What is a Vibration Sensor?

Catalog

Ⅰ Introduction

Vibration is one of the most common phenomena in nature. It exists in everything from the universe to atomic particles. Vibration is ubiquitous in the field of engineering technology, but in many cases, the vibration can be harmful. Vibration reduces machining precision and finish, and aggravates the wear of structural parts. In the field of vehicle and aviation, the vibration of the airframe and structural parts will not only affect the driver's operation but also cause the fracture and disintegration of the airframe and structural parts.

The vibration sensor is used to detect the impact force or acceleration of the sensor. It usually uses piezoelectric devices which will generate charge under a stress. The vibration sensor can be used for long-term monitoring of vibration and displacement, thermal expansion amount of rotor, and chassis. It can be used for on-line automatic detection and control of the production line and a variety of measurements of minute distances and motions in scientific research. Vibration sensors are widely used in energy, chemical industry, medicine, automobile, metallurgy, machine manufacturing, military industry, scientific research and teaching, and many other fields.

Ⅱ Working principle of vibration sensors

There are many ways for vibration sensor to measure vibration, but in summary, the following three principles are mostly adopted:

Mechanical measurement: the engineering vibration is converted into mechanical signals, and then amplified by the mechanical system, and then measured and recorded. The commonly used instruments are lever type vibration meter and Geiger vibration meter. This method has relatively low measurement frequency and accuracy, but it is very convenient to operate.

Optical measurement: the variation of engineering vibration is converted into an optical signal, which is amplified by the optical system and displayed and recorded. Things like laser vibrometers also use this method.

Electrical measurement: the variation of engineering vibration is converted into electrical signals, which are displayed and recorded after amplification. It first converts mechanical vibration into electric quantity and then measures it. According to the corresponding relationship, we can know the magnitude of vibration. This is the most widely used vibration measurement method.

As can be seen from the above three measurement methods, they are completed by three steps: vibration sensor, a signal amplification circuit, and display recording.

Ⅲ Classification of vibration sensors

In terms of the mechanical receiving principle, there are only two kinds of vibration sensors: relative type and inertia type. However, in terms of electromechanical transformation, due to different transformation methods and properties, there are many kinds of vibration sensors and their application range is also very wide. The sensor used in the modern vibration measurement is no longer an independent mechanical measurement device in the traditional concept, it is only a link in the whole measurement system and closely related to the following electronic circuit.

Due to the different principles of electromechanical transformation inside the sensor, the output power is also different. Some change the mechanical quantity into electromotive force and charge, and some change the mechanical vibration quantity into resistance, inductance, and other parameters.

In general, these quantities cannot be directly accepted by the subsequent display, recording, or analytical instruments. Therefore, for different electromechanical transformation principles of the sensors, they must be attached to a special measuring line. The function of the measuring circuit is to change the output power of the sensor into the general voltage signal acceptable to the subsequent display and analysis instrument.

In general, vibration sensors can be classified as follows according to their different functions:

According to the principle of mechanical reception, it is divided into the relative type and inertia type;

According to the principle of mechanical and electrical transformation, it can be divided into electric type, piezoelectric type, eddy current type, inductance type, capacitance type, resistance type, and photoelectric type.

According to the measured mechanical quantity, it is divided into a displacement sensor, a speed sensor, an acceleration sensor.

1 Relative and inertial vibration sensors

The relative vibration sensor is mainly used to measure the motion of the vibration body relative to its vibration reference point. For example, the vibration of the machine tool shaft relative to the machine tool base.

The inertial vibration sensor is mainly used to measure the motion of the vibration body relative to the ground or inertial space, such as the vibration of the machine tool base, the vibration of the ground, the vibration of the aircraft in the sky, etc. The absolute vibration sensor is also called an inertial vibration sensor because it contains an inertial mass block.

The inertial vibration sensor must be installed in contact with the vibration body under test. The relative sensor can be contact or non-contact type.

2 Electrodynamic vibration sensor

The electrodynamic vibration sensor is divided into a relative electrodynamic sensor and an inertial electrodynamic sensor.

 Magnetoelectric vibration sensor

The relative type of electric sensor is based on the principle of electromagnetic induction. That is, when the moving conductor cuts the magnetic field line in the fixed magnetic field, the two ends of the conductor will generate the electromotive force, so the sensor produced by using this principle is called the electrodynamic sensor.

The inertial electric sensor consists of a fixed part, a movable part, and a supporting spring part. To make the sensor work in the displacement sensor state, 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 enough natural frequency.

According to the law of electromagnetic induction, the induced emf 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 structure of the sensor, the inertial electric sensor is a displacement sensor. However, since the output electrical signal is generated by electromagnetic induction, according to the electromagnetic induction law, when the coil moves relative to the magnetic field, the induced electromotive force is proportional to the speed of the coil cutting the magnetic field line.

