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What are capacitive sensors and how they work Advantages and disadvantages

What are capacitive sensors and how they work Advantages and disadvantages

September 14 , 2023 0

Capacitive sensors are various types of capacitors as the sensing element, the measured physical or mechanical quantities into a capacitance change of a conversion device, in fact, is a capacitor with variable parameters. Capacitive sensors are widely used in the measurement of displacement, angle, vibration, speed, pressure, composition analysis, media characteristics and so on. The most commonly used are parallel plate capacitors or cylindrical capacitors.

Operating Principle Of Capacitive Transducer

Capacitive sensors use various types of capacitors as the sensing element. Changes in the capacitance of the capacitor are caused by changes in the measurement, and the change in capacitance is converted to an electrical signal output through the measurement circuit. By measuring the size of the electrical signal, the size of the measurement can be judged. This is the basic working principle of capacitive sensors.

Advantages Of Capacitive Transducer

1, Good Temperature Stability

Capacitive sensor capacitance value is generally unrelated to the electrode material, which is conducive to the selection of low temperature coefficient of the material, but also because of its own heat is very small, affecting the stability is very small. Resistive sensors have copper loss, easy to heat zero drift.

2, Simple Structure

Capacitive sensor structure is simple, easy to manufacture and ensure high accuracy, can be made very small, in order to achieve some special measurements; can work in high temperatures, strong radiation and strong magnetic fields and other harsh environments, can withstand a lot of temperature changes, withstand high pressure, high impact, overload, etc.; can measure the ultra-high temperatures and low-pressure differentials, but also to work with the magnetism of the measurement.

3, Good Dynamic Response

Capacitive sensor due to the electrostatic forces between the charged plate is very small (about a few 10 ^ (-5) N), the role of the energy required is very small, but also because of its movable part can be made very small and thin, that is, the quality is very light, so its intrinsic frequency is very high, the dynamic response time is short, can be in a few megahertz of frequency work, especially suitable for dynamic measurements. And because of its small dielectric loss can be powered by a higher frequency, so the system working frequency is high. It can be used to measure high-speed changes in parameters.

4, Can Be Non-contact Measurement And High Sensitivity

Non-contact measurement of rotary shaft vibration or eccentricity, small ball bearing radial clearance. When non-contact measurement, capacitive sensors have an averaging effect, which can reduce the impact of workpiece surface roughness on the measurement.
Capacitive sensors in addition to the above advantages, but also because of its charged electrostatic gravitational force between the electrode plate is very small, the required input force and input energy is very small, and thus can measure very low pressure, force and very small acceleration, displacement, etc., can be made very sensitive, high resolution, can sense 0.01μm or even smaller displacements. Because of its air and other media loss is small, the use of differential structure and connected to the bridge type when the zero residual is very small, thus allowing the circuit to carry out a high rate of amplification, so that the instrument has a very high sensitivity.

Characteristics of Capacitive Transducer

Advantages

(1) Good temperature stability. The capacitance value of a capacitive sensor is usually independent of the electrode material, which facilitates the selection of materials with low-temperature systems and has little effect on stability because it generates very little heat. Resistive sensors have resistance and generate heat when energised: inductive sensors have copper losses, magnetic flux and eddy current losses, etc., and are prone to zero drift due to heat.　　

(2) Simple structure. Capacitive sensors are simple in structure, easy to manufacture, easy to ensure high accuracy, and can be made very small to achieve some special measurements: they can work in high temperatures, strong vehicles and ships, and strong magnetic fields and other harsh environments, and can withstand large temperature variations, withstand high pressures, high-impact overloads and so on. It can measure ultra-high temperature and low pressure difference, and can also measure magnetic work.　　

(3) Good dynamic response. Since the electrostatic gravitational force between plates with electrodes is very small (about 105N), capacitive sensors require very little energy, and since their moving parts can be made very small and thin, i.e., lightweight, they are very lightweight. The high natural frequency, the short dynamic response time and the fact that they can operate at frequencies of several megahertz make them particularly suitable for dynamic measurements. And because of its low dielectric loss, it can be powered by higher frequencies, so the system has a high operating frequency. It can be used to measure parameters that change at high speeds.　　

(4) Non-contact measurement of averaging effects is possible. For example, non-contact measurement 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 the roughness of the working surface on the measurement. In addition to the above advantages, capacitive sensors also have a small electrostatic attraction between the electrode plates, so the input and input energy is very small, so it can measure very low pressure, as well as small acceleration, displacement, etc., can be made into a very sensitive, high-resolution and sensitive to the displacement of 001m or even smaller sensors: due to its low dielectric loss (such as air), the differential structure is connected to a bridge when the Due to its low dielectric loss (e.g. air), the zero residue generated when connecting the differential structure as a bridge is very small, thus allowing the circuit to be highly amplified, giving the instrument a high sensitivity.

Disadvantages　

(1) High Output Impedance, Poor Loadability - The capacity of capacitive sensors is limited by the common geometry usually only a few pF to a few hundred pF, which makes the output impedance of the sensors very high, especially when using AC power in the audio range, up to 1061082 Therefore, the sensor's loadability is very poor and it is susceptible to external disturbances and instability. In severe cases, it may not even work. Shielding measures must be taken, causing great inconvenience in design and use. Large impedances also require extremely high resistance values (more than a few tens of MQ) in the insulating part of the sensor, otherwise the insulating part will be used as a bypass resistor and will affect the performance of the instrument (e.g., reduce the sensitivity). Therefore, special attention should be paid to the temperature, cleanliness and other surroundings. Do not use high-frequency power supply will reduce the output impedance of the sensor, but high-frequency amplification and transmission is much more complex than low-frequency, and parasitic capacitance has a great impact, it is not easy to ensure the stability of the work.　　

(2) capacitance due to structural and dimensional limitations, the type of sensor capacitance is very small (a few pF to dozens of pF), and lead capacitance connecting sensors and electronic circuits (1-2m wire can reach 800pF), other electronic circuits volume capacitance and the formation of sensors by the inner plate and its surrounding conductors "parasitic capacitance". Relatively large, which not only reduces the sensitivity of the sensor, and these capacitances (cable capacitance) often change randomly, which will make the instrument work very unstable. This not only reduces the sensitivity of the sensor, but these capacitances (cable capacitance) often change randomly, which makes the instrument work very unstable. With the development of materials, technology, electronics and especially integration technology, the advantages of capacitive sensors continue to develop, while the disadvantages are constantly being overcome. Capacitive sensors are gradually becoming a sensor with high sensitivity, high accuracy, and is expected to be developed in the dynamic, low-voltage and some special measurement areas.

Application Areas For Capacitive Transducer

Capacitive transducer can be used to measure linear displacement, angular displacement, vibration amplitude (can be measured up to 0.05 μm of the small amplitude), especially suitable for measuring high-frequency vibration amplitude, precision shaft rotation accuracy, acceleration, and other mechanical quantities, but also can be used to measure the difference between the pressure, level, surface, moisture content of grain, non-metallic materials, coating, oil film thickness, the measurement of the point of the medium of humidity, density, thickness and so on. In the automatic detection and control system is also often used as a position signal generator.　　When measuring the metal surface condition, distance size, vibration amplitude, often using a single electrode type variable pitch type capacitance sensor, then the measured object is a capacitor electrode, the other electrode is in the sensor.　　

Aerospace, automotive manufacturing, petrochemical, brick burning, ceramics, surface treatment, atmospheric environment, environmental chamber, food, beverage, high-tech, ambiguous ventilation, industrial, metallurgy, meteorology, measurement, military, pharmaceutical, paper and other industries are useful.