What is a Capacitive Sensor?

Catalog

Ⅰ Introduction

Since the late 1970s, with the development of integrated circuit technology, there have been capacitive sensors packaged with miniature measuring instruments. This new type of sensor can greatly reduce the influence of distributed capacitance and overcome its inherent shortcomings. The capacitive sensor is a sensor with extremely wide application and it has great development potential.

A typical capacitive sensor consists of upper and lower electrodes, an insulator, and a substrate. When the membrane is subjected to pressure, the membrane will be deformed to a certain extent. Therefore, the distance between the upper and lower electrodes changes to a certain extent, thereby changing the capacitance. However, the relationship between the capacitance of the capacitive pressure sensor and the distance between the upper and lower electrodes is non-linear. Therefore, a measurement circuit with a compensation function should be used to compensate the output capacitance nonlinearly.

Ⅱ Working Principle 

1 Parallel plate capacitors

It is known from physics that the capacitor formed by two parallel metal plates, if the edge effect is not considered, the capacitance is C=εs/d

Where ε--the dielectric constant of the medium between the two plates;

s--the relative effective area of the two polar plates;

d--the distance between two plates

It can be seen from the above formula that there are three methods for changing the capacitance C. One is to change the dielectric constant ε of the medium; the second is to change the effective area of the capacitor; the third is to change the distance between the two plates. The output of the obtained electrical parameter is the increment ΔC of the capacitance value, which becomes a capacitive sensor.

2 Cylindrical capacitors

Capacitive sensors are also often referred to as capacitive level gauges. The capacitance detection element of a capacitive level gauge works according to the principle of a cylindrical capacitor. The capacitor consists of two insulated coaxial cylindrical polar plates with internal electrodes and external electrode composition, when an electrolyte with a dielectric constant of ε is filled between the two cylinders, the capacitance between the two cylinders is

Where L is the length of the part where the two cylinders overlap; D is the diameter of the outer cylinder electrode; d is the diameter of the inner cylinder electrode; e is the dielectric constant of the intermediate medium. In the actual measurement, D, d, and e are basically unchanged, so the level of the liquid can be known by measuring C, which is also one of the reasons that the capacitive sensor has the characteristics of convenient use, simple structure, high sensitivity, and low price.

Capacitive sensors use various types of capacitors as sensing elements. Due to the measured changes, the capacitance of the capacitor will change. Through the measurement circuit, the change in capacitance can be converted into an electrical signal output. By measuring the size of the electrical signal, the measured size can be determined. This is the basic working principle of capacitive sensors.

Ⅲ Classification 

According to the working principle of the sensor, the capacitive sensor can be divided into three types: variable pole distance type, variable area type, and variable medium type. According to the structure of the sensor, capacitive sensors can be divided into three types of structural forms. They can be divided into linear displacement and angular displacement according to the form of displacement, each of which is divided into flat (circular) plate shape and cylindrical shape according to the shape of the sensor plate. Although there are spherical shape and zigzag shape,  they are rarely used. Among them, the differential type is generally better than the single group (unilateral) sensor, which has the characteristics of high sensitivity, wide linear range, and high stability.

Shapes of capacitive sensor 

1. Variable pole distance type capacitive sensor

The variable pole distance capacitive sensor is composed of a fixed pole wrench and a movable pole plate: the movable pole plate is served by the metal plane under test. The distance between the plates of the capacitive sensor changes due to the measured changes.

 variable pole distance capacitive sensor structure

The change of the electrode plate spacing is not a linear relationship, indicating that there is a principle nonlinear error in this type of capacitive sensor. That is, when the range is far less than the initial distance between the two plates, it can be considered a linear relationship. Therefore, this type of sensor is generally used to measure small displacement changes. However, when the distance between the plates is too small, the flatness of the electrode surface should have an effect on the sensitivity, and at the same time, it is easy to cause capacitor breakdown. The distance between the plates cannot be infinitely small. The way to improve is to add a piece of mica or plastic film between the plates.

The relative dielectric constant of mica is 7 times that of air, and its breakdown voltage is not less than l0 to the third power kv/mm, while the breakdown voltage of air is only 3kv/mm. Even if the mica sheet is 0.01mm thick, its breakdown voltage is also less than 10kv/mm. Therefore, after the mica sheet is placed, the initial distance between the plates can be greatly reduced. As long as the mica sheet is properly selected, a better linear relationship of the rope displacement sensor can be obtained. Generally, the initial capacitance of a capacitive sensor is between 20PF and 30pF, the distance between the plates is between 25um and 200um, and the maximum displacement should be greater than 1/10 of the distance.

In actual use, in order to improve the nonlinearity, increase the sensitivity and overcome some external conditions such as the power supply ratio, the influence of environmental temperature changes, etc., capacitive sensors often use the zigzag structure. When the initial state of the measurement is not started, adjust the movable plate to the middle position to make the capacitance on both sides equal; during measurement, the middle plate moves up and down with the measured object, which will cause the capacitance changes of the upper and lower parts. Therefore, the control value of the capacitors on both sides improves the sensitivity in this way, and the linearity made near the zero point is also improved.