3 Piezoelectric vibration sensor

The piezoelectric vibration sensor can also be divided into a piezoelectric acceleration sensor, piezoelectric force sensor, and impedance head.

3.1 Piezoelectric acceleration sensor

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 when some crystals are deformed by external force in a certain direction or undergo deformation, charges will be generated on their crystal or polarized surfaces. This conversion from mechanical energy to electrical energy is called a 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 force sensor. In vibration measurement, as the force of the piezoelectric crystal is the inertial force involved in the mass block, the amount of charge generated is proportional to the magnitude of acceleration, so the piezoelectric sensor is an acceleration sensor.

3.2 Piezoelectric force sensor

In the vibration test, in addition to measuring the vibration, it is often necessary to measure the dynamic excitation force applied to the specimen. Piezoelectric force sensors are widely used because of their wide frequency range, large dynamic range, small size, and lightweight. The working principle of the piezoelectric force sensor is to utilize the piezoelectric effect of a piezoelectric crystal, that is, the output charge signal of the piezoelectric force sensor is proportional to an external force.

3.3 Impedance head

Impedance head is a kind of integrated sensor. It is a piezoelectric force sensor and a piezoelectric acceleration sensor, and its function is to measure the vibration force at the force transfer point and the motion response at the same time.

Impedance head

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 measured 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 (measuring acceleration) is connected to the force applying rod of the vibrator. The signal of excitation force is measured from the "force signal output", and the response signal of acceleration is measured from the "acceleration signal output".

Note that impedance heads can generally only withstand light loads and are therefore suitable only for lightweight structures, mechanical components, and material samples. Both the force transducer and the impedance head are piezo crystals, so the measuring lines should be voltage or charge amplifiers.

4 Eddy current vibration sensor

Eddy current vibration sensor

Eddy current vibration sensor is a relative non-contact sensor. It measures the vibration displacement or amplitude of the object by the change of the distance between the end of the sensor and the measured object.

The eddy current sensor has the advantages of the wide frequency range (0 ~ 10 kHz), large linear working range, high sensitivity, and non-contact measurement, etc. It is mainly applied to the measurement of static displacement, vibration displacement, and vibration measurement of the monitored shaft in rotating machinery.

5 Inductive vibration sensor

An inductive vibration sensor is a kind of vibration sensor designed according to the principle of electromagnetic induction. Inductance-type vibration sensor is equipped with a magnet and magnetic body. When measuring the vibration of the object, the mechanical vibration parameters can be converted into electrical parameter signals. Therefore, the inductance sensor has two forms, one is the variable clearance, the other is the variable magnetic conductivity area. The inductance vibration sensor can be used to measure vibration velocity, acceleration, and other parameters.

6 Capacitive vibration sensor

Capacitive vibration sensor

The capacitive vibration sensor obtains the variable capacitance by changing the gap or the common area, and then the mechanical vibration parameters are obtained by measuring the capacitance. The capacitive vibration sensor can be divided into variable gap type and variable common area type, the former can be used to measure the linear vibration displacement, the latter can be used to measure the angular displacement of torsional vibration.

7 Resistance strain type vibration sensor

The resistance strain vibration sensor converts the measured mechanical vibration amount into the change of the resistance of the sensing element. There are many kinds of sensing elements to realize this electromechanical conversion, among which the most common one is the resistance strain gauge.

resistance strain gauge

The basic structure of the resistance strain gauge is shown in the figure. It is generally composed of the sensitive gate, base, lead, and cover plate. The sensitive grid is formed by bending wires with a diameter of 0.01-0.05mm and a high resistance coefficient. It is a resistor element and a sensitive part of the resistance strain gauge to perceive the strain of the element. The sensitive grid is bonded to the substrate. The function of the substrate should ensure that the strain on the component is transmitted to the sensitive grid accurately.

When specimen stress deformation, strain gauge sensitive gate also gets the same deformation, thus make its resistance with the change, and the resistance change is proportionate to the specimen strain. So if by measuring circuit converts the resistance to voltage or current change, and then use display record instrument to record down, you can know the size of the test pieces shall be variable.

8 Fiber optic vibration sensor

With the development of optical fiber and optoelectronic devices, optical fiber sensing technology has been developed by leaps and bounds. Due to the advantages of small size, lightweight, high precision, fast response, wide dynamic range, fast response,  it is widely used in many fields. It also has good anti-electromagnetic interference, corrosion resistance, and non-conductivity.

Fiber-optic vibration sensors, which measure vibration signals, have been around for more than 30 years. The original optical fiber vibration sensor adopts an interferometric structure. The optical fiber strain generated by vibration causes the phase change of the signal arm of the interferometer. However, this kind of sensor structure is more complex and is not conducive to repeated use.

