Vibration sensor

1 development trend

1. Introducing new technologies to develop new features [1]

As people deepen their understanding of nature, they will continue to discover new physical effects, chemical effects, and biological effects. With these new effects, corresponding new sensors can be developed, thus providing new possibilities for improving sensor performance and extending the application range of sensors. Yang Deyou, product manager and head of technical support at Turck Market Technology Division, told reporters, "The biggest feature of the sensor community is the continuous introduction of new technologies to develop new functions." Such as inductive proximity switches that detect the location of metal products, it uses metal objects to approach The position of the metal product can be detected by the eddy current effect on the measured metal when an oscillating sensor head capable of generating an electromagnetic field is generated. Because the effects of different metal eddy current effects are different, the detection distance of different metals is not the same, especially when faced with various types of alloys, the ordinary inductive proximity switches will become incompetent. This requires manufacturers to work hard to improve product functions. . Since the inductive proximity switch has an internal structure in which a coil is wound on a ferrite core as an inductor coil, and the ferrite core itself limits the inductive sensor from being developed under the existing design concept, only Technically developed products that can replace ferrite coils improve product performance. Turck's inductive proximity switches eliminate ferrite cores and remove core restrictions. In this way, when detecting different metals, the detection distance of the product can be improved through circuit adjustment, and the full metal detection distance is not attenuated, and the anti-interference ability is also improved.

2. Use new materials to develop new products

Sensor material is an important basis for sensor technology. With the advancement of materials science, people can create a variety of new sensors. For example, a temperature sensor made of a polymer film, the optical fiber can be made into a variety of sensors such as pressure, flow, temperature, displacement, etc., and a pressure sensor made of ceramic. Polymers absorb and release water molecules in proportion to the relative humidity of the surrounding environment. The polymer electrolyte is made into a capacitor, and the relative humidity is determined by measuring the change in capacitance. The plasma polymerized polystyrene film temperature sensor made by using this principle has the characteristics of wide humidity measurement range, wide temperature range, fast response speed, small size, and can be used to measure humidity in a small space and has a small temperature coefficient. The ceramic capacitive pressure sensor is a dry pressure sensor without intermediate fluid. Using advanced ceramic technology and thick-film electronic technology, its technical performance is stable. The full-scale error of annual drift is less than 0.1%. The temperature drift is small, and the overload resistance can reach several hundred times of the range.

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

2 test methods

In the field of engineering vibration testing, the testing methods and methods are various, 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

The engineering vibration parameters are converted into mechanical signals, and then measured and recorded after being amplified by a mechanical system. Commonly used instruments include a lever-type vibrometer and a Geiger vibrometer, which can measure a lower frequency and have a higher accuracy. difference. However, it is relatively simple and convenient when testing in the field.

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 microscope and laser vibrometer.

Electrical test

The engineering vibration parameters are converted into electrical signals, which are displayed and recorded after being magnified by electronic circuits. The main point of the electrical measurement method is to first convert the mechanical vibration quantity into electric quantity (electromotive force, charge, and other quantity of electricity), and then measure the quantity of electricity so as to obtain the mechanical quantity to be measured. This is the most widely used measurement method at present.

Although the physical properties of the above three measurement methods are not the same, the measurement systems that are composed of them are basically the same. They all contain three steps: pick-up vibration, measurement amplification circuit, and display and recording.

1, pick-up vibration. The measured mechanical vibrations are converted into mechanical, optical, or electrical signals. The device that completes this conversion is called a sensor.

2, measuring lines. There are many kinds of measuring lines, and they are all designed for the transformation principle of various sensors. For example, measuring lines that are specifically equipped with piezoelectric sensors include voltage amplifiers, charge amplifiers, and the like; in addition, there are integration lines, differential lines, filter lines, normalization devices, and the like.

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

3 reception principle

The vibration sensor is one of the key components in the test technology. Its role is mainly to receive the mechanical quantity and convert it into electricity proportional to it. 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 electrical quantity, but instead uses the original mechanical quantity to be measured as the input quantity of the vibration sensor, which is then received by the mechanical receiving part to form another mechanical quantity suitable for transformation. Finally, the electromechanical conversion part will be converted to electricity. Therefore, the working performance of a sensor is determined by the working performance of the mechanical receiving portion and the electromechanical converting portion.

1. Relative mechanical receiving principle

Since mechanical motion is the simplest form of physical motion, the first thing people think of is measuring vibrations mechanically, thus creating mechanical vibrometers (such as the Geiger vibrometers). The principle of mechanical reception of sensors is based on this. The work receiving principle of the relative vibrometer is to fix the instrument on a stationary support during measurement, so that the vibration direction of the contact rod and the measured object are the same, and the elasticity of the spring contacts the surface of the measured object. When the object vibrates, the stem moves along with it, and pushes the recording stem on the moving tape to depict the curve of the displacement of the vibrating object with time. Based on this recording curve, parameters such as the magnitude and frequency of the displacement can be calculated.

It can be seen that the measured result of the relative mechanical receiving part is the relative vibration of the measured object with respect to the reference body, and the absolute vibration of the measured object can only be measured when the reference body is absolutely not moving. In this way, a problem arises. When absolute vibrations need 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 running diesel locomotive, measuring the vibration of the ground and the building during an earthquake,... There is no immovable reference point. In this case, we must use another measurement method vibrometer to measure, that is, using an inertial vibrometer.