2. Variable area type capacitive sensor

In variable area type capacitive sensor, the flat structure is particularly sensitive to the change of the dust distance, and the measurement accuracy is affected. The cylindrical structure is nearly not affected by the radial change of the weight plate. It has become the most common structure in practical applications. The capacitance change of the capacitive sensor proportional to the linear displacement when the edge effect is ignored.

Variable area type capacitive sensor 

Figure a is a flat-plate linear displacement structure, in which the pole plate 1 can movable, called the moving pole plate. The polar plate 2 is fixed and is called a fixed polar plate. Figure b is a concentric cylindrical deformed area sensor. The outer cylinder does not move, and the inner cylinder moves linearly up and down within the outer cylinder. Figure c is an angular displacement structure. When the axis of the polar plate 2 is driven by the object to be measured and rotates by an angular displacement of q degrees, the covering area A of the two polar plates decreases, so the capacitance decreases accordingly.

3. Variable dielectric constant (e) type capacitive sensor

Variable dielectric constant (c) type capacitive sensors are mostly used to measure the thickness and liquid level of dielectrics. They can also be used to measure the temperature and humidity of dielectric materials according to the change of the dielectric constant of the inter-electrode medium with changes in temperature and humidity. If the edge is ignored, the relationship between the capacitance of the single-group flat linear displacement sensor and the linear displacement of the medium is the length and width of the fixed plate and the length of the measured object entering between the two plates.

Ⅳ Advantage 

1) Good temperature stability

The capacitance value of the capacitive sensor generally has nothing to do with the electrode material, which is conducive to the selection of materials with a low-temperature coefficient, and because of the extremely small heat generated by itself, the stability is not affected. The resistance sensor has a copper loss, which is easy to generate heat and generate zero drift.

2) Simple structure

The capacitive sensor has a simple structure and is easy to manufacture and guarantees high accuracy. It can be made very small to achieve some special measurements; it can work in harsh environments such as high temperature, strong radiation, and strong magnetic field, and can withstand large temperatures Change, withstand high pressure, high impact, overload, etc.; it can measure ultra-high temperature and low-pressure difference.

3) Good dynamic response

Capacitive sensors have very little electrostatic energy (about a few 10^(-5)N) due to the electrostatic attraction between the electrode plates. The movable part can be made very small and thin, so its natural frequency is very high and its dynamic response time is short. It can work at a frequency of a few megahertz, especially suitable for dynamic measurement. Because of its small dielectric loss, it can be powered by a higher frequency and the system operates at a higher frequency. It can be used to measure high-speed changing parameters.

4) Non-contact measurement and high sensitivity

Capacitive sensors can measure the vibration or eccentricity of the rotary shaft, the radial clearance of small ball bearings, etc. through non-contact measurement. When non-contact measurement is used, the capacitive sensor has an average effect, which can reduce the influence of the surface roughness of the workpiece on the measurement.

In addition to the above-mentioned advantages, the capacitive sensor also has a very small electrostatic attraction between the electrode plates, and the required input force and input energy are extremely small, so it can measure extremely low pressure, force, and very small acceleration, displacement, etc. It can be made very sensitive with high resolution, which can sense 0.01μm or less displacement. Due to its low dielectric loss such as air, the zero residuals generated when connected in a bridge structure using a differential structure are extremely small, thus allowing the circuit to be amplified at high magnification and making the instrument highly sensitive.

Ⅴ Disadvantages

①High output impedance and poor load capacity

Regardless of the type of capacitive sensor, limited by the geometry of the electrode plate, its capacitance is very small, generally tens to hundreds of picofarads (pF), so the output impedance of the capacitive sensor is very high. Due to the high output impedance, the output power is small, the load capacity is poor, and it is susceptible to external disturbances causing instability.

②The influence of parasitic capacitance is big

The initial capacitance of the capacitive sensor is very small, but the parasitic capacitance of the lead cable capacitance connecting the sensor and the electronic circuit, the stray capacitance of the electronic circuit is large. The presence of parasitic capacitance not only reduces the measurement sensitivity, but also causes a nonlinear output. Because the parasitic capacitance changes randomly, the sensor is in an unstable working state which can affect the measurement accuracy.

Ⅵ Application 

Capacitive sensors have the advantages of simple structure, high-temperature resistance, radiation resistance, high resolution, and good dynamic response characteristics. They are widely used in pressure, displacement, acceleration, thickness, vibration, liquid level, and other measurements. However, pay attention to the impact of the following aspects on the measurement results: ① reduce the changes in ambient temperature and humidity (which may cause changes in the dielectric constant of certain media or the geometric dimensions and relative positions of the plates); ② reduce Edge effect; ③ Reduce parasitic capacitance; ④ Use shielding electrode and ground (protect the electric field of the sensitive electrode and isolate it from external electric field); ⑤ Pay attention to the leakage resistance, excitation frequency and the insulation of the plate support material.

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