What is a vibration sensor? Introduction to the working principle of vibration sensors

In the highly developed modern industry, the development of modern testing technology towards digitization and informatization has become an inevitable trend. The forefront of the testing system is the sensor, which is the soul of the entire testing system and is listed as a cutting-edge technology by countries around the world. Especially in recent years, the rapid development of IC technology and computer technology has provided a good and reliable scientific and technological foundation for the development of sensors. The development of sensors is rapidly advancing, and digitization, multifunctionality, and intelligence are important features of modern sensor development.

What does a vibration sensor do

Vibration sensors mainly monitor the vibration of rotating machinery, and each device has its own vibration standard. Exceeding the vibration value indicates that the machine has malfunctioned, so vibration sensors play a protective role against vibration.

Vibration sensors are divided into two types: magneto electric and piezoelectric. Magnetoelectric sensors have a simple structure and lower price, but lower accuracy. Currently, piezoelectric sensors are commonly used, which have higher measurement accuracy.

The vibration sensor first senses vibration acceleration, integrates it to obtain velocity, and integrates it twice to obtain displacement. However, acceleration and displacement are affected by frequency. At the same time, the national vibration standard is called vibration intensity, which is the effective value of vibration velocity. Therefore, vibration velocity is usually monitored.

Introduction to the working principle of vibration sensors

1. Classification and characteristics of vibration parameters

A vibration sensor is a single free oscillation system composed of springs, dampers, and inertial mass blocks. Using the inertia of the mass block to establish coordinates in inertial space and measure the vibration acceleration relative to the earth or inertial space. It converts mechanical vibrations into electrical signals that are easy to transmit, transform, process, and store through its transducer components.

There are many forms of vibration sensors, and the common classification is shown in Figure 1.

1.1 Testing principle of piezoelectric vibration sensor

Piezoelectric vibration sensor is one of the commonly used sensors for vibration testing in testing machines. The corresponding standards propose modification requirements for vibration acceleration measurement sensors, but often due to a lack of understanding of the concepts involved, some unreasonable installation methods are used, which to some extent affects the testing accuracy.

To correctly understand and implement standard requirements, it is necessary to have knowledge of relevant background, such as the testing principles, structures, and basic characteristics of sensors.

When some media undergo deformation due to mechanical pressure applied in a certain direction, polarization occurs inside and charges are generated on the surface. When the external force is removed, the electric field and surface charges inside the material also disappear. This characteristic is called piezoelectric effect. Piezoelectric vibration sensors utilize this characteristic to convert the mechanical vibrations sensed by the substrate into electrical energy output.

1.2 Basic Structure of Typical Piezoelectric Vibration Sensor

The typical structure of a piezoelectric vibration sensor is shown in Figure 2.

The piezoelectric crystal is compressed between the mass block and the substrate. When the accelerometer senses vibration, the mass block applies a vibration force to the piezoelectric crystal, generating a variable potential in the crystal. By appropriate design, it can be ensured that the input acceleration is proportional to the output potential within a certain frequency range.

1.3 Characteristics of piezoelectric vibration sensors

1.3.1 Frequency response

Mm is the mass of the mass block pressed on the sensitive element; Mb is the mass of the accelerometer substrate and housing; K is the equivalent stiffness of the system between Mm and Mb. The natural frequency of this system is:

Fo=fm

In the formula, fm is the natural frequency of the mass Mm on the spring K.

According to vibration theory: .

Assuming the accelerometer is rigidly installed on a much heavier structure, Mm/Mb → 0, fo → fm. Thus, the upper limit response frequency of the accelerometer is obtained as fm.

Piezoelectric vibration sensors can accurately detect a wide range of dynamic accelerations, making them suitable for measuring transient impact processes as well as sinusoidal and random vibrations. However, piezoelectric vibration sensors are not suitable for steady-state measurement scenarios, such as Earth's gravity, inertial guidance, or slow transient processes such as engine acceleration and braking.

1.3.2 Sensitivity

The sensitivity of an accelerometer is defined as the ratio of electrical output to mechanical input. From the sensor structure, it can be seen that sensitivity is directional. Due to manufacturing errors in the sensor, its maximum sensitivity direction is not aligned with the geometric axis, and the maximum sensitivity vector can be decomposed into two parts: axial sensitivity and lateral sensitivity.

