, piezoelectric, eddy current, inductive, capacitive, resistive, and photoelectric;

According to the measured mechanical quantity: displacement sensor, speed sensor, acceleration sensor, force sensor, strain sensor, torsional vibration sensor, torque sensor.

The sensors in the above three classifications are compatible.

Vibration sensor classification

Relative

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 forces are induced at both ends of the conductor, so 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 law of electromagnetic induction is applied in the principle of electromechanical conversion, the electromotive force generated is proportional to the measured vibration speed, so it is actually a speed sensor.

Eddy current

An eddy current sensor is a relative non-contact sensor. It measures the vibration displacement or amplitude of an object by changing the distance between the sensor end 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 rotating shafts in rotating machinery.

Inductive

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

Capacitive

Capacitive sensors are generally divided into two types. That is, variable gap 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

The inertial electric sensor is composed of a fixed part, a movable part, and a support spring part. In order to make the sensor work in the state of the displacement sensor, the mass of its movable part should be sufficiently large, and the stiffness of the support spring should be sufficiently small, that is, 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, 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 electric signal it outputs is generated by electromagnetic induction, according to the electromagnetic law of electromagnetic induction, when the coil makes relative movement in a magnetic field, the induced electromotive force is proportional to the speed of the coil cutting the magnetic field lines. So 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

The mechanical receiving part of the piezoelectric acceleration sensor is the principle of inertial acceleration mechanical receiving, and the electromechanical part uses the positive piezoelectric effect of the piezoelectric crystal. The principle is that certain 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.) External forces in a certain direction Under the action or deformation, charges will be generated on its crystal plane or polarized plane. 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 the vibration measurement, since the force on the piezoelectric crystal is the inertial force of the inertial mass, the number of charges generated is proportional to the acceleration. The electric sensor is an acceleration sensor.

Piezoelectric force

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.

Impedance head

The impedance head is a comprehensive sensor. It integrates a piezoelectric force sensor and a piezoelectric acceleration sensor, and its role is to measure the motion response of the point while measuring the exciting force at the point of force transmission. Therefore, the impedance head is composed of two parts, one is a force sensor and the other is an acceleration sensor. Its advantage is that the response of the measurement point is the response of the excitation point. When in use, connect the small head (force measurement end) to the structure, and connect the large head (measure acceleration) to the force applying rod of the shaker. The signal of the exciting force is measured from the "force signal output terminal", and the response signal of the acceleration is measured from the "acceleration signal output terminal".

Note that the impedance head can generally only bear light loads, so it can only be used for the measurement of 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.

Resistance strain gauge

The resistance type strain sensor converts the measured mechanical vibration amount into the change amount 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 attached to a test piece, the test piece is deformed by force, the original length of the strain gauge changes, so that the resistance value of the strain gauge changes. The relative change in sheet resistance is proportional to the relative change in length.

laser

Laser sensor A sensor that uses laser technology to make measurements. It consists of a laser, a laser detector, and a measurement circuit. The laser sensor is a new type of measuring instrument. It has the advantages of non-contact long-distance measurement, fast speed, high accuracy, large range, strong resistance to light and electrical interference, etc., which is very suitable for non-contact measurement applications in industry and laboratories.