Introduction to Capacitive Sensor Basic and Applications

Capacitive sensor is a detect device that can measure physical or mechanical quantities without contact. So it is actually a capacitor with a detection function. It is widely used for measurement of displacement, angle, vibration, speed, pressure, composition analysis, medium properties, etc.

Basic

1. Working Principle

The working principle of the capacitive sensor is based on the change in capacitance caused by the distance between the two electrodes of the sensor, the relative size change, and the change in the medium of capacitor. The formula for calculating the capacitance is as follows:

Where,

ε0=8.85×1012F/m

ε is the relative dielectric constant of the medium between the plates. When the medium is air, ε0=1, and A is the substrate area.

δ is the distance between the plates.

When these parameters change, it will also cause changes in capacitance. If two of the parameters are kept unchanged and only the other parameter is changed, the change in this parameter can be converted into a change in capacitance.

2. Pros and Cons

Advantages

1) Good temperature stability

The capacitance value of capacitive sensors generally has nothing to do with the electrode material, which is better to choosing materials with low temperature coefficients. Since they generate very little heat, they can work stably.

2) Simple structure

Capacitive sensors have a simple structure, are easy to manufacture and ensure high accuracy. They can be made very small to do certain special measurements; they can work in harsh environments such as high temperature, strong radiation and strong magnetic fields, and can withstand high temperature, high voltage, high impact, overload, etc.

3) Good dynamic response

Because the electrostatic attraction between the electrode plates is very small (about a few 10^(-5)N), the capacitive sensor requires very little energy, and because its movable part can be made very small and light, so its natural frequency is very high, the dynamic response time is short. Also it can work at frequencies of several megahertz, which is especially suitable for dynamic measurements. And because its dielectric loss is small, it can be powered at a higher frequency, in addition, the system operating frequency is high.

4) Non-contact measurement with high sensitivity

It can measure the vibration or eccentricity of the rotary shaft, the radial clearance of small ball bearings, etc. without contact. In terms of it, the capacitive sensor has an averaging effect, which can reduce the impact of the surface roughness on the measurement.

In addition to the above advantages, because the electrostatic attraction between the electrode plates is very small, the required input force and input energy are extremely small, so it can measure extremely low pressure, force, very small acceleration, and displacement, etc. As for advantages, it is very sensitive, has high resolution and can sense displacements of 0.xn--01m-yyc or less. Due to its small loss in air and other media, the zero residual generated when using a differential structure and connected in a bridge type is extremely small, thus allowing the circuit to amplify at a high rate, making the instrument highly sensitive.

Disadvantages

1) High output impedance and poor load capacity

Regardless of the type of capacitive sensor, limited by the size of the electrode plate, its capacitance is very small, generally tens to hundreds of picofarads (pF), so the output impedance of it is very high. Under this, the output power is small and the load capacity is poor. So it is easily affected by external interference and even fails to work.

2) Parasitic capacitance influence

The initial capacitance of the capacitive sensor is very small, but the parasitic capacitances such as the capacitance of the lead cable connecting the sensor and the electronic circuit, the stray capacitance of the electronic circuit, and the capacitance formed by the capacitor plate and the surrounding conductors are large. The existence of parasitic capacitance not only reduces the measurement sensitivity, but also causes nonlinear output. Because the parasitic capacitance changes randomly, the sensor is in an unstable working state, which affects measurement accuracy.

3. Classifications

1) Variable plate distance capacitive sensor

When the moving plate moves up and down due to the measured change, the distance δ between the two plates will change, causing a change in capacitance. If the plate coverage area is A, the initial gap is δ, and the dielectric constant is ε, then the initial capacitance is:

When the moving pole plate moves downward Δδ, the distance between the pole plates δ=δ0?Δδ.

The increment of capacitance is:

In a differential capacitive sensor, if the capacitance of one capacitor C1 increases with the displacement, the C2 capacitance decreases accordingly:

where:

The total change in capacitance is:

Its sensitivity is:

The relative error is:

It can be seen from the above formula that the sensitivity is inversely proportional to the square of the gap. If the gap is large, the sensitivity is low. It should be noted that the problem caused by high sensitivity is that nonlinearity will increase.

When such capacitor forms a differential structure, not only its sensitivity is doubled, but the linearity error will also be greatly reduced. Therefore, this type capacitive sensor has high sensitivity, and the measured component can be used as a moving pole plate to achieve non-contact measurement, and can be used to measure small displacement and pressure.

2) Variable area capacitive sensor

The sensitivity S of this capacitive sensor is a constant, which means that the output is linearly related to the input (edge effects are ignored). However, compared with the former type, its sensitivity is lower and it is suitable for the measurement of larger angular displacement and linear displacement. In addition, it can use differential structures to improve static sensitivity, generally doubling the sensitivity.

3) Variable dielectric capacitive sensor

Changing the capacitance by changing the capacitor dielectric is a well-established measurement method. The capacitance C depends on the dielectric constant ε0 and the medium thickness a2. Therefore, when one of these two parameters is known, the other can be determined by the formula:

a. Measure dielectric thickness

b. Measure dielectric position

The figure above shows the form of a dielectric change capacitive sensor with a movable medium. The medium 2 is inserted into the capacitor to a certain depth, which is equivalent to connecting two capacitors in parallel. This principle is often used to measure the liquid level or filling height of non-conductive and loose materials.

c. Measuring dielectric level

If the edge effect is ignored, the cylindrical liquid level sensor is as shown below. The relationship between the capacitance of the sensor and the liquid level is:

It can be seen that the sensor capacitance C has a linear relationship with the measured liquid level height hx.

d. Measure dielectric temperature and humidity

For example, it is used in the detection of grain moisture.

e. Measuring circuit

The figure below shows the measuring circuit and output voltage waveform of a typical condenser pickup. The pickup hardware part consists of a capacitive pickup sensor and supporting circuit. Among them, the basic structure of the capacitive pickup sensor is composed of a back electrode, an inner cavity, a capillary hole, a diaphragm, a damping hole and an insulator. Its function is to convert changes in air pressure caused by sound waves into changes in the capacitance, thereby making the voltage on the resistor R in the detection circuit changes, thus converting the sound wave signal into a voltage signal.

Main Applications

1) Capacitive sensors can detect and measure conductive objects with dielectrics other than air.

2) These sensors are most commonly used to determine changes in the position of conductive targets. However, these types of sensors are also very effective at measuring density, thickness, and location of non-conductors. In addition, non-conductive materials such as plastics have different dielectric constants compared to air.

3) Capacitive sensors are used to detect or measure position, proximity, acceleration, displacement, liquid level and humidity.

4) As input devices, capacitive sensors are used in mobile phones, tablets, digital audio players, etc.

5) These sensors replace mechanical buttons.

Examples

1) Capacitive pressure sensor

The diaphragm and the metal layer plated on the surface of the spherical glass form a differential capacitance. Under the action of the pressure difference ΔP =P1?P2ΔP, the diaphragm moves in the direction of small pressure, causing a change in the differential capacitance C, that is, the capacitance C is proportional to the pressure difference ΔP. It is suitable for measuring the micro pressure difference of liquid (gas).

2) Capacitive speed sensor

How it works is shown in the figure below. When the fixed pole plate is opposite to the tooth top, the capacitance is maximum; when it is opposite to the tooth gap, the capacitance is minimum. When the gear rotates, the capacitive sensor generates a periodic signal, which is converted into a pulse signal by the test circuit, and a frequency meter is used to display the gear speed.

Anyone interested in semiconductor industry, please follow KUKE Electronics to learn more.