8.1 Phase-modulated fiber-optic vibration sensor

The bit-modulated optical fiber vibration sensor utilizes a coherent laser source and two single-mode fibers. The light rays are separated and incident on the fiber. If the interference affects one of the two related optical fibers, it will cause a phase difference, which can be accurately detected. The phase difference can be measured with an interferometer. There are four types of interferometer structures. They include Mach - Zedel, Michelson, Fabriz-Paro, and Segnak interferometers.

The following is based on the optical fiber Sagnac interference principle. A and B are the two sensing arms of the interferometer, which transmit light. C is a piece of optical fiber that is wound into a circle, which is used to receive or sense changes of external information. 22 optical fiber 3dB couplers are used to decompose and synthesize interference beams. The injected light passes through the coupler and is divided into two beams. A beam of light passes from A to C to B and is returned to the coupler. The other beam goes from B to C to A, and then back to the coupler. The two beams meet and cause interference.

3dB coupler

The optical fiber Sagnac interference vibration sensor is based on the optical Sagnac interferometer and is composed of single-mode fiber and 3dB coupler. The sensor can detect weak vibration. The output light intensity of the sensor is modulated by the signal when it is propagated in solid and used as the sensor. By detecting the output light intensity and using the Fourier transform, the frequency characteristics of the signal are obtained.

8.2 Optical stress system fiber optic vibration sensor

In optical fiber communication, based on mature optical fiber coupling technology, people have successfully developed a high-performance coupled optical fiber acoustic vibration sensor of all-optical fiber devices, which has attracted many people's attention due to its advantages such as measurement bandwidth, high sensitivity, demodulation, low production cost, and simple use.

To make the coupler of single-mode fiber be used as a sensor, researchers analyzed the sensitivity mechanism of a single-mode fiber coupling sensor. According to the principle that there is a certain relationship between the coupling output of the sensor and the length of the coupling region of the sensor and the vibration frequency of the coupling region, an optical fiber vibration sensor can be made to realize vibration detection.

When the incident light P0 enters the input end, the two conduction modes begin to overlap as the two waveguides approach each other gradually. In the coupling region of the double-cone structure, the optical power is redistributed. One part of the optical power continues to transmit from the "through arm", while the other part is transmitted from the "coupling arm" to another optical path.

The power difference between the two outputs of the coupler is linearly related to the vibration acceleration of the excitation source. Therefore, the vibration measurement can be realized by measuring the change of coupler output power and calculating the value of sensor acceleration.

This kind of sensor is very sensitive to strain, and its linear relationship of coupling ratio is good. The influence of temperature drift can be stable within 0.5 %. Compared with the measurement of the piezoelectric vibration sensor, the sensor can better realize vibration detection at a low frequency of 0 ~ 50 Hz and a high frequency of 4 kHz.

8.3 Wavelength modulated optical fiber vibration sensor

The principle of wavelength modulation sensing is the interaction between the measured field/parameter and the sensitive fiber, which causes the wavelength change of the transmitted light in the fiber, and then the measured parameter is determined by measuring the variation of the light wavelength.

According to the mathematical expression of Bragg's central wavelength 3.1.3, the sensing information is obtained by modulating the central wavelength of Bragg through external parameters. This process is the sensing principle of fiber Bragg grating.

Where the effective refractive index of the fiber core is n, T is the period of the grating.

According to the equation, the wavelength is determined by the effective refractive index of the grating period and the reverse coupling mode. Any physical process that causes these two parameters to change will cause the grating Bragg wavelength to drift. Among all the external factors that cause grating Bragg wavelength drift, the most direct one is the change of strain parameter.

As shown in the figure below, a fiber Bragg grating vibration sensor is fixed on the package shell by one end of the mechanical suspension beam arm and connected to the platform to be tested. In the measurement of vibration, the vibration source and the platform vibrate at the same time, which causes the suspension beam arm to vibrate.

Two fiber Bragg gratings of the same characteristics, one mounted on the symmetrical position of the lower surface of the suspension arm as a signal demodulation grating, the other mounted on the upper surface of the mechanical suspension arm as a sensing grating.

The mechanical vibration of the cantilever beam under the action of vibration inertia force will drive the two gratings to produce periodic strain stretching or contraction, thus causing FBG's Bragg wavelength to change. Vibration measurement can be realized by detecting whether the wavelength information is consistent.

fiber Bragg grating vibration sensor(1)

Light is sent to sensor head 1 via a 2×2 fiber coupler. Reflected light signal return to the 2 x 2 fiber coupler. After the sensing head 2, the transmission light intensity of sensing head 2 is converted into vibration signals from the light signal by photoelectric conversion. The sensing head 2 is used as a light wavelength filter of sensing head 1, which concerts changes of the wavelength of sensing head 1 into changes of light intensity signal.

fiber Bragg grating vibration sensor(2)

The characteristic of the FBG vibration sensor is a new simple demodulation technology, which can effectively eliminate the chirp of FBG sensitive signal, effectively reduce the temperature cross-sensitivity of the sensor, and significantly improve the vibration measurement accuracy.

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