Capacitive Sensors: Concept, Structure, and Principle of Operation

Capacitive sensors are one of the devices that are widely used in many industries. So what is a capacitive sensor? How is it structured, classified and applied? Let’s explore the details with  RX Tradex through this article.

1. What is a capacitive sensor?

Capacitive sensors (also known as dielectric sensors) work on the principle of capacitance measurement to determine the dielectric constant of the surrounding environment. This sensor typically has a similar structure to a neutron probe, with the access tube section made of PVC and installed in the ground. In addition, the probes can be designed as modular (comb-like) and connected to the recorder.

Capacitive sensors are used to sense the level of liquids, adhesives, or small bulk solids such as powders, plastic granules, cement, sand,… A common application of these sensors is in factories, where they are used to measure and monitor the level of liquids or solids in water tanks, silos, or storage tanks.

2. Structure and principle of operation

A capacitive sensor consists of two electrodes and a layer of dielectric in between, called a capacitive element. This dielectric layer can be either flat or round, with electrodes placed on either side.

The first electrode, called the input electrode, and the second, called the output electrode. As the distance between the two electrodes changes, the capacitance of the sensor also changes.

Capacitive sensors can use a variety of different types of dielectrics, including polymers, oxides, ceramics, and various metals. The capacitance characteristics of the dielectric will affect the performance of the sensor.

Capacitive sensors are capable of measuring changes in temperature, humidity, pressure, vibration, and distance. The specifications of the capacitive sensor will be adjusted according to the specific application.

In industry, common types of capacitive sensors used to detect objects have the following structure:

• The first electrode (positively charged) is located on the sensor surface.
• The detected object acts as the second electrode (negatively charged).
• The environment between the sensor surface and the object to be detected acts as a dielectric.

Besides capacitive sensors, the industry also uses various types of sensors to meet diverse application requirements.

3. Classification of capacitive sensors

Capacitive sensors can be classified according to various criteria such as operating principle, size, and application. Here are some common types of capacitive sensors:

• Distance capacitive sensor: This type of sensor is designed to measure the distance between two objects. It is commonly used in applications such as measuring distance to objects, measuring thickness, and measuring length.
• Humidity capacitive sensor: This type of sensor specializes in measuring the humidity of the surrounding environment. It can measure moisture in soil, air, or other materials, and is often applied in humidity control devices such as air conditioners or medical equipment.
• Temperature capacitive sensor: This type of sensor is used to measure the temperature of an object or the surrounding environment. Its applications include measuring temperature in electronic devices, medical devices, or in temperature control systems.
• Pressure capacitive sensor: This type of sensor is used to measure the pressure of a liquid or gas. Its common applications include pressure measurement in air conditioning systems, medical equipment, and pressure control systems.
• Universal capacitive sensor: This type of sensor is capable of measuring various parameters such as temperature, pressure, humidity, and distance. It is widely used in instrumentation and control applications.

3.1. Capacitive water level sensor

Capacitive level sensors are used to report water levels in open water pipelines in water storage areas that need to be monitored.

The CLS23 capacitive sensor is a type of water level sensor with an extremely short electrode rod, ranging in length from 30mm to 1000mm. It is a specialized sensor series for measuring conductive flows (such as water, aqueous solutions) as well as non-conductive liquids (such as mineral oil, vegetable oil)

A prominent advantage of the CLS-23 capacitive sensor is its ability to accurately measure in tight locations without taking up much space. It is the lowest valued water level sensor among industrial water level sensors.

3.2. Oil and solids level sensors; Adhesives

Capacitive sensors are used to measure the level of water, oil, adhesives, and small-volume solids at low pressure.

This type of sensor has an electrode rod length of up to 6 meters.

Widely applied for level measurement in tanks of lubricants, vegetable oils, diesel oil, gasoline, fine powder and sand.

In particular, this water level sensor can be selected as a capacitive level sensor.

4. Capacitive Sensor Instruction Manual

Capacitive sensors are used to measure solids, liquids, or to indicate full or empty water levels. This device works by attaching the sensor to areas containing water or light-weight solids…

5. Principle of capacitive sensor mounting

The principle of mounting the capacitive sensor is as follows: For measurement positions that require electrode rods with a length of about 400-500 mm or more, it is recommended to mount the measuring sensor vertically from top to bottom for the best effect. As for rods with a length of 300 mm or less, it is recommended to install the sensor horizontally in tanks, silos, or on storage pipes.

6. Pros and Cons of Capacitive Sensors

To better understand what capacitive sensors are and make the right choice, let’s take a look at their pros and cons.

Advantages of capacitive sensors

• High sensitivity: Capacitive sensors are capable of detecting small changes in the capacitance of the dielectric, making accurate measurements of small variations.
• Fast response: Capacitive sensors have a fast response rate to changes in capacitance, allowing for rapid monitoring and recording of fluctuations.
• High reliability: This type of sensor is highly reliable, less affected by peripheral factors such as mechanical vibration, shock, or electromagnetic interference.
• Low cost: Compared to many other types of sensors, capacitive sensors typically have a lower cost.
• Diverse designs: Capacitive sensors can be built in a variety of sizes and shapes, making them easy to integrate into a variety of specific applications.

Disadvantages of capacitive sensors

• Affected by temperature: Temperature can impact the performance of a capacitive sensor, requiring frequent calibration to ensure accuracy.
• Low accuracy: Humidity, pressure, and other environmental factors can reduce the accuracy of the capacitive sensor.
• Poor durability: Capacitive sensors are susceptible to damage due to mechanical impact or shock.
• Application Limitations: Capacitive sensors are not suitable for instrumentation applications in highly corrosive environments.
• Difficult to operate in extreme environments: In extreme environments such as vacuum, or where the temperature is too high or too low, the capacitive sensor may not work effectively.

7. Some applications of capacitive sensors

One application for this device is to measure the water content of the soil, where the volume of water in the total volume of the soil has the greatest influence on the dielectric permeability of the soil. This is because the dielectric constant of water is much higher than that of other components in the soil (mineral soil: 4, organic matter: 4, air: 1). When the water content changes, a probe measures the change in capacitance, as the variation in dielectric permeability is directly related to the change in water content. Currently, capacitive sensors are being commonly used in irrigation planning in agriculture worldwide.

Cure Monitoring of Composite Materials: Dielectric or capacitive sensors are used to measure the electrical reactions of thermoset plastics and composite matrices at different depths on the sensor surface. The model that is mainly applied with these sensors is the electric field model. The correlation between the electrical properties of the material in the field and the measurement results (i.e. capacitance) plays a fundamental role in interpreting the readings from the dielectric sensor.

Measure the distance in the crusher test.

Capacitive sensors can also be used to measure the level of certain solid materials in structures such as hoppers or silos.

8. Capacitive sensor manufacturers

• Omron – Japan
• Autonics – Korea
• …

9. Conclusion

Capacitive sensors are an important device and are widely used in many industries today. Through this article, we hope you have a better understanding of the concept, structure and operating principle of capacitive sensors. In addition, to stay up-to-date with the latest trends in automation and electronics manufacturing technology, as well as discover advanced technologies in the industry, you can register to participate in the  NEPCON Vietnam exhibition, organized by RX Tradex in September 2024. Register here.