Capacitive Sensor: A Full Understanding of It

What is a capacitive sensor?

Capacitive sensors use various types of capacitors as sensing elements to convert the measured physical or mechanical quantities into capacitance quantities. In fact, it is a capacitor with variable parameters. In applications using capacitive technology, the sensitivity of capacitive proximity switches depends on the material properties of the target object. The material properties of metal, plastic, or liquid objects will affect the sensing range and thus the output detection point of the capacitive sensor.

Capacitive sensors are widely used in the measurement of displacement, angle, vibration, speed, pressure, composition analysis, medium properties, etc. The most commonly used are parallel plate capacitors or cylindrical capacitors.

Since the late 1970s, with the development of integrated circuit technology, capacitive sensors packaged with miniature measuring instruments have appeared. This new type of sensor can greatly reduce the influence of distributed capacitance and overcome its inherent shortcomings. The capacitive sensor is a very versatile sensor with great potential for development.

Related Article: Most Comprehensive Introduction to Decoupling Capacitors

A typical capacitive sensor consists of upper and lower electrodes, an insulator, and a substrate. When the film is under pressure, the film will deform to a certain extent, so the distance between the upper and lower electrodes will change to a certain extent, so that the capacitance will change. However, the relationship between the capacitance of the capacitive pressure sensor and the distance between the upper and lower electrodes is a nonlinear relationship. Therefore, a measurement circuit with a compensation function should be used to perform nonlinear compensation on the output capacitance.

How does a Capacitive Sensor Work?

In the following part, Easybom will elaborate on the capacitive sensor working principle.

Structure

There are four basic elements in the capacitive sensor structure, namely sensor plate, oscillator, trigger circuit, and an output terminal. The target object and sensor board form the oscillator feedback circuit. When the object and the sensor board are in close proximity, the feedback capacitance appears; when the large capacitance appears, oscillation starts. A trigger circuit senses the oscillation level and is controlled to change the state of the switching device at the output. The amplitude of the oscillation decreases as the object moves further from the sensor tip, and increases as it gets closer. It is essentially the working principle of a capacitive sensor.

Capacitive sensors are also often referred to as capacitive level gauges. The capacitive detection element of capacitive level gauges works according to the principle of cylindrical capacitors. The capacitor consists of two insulated coaxial cylindrical plates, forming an inner electrode and an outer electrode. When an electrolyte with an electric constant ε between the two cylinders, the capacitance between the two cylinders is

C=2πeL/ln (D/d)

In the formula, L is the length of the overlapping part of the two cylinders; 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 positive intermediate medium. In fact, D, d, and e are mainly unchanged in the accurate measurement, so the liquid level can be known by measuring C, which is also the reason why 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 because the changes in accurate measurement will lead to changes in the capacitance of the capacitors. According to the precise measurement circuit, the changes in capacitance can be converted into electrical signal output. The size of the electrical signal can be measured, and the size that is accurately measured can be distinguished. This is the main working principle of capacitive sensors.

Capacitive Sensor Types

Pole-to-Pole Capacitive Sensor

Working characteristics:

The sensitivity s is inversely proportional to the square of the pole distance, the smaller the pole distance, the higher the sensitivity, but the reduction of the pole pitch is limited by the breakdown voltage between the capacitor plates. Generally δ0=0.1～1mm.

The capacitance C has a nonlinear relationship with the pole pitch δ. In order to reduce the nonlinear error, the variation range of the pole pitch is usually Δδ/δ0≈0.01～0.1.

This type of capacitive sensor is only suitable for the measurement of small displacements (0.01μm to hundreds of microns) and non-contact measurement.

Schematic diagram of the structure of the pole-to-pole capacitive sensor

In practical applications, in order to improve the sensitivity of the sensor, increase the linear working range and overcome the influence of changes in external conditions (such as power supply voltage, ambient temperature, etc.) on the measurement accuracy, differential capacitive sensors are often used.

Variable Area Capacitive Sensor

Since the movable plate of the flat-panel sensor moves slightly in the direction of the pole pitch, it will affect the measurement accuracy. Therefore, in general, the variable-area capacitive sensor is often made into a cylindrical shape.

The capacitance of a cylindrical capacitor is:

C=2πε0εr x/ln (D/d)

When the coverage length x changes, the capacitance changes, and its sensitivity is

S=dC/dx=2πε0εr//ln (D/d) = constant

The advantage of the area-variable capacitive sensor is that the output and input have a linear relationship, but compared with the plate-variable sensor, the sensitivity is lower, and it is suitable for the measurement of large angular displacements and linear displacements.

Variable Dielectric Capacitive Sensor

In the figure, a medium with a thickness of δ2 (ε2 is its permittivity) moves left and right in the capacitor, and the capacitance changes due to the change in the permittivity of the medium in the capacitor. Let the width of the capacitor plate be b, and the width of the medium ε2 is greater than or equal to b.

When there is no medium ε2 between the plates,

C0= bl / (δ/ε1)

The variable dielectric capacitive sensor is often used to measure the thickness, displacement, and liquid level of dielectrics. It can also be used to measure temperature, humidity, and capacity according to the change of the dielectric constant of the medium between the plates with the change of temperature, humidity, and capacity.

Capacitive Sensor Measurement Circuit

At present, bridge circuits, resonant circuits, frequency modulation circuits, operational amplifier circuits, differential pulse width modulation circuits, etc. are commonly used.

Bridge Circuit

Resonant Circuit

The capacitance Cx of the capacitive sensor acts as part of the tuning capacitance of the resonant tank (L2, C2, Cx). The resonant tank obtains the oscillating voltage from a stable high-frequency oscillator through inductive coupling. When the capacitance of the sensor changes, the impedance of the resonant circuit changes accordingly, which is converted into voltage or current, and then the corresponding output can be obtained after amplification and detection.

FM Circuit

Operational Amplifier Circuit

uy=-u0Ÿ(C0/Cx)=-u0·(C0δ/ε0εrA)

The output voltage is linear with the gap.

Pulse Width Modulation Circuit

U0=U1Ÿ(Δδ/δ0)

The DC output voltage is linear with the input (gap variation).

What are the differences between capacitive sensors and inductive sensors?

Capacitive sensor vs inductive sensor

Capacitive sensors can be classified into pole-to-pole type, variable area type, and variable dielectric type. In general, it can be used for more tested materials. Inductive sensors can only be used for metal objects to be measured.

On another level, capacitive sensors need to ensure that the measured environment is free of contamination, such as dust, oil, and water, as these factors can change the dielectric constant and thus the measurement results. The inductive sensor is insensitive to the above factors and can be used with confidence.

In addition, the accuracy of capacitive sensors is relatively higher, but the measurement frequency may be slightly lower. Inductive sensors only detect metal components, but capacitive sensors detect both plastic and metal components.

The range is slightly wider. Capacitive sensor components are relatively small inductive sensors. Inductive sensors use electromagnetic induction to convert the measured physical quantities such as displacement, pressure, flow, vibration, etc. into changes in the self-inductance coefficient and mutual inductance coefficient of the coil. The circuit converts the output of the change of voltage or current to realize the conversion from non-electricity to electricity.