2, inertial mechanical receiving principle

When the inertial mechanical vibrometer measures vibration, the vibrometer is directly fixed on the measuring point of the measured vibrating object. When the sensor housing moves with the measured vibrating object, the inertial mass supported by the elastic will be opposite to the housing. In motion, the stylus pen mounted on the mass can record the relative vibration displacement amplitude of the mass element and the housing, and then use the relationship between the relative vibration displacement of the inertia mass and the housing to obtain the absolute value of the measured object. Vibration displacement waveform.

4 Electromechanical transformation

In general, vibration sensors have only relative and inertial principles in terms of mechanical receiving principle. However, in terms of electromechanical conversion, due to their different methods and properties, their types and applications are extremely wide.

The sensor used in modern vibration measurement is not a traditional conceptually independent mechanical measuring device. It is only a part of the entire measurement system and is closely related to the subsequent electronic circuits.

Due to the different internal electromechanical transformation principle of the sensor, the output power is also different. Some change the change in the mechanical quantity into the electromotive force and the change in the charge, while others convert the change in the mechanical vibration quantity into the change in the electrical parameter such as a resistance and an inductance. In general, these power levels cannot be directly accepted by subsequent display, recording, and analysis instruments. Therefore, sensors for different electromechanical conversion principles must be accompanied by dedicated measurement lines. The function of the measuring circuit is to change the output power of the sensor to a general voltage signal which can be accepted by the analysis instrument in the subsequent display. Therefore, vibration sensors can have the following classification methods according to their functions:

According to the principle of mechanical reception: relative, inertia;

According to electromechanical conversion principle: electric, piezoelectric, eddy current, inductive, capacitive, resistive, photoelectric;

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

The sensors in the above three classification methods are compatible.

Category 5

Relative

Electrodynamic sensors are based on the principle of electromagnetic induction. When a moving conductor cuts magnetic lines of force in a fixed magnetic field, electromotive forces are induced at both ends of the conductor. Therefore, sensors produced using this principle are called electrodynamic sensors.

The relative electric sensor is a displacement sensor from the mechanical receiving principle. Since the electromagnetic induction law is applied in the electromechanical conversion principle, the generated electromotive force is proportional to the measured vibration speed, so it is actually a speed sensor.

Eddy current

The 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 measured object. Eddy current sensors have the advantages of wide frequency range (0 ~ 10 kHZ), large linear working range, high sensitivity, and non-contact measurement. They are mainly used for static displacement measurement, vibration displacement measurement, and rotational shaft vibration measurement in rotating machinery.

Inductive

According to the relative mechanical reception principle of the sensor, the inductive sensor can convert the change of the measured mechanical vibration parameter into the change of the electric parameter signal. Therefore, there are two forms of inductive sensors, one is a variable gap and the other is a variable magnetically conductive area.

Capacitive

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

Inertial

The inertial electric sensor consists of a fixed part, a movable part and a support spring part. In order to operate the sensor in the displacement sensor state, the mass of the movable portion thereof should be large enough, and the rigidity of the supporting 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 rx& is the relative speed of the coil in the magnetic field.

From the sensor's structure, 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 is relatively moved in the magnetic field, the induced electromotive force is proportional to the speed of the coil cutting magnetic force line. Therefore, in terms of the output signal of the sensor, 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 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, generally can be found in the piezoelectric material performance table.) In a certain direction of the external force Under the action or under deformation, there will be a charge on its crystal surface 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 inverse piezoelectric effect.

Therefore, by using the piezoelectric effect of the crystal, a force sensor can be manufactured. In the vibration measurement, the force applied to the piezoelectric crystal is the inertia force of the inertial mass, and the number of charges generated is proportional to the magnitude of the acceleration, so the pressure The electric sensor is an acceleration sensor.

Piezoelectric force

In the vibration test, in addition to measuring the 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, and thus have 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 header

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 vibration response of the point while measuring the excitation force at the force transmission point. Therefore, the impedance head consists of two parts, one is the force sensor and the other is the acceleration sensor. Its advantage is that the response of the measurement point is the response of the excitation point. When used, the small head (measuring end) is connected to the structure, and the big head (measuring acceleration) is connected with the exciter's urging rod. The signal of excitation force is measured from the "force signal output terminal" and the response signal of acceleration is measured from the "acceleration signal output terminal".

Note that impedance heads can generally only withstand light loads and can therefore only be used for the measurement of lightweight structures, mechanical components and material specimens. 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 type

A resistive strain sensor converts the amount of mechanical vibration measured into the amount of change in the resistance of the sensing element. Sensing elements that implement this electromechanical conversion take many forms. The most common of these are resistive strain sensors.

The working principle of the resistance strain gauge is: When the strain gauge is affixed to a certain specimen, the specimen is deformed by force, and the original length of the strain gauge is changed, so that the resistance value of the strain gauge is changed. Experiments show that the strain is within the elastic variation range of the specimen. The relative change in sheet resistance is proportional to the relative change in its length.

laser

Laser sensor A sensor that uses laser technology to make measurements. It consists of a laser, a laser detector and a measuring circuit. The laser sensor is a new type of measuring instrument. Its advantages are that it can realize non-contact long-distance measurement, high speed, high precision, large range, and strong resistance to light and electricity. It is very suitable for non-contact measurement applications in industrial and laboratory applications.