Working principle of capacitive sensor
Capacitive sensors are based on various types of capacitors as sensing elements, because the measured changes will lead to capacitor capacitance changes, through the measurement circuit, the capacitance changes can be converted into electrical signal output. Measure the size of electrical signal, can judge the measured size. That's how capacitive sensors work.
Advantages of capacitive sensors
1, good temperature stability
The capacitance value of the capacitive sensor is generally independent of the electrode material, which is conducive to the selection of materials with low temperature coefficient, and because of its very small heat, little influence on stability. The resistance sensor has copper loss, easy to heat zero drift.
2. Simple structure
Capacitive sensors are simple in structure, easy to manufacture and ensure high accuracy, and can be made very small to achieve some special measurements; Can work in high temperature, strong radiation and strong magnetic field and other harsh environment, can withstand great temperature change, withstand high pressure, high impact, overload, etc. Can measure the difference between high temperature and low pressure, also can measure the magnetic work.
3. Good dynamic response
Capacitive sensors due to the electrostatic attraction between the charged plate is small (about several 10 (5) N), to the role of energy, and because of its moving part can be done very small is very thin, the quality is very light, so its inherent frequency is very high, the dynamic response time is short, can work under a few MHZ frequency, especially suitable for the dynamic measurement. And because its medium loss is small can use higher frequency power supply, so the system working frequency is high. It can be used to measure parameters with high speed variation.
4, non-contact measurement and high sensitivity
Non-contact measurement of vibration or eccentricity of rotating shaft, radial clearance of small ball bearings, etc. When non-contact measurement is used, capacitive sensor has average effect and can reduce the influence of workpiece surface roughness on measurement.
Capacitance sensor in addition to the advantages of the above, but also because of the electrostatic attraction between the charged plate is very small, the required input force and input energy is very small, so it can measure very low pressure, force and very small acceleration, displacement, etc., can do very sensitive, high resolution, can sense 0.01μm or even smaller displacement. Due to the small loss of air and other media, the zero residual generated by the differential structure and connected to the bridge type is very small, so the circuit is allowed to carry out high magnification, so that the instrument has a high sensitivity.
Disadvantages of capacitive sensors
1, high output impedance, poor load capacity.
No matter what type of capacitive sensor, limited by the geometric size of the electrode plate, its capacitance is very small, generally dozens to hundreds of skin method (pF), so that the output impedance of the capacitive sensor is very high, up to ω. Due to the high output impedance, the output power is small, the load capacity is poor, vulnerable to external interference and unstable phenomenon, and even can not work seriously.
2. Parasitic capacitance has great influence.
The initial capacitance of the capacitive sensor is very small, but the capacitance of the lead cable connecting the sensor and the electronic circuit, the stray capacitance of the electronic circuit and the capacitance of the capacitor plate and the surrounding conductor and other parasitic capacitance is large. The existence of parasitic capacitance not only reduces the sensitivity of measurement, but also causes nonlinear output. Because the parasitic capacitance is random. Therefore, the sensor is in an unstable working state. Affect measurement accuracy.