The true representative of the sensitivity of piezoelectric vibration sensors is the charge sensitivity, which is not affected by changes in the internal capacitance of the sensor or cable length, but only depends on the piezoelectric constant of the piezoelectric material. Generally, the charge sensitivity decreases by less than 1% per year.

Piezoelectric vibration sensors are essentially solid-state devices that are very sturdy and durable, and generally do not cause damage in case of misuse. Inside the sensor, there are no adjustment components, which increases the reliability and repeatability of the sensor, making it suitable for extremely harsh environments.

Modification requirements and measurement accuracy of piezoelectric vibration sensors

2.1 Modification requirements for vibration acceleration measurement sensors

The corresponding standards specify the general modification requirements for vibration acceleration measurement sensors:

(1) The measuring axis of the sensor should be parallel to the axis being measured, and the lateral sensitive axis should avoid the direction with the maximum lateral acceleration.

(2) The sensor installation bracket has a small mass, good rigidity, and the natural frequency of the contact surface contact spring is at least five times greater than the natural frequency of the sensor.

(3) Unipolar sensors should be installed with insulation from the bracket.

2.2 Factors affecting measurement accuracy

(1) Insufficient installation stiffness can lower the response frequency and upper limit of the usage range, which is particularly significant in high-frequency measurements.

The standard stipulates that the natural frequency of the contact surface contact spring should be at least five times greater than the natural frequency of the sensor; This places high demands on the stiffness of the sensor mounting bracket and the contact stiffness of the mounting surface. If the sensor is directly installed on the tested structure, the natural frequency of its contact spring can be calculated based on the static deformation of the contact spring:

FM= (AE/L)δL

Here, (AE/L) is a constant related to material properties and geometric shape, while δ L represents deformation.Due to the generally small mass force of sensors, the stiffness of contact springs tends towards infinity; Therefore, the deformation δ is extremely small, and the contact frequency can meet the standard requirements.

If the sensor is installed on the tested structure through an adapter bracket, both the bracket stiffness and the contact stiffness of the two contact surfaces must be considered simultaneously, and the weight of the bracket must be minimized as much as possible while meeting the installation stiffness requirements. If the sensor is installed with insulation from the bracket, using insulation screws and mica gaskets can achieve maximum installation stiffness.

(2) The installation bracket is too heavy, and its mass load changes the original vibration of the structure, resulting in distorted measurement results; If measuring vibration on lighter or thinner structures, this impact cannot be ignored.

(3) The installation direction deviates from the calibration state of the sensor. The axial sensitivity axis direction of the sensor should be as consistent as possible with the required measurement direction. Once deviated, it will cause a decrease in axial response and an increase in lateral response. The accelerometer should be installed on a flat and clean surface, and the lateral sensitivity axis (marked with a red dot on the housing) should avoid the direction of maximum lateral acceleration.

(4) Improper tightening of bolts can cause the bolts to penetrate too deeply into the base, resulting in arch bending deformation of the base and additional electrical output. The tightening torque should be appropriate, too much will damage the threads, and too little will affect the installation stiffness.

2.3 Typical installation methods and key points of piezoelectric vibration sensors

There are four installation methods for piezoelectric vibration sensors: metal bolt installation, ground insulation conversion bolt connection, adhesive bonding, and magnet conversion suction cup connection. Among them, metal bolt installation and adhesive bonding are the most common. Using steel screws to fix the sensor on a polished metal surface can achieve the highest response frequency, while other installation methods may lower the response frequency.

To achieve accurate data conclusions, the first step is to use and install sensors correctly. When designing vibration sensor brackets, fasteners, and implementing installation, the following points should be followed:

(1) Understand the basic information of the measured parameters, such as the amplitude and frequency range of vibration acceleration;

(2) Understand the structural form and characteristics of sensors, including sensor quality, natural frequency, installation dimensions, etc;

(3) Determine the installation method of the transmitter based on the specific situation of the tested structure, and focus on considering the impact of additional mass on the measurement results for low-frequency measurements; For high-frequency measurements, it should be ensured that the installation stiffness meets the standard requirements;

(4) The installation contact surface of sensors and adapter brackets should be flat and smooth to ensure the accuracy and rigidity of sensor installation;

(5) Carefully adjust the installation direction of the sensor. Align the axial sensitive axis with the direction to be measured, and the lateral sensitive axis should avoid the direction with the maximum lateral acceleration;

(6) Control the depth of bolt insertion and tightening torque, with an appropriate tightening torque of 1.8N · m.