TQ What are the design points of capacitive sensors?

Chimicron

Junior Member
The unique characteristics of high sensitivity and high precision of capacitive sensor are inseparable from its correct design, correct material selection and fine processing technology. In the process of designing the sensor, within the required range, temperature and pressure range, try to make it have low cost, high precision, high resolution, stability and reliability and good frequency response, etc., but it is generally difficult to achieve the ideal level. A compromise solution is therefore often used. For capacitive sensor, in order to develop its characteristics and overcome its shortcomings, the following aspects can be considered in the design.

Reduce the error caused by changes in ambient temperature, humidity, etc., to ensure the insulation performance of the insulating material. The temperature change changes the geometric size and mutual position of each part in the sensor and the dielectric constant of some medium, thereby changing the capacitance of the capacitive sensor, resulting in temperature error. Humidity also affects the dielectric constant and insulation resistance values of some media. Therefore, it is necessary to reduce errors such as temperature and ensure that the insulating material has high insulating performance from the aspects of material selection, structure, processing technology, etc.

capacitive sensor


The material of the metal electrode of the capacitive sensor is preferably an iron-nickel alloy with a low temperature system, but it is difficult to process. The process of sputtering alloy or silver on ceramic or quartz can also be used, so that the electrode can be made very thin, which is extremely beneficial to reduce the edge effect.

The electrode surface inside the sensor is inconvenient to clean frequently, so it should be sealed to prevent dust and moisture. If a very thin layer of inert metal (such as rhodium, etc.) is plated on the surface of the electrode, it can replace the seal and play a protective role, which can prevent dust, temperature, moisture and corrosion, and can reduce surface loss and reduce temperature coefficient, but the cost is higher.

In the sensor, the electrode holder should have stable performance in addition to a certain mechanical strength. Therefore, the material with small temperature coefficient, geometric size, good stability, high insulation resistance, low moisture absorption and high surface resistance is selected as the bracket. For example, quartz, mica, gemstones, and various ceramics can be used. Although they are more difficult to process, their performance is much higher than that of materials such as plastics and plexiglass. In the environment where the temperature is not too high, PTFE has good insulating properties, which can also be considered when selecting.

Try to use air or mica and other dielectrics with a temperature coefficient of dielectric constant close to zero (not affected by humidity changes) as the dielectric of the capacitive sensor. If some liquids such as silicone oil, kerosene, etc. are used as the dielectric When the humidity changes, their dielectric constants change accordingly, resulting in errors. Although this error can be compensated by the subsequent electronic circuit, it is not easy to completely eliminate it.

If possible, the differential symmetrical structure should be used in the sensor, so that errors such as temperature can be reduced by some types of electronic circuits (such as bridges).

Selecting 50kHz to several MHz as the power frequency of the capacitive sensor can reduce the insulation requirements for the insulation part of the sensor.

All parts in the sensor should be cleaned and dried before assembly. The sensor should be sealed to prevent the change of capacitance value and the deterioration of insulation caused by water immersion inside. The rigidity of the sensor shell is better to avoid deformation during installation.
 
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