An overview of capacitive sensors and how capacitive sensors work

For general acoustic emission applications, the signal frequency band range is mostly between 25kHz and 750kHz, so resonant sensors are more suitable. Resonant narrowband sensors with a general broadband frequency of 150kHz are commonly used for metal materials and other applications; For acoustic emission signals such as corrosion and leakage, resonant sensors with a resonant frequency of 40kHz are commonly used; Due to the relatively low response sensitivity of wideband sensors in a wide frequency band, wideband sensors are selected for spectrum analysis. Qingcheng acoustic emission sensor is made of high-quality stainless steel material, and the overall shielding design can effectively reduce interference; It is suitable for bridge wire fracture monitoring, valve pipeline leakage monitoring, tool wear monitoring, wind power equipment monitoring, rotating equipment acoustic emission monitoring, structural parts damage monitoring, storage tank floor acoustic emission monitoring.

From the perspective of energy conversion, a capacitive converter is a passive converter, which needs to convert the measured mechanical quantity into voltage or current and then amplify and process it. Linear displacement, angular displacement, spacing, distance, thickness, tension, compression, expansion, deformation, etc. in the mechanical quantities are all closely related to length; These quantities are all measured by length or length ratio, and their measurement methods are closely related to each other. In addition, under some conditions, these mechanical quantities change quite slowly, and the range of change is extremely small, if it is required to have a high resolution when measuring a very small distance or displacement, it is difficult for other sensors to achieve high resolution requirements, and the resolution of the differential transformer sensor commonly used in precision measurement only reaches the order of 1 5 m; A capacitive micrometer with a resolution of four orders of magnitude, a maximum range of 1250 m, and zero hysteresis, is favored for precision small displacement measurements.

A capacitive sensor is a sensor that converts a change in measurement (e.g., size, pressure, etc.) into a change in capacitance. A sensor that converts measured mechanical quantities, such as displacement and pressure, into capacitance changes. The sensitive part of it is the capacitor with variable parameters.

The most commonly used form is a capacitor consisting of two parallel electrodes with air as the medium between the poles. Ignoring the edge effect, the capacitance of the plate capacitor is a δ, where is the dielectric constant of the dielectric between the poles, a is the effective area of the two electrodes covering each other, and δ is the distance between the two electrodes.

A change in any of the three parameters will cause a change in capacitance and can be used for measurement. Therefore, capacitive sensors can be divided into three types: pole distance change type, area change type, and medium change type. Pole distance variation is generally used to measure small linear displacements or pole distance changes due to force, pressure, vibration, etc. (see Capacitive Pressure Sensors).

The area-varying type is generally used to measure angular displacements or large linear displacements. The medium change type is often used for level measurement and the determination of temperature, density and humidity of various media. Since the end of the 70s, with the development of integrated circuit technology, capacitive sensors have appeared packaged with miniature measuring instruments.

This new type of sensor can greatly reduce the influence of distributed capacitance, overcoming its inherent drawbacks. Capacitive sensors are a very versatile sensor with great potential.

Capancdt capacitive displacement sensors are based on the flat plate capacitance principle. The two poles of a capacitor are the sensor and the object being measured. If there is a stable alternating current passing through the sensor, the voltage of the output alternating current will be proportional to the distance between the sensor and the measured object, so that the distance information can be obtained by measuring the change of voltage.

Capacitive displacement sensor is a kind of non-contact capacitive principle of precision measuring instrument, with the general non-contact instrument common friction, non-destructive wear characteristics, but also has the advantages of large signal-to-noise ratio, high sensitivity, zero drift, wide frequency response, small nonlinearity, good accuracy and stability, strong anti-electromagnetic interference ability and easy to use.

In practice, thanks to the unique magnetic shielding ring design, the capacitive sensors from Mi-Iridium can achieve near-perfect linear measurements. However, capacitive sensors require that the dielectric between the probe and the object to be measured must be uniform and constant. The measuring system is very sensitive to dielectric changes in the measuring range.

The capacitance detection element of the capacitive sensor works according to the principle of cylindrical capacitor, the capacitor is composed of two insulated inner electrodes and an outer electrode of the coaxial cylindrical plate, and when the electrolyte with dielectric constant e is charged between the two cylinders, the capacitance between the two cylinders is c = 2 el lnd d, where l is the length of the overlapping part of the two cylinders; d is the diameter of the outer tube electrode; d is the diameter of the inner cylinder electrode; e is the dielectric constant of the intermediate medium. In the actual measurement, d, d, and e are basically unchanged, so the capacitive sensor has the characteristics of easy use, simple structure and high sensitivity.

<>1. The capacitance sensor is also known as the capacitance level meter, and its capacitance detection element works based on the principle of cylindrical capacitor, the cylindrical capacitor is mainly composed of two coaxial cylindrical plates that are insulated from each other, and the medium is filled between the two plates, then the capacity of the capacitor is c=2 el lnd d, wherein, represents the dielectric constant of the medium between the two plates, l represents the length of the two plates that coincide with each other, d represents the diameter of the outer cylindrical plate, and d represents the diameter of the inner cylindrical plate. Since the three parameters of d, d, and e do not change when measured under fixed conditions, the level height can be known according to the measured capacitance.

2. Capacitive sensors have certain advantages over resistive sensors and inductive sensors, but they are not perfect, and they also have shortcomings

3. Advantages: Cheap, affordable; High sensitivity and accuracy; Simple structure; It can also be applied in harsh environments; Good temperature stability; with an averaging effect; Good dynamic responsiveness; High overload capacity.

4. Disadvantages: output is non-linear; The sensitivity and measurement accuracy of parasitic capacitance and distributed capacitance are easily affected and unstable; The connection circuit is more complex.

a.The power is small, and the impact of heating itself is small.

b.The capacitor has a small capacitance and high output impedance.

c.The movable mass is small, the inherent frequency stuffiness and dust rate is high, and the dynamic response characteristics are good.

d.The electrostatic attraction of the plates is small, and the force required for work is very small.

Correct answer: The power is small, and the impact of its own heating is small; The capacitor has a small capacitance and high output impedance. The movable mass is small, the natural frequency is high, and the dynamic response characteristics are good; The electrostatic attraction of the plate is small, and the force required for work is very small.

According to the working principle of the sensor, the capacitive sensor can be divided into three types: variable pole distance type, variable area type and variable medium type.

According to the structure of the sensor, capacitive sensors can be divided into three types of structural forms. They can be divided into two kinds according to the form of displacement: linear displacement and angular displacement, each of which is divided into flat (circular) plate shape and cylindrical (cylindrical) shape according to the shape of the sensor plate, although there are other shapes such as spherical shape and zigzag shape, but generally rarely used. Among them, the differential type is generally better than the single-group (single-sided) sensor, which has the characteristics of high sensitivity, wide linear range and high stability.