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Home > Mall Dynamic > Capacitive Sensor Type And Advantages, Application
Capacitive sensors use various types of capacitors as sensing elements to convert the measured physical or mechanical quantity into a conversion device for capacitance changes, which is actually a capacitor with variable parameters. In applications using capacitive technology, the sensitivity of capacitive proximity switches depends on the material properties of the target object. The material properties of metal, plastic or liquid objects will affect the sensing range and thus the output detection point of the capacitive sensor.
Capacitive sensor can be used to measure linear displacement, angular displacement, vibration amplitude (can be measured to 0.05μm small amplitude), especially suitable for measuring high-frequency vibration amplitude, precision shafting rotation accuracy, acceleration and other mechanical quantities, can also be used to measure the difference pressure, liquid level, material level, food moisture content, non-metallic material coating, oil film thickness, measuring point media humidity, density, thickness Degree and so on. It is also often used as a position signal generator in automatic detection and control systems.
When measuring the metal surface condition, distance size, vibration amplitude, often use a single electrode variable pole distance type capacitance sensor, then the measured object is an electrode of the capacitor, the other electrode is in the sensor. Aerospace, automobile manufacturing, petrochemical, brick burning, ceramics, surface treatment, atmospheric environment, environmental test chamber, food, beverage, high-tech, heating, industry, metallurgy, meteorology, metrology, military, pharmaceutical, paper and other industries are useful.
1, capacitive sensor theory can detect any object, when the detection of high dielectric constant objects, the detection distance is greatly shortened, even if the increase in sensitivity will not be effective.
2, when the inductive load (such as lights, motors, etc.) is used, the transient impulse current is large, which may degrade or damage the AC two-wire capacitive proximity switch. In this case, an intermediate relay should be added as the transfer load.
3, DC two-wire proximity switch static leakage current is 0.5~1mA. In some cases where the leakage current of the proximity switch is large, the DC three-wire proximity switch should be used to improve its anti-interference performance.
4, capacitive proximity switch can not be used in the DC magnetic field environment above 0.02t, otherwise it will occur misoperation.
5, due to the influence of moisture, dust and other factors, capacitive sensors should be regularly maintained, including detecting changes in the installation position of objects and proximity switches, poor contact between wiring and connection parts, and whether there is dust adhesion.
6, to avoid the use of capacitive proximity switches in chemical solvents, especially in the strong acid, strong alkali environment.
Advantage
(1) Good temperature stability. The capacitance value of a capacitive sensor is usually independent of the electrode material, which is advantageous for selecting a material with a cryogenic system, and since it generates little heat, it has little effect on stability. Resistance sensors have resistance and generate heat after energizing: Inductive sensors have copper losses, magnetic flux and eddy current losses, etc., which are prone to zero drift due to heat.
(2) Simple structure. Capacitive sensors are simple in structure, easy to manufacture, easy to ensure high precision, and can be made very small to achieve certain special measurements: they can work in harsh environments such as high temperatures, strong vehicles and ships, and strong magnetic fields, and can withstand large temperature changes, withstand high pressure, high impact overload, etc. It can measure ultra-high temperature and low pressure difference, and can also measure magnetic work.
(3) Good dynamic response. Since the electrostatic attraction between the plates with electrodes is very small (about 105N), the capacitive sensor requires very little energy, and because its moving part can be made very small and thin, that is, very lightweight, it is very lightweight. The natural frequency is very high, the dynamic response time is short, and it can operate at a frequency of several megahertz, which is especially suitable for dynamic measurement. And because of its low dielectric loss, it can be powered through a higher frequency, so the system has a higher operating frequency. It can be used to measure fast-changing parameters.
(4) Non-contact measurement that can achieve an average effect. For example, non-contact measurement of vibration or eccentricity of rotating shafts, radial clearance of ball bearings, etc. When non-contact measurement is used, capacitive sensors have the function of averaging, which can reduce the influence of working surface roughness on measurement. In addition to the above advantages, the capacitive sensor also has a small electrostatic attraction between the electrode plates, so the input and input energy are very small, so it can measure extremely low pressure, as well as small acceleration, displacement, etc., can be made into a very sensitive, high-resolution and sensitive to 001m or even smaller displacement sensor: Due to its low dielectric loss (e.g. air), the zero residue generated when connecting the differential structure into a bridge is very small, thus allowing the circuit to perform high magnification, giving the instrument high sensitivity.
Shortcoming
1) High output impedance, poor load capacity · The capacity of the capacitor sensor is limited by the influence of the general geometry, which is usually only a few pF to several hundred pF, which makes the output impedance of the sensor very high, especially when using the AC power supply in the audio range, the output impedance is as high as 1061082, therefore, the load capacity of the sensor is very poor. It is easily affected by external interference and produces instability. In severe cases, it doesn't even work. Shielding measures must be taken, which brings great inconvenience to design and use. Large impedance also requires extremely high resistance values (more than a few dozen MQ) for the insulated part of the sensor, otherwise the insulated part will act as a bypass resistance and affect the performance of the instrument (e.g. reducing sensitivity). Therefore, special attention should be paid to the surrounding environment such as temperature and cleanliness. Not using a high-frequency power supply will reduce the output impedance of the sensor, but the high-frequency amplification and transmission are much more complex than the low-frequency, and the parasitic capacitance has a great impact, and it is not easy to ensure the stability of the work.
(2) Capacitance Due to structural and size limitations, the capacitance of the type sensor is very small (several pF to tens of pF), and the lead capacitance connects the sensor and the electronic circuit (1-2m wire can reach 800pF), the volume capacitance of other electronic circuits and the "parasitic capacitance" formed by the inner plate of the sensor and its surrounding conductors is relatively large. This not only reduces the sensitivity of the sensor, but these capacitors (cable capacitors) often change randomly, which will make the instrument work very unstable. It affects the measurement accuracy, so there are requirements for the selection, installation and connection of cables. With the development of materials, technology, electronic technology, especially integrated technology, the advantages of capacitive sensors continue to develop, but the disadvantages are constantly overcome. Capacitive sensor is gradually becoming a kind of sensor with high sensitivity and high precision, and is expected to be developed in dynamic, low pressure and some special measurement fields.
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