Working principle of capacitive sensors
Capacitive sensors use various types of capacitors as sensing elements. Changes in the measured quantity cause changes in the capacitor's capacitance. A measuring circuit converts these capacitance changes into an electrical signal output. By measuring the magnitude of this electrical signal, the magnitude of the measured quantity can be determined. This is the basic working principle of capacitive sensors.
Advantages of capacitive sensors
1. Good temperature stability
The capacitance value of capacitive sensors is generally independent of the electrode material, which is advantageous for selecting materials with low temperature coefficients. Furthermore, because they generate very little heat, their stability is minimally affected. In contrast, resistive sensors suffer from copper losses and are prone to overheating, leading to zero drift.
2. Simple structure
Capacitive sensors have a simple structure, are easy to manufacture and ensure high accuracy, and can be made very small to achieve certain special measurements; they can work in harsh environments such as high temperature, strong radiation and strong magnetic field, and can withstand large temperature changes, high pressure, high impact and overload; they can measure ultra-high temperature and low pressure difference, and can also measure magnetic work.
3. Good dynamic response
Capacitive sensors require minimal energy due to the very small electrostatic attraction between the electrode plates (approximately a few 10^(-5) N). Furthermore, their moving parts can be made very small and thin, resulting in very light weight. Consequently, they exhibit high natural frequencies and short dynamic response times, operating at frequencies of several megahertz, making them particularly suitable for dynamic measurements. Their low dielectric loss allows for higher frequency power supply, enabling high system operating frequencies. They can be used to measure rapidly changing parameters.
4. It can perform non-contact measurements and has high sensitivity.
It can perform non-contact measurements of vibration or eccentricity of rotating shafts, radial clearance of small ball bearings, etc. When using non-contact measurement, capacitive sensors have an averaging effect, which can reduce the influence of workpiece surface roughness on the measurement.
In addition to the advantages mentioned above, capacitive sensors also possess the advantage of very low electrostatic attraction between their electrode plates, requiring minimal input force and energy. This allows them to measure extremely low pressures, forces, and very small accelerations and displacements, resulting in high sensitivity and resolution, capable of sensing displacements as small as 0.01 μm or even smaller. Furthermore, due to their low dielectric losses (such as from air), the residual loss when using a differential structure connected in a bridge configuration is minimal, allowing for high-rate amplification and thus giving the instrument very high sensitivity.
Disadvantages of capacitive sensors
1. High output impedance and poor load capacity.
Regardless of the type, capacitive sensors are limited by the geometry of their electrode plates, resulting in very small capacitance, typically ranging from tens to hundreds of picofarads (pF). This leads to very high output impedance, reaching ~Ω. Due to this high output impedance, the output power is low, the load capacity is poor, and the sensor is susceptible to external interference, causing instability and, in severe cases, even malfunction.
2. Parasitic capacitance has a significant impact.
Capacitive sensors have a small initial capacitance, but significant parasitic capacitances, such as the capacitance of the lead cables connecting the sensor and the electronic circuitry, stray capacitance of the electronic circuitry, and the capacitance formed by the capacitor plates and surrounding conductors. The presence of parasitic capacitance not only reduces measurement sensitivity but also causes nonlinear output. Because parasitic capacitance varies randomly, it puts the sensor in an unstable operating state, affecting measurement accuracy.
